A COMPARISON OF WESTERN POND TURTLE ( ACTINEMYS MARMORATA ) MOVEMENTS IN PERENNIAL AND INTERMITTENT PORTIONS OF A NORTHWESTERN CALIFORNIA RIVER SYSTEM
By
Cheryl A. Bondi
A Thesis
Presented to the Faculty of
Humboldt State University
in Partial Fulfillment of the
Requirements for the Degree
Masters of Arts
May 2009
A COMPARISON OF WESTERN POND TURTLE ( ACTINEMYS MARMORATA ) MOVEMENTS IN PERENNIAL AND INTERMITTENT PORTIONS OF A NORTHWESTERN CALIFORNIA RIVER SYSTEM
By
Cheryl A. Bondi
Approved by the Master’s Thesis Committee
Sharyn Marks, Major Professor Date
Bryan Jennings, Committee Member Date
Mark Rizzardi, Committee Member Date
Hartwell Welsh, Committee Member Date
Michael Mesler, Graduate Coordinator Date
Chris A. Hopper, Dean for Research and Graduate Studies Date
ABSTRACT
A comparison of Western Pond Turtle ( Actinemys marmorata ) movements in perennial and intermittent portions of a northwestern California river system
Cheryl Bondi
Many turtle species require aquatic and terrestrial habitats during different times in their life history for foraging, mating, nesting, estivation and overwintering. The status of many turtle populations depends directly on the condition of both environments, as well as the connectivity between the two. Identifying factors that prompt movements between these two environments, or conditions in which one is no longer suitable such as the occurrence of seasonal drying, can aid in predictions on migration timing and habitat patch occupancy. This information in turn can be used in conscious land planning decisions.
I used radiotelemetry to track the seasonal movements of Western Pond Turtles inhabiting perennial and intermittent river reaches of the Mad River drainage in northwestern California. I monitored several environmental characteristics such as water and air temperature, and water availability to describe migrations in the context of changing environmental conditions. In addition, I investigated the effects that seasonal drying may have on adult body size by comparing the carapace length and body mass of individuals captured at both reaches.
I found that while differences in water availability did not significantly affect the extent of turtle movements within aquatic and terrestrial environments, it did affect migration timing. Turtles in the intermittent reach departed the river earlier than those in
iii the perennial reach, apparently in response to declines in surface water area. In the perennial reach, the turtles appeared to depart the river in response to declining air and water temperatures, as well as rises in streamflow. In response to declining water availability, 100% of turtles at the intermittent site estivated in adjacent upland habitat in
2007, and 92% of them did so in 2008; this indicates that this is the dominant strategy for dealing with drying conditions in this population. Turtles from the intermittent reach also migrated to the river earlier in the spring than those in the perennial reach, perhaps in an attempt to maximize foraging opportunities before the severe water loss that occurs mid- summer. Turtles were significantly smaller with regards to both carapace length and body mass at the intermittent reach, suggesting a morphological population response to the extreme conditions associated with seasonal drying.
This research shows that loss of surface water influences the movement patterns
Western Pond Turtles by eliciting an emigration response, and also appears to affect the body size of adult turtles. This result has important conservation implications since it demonstrates that available water may influence the body size of adults, which may have negative affects on survival and fecundity. Anthropogenic induced declines in available surface water due to pond draining for agricultural purposes, water diversions and intermittent flows created by dams, can all result in a depletion of aquatic resources and create aquatic environments unsuitable for resident turtles. Land managers need to evaluate individual populations’ response to decreasing water to develop management plans that alleviate any negative impacts.
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ACKNOWLEDGMENTS
I would like to thank the members of my graduate committee for their thoughtful input and suggestions throughout all stages of this project. I would like to thank my major advisor, Dr. Sharyn Marks, for her support and encouragement throughout this endeavor, and her invaluable editorial suggestions, especially in the final writing stages.
I thank Dr. Hart Welsh for sharing his knowledge and expertise of the species, Dr. Mark
Rizzardi for his statistical guidance and Dr. Bryan Jennings for his advice and sharing his knowledge of turtle biology.
I thank the California Department of Fish and Game for providing significant funding for this project. In particular, I thank Steven Burton for obtaining the funds necessary to purchase the radiotelemetry equipment. I thank the Mad River Ranger
District of the Six Rivers National Forest for providing funding, logistical support and housing for the entirety of this project. I want to extend my biggest of thanks and gratitude to Tracy Cline, former Mad River Ranger District Wildlife Biologist, for her dynamic role as colleague, mentor and a dear friend, and for securing significant financial support. I thank Humboldt State University’s Scholarship and Creative Activities grant and the Department of Biological Sciences Masters Student Grant for providing funding for this project.
I am greatly appreciative to have had amazingly helpful and dedicated assistants in the field. I would like to thank the “turtle girls”, Julie Combes and Amanda
Proudman, for dedicating countless hours in the field and for volunteering from the time
v it was over 100 degrees at Mad River to the days when we were looking for turtles in the snow. I would like to thank the 2007 field crew: Courtney Owens, Jeremiah Preffer, and
Scott Benson; and the 2008 field crew: Jennifer Brown, Steven Escobar, Kana
Kobayashi, and Kenneth Shapiro. I extend my deepest gratitude to my tireless diving crews: Christopher West, Oliver Miano, Michael Best, Ryan Bourque, and Eric Russell.
I would like to thank Jamie Bettasso and Donald Ashton for sharing their knowledge, enthusiasm and passion for studying the Western Pond Turtle with me. There are many other friends that helped in the field that I would like to thank: John Welsh, Jane Cipra,
Maria Cisneros, Erin Hanelley, Carrie Sendak, Pakki Reath, Mathew Lowe, Jada
Howarth and Iris Gray. I could not have done this project without all of your help.
I would like to thank my family for their continued encouragement and support, not just during my graduate experience, but in everything that I do. I thank my family for their patience and understanding of my absence while I moved across the country in pursuit of my degree, and for being there for me always.
Last, but certainly not least, I would like to thank the Western Pond Turtles of the
Mad River, that so patiently allowed me to intrude on their lives to gain insight into their fascinating world.
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TABLE OF CONTENTS
ABSTRACT...... iii
ACKNOWLEDGMENTS ...... v
TABLE OF CONTENTS...... vii
LIST OF TABLES...... ix
LIST OF FIGURES ...... x
INTRODUCTION ...... 1
MATERIALS AND METHODS...... 7
Study Site...... 7
Radiotelemetry and Turtle Movements ...... 8
Environmental Monitoring and Stream Habitat Inventory ...... 19
Turtle Body Size and Mass...... 21
Data Analysis...... 22
RESULTS ...... 25
Summer Aquatic Movements ...... 25
Migrations to Upland Habitat and Terrestrial Habitat Use...... 31
Migrations to the River From Terrestrial Habitats ...... 43
Turtle Carapace Size and Mass...... 44
DISCUSSION...... 51
Turtle Body Sizes...... 59
Conservation Implications ...... 61
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REFERENCES ...... 65
APPENDIX A...... 70
APPENDIX A (Continued)...... 71
APPENDIX A (Continued)...... 72
APPENDIX A (Continued)...... 73
APPENDIX B ...... 74
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LIST OF TABLES
Table Page
1 Sex and size data for turtles fitted with transmitters at the upper reach in June 2007. Carapace length and plastron length are maximum shell measurements to the nearest mm ...... 14
2 Sex and size data for turtles fitted with transmitters at the lower reach in June 2007. Carapace length and plastron length are maximum shell measurements to the nearest mm...... 15
3 Summary of radiotracking data for turtles at the upper reach used in the general linear models. Number of relocations is the number of times a turtle was found throughout the entire study. Exit dates and return date are given as day of calendar year. Total days on land is the number of days spent in terrestrial habitat from June 2007 through June 2008. Means, standard deviations and standard error are given for each of the 3 variables that were response variables in the general linear models...... 27
4 Summary of radiotracking data for turtles at the lower reach used in the general linear model. Number of relocations is the number of times a turtle was found throughout the entire study. Exit dates and return date are given as day of calendar year and total days on land is the number of days spent in terrestrial habitat from June 2007 through June 2008. Means, standard deviations and standard error are given for each of the 3 variables that were response variables in the general linear models...... 28
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LIST OF FIGURES
Figure Page
1 Current distribution of Actinemys marmorata in western North America. The red dots represent occurrence data from museum and literature records. The green shaded area is the projected distribution based on GIS-defined hydrologic unit compartments that were constructed around verified localities. (Reprinted from Bury, 2008, permission to use)...... 9
2 A map of the study area within the Mad River, California within Humboldt and Trinity counties. (A). Inset map showing study reaches as the extent of the area in which turtles with radio transmitters traveled during the 17 month study period. The Lower Reach (shown in red) is approximately 6.3 km linear distance of river and the Upper Reach (yellow) is 2.5 km of river (B)...... 10
3 Photographs of the two study reaches on the Mad River, California. At the upper reach (A-C) streamflow is characterized by intermittent flows. In mid-summer the river is reduced to scattered shallow pools interspersed with long stretches of exposed gravel bar (B-C). The lower reach (D-F) is a perennial reach in which streamflow is controlled by water releases from the dam and flows are slightly raised in the winter (F)...... 11
4 Photographs (A) and (B) of two separate turtles with radiotransmitters attached to their carapaces with gel epoxy. Photographs show placement of the transmitter on the left 3rd pleural scute with antenna left free...... 13
5 Map showing the three movement parameters used in the general linear model analysis. Home range was calculated as the linear distance mid channel along the river between the two farthest aquatic locations. Step length was defined as the shortest straight line distance between consecutive locations. Overwintering distance to water was measured as the shortest straight line distance from hibernation sites to the river. Movements for turtles #538 (male) and # 445 (female) from June 2007 through their return to the river in the spring 2008 are shown...... 18
x
6 Bar graph of average length of aquatic movement for each turtle tracked at the lower (A) and upper (B) reach. Average length of movement was calculated as the total length of aquatic movement divided by the number of relocations. Solid bars indicate the 2007 field season and dashed bars refer to the 2008 field season. Males had greater average length of movement for both field seasons, there was no difference between sites...... 29
7 Linear aquatic home ranges for each turtle at the lower (A) and upper (B) reach. Linear aquatic home ranges are defined as the distance of stream between the two farthest relocations. Solid bars indicate 2007 field season and dashed bars refer to the 2008 field season. Males had larger aquatic home ranges than females for both years and there was no difference between sites...... 30
8 Timelines representing the duration individual turtles spent in each environment (aquatic or terrestrial), calculated as the number of days. Turtles at the upper reach (B) spent a significantly longer time on land than those of the lower reach (A; 241 versus 207 days)...... 33
9 Line graph showing the inverse relationship between turtles on land and declining water levels at the upper reach. Average cross sectional area (meters 2) of stream within a 400 meter survey area at the upper reach for each survey date is on the primary y-axis. On the secondary y-axis is the proportion of turtles on land for each survey date at the upper reach. Turtles began exiting the stream when wetted cross sectional area dropped below 3.9 meters 2 on July 17, 2007 (A) and 6.1 meters 2 on July 11, 2008 (B)...... 34
10 Scatterplots showing the relationship between proportion of individuals on land (primary y-axis) and survey date (x-axis) and weekly average air and water temperatures (secondary y-axis). Turtles at the lower reach (A) exited the water in late fall as water and air temperatures declined. Turtles at the upper reach (B) had already exited the water before a decline in temperatures...... 35
11 Photographs of turtles estivating at the upper reach. A female, on July 29, 2007 (a) with the substrate (about 3 cm. deep) pushed aside for photographing purposes. This shows the organic substrate turtles tended to bury themselves in and the abundant vegetative cover aestivation sites possessed. A male on July 20, 2007 (b) with most of his carapace exposed. This photograph shows the turtle is partially covered under sand substrate and beneath abundant ground cover vegetation...... 36
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12 Frequency of dominant microhabitat characteristics of estivation sites at the upper reach. (A) Cover type (object turtle was found beneath), (B) substrate type (organic =humus or plant material; silt=0.004-0.06 mm grain diameter; sand=0.06-2 mm; gravel=2-64 mm; cobble=64-256 mm; boulder >256 mm), (C) height to first branch (ground<1 m; understory= 1-5 m; overstory>5 m), (D) dominant vegetation type (based upon dominant species in 1 meter 2 plot) and (E) distance from the river for estivation sites. 2007 sites (n=112) represented with light gray shading and 2008 sites (n=100) represented by dark gray...... 37
13 Mean weekly stream discharge (cubic feet per second) from the upper reach (dashed line) and lower reach (solid line) on the Mad River. Streamflow from May through October is essentially nonexistent at the upper reach (0-18 cfs). The black arrows indicate dates when water returned to the upper reach with the onset of fall precipitation in both years. At the lower reach streamflow remains steady in the summer (between 50cfs and 60cfs). Dotted black arrows indicate the dates when there was 16 cfs increase in weekly streamflow in 2007, upon which only one turtle exited the river, and 24 cfs in 2008, when two turtles departed the river. Data were only available through September 30, 2008 at the lower reach...... 40
14 Distance that the turtles overwintered from the river at the lower reach (A) and the upper reach (B). None of the explanatory variables (site, sex, or weight) were significant in the general linear model and the turtles generally traveled the same distance both years...... 41
15 Turtles displayed site fidelity in their selection of overwintering sites by returning to the same areas for this activity in consecutive years. Seventy percent of turtles at the lower reach overwintered within 20 meters of their 2007 overwintering site in 2008...... 42
16 Boxplots showing the median and range of maximum carapace length (mm) (A) and mass (g) (B) for adult turtles during initial dives in 2007 at the two study reaches on the Mad River, California. Both maximum carapace length ( P<0.001) and weight ( P<0.001) were significantly different from each other...... 46
17 Histograms of maximum carapace length (mm) frequencies for all turtles captured during the summer of 2008 at the lower reach (A) and upper reach (B). Turtle of unknown sex were those that did not possess secondary sexual characteristics and were classified as subadults...... 48
xii
18 Boxplots displaying descriptive statistics for maximum carapace length (mm) (A) and mass (g) (B) for adult turtles at two sites on the Mad River, California from dives conducted in 2008. Both maximum carapace length (p<0.001) and weight (p<0.001) were significantly different from one another...... 49
19 Relationship of plastron length (A) and mass (B) to maximum carapace length at two sites on the Mad River, California. Adult turtles at the upper reach (yellow diamonds) have overall smaller body size compared to those of the lower reach (red triangles) as seen by their placement in the lower left corner of the scatterplot...... 50
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INTRODUCTION
Investigating the spatial ecology of mobile animals at the landscape scale is essential for understanding the life history processes and environmental requirements needed to sustain healthy populations, and adds a critical component to conservation efforts. Some species require resources within spatially separated habitat patches at different times during their life history. Individuals often must travel between habitat patches within the landscape to attain the critical resources provided at each through landscape complementation (Dunning et al. 1992). The movement patterns of individuals among these habitat patches, the resources gained at each, and circumstances in which they are no longer suitable, provide insight into some of the ecological and evolutionary reasons for dispersal and migration.
