1993 Proceedings of the Midwest Oak Savanna Conferences
TERRESTRIAL VERTEBRATES OF OAK SAVANNAS AND WOODLANDS IN THE CHICAGO REGION
Kenneth S. Mierzwa
Amphibians, reptiles, and small mammals were sampled at savanna and woodland sites in five Chicago region counties. Drift fences were used to capture amphibians, reptiles and shrews, while small mammals were caught in snap traps. Woodlands with relatively dense canopy cover had the highest species richness and density of salamanders, while sand savannas supported more reptiles and the highest overall species richness of amphibians and reptiles. A northern flatwoods site was poor in species but had an extremely high density of blue-spotted salamanders. The mammalian fauna was relatively similar at all sites sampled, although the more open sites have small populations of a few species typically associated with grasslands.
Oak savannas and woodlands were once widespread in northeastern Illinois and northwestern Indiana (Nuzzo, 1985; Packard, 1986). These unique community types have now been profoundly altered by fire suppression, grazing, agriculture, and development. As a result, the characteristic faunas of the original savannas and woodlands are poorly known and in many cases, misunderstood.
Few authors have specifically addressed small terrestrial vertebrates of savannas and woodlands in the Chicago region. Byers et al. (1986) discussed savanna mammals, and Mierzwa (1989) listed some characteristic amphibians and reptiles based on documented localities and distribution of presettlement plant communities. This paper includes preliminary results of ongoing quantified studies, and discusses factors influencing the distribution of animals found in oak communities.
Study sites were established in five Chicago region counties. Amphibians and reptiles were collected from 1989 to 1992 using drift fences with pitfalls and funnel traps at sites one to seven, in McHenry, Lake, and Cook Counties, Illinois, and Lake County, Indiana (Fig. 1). Some small mammals were also captured by this method. Drift fences are more effective and less biased than other collecting methods for amphibians and reptiles (Campbell and Christman, 1982; Rudis, 1984), are one of the most efficient methods of collecting shrews (Hoffmeister, 1989), and provide easily quantified data. Karns (1986) provides for a detailed description of drift fences. One to four drift fences were used per site. Because effort was unequal between sites, results are presented as captures per 1000 trap-nights. A trap-night is defined as each 24 hour period a 15 meter long drift fence is in operation.
Small mammals were sampled during 1990 and 1991 at sites five to nine with snap traps baited with peanut butter. Traps were positioned at 10 meter intervals along transects, with two mouse traps and one rat trap at each station. Sixty to 90 traps were set simultaneously at each site. Results are presented as catches per 1000 trap-nights.
Location, community type, canopy cover, and soil type were tabulated (Table 1). For analysis, some study sites were grouped based on geographic proximity, similar canopy cover, and soil type. Community classification follows White (1979), except that savanna and woodland are defined as in Chapman (1990).
Sites one to four were used primarily for amphibian and reptile sampling, although some of small mammals were captured in drift fences at site two. Sites five to seven were used for amphibian-reptile and mammal sampling. Only small mammals were sampled at sites eight and nine.
Amphibians and reptiles
A total of 1935 individual amphibians and reptiles belonging to 20 species were captured at the seven study sites. the relative numbers of amphibians and reptiles captured at each study site were calculated (Table 2). Sample sizes are still relatively small, and these results must be considered preliminary.
The two woodland sites supported a moderately diverse assemblage of amphibians and reptiles. Frogs and toads were frequently captured than any other major group, but salamanders and snakes were common.
In general, closed canopy sites had the highest species richness and density of salamanders, but few reptiles. Sand savannas are especially rich in reptiles, including lizards and some snakes not found in black soil communities in this region; several types of frogs are present but salamanders are relatively uncommon. Woodlands are intermediate between these community types, and had a more even mix of amphibians and reptiles. A gradient probably exists from amphibian dominated dense woodlands to open savannas where reptiles are more common. Little information is available for open canopy sites on black soil.
The flatwoods site had relatively low species richness, but blue-spotted salamanders, Ambystoma laterale, reached extremely high densities. Two amphibians, the spotted salamander, Ambystoma maculatum, and the wood frog, Rana sylvatica, which are characteristic of humid eastern forests were present but uncommon. The proximity of mesic and dry-mesic forest and the availability of several vernal ponds for breeding probably influence amphibian usage. Reptiles were uncommon, and were seen only in early spring and late fall when sunlight penetrates the tree canopy.
The dry-mesic forest/woodland site supported nine species of salamanders and frogs, the highest amphibian species richness of any site sampled. This may be attributable in part to the topographic variation of the site; mesic ravines and well-drained low ridges are present near the sample area. Salamanders were the most common major group. Only one reptile species (the common garter snake, Thamnophis sirtalis) has been observed to date, although others are known to occur in more open areas nearby.
The three sand savanna sites had the highest combined amphibian and reptile species richness of the community types sampled, with 13 species. Frogs and snakes were the dominant groups. Two lizard species, the six-lined racerunner, Cnemidophorus sexlineatus, and the slender glass lizard, Ophisaurus attenuatus, which are restricted to sand areas in the Chicago region were present, although only the latter was captured in drift fences. Additional sand area species are known to occur in the Kankakee sand area, (for which quantitative data are not yet available), but not in the smaller Gary sand area studied here.
