Increasingly, the indirect effects of predators on resources are recognized to arise through predator effects on consumer density or consumer traits. Yet, the net effect of predators on consumers does not consistently result in strong top-down control of resources and ecosystem functions. The ways in which consumers alter their behavior to minimize predation risk, while still consuming resources, i.e. adaptive foraging, may provide a mechanistic explanation for these inconsistencies. We examined the influence of predation on a widespread, invasive dominant consumer in aquatic systems, the Asian clam, Corbicula fluminea, which through its filter-feeding activity strongly influences ecosystem functions, such as benthic-pelagic organic matter and nutrient cycling. C. fluminea can also pedal feed and ingest deposited organic matter from sediment—a strategy it can perform while hidden from predators under the sediment. Thus, C. fluminea can adaptively alter its feeding strategy according to predation risk, allowing it to continue to feed. This adaptive behavior has the potential to mitigate otherwise strong indirect influences of predators on resources and ecosystem functions. We conducted a laboratory experiment to quantify and characterize the direct effects of a non-lethal crayfish predator on the growth and behavior of invasive C. fluminea and the ability of C. fluminea to mediate the indirect effects of crayfish on fine benthic (FBOM) and suspended particular (SPOM) organic matter biomass through their ability to adaptively forage.
Results/Conclusions
Results indicate that the presence of crayfish increased the burial behavior and reduced the filtration rates of SPOM by C. fluminea relative to the no predator control. While ordinarily, this direct negative effect of predators on C. fluminea foraging behavior should increase the abundance of FBOM and SPOM relative to the no predator control, resource abundance between the predator and no predator treatments was not significantly different. Furthermore, growth (change in mass) of C. fluminea did not differ between the predator and no predator treatment suggesting that C. fluminea might mediate the indirect effect of crayfish on FBOM by consuming FBOM through its alternative foraging mode (e.g. pedal feeding). Furthermore, isotopic signatures of C. fluminea collected in the field indicate that C. fluminea derives much of its energy from FBOM as opposed to SPOM. This dual foraging mode of C. fluminea is a unique strategy relative to native mollusks in the system (which do not pedal feed as adults), suggesting that this novel consumer might impact ecosystem function following invasion.