ECOLOGICAL DISTRIBUTION OF NOCTURNAL RODENTS IN A PART OF THE SONORAN DESERT
Item Type text; Dissertation-Reproduction (electronic)
Authors Hoagstrom, Carl William
Publisher The University of Arizona.
Rights Copyright © is held by the author. Digital access to this material is made possible by the University Libraries, University of Arizona. Further transmission, reproduction or presentation (such as public display or performance) of protected items is prohibited except with permission of the author.
Download date 28/09/2021 11:56:20
Link to Item http://hdl.handle.net/10150/298432 INFORMATION TO USERS
This material was produced from a microfilm copy of the original document. While the most advanced technological means to photograph and reproduce this document have been used, the quality is heavily dependent upon the quality of the original submitted.
The following explanation of techniques is provided to help you understand markings or patterns which may appear on this reproduction.
1.The sign or "target" for pages apparently lacking from the document photographed is "Missing Page(s)". If it was possible to obtain the missing page(s) or section, they are spliced into the film along with adjacent pages. This may have necessitated cutting thru an image a>id duplicating adjacent pages to insure you complete continuity.
2. When an image on the film is obliterated with a large round black mark, it is an indication that the photographer suspected that the copy may have moved during exposure and thus cause a blurred image. You will find ja good image of the page in the adjacent frame.
3. When a map, drawing or chart, etc., was part of the material being photographed the photographer followed a definite method in "sectioning" the material. It is customary to begin photoing at the upper left hand corner of a large sheet and to continue photoing from left to right in equal sections with a small overlap. If necessary, sectioning is continued again — beginning below the first row and continuing on until complete.
4. The majority of users indicate that the textual content is of greatest value, however, a somewhat higher quality reproduction could be made from "photographs" if essential to the understanding of the dissertation. Silver prints of "photographs" may be ordered at additional charge by writing the Order Department, giving the catalog number, title, author and specific pages you wish reproduced.
5. PLEASE NOTE: Some pages may have indistinct print. Filmed as received.
University Microfilms International 300 North Zeeb Road Ann Arbor, Michigan 48106 USA St. John's Road, Tyler's Green High Wycombe, Bucks, England HP10 BHR 7904977
HGAG5TRUM, CARL WILLIAM ECOLOGICAL DISTRIBUTION OF NOCTURNAL RODENTS IN A PART OF THE SOMDRAN DESERT.
THE UNIVERSITY OF ARIZONA, PH.D., 1978
University Microfilms International 300 n, zeed road, ann arbor, mi 4siog ECOLOGICAL DISTRIBUTION OF NOCTURNAL RODENTS IN
A PART OF THE SONORAN DESERT
by
Carl William Hoagstrom
A Dissertation Submitted to the Faculty of the
DEPARTMENT OF ECOLOGY AND EVOLUTIONARY BIOLOGY
In Partial Fulfillment of the Requirements For the Degree of
DOCTOR OF PHILOSOPHY WITH A MAJOR IN ZOOLOGY
In the Graduate College
THE UNIVERSITY OF ARIZONA
19 7 8 THE UNIVERSITY OF ARIZONA
GRADUATE COLLEGE
I hereby recommend that this dissertation prepared under my
direction by Carl William Hoagstrom
entitled ECOLOGICAL DISTRIBUTION OF NOCTURNAL RODENTS IN A PART
OF THE SONORAN DESERT
be accepted as fulfilling the dissertation requirement for the
degree of Doctor of Philosophy ___
11 "7 £ Dissertation Director Date
As members of the Final Examination Conmittee, we certify
that we have read this dissertation and agree that it may be
presented for final defense.
*5" J i 7 %
^ \AA ' fan. 1^7^*
RMAvlJU? IHTt
Final approval and acceptance of this dissertation is contingent on the candidate's adequate performance and defense thereof at the final oral examination. STATEMENT BY AUTHOR
This dissertation has been submitted in partial fulfillment of requirements for an advanced degree at The University of Arizona and is deposited in the University Library to be made available to bor rowers under rules of the Library.
Brief quotations from this dissertation are allowable without special permission, provided that accurate acknowledgment of source is made. Requests for permission for extended quotation from or re production of this manuscript in whole or in part may be granted by the head of the major department or the Dean of the Graduate College when in his judgment the proposed use of the material is in the in terests of scholarship. In all other instances, however, permission must be obtained from the author.
SIGNED ACKNOWLEDGMENTS
I would especially like to express my appreciation to Dr. E.
Lendell Cockrum for assistance throughout the study. In addition, I
wish to thank Drs. Stephen M. Russell, Charles T. Mason, Willard Van
Asdall, and H. Eonald Pulliam for suggestions during the field work
and/or critical reading of the manuscript; and Dr. Nelson and Janet
Moore for reading the manuscript and contributing constructive criti
cism.
I want to e: Maureen (who also read the manuscript), and to Becky, Chris and Sherwin for their patience and support during the study. The field work was supported by an XBP-Desert Biome grant. iii TABLE OF CONTENTS Page LIST OF TABLES vi LIST OF ILLUSTRATIONS viii ABSTRACT ix INTRODUCTION 1 MATERIALS AND METHODS 7 Study Area 7 Rodents ...... 8 Trapping 9 Preliminary Trapping 9 Within-habitat Lines .»• ...... 10 Miniquadrats 12 Red Hill Grid 12 IBP Lines l*t Collection of Data on Habitat Variables 15 Plant Densities ...... 15 Soil Texture and Gravel Content 16 Surface Characteristics ...... 17 Soil Depth 18 Habitat Variables Used in the Analysis 19 Multiple Discriminant Analysis • 21 The Enclosure 23 RESULTS AND DISCUSSION 28 Trapping • ••...•••••••• ...••• 28 Within-habitat Lines •••• ...... 28 Miniquadrats ...... 30 Red Hill Grid Jk IBP Lines bG Summary of the Discriminant Analysis Results 56 Heteromyid Habitat Associations 6l Dipodomys merriami ...... 62 Perognathus baileyi ...... 63 Perognathus penicillatus 65 Perognathus intermedins ...... 68 Perognathus amplus .. . . 69 iv V TABLE OF CONTENTS—Continued Page Association Between D. merriami and P. amplus ...... 69 Nonhabitat Coexistence Mechanisms ...... 7^ Diel Activity Differences 7^ Other Temporal Differences ...... 77 Food and Coexistence ...... 80 The Enclosure 80 Cricetid Species Habitat Affiliation ...... 86 Neotoma albigula ...... 88 Perooyscus eremicus ...... 90 Community Organization .. 93 Habitat Dimensions and Complementarity of Dimensions . 9^ Other Dimensions and Interaction of Differences on Several Dimensions 9k Dimensionality of Coexistence . 95 Habitats and Community Organization «... 96 SUMMARY 103 LIST OF REFERENCES 108 LIST OF TABLES Table Page 1. Data from the within-habitat lines 29 2. Miniquadrats: means and standard deviations of each habitat variable for each species of heteromyid rodent • 31 3. Grid, September 1972: means and standard deviations of each habitat variable for each species of heteromyid rodent ...... 35 k. Grid, September 1973s means and standard deviations of each habitat variable for each species of heteromyid rodent 36 5. Grid, September 197^: means and standard deviations of each habitat variable for each species of heteromyid rodent 37 6# Grid, October 197^: means and standard deviations of each habitat variable for each species of heteromyid rodent ...... 38 7. IBP lines, spring and fall 1972 and spring 1973! means and standard deviations of each habitat variable for each species of heteromyid rodent ^7 8. IBP lines, fall 1973s means and standard deviations of each habitat variable for each species of heteromyid rodent 48 9. IBP lines, spring 197^: means and standard deviations of each habitat variable for each specieB of heteromyid rodent ^9 10. IBP lines, fall 197^: means and standard deviations of each habitat variable for each species of heteromyid rodent ...... 50 11. Summary of the interpretation of multiple discriminant analysis results ...... 57 12. Coefficients of interspecific association between P. ampluB and D. merriami ...... 71 vi vii LIST OF TABLES—Continued Table Page 13. Contingency Chi square to test for difference in reaction of P. amplus (Pa) and D. merriarai (Dm) to low densities of"shrubs of category k on the grid •••••• 73 l^f. Contingency Chi square to test for difference in reaction of P. amplus (Pa) and D. merriaml (Dm) to low densities of shrubs of category™*! on the IBP lines • • • 73 15. Friedman and Wilcoxon tests for differences between evening and morning captures of the mice on the IBP lines 75 16. Sources of variation in the Analysis of Variance of the Perognathus tracking data in the enclosure • 83 17- Means and standard deviations of the habitat variables for Neotoma albigula and Peromyscus eremicus ...... 87 18. Means and standard deviations of the habitat variables for the trapping systems ...... 89 19. Weight ratios between coexisting species on the Red Hill grid and IBP lines 98 20. Species diversity of the catch from each trapping session on the Red Hill grid and IBP lines 101 LIST OF ILLUSTRATIONS Figure Page 1. Construction of the burrow in the enclosure 25 2. Miniquadrats: discriminant score graphs and corre sponding discriminant functions ... 32 3. Grid, September 1972: discriminant score graphs and corresponding discriminant functions 39 'f. Grid, September 1973: discriminant score graphs and corresponding discriminant functions ...• • 40 5» Grid, September 197^: discriminant score graphs and corresponding discriminant functions *+1 6. Grid, October 197**: discriminant score graphs and corresponding discriminant functions *t2 7. IBP lines, spring and fall 1972 and spring 1973: discriminant score graphB and corresponding discriminant functions '51 8. IBP lines, fall 1973: discriminant score graphs and corresponding discriminant functions ... 52 9. IBP lines, spring 197^: discriminant score graphs and corresponding discriminant functions 53 10* IBP lines, fall 197^: discriminant score graphs and corresponding discriminant functions 5& 11. Rodent catch on the grid by year and species ...••» 78 12* Rodent catch on the IBP lines by year and species .. • • 79 13* Foraging in the enclosure 8l Ik* Tracking in the enclosure 82 viii ABSTRACT The ecological distribution of seven coexisting rodent species was investigated to determine the part habitat differences played in the subdivision of community resources. Nonhabitat factors of poten tial importance in coexistence of the species were also considered, especially in the case of species without apparent habitat differences. The Sonoran Desert community studied was located 60 km NW of Tucson, Arizona, and contained the following rodent species: Perognathus penicillatus, Perognathus baileyi, Perognathus intermedius, Perognathus amplus and Dipodomys merriami of the family Heteromyidae and Peromyscus eremicus and Neotoma albigula of the family Cricetidae. Multiple discriminant analysis was used to find habitat vari ables which discriminated between the habitats of the five heteromyid rodents. The percentage of the surface covered with bedrock and with stones and cobbles were two of the consistently important variables. Both were interpreted as contributors to a soil-surface dimension, which was important to the coexistence of the five species. P» intermedius was most frequently taken in areas of coarse substrate (including both soil and surface factors), ]?. baileyi in areas of in termediate substrate, and the other three heteromyids in areas of less coarse substrate• Another consistently important variable was the density (number of individuals per unit area) of the Acacias and other large spinescent ix X shrubs* Smaller and less spinescent shrubs* as v/ell as trees, were im portant variables in some analyses. Together, these observations were interpreted as indicating importance of a shrub and tree density dimen sion in heteromyid coexistence. P_. intermedius and P. baileyi were taken in areas of relative high densities of such plants. Apparently discriminating more specifically, P. penicillatus was taken in areas of relatively high densities of large, spinescent shrubs. JD. merriami and P. amplus, on the other hand, were taken in areas of relatively low densities of most shrubs and trees. The cricetids were taken in the rocky habitats (both species) and in the areas of high shrub and tree densities (II. albigula only). Specific variables of importance to each species were not determined because of high habitat specificity and relatively small samples. Temporal differences were found between species which were not well separated by habitat differences (P. amplus and D. merriami; Peromyscus eremicus and P. intermedius). Annual activity patterns differed between both pairs. In addition, the first named pair showed different responses to an increase in food supply. P. amplus popula tions increased more quickly. This difference, in conjunction with food storage and periods of winter inactivity, was hypothesized to further contribute to coexistence of P. amplus and D. merriami in the area. Some species pairs with habitat differences also showed tem poral differences under certain conditions. For example, P. penicillatus and D. merriami differed in diel activity times. It xi was suggested that these temporal differences may have supplemented the habitat differences and so contributed to coexistence. The literature suggested that some of the species with similar habitats had different diets. This suggestion was most convincing with respect to Peromyscus eremicus and P. intermediU6, and may help explain how the two species were able to coexist with such apparently similar habitat requirements. It was concluded that at least four dimensions were necessary to explain rodent coexistence in this area: shrub and tree density, soil and surface coarseness, diet, and the timing of activity of various types. The habitat dimensions apparently accounted for most of the resource subdivision in the community, but some species pairs were so similar on habitat dimensions that differences on the other dimensions appeared to be required for their coexistence. INTRODUCTION According to the principle of competitive exclusion, two species cannot exiBt continually in the same area if their use of available resources is too similar (Hardin i960). Many species with sympatric geographic distributions maintain complementary ecological distributions. That is, they make use of different habitats in the same area. Food and temporal differences are also commonly found be tween community members, and are often interpreted as important factors in coexistence. Schoener (197^) gave several examples of each of these resource petitioning devices. In the studies he surveyed, habitat differences were important more often than either food or temporal dif ferences. Desert rodent comnrunities of North America have been the sub ject of several recent studies on coexistence (Hosenzweig and Winakur 1969; Brown and Lieberman 1973; Christopher 1973; Congdon 197^; O'Farrell 197*0• Because they are often populated by several related and ecologically similar species, they are especially good systems for this study. Hosenzweig and several coworkers have pointed this out (Hosenzweig and Winakur 1969) and have made a case for habitat differ ences as the basis of desert rodent coexistence (Hosenzweig and Sterner 1970; Hosenzweig 1973; Smigel and Hosenzweig 197^; Schroder and Hosenzweig 1975)* Other investigators have also found habitat segre gation to be an important factor in coexistence (Hawbecker 1951; Brown 1 2 and Lieberman 1973; Christopher 1973; Congdon 19?k; Kritzman 197*0 • Many other studies, not specifically dealing with the coexistence of species in a community, have reported specific habitat associations for various desert rodent species. The variability of the desert land scape makes several habitats available in a small area, so habitat segregation is a likely mechanism for resource subdivision. All of this suggests that habitat dimensions are probably important in desert rodent community organization. Habitat dimensions considered to be important have been basi cally of two types. Soil and surface factors (surface rockiness, soil texture, Boil depth and so forth) have been found to separate species and BO have vegetative factors. Some investigators have considered the soil-surface variables to be most important (Kritzman 197^0; others have felt vegetative effects were most important (Rosenzweig and Winakur 1969). However, both have been found to contribute in most studies in which both were investigated (Grinnell 191^; Hardy 19*+5i Rosenzweig and Winakur 1969; Kritzman 197*0 • Many specific habitat variables have been associated with the different member species of these communities: foliage height density combinations (Rosenzweig and Winakur 1969), type and amount of plant cover (Hawbecker 1951; Reynolds 1958; Schroder and Rosenzweig 1975), presence of certain plant species (Rosenzweig and Winakur 1969), soil texture (Burt 1938; Hardy 19^5)« presence of rock and stone in the soil and on the surface (Kritzman 197*0, soil depth (Hardy 19^5)1 and others. Information such as this, on the variables associated with species' ecological distributions, can be used to establish habitat 3 differences between species. If these are complementary, they suggest that the variables are important in species coexistence (Rosenzweig and Winakur 1969). The information can also be used in comparisons between communities. Various cross-community changes may suggest coexistence mechanisms (Schoener 197*0. Despite the apparent importance of habitat dimensions, others have also been found to be important. Both dietary (Smith 19^2; Brown and Lieberman 1973; Kenagy 1973) and temporal (Kenagy 1973; Kritzman 1974; O'Farrell 197*0 dimensions have been implicated as important con tributors to coexistence in desert rodent communities. Further study of the rodent communities in the various parts of the desert is needed to establish and test generalizations on the structure and organization of these communities. Generalizations con cerned with the identification of dimensions important to coexistence, their relative importance and the type and number of dimensions in volved are among those of interest in the understanding of these com munities and community structure in general (Schoener 197*0 • This study was undertaken as a contribution to that understanding* Most of the studies mentioned were especially concerned with the rodent family Heteromyidae. This family contributes more than any other to the rodent diversity of the North American deserts. The mem bers are very similar to one another ecologically (Rosenzweig and Winakur 1969). Several species can be trapped in the same general area in many parts of the desert. Host of the studies referred to so far gave examples of this. In short, heteromyids are the basis for the attractiveness of desert communities for rodent coexistence studies. b In this study five heteromyid and two cricetid species were trapped in an area northwest of Tucson, Arizona, All appeared to have specific habitat associations, but in selected parts of the area seven could be taken at the same trap site. In large parts of the area, four of the heterorayids were taken together with only a sugges tion of habitat differences. The ecological distribution of the species in this community and the contribution of this distribution to coexistence were studied by attempting to answer the following ques tions. What habitat variables, if any, are important to each of the species? If certain variables are important to each species, are they the same in areas of obvious habitat affiliation as in areas of less apparent habitat affiliation? Are they constant under different trap ping systems, through time, and under different conditions of popula tion size? Are the habitat variables associated with one species complementary to those of the other species? For example, if one species is associated with rocky areas, do other species avoid these areas? If there are species pairs or groups without habitat differ ences, are they associated with one another? Does the association change when similar habitat variables (those both species respond to in the same way) are eliminated? Are there complementary activities of a nonhabitat nature, and if so, what are they? Most of the above questions are concerned with the existence, constancy and complementarity of species-habitat relationships. If habitat associations exist and are complementary, they may be impor tant contributors to coexistence as well as being important to the species with which they are associated. However, if they are not 5 constant in space and time one should question their importance in both regards* and look for a more complicated coexistence mechanism than simple habitat segregation. If associations are not constant under several different trapping systems, perhaps they are not associations at all, but artifacts of the trapping systems in which they were ob served. If no habitat differences are found between two or more species, other dimensions (dietary or temporal, for example) may be involved in their coexistence. One cannot, of course, rule out the possibility that the study missed the important habitat differences. The last question asked above is of particular