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2004 Landscape Vs. Local Habitat Scale Influences To Communities From Tallgrass Prairie Remnants Kristal J.L. Stoner Nebraska Game and Parks Commission, [email protected]

Anthony Joern Kansas State University, [email protected]

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Stoner, Kristal J.L. and Joern, Anthony, "Landscape Vs. Local Habitat Scale Influences To Insect Communities From Tallgrass Prairie Remnants" (2004). Faculty Publications in the Biological Sciences. 422. https://digitalcommons.unl.edu/bioscifacpub/422

This Article is brought to you for free and open access by the Papers in the Biological Sciences at DigitalCommons@University of Nebraska - Lincoln. It has been accepted for inclusion in Faculty Publications in the Biological Sciences by an authorized administrator of DigitalCommons@University of Nebraska - Lincoln. Ecological Applications, 14(5), 2004, pp. 1306±1320 ᭧ 2004 by the Ecological Society of America

LANDSCAPE VS. LOCAL HABITAT SCALE INFLUENCES TO INSECT COMMUNITIES FROM TALLGRASS PRAIRIE REMNANTS

KRISTAL J. L. STONER1 AND ANTHONY JOERN2 School of Biological Sciences, University of Nebraska, 348 Manter Hall, Lincoln, Nebraska 68588 USA

Abstract. Habitat loss and fragmentation currently threaten ecosystems worldwide, yet remain dif®cult to quantify because within-fragment habitat and landscape-scale in¯uences often interact in unique ways. Although individual respond to fragmentation dif- ferently, large-scale conservation planning must unavoidably target multiple species. Al- though information on a population's response to fragmentation is critical, and measure- ments of species richness provide useful insights, exclusive reliance on these responses may mask important information about the taxonomic composition of assemblages in re- sponse to fragmentation. The North American tallgrass prairie ecosystem is one of the most threatened and fragmented ecosystems in the world, and are signi®cant contributors to its . In remaining grassland fragments, we evaluated within-fragment in¯u- ences in conjunction with landscape-scale responses of representative insect communities from four feeding guilds: generalists, specialists, multiple life stage habitat use, and pred- ators. Fragment-speci®c attributes capable of in¯uencing insect diversity include plant species composition, plant biomass, abundance of blooming ¯owers, and vertical habitat heterogeneity created by the vegetation. Landscape-scale factors expected to in¯uence pat- terns of insect species diversity include fragment size and shape as well as the spatial con®guration of fragments. Ordination techniques were used to summarize composition of each feeding guild assemblage of each fragment, and structural equation modeling was used to examine the direct and indirect effects of fragmentation with in¯uences from local habitats. Generalists (), mixed-modality feeding that changes with life stage (), and specialist herbivores () all responded directly to within- site characterizations of the plant community. Site management from large ungulate grazing or mowing for hay production consistently had an indirect effect on the insect community through in¯uences on plant community composition. The predator assemblage (Coccinel- lidae) was in¯uenced directly by fragment shape. To maintain insect biodiversity in tallgrass prairie fragments, these results indicate that conservation practices should focus on com- munities in order to maintain insect biodiversity in tallgrass prairie fragments. Landscape- scale factors must also be considered when making conservation decisions, primarily be- cause predators (top trophic level organisms) are more likely to respond to regional changes. Key words: conservation; Coccinellidae; Curculionidae; fragmentation; insect com- munities; landscapes; Lepidoptera; LISREL algorithm; Orthoptera; structural equation modeling; tallgrass prairie.

