Fish as a structuring force of aquatic macroinvertebrate communities: Assessing the role of predator type on community structure and its variation with time and drought

Background/Question/Methods

Understanding the relative contributions of deterministic and stochastic process in the structuring of communities has been a cornerstone in community ecology research for the past decade. Systems with harsh ecological niche filters often have a more deterministic basis to community structure. These filters are often assumed to be static in presence and strength, investigations testing their stability through time are largely lacking, particularly in combination with concurrent forms of environmental change such as prolonged droughts. We examined the role of the presence of fish in structuring the aquatic macroinvertebrate communities of wetland habitats to answer the questions: what are the relative strength of fish niche filter in structuring communities, and how does the strength of niche filters vary through time? Over the course of four years, we characterized macroinvertebrate communities of 12 small freshwater ponds with fish and 24 without fish in California during the onset of a severe drought. We conducted field surveys from 2011 – 2014 to assess how both local alpha diversity and gamma diversity responds to different predator types. We performed two types of analysis: within each year, we examined the influence of fish on community diversity (alpha and gamma) and species composition; we then examined how the influence of fish on community diversity and species composition varied across years and with severity of drought.

Results/Conclusions

Across the duration of our study, fish presence had an overall strong, negative effect on both alpha and gamma diversity, reducing alpha diversity by 3 species and gamma by 10 relative to fishless sites. However, we found a significant interaction between the year of sampling and the presence of fish in determining both the mean alpha diversity and species composition. The effects of fish on alpha and gamma diversity progressively weakened over the study period, such that there were no differences by the last year of our study. Moreover, the taxonomic groups largely lacking from fish sites had re-appeared by 2014. This pattern was closely linked with changes in precipitation due to a severe drought recently experienced in California. Correspondingly, incorporating annual precipitation into models offered improved model fit compared to year alone. This suggests the relative strength of niche filters may shift as a result of external forces. The results of our study suggest that the harshness of the niche filter varies significantly through time and regional factors may play important yet variable roles in influencing local community structure.