Anthropogenic noise does not strengthen multiple‐predator effects in a freshwater invasive fish

Abstract

AbstractAnthropogenic noise has the potential to alter community dynamics by modifying the strength of nested ecological interactions such as predation. Direct effects of noise onper capitapredation rates have received much attention but the context in which predation occurs is often oversimplified. For instance, many animals interact with conspecifics while foraging and these nontrophic interactions can positively or negatively influenceper capitapredation rates. These effects are often referred to as multiple‐predator effects (MPEs). The extent to which noise can modulate MPEs and thereby indirectly alterper capitapredation remains unknown. To address this question, we derived the relationship betweenper capitapredation rate and prey density, namely the functional response (FR), of single and pairs of the invasive topmouth gudgeonPseudorasbora parvawhen feeding on water fleas under two noise conditions: control ambient noise estimated at 95 dB re 1 μPa and ambient noise supplemented with motorboat sounds whose relative importance over ambient noise ranged from 4.81 to 27 dB. In addition, we used video recordings to track fish movements. To detect MPEs, we compared the observed group‐level FRs to predicted group‐level FRs inferred from the individual FRs and based on additive effects only. Regardless of the number of fish and the noise condition, the FR was always of type II, showing predation rate in a decelerating rise to an upper asymptote. Compared to the noiseless condition, the predation rate of single fish exposed to noise did not differ at high prey densities but was significantly lower at low prey densities, resulting in an FR with the same asymptote but a less steep initial slope. Noise also reduced fish mobility, which might explain the decrease in predation rate at low prey densities. Conspecific presence suppressed the individual response to noise, the FRs of two fish (observed group‐level FRs) being perfectly similar between the two noise conditions. Although observed and predicted group‐level FRs did not differ significantly, observed group‐level FRs tended to fall in the low range of predicted group‐level FRs, suggesting antagonism and a negative effect of nontrophic interactions on individual foraging performance. Interestingly, the difference between predicted and observed group‐level FRs was not greater with noise, which means that noise did not strengthen MPEs. Our results show that when considering the social context of foraging, here through the presence of a conspecific, anthropogenic noise does not compromise foraging in the invasiveP. parva.