Species that use both terrestrial and aquatic habitats require particular consideration with regards to land management (Pilliod et al. 2002; Ryan et al. 2008).
The loss or degradation of either of the two habitat types and their associated resources would have profound consequences on individual survival and population viability. In addition, a disruption in the connectivity between habitat patches may render them no longer accessible (Mitchell and Klemens 2000). Many species of turtles require aquatic and terrestrial habitats during different times in their life history for activities such as foraging, mating, nesting, estivation and overwintering (Bennett et al. 1970; Gibbons
1970; Gibbons et al. 1990; Reese and Welsh 1997). The status of many turtle populations depends directly on the quality of both habitats (Bodie and Semlitsch 2000;
1 2 Mitchell and Klemens 2000) as well as the connectivity between the two (Roe and
Georges 2007; Semlitsch and Bodie 2003). Identifying environmental factors that
prompt movements between the two habitats, or conditions in which a habitat patch is no
longer suitable and thus abandoned, can aid in predictions on migration timing and
habitat patch occupancy. This information in turn can be used in conscious land planning
decisions that consider all the habitats needed throughout a species’ life history, as well
as maintaining movement corridors that join them. As habitat loss and fragmentation
continue due to anthropogenic disturbances, it is critical to identify migration timing and
routes to conserve critical habitat patches and maintain their connectivity (Browne et al.
2006).
The activity and movement patterns of several freshwater turtle species are
influenced by seasonal changes in the environment, with individuals moving to certain
habitat types for specific life history activities (Gibbons 1986). For example, there exists
an annual cycle of activity reliant upon temperature cues for Painted Turtles ( Chrysemys
picta) in which migrations from shallow ponds to deeper hibernation ponds occur during
the fall (Sexton 1959). Spotted Turtles ( Clemmys guttata) displayed seasonal annual cycles of movement and activity related to winter hibernation, summer basking and mating (Litzgus and Mousseau 2004; Ernst 1976). Adult Mud Turtles ( Kinosternon
subrubrum ) will migrate as far as 1 km from their aquatic environment in the fall to
overwinter on land (Bennett et al. 1970). Wood Turtles ( Glyptemys insculpta ) have
movements associated with seasonal nesting and movements to river overwintering sites
(Arvisais et al. 2002) and increased activity with warming ambient temperatures
3 (Kaufmann 1992). In riverine systems, three different species of Map Turtles ( Graptemys
flavimaculata, Graptemys geographica, and Graptemys pseudogeographica ) have annual seasonal activity patterns associated with aquatic and nesting movements during the warmer months, and movements during fall associated with a change in activity areas for overwintering (Bodie and Semlitsch 2000; Pluto and Bellis 1988; Jones 1996). Seasonal terrestrial movements of six aquatic turtle species are influenced by precipitation and environmental temperatures at a pond in Ellenton Bay, South Carolina (Gibbons et al.
1970).
A decrease in environmental quality, as can be caused by declining water levels, can also influence turtle movements. When water levels decrease turtles may respond by migrating to alternate aquatic resources, congregating in remnant bodies of water, or estivating until water returns (Ligon and Stone 2003). The costs and benefits that are associated with making extensive overland movements in search of alternate aquatic habitat must be weighed against those associated with terrestrial estivation (Roe et al.
2008). The response to unsuitable habitat is often species specific and dependent upon physiological adaptations for prolonged terrestrial estivation (Peterson and Stone 2000).
Interpopulational variation also exists in some species, with a response to drought being based upon landscape features such as hydroperiod and distance to neighboring water bodies (Roe and Georges 2008; Ligon and Peterson 2002). Many aquatic turtle species have been shown to abandon unsuitable habitat during drought related events and migrate to more favorable habitat (Gibbons 1970; Gibbons et al. 1983 and Moll 1990). Drought is usually an unpredictable phenomenon and hence it is difficult to study species response
4 to it. An investigation on drought’s influence on movement patterns of freshwater turtles
provides valuable insight into turtle life history strategies.
The Western Pond Turtle ( Actinemys marmorata ) is a member of the Emydidae, a diverse family consisting of approximately 100 species. These turtles range from being fully aquatic to completely terrestrial, with many species having a semiterrestrial life history (Devaux and Dupres 2006). Western Pond Turtles are the only freshwater aquatic turtles native to California, with a range extending from western Washington south to
Baja California and mainly west of the Cascade-Sierran crest (Figure 1) (Stebbins 2003).
Populations have experienced declines throughout their range in California due to alteration and loss of habitat, such as timber harvesting, conversion of wetlands to agricultural lands, urbanization, water diversions and dams (Brattstrom 1998; Holland and Bury 1998). This species is currently listed as an endangered species in Washington
(Hayes et al. 1999) and identified as a Species of Special Concern in California (Jennings and Hayes 1994).
Western Pond Turtles are habitat generalists that use a variety of aquatic systems such as ponds, lakes, marshes, rivers, and streams. In river systems, turtles are most abundant in slower moving waters such as pools or slack water that provide abundant basking sites and underwater refugia (Bury 1972; Reese and Welsh 1998). Aquatic movements within the watercourse are related to activities such as basking, foraging, moving to suitable microhabitat conditions, and males seeking mates. Western Pond
Turtles have variable home range sizes with males having greater home ranges than females (Bury 1972; Reese 1996). Previous studies have shown that A. marmorata will
5 move extensive distances within the watercourse (Bury 1972; Rathbun et al. 1992; Cook and Martini-Lamb 2004), with males making significant aquatic excursions compared to those of females (Bury 1972).
These turtles frequently migrate from the water to use terrestrial habitat for nesting, hibernation and estivation (Goodman and Stewart 2000; Rathbun et al. 2002;
Rathbun et al. 1992) and movements between these two environments often coincide with changes in the local environment. This relationship changes with geographic location, local climatic conditions, and aquatic water source. Some populations spend the majority of the year in the water and others migrate to terrestrial habitats in response to unfavorable conditions such as declining temperatures and high flows. Turtles inhabiting rivers in the northern extent of their range make extensive movements to upland overwintering sites in the fall, returning to the water in the spring (Reese and Welsh 1997 and Holland 1994). Studies in the Trinity River in northern California have shown that turtles will migrate from aquatic environments in response to increasing streamflow and decreasing air and water temperatures. In contrast, turtles inhabiting ponds in Oregon have been shown to overwinter in the mud along the banks of the water (Holland 1994).
Gravid females will migrate to suitable nesting habitat in adjacent upland areas during the summer (Reese and Welsh 1997).
Western Pond Turtle movements have been shown to be influenced by hydrological flow. A study in southern California showed that females in an intermittent river had significantly larger linear aquatic home ranges then those inhabiting a dammed river where water levels remained higher (Goodman and Stewart 2000). This suggests
6 that the availability of surface water may have a direct influence on the movement of this
species due to increased movements to find suitable habitat and prey (Goodman and
Stewart 2000). While previous studies have described the movement patterns of Western
Pond Turtles in river systems , research is needed to investigate the effects of seasonal drying on this species’ movements.
The primary objective of this thesis was to describe the seasonal movements, between and within aquatic and terrestrial environments, of Western Pond Turtles inhabiting perennial and intermittent portions of the Mad River. I investigated the timing of migrations between aquatic and terrestrial environments in the context of changing environmental conditions, including air and water temperatures, decreases in streamflow and water availability.
This study provides information on how hydrologic flow may influence the seasonal movements and consequently the relative size of Western Pond Turtles. It provides information on how populations may respond to anthropogenic disturbance in aquatic environments such as the creation of intermittent hydrologic systems by damming, water diversions or draining for agricultural purposes. This research may be applicable to other systems in which there exist dramatic seasonal fluctuations in stream flow that may result in seasonal drying.
MATERIALS AND METHODS
Study Site
This study took place within two reaches of the Mad River, California separated from each other by the Ruth Reservoir (Figure 2). The Mad River drains a 497 square mile watershed and flows in a northwest direction through Humboldt and Trinity counties in northwestern California. There is a single dam on the river, Robert Matthews Dam, which was built in 1962 and forms a seven mile long reservoir, Ruth Reservoir.
The intermittent study reach (upper reach) was located in the uppermost portion of the drainage and ranged between 2,840 and 2,800 feet in elevation (Figures 3A-3C).
This segment of the river experiences intermittent streamflows that are heavily reliant upon precipitation inputs from the southern Yolla Bolly Mountains and South Fork
Mountain to the northeast. During late summer, significant portions of the river are subsurface and surface water is only present in scattered remnant pools at spring recharge locations. Turtles at the upper reach were all originally captured within a 500 meter reach of river, but moved as much as 2.5 km throughout the duration of the study. The upper reach flowed directly into Ruth Reservoir. The perennial study reach (lower reach) is located below Ruth Reservoir between approximately 2,480 and 2,400 feet elevation
(Figures 3D-3F). Regulated dam releases result in a perennial flow regime. Mean monthly flows vary between 50 cfs and 70 cfs in the summer, and are the highest between January and May (497 to 743 cfs). At the lower reach turtles were originally captured in two separate 300 meter sections and they moved throughout 6.3 km of river
7 8
over the course of this study. The dominant vegetation at both reaches is conifer forest of
the Douglas-fir series interspersed with oak woodlands and grasslands. Average
precipitation in the study area is approximately 60" annually with the majority of the
moisture falling between October and April, with seasonal snowfall occurring often
above 2,500 feet elevation.
