ABSTRACT
Agriculture has contributed to loss of vertebrate biodiversity in many regions, including the U.S. Corn Belt (Erlich 1988; Risser 1988; Herkert 1991; Soulé 1991; Freemark 1995; McNeely et al. 1995). Amphibian populations, in particular, have experienced widespread and often inexplicable declines, range reductions, and extinctions (Blaustein and Wake 1990; Wake 1991). Amphibians are an important group to monitor and model for many reasons. They respond to both aquatic and terrestrial conditions, and are considered by many to be barometers of ecosystem health. They were historically an important component of the food web in the region. Among non-fish vertebrates, amphibians are the group with the greatest proportion of threatened and endangered species in Iowa. Of the 23 amphibian species known to occur in Iowa, 12 are declining, threatened, or endangered within the state (Christiansen 1981), leading to forecasts that without a reversal, less than one-third of Iowa's present amphibian and reptile fauna will remain in 50 years. Few attempts have been made, however, to model amphibian population dynamics (Gibbs 1993; Halley et al. 1996). Even fewer models have been spatially explicit or have taken landscape complementation (the proximity of two critical and largely independent habitats and the degree to which amphibians can move between them) and climatic variability into account (Vos and Stumpel 1995). The paucity of life history and dispersal data for most amphibian species makes model parameterization difficult (Doak and Mills 1994; Halley et al. 1996). Because many species require both aquatic and terrestrial habitats, they are limited to areas where there is both sufficient moisture for reproduction and survival and access to adjacent terrestrial habitats (Wilbur 1987; John-Alder and Morrin 1990). In the Midwest, climatic variability is also an important component of amphibian population dynamics. Northern prairie wetlands undergo a 10- to 20-year wet–dry cycle, resulting in two- to three-year droughts (Duvick and Blasing 1981; Karl and Riebsame 1984). Droughts have been linked to suppressed amphibian reproductive activity (Dodd 1993), decreased reproductive success (Seale 1982; Semlitsch 1983, 1987; Skelly 1995, 1996), localized extinctions, and population declines (Blair 1957). For these reasons, models that incorporate both climatic variability and landscape complementation are useful in simulating amphibian population dynamics (Dunning et al. 1992; Taylor et al. 1993; Pope et al. 2000).
