Mechanisms underlying lack of functional compensation by insect grazers after tadpole declines in a Neotropical stream
Here we examine mechanisms behind the observed lack of functional compensation by insect grazers following the loss of amphibian grazers in a stream ecosystem. We examine the role of relatively large-bodied tadpoles in structuring the size distribution of smaller-bodied insect grazers and compare effects of tadpoles and insects on algal diatom size distribution. The size distribution of insect grazers and algal diatoms were compared pre- (2004) and post- (2009) tadpole decline in pool and riffle habitats of a forested Panamanian stream. Tadpole and insect preferences for small, medium, and large-sized diatoms were also examined pre- and post- tadpole decline. Piecewise structural equation modeling (PSEM) was used to explore causal explanations for why insects did not functionally compensate for tadpole declines. The size distribution of insect grazers following tadpole declines differed between habitats, becoming uniform in pools and more right-skewed in riffles. In both habitats, tadpoles selectively consumed medium-sized diatoms but avoided the largest-sized diatoms. In contrast, grazing insects selectively consumed small-sized diatoms, but switched to medium-sized diatoms after tadpole declines. The PSEM showed that tadpoles had stronger effects on the size structure of the diatom community than insects and tadpole effects mediated interactions among diatom groups. With the loss of large-bodied tadpoles came the consequent loss of the strongest interactions, including non-consumptive interactions, between consumers and diatoms, interactions that small-bodied consumers could not duplicate. Furthermore, despite similarities in the taxonomic composition of insect and diatom assemblages in riffle and pool habitats, the loss of large-bodied tadpoles led to a different community structure in riffle and pool habitats, suggesting that the loss of a large-bodied species does not necessarily lead to a singular outcome for the remaining community.