Elevational Patterns of Species Richness, Evenness, and Abundance of the Costa Rican Leaf-Litter Herpetofauna

The abundance, species richness, and evenness of the Costa Rican leaf-litter herpetofauna was estimated during the late wet season of 1985 by quantitative sampling of replicate plots at ten sites encompassing an elevation range of 3 to 1670 m. Species richness was positively correlated with leaf-litter depth, and negatively correlated with elevation. Herpetofaunal density also tended to increase with litter depth and decline with elevation. A strong positive correlation existed between species richness and herpetofaunal density. Evenness was highly variable and independent of both leaf-litter depth and elevation. Analysis of a subset of the data, representing an elevational transect from Tortuguero to the Braulio Carrillo National Park Extension, yielded similar results. Tropical leaf-litter reptiles and amphibians appear to be both more diverse and more abundant at lower elevations. Sites with deep leaf litter generally sustain dense and diverse reptile and amphibian populations. Local herpetofaunas typically consist of a few very common species along with a large number of comparatively rare species. REPTILES AND AMPHIBIANS are a major constituent of the fauna inhabiting tropical forest litter. Studies of tropical leaf-litter herpetofaunas have emphasized patterns of abundance and distribution among geographic regions (Lloyd et al. 1968; Heyer & Berven 1973; Scott 1976, 1982; Inger 1980a, b; Heatwole 1982), vegetational zones (Brown & Alcala 1961, Heatwole & Sexton 1966, Heyer 1967), and elevational gradients (Brown & Alcala 1961, Scott 1976) as well as seasonal changes (Lieberman 1982, 1986). It has become generally accepted that leaf-litter reptiles and amphibians are more abundant in the New World Tropics than in Southeast Asia (Scott 1976, Inger 1980b, Duellman & Trueb 1986) although the exact reason remains a subject of debate (May 1980). Similarly, the general consensus is that the abundance of tropical leaf-litter herpetofaunas increases with increasing elevation, while species richness and equitability both decline (Brown & Alcala 1961, Scott 1976, Heatwole 1982, Duellman & Trueb 1986). Scott (1976) attributed this pattern to greater overall forest productivity at intermediate elevations, coupled with increased densities of the most common species at higher elevations. Before attempting to determine the mechanisms responsible for observed patterns of tropical leaf-litter herpetofaunal abundance and diversity, we must document the phenomenon conclusively. Previous quantitative studies have sampled few sites, have been unable to provide replicated samples (Brown & Alcala 1961), or may have confounded site and year effects by sampling different sites in different years (Scott 1976). We surveyed the Costa Rican leaf-litter herpetofauna by quantitatively sampling replicate plots at ten sites, ranging from 3 to 1670 m in elevation, during the late wet season of July-September 1985. Our sampling strategy permitted statistical analyses to determine the relationship between litter depth and elevation, and leaf-litter herpetofaunal richness, evenness and abundance. The results of our study contradict several of the generally accepted patterns of tropical leaf-litter herpetofaunal abundance and diversity.