interest concerning the species without complementary habitat differences. In this case some alternative coexistence mechanism would be required. With regard to species pairs or groups which have complementary habitat differences, the discovery of complementary differences on another dimension might suggest that the habitat segregation is supplemented by the nonhabitat complementarity. In the search for complementary variables, each species' habi tat may be analyzed and comparisons made between species* Alterna tively, because the complementary differences between species are the ones of greatest interest in terms of community organization and prob ably also are most important to the species, one could look specifi cally for those variables to which the species respond in a comple mentary fashion. The second approach was used in this study. Multiple discriminant analysis was employed to look for the habitat variables that best discriminated between species' habitats. These best 6 discriminators should be, or be related to, the complementary variables sought. Another approach to determination of habitat factors important to a species is to simulate factors of suspected importance in the laboratory and give a mouse the opportunity to choose among various factors (Harris 1952)• An enclosure study of habitat selection was carried out to supplement the field studies. Gross habitat differences were simulated and the question asked was simply: are the simulations favored in the enclosure by each species consistent with associations seen in the field? If they are, it is further evidence of a stable habitat relationship; if not, the enclosure reaction may suggest a reason for the inconsistency. If stable and complementary variables can be identified for the species involved, the role of these variables in community organization can be explored. Questions such as those suggested by Schoener (197*0 and referred to earlier can be considered. Another question which is of interest in this community is that of the relationship between com munity organization and available habitats. How tightly is community organization tied to habitat distribution? After the relationships of the habitat variables to the rodent species are analyzed and some potentially important nonhabitat factors are considered, these aspects of community organization will be discussed. MATERIALS AND METHODS Study Area The study area is 60 km northwest of Tucson, Pima County, Arizona, All trapping was done in Township 11 South, Ranges 8 (eastern third), 9 and 10 (western third) East, of the Silver Bell Peak Quad rangle. Elevation in the area increases from 570 m at Los Robles Wash on the eastern edge, to 730 m at the base of Ragged Top Mountain on the western edge. The eastern part is a gently sloping plain. The western, upslope part is much more rugged with small, rocky hills commonly rising from the plain. Dry washes course through the area and drain into Los Robles Wash. Soils are dominantly loam to sandy loam in the eastern plains (United States Department of Agriculture 1972), but become coarser up slope. On the crests and slopes of the rocky hills, the soil is shallow and coarse. Bedrock is eaqiosed in many of these situations and soil is found only in pockets. The dry washes often have gravelly soil. Occasionally, especially in the upslope part of the area, bed rock is exposed along the washes. The area is in the Sonoran Desert and is covered by two common plant communities of that desert, ereosotebush-bursage (Larrea- Franseria) and paloverde-sahuaro (Cereidium-Cereus) (Lowe 1964). Creosotebush (Larrea divaricata Cav.) covers the plains in nearly pure 7 8 stands or in conjunction with triangleleaf bursage (Franseria deltoidea Torr.). Upslope, the littleleaf paloverde (Cercidium microphyllum ^Torr.^ Rose and Johnston) and the sahuaro cactus (Carnegieae gigantea ^Engelnu/ Britton and Rose) become dominant members of the vegetation. Other shrubs and trees join them to form the paloverde-sahuaro commu nity. The rocky hills are generally covered by this community, but much of the western part of the study area is an ecotone in which the two communities mix and interdigitate. Along the washes trees and shrubs grow more densely than in the other communities. These riparian communities are composed of white thorn acacia (Acacia constricta Benth.), catclaw acacia (Acacia greggii Gray), mesquite (Prosopis juliflora ^Swart^ DC), blue paloverde (Cercidium floridum Benth.) and others, including the dominants of the major communities. Plant nomenclature was taken from Kearney and Peebles (1969). Rodents Five heteromyid rodent species were the major subjects of the study: Perognathus amplus taylori Goldman (Arizona pocket mouse), Perognathus intermedius intermedins Merriam (rock pocket mouse), Perognathus penicillatus pricei Allen (desert pocket mouse), Perognathus baileyi baileyi Merriam (Bailey's pocket mouse) and Dipodomys merriami merriami Mearns (Merriam's kangaroo rat). Two 'cricetid species, Neotoma albigula albigula Hartley (white throated woodrat) and Peromyscus eremicus eremicus (Baird) (cactus mouse), were taken regularly and their habitat associations were considered. To 9 avoid confusion between the genera Perognathus and peromyscus, the first will be abbreviated in the normal fashion (P.), and PeromyBCUB will be abbreviated Pm. Other cricetids, Reithrodontomye megalotis (western harvest mouse), Peromyscus merriami (Merriam's mouse), Peromyscua maniculatus (deer mouse), Onycomys torridus (southern grasshopper mouse) and Sigmodon arizonae (Arizona cotton rat), were also taken occasionally. Mus muBculus (house mouse, family Muridae) was taken regularly during preliminary trapping in the summer of 1972 and on the Red Hill grid and IBP lines in the fall of that year, but none were taken at any other time. No attempt was made to determine habitat factors important to Mus or the less common cricetids. Mammal nomenclature was taken from Cockrum (i960) and Jones, Carter and Genoways (1973)• Trying Museum special mouse and commercial rat traps were used for all snap trapping. Live traps used were Sherman, National and a "homemade" one. The last was made of hardware cloth and sheet metal with a trig ger mechanism like that described by Howell (195*0 • It will be called the IEP trap in this report as moBt of its use was on the IBP lines. Live trapped animals were marked by toe clipping. Five trapping sys tems were used and each is described in the following pages. Preliminary Trapping In the winter of 1971-1972 and the summer of 1972, 21 trap lines were run in the area. Each line was set to pass through as many habitats as possible in the part of the area trapped by that line. 10 The lines were distributed throughout the area and the major habitat types were trapped by several lines. Live traps were used in a few cases, but most of this trapping was carried out with snap traps set in a rectangular pattern and spaced about 10 m apart. The long sides of the rectangles were approximately 50 traps (500 m) and the short sides six traps (50 m) long. Traps were set only along the sides of the rectangles. Most sites were trapped one night, a few were trapped two consecutive nights. Traps were set and baited at sundown and picked up at sunrise. The bait used was a peanut butter-rolled oats mixture. The following data was collected at each successful trap location: species, sex and age (juvenile or adult) of the mouse; esti mated distance in meters to the nearest wash; and ground stir face charac ter (a visual estimate of the percentage of the surface covered by rock, stone and cobble; gravel; or sand and smaller particles). The preliminary trapping results were used as the basis for the other trapping designs and for the habitat simulations of the en closure. The animals caught were taken to the laboratory where field identification and sex were checked. Some of these, and some taken later, were prepared as study skins or as skeletons and were deposited in the mammal collection at The University of Arizona as voucher speci mens. Within-habitat Lines In September 1973 and May 197^ five trap sites (numbered 101- 105) were run in the major habitats, as suggested by preliminary trap ping and observation. Each trap site consisted of two approximately parallel lines. There were 16 trap stations in each line with 11 approximately 15 m spacing between stations and between the lines. Two museum specials and one rat trap were used at each trap station at sites 101 and 102, and two Sherman and/or National live traps at each Btation at sites 103, 10*t and 105* Bait used was a mixture of seeds (millet, milo, wheat, oats, and rolled oats). Each site was set and baited just before sundown, checked and rebaited at 2200 and checked and closed just after sunrise for three consecutive nights. Site 102 was in a nearly pure stand of creosotebush and 101 was in a wash dissecting that creosotebush stand. Site 10*+ was in a ereosotebush-bursage stand, with scattered littleleaf paloverde and 103 was in a wash passing through the creosotebush-bursage stand. These four sites were trapped in September 1973- Site 105 was on the upper slope and top of a rocky hill, and was trapped in May of 197^• These sites were used to sample the rodent communities of the rocky hill (105), creosotebush-bursage plain (102, 104) and riparian (101, 103) habitats. The ecotone habitats were not samples. The pur pose of the trapping system was to test for association of the rodent species with these distinct habitats; and so with the variables charac teristic of the habitats. To do so, each trap site was isolated in a single habitat type. An ecotone site would not have contributed to the objective because the habitats and rodent species mix quite freely in the ecotone. However, site 104 is actually at the lower edge of the ecotone, as indicated by the presence of littleleaf paloverde, a domi nant species from the paloverde-sahuaro community (see Table 1, p. 29). 12 Miniquadrats In the summer of 1973 and spring of 197^i 66 miniquadrats, similar to the small quadrats of Hansson (1972), were trapped. The miniquadrats were scattered throughout the area in patches of uniform habitat. Each was typically made up of four trap stations at the four corners of a square with sides 15 m long. Along washes, however, the habitat type was often not 15 m wide. Under these conditions the trap stations were Bet at the corners of a rectangle with the long axis parallel to the wash, and enclosing approximately the same area as the 15 m by 15 m square. Two museum specials and one rat trap, or two Sherman and/or National live traps, or two IBP live traps were placed at each trap station. Except for Sherman and National live traps, trap types were not mixed within a miniquadrat. A peanut butter and rolled oats mix ture was used in the snap traps, and the seed and rolled oats mixture was used in the live traps for bait. The traps were baited at sunset and checked and closed at sunrise for three consecutive nights. The miniquadrats were used to sample the major habitat types and intermediate and mixed habitat types. Their small size allowed them to be placed in uniform habitat even in the highly variable eco- tone. Their purpose was to test for the association of species with habitat variables over the entire area; not just within the most dis tinct habitat types. Bed Hill Grid In early September of 1972, 1973, and 197^ a snap trap grid was run on the lower, eastern slope of Red Hill, a small rocky hill in the 13 area. It was located at Township 11 South, Range 9 East, Section northeastern quarter. The grid consisted of 256 stations set in 16 parallel lines of 16 stations each, with spacing of 15 m between lines and stations. One museum special and one rat trap were set at each station, and baited with a peanut butter and rolled oats mixture. Traps were baited and set at sunset and checked at sunrise. Seven con secutive nights were trapped in 1972, six in 1973* and eleven in 197^. In all three cases the catch had leveled off at the end of the trapping period. The grid was set so that it covered all the major habitats sug gested by preliminary trapping and observation. These included bedrock- covered slopes, patches of creosotebush-bursage and paloverde-sahuaro habitat in the ecotone, and acacia overgrown washes. Gradations among these were also covered by parts of the grid. The objective was to test the association of species with habitat variables in the ecotone, where habitats are thoroughly mixed. Each trap station was analyzed as a sample unit rather than entire sites being so analyzed as they were in the miniquadrats and within-habitat lines. In October of 197^* just 30 days after the September trapping, the grid was trapped again for three consecutive nights using the same trapping procedures. Assuming that the community of animals invading a depopulated area is less stable than a normal community and that invasion and stabilization takes at least 30 days, the objective was to analyze habitat segregation under these relatively unstable con ditions. 14 IBP Lines Each spring and fall from 1972 to 197^, four pairs of lines in the ecotone area were trapped as a population check for the Inter national Biological Program, Sonoran Desert Biome. They were located at Township 11 South, Range 9 East, Section 21, southwestern quarter# These lines were also analyzed for rodent species association with habitat variables. Each line pair consisted of parallel lines spaced as the within-habitat lines were spaced. Two IBP live traps were set at each station. They were baited with a rolled oats and peanut butter mixture in the spring and fall of 1972 and spring of 1973* The bait used on subsequent trap dates was the mixture of seeds and rolled oats. Traps were baited and set at sunset, checked and rebaited at 2250, checked again and closed at Bunrise for three consecutive nights each spring and fall. Two line pairs ran east and west and two ran north and south. They crossed habitat types and the objective, as on the grid, was to test for habitat-variable association in the ecotone, where the species lived in most intimate contact with one another. Each trap station was again treated as a sample unit. Two of the line pairs (one running each direction) were in a natural ecotone area, while the other two were in an area run over at 10 m intervals by off-road vehicles. A contingency Chi square was run to determine if mouse captures on the two line types were different. The Chi square was 1.59 with three degrees of freedom (0.6 In the discriminant analysis of these lines the first three trapping sessions (spring and fall 1972 and spring 1973) were treated together. The sample sizes for P. baileyi and P. penicillatus were too small for independent analysis of each session. Populations of »n three sessions were lowt therefore they were considered together as the low population analysis on the IBP lines. Collection of Data on Habitat Variables Plant densities and soil and surface characteristics were meas ured in an attempt to determine the habitat variables most important to the various species. The measurement of each of the variables for each of the trapping systems is described in this section. Plant Densities The density of each species of woody plant was obtained for each trap station on the within-habitat line and maniquadrat sites by counting all such plants within a circle having a radius of 3 m and its center at the trap station. The plants in four additional 3 m radius circles were counted at each maniquadrat. These were located differ ently for wash and nonwash sites. In the first case the centers of two were located 10 m upwash from the upwash trap stations and the other two 10 m downwash from the downwash trap stations. This was done to keep all vegetation sample sites in the wash habitat. At the nonwash sites they were located outside the miniquadrat square, 10 m from and perpendicular to the centers of the four sides of the square. For both the rciniquadrats and within-habitat lines, all circles from a given site were summed to give the number of each woody plant species for that site. The area of the count for each within-habitat line site was 2 2 900 m and for the miniquadrat sites it was 230 m • On the Red Hill grid and IBP lines, each trap station was a sample site. Plant counts were taken from squares, one at each trap 2 station. The area of each square was 130 m . Its center was at the trap location. Its sides were parallel to the rows and columns of the grid. Soil Texture and Gravel Content The surface material was scraped away from an area of about 400 2 cm and a 200-^00 g sample of soil was taken from that area. In the laboratory, the sample was weighed, sieved using a soil sieve of 2 mm mesh size (number 10), and the remaining sample weighed. Percent by weight of gravel (rock particles larger than 2 mm in diameter) in the sample was calculated from these data. Field determination (also called the feel method) of the tex- tural class of the soil remaining after separation of the gravel was carried out as described in the United States Department of Agriculture, Handbook 18 (1951)* The Boil samples were thus placed in textural classes, each class defined by a percentage range of sand, silt, and clay. Only the percentage of sand was used in the analysis. The mean percent sand of the textural class was used as the percent sand for all samples placed in that class. Eight samples were taken from each within-habitat line site. They were taken in pairs (one from each line) at about 80 m intervals. 17 The average of these samples was used as the measurement for the site for percent gravel and percent sand* Two samples were taken at each miniquadrat, one from the middle of the north and one from the middle of the south sides of the square or rectangle formed by the miniquadrat* If these were within 10$ gravel content of one another and in the same textural class, their averages were used as the gravel and sand percents of the miniquadrat* If they differed, more samples were collected and the possibility of error was checked. Generally, these differences were due to sudden changes in the soil* Along washes, the opposite banks or the high and low part of a given bank often differed with regard to soil characters. Averages of all the samples were used as the measurement for the site in these cases. Gravel and sand percents were mapped on the IBP lines and Bed Hill grid. A number of samples were taken from each of the sites. These were regularly spaced in uniform habitat and otherwise were taken at apparent changes. The results of these analyses were mapped and a second set of samples was collected where needed to clarify the map. This procedure was repeated until soil gravel and sand percent could be reliably mapped* A total of 71 samples were used in mapping the grid and 40 in mapping the IBP lines. Surface Characteristics The percent of the undisturbed surface covered by bedrock, stones (rock fragments larger than 25 cm in diameter) and cobbles (rock fragments between 7 and 25 cm in diameter) was estimated visually using the methods of the U.S.D.A*, Handbook 18 (1951)• A single estimate for 18 each category was made for each within-habitat line and miniquadrat site. The IBP lines and Red Hill grid were mapped by estimating cover ages from station to station* All these estimates resulted in coverage classes similar to the textural classes of the last section. In analy sis, the mean of the coverage clasB to which a site was assigned was used as the percent coverage for that site. Soil Depth Soil depth was found by driving a 91 cm steel stake into the soil with a 2 kg hand sledge hammer. The stake was driven in until it struck an impenetrable layer (presumed to be bedrock) or to the depth of 61 cm. If the stake could not be driven to 61 cm, two other points within 2 m of the first were tried. If all these showed a depth of less than 6l cm, the average was used as the soil depth at that point. If two were over 6l cm, the soil was considered to be deeper than 6l cm. If one was over 61 cm, the stake was driven at a fourth point. If the depth there was over 6l cm, the depth was considered to be over 61 cm. If not, the average of the four was used as the depth at that point. Eight such depth samples, spaced as in the soil samples, were taken at each within-habitat line site, and the average was used as the depth of the site. Two samples, taken from the middle of the north and south sides, were averaged to give the depth of each miniquadrat. Soil depth was mapped on the IBP lines and Red Hill grid in a manner similar to the mapping of gravel and sand percents. Fifty-one depth samples were used to map the lines and 66 to map the grid. 19 Habitat Variables Used in the Analysis Most of the plant species were grouped into growth-form cate gories before actual analysis was carried out. Some grouping of sur face and soil characteristics was done as well. An explanation of each habitat variable analyzed in the study is given here; so are the abbreviations used for the categories in tables and figures* Shrub category 1 (S-l on tables and figures) was made up of very small, at least somewhat shrubby, plants such as Porophyllum gracile Benth. and Cassia covesii Gray. The area covered by a single 2 plant was generally less than 1000 cm • Shrub category 2 (S-2) consisted of larger, low growing shrubs. Major contributors to the category were Krameria grayi Rose and Painter (ratany), Trixis californica Kellogg, Calliandra eriophylla Benth. (fairy duster), and Encelia farinosa Gray (brittlebush). The area 2 covered by a single plant was generally more than 1000 cm and plant height was generally one meter or less. Franseria deltoidea (Fd) and Larrea divaricata (Ld) were so common in the area that they were given individual categories. In growth form, Franseria was similar to shrubs of category 2 and Larrea to those of category 3» Shrub categories 3 (S-3) and ^ (S-A-) were made up of larger shrubs, generally much greater than 1 m in height. Shrub category 3 contained the smaller, less spreading and less spinescent of these shrubs. Lycium berlandieri Dunal. (wolfberry) and Jatropha cardiophylla (Torr.) Muell. Arg. (limberbush) were the major contribu tors to the category. Shrub category k was primarily made up of 20 Acacia constricta and A. greggi. A few other large, spinescent shrubs such as Condalia spp. were also included. The tree category (Tr) consisted primarily of Cercidium microphyllum, but in some areas Olneya tesota Gray (ironwood) was im portant. Prosopis juliflora and Cercidium floridum were also included. Plants were classified as small, intermediate and large when they were counted. The size categories, of course, varied with the plant species being considered. This sizing was only used to eliminate small plants of the Larrea, shrub 3* and *f, and tree categories from the analysis. Small plants of these groups were defined as individuals of the same size or smaller than large plants of shrub category 2. They were eliminated to maintain the height characteristic of these categories. The Opuntia cactus category (C-l) included the cholla cacti. Important species in the area were Opuntia leptocaulis DC., 0. acanthocarpa Engelm. and Bigel. and 0. fulgida Engelm. The other cactus category (C-2) was made up of Carnegieae gigantea, Ferocactus (barrels), Echinocereus (hedge hogs) and Maimnillaria (pincushions). Percent sand (#S), gravel ($5), stones and cobbles (?teC) and bedrock (#Br) were all obtained directly from measurements and esti mates explained in the section on soil and surface methods. Percent stones and cobbles was just the sum of the two percentages involved. Soil depth (Dcm) was the soil depth measured in centimeters. In «n calculations, 61 cm was used as the depth of all soils of that depth or greater. 