Ecological Interactions of Frog Larvae at a Seasonal Tropical Location in Thailand

To determine the adaptive strategies frogs utilize to exploit tropical seasonal environments, a 10 month study of tadpole ecological interactions was undertaken at the Sakaerat Experiment Station, located approximately 250 km NE of Bangkok. The temporal distribution patterns of larval species occurrence for several ponds show the following: 1) all the species have seasonal reproductive cycles, which are correlated with the unavailability of aquatic habitats during the dry season; 2) the greatest number of species using ponds as larvae correlates with the first time that the aquatic habitats retain water long enough for the larvae to complete metamorphosis; 3) patterns of larval species occurrence in response to the same environmental stimuli are varied; and 4) the occurrence of larval species in any single pond seems due to chance. Food analyses and behavioral observations are compared with niche analyses of overlap based on larval species abundances. Food differences do not add any information to that obtained from occurrence patterns in characterizing larval niches. The frog fauna at Sakaerat divides the total environment spatially and temporally at several levels. The most distinctive partitioning of the environment occurs between the wet (14 species) and dry (3 species) season breeders. The rainy season breeders subdivide the available aquatic environment by: 1) utilizing different ponds within the total environment, 2) occupying a single pond at different times, and 3) dividing up the living space within a pond spatially, rather than by selecting different food categories. All species for which data were gathered have short larval life periods which correlate with the uncertain aquatic habitat due to sporadic rainfall. All of the ponds had a much greater larval biomass at an early time rather than at a later time. Perhaps by having a period of larval activity at the beginning of the wet season, newly metamorphosed frogs are assured of a favorable period of growth before having to survive the dry season. Alternatively, the period of heavy larval usage may occur at the beginning of the pond cycle to avoid heavy predation. The single physical factor of rainfall distribution regulates anuran reproductive patterns in tropical areas characterized by a pronounced dry season. The more northerly the environment, the greater the number of physical factors which control the anuran reproductive patterns. * * *