INTRODUCTION of fragmentation are often evaluated using species-spe- Habitat loss, fragmentation, and modi®cation are ci®c responses (Kareiva 1987, Robinson et al. 1992, major anthropogenic processes affecting ecosystems at Banks 1999, Bowers and Dooley 1999) at small plot a global level (Saunders et al. 1991, Kruess and scales, or as a community response quanti®ed using Tscharntke 1994, Forman 1995, Turner et al. 2001). species richness (Panzer and Schwartz 1998, Kruess The consequences of fragmentation and community dy- and Tscharntke 2000; K. J. L. Stoner and A. Joern, namics in response to fragmentation are dif®cult to unpublished manuscript). Although species responses assess because of the inherent complexity caused by to fragmentation can be evaluated for individual spe- cies at small plot scales, it is increasingly evident that biotic and abiotic interactions at multiple scales many management and restoration decisions must fo- (McGarigal and Cushman 2002). As a result, the effects cus on landscape scales at the community level for Manuscript received 18 April 2003; revised 3 November practical reasons (U.S. Fish and Wildlife Service 1994, 2003; accepted 8 November 2003; ®nal version received 6 Jan- Mazerolle and Villard 1999). uary 2004. Corresponding Editor: J. A. Logan. The number of species in an area is indisputably 1 Present address: Nebraska's Natural Legacy Project, Ne- important, but the presence or absence of particular braska Game and Parks Commission, 2200 North 33rd Street, species or overall taxonomic composition of a com- Lincoln, Nebraska 68503 USA. 2 Corresponding author. Present address: Division of Bi- munity can often be an equally important conservation ology, Kansas State University, Manhattan, Kansas 66506 goal. Community species composition may change in USA. accordance with the degree of fragmentation, indepen- 1306 October 2004 COMMUNITY COMPOSITION IN FRAGMENTS 1307 dent of the number of species. For example, the amount ni®cantly affected by landscape-scale factors that in- of edge habitat relative to the amount of fragment in- hibit dispersal. In a simulation exercise, With and Crist terior can in¯uence the density of ``invasive'' or ``edge (1995) found that specialists were less likely than gen- specialist species'' vs. ``interior species'' (Yahner eralists to aggregate at resources and continue random 1988, Forman 1995). searching. Trophic level also plays a role as somewhat The effects of fragmentation on communities are dif- different quantitative responses can be expected. Pred- ®cult to assess as the local habitat within a site may ators may respond to landscape-scale factors based on be in¯uenced by geographic attributes of a fragmented the ``trophic level susceptibility to fragmentation'' hy- landscape (area, shape, spatial arrangement of frag- pothesis (Kruess and Tscharntke 1994; K. J. L. Stoner ments; Fahrig and Merriam 1985, Fahrig and Paloh- and A. Joern, unpublished manuscript). This hypoth- eimo 1988, Robinson et al. 1992, Holt et al. 1995, esis suggests that as local extinction occurs in a frag- Leach and Givnish 1996, Van Dorp et al. 1997, Zschok- ment, re-establishment of prey populations is a pre- ke et al. 2000). In addition, responses to fragmentation requisite for the re-establishment of predator popula- at speci®c trophic levels may impact responses at other tions, making predators more susceptible to fragmen- levels as biotic interactions are transferred through a tation. Recolonization and establishment will depend food chain. For example, the local habitat for consum- heavily on regional factors such as fragment area, ers may be altered as the plant species community is amount of edge, and isolation of a fragment from other affected by fragmentation, perhaps because of altered populations (Kareiva 1987, Kruess and Tscharntke seed dispersal from patch isolation, disturbance re- 1994, With and Crist 1995). Interestingly, while land- gimes, species±area relations, and land management scape context can often be a signi®cant predictor of the practices. This interaction of landscape-scale factors presence and abundance for vertebrate species, this is and local habitat dynamics presents a signi®cant chal- less so for invertebrates (Maserolle and Villard 1999); lenge for determining which factors in¯uence changes local patch characteristics had large in¯uences in most in the resident consumer community (Mazerolle and cases. Villard 1999), with important implications for conser- We examined responses of four representative insect vation planning. In particular, a large number of pos- feeding guilds to fragmentation in remnant grassland sible direct and indirect effects must be uncovered as patches in tallgrass prairie habitat (Table 1). The North a basis for understanding the underlying dynamics be- American tallgrass prairie is currently one of the most tween landscape-scale factors and local attributes. threatened ecosystems in the world (Packard and Mutel 1997); the eastern third of the Great Plains was his- Insect communities torically comprised of extensive, continuous tallgrass Insects dominate terrestrial ecosystems worldwide prairie, but has been reduced to ϳ1% of former land (Pyle 1981, Fisher 1998), such that their diversity and cover (Samson and Knopf 1994). are im- abundance makes them an ideal model for investigating portant to grassland communities, contributing signif- habitat fragmentation and modi®cation (Kremen et al. icantly to their biodiversity (Arenz and Joern 1996, 1993, Panzer and Schwartz 1998, Allan et al. 2003, Kaufman et al. 1998, Panzer and Schwartz 1998). Collinge et al. 2003), and thus they are often used as The Orthoptera (grasshoppers, katydids, and crick- indicators of fragment restoration success (Burger et ets) are generally oligophagous to polyphagous with a al. 2003). Habitat fragmentation is known to affect ar- small number of species that are host specialists (Joern thropods differently depending upon the feeding guild 1979, Bernays and Chapman 1994), but as a group they are associated with and whether they specialize typically select a wide range of food resources (grasses, on few host species or can utilize many food sources forbs, and litter) and can consume large quantities of (Jonsen and Fahrig 1997, Golden and Crist 1999, leaf material. Dispersal is somewhat limited and long- Kruess and Tscharntke 2000, Davies et al. 2001). Gen- range movement may not occur if feeding and mating eralist feeding species are more likely to ®nd suf®cient needs are met within a small area (Capinera and Sechr- resources within a fragment compared to specialists ist 1982, Joern 1983, Johnson et al. 1986, Narisu et al. and are also more likely to utilize corridors that could 1999). We expected that the generalist feeding Or- provide nourishment and additional habitat (Forman thoptera community would respond directly to plant 1995, Haddad 1999, Collinge 2000). Changes in local community changes and amount of food available, and habitat and food sources are then expected to have a indirectly to regional factors of area, shape, and neigh- greater impact on generalist feeders compared to land- boring fragments as these regional factors in¯uenced scape-scale factors. Local food sources often have a the local plant community (Fig. 1A). Management strong, direct impact on herbivores in fragmented stud- practices would also directly in¯uence the plant com- ies compared with regional, geographical factors munity and thus indirectly the Orthoptera. (Kruess and Tscharntke 1994, Kindvall 1996, Dubbert Ladybird (Coccinellidae) prey primarily on et al. 1998, Panzer and Schwartz 1998, Collinge et al. aphids (Borror et al. 1989) and respond directly to frag- 2003). Specialists may need to move between frag- ment isolation in experiments (Kareiva 1987, Banks ments to acquire speci®c resources, and could be sig- 1999). These predators represent an upper trophic level 1308 KRISTAL J. L. STONER AND ANTHONY JOERN Ecological Applications Vol. 14, No. 5

TABLE 1. Species included in community composition analysis.

Lepidoptera Orthoptera Acontia aprica Arphia conspersa Arphia sp. Artogeia rapae Chorthippus curtipennis Atryone logan logan Conocephalus sp. Basilarchia archippus archippus Dichromorpha viridis Caenurgina erechtea Hesperotettix speciosus Celastrina argiolus ladon Hippiscus ocelote Cercyonis pegala Hypochlora alba Charidryas gorgone carlota Melanoplus bivittatus Charidryas nycteis nycteis Melanoplus differentialis Colias eurytheme Melanoplus femurrubrum Colias philodice philodice Mermiria bivitatta Cyenia tenera Neoconocephalus ensiger Danaus plexippus Neomembius variegatus Estigmene acrea Oecanthus sp. Everes comyntas comyntas Orphylella speciosa Haematopis grataria Phoetaliotes nebrascensis Haploa reversa Scudderia texensis Hemiargus isola alce Syrbula admirabilis Junonia coenia Curculionidae Libytheana bachmanii bachmanii Megisto cymela cymela Anthonomus molochinus Ostrinia nubilaslis Aphrastus taeniatus Papilio polyxenes asterius Apion melanarium Phoebis sennae eubule Ceutorhynchus sp. Pholisora catullus Epicaerus imbricatus Phyciodes tharos tharos Geraeus Polites coras Haplorhynchites aeneus Polites themistocles Pantamorus tesselatus Pontia protodice Pseudobaris farcta Pterourus glaucus glaucus Sitona cylindricolis Pyrgus communis Smicronyx lineolatus Pyrisitia lisa lisa Schinia jaguarina Coccinellidae Speyeria cybele cybele Hyperaspis undulata Speyeria idalia Coccinella septempunctata Strymon melinus Coleomegilla maculata Vanessa atalanta rubria Hippodamia convergens Vanessa cardui Hippodamia parenthesis Hyperaspis bigeminata Pseudoscymnus tsugae Scymnus sp.

