Status and distribution of frogs and toads in southern Michigan: Population trends and the influence of habitat and landscape characteristics

Declining amphibian populations in many disturbed and pristine habitats worldwide have been the source of great concern and research in amphibian biology and conservation. Research conducted at multiple spatial scales, life history stages, and over long periods of time (at least 10--15 years) needs to be conducted and synthesized to fully understand amphibian population dynamics so that effective management and conservation strategies can be formulated. I investigated some of these issues for 12 species of anurans native to southern Michigan. My objectives were to: (1) assess the occurrence, distribution, and status of each species using multiple methods to analyze data collected by volunteers in the Michigan Frog and Toad Survey (MFTS) from 1996 to 2002, (2) evaluate the quality and consistency of data collected by observers in the MFTS and assess implications for population trends analyses of volunteer-collected data, (3) investigate relationships between anuran abundance and distribution and land cover adjacent to wetland breeding sites and within potential dispersal distances using GIS analyses, and (4) determine the potential influences of habitat characteristics on growth, development, and survival of larval Pseudacris crucifer in natural wetlands. Populations of all Michigan anurans were variable over the seven-year duration of the study, but no major declines were detected. Seven species did show statistically significant trends in site occupancy or abundance (assessed by calling intensity of breeding males), but trends were not consistent in direction across species and were small in magnitude.{09}These trends need to be evaluated over longer time periods to determine whether they represent significant long-term population trends or short-term changes related to climatic variation. Volunteer observers in the MFTS were reliable more than 80 percent of the time in identifying all species of frogs and toads combined from their species-specific mating calls, but there was considerable variability in assessing abundance (i.e., assigning of categorical call index values to calling males). Volunteer background or prior experience in wildlife biology had little influence on data quality. Given variability among volunteers (and years) in abundance estimation, the poor understanding of empirical relationships between call index values and true breeding population size, the most robust analyses of MFTS data are those that use presence/absence (i.e., detection/non-detection) data. Food availability, hydroperiod, canopy cover, and predators affected P. crucifer tadpole development, survival and growth in wetlands in southwestern Michigan. Growth, development, and survival were greatest at sites with intermediate hydroperiods, partial canopy cover, and few (if any) fish predators. Land cover types influenced presence and abundance of anurans at wetland breeding sites. Land cover types indicating habitat alteration or loss (e.g., roads, urban) negatively influenced presence and/or abundance of anurans while land cover types representing important foraging and breeding habitats (e.g., open land, wetlands) represented positive influences. Species richness was not correlated with either the amount of adjacent forest cover or the amount of forest cover in the landscape within 1000 meters of breeding sites. Associations between anuran presence and abundance at breeding sites and land cover adjacent to and within 1000 meters of the breeding sites combined with data from population trends analyses provide valuable insights that can be used to identify critical habitats for management and conservation of these species.