Radiotelemetry and Turtle Movements
I documented the seasonal movement patterns of turtles by radio tracking individuals from each reach. Turtles were captured by hand during snorkel surveys conducted in early June 2007. Upon capture individuals were sexed using morphological characters. Males posses a slightly concave plastron, thicker tail, and a cloaca that extends posterior to the edge of the carapace. Females generally have flat plastrons, and the cloaca is anterior to the edge of the plastron. In addition, adult females have dark flecking on the throat while males tend to have pale throats. Turtles that could not be sexed were categorized as “unknowns”; these were subadults and did not yet possess secondary sexual characteristics.
Turtles were weighed with a 1,000 gram Pesola hand scale, and maximum carapace length and width, shell height and maximum plastron length were measured with 200 mm calipers. Twenty-eight turtles (14 turtles from each reach) were fitted with
ATS R1800 transmitters, each weighing 12 grams (ATS Tracking Systems, Isanti, MN), with five minute waterproof gel epoxy (used for smaller turtles to reduce total weight) or putty epoxy.
9
Figure 1. Current distribution of Actinemys marmorata in western North America. The red dots represent occurrence data from museum and literature records. The green shaded area is the projected distribution based on GIS-defined hydrologic unit compartments that were constructed around verified localities. (Reprinted from Bury, 2008, permission to use)
10
B.
A.
Figure 2. A map of the study area within the Mad River, California within Humboldt and Trinity counties. (A). Inset map showing study reaches as the extent of the area in which turtles with radio transmitters traveled during the 17 month study period. The Lower Reach (shown in red) is approximately 6.3 km linear distance of river and the Upper Reach (yellow) is 2.5 km of river (B).
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Figure 3. Photographs of the two study reaches on the Mad River, California. At the upper reach (A-C) streamflow is characterized by intermittent flows. In mid-summer the river is reduced to scattered shallow pools interspersed with long stretches of exposed gravel bar (B-C). The lower reach (D- F) is a perennial reach in which streamflow is controlled by water releases from the dam and flows are slightly raised in the winter (F).
12
I glued transmitters to the third pleural scute to avoid interference during mating
(Boarman et al. 1998) and the antenna was left free to prevent snagging on vegetation
(Figure 4). Only turtles weighing over 320 grams were fitted with transmitters in order to remain within the suggested 3-5% body weight ratio to avoid alteration of turtle behavior, and efforts were made to include an even sample of males and females (Tables 1 and 2).
All turtle surveys and handling of animals followed the protocol of Humboldt State
University IUCAC 06/07.B.163.A.
I relocated turtles using a Telonics TR-4 model hand held receiver (Telonics,
Mesa, AZ) with a three element antenna. Relocations were attempted approximately once a week from June 19 through September 15, 2007, and May 31 through September
26, 2008. During Fall 2007 (October through December), Fall 2008 (October only) and
Spring 2008 (March through May) turtles were located twice a month to document migrations to and from the river channel. Between December 2007 and February 2008 turtles were located only once a month to verify hibernation locations and habitat features associated with overwintering hibernacula. Turtles were tracked for a total of 496 days
(except turtle #266 which was tracked for 505 days), thereby capturing an entire annual cycle of movement including two seasons of summer aquatic movements and two emigrations from the river (Tables 3 and 4). The sample size declined over the course of this study due to two mortalities, three transmitters falling off, and two turtles that went missing. To counteract this, two additional turtles were entered into the study at the upper reach, one in July 2007 and another in June 2008 (Table 2).
13
A.
B.
Figure 4. Photographs (A) and (B) of two separate turtles with radiotransmitters attached to their carapaces with gel epoxy. Photographs show placement of the transmitter on the left 3rd pleural scute with antenna left free.
14
Table 1. Sex and size data for turtles fitted with transmitters at the upper reach in June 2007. Carapace length and plastron length are maximum shell measurements to the nearest mm
Identification Carapace Carapace Plastron Shell Sex Weight Number Length Width Length Height (g) (mm) (mm) (mm) (mm)
517 F 152 114 138 60 530 677 F 147 108 132 54 430 286 F 140 106 127 59 425 226 F 138 105 127 48 420 345 F 145 110 135 55 525 368 F 135 101 124 49 385 558 F 139 102 125 46 390 735 M 138 104 128 43 385 246 M 140 98 129 43 390 696 M 139 97 124 48 390 776 M 147 101 129 43 425 575 M 144 93 128 49 410 405 M 132 100 118 41 320 1 308 M 135 97 128 47 370 2 635 M 136 97 117 NA 375 Average 140 102 127 49 411
1 Turtle #308 was included in the study in July 2007. 2 Turtle #635 was included in the study in June 2008.
15
Table 2. Sex and size data for turtles fitted with transmitters at the lower reach in June 2007. Carapace length and plastron length are maximum shell measurements to the nearest mm.
Identification Carapace Carapace Plastron Shell Sex Weight Number Length Width Length Height (g) (mm) (mm) (mm) (mm)
596 F 165 124 155 61 660 466 F 162 134 145 60 685 427 F 161 124 147 60 650 326 F 151 118 137 58 510 658 F 157 130 146 60 520 266 F 132 100 122 46 340 445 F 132 104 127 48 335 3 636 F 153 116 142 51 575 485 M 159 124 141 52 570 617 M 169 128 152 52 610 386 M 174 132 154 61 770 755 M 172 130 159 60 720 715 M 145 114 132 53 440 538 M 152 116 138 48 495 Average 156 121 143 55 563
3 Turtle #636 was found dead on July 4, 2007 and was excluded from all further analyses.
16
Turtles were relocated to within approximately 1 meter 2 either through visual confirmation or by removing the antenna and coaxial cable and determining the strongest signal with the receiver box only. Only on a few occasions was the location estimated from across the river when conditions were unsafe to cross. Turtle locations were recorded with a Garmin eTrex Vista Cx Global Positioning System (Garmin, Olathe, KS) hand held unit in NAD83 Universal Trans-Mercator units. The average accuracy for the
GPS was 6 meters at the lower reach and 4 meters at the upper reach. Turtle locations were imported into ArcMap 9.3 (ESRI, Redlands, CA) onto a digital elevation model
(United States Geological Survey Seamless Server) and aerial photographs (National
Agricultural Imagery Program, United States Department of Agriculture, 2004).
Locations that were incorrectly placed on the aerial photographs due to GPS inaccuracies were manually corrected by moving to the known locality on the river and recomputing the site information.
Movement parameters were calculated using ArcView 9.3 Hawth’s Analysis
Tools extension (ESRI, Redlands, CA). This extension creates a line feature layer of connected paths between consecutive locations and calculates the shortest straight line distance between them (Figure 5). To describe how animals moved within aquatic and terrestrial environments the data was divided into subsets, according to whether it occurred in either aquatic or terrestrial environment. For summer aquatic movements the average length of movement was calculated by taking the cumulative distance of movement and dividing by the number of relocations (Bodie and Semlitsch 2000). This was done to adjust for unequal tracking effort over time between individuals. I defined
17 linear aquatic home range as the maximum distance between relocation points as measured along the middle of the river channel (Jones 1996; Travis et al. 2008) (Figure
5). I chose this method, as opposed to others such as minimum convex polygons, since these methods often include large areas that may not be used by an animal. A turtle was considered to be overwintering when it ceased to make any more terrestrial movements and remained inactive for several consecutive visits. Since I was unable to determine the actual direction that a turtle took to reach its overwintering site when leaving the river, I used the distance between overwintering site and the river to represent the distance traveled through terrestrial habitat to overwintering sites. I measured the shortest straight line distance between overwintering sites and the river on aerial photographs using the
“nearest” feature in ArcView 9.3 (Figure 5).
Summer terrestrial locations for turtles at the upper reach were classified as estivation sites until the turtle reached its overwintering destination in late fall. The two were distinguished from each other in that once a turtle reached its hibernation site it no longer moved. Turtles would often shift estivation sites by making short distance terrestrial movements (<10 meters), although a few were greater than 10 meters when en route to overwintering sites. Estivation sites were pooled together for each individual for each year, and the mode for each categorical variable and the mean for distance to water was used to express the dominant characteristics for each turtle. This value was used to describe the frequency of dominant microhabitat characteristics for all the turtles, which were calculated as a percent of turtles in which each variable was dominant.
18
Figure 5. Map showing the three movement parameters used in the general linear model analysis. Home range was calculated as the linear distance mid channel along the river between the two farthest aquatic locations. Step length was defined as the shortest straight line distance between consecutive locations. Overwintering distance to water was measured as the shortest straight line distance from hibernation sites to the river. Movements for turtles #538 (male) and # 445 (female) from June 2007 through their return to the river in the spring 2008 are shown.
19
I recorded several microhabitat characteristics within a one meter 2 radius of each
estivation site. I visually estimated canopy cover as one of five categories: absent (0-
20%), sparse (20-40%), moderate (40-60%), heavy (60-80%) and dense (80-100%). I
categorized dominant substrate type as either organic (humus, plant material), silt (0.004-
0.06 mm grain diameter), sand (0.06-2 mm), gravel (2-64 mm), cobble (64-256 mm) or
boulder (>256 mm). I visually estimated the height to the first branch of vegetation that
was within the one meter 2 plot and categorized it as: ground cover (<1 meter), understory
canopy (1-5 meters) or overstory canopy (>5 meters). Turtle cover was the physical
object that at least partially covered the carapace and was categorized as follows: none
(carapace completely exposed), leaf litter (carapace covered by leaves), substrate
(carapace completely buried in ground), downed woody debris, and vegetation. The
dominant plant species present within the one meter2 plot was recorded. The distance to
the river was determined by measuring the shortest straight line distance between
estivation sites and the middle of the river channel using ArcView 9.3 “nearest” feature.
Environmental Monitoring and Stream Habitat Inventory
To investigate whether turtle migrations from the river were related to seasonal changes in environmental conditions, I monitored several environmental variables throughout the study. I chose to monitor air temperature, water temperature, and streamflow because previous research has shown that this species’ migrations can be influenced by seasonal changes in these variables. I also monitored water availability,
20 represented as the wetted cross sectional area of stream present at each survey, at the upper reach only because the hydrologic regime changes here seasonally.
HOBO environmental data loggers (Onset, Bourne, MA) were placed in deep and shallow locations within the river at both reaches. Loggers took water temperatures every four hours, and from these data daily and weekly averages were calculated. Daily and weekly air temperatures were obtained from the Western Regional Climate Center
(Desert Research Institute, Reno, Nevada). I received data on daily streamflow for the lower reach from the Humboldt Municipal Water District (Eureka, California) records of dam releases. The gaging station is located approximately 14 kilometers upstream from the lower reach. I was only able to obtain records from June 2007 through October 2008.
Data on daily and weekly streamflow data for the upper reach were gathered from the
United States Geological Survey Real-Time Weather Database (Menlo Park, California) from the gaging station on Mad River above Ruth Reservoir located about 12 kilometers downstream from the upper reach.
Streamflow at the upper reach declines quickly after the end of the rainy season and little variability exists in the flow during the summer. To more accurately describe changes in available surface water I did stream cross sectional profiles within the upper reach to quantify declines in aquatic habitat. I used a protocol for stream habitat classification and inventory procedures by McCain et al. (1990). Five meter wide transect belts were placed systematically in 400 meters of the upper reach. Transect belts were spaced every 30 meters from a random start location that was chosen by selecting a number between one and 13. I measured stream width at three locations within each belt
21
(the center of the belt, upstream 2.5 meters and downstream 2.5 meters) using a tape measure. The three width measurements were averaged to calculate the mean top width
(distance from wetted edge to opposite wetted edge of stream) of the river for each transect.
Exposed gravel bars that had a surface area greater than 10% of the transect belt were measured and subtracted from the top width. Along the center width five evenly spaced depth measurements and the transect thalweg were taken with a graduated rod. I conducted these surveys once per radiotelemetry survey until the water at the upper reach was completely absent in late summer. Cross sectional area of stream channel was calculated for all of the transects for each survey date (average width*average depth).