and are predicted to respond directly to landscape level them to readily move between fragments to assess re- factors (Fig. 1B). The resident aphid community would sources, therefore eliminating the regional effects of also in¯uence the ladybird beetles; we have no direct fragmentation. Despite their relatively high dispersal measurement of the aphid community in this study, but capability, individual Lepidoptera species are affected they are often common in both grasslands and neigh- by area, edge, and spatial arrangement of fragments boring crops. We predicted that ladybird beetles, as a (Fahrig and Paloheimo 1987, 1988, Brommer and Fred representative insect predator community, would be in- 1999, Schultz and Crone 2001). For example, results ¯uenced directly by regional factors of area, shape, and of available studies indicated that 60±97% of species neighboring fragments (Fig. 1B). Because of the great examined remain in their natal patch (Thomas and Han- reliance of the host specialist prey, ladybird beetles ski 1997, Hanski 1999), and many species disperse no would also be in¯uenced directly by the local plant more than a few kilometers (Hanski and Kuussaari community, which we predicted would be in¯uenced 1995, Hanski 1999, Haddad 1999). Because Lepidop- by management and regional factors. tera belong to different feeding guilds in nymphal and Lepidoptera are conspicuous and taxonomically well adult life stages, they could be affected by many in- known insects, which makes them ideal for a com- teracting factors at both the landscape scale (fragment munity study of fragmentation. This group represents area, juxtaposition of fragments) and the local scale a feeding guild that is in¯uenced by both the adult (blooming ¯ower community and larval plant species nectar sources of food and larval host plants (Gross- abundances) as predicted in Fig. 1C. mueller and Lederhouse 1987, Collinge et al. 2003). Curculionidae () are typically specialist feed- Lepidoptera have dispersal capabilities that could allow ers that often select one family, genus, or even species October 2004 COMMUNITY COMPOSITION IN FRAGMENTS 1309

