The relationships between stressors, macroinvertebrate community structure and leaf processing in stream ecosystems.

In the face of major threats to global biodiversity, and in order that ecosystem managers might act to reduce the impacts of anthropogenic stressors on ecosystems, it is critical that we understand how ecosystem structure and function respond to stressors. I focussed on investigating the relationship between macroinvertebrate community structure and function, measured as the rate of leaf processing. An initial meta-analysis of previous studies revealed no association between structure and function in streams exposed to three distinct pollutant stressors (heavy metal contamination, acidification and organic pollution). Interpretation of patterns was hindered by low sample sizes, and so a field study was conducted to clarify patterns in response to heavy metal contamination. Stream sites were located in Cornwall and Lanarkshire. Associations between structure and function were driven by the direct effects of stressors in Cornwall, but not in Lanarkshire. The results indicate that the only way to assess function effectively in natural streams may be to make direct assessment of functional aspects of the system, in addition to structural assessment.
Experimental stream mesocosms were used to determine whether structure reveals function, in so much that the rates of leaf processing by mixed-species assemblages were predictable from the rates of species in isolation. Rates of leaf processing were greater than predicted, indicative of complementarity between shredder species. Finally, species-specific feeding trials were used to determine the effect of fungal species richness on rates of leaf processing by macroinvertebrate shredders. While there was some evidence for complementarity between fungal species, which resulted in increases in leaf processing between 1 and 3 fungal species, overall there was no effect of increasing fungal species richness. Results of both experimental studies indicate that the relationship between structure and function is idiosyncratic. The implication of this for the management of freshwater ecosystems is that it is difficult to predict the consequences of species’ losses for ecosystem processes.