All 13 transects were averaged for each survey to calculate average cross sectional area of stream (Appendix A). I monitored the same transect cross sections during the 2007 and 2008 field seasons.
Turtle Body Size and Mass
I investigated body size differences between turtles at the upper and lower reach by comparing carapace size and turtle mass data from a random sample of turtles within each study reach. Visual encounter surveys were conducted in early June 2007 and summer 2008; one snorkel survey was conducted at the upper reach on June 26, 2008 and two snorkel surveys occurred at the lower reach on September 1 and September 7, 2008.
Surveys in 2008 at the two reaches did not occur closer together due to land access restrictions caused by wildfires. The temporal separation between these surveys most
22
likely affected the abundance of individuals captured since some the turtles may have
already begun leaving the river at the lower reach in September when the survey
occurred. In addition, hatchling numbers decline later in the season due to dispersal and
mortality. To compensate for this and to compare my results with previous research the
analysis included adult individuals exclusively (maximum carapace length >110 mm).
The surveys consisted of two to three people diving systematically downstream
within each of the two reaches and capturing all turtles encountered. Surveyors searched
under all boulders, root cavities and undercut banks for turtles. To supplement low
captures at the lower reach in 2008 baited aquatic funnel traps were placed in backwater
pools, but only one turtle was captured using this approach. Upon capture each turtle was
weighed, measured, and marked with a permanent marker on the plastron to avoid
resampling.
Data Analysis
I used paired t-tests to determine if there was a significant effect of year on average length of movement and linear aquatic home range. Since the same turtles were tracked for both years (with the exception of turtle #635 which was added into the study in 2008) the data for each year were analyzed separately to avoid pseudoreplication. For the aquatic movements general linear models were used to test for the effects of the indicator variables (sex, site, and body mass) on the two response variables (linear aquatic home range and average length of movement). In addition, the significance of the interaction term of sex*mass was investigated. Multicolinearity existed in the
23 explanatory variables with site and body mass being marginally correlated with one another (0.59), with smaller turtles present at the upper reach. To resolve the issue of the two variables explaining the same variance I only included body mass as an indicator variable in the general linear model.
I calculated the amount of time that each individual spent in aquatic versus terrestrial environments. This was done by adding the total number of days a turtle spent in each environment from June 2007 through its return to the water in spring 2008.
When a turtle was found in a new environment it was assumed to be its first day there, and if a turtle was found in the same habitat type on two consecutive relocations it was assumed to have remained in that habitat type between surveys. Exit date for each turtle was the survey date in which the turtle left the stream channel and began to permanently use the terrestrial environment (i.e., did not return to the water). Return date was the survey date in which an individual had returned to the river in the spring of 2008. I used a paired t-test to determine differences between exit date for the 2007 and 2008 season, and these data were analyzed separately to avoid pseudoreplication since the same turtles were used both years. I calculated the total amount of time each individual occupied terrestrial environment as the number of days between June 2007 and June 2008 on land.
I tested for the effects of the indicator variables (sex, site and mass) on the three response variables (exit date, return date and total time on land) using general linear models.
To examine the correlation between the timing of turtles leaving the water and changes in environmental variables, I compared weekly air and water temperatures to the number of turtles on land. The proportion of individuals on land at each survey date and
24 temperatures were plotted to describe seasonal terrestrial use in the context of changing ambient conditions. In addition, at the upper reach the proportion of turtles on land was compared to available aquatic habitats as represented by the average cross sectional area of stream. I also compared the proportion of turtles on land to streamflow at the upper and lower reaches.
I investigated differences in the distance individuals traveled on land to overwintering sites at both reaches. General linear models were used to test for the effects of sex, site and body mass on the response variable, distance of overwintering site to the water.
For the turtle size data I used the cut-off of 110 mm carapace length for inclusion in the analysis to be consistent with previous Western Pond Turtle research as there is evidence that individuals become sexually mature by this size (Bury and Germano 2008;
Lubcke and Wilson 2008). I used a two sample t-test to detect differences between average adult maximum carapace length and mass between the two reaches. I used a linear regression of maximum carapace length against weight and plastron length to investigate differences in overall body size between adults of the two populations. Data were stored and managed in Microsoft Excel and all data analyses were performed in
NCSS and MiniTab statistical software.
RESULTS
Of the 27 turtles that originally received transmitters in June 2007, 21 turtles were tracked for the entire 17 month duration of the study, 12 at the upper reach (Table 3) and nine at the lower reach (Table 4). Over the course of this study three turtles lost their transmitters, two turtles went missing due to either transmitter failure or predation, and there was a single mortality. Turtle #755 could not be located for the first month after he was fitted with a transmitter. Turtle #635 was added into the study in June 2008 to compensate for the decline in sample size at the upper reach. In 2007, 27 turtles were tracked during their summer aquatic and fall terrestrial migrations (14 upper and 13 lower reach) and in the summer of 2008 the sample size dropped to 22 (12 upper and 10 lower).
There were a total of 1,160 turtle relocations from June 19, 2007 to November 6, 2008.
Summer Aquatic Movements
There was no difference between average length of aquatic movements made by turtles at both sites between the two years ( P=0.66). However, linear aquatic home ranges were significantly larger in 2008 compares with the 2007 season (P= 0.04). This is most likely due to the fact that turtles were tracked for a longer period during the second field season. In 2008 aquatic tracking began upon turtle reentry in the spring (as early as March 23, 2008), therefore turtles were tracked on average two months longer than in the 2007 season.
25 26
Turtles at both sites had similar linear aquatic home range sizes and average
length of aquatic movements, with the effects of site being insignificant for both home
range ( P=0.141 and P=0.694) and average aquatic movement ( P=0.41 and P=0.14) for
2007 and 2008, respectively. A correlation matrix revealed site and weight were marginally correlated with one another (0.59), since turtles at the lower reach were larger.
I removed site from further analyses of the movement data to avoid multicolinearity in the data..
Male turtles made larger average movements than did females, with sex being significant in the general linear model for 2007 (P=0.02) and highly significant in 2008 season (P<0.001; Figure 6). In addition, mass was found to be statistically significant for
both years ( P=0.04 for 2007 and P=0.03 for 2008) with heavier turtles moving farther.
The interaction of sex and mass was not significant ( P=0.20 for 2007 and P=0.08 for
2008).
Males and heavier turtles also had larger linear aquatic home ranges. In 2007 and
2008, Sex ( P=0.036 and P<0.001; Figure 7) and mass ( P=0.028 and P=0.003) were
statistically significant. Furthermore, analysis investigating for interaction was
statistically significant ( P=0.03) in 2008, suggesting that the effect of mass on linear
aquatic home range was greater for males than females.
27
Table 3. Summary of radiotracking data for turtles at the upper reach used in the general linear models. Number of relocations is the number of times a turtle was found throughout the entire study. Exit dates and return date are given as day of calendar year. Total days on land is the number of days spent in terrestrial habitat from June 2007 through June 2008. Means, standard deviations and standard error are given for each of the 3 variables that were response variables in the general linear models.
Turtle Number of Exit Date Return Exit Date Total Days Total Days identification # relocations 2007 Date 2008 2008 on Land tracked Males 735 46 212 83 193 237 496 776 46 231 83 222 218 496 246 46 220 83 222 229 496 696 46 231 83 235 218 496 405 46 237 83 211 237 496 575 46 206 83 206 243 496 308 41 206 103 200 263 475 635 17 NA NA 310 NA 153
Average (+SD) 220 (12.8) 86 (7.6) 224 (36.9) 235 (15.7) Standard Error 4.8 2.9 13.1 5.9
Females 368 46 220 103 222 249 496 345 45 231 103 230 252 496 677 36 237 111 NA 240 466 517 46 212 111 222 271 496 558 46 220 83 222 229 496 286 27 206 90 NA 257 496 226 25 220 83 NA 229 320
Average(+SD) 221 (10.5) 98 (12.3) 221 (7.8) 248 (15.3) Standard Error 4.0 4.6 3.9 5.8 Average, males and females 219 (10.4) 92 (11.6) 225 (29.5) 241 (16.1) Standard Error 2.8 3.1 8.5 4.3
28
Table 4. Summary of radiotracking data for turtles at the lower reach used in the general linear model. Number of relocations is the number of times a turtle was found throughout the entire study. Exit dates and return date are given as day of calendar year and total days on land is the number of days spent in terrestrial habitat from June 2007 through June 2008. Means, standard deviations and standard error are given for each of the 3 variables that were response variables in the general linear models.
Number of Exit Date Return Exit Date Total Days Total Days relocations 2007 Date 2008 2008 on Land tracked Males 617 46 334 110 NA 121 424 755 46 294 68 310 146 464 538 46 294 82 286 154 495 715 46 294 110 286 182 495 386 46 211 110 NA 287 424 485 46 279 82 286 169 495
Average (+SD) 284 (40.4) 93.7 (18.6) 292 (12.0) 176.5 (58.0) Standard Error 16.5 7.6 6.0 23.7
Females 445 47 294 125 256 197 495 326 47 230 110 243 251 495 466 42 279 125 NA 243 451 658 47 279 135 256 222 495 427 47 230 135 256 310 495 266 48 294 110 310 195 505 596 47 294 135 256 213 495
Average(+SD) 268 (29.1) 120 (11.2) 250 (23.7) 230 (40.0) Standard Error 11.0 4.2 9.7 15.1 Average, males and females 277 (33.8) 109 (22.5) 275 (24.2) 207 (55.3) Standard Error 9.4 6.2 7.7 15.3
29
A.
800 Lower Reach 743 750 700 Male 650 621 600 Female 550 500 472 450 400 341 350 297 300 263 Legnth of of Legnth 250 224 224 200 168
Movement (meters) Movement 150 105 92 86 96 100 60 85 81 65 44 56 57 60 49 50 33 9 20 25 0
Turtle ID#
B.
350 Upper Reach 333
300
250 223
200 186 188 179 177 163 163 150 142 Length of Length 119 102
Movement (meters) Movement 100 85 87 87 79 71 70 66
50 29 36 29 24 27 12 11 10 0
Turtle ID#
Figure 6. Bar graph of average length of aquatic movement for each turtle tracked at the lower (A) and upper (B) reach. Average length of movement was calculated as the total length of aquatic movement divided by the number of relocations. Solid bars indicate the 2007 field season and dashed bars refer to the 2008 field season. Males had greater average length of movement for both field seasons, there was no difference between sites.
30
A.
3500 Lower Reach 3113
3000 Male 2883 2807 Female 2512 2500 2317
2000 1737
1500 1239
(meters) 1000 716 713 609 601 604 606 320 339 500 282 333 214 278 268 204 172
Linear Aquatic Home Range Home Aquatic Linear 109 20 114 32 0
Turtle ID# B. 1400 Upper Reach 1298 1289 1271
1200 1043 1009 1000
800 642 612 600 (meters)
409 400 333 368 347 276 258 248
Linear Aquatic Home Range Range Home Aquatic Linear 199 178 200 120 135 94 65 91 42 80 52 53 34 0
Turtle ID#
Figure 7. Linear aquatic home ranges for each turtle at the lower (A) and upper (B) reach. Linear aquatic home ranges are defined as the distance of stream between the two farthest relocations. Solid bars indicate 2007 field season and dashed bars refer to the 2008 field season. Males had larger aquatic home ranges than females for both years and there was no difference between sites.
31
Migrations to Upland Habitat and Terrestrial Habitat Use
Turtles at the upper reach exited the river to migrate to upland environment earlier
(Table 3 and Figure 8b) than the turtles at the lower reach (Table 4 and Figure 8a). The average exit date for turtles at the upper reach was August 8 (day of year 219) in 2007 and August 12 (day of year 225) in 2008. In comparison, average exit date for lower reach turtles was October 3 (277) in 2007 and October 1 (275) in 2008. Site was a significant explanatory variable for exit date in both years ( P<0.0001 and P<0.001) whereas sex was not (p=0.54 and p=0.48); this model for exit date had an r-squared of
59.11%. Exit date did not differ between years as shown by the pairwise t-test ( P<0.001) for either site.
The turtles at the upper reach exited the river in mid-summer coincident with decreasing surface water. This is illustrated by the inverse relationship between the proportion of individuals on land and the decrease in available surface water (Figure 9).