FIG. 1. Initial hypotheses of ``causal relationships'' for each insect consumer group. The variable ``area/shape 1 & 2'' represents both principal component scores for area and shape. ``Blooming ¯ower community'' represents both blooming ¯ower density and diversity. ``Management level'' indicates hay, moderate grazing, or intense grazing. ``Neighboring frag- ments'' is the result of the proximity analysis. ``Plant'' and all ``insect communities'' (e.g., ``Orthoptera community,'' ``Coccinellidae community,'' etc.) are ordination scores. Each path diagram has representative ®t statistics in Table 2. of vascular plant. Different species often specialize on lated. Kruess and Tscharntke (1994) found signi®cant different parts of plants (e.g., roots, stems, or seeds; decreases in density of several curculionid species in Morris 1991). Most species have the potential response to habitat isolation. As specialists, this group to ¯y (fully developed wings), but rarely do so except may be responding directly to changes in the blooming in extremely hot weather (Borror et al. 1989, Morris ¯ower community, predators, local plant community, 1991). They are more likely to escape predators by proximity of neighboring fragments, area, and shape, dropping to the ground than by ¯ight (Borror et al. with many interacting indirect in¯uences (Fig. 1D). 1989). Limited dispersal tendencies coupled with food Currently, insect diversity in regional tallgrass rem- source specialization are characteristics that suggest nant prairie in an agricultural landscape such as eastern Curculionidae may be less tolerant of fragmented sys- Nebraska, USA, depends primarily on two remaining tems as speci®c resources become rare and more iso- types of habitat: grassland grazed by cattle and horses, 1310 KRISTAL J. L. STONER AND ANTHONY JOERN Ecological Applications Vol. 14, No. 5 and prairie managed for hay production through mow- this region, native prairie that has not been impacted ing. These two management practices produce different in some way by intensive row crop agriculture is rare. disturbance regimes, and both were sampled in this Prairie fragments were selected based on a survey of study. Sites included in this study rarely, if ever, em- native prairie sites conducted by the Lancaster County ployed ®re for management. We examined regional ef- Ecological Advisory Committee in 1985. Thirty-six fects of fragmentation on the composition of insect sites varying in size from 0.10 ha to 242.8 ha were species assemblies, including area, shape, and spatial included in this study; 19 were managed for hay pro- relationship of fragments, and then evaluated how these duction and 17 were grazed primarily by cattle. In Lan- factors contributed to explaining insect species assem- caster County, it is common practice for hay ®elds to blies compared with contributions from local biotic be mowed annually in early August (B. E. Anderson, characteristics of each individual fragment. Local fac- personal communication). For inclusion in this study, tors in this study include plant species composition, we required that fragments be isolated and contain sig- plant biomass, abundance and diversity of blooming ni®cant amounts of native vegetation. All sites also ¯owers, and vertical habitat heterogeneity created by included non-native plant species and some degree of the vegetation. The response of these communities is human impact. Sites were considered isolated if they evaluated for changes in the context of tallgrass prairie were separated from other prairie habitat by a road, fragmentation consisting of both regional geographic agriculture, or a continuous line of trees. Sites sup- and local biotic factors. porting intensive row crop agriculture were not studied as they contain limited numbers of host plants and pes- METHODS AND MATERIALS ticides are widely and often heavily used at these sites. We used ordination techniques coupled with struc- tural equation modeling to examine the direct and in- Regional factors direct effects of fragmentation and local habitat attri- Fragment area, shape, and spatial juxtaposition of butes on species assemblages of these four insect fragments can disrupt dispersal, affect colonization and groups in eastern Nebraska, USA. Ordination succinct- extinction rates and alter the community composition ly summarizes community data in which sites with sim- within remaining fragments (Forman 1995, With and ilar species compositions are closely situated within n- Crist 1995, Banks 1998). The area and perimeter of dimensional ordination space (Beals 1973, Pielou each fragment were estimated with ENVI 3.4 software 1984, Quinn and Walgenbach 1990, ter Braak 1995, based on GPS coordinates of each corner (Research Wardle et al. 1999). A community score is then cal- Systems, Boulder, Colorado, USA). Shape was char- culated for each fragment based on its location in or- acterized using a perimeter to area ratio of each frag- dination space. This score can then be used as a re- ment (Helzer and Jelinski 1999). Since the shape index sponse variable to determine which environmental var- is a ratio, the area and shape variables are highly cor- iables are important to changes in species composition related. Structural equation modeling using LISREL between sites. software allows relationships between independent var- Structural equation modeling, a general form of path iables to be speci®ed, although collinearity may have analysis, is a powerful method of extracting direct and serious impacts on stability of correlation coef®cients indirect effects (Johnson et al. 1991, Shipley 1999). (Maruyama 1998). We eliminated collinearity with Structural equation modeling supports rigorous hy- principal components analysis applied to area and pothesis testing as complex relationships are estab- shape (perimeter/area ratio); the ®rst two principal lished a priori between variables using a series of re- components scores were used to create new indices gression equations. One may incorporate both regional (Scheiner and Gurevitch 2001). The principal com- and local factors in a comprehensive assessment with ponent scores, named ``area/shape 1'' and ``area/shape this approach. The entire hypothesized model can be 2,'' were used in analyses that follow. analyzed simultaneously using computer algorithms Although each fragment may be isolated from other such as LISREL (Johnson et al. 1991, JoÈreskog and fragments, some insects may move between grassland SoÈrbom 1996, Grace and Pugesek 1998, Shipley 1999), fragments, requiring that spatial relationships be eval- and results are easily visualized using a path diagram. uated. Large fragments that support large populations Native vegetation of eastern Nebraska was histori- with high species diversity within dispersal distance cally comprised of extensive, continuous tallgrass prai- may strongly impact the insect community present in rie (Johnsgard 2001), a situation that no longer exists. an isolated fragment (Simberloff and Abele 1976, Most remaining tallgrass grassland is currently reduced Shmida and Wilson 1985, Rosenzweig 1995, With and to remnant fragments, largely due to varying degrees King 1999). To evaluate the possible impact of neigh- of human impact. Grassland fragments from eastern boring fragments within the matrix of a fragmented Nebraska, mostly within Lancaster County (Lancaster landscape, a principal components analysis was applied County, Nebraska; northeast corner of rectangular to two Landsat Thematic Mapper scenes (path 28, row county, 41Њ2Ј42Љ N, 96Њ27Ј50Љ W, southwest corner, 32) from 4 April and 26 August 1991. The resulting 40Њ31Ј25Љ N, 96Њ54Ј44Љ N) were used in this study. In 30-m resolution image (G. Henebry, unpublished man- October 2004 COMMUNITY COMPOSITION IN FRAGMENTS 1311 uscript) was displayed as red, green, and blue pixels, accumulation curve was constructed. The sampling ef- which are the sixth, ®rst, and third principal compo- fort required for a reliable estimate of species richness nents. This image was used to identify neighboring with ®ve transects was 125 sweeps (data not shown). grassland fragments within an 8-km radius of each of In our analysis, ®ve transects of 150 sweeps were made the 36 sampled fragments. This distance was selected at each site. Insects were frozen and later identi®ed to based on estimates of butter¯y dispersal (Hanski and genus and species. Lepidoptera were censused using Kuussaari 1995). The area of each of these neighboring Pollard visual transects 150 m long in which all species fragments and distance to the sampled fragment was seen5moneither side of the transect were recorded determined. The distance was measured from the (Pollard and Yates 1993). Five sites were smaller than neighboring patch edge to the approximate location of the transect area, so these areas were considered to be sampling within the remnant. A proximity analysis was exhaustively sampled. Insects were collected and cen- performed to obtain an index of the abundance and sused only on sunny days between 1000 and 1600 hours proximity of neighboring fragments (Gustafson and if the vegetation was dry and the wind speed was Ͼ15 Parker 1992). This index is the sum of all fragment mph (Ͼ24 km/h). areas of the neighboring fragments divided by the dis- tance to the sampled fragment. High values indicate Ordination greater connectivity. Principal components analysis (PCA) is a basic ei- genanalysis ordination technique that maximizes the Local factors variance explained by each successive orthogonal axis Habitat attributes of each fragment were character- (McCune and Mefford 1999). Prior to ordination using ized in several ways to assess the in¯uence of local PC-ORD, an outlier analysis was performed as extreme (within-fragment) biotic relationships. The plant com- values can have a profound in¯uence on multivariate munity was characterized by counting the number of analysis that leads to extreme non-normality. Sites with plant species in a 0.25-m2 rectangular frame at 30 lo- values more than two standard deviations away from cations in each fragment. Sampling locations were at the mean were considered outliers and removed from six randomly predetermined intervals along ®ve tran- further analysis. Different sites for each group of in- sects centrally located in each fragment. Sampling was sects were outliers. We used principal components completed over a period of 21 days in June 2001. The analysis to extract axes that summarized the taxonomic forb blooming ¯ower community, plant biomass, and composition of each representative insect community habitat structure were all measured in 17 days from the and plant community for each site. Site scores of the end of July to the beginning of August 2001 on the ®rst PCA axis summarized multivariate community same day insects were collected, and before mowing composition of insect assemblies as a single value that commenced. Each of these local factors (forb blooming was then treated as a response variable in structural ¯ower community, plant biomass, and habitat struc- equation modeling. Using ordination to summarize dif- ture) were measured at predetermined random distanc- ferent community assemblages by site is a powerful es along each of the ®ve insect sweep transects. Bloom- way to extract which environmental variables in¯uence ing ¯ower diversity and density were measured by the changes seen in the assemblage composition. The counting the number of blooming plants (forbs with at ®rst three PCA axes were examined as response vari- least one open bloom) within 10, 4 ϫ 4-m plots. Ten ables in structural equation models to determine how plant biomass samples clipped in 1.0 ϫ 0.10-m strips regional landscape factors, local factors, and manage- were randomly collected, dried, and weighed for each ment practices interact to explain the resulting insect fragment. Habitat structure was measured as the vari- species composition. Species composition of the plant ability in vegetation canopy height (or vertical hetero- community was also summarized using principal com- geneity). A meter stick was placed into the vegetation ponents analysis and the ®rst axis scores were used to perpendicular to the ground and the height of all veg- represent the variable ``plant community composition'' etation touching the meter stick was recorded at 10 in the structural equation modeling analysis. randomly selected points. The diversity of canopy heights (using a Shannon index) was employed as a Structural equation modeling measure of vertical structural heterogeneity. Structural equation modeling (SEM) is a hypothesis- testing approach to traditional multivariate analysis. An Survey techniques initial model represented visually as a path diagram Orthoptera (families , Gryllidae, and Tet- was formulated for each insect group based on putative tigoniidae), Coccinellidae, and Curculionidae were col- causal relationships based on a priori relationships ex- lected using a sweep net. Sweep net sampling is an pected from literature reports (Fig. 1). In the process ef®cient way to achieve estimates of relative abundance of SEM, initial model hypotheses should include all of foliage dwelling arthropods (Evans et al. 1983), and expected possible relationships. Anticipating that the is useful for conservation managers because of its rel- initial model hypothesis, which included all possible ative ease of use. Before beginning sampling, a species relationships (Mitchell 2001), would not generate the 1312 KRISTAL J. L. STONER AND ANTHONY JOERN Ecological Applications Vol. 14, No. 5

TABLE 2. LISREL ®t statistics for each hypothesis.

