Evaluating Swamp Rabbits as Indicators of Habitat Quality in Bottomland Hardwood Forests

Photos courtesy of Southern Illinois University–Carbondale.

Bottomland hardwood (BLH) forests are lowland forested floodplains along major rivers and creeks that contain a build-up of rich alluvial soils. BLH forest habitats have been degraded throughout the Lower Mississippi Alluvial Valley through loss and fragmentation, largely attributed to past agricultural conversion. Flood-control structures, reservoirs, urban development, unsustainable natural resource extraction and invasive species have also contributed to the substantial decrease in habitat quality of BLH forests. Efforts in recent years have focused on restoring BLH forests to enhance ecosystems structure, function and diversity. However, the lingering effects of agricultural practices on recently reforested bottomlands have resulted in altered drainage, hydrology, soil conditions, and vegetation composition when compared to BLH forests that were not converted to agricultural uses.

These human-caused alterations have adversely impacted wildlife populations, both for species that utilize BLH forest habitat year-round as well as during migration and/or breeding. Southern Illinois bottomland hardwood forests are home to a diverse number of avifauna and reptile and amphibian (herpetofauna) species, including those of high conservation concern. The spatial and structural diversity of BLH forests include a wide variety of habitat suitability that support a wide array of forest-dependent bird species. Remaining BLH forests have been identified as regions of conservation concern for neotropical migratory bird species that have been suffering long-term decline. The interconnected mosaic of wet and dry soils within BLH forests provide high quality wildlife habitat for many bird species, and also serve important functions for herpetofauna throughout various stages of their lives.

Due to the complexity of BLH forest habitat and varied habitat needs of different species, managers find it difficult to develop restoration plans aimed to improve wildlife habitat conditions for all key species. Faunal indicator species may provide a way to guide BLH forest restoration and management to ensure that these wide range of habitat needs are being met. An effective BLH indicator species must be easily monitored and have habitat needs that represent those of other species with similar habitat requirements within the BLH forest ecosystem.

A recently published review article by our research team at Southern Illinois University Forestry discussed how the swamp rabbit (Sylvilagus aquaticus) may be a suitable indicator species within BLH forests. We concluded that management of habitat to benefit swamp rabbits may also positively affect other wildlife species. Swamp rabbits are relatively easy to survey via pellet counts on natural and artificial latrine logs and have evolved within and remain endemic to south-central and southeastern BLH ecosystems in the United States. In southern Illinois and throughout the northern edge of their range, swamp rabbits exist in metapopulations that are limited to the isolated patches of optimal habitat and environmental conditions. Remaining BLH habitat occurs disproportionately at low-elevations more prone to flooding and poor hydrological conditions, thereby restricting the amount of potential ideal habitat conditions for swamp rabbits.

Swamp rabbits are BLH specialists that rely on complex spatial and structural characteristics found in both mature and young stands within BLH forests. For example, understory structures, including woody stems, shrubs, herbs and grasses, provide suitable cover for swamp rabbits. In addition, coarse woody debris, decomposing stumps and logs may be used as resting sites for swamp rabbits and also provide the elevated surfaces swamp rabbits prefer to use as latrines. Similarly, structurally diverse understories provide perching, foraging and nesting sites for a variety of forest avifauna and provide critical refuge to a variety of herpetofauna species. Swamp rabbits occupy sites in close proximity to water, which provide escape routes from predators. Similarly, the prothonotary warbler (Protonotaria citrea) prefers to nest above water to protect their nests from predators. Herpetofauna also need access to water sources connected to forest uplands. Changes in water management practices can expose the remaining low-elevation BLH stands to more extreme, frequent and inconsistent inundation compared to historical conditions. Swamp rabbits inhabiting low-lying forested wetlands benefit from areas less prone to flooding (i.e., uplands and flood-free landscape features) which provide refuge during flood events. Similarly, some forest-dependent birds, like the Swainson’s warbler (Limnothlypis swainsonii), abandon areas with frequent flooding and tend to occupy drier patches or upland forests nearby for breeding and nesting. Furthermore, several herpetofauna species depend on upland areas during their active seasons and for overwintering (hibernacula).

Due to the clear overlap of habitat-characteristic associations in BLH forests, swamp rabbit site occupancy may relate to avifauna and herpetofauna occupancy and, therefore could serve as useful indicators of habitat quality for many species. In order to best manage the remaining BLH forest habitat in the Lower Mississippi Alluvial Valley, it is important to determine which habitat characteristics and conditions most strongly influence specialist species occupancy across taxa. Identifying specialist indicator species will help managers monitor habitat conditions for BLH wildlife by creating more efficient and cost-effective ways to monitor and manage the entire wildlife community.

Our research seeks to determine the effectiveness of swamp rabbits to serve as useful indicators of quality habitat for other wildlife in BLH ecosystems. We surveyed for swamp rabbit, herpetofauna, and avifauna presence across Cypress Creek National Wildlife Refuge (CCNWR) for two field seasons (March – June 2019 and April – July 2020). This portion of southern Illinois represents the northernmost extension of the Lower Mississippi Alluvial Valley and houses the most diverse communities of bottomland tree species in Illinois.

We used non-invasive survey methods to detect species presence. These methods included swamp rabbit pellet surveys, PVC pipe tubes, tin and wood coverboards for herpetofauna species detection and fixed-radius point-count surveys for avifauna species detection. With the addition of habitat measurements and survey-specific conditions, we are able to account for imperfect detection of species in our analysis. Our goal is to better understand how swamp rabbits may improve management of BLH forests throughout their current range.

Jessica Esposito is a graduate student in the Cooperative Wildlife Research Laboratory and the Department of Forestry at Southern Illinois University Carbondale. She earned her bachelor’s degree in Environmental Studies with a minor in Animal Behavior from Manhattanville College in Purchase, NY. Her graduate thesis focuses on evaluating swamp rabbits as effective indicators of habitat quality for other wildlife species in bottomland hardwood forests. This research study is funded by the USDA McIntire-Stennis Cooperative Forestry Research Program through the SIUC Department of Forestry. Her broad research interests involve forest wildlife conservation and management. She is co-advised by Dr. Clay K. Nielsen and Dr. John W. Groninger.