The relationship between turtles leaving the water, and water and air temperatures
(Figure 10B) indicates that turtles at the upper reach had already exited the river before a seasonal decrease in ambient temperatures. In 2007, all 14 (100%) of the turtles at the upper reach had left the water by August 25. In 2008, 11 out of the 12 (92%) turtles had left the water by August 22; whereas a significant decrease in ambient temperatures
(more than -3˚C from the summer average water and air temperatures) did not occur until mid-September. Turtles began to permanently leave the water on July 17, 2007 when cross sectional area of the upper reach had declined to 3.9m2, and on July 11, 2008 when water levels began to decline at 6.0m2. At this point, the upper reach had become
32 reduced to small pools with intermittent stretches of exposed gravel bar (Figure 3c).
Although turtles began to depart the river one week earlier in 2008, an increase in turtle departures from the river coincides with the timing of severe water declines in both years.
The similar response in both years to drying conditions suggests a threshold in the amount of water available at which the aquatic habitats are no longer suitable and abandoned.
Turtles responded to channel drying at the upper reach by estivating in the adjacent upland environment. In 2007 all 14 turtles engaged in terrestrial estivation at the onset of drying, and 11 out of 12 estivated in 2008. In 2008 a single male (turtle #635) remained in a shallow remnant pool until November 6, 2008, upon which he migrated upland to overwinter. Estivation behavior involved turtles burying themselves beneath the substrate, which usually consisted of organic debris (humus) or silt, or covering their carapace with leaf litter (Figures 11 and 12A and B). Estivation sites were characterized as having ground vegetation cover (height to first branch <1 m, Figure 12C) consisting of dense, ground cover type shrubs (Figure 12D). During estivation turtles only made small terrestrial movements averaging less than 10 meters, to change sites and occasionally expose their carapace for basking.
Water returned to the river at the upper reach in late October with the onset of fall rains in both years (Figure 13). Despite the return of water, all of the estivating turtles went directly into hibernation both years, with none of the turtles returning to the stream channel. Most turtles only moved a few meters to overwintering sites in the late fall from estivation sites, although one male (turtle #735) moved approximately 200 meters to reach its overwintering site in both years.
33
A.
Lower Reach Aquatic Terrestrial 266(F) 658(F) Potential nesting event 596(F) 427(F) 466(F) 326(F) 445(F) 617(M) Turtle ID# Turtle 755(M) 715(M) 386(M) 485(M) 538(M) Jul-07 Jul-08 Jan-08 Jun-07 Jun-08 Oct-07 Oct-08 Feb-08 Apr-08 Sep-07 Sep-08 Dec-07 Aug-07 Aug-08 Nov-07 Nov-08 Mar-08 May-08
Date
B.
Upper Reach 368 (F) 345 (F) 677 (F) 517 (F) 226 (F) 286 (F) 558 (F) 405 (M) 575 (M) Turtle ID# Turtle 696 (M) 246 (M) 308 (M) 776 (M) 735 (M) 635 (M) Jul-08 Jul-07 Jan-08 Jun-08 Jun-07 Oct-08 Oct-07 Feb-08 Sep-08 Apr-08 Sep-07 Dec-07 Aug-08 Aug-07 Nov-08 Nov-07 Mar-08 May-08 Date
Figure 8. Timelines representing the duration individual turtles spent in each environment (aquatic or terrestrial), calculated as the number of days. Turtles at the upper reach (B) spent a significantly longer time on land than those of the lower reach (A; 241 versus 207 days).
34
A.
B.
Figure 9. Line graph showing the inverse relationship between turtles on land and declining water levels at the upper reach. Average cross sectional area (meters 2) of stream within a 400 meter survey area at the upper reach for each survey date is on the primary y-axis. On the secondary y-axis is the proportion of turtles on land for each survey date at the upper reach. Turtles began permanently exiting the stream when wetted cross sectional area dropped below 3.9 meters 2 on July 17, 2007 (A) and 6.1 meters 2 on July 11, 2008 (B).
35
A.
Lower reach Turtles on land Air Temperature 1.1 26 Water Temperature 1 24 22 0.9 20 0.8 18 0.7 16
on land land on 0.6 14 0.5 12
Proportion of turtles of Proportion 0.4 10
8 (°C) Temperature 0.3 6 0.2 4 0.1 2 0 0 July-08 July-07 May-08 June-08 June-07 April-08 March-08 August-08 August-07 January-08 October-08 October-07 February-08 December-07 November-08 November-07 September-08 September-07 Date
B.
Upper reach Turtles on land Air temperature
1.1 Water temperature 26 1.0 24 0.9 22 20 0.8 18 0.7 16 0.6 14 0.5 12 on land on 0.4 10 8 0.3 6 ºC) ( Temperature Proportion of turtles turtles of Proportion 0.2 4 0.1 2 0.0 0 July-07 July-08 May-08 June-07 June-08 April-08 March-08 August-07 August-08 January-08 October-07 October-08 February-08 December-07 November-07 November-08 September-07 September-08 Date
Figure 10. Scatterplots showing the relationship between proportion of individuals on land (primary y- axis) and survey date (x-axis) and weekly average air and water temperatures (secondary y-axis). Turtles at the lower reach (A) exited the water in late fall as water and air temperatures declined. Turtles at the upper reach (B) had already exited the water before a decline in temperatures.
36
A.
B.
Figure 11. Photographs of turtles estivating at the upper reach. A female, on July 29, 2007 (a) with the substrate (about 3 cm. deep) pushed aside for photographing purposes. This shows the organic substrate turtles tended to bury themselves in and the abundant vegetative cover aestivation sites possessed. A male on July 20, 2007 (b) with most of his carapace exposed. This photograph shows the turtle is partially covered under sand substrate and beneath abundant ground cover vegetation.
37
A. B.
70 63 70 65 58 2007 2007 60 60 2008 52 2008 50 50
40 40 26 30 23 23 30 23 20 20 16 Estivation sites (%) sites Estivation
Estivation sites (%) sites Estivation 12 8 6 6 6 10 3 4 10 4 0 01 0 1 0 0 Leaf Litter Substrate Downed Vegetation Open Woody Debris Cover Type Substrate Type
C. D.
100 94 80 2007 90 69 90 2007 70 2008 80 2008 60 70 50 60 40 36 50 29 30 40 19
Estivation sites (%) sites Estivation 20 30 11 Estivation sites (%) sites Estivation 9 7 20 10 4 4 3 5 4 9 1 0 10 4 2 1 0 0 Ground Cover Understory Overstory
Height to first branch of vegetation Dominant vegetation
E.
80 2007 69 70 2008 58 60 50 40 30 21 23
Estivation sites (%) sites Estivation 20 16 9 10 1 3 0 0-49 50-99 100-149 150-199 Distance to river (meters)
Figure 12. Frequency of dominant microhabitat characteristics of estivation sites at the upper reach. (A) Cover type (object turtle was found beneath), (B) substrate type (organic =humus or plant material; silt=0.004-0.06 mm grain diameter; sand=0.06-2 mm; gravel=2-64 mm; cobble=64-256 mm;boulder >256 mm), (C) height to first branch (ground<1 m; understory=1-5 m; overstory>5 m), (D) dominant vegetation type (based upon dominant species in 1 meter 2 plot) and (E) distance from the river for estivation sites. 2007 sites (n=112) represented with light gray shading and 2008 sites (n=100) represented by dark gray.
38
The results from the lower reach show a combination of seasonal changes cue turtle migrations. Turtles appear to respond to decreasing air and water temperatures, and slight increases in discharge in early fall, by migrating from the river to upland habitats.
The proportion of turtles on land increased as ambient temperatures decreased, as indicated by the inverse relationship between turtles on land and the two temperatures
(Figure 10A). In addition, increases in stream discharge may also have influenced some turtle migrations. In 2007 between September 8 and October 21 70% of turtles migrated from the river. During this time the weekly water temperature dropped from 5˚C, air temperature dropped 10.1˚C, and streamflow increased by 21 cfs (Figure 13). A quick change in streamflow occurred between September 18 and September 25, 2008 (16 cfs), and three turtles departed the river. During the week of June 26, 2007 streamflow raised unexpectedly from 51 cfs to 77 cfs and one turtle departed the river. In 2008 an increase in streamflow occurred earlier; streamflow rose 24 cfs between August 5 and August 26, during this time two turtles (18%) departed the river. The majority of the migrations
(60%) occurred between September 7 and October 25, when water temperature decreased from 16.4˚C to 15.5˚C and air temperature dropped 8.0˚C. It appears that streamflow was not a major factor in most migrations, but may influence turtle migrations in conjunction with decreasing air and water temperatures. All 14 turtles were out of the water by
November 30, 2007, and all 13 turtles were out by November 6, 2008.
There were no differences between sites in the distance traveled on land from the river to overwintering locations (Figure 14). The effects of sex ( P=0.40 and 0.43) and body mass ( P=0.76 and 0.76 ) were nonsignificant in both 2007 and 2008. There was no
39 significant difference in the distance traveled to overwintering sites between years. The average distance of overwintering sites from the stream was 101 meters in 2007 and 119 meters in 2008 for turtles at the upper reach. At the lower reach the average distance to overwintering sites was 91 meters in 2007 and 90 meters in 2008. The same two males in both years traveled the longest distance to overwintering sites. Turtle #735 at the upper reach moved 263 meters in 2007 and 269 meters in 2008, and turtle #715 traveled 214 meters in 2007 and 213 meters in 2008.
Turtles displayed site fidelity to overwintering sites by returning to the same area in consecutive years at both reaches (Figure 15). Thirty percent of turtles at the lower reach and forty five percent at the upper reach returned to within 10 meters of their 2007 overwintering sites in 2008. Increasing the buffer to 20 meters around the 2007 overwintering sites, showed seventy percent of turtles at the lower reach, and sixty four percent at the upper reach returned to the same area of the previous year. Many of the turtles returned to the same exact cover object (i.e., shrub, downed log) as the previous year.
40
950 900 Upper reach 850 800 750 Lower reach 700 650 600 550 500 450 400 Discharge (cfs) Discharge 350 300 250 200 150 100 50 0 July-08 July-07 May-08 June-08 June-07 April-08 March-08 August-08 August-07 January-08 October-08 October-07 February-08 December-07 November-08 November-07 September-08 September-07
Date
Figure 13. Mean weekly stream discharge (cubic feet per second) from the upper reach (dashed line) and lower reach (solid line) on the Mad River. Streamflow from May through October is essentially nonexistent at the upper reach (0-18 cfs). The black arrows indicate dates when water returned to the upper reach with the onset of fall precipitation in both years. At the lower reach streamflow remains steady in the summer (between 50cfs and 60cfs). Dotted black arrows indicate the dates when there was 16 cfs increase in weekly streamflow in 2007, upon which only one turtle exited the river, and 24 cfs in 2008, when two turtles departed the river. Data were only available through September 30, 2008 at the lower reach.
41
A.
Lower reach
250 Male Female 214 213 200
153 151 150 143 (meters) 129 106 104 100
Distance from river from Distance 100 86 84 77 71 70 69 61 57 62 50 47 47 45 37 27
0
Turtle ID#
B.
Upper reach
300 269 263 250
193 200 183
139 142 150 133 133 (meters) 124 129 100 91 Distance from river from Distance 77 75 58 54 63 58 44 50 34 38 37 36 35 40 35 23
0
Turtle ID#
Figure 14. Distance that the turtles overwintered from the river at the lower reach (A) and the upper reach (B). None of the explanatory variables (site, sex, or weight) were significant in the general linear model and the turtles generally traveled the same distance both years.
42
Figure 15 Turtles displayed site fidelity in their selection of overwintering sites by returning to the same areas for this activity in consecutive years. Seventy percent of turtles at the lower reach overwintered within 20 meters of their 2007 overwintering site in 2008.
43
Migrations to the River From Terrestrial Habitats
Turtles at the upper reach (Table 3 and Figure 10B) migrated back to the river earlier than those at the lower reach (Table 4 and Figure 10A). In addition, males returned to the water earlier than females at both sites (Figure 8). Both Sex ( P<0.001) and Site ( P=0.007) were significant explanatory variables in the general linear model for
Return Date with an R-Square of 53.3%. Many turtles made several stops at alternate water bodies, such as small backwater pools, during their migration back to the river.
