Examining why grazing mayflies do not functionally compensate for the top-down control of algal communities following disease-driven tadpole declines in a Neotropical stream
Quantifying functional trait diversity in an assemblage is essential to understanding the potential for functional redundancy following an extinction event. We examined an apparent lack of functional redundancy by algae grazing mayflies following the disease-driven loss of stream-dwelling tadpoles from a Panamanian montane stream. Prior to their extirpation, four species of grazing tadpoles reduced algal biomass and we predicted that insect grazers would functionally compensate for tadpoles given that 9 genera of mayflies (Insecta: Ephemeroptera) in our study stream also consume algae. However, algal biomass increased 4-fold following tadpole declines, maintaining high levels for 4 yrs post-extirpation and was accompanied by shifts in diatom assemblages from small- to large-sized taxa. We then hypothesized that mayflies are gape-limited and unable to consume large-bodied diatom taxa. We compared mayfly abundances and diatom community composition by collecting monthly pre-extirpation samples (N=3) in 2004 and monthly post-extirpation samples (N=3) in 2009. We also assessed pre-extirpation tadpole diets and pre- and post-extirpation mayfly diets via gut analysis. Effects of tadpoles and mayflies on the algal assemblage were estimated using a structural equation model (SEM), and tadpole/mayfly preferences for different diatom size classes were calculated via an electivity index.
The abundance of mayflies with the smallest gape size (200-300 μm) declined by 90% from 2004 to 2009 while total mayfly abundance declined by 83% from 2004 to 2009. Populations of small diatom taxa doubled while large diatom taxa increased 5-fold from 2004 to 2009. The SEM showed that tadpoles had negative effects on both small- and large diatom taxa, but no effect on medium-sized diatom taxa, while mayflies had a weak negative effect on large diatoms. The electivity index indicated that tadpoles preferred medium-sized diatoms, but avoided the largest diatoms, while mayflies preferred the smallest diatoms. Following the amphibian decline, mayflies switched to medium-sized diatom taxa. Consumer functional trait richness also declined from 2.71 in 2004 to 0.007 in 2009. Findings suggest tadpoles had direct consumptive effects on small- and medium-sized diatoms, and non-consumptive effects on large diatoms. Non-consumptive effects by tadpoles may have been due to bioturbation associated with tadpole foraging. The decline in functional trait richness suggests tadpoles possessed traits that were lost from the algal-grazing community, preventing functional compensation by mayflies after the decline. Our results also suggest that the loss of consumer trait diversity can be linked to loss of ecosystem function.