21 Multiple Discriminant Analysis Green (1971) interpreted multiple discriminant analysis as identifying important ecological factors which distinguished species' niches from one another. He explained the rationale for its use in such studies. McCloskey and Fieldwick (1975) used it in the separation of the structural habitat of two mouse species. In the present study it was used to find the habitat factors that best and most consistently distinguished between the habitats of the five heteromyid species in the area. This analysis was carried out using the discriminant analy sis subprogram in the Statistical Package for the Social Sciences, version 5*8. Nie et al. (1975) describe version 6.0t which is very similar to 5*8. The analysis was run on the CDC 6400 computer at The University of Arizona Computer Center. There was a set of l*f habitat measurements (described in the last section) associated with each mouse capture. These data, for all captures of a given snap trapping session, were subjected to multiple discriminant analysis. In live trap studies, the second and all sub sequent captures of an individual at a given trap station were elimi nated from the analysis. Captures were grouped according to species, and the analysis separated species' habitats from one another on the basis of the habitat variables measured. The importance of each habitat variable in the separation was indicated by the size of its coefficient in the standardized discriminant function. A discriminant score was calculated for each capture, on each discriminant function. This score represents that capture with respect to the variables as they were weighted in the discriminant function. The mean of all the 22 discriminant scores for a species vae interpreted as the species' average reaction on that discriminant function. These reactions were compared for the different species. Graphs of the means and standard deviations of the discriminant scores were used for this comparison (refer to Figs. 2-10 in the Results and Discussion section). The graphs were used to determine which species' habitatB were separated by that discriminant function. The relative sizes of the standardized discriminant function coefficients were used to determine which habitat variables were important in that separation. The data reported here are not normally distributed and so do not fit the model for a test of statistical significance of habitat separations (Green 1971)• A Chi square test for significance of dis crimination was run and is reported, but analysis was based on the habitat segregation indicated by graphs of the discriminant score means and standard deviations of the species and on the relative sizes of the standardized discriminant function coefficients which indicated habitat factors of importance in the separations. The habitat factors which consistently discriminated between certain species' habitats were interpreted as factors important to those species. The comple mentary reactions of the species to the discriminating factors were interpreted as evidence that the factors were also important to habitat segregation and hence to species coexistence in the area. Five heteromyid species were taken on the grid and miniquadrats and so there were five groups in the discriminant analyses at these sites. Discriminant analysis reports one less discriminant function than there are groups to be separated. Therefore, there were four 25 discriminEtnt functions in each discriminant analysis of the data from these sites. However, for the reasons discussed in the following paragraph, only one of these discriminant functions was used. P. intermedius was not taken on the IBP lines often enough to be included in the analysis. In all other multiple discriminent analy ses (four on the grid and one on the miniquadrats), P_. intermedius habitat was distinctly different from that of the other species. Therefore, in addition to the five group analysis, a four group analy sis excluding P. intermedius was run on these sites. This made com parison between IBP lines and the other systems possible. It also afforded a better look at habitat segregation among the four species with the most similar habitats. Since the first discriminant function of the five group analysis separated P. intermedius in all cases, and the factors discriminating between the other four species were analyzed in the four group analysis, only the first discriminant function of the five group analysis was reported. The four group discriminant analysis generated three discriminant functions, but only the first two of these were reported. The third contributed little to the separation (always less than 20# of the total obtained by the analysis), and was neither consistent nor easily interpretable. The Enclosure An outdoor enclosure was built to test reactions of the species to habitat simulations. It was located in the back yard of a Tucson residence. The ground in the enclosure was nearly bare, and the grass that was present was kept cut. An area of 32 m (8 m by k m) was en closed by a sheet metal fence buried 30 cm and standing about 76 cm 2b above ground. The sheet metal sections were connected to one another, and support was given to the fence, by attaching two-by-four boards to the outside of the sheet metal with wood screws. An artificial burrow was placed in the center of the enclosure. Figure 1 shows the struc ture of the burrow. The four quarters of the enclosure (each 2 m by U m) were treated as follows: one was covered with stones (stone cover), another with piles of brush (brush cover). The third was partially covered (kC%> to 6C$) with cobbles and small stones (scattered cobble cover). The fourth was left uncovered (open). Mouse activity was monitored in two ways. Eight sand tracking squares, 30 cm on a side, were placed in each quarter. These were just piles of sand about one-half inch deep. They were centered on each of the eight square meters of each quarter. They were smoothed each evening at dusk and checked the next morning at sunrise. A frame, divided into *f0 rectangles, was placed over each tracking square and the number of rectangles marked by the mouse (footprints, tail drags and other marks) was counted. The sum of these in one cover type waB used as an indication of the amount of activity of the mouse in the cover type. Secondly, two dishes, each containing 50 E of mixed seed (milo, millet, wheat and oats), were placed in each quarter. The Beed was weighed and placed in the dishes at dusk and weighed again at sunrise. The weight of the seeds removed was used as a meastire of the amount of foraging by the mouse in that cover type. CT CC CC Figure 1. Construction of the burrow in the enclosure. — Two, three pound coffee cane (marked CC in the diagram) were buried and covered with square boards (B). The boards were covered with 7 cm of soil and cm of soil was placed in each can. Each can had three holes to accommodate the connecting tubes (CT) and an access tube (AT). The connecting tubes were 50 cm long and the access tubes were 40 cm long. Their in ternal diameter was approximately 6 cm. The tubes were made of carpet fragments rolled inside a waterproof paper. Line "S" in the diagram represents the surface of the soil. The diagram is not to scale. r\j \JI 26 A single mouse was placed in one of the coffee cans of the artificial burrow system before noon of the first day of observation. Both exits from the burrow were covered with stones so that the mouse would have to stay in the burrow the rest of the day. Thirty milo, millet and wheat seeds (ten each) were put in with the mouse. That evening the seed dishes were placed, the tracking squares smoothed, and the stones removed from the burrow entrances. The next morning data were collected from seed dishes and tracking boards. The mouse was run for three consecutive nights. The artificial burrow was dug up the last morning and the mouse removed. The cans and tubes were washed, dried and replaced, A new mouse was introduced for three nights of observation. Eight individuals of each species of pocket mouse were run in this way. All were trapped in the area of investigation and housed in The University of Arizona, Department of Biological Science animal quarters in plastic or metal mouse cages with 2 cm of sandy soil on the bottom. They were fed the same seed mixture that was used in the en closure. The order in which the species were used was rotated so that each species followed every other twice. Sexes were alternated so that half of the subjects of each species were male and half female. The cover types were rotated one quarter clockwise after one mouse of each species was run. Each individual pocket mouse tracked a number of rectangles in each of the four cover types for the three night trial. Because cover types were rotated between quarters in four different configurations, each cover type-quarter-species combination occurred twice. To test whether the quarter of the enclosure, the cover type, the species of mouse or some interaction among them was responsible for the results obtained, a three way analysis of variance was run on the tracking data. Six D. merriami were run consecutively with cover types rotated after two runs (one male and one female). Other procedures were the same as for the pocket mice. Data for D. merriami were not included in the analysis of variance because the kangaroo rats were not nm in the same rotation as the pocket mice. RESULTS AND DISCUSSION Trapping Within-habitat Lines Table 1 gives the results of trapping on the within-habitat lines and values of the habitat variables for each site. The differ ences between sites are obvious, both in terms of animals caught and habitat variables. There were apparent differences in habitat utili zation for the mouse species and these were, to a great extent, complementary so that habitat segregation resulted. Of the hetero- rayids, only P. amplus and D. merriami appeared to have peak popula tions in the same habitat. There was an apparent similarity between P. baileyi and P. intermedjus habitat, but this was due to the fact that no peak P. baileyi sites were trapped by the within-habitat lines. Tables 3 through 6 (pp. 35-38) show that P. baileyi was the most numerous species taken on the Red Hill grid. The grid was an ecotone site, and the ecotone was avoided in the selection of within-habitat line sites, for reasons given in the Methods section. P. baileyi was captured most often in the ecotone and so its habitat relationships were not adequately reflected in the within-habitat lines data. The preliminary trapping gave the same sort of results as those shown in Table 1 as long as the ecotone was avoided. In the ecotone area, however, a given trap line (or station) could take all five 28 29 Table 1. Data from the within-habitat lines. — Animal data are the numbers of individuals caught in three nights of trapping* Plant data are numbers of individuals counted in 900 m2 of the sample area. Sites are described in the Methods section. Abbreviations are as follows: Dm is Dipodomys merriami, Pb is Perognathus baileyif Pp is P. penicillatust Pi is P» intermedins. Pa is P• ampins, Pme is Peromyscus eremicus, and Na is Neotoma albigula; S-l through S-H are shrub cate gories 1 through 4, Fd is Franseria deltoidea, Ld is Larrea divaricata, Tr is trees, C-l and C-2 are cholla cacti, and sahuaro and others respectively, #S, #G, #SC, and %Br are percent sand, gravel, stone and cobble cover, and bedrock cover, respectively; Dcm is soil depth in centimeters. Measurement of the variables is described in the Methods section. Site Species "lOl 102 103 loS 105 Dm 0 5 0 3 0 Pb 0 0 1 0 7 Pp 25 0 46 9 0 Pi 0 0 0 0 46 Pa 1 11 0 30 0 Pme 0 0 0 0 20 Na 6 0 8 0 6 Habitat Factors S-l 0 0 0 0 71 S-2 22 1 7 3 346 Fd 6 0 20 128 99 Ld 90 3^5 106 193 . 98 S-3 3 0 6 0 50 s-4 62 1 63 0 8 Tr 14 0 11 5 39 C-l 0 1 0 1 5 C-2 0 3 0 0 25 ste 64 57 69 46 27 #5 8 26 9 27 44 9SSC < 1 < 1 < 1 < 1 15 #Br 0 0 0 0 70 Dcm >61 >61 >61 >61 18 heteromyid species. Perhaps this is not surprising because the habi tats of the area were mixed in the ecotone, both in the sense of inter mediate habitat and of mixed patches of the various habitats. If the habitats of the species were more intimately associated, the species should also be more intimately associated. It is in this ecotone situation that most of the questions asked in the Introduction can be approached. The other trapping systems all involve the ecotone. M iniquadrats The miniquadrats were also concerned with overall habitat associations* but the sites were smaller and some were placed in the ecotone. The results gave a somewhat closer look at habitat-variable association than did the within-habitat lines results. Table 2 gives the means and standard deviations of the habitat variables for each mouse species* over all miniquadrat captures. AB in the within-habitat lines, habitat differences among species are obvious, but several variables are involved in differences between most species. Some of these must be more important than others. Multiple discriminant analy sis was carried out on the miniquadrat data to search for these more important variables. The results are shown in Figure 2. The first discriminant function is given for the five group analysis and the first two are given for the four group analysis* The five group discriminant score graph shows the distinct segregation of P. intermedius habitat from the habitats of the other heterorayid species. P. baileyi habitat was intermediate but more similar to those of the other three mice than to that of P. intermedius. Table 2. Miniquadrats: means and standard deviations of each habitat variable for each species of heteromyid rodent. — The column headed Total contains the means for all species together. N is the number of captures in the analysis. Each mean iB equal to m.h./ .i if ill«nr,« | wherewuci c in*m. is_lo the number of mice uiof a given Bpeciesxc• takenbcuvcii aiat site • i* and h,. is the count or1measurement of the given habitat variable at site 1 i*. Plant means are in numbers of individuals counted in 230 m . Variable measurement is explained in the Methods section. Abbreviations are as in Table 1. Habitat Species Variables Dm Pb Pp Pi Pa Total + S-l 4.2 i 11.3 10.2 i 9.8 2.1 j 11.4 5.1 - 11.4 0.9 + 2.5 7.1 T 13.8 S-2 0.8 t 1.4 0.3 + 0.7 4.9 13.1 1.3 J 3.2 24.7 x 31.6 5-5 x !6.1 Fd 62.0 + 27.2 70.0 42.1 49-3 J 36.1 50.0 t 44.2 56.9 j 50.2 57.1 i 41.0 Ld 41.2 11.0 i 40.0 - 27.4 + 25.7 20.7 14.2 28.5 - 21.6 9.9 27.4 j 22.6 s-3 0.8 6.4 t 2.4 i 3.3 + 1.6 3-7 3.6 1.1 J 1.9 3.7 0.7 x 1*5 S—4 1.3 6.4 10.1 14.3 1.6 i + 3.5 11.2 J 12.8 18.2 3.6 7.8 J 12.3 Tr M 4.0 4.0 2.0 3.6 t 2.6 + 2.5 2.5 3.9 + 3.0 2.7 ? 2.7 2.7 C-l 2.5 7.1 4.1 2.0 1 4.4 2.7 + 7.1 5.6 3.3 + 3.0 2.7 4.5 r C-2 1.1 6.2 2.ii 1.7 + 1.8 2.3 3.8 + 1.6 5.1 1.2 J 1.7 3.5 8 60 10 5 12 6l 55 + 63 + 9 67 57 t 10 #5 12 44 29 + 9 38 13 27 13 30 9 33 t 13 SfeC 10 1 21 4 8 4 + 7 13 6 I 4 22 J 15 #Br 10 16 31 11 I 20 3 + 6 3 43 2 ? 5 Dcm 61 1 54 14 6l 5 x5 36 19 58 t 9 55 t 14 N 27 54 74 34 36 225 h 32 co c o 5 Groups 4 Groups •p•h to cfl d) •rt h Discriminant Discriminant t> o Discriminant 0) o Function 1 Function 1 Function 2 o w t3 -p as= .s ts e 4-> -h fif ^ w b o •a to §~ -hq -2 tq Species (Groups) S-l +0.2 -0.1 -0.1 S-2 -0.2 +0.1 +0.3 c Fd -0.1 +0.2 -0.1 o •h•p Ld +0.1 0.0 -0.2 o co c -+j S-3 0.0 +0.5 -0.2 3 c S-4 -0.2 -0.2 +0.8 & a> •ri Tr • +0.4 0.0 +> o +0.1 d *h C-l 3 «h -0.3 +0.5 +0.6 •h q) C-2 +0.1 0.0 +0.2 e o 56S -0.1 -0.3 +0.4 •h o tt o -0.5 +0.1 -0.1 m +0.1 -0.2 +0.2 •h #sc o foBr -1.0 +0.4 -0.5 Dcm +0.1 0.0 +0.1 38 *2 73 57 x2 303 128 57 df 56 42 26 P 0.00 0.00 0.00 Figure 2* Miniquadrats: discriminant score graphs and corresponding discriminant functions* —• The graphs show the mean and standard deviation of the discriminant scores for each species. Abbreviations are as in Table 1. Sample sizes are given in Table 2. Discriminant function coefficients are underlined if they are 1C$ or more of the sum of the discriminant function coefficients. % is the percent of the total separation in the discriminant analysis which is due to that discriminant function. X2 is the Chi square of that discriminant function, and df is the degrees of freedom, p is the probability that the separation is due to chance. 