Community structure of tadpoles along environmental gradients in the Bolivian Gran Chaco

Background/Question/Methods: Communities of tadpole species are assembled in association with multiple environmental gradients, in particular pond hydroperiod and canopy cover, resulting in non-random distribution of species in breeding ponds and across entire landscapes.  The relative strengths of these abiotic and biotic factors that constrain species’ distributions often change along environmental gradients.  Species distibutions often reflect their ability to cope with these abiotic and biotic constraints.  I studied the role of environmental gradients contributing to assembly of tadpole communities in the Bolivian Gran Chaco, a semiarid thorn forest that receives approximately 450 mm of rainfall annually.  Rainfall is sporadic and results in a highly dynamic pond presence across the landscape.  Ponds in this region not only contain invertebrate and fish predators, but the carnivorous tadpoles of Ceratophrys cranwelli.  I sampled 35 ponds to determine community composition and species abundance of tadpole species associated with these environmental gradients across the landscape.   At each pond, I quantified abiotic variables (pond dimensions, physiochemical factors, and canopy cover) and predator abundance and diversity.  

Results/Conclusions:

A Principal Coordinates Analysis ordination showed that community structure was primarily associated with abiotic factors, specifically forest canopy cover, pond depth, and pond area.  Both tadpole richness and abundance increased with increasing pond area and pond depth, but decreased with increasing canopy cover.  These results suggest greater species diversity and abundance in these ponds is dependent on large, open canopy ponds.  These species may be unable to cope with increasing canopy cover due to the low productivity within these pond types.  Tadpole richness and abundance had weak associations with predator abundance and diversity.  Tropical dry forests, such as the Bolivian Gran Chaco, are currently threatened with habitat loss and among the most endangered ecosystems in the world.  The implications for conservation of this study are that both species diversity and variation in community structure depends on pond structure.  Conservation strategies may be developed that take into account the environmental factors associated with community assembly as well as maintenance of species diversity.