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	A multi agency project funded by US EPA's STAR Program

Results: Amphibians of Coastal Wetlands

Project Summary

Amphibians are often recognized as important indicators of environmental health or ecological condition due to their dependence on both water quality and wetland habitat. During 2002-2003, researchers from the University of Wisconsin-Green Bay, University of Minnesota Duluth’s Natural Resources Research Institute, and Cornell University completed an extensive field survey of breeding frogs and toads (anurans) at 331 points in 220 coastal segments in the U.S. portion of the Great Lakes coastal zone. Methods followed guidelines outlined by MMP for conducting amphibian calling surveys (Weeber and Vallianatos 2001). Three calling surveys were conducted at each site during early spring (when night-time temperatures reached 5C), mid-spring (when over-night temperatures reached 10°C), and early summer (when temperatures reached 17°C). Calling level codes were assigned to each species, although this analysis explores simply the presence or absence of a species among all three counts.
     
Field data were complemented by GIS analysis of land cover data and other environmental variables such as human population density, pollution emissions, and agricultural impacts. Multivariate analysis of these variables yielded a gradient of “reference condition” ranging from 0 (maximally impacted by humans) to 10 (minimally impacted by humans). Wetland sites with similar reference condition were grouped into categories, and the proportion of sites where the species was recorded at least once was used as the probability of finding the species in that category. Separate analyses were conducted for the northern Laurentian Mixed Forest Ecological Province and the southern Eastern Deciduous Forest Ecological Province. More detailed, local scale variables were collected for a subset of sites by Steven Price as part of his master’s thesis work at the University of Wisconsin-Green Bay.

Results

Observers recorded 13 species of frogs and toads, four of which were too rare to warrant quantitative analysis. Responses of each anuran species to the reference gradient (Figure 1) were plotted and evaluated in the context of a new, probability-based indicator method developed by scientists in the GLEI project (Howe et al., in review). In general, frog and toad species did not show as close relationships with the reference gradient as did birds, although one species, Spring Peeper, was consistently sensitive to reference condition in both the northern (Figure 1a) and southern ecological provinces. Several species (e.g., Gray Treefrog) showed little or no relationship with reference condition in one ecological province (Figure 1b) but a stronger or opposite relationship in the other ecological province (Figure 1c).

An Amphibian-based Indicator of Ecological Condition

Our findings suggest that the occurrence of just a single anuran species, Spring Peeper, is the most defensible and robust indicator of ecological condition with respect to our reference gradient. A method for applying this method follows these steps:

1. Using a protocol similar to the MMP methods, estimate the probability of recording Spring Peepers in an area of interest. For example, use relative frequency of occurrence from multiple points as the probability or use simply 1 = present and 0 = absent.
   
2. The SSD function for Spring Peeper (e.g., Fig. 1a) will give a value or range of condition values that best “fits” the observed probability.

In Fig. 1a, a relative frequency of 0.6 will indicate a condition of approximately 1.5 in the Laurentian Mixed Forest Ecological Province. A relative frequency of 0.9 will indicate a condition between 6 and 10. This method is most useful when combined with simultaneous condition estimates from other species such as wetland birds. In other words, information from Spring Peepers can contribute to an estimate of condition based on information from multiple species, as described by Howe et al. (in review). Our findings suggest that this procedure should be applied separately for the two ecological provinces in the Great Lakes coastal zone. The inconsistent stress-response relationships of other species of frogs and toads (anurans) suggest that species richness of anurans in Great Lakes coastal wetlands clearly is not a reliable indicator of ecological condition.