Mammalian captures were similar to those reported elsewhere (Byers et al., 1986). Although several types of small mammals were consistently captured in savanna and woodland communities (Table 3), none were restricted to those habitats. White-footed mice, Peromyscus leucopus, are often the most common small mammal in savanna and woodland communities; they occur at every site sampled during this study. They also are known from forest and successional field habitats. Chipmunks, Tamias striatus, are known from the full range of wooded communities. The shrews Sorex cinereus and Blarina brevicauda occur in a variety of upland and wetland habitats, including those covered by this paper.
Several small mammals usually associated with grasslands were trapped in small numbers at some sites. Meadow voles, Microtus pennsylvanicus, meadow jumping mice, Zapus hudsonius, and eastern cottontails, Sylvilagus floridanus, were all relatively uncommon and were taken only at the more open sites. Meadow jumping mice, Zapus hudsonius, were captured in funnel traps at Forbes Savanna.
Little difference in mammalian species composition was noted between savanna and woodland communities, except that the sand area sites were generally less productive. Fox squirrels, Sciurus niger, were observed but not captured at relatively open black soil sites. Only gray squirrels, Sciurus carolinensis, were captured in sand savannas. The single Franklin's ground squirrel, Spermophilus franklinii, captured during the study was taken in a sand savanna.
Amphibians are subject to rapid evaporative water loss (Feder and Burggren, 1992). Some species have developed behavioral or morphological adaptations to cope with lowered humidity, and many Chicago region amphibians are characteristic of habitats other than forest. However, some species are restricted to the vicinity of tree cover and the associated shade and lower wind velocities. The availability of suitable breeding ponds is also important; optimal terrestrial habitat will support few amphibians if ponds are not present nearby.
Many types of reptiles bask to raise their body temperatures; the mosaic of sunlight and shade typical of savannas provides ideal thermoregulatory conditions (Mierzwa, in press). It is not unusual to find basking snakes partially in the sun and partially in shade. Savannas or open woodlands, with their relatively sparse canopy cover, are likely to maximize opportunities for thermoregulation; in denser woodlands, patches of sunlight may be small and widely scattered.
Soil types are important to amphibian and reptile species presence. Sand areas are almost always populated by several lizards and snakes more typical of the great plains or the south. In some cases, these populations are xerothermic relicts separated from the main body of their ranges (Smith, 1957).
Some species, such as the tiger salamander, Ambystoma tigrinum, the common garter snake, Thamnophis sirtalis, and the brown snake, Storeria dekayi, are frequently captured or observed in savannas and woodlands. They have been referred to as "forest-edge" inhabitants by some previous authors (for example, Smith, 1961). Others, like the American toad, Bufo americanus, and the western chorus frog, Pseudacris triseriata, are usually present in savannas and woodlands but may be equally abundant in grassland habitats.
It is important to remember that a single species of amphibian or reptile often has different habitat requirements in separate parts of its range. Chicago region habitat utilization is not necessarily identical to that in other parts of the country.
The information presented above describes broad patterns of habitat utilization. More and larger samples, from additional locations, are necessary. Sites with less than 40% canopy cover are poorly represented. Once sufficient data have been accumulated, statistical analysis may identify clear differences in utilization of various community types. Block et al., (1988) analyzed several types of oak woodlands in California in this manner. Their study differentiated woodland types by tree species and moisture gradient.
Certain animal species assemblages are associated with landscape level community mosaics, of which woodlands and savannas are or were a dominant or significant component. Because of the dynamic nature of the combined woodland/savanna/grassland/wetland ecosystem, assigning a particular species to a discrete community type can sometimes obscure landscape level interactions which occur over long periods of time. Studies of sites large enough to encompass a diversity of community types will be necessary to reach an understanding of these interactions. Studies of large savanna and woodland restorations, where temporal changes are accelerated by management, may also be useful.
A hypothetical and untested model was prepared to indicate the relationships between tree canopy closure and distribution of several forest, woodland, savanna, and prairie species (Fig. 2). The ranges shown are based on observations at a limited number of sites. They can in theory be adjusted and verified by sampling the plant community around drift fence or trap line locations at additional study areas; other habitat variables can be measured as well.
Herpetologists and mammalogists are often unfamiliar with community types outside their local region. Increased familiarization with savanna/woodland concepts and classifications, and a wider understanding of the extent of temporal change in canopy closure and community structure caused by fire suppression, will facilitate communication among zoologists, botanists, community ecologists, land managers, policy makers, and educators in various regions and lead to a better understanding of the entire oak savanna and woodland ecosystem.
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Figure 2. Hypothetical model of relationships between tree canopy closure and the occurrence of several forest, woodland, savanna, and prairie amphibians and reptiles of the Chicago region. Solid line = common; dashed line = uncommon or occasional. Graphic by Ellin Beltz.