The environmental stress hypothesis (ESH) predicts decreasing top-down control by consumers as the frequency and intensity of environmental stress increases. Often applied at the landscape level such as that of an exposed rocky shore, we propose a more nuanced definition of stress at the level of the individual organism. In particular, if an animal is able to move into, feed in, and exit a resource patch faster than the return time (periodicity) of the stressor, then it has the potential to exert top-down effects regardless of stress. We studied the composition of urchin and fish herbivore assemblages on rocky subtidal reefs in the Galapagos, and hypothesized that their potential for top-down control on algal communities across a wave stress gradient would vary according to species mobility. We quantified wave stress at paired exposed/protected locations measuring flow every 3s. Fish and macroinvertebrate assemblages and bite rates on the substrate were quantified by visual census and video. In a manipulative experiment, cages were bolted to the rock in 4 treatments: open (+fish +urchins), cage (-fish - urchins), fence (+fish -urchins), and roof (-fish +urchins). Algal settlement plates were recovered after two months and all algae were weighed and identified.
Results/Conclusions
Despite peak wave velocities of 289.6 cm/s, herbivorous fish biomass was 67.8 times greater at exposed sites. Several families of fishes (primarily Acanthuridae) were observed feeding in this high-energy environment, where bite rates were concentrated during lulls in flow speeds in between wave sets. In contrast, urchin densities were 2.7 times greater at protected sites, where 28% of individuals were observed feeding outside of refuges (<1% at exposed sites). Benthic cover at protected sites was exclusively crustose coralline algae, whereas exposed sites contained a diverse algal community. Allowing access only to urchins, experimental fish exclusions (roofs) resulted in algal biomass that was 502 times greater at exposed vs protected locations, following ESH predictions. In contrast, when urchins were excluded, algal biomass was only 23 times greater at exposed sites indicating minimal flow limitation of fish herbivory. Combined, our results indicate that fish herbivory and top-down control is only partially suppressed at extreme flow conditions relative to urchins. We use this model to illustrate the utility of a “stress-mobility” hypothesis, in which the movement capacity of a foraging animal relative to the frequency of environmental stress predicts the extent to which ESH applies.