Impact of Urban Fragmentation
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The Des Plaines River historically influenced the landscape of the north suburban Chicago region by acting as a firebreak. Areas west of the river were treeless prairie, while east of the river a band of mesic forest and woodland developed in the absence of fire. Farther east, oak savanna, flatwoods, and small prairie openings were present. Distribution of 13 amphibian species coincided with different degrees of tree canopy and herbaceous cover along this vegetational gradient. Today, upscale residential neighborhoods and corporate campuses have fragmented the original ecosystem, leaving part of the landscape protected in a series of preserves that range in size from about 15 to 300 hectares. We compared recent quantitative inventory data with historical information available from museum collections and the field notes and correspondence of individuals who collected in the 1940s and 1970s. We have documented a decline in overall species richness, and in the abundance of at least some species. Sites with high species richness and with populations of rare species tend to be relatively large, have multiple breeding ponds, and include areas of high natural quality. IntroductionUntil at least 1835 southeastern Lake County, Illinois was characterized by a diverse mosaic of natural landscapes. Areas west of the DesPlaines River were swept by frequent fire, and were mostly maintained as open prairie. On the east side of the river, a narrow band of floodplain forest merged into mesic upland forest and woodland with Acer saccharum, Tilia americana, and Quercus rubra. To the east on morainal soils with high clay content, a northern flatwoods community characterized by Quercus bicolor and with numerous ephemeral ponds gradually gave way to a more extensive open savanna with small prairie and wetland openings. Because of the very different structure, microclimate, and moisture regime of these diverse communities, each segment of the gradient must have supported a unique assemblage of animals.As the region was subjected first to agricultural and then suburban disturbance and fragmentation, the Lake County Forest Preserve District and various local entities set aside most of the best remaining examples of natural areas. This network of preserves is extensive and often of good quality, but it remains fragmented by non-compatible land uses. This paper is intended to provide background context for various quantitative studies we have recently completed or which are currently underway on habitat structure, amphibian population response to restoration activities, and genetics. Because historical information is incomplete, mostly qualitative, and of uneven geographic and seasonal coverage, we are able to discern trends only where they are unusually distinct. However, it is clear that some amphibians once found on every collecting trip are today, only a little over 50 years later, seldom encountered despite the use of more efficient collecting techniques. MethodsWe obtained historical amphibian locality data for specimens in the collection of the Field Museum of Natural History and the Chicago Academy of Sciences. Most relevant specimens in the latter institution were collected by Richard A. Edgren Jr., and for those we worked directly from a copy of the original field catalog and notes. Recent correspondance with Edgren helped to establish the exact location of some historical localities. Additional information was taken from Smith (1911), Stille and Edgren (1948), and Pope (1964). Historical collections were, as far as we have been able to determine, by visual encounter survey.Recent distribution, status, and habitat information is from studies conducted by Nuzzo and Mierzwa (2000), Mierzwa and Beltz (1994, 1999), Hendricks (unpublished field notes) and Schlosser (in prep.). Recent inventories have relied mostly on drift fence arrays with funnel traps and pitfall traps, with visual encounter surveys, frog call surveys, and larval sampling as secondary techniques. In general recent efforts were longer lasting (over 60 days per year) and probably more intensive than historical inventories. Information on presettlement vegetation is from Moran (1978) and the Lake County Forest Preserve District (K. Klick pers. comm.). ResultsNo high quality prairie sites remain immediately west of the river and it is likely that most had been plowed before historical specimens were collected. Small prairies and savannas of good quality have been preserved in the eastern part of the study area. Numerous wooded tracts from a few to 300 hectares in size have been preserved in the DesPlaines River corridor.Most prairie and savanna amphibians collected in historical samples remain common. If declines or losses occurred, they must have happened before the earliest inventory work. Several forest and woodland amphibians appear to have disappeared or declined dramatically, despite the preservation of a considerable amount of habitat. Prairie AmphibiansA few habitat generalists occur in prairies, and no local species is restricted to grasslands. Except for Acris crepitans, which has disappeared from much of the north-central U.S., prairie amphibians remain common and populations at some restored sites have rapidly expanded.Savanna AmphibiansAmbystoma tigrinum and Rana pipiens are abundant today in oak savannas. The only amphibian noted in historical (1953) samples and not captured recently is a member of the Hyla versicolor complex, presumably H. chrysoscelis.Forest AmphibiansIn the Chicago region several types of amphibians occur only in forest, woodland, or northern flatwoods communities. Extensive historical and recent information is available from wooded sites. Some species (Ambystoma laterale, Pseudacris crucifer, Pseudacris triseriata) were common in most historical and recent samples. Two species (Ambystoma maculatum, Rana sylvatica) were common in 1941-48 samples, rare in 1983-89, and either absent or extremely rare in 1999-2000. Edgren observed 70 A. maculatum in a single night in 1948, while we captured 27 in 1986 and none in 1999 during intensive multi-week sampling at three nearby ponds. Two other species (Hemidactylium scutatum, Notophthalmus viridescens) were apparently always rare in the area, and have not been reported recently.We documented a complete loss of all amphibians at one wooded site between 1953 and 1999. That site is relatively small, isolated by highways and residential development, and degraded by hydrological alteration and buckthorn encroachment. In contrast, the highest amphibian species richness and abundance was correlated with the presence of multiple ponds, longer lasting (into mid-July) ponds, and a dense herbaceous understory (Nuzzo and Mierzwa, 2000). DiscussionAt forest, woodland, and northern flatwoods sites some once common amphibian species have been declining for at least several decades, and they may now be extirpated at many locations. Recent work has shown that hydrological alteration has almost certainly contributed to species loss. Most ephemeral ponds in wooded habitat within the study area now dry too early to allow larvae of several species to achieve metamorphosis. Secondarily, changes in forest structure including exotic shrub encroachment and loss of groundlayer herbaceous vegetation correlate with lower amphibian species richness and abundance. Forested sites with more amphibians tend to be relatively large, have multiple breeding ponds which hold water into midsummer, and have a mature tree canopy and extensive herbaceous vegetation. Sites with few amphibians tend to be smaller and more isolated, have fewer and earlier drying ponds, or have some combination of a younger tree canopy, dense stands of exotic shrubs, and sparse groundlayer vegetation.At Prairie Wolf Slough, a prairie and wetland restoration on former cropland within a landscape-level savanna context, Bufo americanus, Pseudacris triseriata and Rana pipiens have become abundant in only a few years. Indeed, studies in other savanna and grassland sites not far away have shown that amphibians can rapidly increase in numbers when both upland and wetland habitat is restored on a larger site (Mierzwa, 1998). ConclusionsRestoration efforts have been underway for some time at Chicago region prairie and savanna sites, and have already had beneficial effects for some amphibians. Attempts to restore wooded communities have only recently been initiated. We suggest that woodland restoration efforts should focus on both hydrology and vegetation structure, and that results be monitored. The ongoing decline of some types of amphibians suggests that much remains to be learned about the restoration and management of Chicago region wooded ecosystems.AcknowledgementsWe thank Ken Klick and Jim Anderson of the Lake County Forest Preserve District for providing maps and aerial photographs. Richard A. Edgren, Jr. shared detailed reminiscenses; without his comments it would not have been possible to complete this paper. Some of the studies referenced above were funded by the U.S. Environmental Protection Agency, Lake County Forest Preserve District, Forest Preserve District of Will County, McHenry County Conservation District, and Lake Forest Open Lands Association. TAMS Consultants, Inc. provided graphics and administrative support, and Amanda McTigue assisted with graphic design.Literature Cited
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| Updated: October 6, 2007
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