33 The factors most important in the graphed separation are given by the discriminant function below the graph# Bedrock exposure was obviously the most important discriminator between these habitats, though gravel percent in the upper layer of soil, cholla cactus and tree densities also contributed more than the other variables to the discrimination. The interpretation given these results was that P. intermedius occupied areas high in bedrock exposure while the other species did not. P. baileyi was associated with an intermediate level of bedrock ex posure, while the other three species inhabited areas with little exposed bedrock. The separation of these last four can be better ana lyzed in the four group multiple discriminant analysis* For the reasons discussed in the methods section, P. intermedius was removed and the four group discriminant functions were derived. The first of these separated P. baileyi from the other three species, in keeping with its intermediate position in the five group analysis. Cholla cactus density, shrub category 3 density, bedrock exposure, and sand percent in the soil were the important variables. Interpretation is easier after description of the second discriminant function. It shows P. baileyi and P. penicillatus separated from P. amplus and I). merriami. P. baileyi is in an intermediate position, though more closely associated with P. penicillatus. The discriminant function gives shrub ^ density as the most important discriminator with cholla density, bedrock percent and percent sand in the upper soil of lesser importance. The following interpretation was given to the four group re sults. P. baileyi inhabited areas of high colla cactus and shrub 3 3*+ and 4 densities. P. penicillatus occupied areas with high densities of shrub 4 and (perhaps) sandy soil. These two species* habitats were apparently differentiated on the basis of association with different shrubs and P. baileyi's stronger association with bedrock exposure. P_. amplus and D. merriami inhabited areas with low cholla cactus, shrub 3 and k densities and bedrock exposure. No habitat separation between then was apparent. These results and their interpretation are consistent with the results of the within-habitat lines. All the factors of importance in the multiple discriminant analysis of the miniquadrats were associated with the same species on the lineB. D. merriami and P. amplus were associated with the same factors in the miniquadrat discriminant analy sis, and were taken on the same line sets in the within-habitat l'.nes. Red Hill Grid The trapping systems analyzed so far were designed to study the overall habitat segregation in the area. Both the Bed Hill grid and the IBP lines were intended to explore the less obvious habitat associ ations in the ecotone. Both were set in this area where habitats were mixed. No attempt was made to isolate a site within a habitat type. Instead the trap stations were set at regular 15 m intervals, and each trap station was analyzed as a trapping site analogous to a miniquadrat or within-habitat line. Tables 3-6 and Figures 3-6 give the results of the four grid trapping periods. In each run P. intermedins habitat was distinctly separated from the habitats of the other species. Again, the first Table 3. Grid, September 1972: means and standard deviations of each habitat variable for each species of heteromyid rodent. — The column headed Total contains the means for all species together. N is the number of captures in the analysis. Plant means are in numbers of individuals counted in 130 m^. Variable measurement is explained in the Methods section. Abbreviations are as in Table 1. Means were calculated as in Table 2. Habitat Species Variables Dm Pb Pp Pi Pa Total + + S-l o.4± 0.9 2.2 ± 4.3 0.2 0.6 4.2 i 5.6 1.7 3.7 2.0 t 4.0 + + S-2 0.6 - 0.9 5.9 0.1 0.3 10.8 i 10.0 2.2 5.3 3.6 i 6.7 3-5| + + Fd 75-8 t 16.7 74.5 x 26.9 55.2 • 20.7 84.0 t 31.3 77.7 20.4 75.1 X 25.4 T T Ld 6.2 t 4.0 8.9 x 6.0 10.7 5.3 5.4 t 5.5 6.7 t 5.5 7.7 X 5-7 + *r i 2.2 S-3 0.2 i 0.4 2.5 0.6 * 0.7 2.7 0.3 t 0.7 1.2 1.3 x T T S-4 o.l - 0.2 2.6 5.1 4.9 2.2 t 3.3 1.1 1.5 1.6 t 2.8 1.5 x + + Tr 0.7| 0.7 i.i i 1.4 0.6 0.9 2.2 J 1.3 1-3 1.0 1.2 i 1.3 + + C-l 2.6 ± 1.4 2.1 4.1 2.6 2.6 3.1 2.8 3-1 i 2.3 2.9 x + 3.7 J + C-2 3-0 2.2 7 2.9 0.7 1.5 4.0 i 6.8 2.2 2.3 2.3 2.3. J + + X 5.6 67 | 5 65 x 6 68 5 66 i 6 63 6 65 ± 6 41 - 4 + 6 33 J 6 38 J 9 32 2 37 + 37 I 8 2 1 + 4 6 4 6 1 6 J 6 2 « ? + 5 x 6 %Br o i 0 12 22 2 5 36 i 31 11 23 13 t 24 X + + Bern 4i i 14 39 - 16 53 12 26 ± 16 39 15 39 1 16 N 20 59 11 20 26 136 Table 4. Grid, September 1973* means and standard deviations of each habitat variable for each species of heteromyid rodent. — The column headed Total contains the means for all species together, N is the number of captures in the analysis. Plant means are in numbers of individuals counted in 130 m^. Variable measurement is explained in the Methods section. Abbreviations are as in Table 1. Means were calculated as in Table 2. Habitat Species Variables Dm Pb Pp Pi Pa Total + + + + 4 S-l 0.2 0.4 3.1 4.9 0.6 J 1.3 5.4 7.7 2.0 3.8 2.5 4.6 + + + + + S-2 0.5 0.1 5.2 8.6 1.3 x 2.8 10.3 10.2 2.0 5.4 3.8 7.5 + + + + + Fd 85.9 21.9 81.5 24.2 62.0 t 32.4 76.6 23.6 84.8 23.9 81.8 24.6 + + + + + Ld 6.8 t 4.8 6.6 5.5 » 2.9 7.3 * 5.8 4.7 5-1 • • 4.3 6.7 • 4.9 *T" T T T S-3 0.1 0.5 1.9 * 2.8 2.2 t 3.7 2.9 3.2 1.0 2.0 1.5 2.5 4* + + + + S-4 0.5 0.8 1.6 2.1 3-0 t 3.4 2.5 3.2 1.2 1.7 1.5 2.1 + + + + + Tr 0.7 0.7 1.6 1.1 0.8 i 0.8 2.6 2.3 1.0 1.1 1.3 1.3 + + + C-l 3.1 2.1 3.6 3-1 3.0 i 2.7 3.8 3.4 3.2 2.5 3.4 2.8 + + + + + C-2 1.9 3.3 2.5 3.6 2.3 x 4.1 3.0 5.9 3-1 4.7 2.7 4.2 + + + 4* + 65 6 64 6 6k t 6 64 6 63 6 64 6 + + + + + 37 6 38 6 35 |8 40 5 38 6 38 6 + + + + + #SC 3 5 6 6 k t 6 11 7 4 5 5 6 + + 4* + #Br 2 5 17 2k 2 £ 5 40 31 11 19 14 23 + + + + + Dcm 42 14 34 16 48 - 13 33 18 35 14 36 15 N 19 82 10 15 71 197 Table 5* Grid, September 1974: means and standard deviations of each habitat variable for each species of heteromyid rodent. -- The column headed Total contains the means for all species together. N is the number of captures in the analysis. Plant means are in numbers of individuals counted in 130 m^. Variable measurement is explained in the Methods section. Abbreviations are as in Table 1. Means were calculated as in Table 2. Habitat Species Variables Dm Pb Pp Pi Pa Total S-l o.6 i 1.5 2.9 x 5-^ 1.1 x 2.9 5.6 t 6.9 1.0 t 2.9 2.5 i 5.0 S-2 0.4 - 0.7 4.2 ± 7.4 2.8 i 5.5 13.5 x 12.0 o.6 i 1.9 4.2 ± 7.8 Fd 73.2 2 17.0 79-5 x 27.9 64.2 - 29.7 75.9 x 27.0 78.5 J 19.5 76.4 t 27.1 Ld 7.0 t. 4.8 6.5 J 5.^ 7.5 x 5.2 i 4-9 7.6 t 5.1 6.7 J 5.3 S-3 0.4 t 1.2 1.6; 2.8 !.2 J 2.0 2.9 x 4.4 0.3 J 1.0 !.4 x 2.8 S-4 0.5 x 1.1 1.6 i 2.3 2.8 ± 3.5 2.0 J 2.5 0.5 x 1.4 1.6 i 2.5 Tr 0.6 - 0.8 1.4 - 1.4 1.3; 1.3 2.7 x 2.2 0.8 t 0.9 i.4i 1.5 C-l 2.4 - 1.8 3.2 i 2.4 3*5 x 2.8 2.5 i 2.2 2.8 - 1.8 3.1 x 2.4 C-2 1.6 t 2.3 2.3 x 3.6 2.2 3.0 i M i.5- 3.3 2.0 2 3.5 64 t 6 65 x 6 66 | 6 67 t 5 64 t 6 65 x 6 8 41 - 36 i 55 i 5 38 x 3tf I 7 7 7 37 J 7 2 ± 3 6 t 6 3 2 4 12 X 7 3 x 4 5 2 6 #Br 3 ± 11 25 15 32 13 24 17 x 5 x 59 J 6 * 15 I Dcm 40 - 13 37 - 15 48 - 14 32 i 17 40 ± 13 39 - 16 N 44 421 108 67 87 727 vjj Table 6. Grid, October 1974: means and standard deviations of each habitat variable for each species of heteromyid rodent. — The column headed Total contains the means for all species together, N is the number of captures in the analysis. Plant meanB are in numbers of individuals counted in 130 m^. Variable measurement is explained in the Methods section. Abbreviations are as in Table 1. Means were calculated as in Table 2. Habitat Species Variables Dm Pb Pp Pi Pa Total + + 0.8 t 1.0 t 1.8 S-l 2.4 2 4.9 2.0 4.3 • 5.6 3.2 2.4 - 4.4 -9 £ + + S-2 0.6 i 1.3 6.6 1-3| 2.8 13.7 11.8 0.5 1.2 4.2 t 7.8 3.7 I + + Fd 21.6 79.6 t 26.6 30.0 77-4 30.8 77.0 12.8 77.5 j 26.2 75-5 2 73-5 j + +t Ld 6.9 - 4.5 6.7 i 5.2 5.6 4.8 3.9 6.1 4.9 6.5 1 5.0 7-0 1 + o.kt 0.8 1.6 S-3 1.0 1.6 i 3.1 1 1 1.6 2.1 2.0 » 1.3 J 2.5 ' 7 + *r s-4 o.4 t 1.1 1.9 3.ii 3.4 2.2 2.6 0.9 • 1.1 1.5 J 2.3 1.3 J + Tr 0.6 - 2.8 2.2 T 0.8 7 0.7 1.2 - 1.1 1.2 J 1.1 + 0.7 + 1.3 1.4 C-l 2.6 i 1.9 3.3 i 2.6 3.6 t 2.9 3.3 2.7 2.7 1.5 3.21 2.5 T + C-2 2.3 2.4 ± 1.6 3.2 5.4 1.0 1.5 2.2 J 3.6 1.9 x 3.7 + + 65 i 6 64 i 6 64 t 6 67 5 64 6 65 x 6 40 + + 38 i 7 37 i 7 38 t 8 35 ~ 5 + 7 36 + 7 stec 4 t 6 i 6 4 t 5 11 7 3 2 6 i 6 5 + + 2 #Br 19 17 24 5 i 14 39 32 6 X 8 17 x 5 9 I I T Dcm 39 - 12 35 - 15 46 i 15 29 17 38 13 37 - 15 N 40 103 28 33 12 216 39 § •H (0 5 Groups 4 Groups +> 0> nj ti •h o > O Discriminant Discriminant Discriminant q03 W Function 1 Function 1 Function 2 +> 13 13& -h1 3 5 +> ^ h w o t3 »rtto U §s Dm Pb PP Pi PQ Dm Pb Pp Pa Dm Pb Pp Pa Species (Groups) S-l +0.3 -0.1 +0.3 S-2 -0.6 +0.2 +0.4 Fd -0.2 +0.1 +0.1 c o Ld -0.4 -0.4 +0.6 •rl •P S-3 -0.4 0.0 +0.3' oe +>to 3 c S-4 -0.2 -0.9 -0.4 m o -0.1 •h Tr ' +0.1 +0.1 +> o C-l -0.2 0.0 0.0 h «m C-2 0.0 +0.1 -0.1 •H O 0.0 e o #S —0.1 -0.1 •H U -0.2 fci -0.1 +0.5 u co %sc -0.6 0.0 +0.1 •rl -0.1 o %Br +0.1 -0.5 Dcm +0.2 -0.2 +0.1 55 48 33 ^2 X2 123 70 37 df 56 42 26 P 0.00 0.00 0.07 Figure 3* Grid, September 1972: discriminant score graphs and cor responding discriminant functions. — The graphs show the mean and standard deviation of the discriminant scores for each species. Abbreviations are as in Table 1. Sample sizes are given in Table 3« Other information is as in Figure 2. 40 to c 5 Groups 4 Groups •ho (0 -p (u d k •h o Discriminant Discriminant Discriminant > o q0) co - Function 1 Function 1 Function 2 +> 13 c t)& 'h£ c e cd *h W V t3 *hto §s (0 o c« aj -A T, Dm Pb Pp PI Po Dm Pb Pp Po Dm Pb Pp Pa Species (Groups) S-l -0.1 +0.3 +0.2 S-2 -0.1 +0.2 0.0 Fd +0.1 0.0 +0.5 O •h Ld -0.4 +0.7 +0,3 +> u id c +» S-3 —0.2 +0.2 -0.5. P C s-4 -0.2 +0.1 -0.4 -p •ho Tr -0.3 +0.3 +0.2 C >rl C-l -0.1 +0.1 +0.1 3 c #2 59 56 30 yr 98 70 32 df 56 42 26 p 0.00 0.00 0.20 Figure 4. Grid, September 1973i discriminant score graphs and cor responding discriminant functions. — The graphs show the mean and standard deviation of the discriminant scores for each species. Abbreviations are as in Table 1. Sample sizes are given in Table 4. Other information is as in Figure 2. 41 8 o 3 Groups 4 Groups h to 4j 0) ctj Discriminant Discriminant h Discriminant > o Function 1 Function 1 Function 2 qa> w -p t3 fl U& -HS A E Cd Species (Groups) S-l 0.0 0.0 +0.3 S-2 -0.3 0.0 -0.3 c Fd -0.1 -0.3 +0.1 o Ld -0.2 -0.1 0.0 , •p•H O to S-3 -0.1 -0.2 0.0 C -P 3 a S—4 -0.3 -0.6 -0.5 1*4 0> •H Tr -0.3 -0.3 -0.2 -P O C-l +0.1 -0.1 -0.2 3S3 *H 68 54 42 X2 305 163 75 df 56 42 26 P 0.00 0.00 0.00 Figure 5* Grid, September 197^s discriminant score graphs and cor responding discriminant functions. — The graphs show the mean and standard deviation of the discriminant scores for each species. Abbreviations are as in Table 1. Sample sizes are given in Table 5» Other information is as in Figure 2. b2 5 Groups k Groups •ho tq •p 0) ce u •h o Discriminant Discriminant Discriminant > o U U3 Fimction 1 Function 1 Function 2 n •a {3 73 •h Gm 'he tn h ea o n (0 13 I" a s -2 ^ |- 10 o 'V § Species (Groups) S-l +0.3 -0.2 -0.2 S-2 -0.6 0.0 -0.3 c Fd -0.1 +0.2 0.0 o •H Ld -0.2 +0.1 -0.3 +> +0.2 -0.1 co -pto s-3 -0.3' P e £-4 -0.5 +1.0 +0.2 •h Tr -0.5 +0.5 -0.1 •p o C-l -0.2 +0.2 -0.2 ss K «h C-2 -0.1 0.0 -0.3 *h o e o -0.1 -0.2 -0.2 •h o +0.2 0.0 +0.2 oh to -0.5 +0.2 -0.5 #Br +0.1 -0.1 +0.1 Dcm +0.4 -0.1 +0.5 76 53 kz %X2 156 69 33 df 56 42 26 P 0.00 0.00 0.17 Figure 6. Grid, October 197^: discriminant score graphs and cor responding discriminant functions. — The graphs show the mean and standard deviation of the discriminant scores for each species. Abbreviations are as in Table 1. Sample sizes are given in Table 6. Other information is as in Figure 2. discriminant function of the five group analysis and the first two for the four group analysis are shown in each figure. The five group discriminant functions are similar to one an other with the exception of the 1973 trapping. In that year's dis criminant function bedrock exposure had the largest coefficient, as it did in the miniquadrat analysis. Larrea and tree densities were of secondary importance in the discriminant function. In the other three runs bedrock exposure did not contribute much to the discriminant func tions. In each of these runs stone and cobble cover and shrub 2 density were the largest contributors to the discriminant function. Both of these variables were strongly correlated with exposed bedrock (correlation coefficients ranging from 0.6? to 0.70 for stones and cobbles with bedrock percent and 0.60 to O.76 for shrub 2 with bedrock percent). This suggests that bedrock may still have been involved in the segregation of P. intermedins from the other Bpecies, but that at the edge of bedrock exposures and at the microhabitat level, these associates of bedrock became important keys for the species. At any rate, there was a distinct habitat segregation between P. intermedins and the other species based on exposed bedrock and/or associated habi tat factors as in the miniquadrats. Again, P. baileyi habitat was intermediate, though more similar to that of the other three species than to P. intermedius habitat. Other important contributors to the five group discriminant functions appeared to be contributing more to the separation within the group of four than to separation of P. intermedius habitat from the 44 others. This was especially true of shrub 4 density and perhaps also of tree density. The two discriminant functions of the four group analyses are best considered together. In each discriminant function in which P. penicillatus habitat was the most distinctly separate of the four (discriminant function 1 of Figures 3 and 6 and discriminant function 2 of Figures 4 and 5)* shrub 4 density was an important member of the discriminant function. In three trapping sessions it had the largest coefficient, and in the other (Fig, 4) it was third largest following shrub 3 and Franseria densities. The shrub 3 category contained the shrubs most similar to those in category 4 and perhaps could substi tute in part for shrub 4 in the habitat of P. penicillatus. The habitat of P. baileyi was separated on the other discrimi nant function of each pair (discriminant function 2 of Figures 3 and 6 and discriminant function 1 of Figures 4 and 5)» but the factors in volved were not as clear. In each case, however, one of the two largest discriminant function coefficients was a surface or soil fac tor: gravel percent (Fig. 3)* bedrock exposure (Fig. 4), stone and cobble percent (Fig. 5), and stone and cobble percent and shallow soil (Fig. 6). All of these indicated an association of P. baileyi with coarse soil-surface factors as did the bedrock association on the mini- quadrats. In two of the runs (Figs. 3 and 4) Larrea was an important contributor to the discriminant function in which P. baileyi was sepa rated, in another (Fig. 5) shrub category 4 was important. Trees also had fairly large coefficients in some runs (Figs. 4 and 5)» Also in P. penicillatus separations, which were based on high shrub 4 density, k5 P. baileyi was always intermediate between P. penicillatus and the other two species. All of this suggests an association with shrubby vegetative cover for P. baileyi, similar to that shown on the mini- quadrats. P. amplus and D. merriami lived in areas which were low in bedrock esqposure, stone and cobble surface cover, and shrub and tree density. Their habitats appeared to be very similar to one another, as in the other trapping systems. Comparison of the Red Hill Grid with the Miniquadrats. The habitat-variable associations shown by the animals on the grid were very similar to those of the miniquadrats. P. intermedius lived in rocky sites, though the keys on the grid may have been associates of bedrock rather than bedrock itself a6 on the miniquadrats. P. penicillatus occupied sites of high shrub k density away from rocky sites. D. merriami and P. amplus were taken where shrub and tree den sities were low and away from the rocky sites. These reactions were all very similar to those on the miniquadrats. Comparisons of P. baileyi reactions were not so clear, but it appeared to be associated with coarse soil-surface factors, and large shrub and tree densities. P. baileyi appeared to be associated with these same general factors in the miniquadrat analysis. Population Fluctuation and Habitat Associations. Habitat- variable associations were very similar for the high (September 197^, Fig. 5) and low (1972, Fig. 3) population trapping sessions, and from one trapping session to another generally. Only P. baileyi seemed to ke have different associations in the high and low years, and these were consistent in the context of baileyi1s general associations with shrubs and coarse soil-surface factors. Population fluctuation on the grid seemed to have little effect on the habitat-variable associations of the species. Habitat Associations of an Invasion Population. Since the grid had been essentially trapped out in September, it was assumed that the grid trapping of October 197^ took animals which were either actively invading the area or had recently done so. One might expect habitat associations to break down under these conditions. The similarity of the animal separations and the habitat factors involved between the two 197^ trapping sessions indicated that no such breakdown occurred or that the associations were re-established within a month. IBP Lines Like the grid, these lines were set in the ecotone and the re sults gave more insight into the habitat relationships of that area. The lines did not include rocky hill habitat, and few P. intermedius were taken. In fact, bedrock exposure was not included in the analysis because it was so rare in the area, and no five group discriminant analysis was run because too few P. intermedius were caught. Means, for the species, and standard deviations of the habitat variables are shown in Tables 7-10. Results of the multiple discriminant analyses are shown in Figures 7-10» The first two discriminant functions are shown for the four group discriminant analysis. In each analysis, one of the discriminant Table 7» IBP lines, spring and fall 1972 and spring 1973s means and standard deviations of each habitat variable for each species of heteromyid rodent. — The column headed Total contains the means for all species together. N is the number of captures in the analy sis. Plant means are in numbers of individuals counted in 130 m^. Variable measurement is explained in the Methods section. Abbreviations are as in Table 1. Means were calculated as in Table 2. Habitat Species Variables Dm Pb Pp Pa Total S-l 0.4 - 1.2 0.5| 0.8 1.0 i 2.2 0.8 ± 1.8 0.7 J 1.6 S-2 0.2 ± 0.5 o.i i 0.3 0.6 - 2.5 0.3 i 0.6 0.3 j 1.1 Fd 54.° t 24.1 ko.k - 17.4 57.8 i 26.7 55.3 i 29.6 54.3 j 26.7 Ld 12.6 t 8.6 14.0 7 9.5 12.6 i 9.1 14.3 f 8.1 13.^ r 8.5 S-3 0.1 i 0.3 0.2 - 0.4 o.i i 0.5 0.2 i 0.5 0.2 j 0.5 S-4 0.5 ± 1.4 i.5 I 2.1 2.1 - 3.1 0.6 i 1.2 0.8 t 1.8 Tr 1.1 i 1.1 1.8| 2.0 2.0 ± 1.8 i.2 i 1.2 1.3 7 1.4 C-l 1.1 - 1.6 2.4 t 3.0 2.4 i 2.3 1.6 - 2.5 1.6 - 2.2 C-2 0.5 ± 0.9 1.9 2 2.4 1.1 t 1.9 0.8 - 1.3 0.8 t i.if %S 64 t 9 66 - 9 64 J 9 65 J 9 & i 9 36 i 10 38 J 12 37 J 8 38 |12 37 £ 11 6 i 8 t sfec 7 12 2 5 io i 7 8 8 t 7 Dcm 58 i 7 57 ~ 9 56 i 8 57 ± 9 57-8 N 94 14 36 102 2b6 Table 8. IBP lines, fall 1973s means and standard deviations of each habitat variable for each species of heteromyid rodent. — The column headed Total contains the means for all Bpecies together. N is the number of captures in the analysis. Plant means are in numbers of individuals counted in 130 m2. Variable measurement is explained in the Methods section. Abbreviations are as in Table 1. Means were calculated as in Table 2. Habitat Species Variables Dm Pb Pp Pa Total S-l |0.3 1.0 1.5 i 2.2 1.6 t 2.8 0.6 i 1.5 1.1 x 2-3 S-2 |0.3 0.7 0.6 - 1.8 0.7 - 2.3 0.3 i 0.7 0.5 x 1-7 Fd 52.4 i 23.5 ^3.6 t 19.1 56.4 J 25.5 52.4 i 25.0 53.6 t 24.8 Ld i4.4 ± 6.3 13.7 T 8.3 14.6 t 8.6 14.9 J 9.0 14.6 i 8.5 S-3 0.! t 0.4 0.11 0.3 0.1 ± 0.4 o.l t 0.3 0.1 i 0.3 S-4 o.4± 1.1 1.6 i 2.2 2.6 0.6 - 1.2 1.1 - 2.1 1-5 7 Tr 0.7 I 1.1 2.2 ^ 2.0 L7- 1.6 1.2 1 1.3 1.5 2 x-5 C-l 1.0 - 1.3 2.6 t 2.4 2.3 2.6 1.5| 2.2 1.9 J 2.4 0.8 i C-2 0.6 - 1.1 1-lf 7 2.1 °.9 x 1.3 1.5 0.9 7 1.4 64 t 9 64 i 10 64 - 9 63 |10 63 ; 9 <%G 36 ± 38 ± 11 J 10 9 38 £ 11 37 f 11 #SC 4 t 6 11 t 8 7 J 7 8 t 8 7 J 7 Dcm 57 i 8 54 i 10 54 ± 9 56 - 8 55 i 9 N 4i 25 157 124 347 Table 9* IBP lines, spring 197^s means and standard deviations of each habitat variable for each species of heteromyid rodent. — The column headed Total contains the means for all species together. N is the number of captures in the analysis. Plant means are in numbers of individuals counted in 130 m^. Variable measurement is explained in the Methods section. Abbreviations are as in Table 1. Means were calculated as in Table 2. Habitat Species iriables Dm Pb Pp Pa Total S-l 0.8 - 2.0 0.8 -* 1.8 1.2 i 2.6 o.7 i 1.8 0.9 * 2.2 S-2 0.4 t 0.9 0.8 - 2.0 0.3 i 1.5 0.5 i 1.7 0.4 i 1.4 Fd 52.9 2 23.1 48.9 j 25.9 54.3 x 27.1 52.1 i 25.1 52.8 - 25.2 Ld 14.2 i 8.7 12.2 i 8.1 13.1 J 8.3 13.7 i 8.4 13.5 x 8-4 s-3 0.1 i 0.4 0.1 t 0.3 0.1 - 0.5 0.1 t 0.3 0.1 t o.4 S—4 0.5 x 0.9 0.9 J 1-5 1.1 t 2.0 0.8 t 1.9 0.8 ± 1.7 Tr 1.2 i.5 2.0 1.5 1.0 - 1.1 1.3| lA 1#1; 1,5 £ C-l 1.8 i 2.6 2.4 J 3.2 1.9 i 2.1 1.9 J 2.8 1.9 x 2.5 C-2 0.8 - 1.3 1.2 - 2.1 0.8 ± 1.4 0.7 x 1.3 0.8 t 1.4 62 i 10 66 i 10 62 ± 10 63 j 10 63 x 10 #3 37 x 11 37 I 11 37 J 10 3V x 11 37 1 11 %sc 6 i 11 i 6 8 - 7 7 6 T 7 7 I 7 Dcm 56 i 8 57 - 8 54 - 9 55 i 9 55 - 9 N 164 52 172 98 486 Table 10. IBP lines, fall 1974: means and standard deviations of each habitat variable for each species of heteromyid rodent. — The column headed Total contains the means for all species together. N is the number of captures in the analysis. Plant means are in numbers of individuals counted in 130 m^. Variable measurement is explained in the Methods section. Abbreviations are as in Table 1. Means were calculated as in Table 2. Habitat Species Variables Dm Pb Pp Pa Total S-1 0.6 i 1.6 1.3 - 2.6 1.3 j 2.8 0.6 - 1.7 l.l - 2.5 S-2 0.4 i 1.1 0.8 t 2.2 0.4 i 1.6 |°-5 0.6 0.5 x 1.7 Fd 55.2 t 27.0 48.8 i 26.1 55.5 x 25.9 50.1 i 27.4 53-6 t 26.3 Ld !3.2 j 7.6 14.2 t 8.3 15.5 i 8.4 13.8 t 8.3 13.6 J 8.2 S-3 0.1 - 0.3 0.2 t 0.5 0.2 - 0.4 0.1 i 0.3 0.2 J 0.4 S-4 0.5 ± 1.0 1.5 i 2.7 1.3 i 2.2 o.4 i 0.7 2.1 1-1 £ Tr 1.0 ± 1.1 1.8 - 1.9 1.5 i 1.5 1.11 1.2 1.4 t 1.5 G-1 2.5 3.0 2.2 1.8 x 2.4 2.0 t 2.5 i.7 I 2.5 J 1#9 T C-2 0.9 i 0.9 i 1.8 0.9 i 1.4 0.9 x 1.6 ia 7 1.5 1.3 #S 62 t 10 65 I 9 63 J 9 62 t 10 63 x 9 37 J 11 11 37 x 10 38 i 13 37 x 11 #sc 6 i i 8 i 6 10 7 7 x 7 11 2 7 7 Dcm 57 - 7 56 ± 9 55 - 9 55 - 9 55 - 9 N 109 129 282 31 551 51 Discriminant Discriminant to £ Function 1 Function 2 •ho tq o o o co t3 s3 u (d 3 B 13 "h 9 5 \ +> (h co o \ 10 'h -2 §° CO o § Species (Groups) S-l 0.0 S-2 0.0 s: o Fd -0.1 •h4j O to Ld -0.2 S3 +> S-3 +0.1 s a •h -0.6 4-> c Tr si C-l •h 0j C-2 e o •rl CJ k %S u (0 5& •h« ?6SC Dcra *2 yr df p Figure 7. IBP lines, spring and fall 1972 and spring 1973* discrimi nant score graphs and corresponding discriminant functions* — The graphs show the mean and standard deviation of the discriminant scores for each species. Abbreviations are as in Table 1. Sample sizes are given in Table 7* Other information is as in Figure 2. 52 to Discriminant Discriminant c o Function 1 Function 2 •rl+> rj •rl > q4) CO +j 13 C & g u "h g£ 0 4-> U w o u l\\[ §° -2 S-l -o.k +0.1 S-2 0.0 +0.1 d o Fd -0.1 +0.6 •H +> Ld -o.i* +0.3 o to C *P S-3 -0.1 +0.2 p c Pn 0> srk —0.6 +0.4 •H •P O Tr -O.k -0.3 C *rl § %P 59 31 x2 89 38 df 39 2k P 0.00 0.0k Figure 8, IBP lines, fall 1973! discriminant score graphs and corresponding discriminant functions. — The graphs show the mean and standard deviation of the discriminant scores for each species. Abbreviations are as in Table 1. Sample sizes are given in Table 8* Other information is as in Figure 2. 53 (0 Discriminant Discriminant a o Function 1 Function 2 •ri (0 +> 0) at •h o > o 0) w a •a c ib& -h1 s5 4j (h I- w o (0 tj *h -2 s£ S-l -0.2 0.0 S-2 +0.4 0.0 £ o +0.2 •h Fd -0.3 4-> o (0 Ld -0.2 +0.2 c -p 0.0 3 c s-3 -0.1 +0.6 •h s-b -0.3 +> o Tr 0.0 +0.6 3C CH'rl c 60 26 %X2 88 36 df 39 2k P 0.00 0.06 Figure 9» IBP lines* spring 197^: discriminant score graphs and corresponding discriminant functions* ~ The graphs show the mean and standard deviation of the discriminant scores for each species. Abbreviations are as in Table 1. Sample sizes are given in Table 9* Other information is as in Figure 2. 