Vocal behavior and the nonrandom structure of two anuran breeding assemblages in Guyana

Factors affecting the structure of communities and the distribution of species are contentious topics in ecology. One of the principal problems in community ecology is identifying patterns or structure in data sets. A review of ecological community literature indicates that many studies of communities usually assume that their component species exhibit nonrandom patterns organized through biotic and abiotic factors. These assumptions are usually not tested, yet the analyses and conclusions depend on the presence of nonrandom patterns. All studies of frog breeding communities or assemblages fall into this category. Many frogs gather at ephemeral bodies of water, and here, males form mixed species breeding groups or choruses by producing calls that attract gravid conspecific females who are ready to mate. Thus, I determined whether null models could test for nonrandom patterns in pond breeding assemblages comprised of 16 and 14 species in Guyana, South America. Comparisons of empirical data against two null models indicated that observed patterns of temporal, spatial and acoustical variables, and calling site use by heterospecific frogs were significantly nonrandom patterns. Observed patterns of spectral and calling site partitioning appeared to contribute to high species richness of these Neotropical frog breeding assemblages. Principal component analysis of advertisement calling variables of sympatric treefrogs (Hylidae) suggested that species differed in dominant frequency, fundamental frequency, pulse rate, note duration, call duration, and call rate. Discriminant function analysis of these same call variables for the numerically three most common treefrog species in the genus Scinax indicated that fundamental frequency and pulse rate alone separated the three species. Overall null models and multivariate statistical techniques indicated that nonrandom patterns of acoustic and calling site usage were implicated in the apparent nonrandom structure of these Neotropical pond breeding anuran assemblages.