Forest canopy affects stream macroinvertebrate assemblage structure but not trophic stability

Understanding the determinates of community structure and function is a central theme in community ecology. The form in which energy is supplied to foodwebs can strongly influence community structure and function. Trophic stability is also thought to be affected by the nature of food web energy pathways and whether they are external (allochthonous) or internal (autochthonous) to the ecosystem. In this study, we assessed whether stream invertebrate assemblages differ in taxonomic composition, energetic network structure, trophic network stability, and assemblage temporal variability based on whether the streams they occupy occur under forested canopies or in open grasslands. We assumed that forested sites would receive more allochthonous inputs, whereas grassland sites would rely more on autochthonous resources. We also tested whether food web stability (robustness, the conservation of energy flow) changed after simulated species extinctions. We found that the forest stream assemblages were dominated by filter feeders and shredders, whereas the grassland assemblages were dominated by grazers. In spite of these differences in assemblage composition, we found no significant differences in trophic network structure, stability, or temporal variability among different site types. Many stream systems, particularly in mountainous areas, such as New Zealand, are exposed to regular and large physical disturbance from flooding, which may result in assemblages with similarly generic diets irrespective of the energy source