Extreme flooding decreases stream consumer autochthony by increasing detrital resource availability

Abstract

Extreme disturbances, those high magnitude events that are statistically rare in a particular system, may affect consumer resource use through multiple mechanisms, such as differential consumer mortality and modified resource availability and quality. Documenting the ecological importance of these rare events is difficult, but essential, as the frequency of extreme events is predicted to increase under many climate change scenarios. We quantified changes in stream insect resource use following intense, nonseasonal flooding in the Rocky Mountains of northern Colorado during September 2013. We hypothesised that taxonomic identity, functional feeding group membership and disturbance-caused alterations in resource quantity and quality (C:N ratios) would determine consumer reliance on autochthonous and allochthonous resources and community structure following the disturbance. In summer 2014, we collected basal resources and stream insects for carbon, nitrogen and deuterium bulk stable isotope analysis, and community composition. Basal resource quantity and quality (C:N ratios) were collected at seven headwater streams along a 2013 flood intensity gradient. Using stable isotope mixing models, we analysed consumer autochthony. We also used baseline community composition data from 2011 to compare functional feeding group abundance prior to and following the flood events. We found that consumer resource use was primarily associated with detrital resource quantity, which was positively correlated with disturbance intensity and elevation. Functional feeding group membership did not predict resource use. However, consumers in functional feeding groups did experience differential mortality following flooding. Herbivore relative abundance significantly declined along the disturbance gradient, and predator relative abundance generally declined across all streams from 2011 to 2014. Our results suggest that changes in resource quantity from extreme disturbances can be associated with shifts in consumer resource use. Following this flood event, detritus standing stocks increased, resulting in a concomitant increase in consumer reliance on allochthonous sources.