5b to c Discriminant Discriminant o •p*ri Function 1 Function 2 •has > FL> 03 n t3 c tj& c as +> o w (0 t3 *h 1" ^ ^ -2 §£ CO o §a) £ -4 Dm Pb Pp Pa Dm Pb Pp Pa Species (Groups) S-l 0.0 -0.^ d S-2 -0.1 +0.3 o Fd +0.3 -O.if •rl 4-> Ld -0.2 -0.3 cC -pCO 3 c S-3 -0.2 -0.3 [=4 v S-4 -0.2 -0.9 •H +> o Tr -O.k -0.1 -RI (H*•"' |3 Figure 10. IBP lines, fall 197k: discriminant score graphs and corresponding discriminant functions. -- The graphs show the mean and standard deviation of the discriminant scores for each species. Abbreviations are as in Table 1, Sample sizes are given in Table 10. Other information is as in Figure 2. 55 functions separated P. baileyi habitat from P. penicillatus and D. merriami habitat. P. amplus habitat was somewhat intermediate, but generally tended to be closer to D. merriami and P. penicillatus habi tat (discriminant function 2 of Figures 7 and 8 and discriminant func tion 1 of Figures 9 and 10), The other discriminant function separated P. penicillatus and P. baileyi from P. amplus and D. merriami (discrimi nant function 1 of Figures 7 and 8 and discriminant function 2 of Figures 9 and 10). In the fall of 197^ trapping session (Fig. 10) P. baileyi was intermediate between P_. penicillatus and the other two, but the pattern appears to be as described. Separations were not as dis tinct as in previously analyzed trapping systems. This area was the one with the most intimate habitat overlap and mixing, so the minimal separations were not surprising. Because the separations found were consistent with those found in the other trapping systems, they were interpreted as being biologically significant. The habitat variables which consistently showed the largest contribution to the discriminant functions were shrub U density, stone and cobble coverage, and tree density. Shrub h was consistently im portant in the separation of P. penicillatus habitat from all others and seemed to contribute to separation of P. baileyi habitat from D« merriami and P. amplus habitat. Stone and cobble surface cover was important in the separations of P. baileyi habitat from all others. P. amplus tended to be associated with stone and cobble cover more than was D. merriami. Trees appeared to be important in separation of P. penicillatus and P. baileyi from P. amplus and D. merriami. 56 Comparison of the Lines with Other Trapping; Systems. Habitat associations were similar to those at other sites. P. baileyi was associated with both coarse surface-soil factors (high percent stone and cobble surface cover) and high shrub and tree densities as in other trapping. The specific discriminators that P. baileyi separation was based on were more consistent on the lines than on the grid, but in comparison between trapping systems, P. baileyi was still the least consistent mouse, P. intermedins was essentially absent and so was the bedrock with which (or with correlates of which) it was associated in other trapping systems. Shrub k density separated the other three species in exactly the same way it did in all other trapping. Percent stone and cobble surface cover also separated the habitats of the mice in ways consistent with the separations due to coarse soil factors in other trapping systems. P. antplus again demonstrated a greater toler ance for coarse soil-surface factors than D. merriami, but the two were very Bimilar in keeping with reactions in the other trapping systems. Comparison of High and Low Populations on the IBP Lines. As on the grid, low (1972 and spring 1973) and high (all other trapping sessions) populations had the same general pattern of habitat associ ations. The same factors were associated with the same species in each analysis. Summary of the Discriminant Analysis Results Table 11 summarizes the interpretation given the discriminant analysis results in this study. The table is a summary of the results Table 11. Summary of the interpretation of multiple discriminant analysis results. — A "+" indi cates a positive reaction to a variable, and a indicates a negative reaction. A "0" indicates no apparent reaction to the variable. Variables not accounting for at least 1C$ of the sum of the discriminant function coefficients are indicated as "no apparent reaction." The top line of each entry is the miniquadrat reaction, the second line contains the Red Hill grid runs in chronological order, and the third contains the IBP line runs in chronological order. Abbreviations are as in Table 1. Habitat Variables Trap Species S-l S-2 Fd Ld S-3 s-<+ Tr C-l C-2 95s %sc Br Dcm System 0 0 0 0 0 0 0 0 0 0 MQ Dm 0000 -0-- 0000 0-00 0-00 0 0 0000 0000 0000 -+00 -0— 0-00 0000 RH 0-00 0000 0000 0000 0000 0 0000 -000 -000 -000 0000 ibp 0 0 0 0 + + 0 + 0 0 + 0 + 0 MQ Pb 00+0 +000 0000 ++00 +000 0—++ o+o+ 0000 0000 0000 ++00 +0++ 0+00 0000 RH Of00 OOfO 0—0 0000 0000 +00+ ++++ 0000 +000 +00+ +000 ++++ 0000 ibp 0 0 0 0 + 0 0 0 + 0 0 MQ Pp 0000 -0— 0000 ++00 0+00 ++++ 0-0+ 0000 0000 0000 —00 -0— 0-00 00++ RH Of 00 0000 0—0 0000 0000 ++++ +0+0 0000 0000 -000 -000 +0-0 0000 ibp 0 0 0 0 0 0 0 0 0 0 + 0 + 0 MQ Pi 0000 +0++ 0000 0000 +000 0000 0+++ 0000 0000 0000 0000 +0++ Of00 000- RH ibp 0 0 0 0 0 0 0 0 0 0 MQ Pa 0000 -0— 0000 0000 0-00 o 0 0000 0000 0000 ++00 —0—— 0-00 0000 RH 0-00 0000 0000 0000 0000 —0- -0-0 0000 -000 +000 +000 000+ 0000 ibp just discussed which compared trapping systems, trapping sessions in the systems, and complementarity of species' associations with habitat variables. It does not show all the reactions of each species to each variable, only those which were important discriminators among the habitats of the various species. A plus sign implies that: (1) the factor was one whose discriminant function coefficient was 10# or more of the sum of the discriminant function coefficients for that analysis; and (2) the species' response to the factor was interpreted as a posi tive response. A negative sign carries the same implications except that the response to the factor was interpreted as a negative one. An entry of zero implies either that the factor's discriminant function coefficient was less than 10$ of the sum of the discriminant function coefficients or (if the discriminant function coefficient was 10$ or more of the sum) that the species' response was interpreted as a neutral one. It is important to recognize that if two variables are highly correlated, only one may show up as a discriminating variable in the multiple discriminant analysis even though the species show strong com plementary responses to both. Once the species' response to one factor has been considered in an analysis, the other may contain no further discriminating power. All its discriminating potential was accounted for in the discrimination produced by its correlate. Presumably, if one of the correlated factors was the variable actually keyed on by the mice, it would be the one diagnosed as most important in every analysis. The species' response to the other would then be interpreted as being due to its correlation with the variable keyed on by the animals. 59 Another possibility under the above conditions, is that the animals did not key on one of the factors but on an environmental con dition due to a combination of the correlated factors (and perhaps other factors as well). Under this possibility, one would not expect either of the factors to always be given as a discriminator. Instead, one analysis should be expected to result in a given variable being an important discriminator and the other appearing to be unimportant due to the correlation. In another analysis, however, this relationship should be reversed. If two or more correlated variables are alterna tively selected as discriminating variables, the simplest interpreta tion would seem to be that they formed (or helped form) an environ mental complex to which the species responded. Table 11 shows that in this study bedrock exposure (Br) and stone and cobble surface cover (#SC) formed such a complex. In the trapping sessions in which bedrock exposure was an important part of the habitat (miniquadrats and grid) the two consistently discriminated between two groups of mice, £. intermedjus and P. baileyi which re acted positively and the other three species which reacted negatively. In two trapping sessions bedrock exposure was important and the surface cover of stones and cobbles was not. In the other three, bedrock was not an important discriminator and the stone and cobble cover of the surface was. The correlation coefficients for the factors ranged from .67 to .70 on the grid and was .36 on the miniquadrats. On the IBP lines, where bedrock was too scarce to be included in the analysis, surface stone and cobble cover was still an important discriminator. The two variables appeared to be part of a surface-soil dimension to 6o which the various species reacted differently. Neither of the vari ables was the key for the animals, but the soil-surface dimension to which both contributed did discriminate consistently between the species in the community. A shrub-tree dimension is alBo shown in Table 11. The comple mentary reactions of P. penicillatus on the one hand and D. merriami and P. amplus on the other to spinescent shrubs (S-4) is evident. Consideration of the tree densities (Tr) generally strengthens that complementarity. Together, the two variables show P. baileyi1s posi tive response to shrub and tree cover as well. In this case, other shrub groups also contribute. Densities of shrubs of category k and trees were not as highly correlated as were the soil surface factors (correlation coefficients ranging from .06 to .21 on the grid, .18 to .30 on the IBP lines and calculated as 0.l4 on the miniquadrats). The two supported one another (a given species responded in the same way to both factors) in some analyses and complemented one another (where one was neutral, the other was a discriminator and vice versa) in others. This sort of relationship would be expected between variables, either or both of which, could supply a given requirement of the species (in this case probably cover). Note in this context, that discrimination of P. baileyi habitat from P. amplus and D. merriami required densities of tree and shrub groups in addition to the density of shrubs of category k. On the other hand, that of P. penicillatus habitat was strongly based on the discrimination due to the spinescent shrubs (S—*t) alone. 61 The summary table and above discussion demonstrated a variety of responses by the community to the variables chosen for analysis. A single variable (S-4) discriminated between the habitats of some species CP, amplus and D. merriami vs. P. penicillatus). A combination of similar variables (various shrub and tree densities) were required to discriminate between others (P. baileyi vs. P. amplus and D. merriami). A combination of two highly correlated variables (Br and #SC) essentially to form a complex variable (soil-surface coarseness) discriminated between two groups of mice (P. baileyi and P. intermedius vs. D. merriami, P. amplus and P. penicillatus). And finally, none of the variables or combinations of variables could be used to distinguish consistently between one pair (D. merriami and P. amplus). This vari ety is probably just a sample of the total complexity of reactions and interactions which are ultimately responsible for the coexistence of the species in the area. Heteromyid Habitat Associations Certain habitat variables have been consistently associated with each species under four different trapping systems. In some of these systems each trap site was placed in a homogeneous area; other systems trapped heterogeneous areas with regular distribution of trap sites. The same associations were found under different population conditions and in an invasion situation. This consistency was inter preted as strong evidence that the habitat variables were important to the ecological distribution of the species. In this section, the habitat-variable associations will be dis cussed for each species and compared with the literature. All of the literature studies used techniques different from those used in this study. Many were not carried out with the same objectives in mind. Rigorous comparability cannot be expected. However, if the factors of importance here, or similar ones, are important in other systems they Bhould often be recognizable despite the variation in techniques. Dipodomys merriami This species was not taken frequently in areas of high densi ties of most shrubs or stone and cobble and bedrock covered surfaces. An association of D. merriami with open habitat is suggested by the reaction to shrub densities, and has been reported many times (Dice 1930? Monson and Kessler 19^0* Spencer 19^1; Hardy 19^5? Reynolds 1950; Hoffmeister and Goodpaster 195^5 Denyes 1956; Reynolds 1958; Lidicker I960; Massion 1971)• There are some apparently exceptional reports, however. For example, Ivey (1957) reported taking them from mesquite filled washes, and Hoffmeister (1956) trapped them among grasses and bushes. Still, the results obtained in this study are in agreement with most of the literature. D. merriami was regularly found to be associated with low shrub densities, or more generally with habitats low in vegetative cover. The fact that few were taken in areas of rocky and stony sur face is not as well supported in the literature. They have been re ported from sandy soils (Grinnell 191^; Bailey 1931; Spencer 19^1; Borell and Bryant 19^2; Huey 1951; Bateman 1967)* and from soils that are quite coarse (Long 19^0; Hardy 19^5; Baker 1956; Baker and Greer 1962; Darden 1965; Drabek 1967; Findley et al. 1975)• Baker (1956) 65 found them in rocky situations, but Reynolds (1958) felt they avoided soils with lots of rock. Huey (1951) also felt that hard, rocky soils were too difficult for them to burrow into, and so were avoided. Dalquest (1953) found them in all open desert and rocky hill areas in which there was enough soil for burrows. Obviously, the literature reports are not as consistent concerning I), merriami's soil-surface associations as they seemed to be with regard to the vegetative associ ations. In this study, few were taken in areas high in surface bedrock. These areas were associated with shallow soil which probably interfered with burrow construction as suggested by Dalquest (1953)* Also, few occurred in stone and cobble covered areas. It may be that a rough surface interferes with the rapid bipedal movement and escape mechanism of D. merriami. This was the common reason given for the species avoidance of heavily overgrown areas (Reynolds 1950? Rosenzweig and Winakur 19&9)• The vegetation was thought to interfere with both de tection of predators and escape movements (Hatt 1932; Reynolds 1950). The ricochetal movement that kangaroo rats use in escape might also be disturbed by rough ground, as in an area high in surface stones and cobbles. Bartholomew and Caswell (1951) suggested that open, smooth surfaced areas may be required by D. merriami for this reason. However, the hypothesis must be treated with reservation because of the several literature reports of D. merriami in rocky areas. Perognathus baileyi This was the most difficult heteromyid to associate with spe cific habitat variables. P. baileyi seemed to be associated with 6k coarse soil-surface factors though not with the high bedrock and stone and cobble soils of the hill tops. No one aspect of soil-surface coarseness showed up in all cases, but percent stone and cobble cover of the surface was an important contributor to several of the dis criminant functions on which P. baileyi was separated. A second association appeared to be with high shrub and tree densities. This association was also quite general with different shrub and tree groups contributing large discriminant function coefficients in different trapping sessions. Several studies have found P. baileyi associated with coarse soils, from gravelly (Burt 1938; V/ondolleck 1975) to rocky (Grinnell 1933; Hoffmeister 1956; Findley 1959; Bateman 1967; Drabek 1967; Massion 1971). Rosenzweig and Winakur (1969) took them on sandy soil without much rock fragment, but they also took them on the coarser soils in the area. However, Patton and Jones (1972) took P. baileyi in an area in which the soils were almost devoid of even small rock mate rial. A shrubby vegetation association has been reported by several authors (Burt 1938; Findley 1959? Massion 1971; Patton and Jones 1972). Rosenzweig and Winakur (1969) grouped £. baileyi with P_. penicillatus and several other mice and found that together their numbers correlated with the proportion of vegetation above ^6 cm. These mice were called bush mice by the authors because this vegetative cover was primarily contributed by shrubs. Reynolds and Haskell (19^9) found P. baileyi associated with heavy grass cover, a vegetated situation, but hardly the same as the shrub and tree association found in this study and in 65 the others just referenced. There were scattered trees and shrubs in the area of Reynolds' and Haskell's study, however. Bateman (1967) Drabek (1967) also took them in sparse, shrubby vegetation. In nearly all these reports P. baileyi was associated with one or (usually) both of the habitat factors with which it was associated in this study. Pocket mice are not as specialized for bipedal, rico- chetal locomotion as are kangaroo rats (Hatt 1932; Bartholomew and Cary 195*0. Eisenberg (1963) pointed out that Dipodomys as a genus has adapted to the open areas of their range and that pocket mice have re mained small and cryptic occupants of that same range. Rosenzweig and Winakur (1969) suggested that pocket mice generally need vegetative cover for protection from predation. Perhaps this is the basis of the shrub and tree association shown in this study by £. baileyi; these types of plants give cover and protection from predators. The coarse soil-surface character of P. baileyi habitat may be a result of compe tition with other heteromyids. Perhaps not current competition, the association may have been molded by past competition as Schroder and Rosenzweig (1975) suggested for Dipodomys ordii and D. merriami• This would seem to be consistent with the stability of the ecological dis tributions of the community members under different trapping and popu lation conditions. Perognathus penicillatus This species was taken in association with shrubs of category k (acacias and other large spinescent shrubs), and away from large bed rock exposures and heavy stone and cobble surface cover. Literature reports commonly ascribe a shrub and tree association to P. penicillatus 66 (Arnold 19*+2; Hall 19^6; Hoffmeister and Goodpaster 195M Denyes 1956; Darden 1965; Bateman 196?; Rosenzweig and Winakur 1969). The shrubs and trees involved included mesquite, desert broom* creosotebush, cottonwood, desert willow, tamarisk and others. In this study acacias were the most important plants in the shrub association of the species. Evidently the species of tree or shrub is not an important factor in that association. Reynolds and Haskell (19^9) found P. penicillatus in heavy grass, and other reports indicated that dense shrub cover was not a requirement for their presence (Dice 1930; Grinnell 1933; Dalquest 1953; Hoffmeister 1956; Baker and Greer 1962; Drabek 1967). In some of these cases (Dice 1930? Reynolds and Haskell 19^9; Hoffmeister 1956) scattered trees or shrubs were present. In this study the con sistent segregation of P. penicillatus habitat on the basis of shrub ^ density was based, to a large extent, on P. amplus and D. merriami avoiding high densities of these shrubs. P. penicillatus average habi tat (Tables 2-10) was somewhat above (grid and miniquadrats) or just above (IBP lines) the overall site averages (see Table 18, p. 89) for these densities. In the ecotone area, P. penicillatus was not re stricted to dense growth of the shrubs, but was more frequently taken in areas containing at least scattered shrubs of category 4 or trees. Rosenzweig (1973) has demonstrated that P. penicillatus will cross areas cleared of vegetation to get to a trap. As the cleared area was increased in size, however, the captures of P. penicillatus decreased. Perhaps this was similar to the natural situations in which they were taken in sparse vegetation; open areas were occupied as long as there 67 were shrubs close enough to afford protection when needed. As dis cussed in the section on P. baileyi, the shrub cover waB probably im portant to P. penicillatus as protection from predators. Literature reports have consistently associated P. penicillatus with fine, silty, sandy, friable or mellow soils (Grinnell 191^; Bailey 1931; Grinnell 1933; Borell and Bryant 19^2; Hall 19^6; Dalauest 1953; Baker 1956; Denyes 1956; Baker and Greer 1962; Darden 1965; Bateman 1967; Drabek 1967; Findley et al. 1975)- Occasionally, in some of these studies, and quite regularly in other studies, they have been found on coarser soils. Burt (1938) found them in gravelly as well as sandy soils. Massion (1971) took 21# of the P. penicillatus of his study on the coarse coils of the site. Rosenzweig and Winakur (1969) took them on firm, but not on the hardest soils. In the present study, P. penicillatus