Descriptive Ecology of a Turtle Assemblage in an Urban Landscape

Article excerpt

ABSTRACT.-

We studied turtle populations inhabiting a canal and a lake (both man-made) within a heavily disturbed, urban setting. Six aquatic and semi-aquatic turtle species were collected in both habitats: spiny softshell turtle (Apolone spinifera), painted turtle (Chrysemys picta), common snapping turtle (Chelydra serpentina), common map turtle (Graptemys geographica), common musk turtle (Sternotherus odoratus) and red-eared slider (Trachemys scripta). While G. geographica was the most common species in the canal habitat, T. scripta was most common in the lake habitat. We describe patterns of sexual size dimorphism and sex ratios for the three most abundant species (G. geographica, T. scripta and S. odoratus). We discuss our data in light of problems facing turtle assemblages in urban settings.

INTRODUCTION

Habitat conversion and degradation is generally recognized as the most pervasive and important of the six major threats to biodiversity (other threats being invasive species, environmental pollution, disease/parasitism, unsustainable use and global climate change; Gibbons et al., 2000). The major effect of habitat conversion is the outright loss of critical habitats for essential life functions, including feeding (Vickery et al., 2001), courting and nesting (Heckert et al., 2003) and hibernation (Ball, 2002). Habitat conversion as the result of increasing urbanization, in particular, affects a wide array of organisms, from large carnivores (Reilly et al., 2003) to butterflies (Collinge et al., 2003) to plants (Fransisco-Ortgea et al., 2000) in terrestrial situations and from salamanders (Willson and Dorcas, 2003) to fish (Paul and Meyer, 2001) to algae (Fore and Grafe, 2002) in aquatic environments.

Turtle populations have been significantly impacted by human activity, development and urbanization. Negative effects include fragmentation of genetic structure (Rubin et al., 2001), demographic effects (Garber and Burger, 1995; Lindsay and Dorcas, 2001) and direct mortality (e.g., through collision with automobiles, Gibbs and Shriver, 2002). Nonetheless, some turtle species may be very resilient in the face of human activity and continue to exist in highly modified habitats when other wildlife is extirpated (Mitchell, 1988). Data on the specific impacts of human activity on turtle populations and assemblages, and how these effects may be ameliorated, provide essential components to sound conservation practices in human-dominated landscapes. The purpose of the present study is to understand the basic ecology of a turtle assemblage living within an urban landscape. These descriptive population and community ecology data can then serve as a baseline for more thorough investigations of the effects of urbanization.

MATERIALS AND METHODS

The Central Canal is a man-made riverine habitat created in the 1830s in Indianapolis, Indiana, the 12th largest city in the USA (2000 census population 791,900+residents). The remnant of a much larger uncompleted canal system, the Central Canal originates from the White River and flows south through commercial, residential and recreational areas for 11.2 km. At least a dozen roads cross the canal, including four major thoroughfares and one interstate highway. At the southern terminus, the canal enters a water treatment facility operated by the Indianapolis Water Company (IWC). The canal transports approximately 70% of the city's annual water use; water level, flow rate, submergent and emergent aquatic vegetation are all controlled in part by the IWC. The canal varies from 15 to 25 m wide and is usually less than 2 m at its deepest points. Shorelines are practically non-existent in most places, with banks 1-2 m high on either side. Fragmented woodlots border portions of the canal and fallen trees and snags serve as basking sites; however, many of these basking sites are removed on a regular basis. Approximately 8.5 km of the canal (76%) is bordered by a greenway (the Central Canal Towpath) maintained by IndyParks, the City of Indianapolis Department of Parks and Recreation. …