The adaptive capacity of aquatic food webs in fluctuating environments
How species and food webs respond to environmental variability is one of the most relevant questions facing ecologists today. The ability of generalist consumers to rapidly respond to declining prey density via increased consumption of their prey's prey, i.e. omnivory, can stabilize spatially compressed food webs by dampening otherwise strong predator-consumer (P-C) interactions. However, it remains unclear if shifts in the degree of omnivory also occur through time, in response to variable environmental conditions. Here, we use a combination of theory and empirical data to determine how omnivory changes with season in response to a specific environmental fluctuation, the flood pulse of floodplain ecosystems. The rising waters of the flood pulse are well known to expand fish habitat and replenish available nutrients for primary production, which, in turn, might predictably impact fish feeding behavior. Model food webs were first used to explore how changing the extent of water coverage within a year impacted the relative production of consumers and resources (C:R) and the extent of predator omnivory. New and previously published empirical food web data from both dry and wet seasons were then compared to model predictions.
Model food webs exhibited decreasing C:R and increasing predator omnivory with declining water levels. Empirical data supported these predictions because fish predators exhibited increased omnivory during the dry compared to the wet season in both North and South American floodplains. Increased omnivory by generalist predators during the dry season was apparently driven by both increased accessibility to lower trophic level resources (due to habitat compression) and reduced consumer production (lower C:R) due to declines in food availability and refugia. Importantly, increased omnivory by predators at precisely the time (dry periods) when food webs experience both spatial compression and diminished consumer production (lower C:R) should alleviate otherwise strong P-C interactions. Our findings therefore indicate that omnivory in floodplain food webs changes predictably with season in such a way that might promote their stability. More generally, we show that seasonal omnivory is one way in which predators and food webs change adaptably in the face of environmental variability.