was taken in coarse soils, but not in the bedrock covered areas. The avoidance of stone and cobble covered areas was not complete. While the species may pre fer the most workable soils, it was only restricted from those with a high percentage of exposed bedrock. This restriction may have been based on the difficulty of burrow construction as discussed in the case of D. merriami. Denyes (195^) has shown that hard soils should not restrict them because they chew through these soils. However, bedrock would be more difficult to burrow through in this manner and surface and shallow bedrock could thus exclude a deep burrower. The sandy soils may have been occupied in preference to the hard, coarse ones without bedrock simply because burrowing is easier. Adaptation to these soils may be a direct result of past competition with other 68 pocket mice as discussed in the section on P. baileyi. In fact, co existence with P. baileyi may depend on the soil-surface complemen tarity in light of the similarity of the two in their reaction to shrubs. Perognathus intertnedius The only place resident populations of this mouse were taken was on and adjacent to the rocky hills. The habitat segregation was clear, no other heteromyid was taken regularly in rocky hill situations. The factors important in the habitat were not so clear, because a group of factors (high bedrock, stone and cobble surface cover, high shrub 2 density) occurred together in these situations. Soil-surface factors appeared to be the principle discriminators, bedrock on the miniquad- rats and stones and cobbles on the grid. This association of P. intertnedius with rocky sites is consistently reported in the literature (Grinnell 191^; Dice 1930; Bailey 1931; Hoffmeister and Goodpaster 195**; Ivey 1957; Drabek 19&7; Findley et al. 1975)- The species common name (rock pocket mouse) reflects how regularly it has been found associated with rocky and stony sites. E" iPtsrmedius must be adapted to burrowing in the soil pockets and using the rock crevices as home sites. As was suggested in the case of P. baileyi, the ultimate cause of this adaptation may have been competition with other mice. Shrub densities are generally high in the rocky hill areas (Table 1 and P. intermedius averages in other trapping systems), so the suggestion that pocket mice generally need cover in their habitat is supported. In addition to the shrubs, the rock 69 crevices and boulder piles must provide protection from predators, further supporting the suggestion. Perognathus amplus P. amplus habitat was very similar to D. merriami habitat. Open areas away from bedrock were the characteristic sites inhabited. However, tolerance for coarse soil and surface factors seemed to be greater than it was in D. merriami. Other studies have noted the association of P. amplus with sparse vegetation (Bateman 1967; Drabek 1967; Massion 1971)- P> amplus is a small (13 g), yellow mouse, and this combination of size and cryptic coloration may be responsible for the mouse's ability to exist in the sparsely covered habitats. Massion (1971) found them predominantly on the coarse soils of his sites, and Rosenzweig and Y/inakur (1969) found a correlation be tween P. amplus and intermediate soil strength. These two observations are in general agreement with the tendency found in this study toward coarse soil-surface factors. In this study, however, the tendency was only relative to that of D. merriami, and the overall reaction was to avoid areas with a lot of exposed bedrock and surfaces heavily covered with stones and cobbles. The bedrock covered areas may exclude P. amplus because burrowing is restricted, as suggested for P. penicillatus and D. merriami. Association Between D. merriami and P. amplus The differences in soil-surface utilization between D. merriami and P. amplus discussed in the last section did not appear to be 70 sufficient to account for their coexistence. In fact, the similar habitat associations of the two suggested that they might subdivide resources in some way other than habitat segregation. Possible alter natives are considered in appropriate places in the remainder of the paper. In this section, the possibility of association (positive or negative) between them is explored. Cole's (19^9) measure of interspecific association was run on all trapping sessions of the grid and IBP lines. First, all trap sites were analyzed, then sites without shrubs of category 4 were analyzed. These shrubs were consistently avoided by both species. If an associ ation was shown in the analysis of all sites and it was due to a common reaction to shrubs of category 4, the coefficient should have decreased in the analysis of sites without such shrubs. If no association was shown in the analysis of all sites, perhaps a negative association would appear in analysis of sites lacking shrubs of category 4. This result could be interpreted as an indication that the species avoided one another within their similar habitat. Table 12 shows the coefficients of interspecific association for all trapping sessions of the Red Hill grid and IBP lines. The only statistically significant associations were in the October 197^ grid trapping session. This was the reinvasion trapping, and the difference may be due to the resultant instability. However, only 12 P. amplus were caught in that trapping period so it was also probably the least reliable. Three other grid runs and six IBP line runs showed no association between the two. The same results were obtained in the absence of category k shrubs, perhaps indicating a different mechanism 71 Table 12. Coefficients of interspecific association between 1?. amplus and D. merriami. -- An asterisk indicates significant association at the 0.05 level. N is the number of sites in the analysis. Cole's (19^9) Coefficient of Association Trapping System Sites Without and Date All Sites Shrub 4 Red Hill Grid Sept. 1972 +.07 +.15 Sept. 1973 +.05 +.03 Sept. 1974 +.11 +.03 Oct. 197*+ +.42* +.73* N 256 117 IBP Lines June 1972 +.00 -.03 Oct. 1972 -.11 -.01 May 1973 +.10 +.07 Sept. 1973 -.04 -.01 May 1974 +.13 + .08 Sept. 1974 + .16 -.02 N 128 75 72 of reaction to these shrubs. One simple possibility is that each species reacted to different levels of shrub density; one was found more in complete absence of the shrubs and the other in low densities* A contingency Chi square test was run on all the grid trapping sessions combined and on the combined IBP lines trapping sessions to test this hypothesis. Tables 13 and l*f show the results for the grid and lines respectively. P. amplus was taken more frequently at low 2 densities (one or two shrubs per 130 m on the grid and one shrub per 2 130 m on the lines') than D. merr iami, which was taken more often in the absence of the shrubs. This difference in reaction was significant for grid data, but not for the IBP lines. Perhaps other shrubs were involved in distinguishing the two species' habitats in this way. An observation consistent with this suggestion was that average shrub densities were generally slightly higher for P. amplus than for D. merriami in all shrub categories except Larrea and Franseria. These observations suggest that P. amplus and D. merriami reacted somewhat differently to shrub cover. It is also interesting that on the IBP lines, where segregation based on shrub 4 density appeared to be least, segregation based on stone and cobble surface cover was most strongly suggested in the dis criminant analysis. Perhaps these two habitat dimensions, which separated the other species' habitats from one another, were also effective in habitat segregation between this pair. However, it does not seem to be at a level of habitat segregation sufficient to explain coexistence between the two. 73 Table 13. Contingency Chi square to test for difference in reaction of P. amplus (Pa) and D. merriami (Dm) to low densities of shrubs of category 7 on the grid. — Each entry in the table consists of the number of mice captured in each density category of shrub 4, and the Chi square for that entry (in parentheses). The sign indicates direction of the observed deviation o Number of individuals of shrub 4 in 130 m plant sample area Mice 0 1 2 >2 Total Dm 98 (+3.4) 17 (-2.2) 1 (-3.6) 7 (-2.1) 123 Pa 113 (-2.1) 46 (+1.4) 13 (+2.3) 24 (+1.3) 196 Total 211 63 14 31 319 3 degrees of freedom, Chi square = 18.4, p< 0.001 Table 14. Contingency Chi square to test for difference in reaction of P. amplus (Pa) and D. merriami (Dm) to low densities of shrubs of category on the IBP lines. — Each entry in the table consists of the number of mice captured in each den sity category of shrub 4, and the Chi square for that entry (in parentheses). The sign indicates direction of the observed deviation. 2 Number of individuals of shrub 4 in 130 m plant sample area Mice 0 1 2 >2 Total Dm 312 (+0.4) 49 (-2.0) 33 (-0.1) 22 (+0.1) 416 Pa 249 (-0.5) 63 (+2.3) 32 (+0.1) 16 (-0.1) 360 Total 561 112 65 38 776 3 degrees of freedom, Chi square = 5*6, 0.2 >p >0.1 ?k Nonhabitat Coexistence Mechanisms The results discussed so far suggest that complementary habitat associations may be important to the coexistence of the heteromyid rodents of the Silver Bell area. The emphasis of this study was on these habitat associations. However, some results obtained suggested that nonhabitat mechanisms may also contribute to the coexistence of the five heteromyid species in this area. These results are discussed below. Diel Activity Differences Table 15 gives the results of Friedman and Wilcoxon tests on the differences between evening and morning captures of the four species taken regularly on the IBP lines. The Friedman test indicates a sta tistically significant difference among the species' activity patterns and the Wilcoxon tests of signed rank (run on each species separately) show that D. merriami and P. penicillatus were responsible for the difference. The former was captured more often on the morning check than the evening and the latter reversed this pattern. Brown and Lieberman (1973) discussed the difficulty of inter preting the advantages of forage time differences among seed eating rodents, A resource removed by one species is unavailable to the other, regardless of the time of removal. In fact, it would seem that the later forager would be at a disadvantage since the removal of re sources by the early forager decreases availability for the later one. However, if the species interfere with each other while foraging it is possible that a separation of foraging times would benefit both. 75 Table 15. Friedman and Wilcoxon tests for differences between evening and morning captures of the mice on the IBP lines. — The first number of each entry is evening captures minus morning captures. The next (in parentheses) is the rank of the entry in its row. It was used in the Friedman test for matched groups. Ties were broken conservatively. The third number is the rank of the absolute value of the number in its column. It was used, in conjunction with the sign of the first number, in the Wilcoxon test for matched pairs. Both rankings are from low to high values. Abbreviations are as in Table 1. Date Dm Pb Pp Pa 6/ 5/72 - 2 (1) 4.5 - 1 (3) 5-5 + 1 (4) 2.5 - 1 (2) 4.5 6/ 6/72 0 (1) - + 1 (2) 5-5 + 5 (4) 9 + 3 (3) 9 6/ 7/72 + 2 (2) 4.5 + 1 (1) 5-5 + 4 (4) 7-5 + 3 (3) 9 10/ 6/72 - 9 (1) 14.5 0 (4) 1-5 - 3 (3) 6 - 5 (2) 13.5 10/ 7/72 - it (1) 10 + 2 (3) 10 + 1 (2) 2.5 + 5 (4) 13.5 10/ 8/72 + 2 (2) 4.5 + 1 (1) 5-5 + 8 (4) 10 + 3 (3) 9 5/23/73 - 5 (1) 11.5 0 (4) 1-5 - 1 (2) 2.5 0 (3) 1.5 5/24/73 - 1 (3) 1-5 - 3 (1) 12.5 - 2 (2) 5 0 (4) 1.5 5/25/73 - 2 (2) 4.5 - 3 (1) 12.5 - 1 (3) 2.5 + 1 (4) 4.5 8/30/73 - 8 (2) 13 - 2 (4) 10 - 4 (3) 7.5 -12 (1) 18 8/31/73 - 1 (2) 1.5 + 2 (3) 10 +19 (4) 14 - 8 (1) 16 9/ 1/73 - 9 (3) 14.5 -10 (1) 16.5 0 (4) •• - 9 (2) 17 5/20/74 + 3 (1) 8 +11 (3) 18 +33 (4) 17 + 6 (2) 15 5/21/74 - 5 (1) 11-5 + 1 (3) 5.5 +18 (4) 13 - 4 (2) 11.5 5/22/74 —10 (1) 16 + 5 (3) 14 +13 (4) 11.5 + 1 (2) 4.5 9/13/74 + 3 (3) 8 -10 (1) 16.5 +13 (4) 11.5 + 1 (2) 4.5 9/14/74 -11 (1) 17 + 8 (3) 15 +22 (4) 15 - 2 (2) 7 9/15/74 - 3 (1) 8 + 1 (2) 5.5 +25 (4) 16 + 4_ (3) 11.5 (29) (43) (63) (45) Friedman: degrees of freedom = 3» Chi square = 19.5* p < 0.001 Wilcoxon: z for Dm 2.44, p<0.01 z for Fb 0.46, p= 0.30 z for Pp 2.51, p<0.01 z for Pa 0.15, p = o.44 76 Wondolleck (1975) has observed D. merriami chasing P. penicillatus from baited seed piles* Kenagy (1973) reported a diel separation of activity between heteromyids. More D. merriami were captured' in the morning trap ses sions and more Dipodomys microps and Perognathus longimembris were cap tured in the evening. He interpreted this as D. merriami avoiding the dominant D. microps, and he observed I), microps chasing D. merriami. Because D. merriami is larger than P. penicillatus, and in light of V/ondolleck's observation, one would probably interpret the results obtained in this study as P. penicillatus avoiding the major activity period of D. merriami. If this interaction hypothesis is the real reason for the separation of activity times, there should be less difference between the two at low populations and in areas where they are not so intimately associated. The first three IBP trapping ses sions were relatively low population periods« and D. merriami and P« penicillatus activity was more similar than in the last three sessions (Table 15)» as the above hypothesis predicted. The within-habitat lines (101-10*0 can be used as an area of less intimate association to make the second comparison. All D. merriami taken on the within-habitat lines (only 9 captures were made) were caught on the morning checks. P. penicillatus captures were as follows: 30 on evening checks and 66 on morning checks. There were more captures on the morning than evening checks all three days. Both these observations are consistent with the interpretation that inter action is responsible for the differences in time of activity on the IBP lines. 77 Other Temporal Differences Annual activity patterns differ among the rodents (Reichman and van de Graaf 1973)• P> amplus and P. penicillatus were inactive above ground in the winter months. They were not taken from November 1971 to February 1972 in the preliminary trapping. A total of 650 trap nights were run in appropriate habitats in the period. I). merriami was trapped regularly all winter. Brown and Lieberman (1973) argued that it is difficult to imagine the contribution this kind of behavioral difference could make to coexistence. In conjunction with the follow ing observation of another difference in temporal pattern, however, it may have significance. Both the Red Hill grid and the IBP lines followed population changes over the three year period. The patterns of these changes are shown in Figures 11 and 12 respectively. The fall, winter and spring rains of 1972-1973 were especially heavy, and winter and spring annual plant production was much higher than any of the other years of the study. Presumably as a result of this, the rodent populations in creased greatly. The pattern of this increase is of possible interest in terms of coexistence. P. amplus populations increased more quickly than those of I), merriami. In light of the similar habitats of the two, the quick response of P. amplus may have been important to the species. It may have made large amounts of seed storage possible under conditions of maximal availability and minimal interspecific competi tion. These stores may have been used as a buffer against times when the foraging was not so good because seeds were less abundant (winter rains were light the next year), and because the competing mouse 78 400 <0 c o •H CO 10 0) w 300 to c •H Pt a c . — .-Dm •H T3 Pb OJ 200 h 4J Pp o Pi (Q Pa H (C E •H B < 100 - (H O 0) JS 72 73 74 Time (trapping sessions) Figure 11. Rodent catch on the grid by year and species. ~ Trapping sessions were in September of the indicated year (72 indicates 1972, etc.). The heavy rains discussed in the text came in the winter and spring of 1972-1973* Abbreviations of the names of the mice are as in Table 1. 79 . • — .Dm to Pb Ei 0 •H 200 Pp (0to 01 Pi w Pa to G •H Pi £ £ •rl 13 V +>& o 100 to '« B •rtC < O / S3 / >A •§ a Time (trapping sessions) Figure 12. Rodent catch on the IBP lines by year and species. — Trapping sessions were in the spring (S) or autumn (A) of the indicated year. Other symbolism is as in Figure 11. 80 populations were higher (Figs. 11 and 12). Seasonal inactivity would make this store last much longer (Chew, Lindberg and Hayden 1965)- However, the 1972-1973 type of rains and seed crop are rare in the Sonoran Desert. Time between such rains considerably exceeds the life span of the proposed benefactors. Such years cannot be a factor of co existence, unless other, less favorable but more frequent years result in the same sort of response on a smaller scale. Perhaps quick repro ductive response to favorable conditions, storage of seeds in the burrow and considerable inactivity during adverse conditions are adap tations of P_. ampluE that help allow for existence in the same habitats as D. merriami. Food and Coexistence Food resource allocation was not a part of this research. How ever, Reichman (1975) has studied the diets of the heteromyids in the Silver Bell area and concluded that they were broadly similar. He did suggest that D. merriami and P. amplus may partition resources to some extent on the basis of dietary differences. The Enclosure Figures 13 and Ik are graphs of foraging and tracking data from the enclosure. The data are extremely variable, and the patterns are similar for the four species of Perognathus. A three way analysis of variance for the Perognathus tracking data is summarized in Table 16. Host of the variance is due to differences between species and cover types. P_. baileyi tracked more than any other species and the two cover types that afforded relatively heavy cover (stones and brush) Figure 13# Foraging in the enclosure. Graphs show means and standard deviations of the mass of seeds (in grams) removed from each cover type by each species. Cover abbreviations are as follows: Br = brush cover St = stone cover Op = open (quarter with no cover) Sc = scattered cobble cover Abbreviations of mouse names are as in Table 1. N = 8 for the pocket mice (perognathus) and 6 for the kangaroo rat (Dipodomys). Dm Pb Pp Pi Pa \ \ /\ i\ \ / r Br St Op Sc Br St Op Sc Br St Op 5c Br St Op Sc Br St Op Sc Cover Type Figure 13. Foraging in the enclosure. CO 900 Dm Pb Pp Pi Pa !