Chi-square RMSEA Hypothesis X2 P RMSEA P RMR CFI ECVI CAIC Initial Coccinellidae 1.84 0.17469 0.165 0.19 0.027 0.98 1.35 92.08 Curculionidae 21.38 0.01862 0.192 0.03 0.094 0.8 2.37 142.1 Lepidoptera 18.61 0.00094 0.338 0.0017 0.11 0.77 1.64 131.36 Orthoptera 9.94 0.12707 0.143 0.16 0.12 0.93 1.25 122.31 Alternative Curculionidae 0.79 0.67407 0.000 0.7 0.045 1.00 0.55 31.23 Lepidoptera 0.01 0.93261 0.000 0.94 0.0022 1.00 0.56 75.72 Orthoptera 0.22 0.63759 0.000 0.65 0.017 1.00 0.32 58.74 Notes: Boldface type indicates a good ®t. A nonsigni®cant low X2 with an associated high P value indicates a close ®t between the hypothesized model and a perfect ®t. Key to abbre- viations: RMSEA, root mean square error of approximation (RMSEA Ͻ0.05 with P Ͼ 0.50 indicates a good ®t); RMR, standardized root mean square residual (the mean of all standardized residual values; RMR Ͻ0.05 indicates a good ®t); CFI, comparative ®t index (accounts for sample size; a CFI value close to 1.0 is acceptable); ECVI, expected cross-validation index; CAIC, information criterion index (consistent version). CAIC and ECVI are used to compare models with similar ®ts with smaller values indicating greater potential for replication and a parsimonious model (indicated particularly with CAIC).

best explanation of the data many alternate hypotheses After selecting a model, the direct and indirect ef- were also formulated (Wootton 1994). These alterna- fects were evaluated. LISREL provides a standardized tive models are revisions of the initial hypothesis, as solution of direct and indirect effects where variables they were also grounded in theory, but contained select were rescaled to have a variance of 1 and all variables variables with relationships consistent with the predic- were then given a value between 0 and 1. This allows tions made for each insect group. Individual paths were comparison of the relative strength of each effect added or removed to generate a model with the best (Johnson et al. 1991). The correlation coef®cient was ®t, making it valuable for prediction purposes in insect also examined for each of the relationships speci®ed conservation. All variables were transformed to have in the model for amount of variance explained. a normal distribution using z scores and log transfor- mation prior to analysis. Maximum-likelihood esti- RESULTS mation of parameters on covariance matrices were used LISREL analysis for each insect consumer group in LISREL. yielded signi®cant yet different combinations of en- Multiple goodness of ®t statistics are available using vironmental variables that predicted changes in com- LISREL programming. Selection of a best ®t model munity composition. For each consumer group, the ini- can be greatly re®ned by using multiple ®t statistics tial model hypothesis contained all relationships and that analyze different aspects of the model, such as variables measured that were predicted from the lit- sample size and parsimony. Because our data included erature (Fig. 1). Alternate model hypotheses were gen- relatively few samples (36 sites), ®t statistics were se- erated for each consumer group that contained a subset lected that included sample size to determine which of variables, more consistent with speci®c predictions model has the best ®t (Table 2). The foremost test of for each group. For herbivorous taxa, generalist feed- ®t is the chi-square. A nonsigni®cant chi-square value ers, life-stage dependent mixed-modality feeders, and with an associated high P value is desired, as this in- feeding specialists were best explained by an accepted dicates close agreement between the hypothesized alternate model that identi®ed strong direct and indirect model and a perfect ®t (Bryne 1998). The root mean relationships. For each group, the ®rst PCA scores were square error of approximation (RMSEA) is currently used in path diagrams. The second and third PCA recognized as an important goodness-of-®t statistic in scores did not correlate with any environmental vari- model selection (Bryne 1998). This statistic is sensitive ables. Characteristics of the plant community and man- to the number of parameters selected in the model. agement were consistently seen in resulting modi®ed Values Ͻ0.08 represent an acceptable ®t and values path models. The variables ``neighboring fragments,'' Ͻ0.05 indicate a good ®t (Bryne 1998). A test for the ``area/shape 1,'' ``area/shape 2,'' and ``plant biomass'' closeness of ®t of the RMSEA is also included; P values were not retained in any accepted alternate model. Ͼ0.50 are suggested targets (JoÈreskog and SoÈrbom Shape, characterized as perimeter/area ratio, was the 1996). Other ®t statistics are included in model selec- only variable that signi®cantly correlated with the pred- tion (Table 2). ator community (represented as the ®rst PCA axis October 2004 COMMUNITY COMPOSITION IN FRAGMENTS 1313

FIG. 2. Numbers associated with each path arrow are the standardized direct effects of each variable. Numbers in parentheses below the var- iables are the total standardized effects (total of direct and indirect) of each variable on the in- sect community. (A) Grasshopper community path diagram. Plant community composition ex- plains 38.4% of the variation of the grasshopper community composition (P Ͻ 0.0001). Man- agement level indicates hay, moderate grazing, or intense grazing. (B) Lepidoptera community path diagram. Plant community composition and blooming ¯ower diversity explain 40% of the variation seen in the lepidoptera community (P ϭ 0.0002). Plant community composition and management level explain 29.6% of the var- iation in blooming ¯ower diversity (P ϭ 0.0031). Management level explains 54% of the variation in plant community composition (P Ͻ 0.0001). (C) Weevil community path diagram. Blooming ¯ower density and predator com- munity composition explain 24.1% (P ϭ 0.0158) of the variation in weevil community composition. Management level explains 8.1% (P ϭ 0.0923) of the variation in blooming ¯ow- er density.

