Although it is generally assumed that the intensifying abiotic environment is the primary effect of drought on aquatic organisms, drought-induced top predator extinctions may be an important underlying mechanism. I used manipulative experiments to disentangle the impacts of drying and top predator extinctions on arid-land aquatic invertebrate communities. I then created a general conceptual framework that specifies how biotic and abiotic disturbances affect the composition of biological traits of species in a community (functional trait composition) and tested the framework with data from the manipulative experiments and a field study. Finally, I proposed a new metric to calculate the difference in functional trait composition between undisturbed and disturbed communities (called “functional distance”) and used it to understand trait turnover between undisturbed and disturbed arid-land stream communities.
In Chapter 2, I describe two manipulative experiments in which I removed an invertebrate top predator from mesocosms containing arid-land stream invertebrates and recorded changes in the aquatic community. I found that top predator removal consistently decreased the abundance of detritivores and increased the abundance of mesopredators, even under different background environmental conditions.
Chapter 3 describes a second mesocosm study in which I manipulated drying severity and measured aquatic community responses. My severe drying treatment allowed mesocosms to desiccate to a depth of ~1cm, yet I still I found that taxonomic and functional trait composition did not vary between treatments. The only discernable effect of drying was a decrease in abundance and increase in density of invertebrates. This result suggests that arid-land aquatic communities are highly resistant to drying disturbance that falls within the range of natural seasonal and interannual droughts but not resistant to the novel disturbance of top predator extinctions.
In Chapter 4 I describe a conceptual framework that uses functional trait diversity to understand the mechanisms behind community responses to disturbances. I applied the framework to datasets from Chapters 2 and 3 and an observational field study during severe drought. While the biotic disturbance of top predator removal did not affect species diversity in the taxonomic analysis, it increased the overall functional trait diversity and favored trait combinations associated with aerial dispersal and predatory feeding modes. Interestingly, although natural stream drying occurred concurrently with the local extinction of the invertebrate top predator in the field, this extreme abiotic disturbance was associated with a reduction in functional trait diversity. The contradictory effects of these two novel disturbance types highlight the importance of colonization and spatial context in the resilience of arid-land aquatic communities to future disturbances; differences between disturbed and undisturbed communities in both top predator removal and catastrophic stream drying studies were associated with aerially dispersing invertebrates. My work suggests that combining taxonomic and functional trait analyses in a rigorous hypothesis-testing framework can reveal hidden mechanisms behind the effects of drought on aquatic communities. Ultimately, I hope that this framework can be applied to other disturbed systems to better understand the effects of human actions on ecological communities.