\ / Jj 600 u rt n Q) / -p3 o I \ K /f ^ 300 / U I \ 1/ Br St Op Sc Br St Op Sc Br St Op Sc Br Si Op Sc Br St Op Sc Cover^ Type Figure l*f. Tracking in the enclosure. — Graphs show the mean and standard deviation of the number of rectangles tracked in each cover type by each species. Abbreviations and sample sizes are as in Figure 13« 83 Table 16. Sources of variation in the Analysis of Variance of the Perognathus tracking data in the enclosure. — An asterisk indicates significance at the 0.05 level. Degrees Source of of _ . Variation Freedom SS x 10 MS x 10" F Species (S) 3 22.8 76.0 50.7! Cover type (C) 3 15.6 52.0 34.7' Quarter (Q) 3 0.9 3.0 2.0 S x C 9 1.5 1.7 1.1 S x Q 9 1.8 2.0 1.3 Q x C 9 1.0 1.1 0.7 S x Q x C 27 11.5 4.3 2.9' Error 64 9.3 1.5 Total 127 64.4 were tracked much more than the two which were poorly covered (open and scattered cobbles). The lack of significance in the first level inter action terms indicates that the species were reacting similarly to the cover types. All tracked more in the well covered quarters. The second level interaction term is significant, indicating some species- cover-quarter interaction. The importance of the interaction would be difficult to interpret. No analysis of variance was run on the forag ing data because the statistical model fit the data so poorly. However, the pattern is very similar to that of the tracking data. Despite the insignificance of the species-cover type interac tion terms, three of the Perognathus species do seem to have reacted in a manner relatively consistent with their habitats in nature. P. intermedius distinguished between brush and stone cover more distinctly than did the others. In fact, each of the eight P. intermedius tracked more in the stone-covered quarter than in any other. Only two of the eight foraged in the brush at all, and both of these foraged much more in the stone-covered quarter. Both P. baileyi and p. amplus favored the stone cover over the brush cover, but neither approached the consistency of P. intermedius. These observations are consistent with the field data for both P. intermedius and P. baileyi. P. intermedius habitat is the most rocky-stony-cobbly habitat available. Brush and rock, stone and cobble contributed to the average field habitat of P. baileyi, and both were used by the species in the en closure. P. penicillatus, on the other hand, tracked and foraged more in the brush cover than in the stone cover, in keeping with the shrub association found for this species in the field. Although all of these 85 observations are consistent with field data in a relative way, none (except the P. intermedius-stones relationship) is a very convincing demonstration of a laboratory selection of field associated cover type. The association of all three with some form of cover rather than no cover is convincing, and consistent with trapping results. According to field data, the fourth pocket mouse, P. amplus, should have tracked and foraged more in the open areas than any of the other mice. In the enclosure, however, the members of this species were as cover oriented as any of the pocket mice. This inconsistency between field and enclosure will be discussed later, after D. merriami enclosure results have been considered. With the similar reactions of the pocket mice in mind, the six I), merriami were run, and they reacted differently (Figs. 13 and 14). Brush-covered and open quarters were used about equally, and the stone and scattered cobble quarter were used less. This pattern is con sistent in terms of the avoidance of rock-stone-cobble areas in the field. It is also consistent, relative to the pocket mice, in terms of their reaction to shrubs in the field because merriami did much more foraging and tracking in the open quarter than did any of the pocket mice. The difference between P. amplus and D. merriami reactions in the enclosure suggests that the two may subdivide their field habitat at a level below that measured in this study. Perhaps Dipodomys is more active and forages more in the open parts of the habitat while P. amplus uses the cover that is available more intensely in its foraging and other activities. This hypothesis is consistent with 86 the slightly higher number of P. amplus associated with low densities of category k shrubs (Tables 13 and 14). It is also consistent with the apparent predator-avoidance specializations of the two species: Dipodomys is specialized for detection of and escape from predators in open situations, and P. amplus is specialized to avoid being detected. Wondolleck (1975) found that P. amplus foraged more under bushes and I), merriami more away from such cover. Brown and Lieberman (1973) captured P. longimembris (a small, silky pocket mouse similar to P. amplus in size and activity) more often in the middle of bushes and D. merriami more often at the edges of and away from bushes. Per haps in this area as well, the large (kl g) kangaroo rat and small (13 g) pocket mouse coexist in the same habitat by foraging in dif ferent microhabitats. Cricetid Species Habitat Affiliation Neotoma albigula and Peromyscus eremicus were the only crice- tids taken often enough to give information on their habitat relation ships. Even they were not taken often enough to analyze for important habitat factors in the way the heteromyid species were handled. Their specific association with habitat types also made it difficult to single out habitat variables that were most important to them because several variables were associated with one another and with the mice in the habitats in which they lived. Nevertheless, their habitat associations are of interest in the context of community organization. Table 2 shows the populations of the two species in the within-habitat lines, and Table 17 gives means of habitat variables for all captures 87 Table 17. Means and standard deviations of the habitat variables for Neotoma albigula and Peromyscus eremicus. — Means are given for trapping systems in which some captures were made# Standard deviations are not given for small sample sizes. Abbreviations are as in Table 1. Trapping System Habitat Miniquadrats Red Hill Grid IBP Lines Variables Na Prae Na Pme Na S-l 5.3; 7.5 12 3.5 J 5.3 6.1| 5.9 4.2 S-2 23.5 7 29.8 19 9-1| 9.9 14.5 i 11.7 0.8 Fd 48.5 l 56.6 64 72.8 J 25.4 81.9 J 26.8 64.0 Ld 14.4 2 11.2 8 7.1 2 5.0 4.8 - 4.7 18.5 S-3 6.1| 6.2 4 3.3 I 3.1 2.8 ± 2.6 0.0 S-4 8.7 i 13.1 6 2.9 J 2.1 1.6 2 1.5 1.9 Tr 3-2 i 2.0 2 2.4 t 1.8 2.6 t 1.6 3.2 C-l 4.2 t 5.4 14 3.7 J 3.0 3.1 J 2.1 5.4 4.4 i C-2 7.4 i 9.6 8 2-^ r 2.4 6.6 2.6 %S 64 ± 9 60 65 J 6 66 i 6 70 %G 23 35 38 |6 2+1 J 6 39 %SC 31 15 8 10 27 I 9 T 12 J 7 27 I 33 70 19 J 27 44 t 29 Dcm 44 i 23 13 36 t 17 25 - 14 45 N 17 2 30 40 1 88 in each of the other trapping systems. Table 18 gives the means and standard deviations of habitat variables for the trapping systems them selves for comparison with the cricetid averages. Neotoma albigula As shown on Table 2, woodrat densities were approximately equal in the wash and rocky hill habitats. With one exception, every Neotoma was taken in or near such habitats. Even in the ecotone, they were taken only along vegetated washes. The restriction of the woodrat to either rocky or heavily covered habitats has been commonly reported (Grinnell 191^; Dice 1930; Baker 1956; Ivey 1957; Bateman 1967; Massion 1971). Other investigators have found them in cactus-covered areas as well (Bailey 1931; Monson and Kessler 19^0; Vorhies and Taylor 19**0; Hooper 19^1; Spencer and Spencer 19^1; Brabek 1967). No association with cactus was found in this study* Olsen (1973) argued that a primary requirement in N. albigula habitat was low, dense cover for house or den sites. In this study the one individual taken some distance from rocky or heavily covered habi tat was taken on a miniquadrat on which a paloverde tree had fallen. A woodrat house was under the branches of the fallen tree. The results of this study were consistent with Olsen1s suggestion. Cover was re quired for the presence of N. albigula at a site, and the single cap ture of a woodrat away from such sites suggested that one of the limiting factors in open areas was the lack of low cover for home sites. N. albigula was taken in the same habitat types as P. intermedius and P. penicillatus. Neotoma is ecologically very 89 Table 18. Means and standard deviations of the habitat variables for the trapping systems. — N is the number of sites in each system. Area is the sample area in which plants were counted at each site. Abbreviations are as in Table 1. Habitat Trapping Systems Variables Kiniquadrats Red Hill Grid IBP Lines S-l ^.5: 10.0 3.0 ^ 5.5 1.2 x 2.4 S-2 6.6 t 15.5 4.2 j 7.6 0.5 x 1-7 Fd 47.5 J 47.1 79.9 1 25.5 53.2 I 26.1 Ld 26.5 J 27.0 6.4 ± 5.0 13.5 x 8«5 3.9 1.6 ± 3.3 0.2 J 0.4 s-3 I'6 S-4 8.2 iI 12.7 1.5 z 2.2 1.1 r 2.2 Tr 3.7 I 3.1 1.4 i 1.4 1.5 x 1-7 C-l 3.9 J 4.9 3.3 j 2.5 2.2 J 2.7 C-2 1.8 i 3.8 2.2 - 2.4 0-9 x ^ %S 61 t 12 65 6 63 x 10 <#& 33 J 16 38 7 38 i 10 6 6 10 18 t 8^7 16 t 2k #Br I 19 _±_ 7 J Dcm 55 i 14 37 - 15 55-9 N 66 n 256 - 128 Area 230 m 130 m 130 m 90 different from the pocket mice. Differences in diet alone are probably sufficient to explain coexistence. Heteromyids are primarily seed eaters (Eisenberg 1963)i while N. albigula eats more vegetative plant parts (Spencer and Spencer 19^1; Vorhies and Taylor 19^0). Peromyscus eremicus The cactus mouse habitat was very similar to that of Perognathus intermedius (Table 2, also compare Pm. eremicus averages on Table 17 with P. intermedius averages on Tables 2-6). Except for a few which strayed to adjacent habitats on the grid, all were taken from rocky hills. Several authors have reported that most Pm. eremicuB were taken from rocky situations (Dice 1930; Bailey 1931; Commissaris I960; Bateman 196?). Others have reported just the opposite: that Pm. eremicus inhabited riparian situations and sandy, deep soil, and were rare in rocky areas (Grinnell 191^» Long 19^0; Darden 1965; Bradley and Mauer 1973)* In other reports, they were found in both habitat types and in others as well (Hardy 19^5; Baker 1956; Denyes 1956; Drabek 1967). The distinct contrast between these reported habitats is greater than for any other rodent in this study. Perhaps some explana tion is apparent in the reports themselves. In the studies mentioned in which Pm. eremicus was absent or rare at rocky sites, Peromyscus crinitus was present at these sites. Hardy (19^5) found Pm. eremicus in rocks with Pm. crinitus, but he also took Pm. eremicus on sandy soil and felt that they favored the latter. Studies that reported Pm. eremicus to be most common in the rocky sites did not report taking 91 Pnu crinitus. Pm. eremicus appears to undergo a habitat shift in the presence of Pm. crinitus. Of the references which reported a rock preference for Pra. eremicus, only Commissaris (19^0) reported a restriction to rocky hills similar to that found in this study. Commissaris's rock hill site was adjacent to a mesquite forest in which Peromyscus merriami waB taken. He suggested that distinct habitat requirements separated the two. In this study Pm. merriami was taken from Los Robles V/ash and occasionally from major washes in the study area. There was no indication that direct interaction maintained the habitat segregation as it might have in Commissaris1s situation. As he suggested, the two mice seem to have complementary habitats in desert areas where their ranges overlap. In this case the habitat shift of Pm. eremicus was in an exactly opposite direction from its shift in the presence of Pm. crinitus. In areas without Pm. crinitus or Pm. merriami, Pm. eremicus had a broader habi tat association, often including both of their habitats. In terms of the rodent community under consideration here, Pm. eremicus habitat was clearly distinct from that of all other rodents except N. albigula and P. intermedius. There is probably greater over lap in diet between Neotoma and Peromyscus than between Perognathus and Neotoma, but little problem regarding coexistence of the two is likely. Neotoma had a broader habitat association in the area, including the heavily covered washes as well as the rocky hills. The size difference between the two (Neotoma weighs about 160 g and Pm. eremicus about 18 g) suggests that they are capable of different use of their shared habi tat. Herritt (197*0 suggested that Neotoma fuscipes was necessary for 92 Peromyscus californicus to inhabit an area, further suggesting that Neotoma and Peromyscus are generally compatible members of a community. As mentioned before, Pm. eremicus and P. intermedius habitats were extremely similar in this study. Perhaps the two subdivided the rocky hill habitat by a microhabitat segregation not apparent in a comparison of averages. Or perhaps they interacted in some way that minimized contact and competition. Either of these mechanisms could result in a negative association between the two. Cole's (19^9) co efficient of interspecific association was run on the within-habitat line captures of the two species. The association coefficient was +0.^+1, indicating that they do use the same areas within the habitat that they share. Certainly there is no indication that they make use of complementary microhabitats. Apparently, habitat segregation is not an important aspect of these species' coexistence. Other dimen sions must contribute to their coexistence. Pm. eremicus in California used far more vegetative plant mat ter and animal matter and fewer seeds than pocket mice in the same area (Meserve 1976). This is a generally accepted difference between members of the two genera. Working in the Silver Bell area Reichman C1975) found that over 82$ of the diet of P. intermedius was seed, and Reichman and van de Graaf (1973) commented that a "major portion" of Pm. eremicus diet was insects. Pm. eremicus is known to estivate in the heat of the summer (MacMillen 1965) and £. intermedius is inactive above ground in the coldest months (Reichman and van de Graaf 1973)* Several authors have reported or suggested this type of difference as the basis of a 93 temporal coexistence mechanism between heteromyids and cricetids (Rosenzweig and Winakur 1969; Kritzman 197^» O'Farrell 1975). Data from the preliminary trapping indicated that Pm. eremicus was inactive in the hot part of 1972. Only six were taken in more than 500 trap nights in rocky hill habitat from June to August that year. In addi tion, only two were taken on the miniquadrats which were trapped from June to August 1973• Twelve of those miniquadrats were in rocky hill habitat. However, the data reported by Reichman and van de Graaf (1973) gave no indication of the estivation of Pm. eremicus in this area in 1971» This demonstrates the need for long term studies of these conununities to seek expleinations for such apparent differences in activity from year to year. Evidence for winter inactivity of P. intermedius during this study was given by preliminary live trapping results in which P. intermedius was not trapped between December 23, 1971 and March 2, 1972. Four different one night sessions and 320 trap-nights were in volved in the period with no P. intermedius captures. In contrast, four were taken in 6b trap-nights on December 23rd and three in 6^ trap-nights on March 2nd. At least two were taken in each comparable session before (3 sessions) and after sessions) the period of no captures. In the absence of habitat differences between these species, it appears that dietary and temporal dimensions may be important to their coexistence. Community Organization Habitat, food and temporal dimensions all appeared to be im portant to the coexistence of species in this study. In this section, 3k these dimensions are discussed in the context of the whole community. The different habitats of the area house different species. A possible role for these habitats in community stability is explored. Habitat Dimensions and Complementarity of Dimensions Both soil-surface factors and vegetative factors made important contributions to habitat segregation of species and presumably to co existence. The soil-surface factors of greatest importance were the percentage of the surface covered by bedrock and stones and cobbles. The density of a single shrub group (primarily composed of the acacias) was the single most important vegetative factor. These two dimensions were complementary (in the sense of Schoener 197*0» in that P. penicillatus, D. merriami and P. amplus reacted similarly in terms of the soil-surface dimension and differently to the shrub dimension. On the other hand, P. baileyi and penicillatus were more similar in the shrub density of their habitats and dissimilar in the soil-surface aspects. Other Dimensions and Interaction of Differences on Several Dimensions D. merriami and P. amplus were very similar in terms of the habitat variables measured in this study. P. amplus tended to be associated more with shrubs and stony-cobbly soils, but only slightly. Enclosure results suggested microhabitat segregation of forage areas between the two. The grid and IBP line trapping results suggested that a combination of different temporal reactions might result in reduced competition. Reichman (1975) suggested that some dietary differences may exist between the two in this area. None of these differences appeared to be sufficient to allow coexistence of the two, but perhaps the total of these small differences on various dimensions does produce a sufficient difference in their resource use. Another possible ex ample of this interaction of dimensions is the apparent complementarity in forage times between P. penicillatus and D. merriami on the IBP lines. The two were apparently well separated on the basis of habitat variables. However, this habitat segregation was least distinct on the IBP lines, and perhaps separation on a second dimension was re quired there. The habitat separation of P. baileyi from D. merriami and P. amplus may be a third example. P. baileyi reacted differently than the other two on both habitat dimensions. In all of these cases different reactions on two or more dimensions may have acted to in crease the difference in resource use between species. Dimensionality of Coexistence The results of this study indicate that several dimensions were important in the subdivision of the resources available in the area. Complementary responses to habitat variables may account for a great deal of resource subdivision. As in most desert rodent communities studied, habitat dimensions were almost certainly the most important dimension type. Here, soil-surface and vegetative dimensions seemed to be roughly equal in importance. Despite their importance, they were not found to be sufficient to explain the coexistence of all the rodents in the community. No habitat complementarity was found for one species pair (Pm. eremicus and P. intermedins). Another appeared 96 to require more than the small habitat differences found between them in order to avoid competition (D. merriaai and P. amplus). In both cases differences in temporal and dietary dimensions appeared to be important to coexistence. In this community of nocturnal rodents, at least four dimensions appeared to be important in subdividing resources. The two habitat dimensions were most important, but temporal and dietary dimensions (at least one of each) were also necessary to sepa rate resource use patterns of some species from each other. Habitats and Community Organization Hawbecker (1951) found that three kangaroo rats each occupied a certain habitat in which other rodents seldom occurred. In other habitats, more heterogeneous than these three, the kangaroo rats were taken together and with other rodents. The pattern he reported is similar to that found in this study. Each distinct habitat was popu lated by two or three species rather than one, but the habitats were nearly mutually exclusive with regard to the rodent species taken (Table 2). In the more heterogeneous areas (ecotone), all the species mixed to some extent. Discriminant analysis showed that, though they mixed, each retained the habitat affinities of the distinct habitat in which it occurred. These affinities appeared to be the bases for coexistence in the area* Within the distinct habitats, coexistence was between the most diverse organisms in the assemblage: N. albigula and P. penicillatus in the overgrown washes; Neotoma, Pm. eremicus and P. intermedius on the rocky hills; D. merriami and P. amplUB on the creosotebush plains. 97 It is generally accepted that closely related species are more likely to be competitors than are more distant relatives (Hardin I960). In the distinct habitats of this study, no major resident species were members of the same genus. Size difference may also enhance coexistence. Brown (1973) found that rodent species pairs with weight ratios less than 1.5 seldom occurred on the same sand dune. In the present study the smallest weight ratio of a pair sharing a distinct habitat violates this gener alization (Pm. eremicus and P. intermedius, 1.3)? but the two are mem bers of different families. The smallest ratio between two members of the same family which also occurred in the same distinct habitat was 3.2 (D. merriami and P. anrolus)• Table 19 shows the weight ratios of resident species on the grid and IBP lines. The ratios are generally smaller than those of the distinct habitats, especially between related species. These observations emphasize the differences between the species which coexisted in these homogeneous areas. The differences should minimize competition between these species. It is possible that these distinct habitats are very important to the maintenance of the various species in the area and hence to the stability of the community. There may be times when the species dis appear except in the specific habitat to which they are best adapted and in which competition is minimal. These habitats would then save them from extinction in the entire area and serve as a source of ani mals for recolonization of other habitats at more favorable times. Compare the IBP line populations of P_. penicillatus and P. baileyi (Fig. 12), and the grid population of P. penicillatus (Fig. 11) before 98 Table 19• Weight ratios between coexisting species on the Red Hill grid and IBP lines. — Abbreviations are as in Table 1. Average Ratios on Ratios on Species Weight the Grid the IBP lines Dm 4l g Dro/Pb = 1,4 Dm/Pb = 1.4 Pb 29 g Pb/Pp = 1.5 Pb/Pp = 1.5 Pp 19 g Pp/Pme=: 1.1 Prae 18 g Pme/Pi= 1.3 Pp/fc>a = 1.5 Pi 14 g Pi/Pa = 1.1 Pa 13 g 99 and after the rains and spring annual plant production of 1972-1973 in light of this hypothesis. Only a few individuals of each species were taken before relatively favorable conditions developed (the presumed food increase due to the rain), but each species became a major com ponent of the catch after the change. Temporal Species Diversity Patterns. A change in community organization that might be expected under the above hypothesis was an increase in species diversity on the ecotone sites with the advent of favorable conditions. Evenness, number of species or both might in crease as the various populations expand from their distinct habitats into the different parts of the ecotone. Whitford (1976) found that rainfall and productivity were positively correlated with species di versity through time at a New Mexico site. This is the sort of result one should expect under the above hypothesis. The diversity data were available from the present study, so diversities were calculated to see if they were consistent with the hypothesis. Productivity data were not collected in this study, but the heavy rains and sensational spring annual display of 1973 have already been discussed. Rodent populations increased after this period (Figs. 11 and 12), presumably in response to an increased food supply. Assuming that the spring of 1973 was a point of high food production, and that rodent species diversity in the ecotone responded according to the prediction of the last paragraph, one should expect the spring or fall of 1973 and subsequent catches to show greater rodent species diversity than the catches of previous trapping sessions. 