scores). The plant community composition analysis ex- ing along axis 1. The tettigoniids Conocephalus spp. tracted 27.02% of the variance in the ®rst axis. and Neoconocephalus ensiger and the acridids Hes- perotettix speciosus and Dichromorpha viridis were all Orthoptera highly correlated with axis 1 (R ϭ 0.70, R ϭ 0.67, R The Orthoptera PCA analysis extracted 15.54, 11.41, ϭ 0.54, R ϭ 0.43), respectively (Fig. 3A). and 11.25% of the variance in the ®rst three axes, re- spectively, for a cumulative total of 38.20% of variance Coccinellidae explained. Although the initial structural equation modeling (SEM) model hypothesis of Orthoptera pro- The Coccinellidae PCA analysis extracted 26.34, duced a nonsigni®cant chi-square (at the 0.05 level), 20.96, and 16.15% of the variance in the ®rst three PC it was rejected based on root mean square error of axes, respectively, for a cumulative total of 63.45% approximation (RMSEA) and standardized root mean variance of Coccinellidae community explained by the square residual (Table 2). The alternative model for analysis. The initial LISREL model was rejected by Orthoptera ®t all criteria and had much lower values RMSEA and associated P value (Table 2). No alter- for both the ECVI (expected cross-validation index) native model had an acceptable ®t, nor did stepwise and CAIC (consistent information criterion index) regression retain any variables included in the initial comparison indices when compared to the initial hy- model hypothesis (␣ϭ0.05; results not shown). Other pothesis (Fig. 2A, Table 2). The standardized solution studies have shown that regional effects such as area of the direct and total effects for the alternative model and shape have signi®cant impacts on predators. More- indicated that plant community composition is strongly over, introduced coccinellid species for biocontrol of correlated with Orthoptera community composition. aphids (Hodek 1973) are rapidly moving across the Management had a strong direct effect on the plant United States. The natural history, biogeography, and community, and a corresponding indirect effect on the fragmentation experiments focused on this fam- Orthoptera community composition (Fig. 2A). Conclu- ily prompted additional simple regression analysis to sions were reinforced with stepwise regression analyses see if there were correlations between the original var- where plant community composition was the only var- iables of perimeter (often created by row crops) to area iable retained in the model of Orthoptera community ratio (shape) and area, each analyzed in a simple re- composition (␣ϭ0.05). gression. Coccinellid community scores were signi®- Sites scores appeared to segregate according to man- cantly correlated with shape (R ϭ 0.36, P ϭ 0.038; agement which is logical, as the plant community com- Fig. 4), but with much unexplained variation. Addi- position was the primary environmental factor chang- tional analyses using site scores from axis 1 as the 1314 KRISTAL J. L. STONER AND ANTHONY JOERN Ecological Applications Vol. 14, No. 5

composition and somewhat correlated with blooming ¯ower diversity (Fig. 2B). Plant community compo- sition had an additional indirect effect through its in- ¯uence on the blooming ¯ower diversity. In addition, management had an indirect effect on Lepidoptera through its large effect on plant community composi- tion and small effect on blooming ¯ower diversity. These results were consistent with stepwise multiple regression, where the variable ``plant species com- munity composition'' was retained in the Lepidoptera community model (␣ϭ0.05). Visual inspection of the graph of sites organized by Lepidoptera scores in ordination space indicated that grazed sites tended to cluster toward the right of the diagram and hayed sites were on the left (Fig. 3C). Distributions of four species were highly correlated with axis 1. The great spangled fritillary (Speyeria cy- bele cybele)(R ϭ 0.51) and the eastern tailed blue (Everes comyntas comyntas)(R ϭ 0.88) are found on the left side of the ordination diagram, the wood nymph (Cercyonis pegala)(R ϭ 0.70) is centrally located on the ordination graph and gorgone checkerspot (Char- idryas gorgone carlota)(R ϭ 0.50) is found on the far right side of the diagram.

Curculionidae The Curculionidae PCA analysis extracted 26.18%, 18.35%, and 13.94% of the variation for axis 1, 2, and 3, respectively, with a cumulative total of 58.5% of the variance explained. The initial curculionid model was rejected based on all ®t statistics (Table 2). The alter- FIG. 3. Axis 1 ordination site scores for each feeding guild. Solid circles are grazed sites; open circles are sites nate model hypothesis indicated that blooming ¯ower managed for hay production. Species that have high corre- density and the insect predator community had a strong lations with axis 1 are shown above the site scores. They are direct in¯uence on the curculionid community struc- positioned where they are approximately located in ordination ture. Management had a smaller indirect effect on Cur- space. Inference can then be made regarding where they are located along axis 1 and which environmental variable is culionidae through grazing effects on the blooming correlated with axis 1. ¯ower density (Fig. 2C). These results were reinforced with stepwise regression (␣ϭ0.05) where blooming ¯ower density and insect predator community were re- coccinellid response variable revealed no signi®cant relationships. Site scores for speci®c species displayed no obvious aggregation by management (Fig. 3B). Three species showed high correlation with axis 1: Scymus sp. (R ϭ 0.74), Hyperaspis sp. (R ϭ 0.77), and Hyperaspis un- dulata (R ϭ 0.72) and all were located in the center right of the ordination diagram (Fig. 3B). Lepidoptera In the principal components analysis for Lepidop- tera, 13.3, 10.77, and 9.51% of the variation was ex- tracted for axis 1, 2, and 3, respectively, with a cu- mulative total of 33.60%. The initial Lepidoptera mod- el hypothesis was rejected using SEM (Table 2). The path diagram for the alternative model hypothesis sug- FIG. 4. The Coccinellidae community is signi®cantly cor- gested that Lepidoptera community composition was related (using principal components analysis) with the perim- strongly and directly correlated with plant community eter/area ratio (R2 ϭ 0.127, P ϭ 0.038). October 2004 COMMUNITY COMPOSITION IN FRAGMENTS 1315 tained in the model and explained the most variation of weevils (R ϭ 0.49, P ϭ 0.0158). The ordination plot of sites based on Curculionidae community species composition does not reveal clear clumping associated with management (Fig. 3D). Ep- icareus imbricatus (R ϭ 0.81), Pseudobaris farcta (R ϭ 0.84), and Sitona cylindricolis (R ϭ 0.80) tended to group on the left side of the ordination diagram (Fig. 3D).