Between March 23 and late April, 2008 there were 17 occasions in which turtles took temporary residence in backwater pools. All 14 turtles at the upper reach were back in the water by April 20, 2008, whereas at the lower reach all 13 turtles were back in the river May 13, 2008 (Figures 8 and 10). The turtles at the upper reach (Table 3) had less dispersion in their Return Date (Figure 8) compared to the lower reach (Table 4) as shown by a smaller standard deviation (11.6 and 22.5, respectively).
Many of the males at both reaches made long distance movements upon re- entering the river. At the lower reach, several males moved long distances in early April
2008; turtle #485 moved 2030 meters upstream and turtle #386 traveled 1742 meters downstream. Several turtles at the upper reach made long distance movements upstream in the early spring to the confluence of several tributaries with the main channel. For example, one male traveled as far as 1035 meters (#246) in two weeks to this area.
During April 2008, several male turtles (#246, #776, #696) were observed in this confluence area;all traveling greater than 600 meters to get there. On April 12 th 2008,
44 eight turtles without transmitters were observed in this area within two separate pools, five of which were observed foraging on rocks under water.
Turtles at the upper reach (Table 3 and Figure 8B) spent significantly more time on land than those at the lower reach (Table 4 and Figure 8A, P=0.021; 241 days versus
207 days). There was greater individual variability in the amount of time turtles spent on land at the lower reach as seen by the greater standard deviation (Table 4). Two turtles had extremely long terrestrial occupancy; turtle #427 (female) spent a total of 310 days on land, and turtle #386 (male) spent 287 days on land. In addition, females spent more time in terrestrial habitats than did males ( P=0.028). This is because female turtles make summer terrestrial excursions for nesting. Although no actual nests were observed during this study efforts were made to avoid disturbing the females during these possible nesting excursions (Figure 8).
Turtle Carapace Size and Mass
During the 2007 visual encounter dive surveys there were 18 adults (7 female, 9 male and 2 unknown) captured at the lower reach and 29 adults (14 female, 12 male and
3 unknown) captured at the upper reach. Since the original objective of these dives were to capture turtles for inclusion in the radiotelemetry study these data were only analyzed to investigate size differences between a sample of adults from each population. This preliminary analysis showed that turtles at the lower reach were significantly larger
(maximum carapace length; P<0.001) and heavier (mass; P<0.001) than those at the upper reach. The average maximum carapace length of adult turtles at the upper reach
45 was 131.5 mm compared to 148.3 mm at the lower reach (Figure 16A). The average weight of turtles at the upper reach was 334.5 g compared to 495 g average weight of turtles at the lower reach (Figure 16B). There was a greater range of sizes for both maximum carapace length and weight found for turtles at the lower reach as shown by a larger range and greater standard deviation.
In 2008 there were 71 turtles caught at the upper reach (31 females, 14 males and
26 unknown) and 23 (12 females, eight males and three unknown) at the lower reach.
Fifty seven percent of the turtles captured at the upper reach were subadults (not sexually mature), whereas only adults were caught at the lower reach (Figure 17). The greater numbers at the upper reach are likely attributed to differences in survey time. At both sites there was no difference in maximum carapace length or weight between adult (>110 mm) males and females. At the lower reach females had an average maximum carapace length of 142.1 mm compared to males with 151.8 mm ( P=0.24), and at the upper reach females averaged 123.1 mm and males 127.3 mm ( P=0.22). In addition, there was no difference in weight between males and females at the upper reach (females 276.9 g and males 292.5 g; P=0.48) or the lower reach (females 467.0 g and males 496.7 g; P=0.73).
46
A.
180
170
160
150
140
130
carapace (mm) length Maximum 120
110
LOWER UPPER B.
800
700
600
500
Mass (g) 400
300
200
LOWER UPPER Figure 16. Boxplots showing the median and range of maximum carapace length (mm) (A) and mass (g) (B) for adult turtles during initial dives in 2007 at the two study reaches on the Mad River, California. Both maximum carapace length ( P<0.001) and weight ( P<0.001) were significantly different from each other.
47
Adult turtles at the upper reach were significantly smaller than those of the lower
reach in regards to maximum carapace length ( P<0.001) and mass ( P<0.001). The average maximum carapace length was 124.6 mm for adult turtles at the upper reach compared to 143.7 mm at the lower reach (Figure 18A). The average mass of adults was
281.2 grams at the upper reach compared to 452.0 grams at the lower reach (Figure 18B).
There is more dispersion in the data for both variables at the lower reach as shown by the wider range in values. Both sites showed a strong positive linear relationship between maximum carapace and plastron length and carapace length and body mass (Figure 19).
48
A.
35 Lower reach Unknown Female 30 Male 25
20
15
10
5
Percent of count (%) count of Percent 0
Maximum carapace length (mm)
B.
Upper reach Unknown 18 Female 16 Male 14 12 10 8 6 4 Percent of count (%) count of Percent 2 0
Maximum carapace length (mm)
Figure 17. Histograms of maximum carapace length (mm) frequencies for all turtles captured during the summer of 2008 at the lower reach (A) and upper reach (B). Turtle of unknown sex were those that did not possess secondary sexual characteristics and were classified as subadults.
49
A.
170
160
150
140
130
120 Maximum carapace (mm) length Maximum
110
Lower Upper
B
700
600
500
400 Weight (g) Weight 300
200
100 Lower Upper
Figure 18. Boxplots displaying descriptive statistics for maximum carapace length (mm) (A) and mass (g) (B) for adult turtles at two sites on the Mad River, California from dives conducted in 2008. Both maximum carapace length (p<0.001) and weight (p<0.001) were significantly different from one another.
50
A.
170 Lower reach
160 Upper reach R² = 0.8956 150
140
130 R² = 0.9136 120
110
Maximum carapace length (mm) length carapace Maximum 90 100 110 120 130 140 150 160 170 Plastron length (mm)
B.
180 Lower reach 170 R² = 0.8618 Upper reach 160
150
140
130
120 R² = 0.8676
110 Maximum carapace length (mm) length carapace Maximum
Weight (grams)
Figure 19. Relationship of plastron length (A) and mass (B) to maximum carapace length at two sites on the Mad River, California. Adult turtles at the upper reach (yellow diamonds) have overall smaller body size compared to those of the lower reach (red triangles) as seen by their placement in the lower left corner of the scatterplot.
DISCUSSION
This study showed that while hydrologic flow did not appear to affect the extent of aquatic movements made by turtles, it did influence the timing of migration to terrestrial environment. More specifically, it showed that turtles at the intermittent study reach migrated from the river to adjacent upland habitats and estivated in response to declining water levels. In the perennial reach, turtles did not migrate from the river until the onset of declining air and water temperatures associated with changes in season. The variation in migratory patterns demonstrates the plasticity in Western Pond Turtle life history and their ability to withstand unfavorable conditions. Furthermore, the difference in size distributions of adult turtles from both reaches suggests a morphological population response to the extreme conditions associated with intermittent hydrology, with turtles being significantly smaller at the upper reach.
Previous research on Western Pond Turtles has shown that home range size may be influenced by hydrologic flow. A study in southern California found that female turtles have significantly longer linear home ranges in an undammed intermittent river compared to those in a dammed perennial river (1273.0 meters compared to 335.2 meters, respectively; Goodman and Stewart 2000). Goodman and Stewart (2000) claim that linear home ranges may be influenced by water availability, and that the turtles in the undammed intermittent river must travel greater distances to cover the same amount of aquatic area. In contrast, my results showed that differences in hydrologic flow had no influence on the distances traveled by turtles in the watercourse. The difference in results
51 52 between my study and that of Goodman and Stewart (2000) could be that their study was limited to females and did not include weight as a covariate. In addition, the Mad River is in a very different climatic zone than their warmer site in southern California. The site in Southern California experiences less extreme seasonal fluctuations in air and water temperatures, which may affect turtle movements.
Home range sizes at the perennial lower reach were larger than those at the upper reach only when site was investigated separately in a two sample t-test, and the difference was significant only for the 2007 data. This is most likely due to the fact that turtles were tracked for a shorter period in 2007. Home range sizes in 2008 were larger at both sites, but much more so at the upper reach. In 2007 tracking did not begin until late June, whereas in 2008, turtles were tracked from the moment they returned to the water in the early spring. As a result I likely missed many long distance movements in 2007 that I observed turtles make in the early spring 2008, resulting in smaller home ranges for 2007.
It appears the 2008 dataset gives a more complete picture of the movements the turtles make within the watercourse.
My study showed that males had larger home range sizes and greater average length of aquatic movements than did females at both sites. These results are similar to those of previous research on Western Pond Turtle movements in northern California.
Males have greater home ranges (1 hectare) compared to those of females (0.3 hectares) and juveniles (0.4 hectares) at Hayfork Creek in northern California (Bury 1972). On the mainstem of the Trinity River, males have average linear home ranges of 1244 meters compared to 598 meters for females (Reese 1996). In addition, other studies have shown
53 males to move on average further and more frequently than females, with males moving up to 188 meters per day on average, in a river in northern California (Holland 1994).
My sampling intervals were longer (weekly) so it is difficult to make straightforward comparisons, but my results are consistent with previous work in that males had greater average length of movements (222 meters in 2007 and 206 meters in 2008) than females
(67 and 49 meters, respectively). In addition, some turtles made extensive movements within the watercourse in a relatively short amount of time. Turtle #485 (male) made large movements greater than 2 km in one week on eight separate occasions. Seven
(54%) of the other males (three from the lower reach and four from the upper reach) made movements greater than 800 meters between weekly surveys at both reaches.
These findings are in accordance with those of Bury (1972), in which one marked turtle moved 1.5 km in two weeks.
The tendency for males to have larger home ranges and average movements than females is a common trend seen in other turtle species that is attributed to mate seeking
(Gibbons 1990). While reproductive success in females is limited by the number of clutches that can be laid in a single season, reproductive success of males is limited by the number of encounters with females. Long distance movements are energetically costly and increase predation risk. Consequently, it is unfavorable for females to make extensive movements since they would not benefit from multiple mating events. In contrast, males are maximizing their mating potential by increasing their chances of encounters with females (Gibbons et al. 1990).
54
When drying conditions arrive turtles may respond with different strategies, such as converging within remnant pools, relocating to alternate water sources or engaging in terrestrial estivation (Ligon and Stone 2003). Studies in South Carolina showed that two species of Pseudemys (P. scripta and P. floridana ) emigrated from a drying pond during severe drought and the majority of animals leaving the pond moved in the direction of the nearest body of water up to 600 meters away (Gibbons et al. 1983). Painted Turtles
(Chrysemys picta ) and Red-eared Sliders ( Pseudemys scripta) have been reported to leave unsuitable ephemeral habitat, whereas Snapping Turtles ( Chelydra serpentina ) remained buried in the mud of the same drying lake (Cagle 1944). Many species such a
Spotted Turtles ( Clemmys guttata ), Mud Turtles ( Kinosternon sp.), and Painted Turtles will estivate either within the same habitat patch or in adjacent terrestrial habitat and wait for the water to return (Gibbons et al. 1983; Ligon and Stone 2003; Milam and Melvin
2001).
The turtles at the upper reach responded to channel drying by migrating to upland habitat and estivating for the remainder of the summer. These results indicate that this is the dominant strategy in this population of Western Pond Turtles, although a small amount of intra-populational variance may exist (as seen by turtle #635 that remained in a remnant body of water in 2008). Other studies have suggested Western Pond Turtles will estivate in response to drying conditions, although intra-populational variability existed in response (Reese 1996; Goodman 1997). The majority of Western Pond Turtles in an intermittent arroyo in southern California remained in remnant wet parts of the creek when other portions of the creek had completely dried up. Only one turtle out of the
55
twelve monitored moved onto land to estivate, showing that there exists variation in
individuals’ response to drying conditions (Goodman 1997). Two out of 15 turtles
departed from aquatic habitat to estivate when drying occurred in a vernal pool in Santa
Rosa, California (Reese 1996). Whereas these studies show considerable individual
variation in response to drying conditions whereas the majority of turtles at the upper
reach (1005 in 2007 and 92% in 2008) of the Mad River chose to estivate.
