PATCH DYNAMICS OF AMPHIBIANS AND REPTILES IN
NORTHEASTERN ILLINOIS SAVANNA LANDSCAPES
Kenneth S. Mierzwa
Based on samples at numerous Chicago region localities, earlier studies identified characteristic savanna and woodland herptile species (Mierzwa, In press). The present effort examines spatial distribution of amphibians and reptiles among woodland, grassland, and wetland communities within the larger savanna ecosystem. Although results are quantified, this study is intended only to help develop hypotheses which can later be tested with a randomized, replicated sample design.
Results are reported for two northeastern Illinois study sites with extensive capture data. Both sites include large areas of oak savanna well interspersed with prairie and wetland communities. The two preserves differ in topography, soil type, and soil moisture.
Glacial Park is a McHenry County Conservation District preserve covering 7,178 ha southwest of Richmond, Illinois. The central core includes areas of relatively steep terrain, with a series of gravel delta kames, a kettle marsh, and a low shrub bog. Dry-mesic savanna dominated by Quercus macrocarpa covers much of the core area. Considerable restoration work, including clearing of non-native shrubs and repeated controlled burning, has taken place here over the last several years. Surrounding outwash plains are more nearly level and much more open, with extensive sedge meadow-marsh-successional field complexes. Quantitative data are available for 1989-1993 (Mierzwa and Beltz 1994a).
Middle Fork Savanna is an approximately 1,235 ha Lake County Forest Preserve District and Lake Forest Open Lands Association preserve located at the western edge of Lake Forest, Illinois. The site is nearly level, and soils are high in clay content. The preserve consists of interspersed savannas and woodlands, grassland openings, and numerous marshes. Sampling took place in a high-quality mesic to wet-mesic savanna dominated by Quercus alba and Quercus macrocarpa with 30% canopy cover, in a mesic woodland consisting mostly of Quercus alba with 60% canopy cover, and in a mesic to wet prairie opening. Additional community types were sampled less intensively. All sample locations were within 50 m of wetlands. Sampling took place during the spring, summer and fall of 1993 (Mierzwa and Beltz 1994b).
Amphibians and reptiles were sampled primarily with drift fence arrays. Each array consisted of two 15 m fences constructed from 51 cm high aluminum flashing, with a funnel trap at each end and a 20 liter pitfall trap at the center of each fence. Each pair of fences was placed in an "L" shape with at least 15 m between the two arms of the array. Drift fence techniques are described in greater detail by Heyer et al. (1994). Capture results are reported as catches per 1,000 trap nights to allow comparison of data from arrays run for different lengths of time. A trap night was defined as a 24 hour period for each 15 m fence within an array. Drift fence sampling was supplemented with random sampling, including monitoring of cover objects (Fitch 1992) and time constrained random searches (Karns 1986). Breeding amphibians were identified at several wetlands within each site by monitoring frog calls and by seining for aquatic larvae.
Eighteen species of amphibians and reptiles were documented at Glacial Park, but a few of these are aquatic or semi-aquatic species restricted to the vicinity of Nippersink Creek thus not normally captured in terrestrial communities. The highest species richness was in the core savanna, where 13 species were found (Table 1). Nine species were taken in the sedge meadow/marsh/ successional field complex, and nine species were captured in prairie restorations.
Twelve species of amphibians and reptiles were found at Middle Fork Savanna, with roughly equivalent species richness in woodland, savanna, and prairie communities. Adult salamanders were captured only in savanna and woodland communities, but recently transformed juvenile salamanders and frogs of all ages were found throughout the preserve. Snakes were most common in prairie openings, and most captures in wooded communities took place in the spring before the leaf canopy was complete or late in the fall. The total number of individual amphibians and reptiles captured was much lower than at Glacial Park.
With one exception, a similar regional pool of amphibian and reptile species is present around the two study areas. Ambystoma laterale is restricted to areas east of the Fox River and is common in the vicinity of Middle Fork Savanna but absent near Glacial Park. The higher species richness and abundance of individuals at Glacial Park is probably attributable to the much larger size of the preserve, the presence of a third-order stream and semi-permanent wetlands, and the less intensive level of development on surrounding land. Middle Fork Savanna also has a less than optimal long and narrow shape.
Slope aspect, soil moisture, and tree canopy cover may have influenced capture rates. Within the Glacial Park core savanna, all snake captures were on well-drained west or south facing slopes or more mesic level areas, in relatively open and sunlit situations. No snakes were captured on shaded east facing slopes. Amphibians were captured at all savanna sample locations.
Within the Glacial Park core savanna, species considered characteristic of that community were generally common. Some of these same species were captured in much smaller numbers within nearby open communities. Holt (1993) suggested that the presence of a core area of optimal habitat may allow some species to disperse into habitats where they would otherwise not be found. This "source-sink" condition may be functioning at Glacial Park, and deserves further study.