DISCUSSION Habitat fragmentation presents a challenge for de- veloping conservation measures to preserve biodiver- sity worldwide. The effects of fragmentation are dif- ®cult to assess as landscape-scale factors such as frag- ment area, isolation, and juxtaposition of fragments interact with aspects of the fragment habitat creating FIG. 5. Plant species richness is signi®cantly negatively correlated with increasing management intensity (R2 ϭ 0.344, a complex interaction of direct and indirect effects that P ϭ 0.0002). Management was reclassi®ed into three cate- potentially in¯uence fragment residents in a scale-de- gories based on the percentage of bare ground: hay produc- pendent fashion. This complexity, coupled with chal- tion, moderate grazing, and intense grazing. Plant species lenges associated with large-scale experimental ma- richness was estimated using a rarefaction technique, Chao 2 (Colwell and Coddington 1994). nipulations, has limited many fragmentation studies to focus on one or few species. Although individual spe- cies do respond to fragmentation differently, con- Management had a dramatic in¯uence on the local strained by their speci®c natural history (Harris 1988, plant community, thus consistently having an indirect Yahner 1988, Forman 1995) and scale at which they effect on insect community composition. Grazing by utilize the environment (Wiens 1976), conservation large herbivores has many effects on vegetation and is planning now often targets multiple-species conser- known to affect insect communities (Quinn and Wal- vation needs (Blockstein 1988, U.S. Fish and Wildlife genbach 1990, Onsager 2000). Bison grazing in tall- Service 1994). Community responses indexed as spe- grass prairie has been shown to increase plant species cies richness to fragmentation provide important in- diversity, increase spatial heterogeneity of vegetation, sights, but this approach masks important information and affect multiple ecosystem processes through graz- about the taxonomic composition of assemblages, ing and other behaviors (Hartnett et al. 1996, Knapp which may be as or more important. Moreover, con- et al. 1999). Cattle grazing also has varied effects, with servation decisions require that one recognize and the potential to increase structural heterogeneity, plant quantify direct and indirect effects as the manipulation diversity, and production at moderate grazing intensity of one critical aspect may trigger undesirable responses (Collins 1987, Dyer et al. 1993, Lauenroth et al. 1994, due to unknown indirect effects. Cid and Brizuela 1998, Knapp et al. 1999). In eastern Summarizing insect community composition using Nebraska, USA, common ragweed (Ambrosia artemi- ordination techniques and using structural equation siifolia), western ironweed (Vernonia baldwinii), modeling (SEM) proved to be useful in detecting which snow-on-the-mountain (Euphorbia marginata), buffalo variable had the most signi®cant direct and indirect bur (Solanum rostratum), and sun¯ower (Helianthus effects on insect communities of tallgrass prairie frag- annuus) are more likely to be found in heavily grazed ments in this study. Plant community composition, areas (Stubbendieck et al. 1995; K. J. L. Stoner and A. blooming ¯ower diversity, and density characterized Joern, personal observations). Hayed areas are domi- local habitats and had strong direct in¯uences on gen- nated by native grasses and forbs such as heath aster eralist, specialist, and insect herbivores that shift feed- (Aster ericoides), goldenrod (Solidago missouriensis), ing modalities throughout their life cycle. This rela- and leadplant (Amorpha canescens) (K. J. L. Stoner tionship between herbivorous insects and patterns of and A. Joern, personal observations). There is a sig- vegetation serving as habitat and food is well estab- ni®cant decline in plant species richness as grazing lished (Southwood 1977, Kemp et al. 1990, Bernays intensity increases in the sites examined here (Fig. 5). and Chapman 1994, Panzer and Schwartz 1998, Coll- Plant species composition did not respond to any inge et al. 2003). Our study suggests that for foliar landscape-scale in¯uences as originally predicted. insect feeding guilds in remaining tallgrass prairie frag- Plants were expected to respond to fragment size and ments, changes in local habitat may supercede land- fragment isolation, based on the assumption that both scape factors as important in¯uences on insect guilds, colonization from neighboring ``source'' fragments and at least at the scale we examined the problem. local extinction are important. Plant species in tallgrass 1316 KRISTAL J. L. STONER AND ANTHONY JOERN Ecological Applications Vol. 14, No. 5 prairie fragments may be temporarily removed through would respond to both adult and larval food sources herbivory or mowing, but could remain in the fragment as well as regional factors of area and neighboring as a seed bank or through asexual reproduction. Holt fragments. Only local effects of plant community com- et al. (1995) found that patch size and level of isolation position and blooming ¯ower diversity predicted Lep- did not in¯uence secondary succession in old ®eld hab- idoptera community species composition. This rela- itats, suggesting that abandoned agricultural ®elds con- tionship showed both a large direct and a smaller in- tain a suf®cient seed bank to repopulate a fragment. direct effect. Management was also included in the best-®t model as it indirectly in¯uenced the Lepidop- Generalist and specialist feeders tera community composition by in¯uencing the plant Consistent with our predictions, the Orthoptera com- species community and blooming ¯ower diversity. The munity responded directly and strongly to changes in total effect management had on the Lepidoptera com- the plant species community. We also found that the munity was stronger than the blooming ¯ower density plant species community was in¯uenced strongly by (Fig. 2B). management, and less by regional aspects such as area Butter¯y species are in¯uenced by regional aspects reduction, edge, and proximity of neighboring frag- of fragment area and isolation levels (Brommer and ments as predicted. Management practices acted indi- Fred 1999). However, studies show that butter¯ies uti- rectly on Orthoptera community composition (Fig. 2A). lize corridors between patches of habitat, but when We predicted that curculionid species, representing patches are in close proximity (within 64 m), butter¯ies specialists, would respond to local biotic changes with- do not necessarily utilize corridors and can simply in a fragment and to regional aspects of fragment area, move between patches (Haddad 1999) suggesting that, shape, and isolation. Curculionidae responded with ap- at this distance, the butter¯ies could detect neighboring proximately equivalent strength to both the blooming patches. Baguette and Neve (1994) found that butter¯y ¯ower density and insect predator community. Man- species are likely to readily move between patches in agement contributed a smaller indirect effect by in¯u- a highly fragmented system. Collinge et al. (2003) encing the blooming ¯ower density (Fig. 2C). found that local habitat