100 Table 20 shows the diversities of all the runs of the grid and IBP lines. The diversity index of Shannon was used, but the same pat tern was obtained with the Brillouin index. The tables of Lloyd, Zar and Karr (1968) were used in the calculations. Species diversity de creased after the 1972-1973 rains on both the Red Hill grid and IBP lines. The lowest diversities were associated with high populations (and presumably favorable conditions) on both sites. Favorable con ditions and diversity increases were not correlated in this study. Species number did not increase at a site, nor did evenness. At each site, one species (P. baileyi on the grid ^Fig. 137 and P. penicillatus on the IBP lines ^Fig. 127) responded so much more to the conditions than did other species that despite increases in populations of all the species there was no evenness increase. No Bpecies moved onto a site that it had not occupied before the change in conditions either. The fluctuations in species number on the table were contributed by one or a very few individuals of various transient species. The one exception occurred in the fall of 1972 when Mus musculus was present in fairly high numbers on both the lines (20) and the grid (1*0. The highest diversity on each site was due to the pres ence of Mus which increased both species number and evenness. Evenness adjustments were more important than species number in the other fluc tuations that did occur from trapping session to trapping session at both sites. Mus was not taken on either site before or after the fall of 1972, so the generalization that species did not move onto the sites with change in conditions holds for the resident species. 101 Table 20, Species diversity of the catch from each trapping session on the Red Hill grid and IBP lines. — The diversity measure of Shannon was used. Base 2 logarithms were used. H' = diver sity, S a number of species captured, J = evenness, N = total number of individuals captured. Trapping System and Date H1 S J N Red Hill Grid Sept. 1972 2.7 9 0.84 162 Sept. 1973 2.1 8 0.70 206 Sept. 197^ 2.0 8 0.67 761 Oct. 197^ 2.3 8 0.77 238 IBP Lines June 1972 1.8 5 0.78 85 Oct. 1972 2.2 6 0.85 75 May 1973 2.0 6 0.77 62 Sept. 1973 1.7 6 0.65 323 May 197^ 2.0 7 0.71 359 Sept. 197^ 1.8 6 0.69 kk7 102 Other Community Patterns. Despite the fact that species diver sity did not change in the expected direction, and that habitat segre gation patterns were constant, the rodent community organization was not constant throughout the study. Figures 11 and 12 Bhow that the various populations did not increase proportionally, but that relative species populations changed dramatically, especially on the IBP lines. Long term studies are probably required to understand the meaning of these changes and the role of the distinct habitats in community sta bility. SUMMARY Trapping and enclosure studies were carried out in a nocturnal rodent community in the Sonoran Desert near Tucson, Arizona, to de termine the ecological distribution of, and habitat variables important to, the species in the community. Community organization was then dis cussed in light of the habitat and other differences among the rodents. Preliminary trapping results indicated that the various habi tats of the area were occupied by different rodent species as follows: the overgrown washes had Perognathus penicillatus (Heteromyidae), and Neotoma albigula (Cricetidae). The creosotebush plains contained Dipodomys merriami (Heteromyidae), and Perognathus amplus (Hetero myidae). The tops of rocky hills were occupied by Neotoma albigula, Peromyscus eremicus (Cricetidae), and Perognathus intermedius (Hetero myidae). However, all the species occurred together in parts of the ecotone, where the characteristics of the other three habitats were mixed. A seventh species, Perognathus baileyi (Heteromyidae), was taken most commonly in parts of the ecotone. D. merriami, P. penicillatus and P. amplus occurred with P. baileyi in much of the ecotone. The habitat segregation, which was so obvious when only the three habitats mentioned first were considered, was not readily dis tinguishable in the ecotone. Multiple discriminant analysis was used to find habitat vari ables which consistently discriminated between the heteromyid species, 103 104 even in the ecotone. P. intermedius was associated with surfaces having high percentages of exposed bedrock, and stone and cobble cover. P. baileyi was associated with intermediate levels of soil-surface factors (not as coarse as those associated with P. intermedius, but coarser than those associated with the other heteromyids), and with relatively high densities of shrubs such as Larrea, Lycium and Acacia. P. penicillatus was associated with the less coarse soil-surface fac tors and with relatively high densities of a single group of shrubs, primarily made up of Acacia. P. amplus and JD. nterriami were both associated with the less coarse soil-surface factors and with low densities of Acacia and other woody plants. The variables considered in the last paragraph were consis tently associated with the same species under different trapping systems, habitat types (ecotone vs. distinct habitats), and popula tions, as well as through time. Even invading populations maintained the same associations with these variables. This constancy was in terpreted as evidence for the importance of the variables to the species with which they were associated, and to the coexistence of the animals in the area. An enclosure experiment was run to test heteromyid responses to crude simulations of habitat variables judged to be important on the basis of preliminary trapping. Three quarters of the enclosure were covered: one with stone, one with brush and one with scattered cobbles. The fourth was left uncovered. P. intermedius, P. baileyi and P. penicillatus responded in a manner relatively consistent with their field responses. The enclosure responses were not significantly 105 different from one another, however, because all favored covered areas over uncovered areas. P. amplus responded in a manner inconsistent with field results- In the field it was associated with the open creo- sotebush plain, but in the enclosure it favored cover. The enclosure response of D. merriami was relatively consistent with field results. D. merriami avoided stones and tracked and foraged more in the open areas than did any of the pocket mice. The similarity of D. merriami and P. amplus habitat suggested that their coexistence is not based on habitat segregation alone. The enclosure results suggested a possible microhabitat difference at a level below the resolution of this study. Perhaps P. amplus uses the cover available while D. merriami makes more intense use of the open areas in their shared habitat. The two cricetids occurred in quite specific habitats. Both N. albigula and Peromyscus eremicus were on the rocky hills, and N. albigula also occurred in the overgrown washes. The literature and some trapping results indicated that dietary and temporal differences may be responsible for coexistence with heteromyids of the same habi tats. Temporal and dietary differences were also found between the heteromyids. A quick reproductive response to favorable conditions may contribute to the ability of P. amplus to coexist with D. merriami. Peromyscus eremicus and P. intermedius appeared to have annual activity differences which may contribute to their coexistence. Literature re ports indicated that dietary differences may also contribute to co existence of both of these pairs. 106 An explanation of coexistence in this Sonoran Desert community probably requires at least two habitat dimensions: a woody plant den sity dimension and a fine to coarse surface-soil dimension. These appear to account for much of the required subdivision of resources* At least one food dimension (difference in food type — vegetative vs. seed) is probably involved in heteromyid coexistence with cricetids. Several temporal dimensions (daily activity, annual activity, different responses to favorable conditions) may be important components in coexistence of some species pairs. The dimensions were complementary to one another. Similarity of P. intermedius and Feromyscus eremicus on both habitat dimensions was complemented by differences on temporal and food type dimensions. Similarity of P. penicillatus to D. merriami and P. amplus in avoiding coarse surface-soil factors was complemented by the different reaction of P. penicillatus on the shrub density dimension. In other cases different reactions on various dimensions appeared to interact to produce differences in resource use. P. ampluB and merriami were slightly different on both habitat dimensions, in addition to the microhabitat, temporal and dietary differences already mentioned. None of these differences appeared to be sufficient to allow coexistence of the two species, and it was suggested that P. amplus and I), merriami coexisted by the interaction of all these dimen sions. There were other examples. D. merriami and P. penicillatus had daily activity differences in addition to their complementary reaction to shrub density. D. merriami and P. amplus differed from baileyi on both habitat dimensions. 107 The distinct differences between the ecotone and the homoge neous habitats with regard to species mixing, suggested that.species are best adapted for certain habitats, and that they may occupy those habitats at all times and expand into the other habitats in favorable times. If so these havens would be important to stability of the various populations of the area and hence to community stability. Species diversity, however, did not increase in the ecotone in periods of population increase and apparently high productivity. One might expect such a change if the community was organized around the homo geneous habitats as suggested. Although diversity did not change as expected, the species' populations did change proportionally with con ditions. This might indicate a role for the homogeneous habitats in community dynamics. Understanding the relationship of the homogeneous habitats and ecotone in community stability probably requires long term studies of the community. LIST OF REFERENCES Arnold, Lee W. 19^2. Notes on the life history of the sand pocket mouse. Journal of Mammalogy 23:339-3^1* Bailey, Vernon. 1931# Mammals of New Mexico. North American Fauna 53:1-^12. Baker, Rollin H. 1956. Mammals of Coahuila, Mexico. University of Kansas Publications of the Museum of Natural History 9:125-335* Baker, Rollin H. and J. Keever Greer. 1962. Mammals of the Mexican State of Durango. Publications of the Museum of Michigan State University, Biological Series 2:28-15^. Bartholomew, George A., Jr. and Grant Reynolds Gary. 195^* Locomotion in pocket mice. Journal of Mammalogy 55:386-392. Bartholomew, George A., Jr. and Herbert H. Caswell, Jr. 1951* Locomotion in kangaroo rats and its adaptive significance. Journal of Mammalogy 32:155-169* Bateman, Gary C. 1967. Home range studies of a desert nocturnal rodent fauna. Doctoral dissertation, University of Arizona, Tucson. Borell, Adrey E. and Monroe D. Bryant. 19^2. Mammals of the Big Bend area of Texas. University of California Publications in Zoology 48:1-62. Bradley, W. Glen and Roger A. Mauer. 1973* Rodents of a creosotebush community in southern Nevada. Southwestern Naturalist 17: 333-3M*. Brown, James H. 1973* Species diversity of seed-eating desert rodents in sand dune habitats. Ecology 5^:775-787* Brown, James H. and Gerald A. Lieberman. 1973* Resource utilization and coexistence of seed-eating desert rodents in sand dune habitats. Ecology 5^:788-797* Burt, William H. 1938. Faunal relationships and geographic distri bution of mammals in Sonora, Mexico. Miscellaneous Publica tions of the Museum of Zoology, University of Michigan 39:1-77* 108 109 Chew, Robert M., Robert G. Lindberg and Page Hayden. 1965. Circadian rhythm of metabolic rate in pocket mice. Journal of Mammalogy *»6:477-W- Christopher, Ernest A. 1973» Sympatric relationships of the kangaroo rats, Dipodomys merriami and Dipodomys agilis. Journal of Mammalogy 5^!317-326. Cockrum, B. Lendell. I960. The Recent Mammals of Arizona: Their Taxonomy and Distribution. The University of Arizona Press, Tucson. Cole, LaMont C. 19^9* The measurement of interspecific association. Ecology 30:^11-^2^. Commissaris, Larry R. I960. Morphological and ecological differen tiation of Peromyscus merriami from southern Arizona. Journal of Mammalogy ^1:305-310. Congdon, Justin. 197^« Effect of habitat quality on distributions of three sympatric species of desert rodents. Journal of Mammalogy 55•659-662. Dalquest, Walter W. 1953- Mammals of the Mexican State of San Luis Potosi. L.S.U. Studies in Biological Sciences, Series 1:1-229. Darden, T. R. 1965* Ecology and distribution of the rodents of northern Mohave County, Arizona. Master'6 ThesiB, University of Arizona, Tucson. Denyes, H. A. 195^» Habitat restriction and the digging ability of certain pocket mice. Journal of Mammalogy 35i1*53» Denyes, H. A. 1956- Natural terrestrial communities of Brewster County, Texas, with special references to distribution of mammals. American Midland Naturalist 55:289-320. Dice, Lee R. 1930. Mammal distribution in the Alainogordo region, New Mexico. Occasional Papers of the Museum of Zoology, Univer sity of Michigan 213:1-32. Drabek, C. M. 1967* Ecological distribution of the mammalian fauna of the Desert Biology Station Area. Master's Thesis, University of Arizona, Tucson. Eisenberg, John F. 1963* The behavior of heteromyid rodents. Uni versity of California Publications in Zoology 69:1-100. Findley, James S. 1959. The Bailey pocket mouse in New Mexico. Journal of Mammalogy 40:1^5* 110 Findley, James S., A. H. Harris, D. E. Wilson and Clyde Jones. 1975. Mammals of New Mexico. University of New Mexico Press, Albuquerque, New Mexico. Green, Roger H. 1971• A multivariate statistical approach to the Hutchinsonian niche: Bivalve molluscs of central Canada* Ecology 52:5^3-556. Grinnell, Joseph. 191^* Accounts of the Mammals and Birds of the lower Colorado River valley with special reference to the distribution problems presented. University of California Publications in Zoology 12:51-29^• Grinnell, Joseph. 1933- Review of the recent mammal fauna of Cali fornia. University of California Publications in Zoology 40:71-23^. Hall, E. Raymond. 19^6. Mammals of Nevada. University of California Press, Berkeley and Los Angeles. Hansson, Lennart. 1972. Evaluation of the small quadrat method of censueing small mammals. Ann. Zool. Pennici 9:184-190. Hardin, Garrett, i960. The competitive exclusion principle. Science 131:1292-1297. Hardy, Ross. 19^5* The influence of types of soil upon the local distribution of some mammals in southwestern Utah. Ecological Monographs 15:73-108. Harris, V. T. 1952. Habitat selection in deer mice. Contributions, Laboratory of Vertebrate Biology, University of Michigan 56: 1-56. Hatt, Robert Torrens. 1932. The vertebral columns of ricochetal rodents. Bulletin of the American Museum of Natural History 63:599-739- Hawbecker, Albert C. 1951* Small mammal relationships in an Ephedra community. Journal of Mammalogy 32:50-60. Hoffmeister, Donald F. 1956. Mammals of the Graham (Pianleno) Mountains, Arizona. American Midland Naturalist 55:257-288. Hoffmeister, Donald F. and Woodrow Goodpaster. 195^» Mammals of the Huachuca Mountains. Illinois Biological Monograph 2*4-:1-152. Hooper, Emmet T. 19^1. Mammals of the lava fields and adjoining areas in Valencia County, New Mexico. Miscellaneous Publications, Museum of Zoology, University of Michigan 51s1-^7• Ill Howell, Joseph C. 1954• Populations and home ranges of small mammals on an overgrown field. Journal of Mammalogy 35!177-186. Huey, Laurence M. 1951* The kangaroo rats (Dipodamys) of Baja California, Mexico. Transactions of the San Diego Society of Natural History 11:205-256. Ivey, R. DeWitt. 1957• Ecological notes on the mammals of Benalillo County, New Mexico. Journal of Mammalogy 38:490-502. j Jones, J. Knox, Jr., Dilford C. Carter and Hugh H. Genoways. 1973. Checklist of North American mammals north of Mexico. Occasion al Papers, the Museum, Texas Tech University 12:1-14. Kearney, Thomas H. and Robert H. Peebles. 1969- Arizona Flora. University of California Press, Berkeley and Los Angeles. Kenagy, C. J. 1973- Dailey and seasonal patterns of activity and energetics in a heteromyid rodent community. Ecology 54: 1201-1219. Kritzman, Ellen B. 1974. Ecological relationships of Peromyscus maniculatus and Perognathus parvus in eastern Washington, Journal of Mammalogy 55:172-188. Lidicker, W. Z• I960. An analysis of intraspecific variation in the kangaroo rat Dipodomys merriami. University of California Publications in Zoology 67:125-218. Lloyd, Monte, Jerrold H. Zar, and James R. Karr. 1968. On the cal culation of information-theoretical measures of diversity. The American Midland Naturalist 79:257-272. Long, V/. S. 1940. Notes on the life histories of some Utah mammals. Journal of Mammalogy 21:170-180. Lowe, Charles H. 1964. The Vertebrates of Arizona. The University of Arizona Press, Tucson, Arizona. MacMillen, Richard E. 1965. Aestivation in the cactus mouse, Peromyscus eremicus. Comp. Biochem. Physiol. 16:227-248. Massion, Dennis D. 1971« Population studies of nocturnal rodents in two desert habitats. Master's Thesis, University of Arizona, Tucson. Arizona. ! McCloskey, Robert T. and Brian Fieldwick. 1975* Ecological separa tion of sympatric rodents (Peromyscus and Microtus). Journal of Mammalogy 56:119-129» 112 Merritt, Joseph F. 197^• Factors influencing the local distribution of Peromyscus californicus in northern California. Journal of Mammalogy 55:102-114. Meserve, Peter L. 1976- Food relationships of a rodent fauna in a California coastal sage scrub community. Journal of Mammalogy 57:300-319. Monson, Gale, and Wayne Kessler. 1940. Life history notes on the banner tail kangaroo rat (Dipodomys spectabilis), Merriam1s kangaroo rat (Dipodomys merriamiJtand the white throated woodrat (Neotoma albigula) in Arizona and New Mexico. Journal of Wildlife Management 4:37-^3. Nie, Norman H., C. Hadlai Hull, Jean G. Jenkins, Karin Steinbrenner and Dale H. Bent. 1975- Statistical Package for the Social Sciences, 2nd ed. McGraw-Hill Book Company, New York. O'Farrell, Michael J. 197^• Seasonal activity patterns of rodents in a sagebrush community. Journal of Mammalogy 55'809-823. O'Farrell, Thomas P. 1975- Seasonal and altitudinal variation in populations of small mammals on Rattlesnake Mountain, Washing ton. The American Midland Naturalist 9^:190-204. Olsen, Ronald W. 1973- Shelter-site selection in the white-throated woodrat Neotoma albigula. Journal of Mammalogy 5^:59^610. Patton, James L. and J. Knox Jones. 1972. First records of Perog - nathus baileyi from Sinaloa, Mexico. Journal of Mammalogy 53:371-372. Reichman, 0. J. 1975- Relation of desert rodent diets to available resources. Journal of Mammalogy 56:731-751. Reichman, 0. J. and Kent M. van de Graaf. 1973. Seasonal activity and reproductive patterns of five species of Sonoran Desert Rodents. The American Midlands Naturalist 90:118-126* Reynolds, Hudson G. 1950. Relation of Dipodomys merriami to range vegetation in southern Arizona. Ecology 31:456-463. Reynolds, Hudson G. 1958. The ecology of the Merriam kangaroo rat (Dipodomys merriami Mearns) on the grazing lands of southern Arizona. Ecological Monograph 28:111-127. Reynolds, Hudson G. and Horace S. Haskell. 1949. Life history notes on Price and Bailey pocket mice of southern Arizona. Journal of Mammalogy 30:150-156. 113 Rosenzweig, Michael L. 1973- Habitat selection experiments with a pair of coexisting heteromyid rodent species. Ecology 54: 111-117. Rosenzweig, Michael L. and Philip W. Sterner. 1970* Population ecology of desert rodent communities: Body size and seed- husking as bases for heteromyid coexistence. Ecology 51:217- 224. Rosenzweig, Michael L. and Jerald Winakur. 1969. Population ecology of desert rodent communities: Habitats and environmental complexity. Ecology 50:558-572. Schoener, Thomas W. 1974. Resource partitioning in ecological com munities. Science 185:27-39. Schroder, Gene D. and Michael L. Rosenzweig. 1975« Perturbation analysis of competition and overlap in habitat utilization between Dipodomys ordii and Dipodomys merriaai. Oecologia 19:9-28. Smigel, Barbara \U and Michael L. Rosenzweig. 1974. Seed selection in Dipodomys merriami and Perognathus penicillatus. Ecology 55:329-339. Smith, C. F. 1942. Fall food of the brushfield pocket mice. Journal of Mammalogy 23:337-339* Spencer, Donald A. 1941. A small mammal community in the upper Sonoran Desert. Ecology 22:421-425. Spencer, Donald A. and Alice L. Spencer. 1941. Food habits of the white-throated woodrat (Neotoma albigula) in Arizona. Journal of Mammalogy 22:280-284. United States Department of Agriculture. 1951* Soil Survey Manual. United States Department of Agriculture Handbook 18. United States Department of Agriculture, in cooperation with University of Arizona Agricultural Eaqaeriment Station. 1972. Super intendent of Documents, United States Government Printing Office, Washington, D. C. Vorhies, C. T. and V/. P. Taylor. 1940. The life history and ecology of the white-throated woodrat, Neotoma albigula -albigula Hartley, in relation to grazing in Arizona. University of Arizona Agricultural Experimental Station, Technical Bulletin 86:453-529. Whitford, Walter G. 1976. Temporal fluctuations in density and diversity of desert rodent populations. Journal of Mammalogy 57:351-369. lnA Wondolleck, John Thomas. 1975- The influence of interspecific inter actions on the forage areas of heteromyid rodents. Master's Thesis, University of Arizona, Tucson, Arizona.