Because almost all northeastern Illinois amphibians require water for reproduction, the amount, distribution, type, and quality of wetland communities within any preserve is an important factor controlling the presence and abundance of salamanders and frogs. At Glacial Park and especially at Middle Fork Savanna, each wetland sampled contained a unique mix of larval amphibians. At least three factors influencing the larval species mix have been identified. Hydroperiod is perhaps the most obvious; only Bufo americanus and Pseudacris triseriata routinely inhabit ephemeral wetlands. Ambystoma laterale, Ambystoma tigrinum, Hyla chrysocelis, and Rana pipiens require wetlands which retain water into at least July in most years. Rana catesbeiana and Rana clamitans overwinter as tadpoles, and require permanent water.
Hydroperiod varies considerably from year to year, and a succession of drought years can extirpate some amphibian species from entire preserves, or from substantial portions of a preserve. In wet years, juveniles once again disperse into underutilized sections of the preserve. At both study sites, long-distance dispersal of amphibians was documented during 1993. Because of unusually cool and wet weather, large numbers of larval amphibians reached metamorphosis even in usually ephemeral wetlands, and then remained surface active through the late summer and fall. At Glacial Park staff members observed "hundreds" of juvenile tiger salamanders crossing roads on rainy nights. A drift fence array placed within 50 m of the breeding marshes in the core savanna captured 523 salamanders; another array 0.6 km distant from the marsh captured 34 salamanders; and an array 1.5 km from the marsh caught two salamanders. Adult salamanders had been absent or rare in these areas previously. In some cases, such dispersal would have been difficult or impossible a few years earlier before intervening agricultural fields had been restored to prairie. Amphibians are apparently adept at tracking "the shifting spatial mosaic of suitable environmental conditions" (Thomas 1994).
Habitat structure is also important. In semi-permanent ponds without extensive emergent vegetation, larger predators such as Ambystoma tigrinum held other amphibian species to very low levels. In nearby densely vegetated ponds, the greater amount of cover allowed several species to maintain high numbers. Finally, surrounding terrestrial habitat influences the ability of adult amphibians to reach the wetland to breed. Species restricted to woodland are unlikely to breed in significant numbers in a wetland with no woody vegetation nearby. The role of the herbaceous layer is less well understood, but it may be of physiological importance to at least some amphibians. Feder and Burggren (1992) report that Rana pipiens rehydrates using morning dew on grass.
Snakes were most common in small prairie openings within savanna and in very open portions of savannas with only 5-10% canopy cover. Snake species richness and abundance were usually lower in extensive treeless areas and in dense woodlands. A few characteristic prairie species, including Opheodrys vernalis and Thamnophis radix, were present but essentially limited to the larger prairie openings.
The remaining elements of the savanna ecosystem evidently function as a unit incorporating the various community types. Because of the importance of spatial and temporal heterogeneity, large restored savanna preserves with wooded, grassland, wetland, and stream systems all present are likely to be much more diverse than smaller, more homogenous preserves.
The Illinois Department of Conservation Non-game checkoff fund, the McHenry County Conservation District, and the Lake Forest Open Lands Association provided funding for this study. Ellin Beltz participated in work on both study sites, and co-authored the resulting reports. Numerous individuals assisted with field or support work, including W. Schennum, E. Collins, B. Woodson, S. Hayden, R. Sliwinski, T. Anton, M. Redmer, S. Busack, S. Crispin, S. Christy, J. Plumb, M. Felix, T. Radke, J. Levell, and J. Bentley.
Feder, M. E., and W. W. Burggren. 1992. Environmental physiology of the amphibians. University of Chicago Press, Chicago, IL. viii + 646 p.
Fitch, H. S. 1992. Methods of sampling snake populations and their relative success. Herpetological Review 23(1):17-19.
Heyer, W. R., M. A. Donnelly, R. W. McDiarmid, L. C. Hayek, and M. S. Foster. 1994. Measuring and monitoring biological diversity: Standard methods for amphibians. Smithsonian Institution Press, Washington, D.C. xix + 364 p.
Holt, R. D. 1993. Ecology at the mesoscale: The influence of regional processes on local communities. In: R. E. Ricklefs and D. Schluter (eds.), Species diversity in ecological communities: Historical and geographical perspectives. P. 78-88. University of Chicago Press, Chicago, IL. 414 p.
Karns, D. R. 1986. Field herpetology: methods for the study of amphibians and reptiles in Minnesota. James Ford Bell Museum of Natural History Occasional Paper 18. 88 p.
Mierzwa, K. S. In press. Terrestrial vertebrate utilization of oak savannas and woodlands in the Chicago region. In: Proceedings of the Oak Savanna Conference, Northeastern Illinois University. February 1993. U.S. Environmental Protection Agency Region V, Chicago, IL.
Thomas, C. D. 1994. Extinction, colonization, and metapopulations: Environmental tracking by rare species. Conservation Biology 8(2):373-378