The differences between my results and those of previous studies could be due
differences in the rate at which the water drops at different localities. This may influence
the response of Western Pond Turtles to seasonal drying, as seen in other species. The
Chicken Turtle ( Deirochelys reticularia ) did not respond to gradual pond drying during a drought in South Carolina, but emigrated from a pond that underwent a rapid decline in water levels during experimental pond drying (Gibbons 1990). The upper reach of the
Mad River exhibited a fairly rapid decline in water levels whereby wetted area of river channel decreased by half over the course of as little as four weeks. Turtles began emigrating from the river at the upper reach when the mean cross sectional area of stream began rapidly declining in mid-July in both years indicating a threshold at which the aquatic habitats are no longer suitable and a cue for migrations. At this point the river had been reduced to a series of disconnected pools each approximately 2-3 meters 2 in
surface area and distributed about 300-500 meters apart. The water in these remaining
pools was stagnant, very warm (>20.0ºC) with abundant algae, and was possibly anoxic.
For turtles to congregate in these remnant pools or make overland movements to
reach them would be disadvantageous since they would not likely find adequate
56 supporting food resources. High temperatures and nutrient buildup from lack of flow cause algae blooms, resulting in an anoxic environment that many species of aquatic invertebrates cannot tolerate (Larimore et al. 1959; Towns 1991). The combination of these factors cause changes in food availability by altering aquatic invertebrate abundance and diversity; these invertebrates are a dominant source of prey for Western
Pond Turtles (Bury 1986). In addition, the high abundance of turtles within the pools would create a highly competitive environment and increase vulnerability to predation by terrestrial vertebrates such as birds and raccoons. Predator densities may also increase with the formation of pools since they are attracted to the high densities of prey (Lake
2003). Making overland movements to remaining pools would increase the risk of predation and desiccation, and would have a high energy cost. The resources turtles could gain at these remnant pools would be less than the energy saved by estivating (Roe et al. 2008). While more research is needed to investigate the physiological consequences of remaining in these remnant pools, my results suggest that estivation is the more energetically favorable response to seasonal drying when alternate suitable aquatic habitat is not available. Perhaps turtles would expend the energy to migrate to alternate bodies of water if they contained enough food resources and refugia for turtles to benefit.
My study showed that turtles require upland habitats to estivate usually within 50 meters from the river but may change position in the fall up to 263 meters. Turtles selected estivation sites beneath abundant ground cover, which would provide protection from terrestrial predators at a time when turtles are most vulnerable. In addition turtles
57 burrowed beneath the substrate which would provide refuge from hot temperatures (air temperatures reached up to 102˚F in mid-summer at the upper reach when turtles were estivating) and minimizes evaporative water loss. It is necessary to maintain accessible terrestrial habitats for Western Pond Turtle populations that estivate to maximize individual survival during this physiologically demanding time.
Migration to terrestrial habitat at the lower reach did not occur until late fall and coincided with declining ambient temperatures and increases in streamflow. These results are consistent with the migration behaviors of other populations of Western Pond
Turtles in northern California, in which turtles depart the water in late October (Holland
1994). Turtles at the lower reach of the Mad River move somewhat later than Trinity
River populations that leave by early September (Reese and Welsh 1997). The authors state that turtles emigrated from the river in response to high flows associated with the fall and winter (Reese and Welsh 1997). While increases in flows may contribute to departures from the river, my results suggest that decreasing air and water temperature played the major role in triggering turtle migrations at the lower reach, since all the turtles had left the water by the times the flows increased significantly. Photoperiod may also be a cue for emigrations from the river (H. Welsh, personal communication). This may well influence turtle departures since a few turtles began departing the river in early
September before temperatures began dropping but when days began to get shorter.
Turtles spent a significant part of the year in terrestrial habitat at both reaches, but those at the upper reach were on land considerably longer due to their earlier departure from the water. Turtles spent on average 241 days on land at the upper reach versus 207
58 days at the lower reach. Females spent more time on land annually than males at both reaches, due to frequent summer terrestrial visits associated with nesting, and later return dates to the river in the spring. These results are similar to those of the Trinity River turtles which spent on average 239 days (females) and 168 days (males) on land during overwintering (Reese 1996). In contrast, central California turtles spent on average 111 days in terrestrial refugia with no difference between sexes (Rathbun 2002). This difference is most likely due to habitat and climatic differences.
There were no differences in the distance that turtles traveled on land to overwintering between the upper and lower reach or between sexes. These results concur with those of the mainstem (below dam) and Southfork (undammed) Trinity River turtles, where there was no difference between the sexes or between the two sites with regards to the distances traveled to overwintering sites (Reese 1996). There were two males that overwintered as far as 213 and 269 meters from the river, at the lower and upper reach, respectively. Turtles overwintered on average 95 meters from the river at both sites.
Turtles showed high site fidelity with many returning to the same areas (within less than
20 meters) in consecutive years.
Both within and between population variation existed for spring migrations back to the river. Males returned to the river earlier than females, and turtles at the upper reach returned to the water earlier than those of the lower reach. In the mainstem Trinity
River, males also returned to the river earlier than females (Reese 1996). In the Mad
River many males made long distance movements upon reentering the river. Many of these long distance movements may be associated with increased foraging opportunities
59
or mate seeking by males early in the season. At the upper reach, the early return of
males and females to aquatic habitats may be to maximize foraging time. These turtles
forage exclusively in aquatic environment (Bury 1972) and since water declines as early
as late June it would be beneficial for these turtles to return to the river as early as
possible and maximize foraging time.
Turtle Body Sizes
Many turtle species display phenotypic plasticity in that interpopulation variation
exists for body size, growth rates and sexual dimorphism. Differences in morphology
between geographically separated conspecifics have been attributed to a variety of biotic
and abiotic factors. For example, substrate type influences growth rates in Map Turtles
(Graptemys pseudographica ouachitensis ) via food availability (Moll 1976) and food quality increases growth rate and overall body size of Painted Turtles (Chysemys picta ;
Gibbons 1967; Gibbons and Tinkle 1969). Thermal characteristics of the environment also play a significant role in growth by regulating metabolic processes of ectotherms
(Congdon 1989) indicated by turtles being smaller in colder environments (Reese 1996).
My research suggests that channel drying imposes a decrease in growth as shown by significantly smaller adult turtles at the upper reach. This finding agrees with Gibbons’
(1990) research showing that shortened activity period negatively affects the growth rate of Slider Turtles ( Trachemys scripta ). In the Mad River, due to seasonal drying of the upper reach, foraging time is limited to a few months in the spring and early summer.
These turtles were in the water an average of 125 days out of one year, shorter than that
60 found with Trinity River populations (Reese and Welsh 1997). In addition, the upper reach population spends an extended period in a resting, or dormant state of either estivation (in the summer) or hibernation (in the winter) in which metabolic depression occurs (Congdon 1989).
Western Pond Turtles exhibit variation in carapace length (Appendix B) and body condition over their range. Adult turtles at a perennial site (average CL 132.5 mm) are smaller, than conspecifics at an intermittent stream (average CL 140.4 mm) in southern
California (Goodman 1997). The author attributes this to cooler temperatures and differences in prey availability. At three sites in northern California, significantly smaller turtles were found in lotic environments with cooler water and sandy substrates compared to those in slow moving habitats with warmer water and organic substrate (Lubcke and
Wilson 2007). The authors suggested that an increase in prey availability at sites with organic substrate, as well as warmer temperatures and more abundant basking sites, may contribute to larger body size.
My research provides another example of environmental factors that result in body size differences between conspecific populations of Western Pond Turtles. The upper and lower reach experienced the same air temperature and precipitation. Water temperature at the lower reach was significantly colder than at the upper reach, yet turtles at the upper reach were significantly smaller, indicating that seasonal drying is the likely cause of reduced body size. During estivation, turtles engage in metabolic depression
(Guppy and Withers 1999), increases in plasma osmolality and starvation (Peterson and
Stone 2000). This prolonged period of reduced metabolic activity and fasting likely
61
affects the growth of this population, causing turtles to be smaller and potentially
influencing their survivorship. Although a direct cause and effect relationship cannot be
determined without controlled experiments, this research provides evidence that this may
occur in natural systems. To further solidify this hypothesis a more thorough
investigation of the Mad River populations’ demographics is needed.
Conservation Implications
This research showed that seasonal drying influences the movement patterns of this species by eliciting an emigration response. There appears to be a threshold limit in which aquatic habitats become no longer suitable and are abandoned in favor of estivation in upland habitat. These results have important management implications because this species is abundant in the rivers and ponds of northern California. Excessive pond draining for agricultural purposes, water diversions and intermittent flows created by dams, can all result in a depletion of aquatic resources that can render conditions unsuitable for resident turtles. Land managers should evaluate individual populations’ response to decreasing water. It should be recognized that the ability to estivate for extended periods is most likely a population-specific response due to genetics or local physiological adaptations, and drying could have severe negative consequences on local populations. It has been suggested that an extreme drying event that occurred in southern
California may have been responsible for an 85% reduction in Western Pond Turtle populations during the 1987-1992 drought (Holland 1994). Research is needed to further investigate the physiological capacity of Western Pond Turtles to estivate for extended
62 periods of time to determine to what extent other populations can survive this stressor. In addition, my results were adult biased, and research is needed to investigate the response of subadults and hatchlings to drying. Smaller turtles may be more susceptible to this stressor because they have fewer energy reserves for extended estivation.
For populations that migrate upland to estivate, such as the turtles at the upper reach of the Mad River, land managers need to be aware of their occupancy in terrestrial habitat within 50 meters and up to over 250 meters from the river. Terrestrial zones adjacent to the river need to provide suitable microhabitats to accommodate the physiological challenges of estivation and provide protection. Microhabitats characterized by abundant ground cover and soft substrate composed of organic or silt material in which turtles can burrow a few centimeters need to be accessible.
Disturbances to ground cover vegetation or destabilization of these soft substrates along the river from activities such as cattle grazing, off road vehicle use and brush removal, need to be restricted during times when turtles are estivating.
It is important to retain a high level of connectivity between the river and adjacent upland habitats for semiaquatic turtle species (Roe and Georges 2007). My research and that of others have demonstrated that Western Pond Turtles have a high affinity for terrestrial habitats for various life history activities. The timing and extent to which individuals use adjacent terrestrial habitats seems to be population specific, as evidenced by the variable results between studies. Migrations to terrestrial environment are triggered by water and air temperatures, seasonal high flows, and seasonal drying in intermittent systems. Turtles at both reaches on the Mad River made long distance
63 movements to upland habitats for overwintering (average 90 meters, but as far as 263 meters). Maintaining migration corridors, as much as 300 meters from the river, is necessary for managing both the upper and lower reach populations on the Mad River.
The construction of roads or loss of habitat within migration routes may disrupt turtles’ movements to a habitat patch that is necessary for estivation and overwintering.
Finally, turtles inhabiting an intermittent river system have significantly smaller body size than those residing in a perennial system. These findings have important conservation implications since it demonstrates that drying can not only alter Western
Pond Turtle life history, but consequently alter population morphology as well. Seasonal drying is a natural phenomenon in this system, so this population has evolved with the selective pressures it presents. Anthropogenic induced water loss due to water diversions, dams, and irrigation, may effectively alter life history and body size of local populations over time. Smaller body size could have negative effects on survival and reproductive output in populations that are not adapted to drying conditions. Previous research has shown that smaller females lay smaller clutch sizes (Goodman 1997;
Holland 1994), thus a reduction in body size may negatively affect reproductive output.
Western Pond Turtles occupy a variety of aquatic habitats across a wide range of climatic zones, and posses a variable life history. Consequently, management strategies must be developed based upon assessments of individual populations. Further investigations are needed on other populations that experience seasonal drying to evaluate population specific responses. I also recommend further research investigating the physiological consequences associated with prolonged estivation that may cause
64 decreases in body condition. The effects that human introduced drying may have on the life history and population demographics need to be taken into consideration during the development of watershed management policy for rivers that have known populations of
Western Pond Turtles.
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APPENDIX A
Mean depth, mean width and the cross sectional area for each transect on survey date. Stream habitat inventory was performed at the upper reach per survey between June and September 2007 and 2008.