characteristics explained pat- A generalist may have suf®cient resources to support terns of butter¯y diversity rather than the juxtaposition a population in fragments of reduced size even though of fragments in an urban landscape, and edge-mediated the diversity of resources would be less, although a movement behavior can increase residence time in local specialist feeder using fewer host plants may not have habitat patches (Schultz and Crone 2001). In tallgrass appropriate resources in the same fragment. Generalist prairie remnants in eastern Nebraska, butter¯ies were feeders may not require conservation efforts at a land- not obviously responding to area, edge, or isolation scape scale, as long as suf®cient resources are readily based on our analysis in the sense of large-scale dis- available in close proximity. Generalists can utilize a tributional patterns, and local plant communities ap- variety of habitat types and may readily use corridors peared most important. Behavioral controls on move- and move between fragments, so a landscape may have ment often alter individual dispersal near patch bound- much greater connectivity to a generalist. Habitat gen- aries, and individuals may remain in patches more often eralists may ``aggregate'' at higher levels of fragmen- than expected from random movements (Jones 1977, tation in the landscape (where a lower percent of the Kareiva and Shigesada 1983, Kindvall 1996, Fagan et landscape is habitat) compared with specialists (With al. 1999, Schultz and Crone 2001). Fragments in this and Crist 1995). Because isolation levels in¯uence col- landscape may be within dispersal distances and with- onization and establishment of both food plants and the out dispersal barriers, so Lepidoptera species in this insect specialist populations, food specialists were ex- study may not perceive the habitat as fragmented. Areas pected to respond to regional effects of fragmentation. considered inhospitable to Lepidoptera in agricultural This effect was not seen, suggesting that these species landscapes (row crops) may be sometimes used as hab- may perceive the landscapes at a much smaller scale itat, such as the monarch butter¯y that will lay eggs than considered here, such that the habitat is not frag- on milkweed plants in row crop ®elds (Sears et al. mented from their perspective. For example, specialists 2001), but this is probably an unsuitable habitat matrix chosen in this study may utilize vegetation found in to most species. corridors (ditches) that connect larger habitat patches, or habitat fragments may be situated well within dis- Predators persal distances. Consistent with this view, Jonsen and Predators represented in this study by coccinellids Fahrig (1997) found that isolation had no effect on responded only to regional aspects of fragmentation in specialist plant-hoppers at the landscape scale. this analysis. Fragment shape was the most important variable in explaining changes in Coccinellidae com- Lepidoptera munity composition. Unexpectedly, management had Lepidoptera change feeding modalities from most no in¯uence on coccinellids in this study (Fig. 3B). specialist feeding larvae to generalist nectar and pollen Many studies have found that top trophic-level species feeders as adults. We predicted that Lepidoptera species richness, represented by a few taxa or an individual October 2004 COMMUNITY COMPOSITION IN FRAGMENTS 1317 predator, is in¯uenced directly by regional factors, and examining small assumed to respond only to regional factors such as fragment area and isolation are small-scale changes. At present, much additional work more important than regional biotic factors (Kruess and remains before we will understand landscape responses Tscharntke 1994, 2000, Tscharntke et al. 2002). Our in suf®cient detail to guide conservation efforts for results reinforce the trophic susceptibility to fragmen- insect predators. tation hypothesis that states species associated with higher trophic levels are more susceptible to fragmen- ACKNOWLEDGMENTS tation due to the challenge of recolonization after a We thank Brad Danner, Kathleen Keeler, Diana Pilson, and local extinction event (K. J. L. Stoner and A. Joern, two anonymous reviewers for helpful comments and insights. We also gratefully acknowledge Goeff Henebry for providing unpublished manuscript). the image ``Prospecting for Tallgrass Prairie'' and additional insight to the complex world of landscape ecology. We also Direct and indirect effects thank Matt Paulsen for help with insect identi®cation. Special Causal mechanisms that link species responses to thanks to Gerald Kubic, David Barry, James Kumm, Don Crawford, Charles Henkelmann, Wayne Schroeder, Ervin habitat fragmentation are still poorly understood due Maahs, Theresa Retzlaff, Owen Hertner, Eleanor Francke, to the dif®culty in performing large-scale landscape Robert Lemnke, Charles Werchert, Harold Minke, The Au- manipulation experiments, and the dif®culty of eval- dubon Society±Nebraska Chapter at Spring Creek Prairie, uating contributions from direct and indirect effects Steven Kraus, Henery Luers, David Brhel, Chere Atkins, Lyle that can have opposing effects on species responses Mayer, Ronald Trouba, Calvin Sterns, John Sterns, Larry Dvorak, Don Brubaker, Eldon Kratochvil, Larry England, (Haila 2002, McGarigal and Cushman 2002). Although Jimmy Monnier, Raymond Siekmann, David Predmore, Daryl experimental manipulations best identify causal inter- Mitchell, Harley Batie, Lillie Larsen, John Marshalek, Mi- actions when hypotheses can be properly framed, this chael Norman, Terrance Vaverka, and Jake Kare at U.S. Bank direct approach may not be possible if experiments Farm Management for use of their land. Financial support was provided by the National Science Foundation grant cannot be performed at the scale needed (e.g., regional 0087259, The Nature Conservancy±Nebraska Chapter's J. E. landscapes), or if the putative causal relationships are Weaver Competitive Grants Program, and the School of Bi- too poorly de®ned and appropriate hypotheses cannot ological Sciences and Initiative for Ecological and Evolu- be devised for rigorous testing. Structural equation tionary Analysis at the University of Nebraska at Lincoln. modeling helps delineate possible direct and indirect LITERATURE CITED effects (Johnson et al. 1991, Grace and Pugesek 1998) Allan, B. F., F. Keesing, and R. S. Ostfeld. 2003. Effect of from data that can be readily obtained, and is useful forest fragmentation on lyme disease risk. Conservation for analyzing the impact of fragmentation. The LISREL Biology 17:267±272. algorithm is helpful as it provides extensive quantita- Arenz, C. L., and A. Joern. 1996. Prairie legacies-inverte- tive insight to tailor selection of alternative models to brates. Pages 91±110 in F. B. Samson and F. L. Knopf, the speci®cations of the study. The resulting best-®t editors. Prairie conservation: preserving North America's most endangered ecosystem. Island Press, Washington, model illustrated as a path diagram suggests individual D.C., USA. hypotheses that can be tested experimentally or further Baguette, M., and G. Neve. 1994. 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