June 18, 2007 June 27, 2007 July 3, 2007 July 10, 2007 July 17, 2007 Transect Mean Mean Sectional Mean Mean Sectional Mean Mean Sectional Mean Mean Sectional Mean Mean Sectional Depth Width Area Depth Width Area Depth Width Area Depth Width Area Depth Width Area 0.7 20.1 14.1 0.6 18.9 11.3 0.5 16.6 8.3 0.5 13.3 6.7 0.6 10.1 6.1 12 0.8 21.6 17.3 0.7 21.7 15.2 0.7 21.5 15.1 0.6 21.1 12.7 0.6 19.8 11.9 42 0.5 27.1 13.6 0.6 27.1 16.3 0.5 27.3 13.7 0.4 26.8 10.7 0.3 25.9 7.8 72 0.6 31.0 18.6 0.5 32.3 16.2 0.5 30.3 15.2 0.4 29.8 11.9 0.4 21.5 8.6 102 0.4 26.7 10.7 0.4 26.1 10.4 0.3 25.3 7.6 0.4 15.5 6.2 0.3 17.9 5.4 132 0.2 19.9 4.0 0.3 16.2 4.9 0.3 15.2 4.6 0.1 2.5 0.3 0.3 5.3 1.6 162 0.4 22.6 9.0 0.3 18.7 5.6 0.4 16.2 6.5 0.2 12.0 2.4 0.3 7.7 2.3 192 0.4 20.4 8.2 0.3 21.2 6.4 0.4 18.6 7.4 0.3 16.7 5.0 0.1 10.2 1.0 222 0.2 21.4 4.3 0.1 16.7 1.7 0.2 20.8 4.2 0.1 11.9 1.2 0.0 0.0 0.0 252 0.1 16.2 1.6 0.0 0.0 0.0 0.1 10.6 1.1 0.1 5.4 0.5 0.0 0.0 0.0 282 0.1 5.4 0.5 0.3 5.2 1.6 0.1 8.2 0.8 0.1 4.0 0.4 0.0 0.0 0.0 312 0.2 9.2 1.8 0.2 9.6 1.9 0.2 9.7 1.9 0.1 7.8 0.8 0.1 8.4 0.8 342 0.2 9.4 1.9 0.2 9.8 2.0 0.1 9.5 1.0 0.1 6.1 0.6 0.1 3.1 0.3 372 0.6 17.5 10.5 0.5 18.5 9.3 0.5 18.7 9.4 0.4 18.6 7.4 0.5 17.8 8.9 402 Average 0.4 19.2 8.4 0.4 18.6 7.3 0.3 17.7 6.9 0.3 13.7 4.8 0.3 10.5 3.8 ∆ last NA NA NA 0.0 0.6 1.1 0.1 0.9 0.4 0.1 4.0 2.1 0.0 3.2 1.0 survey
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APPENDIX A (Continued)
July 25, 2007 July 31, 2007 August 8, 2007 August 19, 2007 August 25, 2007 Transect Mean Mean Sectional Mean Mean Sectional Mean Mean Sectional Mean Mean Sectional Mean Mean Secti onal Depth Width Area Depth Width Area Depth Width Area Depth Width Area Depth Width Area 0.6 9.6 5.8 0.5 8.5 4.3 0.3 5.8 1.7 0.2 2.9 0.6 0.1 2.9 0.4 12 0.6 20.2 12.1 0.5 15.4 7.7 0.3 11.1 3.3 0.2 7.0 1.4 0.2 1.7 0.3 42 0.3 25.8 7.7 0.1 14.2 1.4 0.0 0.0 0.0 0.0 0.0 0.0 0.0 0.0 0.0 72 0.4 20.4 8.2 0.2 11.3 2.3 0.2 3.0 0.6 0.0 0.0 0.0 0.0 0.0 0.0 102 0.3 12.1 3.6 0.2 8.8 1.8 0.0 0.0 0.0 0.0 0.0 0.0 0.0 0.0 0.0 132 0.2 5.4 1.1 0.1 3.4 0.3 0.0 0.0 0.0 0.0 0.0 0.0 0.0 0.0 0.0 162 0.2 6.4 1.3 0.0 0.0 0.0 0.0 0.0 0.0 0.0 0.0 0.0 0.0 0.0 0.0 192 0.0 0.0 0.0 0.0 0.0 0.0 0.0 0.0 0.0 0.0 0.0 0.0 0.0 0.0 0.0 222 0.0 0.0 0.0 0.0 0.0 0.0 0.0 0.0 0.0 0.0 0.0 0.0 0.0 0.0 0.0 252 0.0 0.0 0.0 0.0 0.0 0.0 0.0 0.0 0.0 0.0 0.0 0.0 0.0 0.0 0.0 282 0.0 0.0 0.0 0.0 0.0 0.0 0.0 0.0 0.0 0.0 0.0 0.0 0.0 0.0 0.0 312 0.1 3.7 0.4 0.1 3.5 0.4 0.0 0.0 0.0 0.0 0.0 0.0 0.0 0.0 0.0 342 0.0 0.0 0.0 0.0 0.0 0.0 0.0 0.0 0.0 0.0 0.0 0.0 0.0 0.0 0.0 372 0.5 17.9 9.0 0.4 16.7 6.7 0.2 11.1 2.2 0.1 0.2 0.0 0.0 0.0 0.0 402 Average 0.2 8.7 3.5 0.2 5.8 1.8 0.1 2.2 0.6 0.0 0.7 0.1 0.0 0.3 0.1 ∆ last 0.1 1.8 0.3 0.0 2.9 1.7 0.1 3.6 1.2 0.1 1.5 0.5 0.0 0.4 0.0 survey
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APPENDIX A (Continued)
June 5, 2008 June 12, 2008 June 20, 2008 July 3, 2008 July 11, 2008 Transect Mean Mean Sectional Mean Mean Sectional Mean Mean Sectional Mean Mean Sectional Mean Mean Sectional Depth Width Area Depth Width Area Depth Width Area Depth Width Area Depth Width Area 0.8 22.6 18.1 0.7 23.4 15.7 0.6 22.3 13.4 0.4 22.1 9.4 0.6 22.2 13.3 12 0.8 22.7 18.2 0.8 23.1 18.4 0.8 22.5 18.0 0.8 22.8 17.2 0.7 22.5 15.8 42 0.5 28.1 14.1 0.5 28.3 13.6 0.5 28.0 14.0 0.4 27.7 12.0 0.4 27.1 10.8 72 0.5 31.2 15.6 0.4 30.4 11.5 0.6 31.0 18.6 0.4 30.2 12.3 0.3 27.3 8.2 102 0.4 28.5 11.4 0.3 28.7 9.5 0.4 26.0 10.4 0.4 24.7 9.9 0.5 14.2 7.1 132 0.4 24.8 9.9 0.3 24.1 8.4 0.5 13.2 6.6 0.4 12.9 5.0 0.4 8.5 3.4 162 0.3 22.0 6.6 0.4 23.8 10.1 0.3 19.8 5.9 0.3 18.1 5.7 0.3 10.5 3.2 192 0.3 20.9 6.3 0.4 20.3 7.4 0.4 19.0 7.6 0.2 21.7 5.2 0.4 17.6 7.0 222 0.2 30.8 6.2 0.2 28.4 6.3 0.2 21.5 4.3 0.1 29.7 2.7 0.2 17.0 3.4 252 0.1 27.1 2.7 0.1 24.6 3.1 0.2 23.0 4.6 0.1 0.4 0.0 0.1 4.6 0.5 282 0.2 10.8 2.2 0.2 9.0 2.1 0.2 6.8 1.4 0.2 11.0 1.9 0.2 3.1 0.6 312 0.3 11.6 3.5 0.3 10.2 2.8 0.2 9.3 1.9 0.2 9.4 1.4 0.2 9.5 1.9 342 0.2 11.0 2.2 0.2 10.7 2.4 0.2 10.8 2.2 0.2 11.9 2.8 0.1 8.5 0.9 372 0.5 19.1 9.6 0.5 19.3 10.4 0.5 18.4 9.2 0.4 19.0 8.3 0.5 17.9 9.0 402 Average 0.4 22.2 9.0 0.4 21.7 8.7 0.4 19.4 8.4 0.3 18.7 6.7 0.4 15.0 6.1
∆ last NA NA NA 0.0 0.5 0.3 0.0 2.3 0.3 0.1 0.7 1.7 0.1 3.7 0.6 survey
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APPENDIX A (Continued)
July 18, 2008 July 24, 2008 July 29, 2008 August 10, 2008 August 17, 2008 Transect Mean Mean Sectional Mean Mean Sectional Mean Mean Sectional Mean Mean Sectional Mean Mean Sectional Depth Width Area Depth Width Area Depth Width Area Depth Width Area Depth Width Area 0.5 19.5 9.8 0.4 18.3 7.3 0.4 13.5 5.4 0.2 6.3 1.5 0.2 3.5 0.7 12 0.6 22.4 13.4 0.6 21.5 12.9 0.4 20.1 7.6 0.2 9.1 2.2 0.1 5.9 0.6 42 0.3 26.8 8.0 0.3 22.7 6.8 0.1 16.2 2.3 0.0 0.0 0 0.0 0.0 0.0 72 0.4 25.1 10.0 0.3 18.5 5.6 0.2 26.7 5.0 0.0 0.0 0 0.0 0.0 0.0 102 0.3 13.0 3.9 0.3 10.7 3.2 0.1 2.7 0.2 0.0 0.0 0 0.0 0.0 0.0 132 0.3 6.7 2.0 0.1 4.6 0.5 0.0 2.3 0.1 0.0 0.0 0 0.0 0.0 0.0 162 0.3 8.3 2.5 0.0 0.0 0.0 0.0 0.0 0 0.0 0.0 0 0.0 0.0 0.0 192 0.2 12.9 2.6 0.0 0.0 0.0 0.0 0.0 0 0.0 0.0 0 0.0 0.0 0.0 222 0.1 4.7 0.5 0.0 0.0 0.0 0.0 0.0 0 0.0 0.0 0 0.0 0.0 0.0 252 0.0 0.0 0.0 0.0 0.0 0.0 0.0 0.0 0 0.0 0.0 0 0.0 0.0 0.0 282 0.0 0.0 0.0 0.0 0.0 0.0 0.0 0.0 0 0.0 0.0 0 0.0 0.0 0.0 312 0.2 8.3 1.7 0.2 6.5 1.3 0.1 2.5 0.2 0.0 0.0 0 0.0 0.0 0.0 342 0.1 4.1 0.4 0.0 0.0 0.0 0.0 0.0 0 0.0 0.0 0 0.0 0.0 0.0 372 0.4 17.8 7.1 0.3 15.4 4.6 0.2 13.3 3.3 0.2 4.9 0.8 0.1 0.9 0.1 402 Average 0.3 12.1 4.4 0.2 8.4 3.0 0.1 7.0 1.7 0.0 1.5 0.3 0.0 0.7 0.1 ∆ last 0.1 2.9 1.7 0.1 3.7 1.4 0.1 1.4 1.3 0.1 5.5 1.4 0.0 0.8 0.2 survey
APPENDIX B
Summary of Western Pond Turtle adult maximum carapace length from various locations throughout its range in California. Location Maximum Carapace Source Length Mad River, upper reach, NC average 124.6 mm, +8.6 This study
Mad River, lower reach, NC average 143.7 mm, +17.9 This study
Russian River, NC average 169.4 mm, +14.3 Cook and Martini-Lamb, 172.6 mm, + 17.5 (Male), 2004 165.4 + 6.2 (female) Aliso Creek, Chino Hills State Park, SC average 140.4 mm +12.1 Goodman, 1997 West Fork San Gabriel River, SC average 132.6 mm + 13.2 Goodman, 1997
Sacramento River (Phelan Island), NC 176.0 mm +2.06 Lubcke, 2008 Big Chico Creek, NC 150.2 mm +0.67 Lubcke, 2008 Howard Slough Unit, NC 185.1 mm +1.41 Lubcke, 2008 137 mm, +1.1(male), Lovich and Meyer, 2002 Mojave River, SC 144 mm , +0.8 (female)
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