Exploiting fear in restoration? The non-consumptive effects of avian predators on fish and seagrass associated fauna

Background/Question/Methods

Since the 1980s, bird roosting stakes have been used to restore damaged seagrass beds, where the addition of bird roosting stakes increases local nutrients through the input of bird guano and promotes the growth of seagrasses. Although these bottom-up processes are well described, the predatory top-down effects of birds in these estuarine systems have often been overlooked. Specifically, the bird stake restoration method increases traffic of predatory birds such as cormorants, pelicans, seagulls, and herons which may have multiple consumptive and non-consumptive direct and indirect effects on seagrass faunal communities.  In order to examine the impact of birds and the bird stake restoration method on seagrass faunal communities, we examined the direct and indirect effects of avian predators on pinfish and associated seagrass fauna using laboratory and field experiments.

Results/Conclusions

In laboratory mesocosms, pinfish swimming activity was greatly reduced by models of herons but only slightly by seagulls, suggesting that the response to avian predators was species-specific and herons were perceived as the greatest threat. This effect was confirmed in the field where traps adjacent to heron models also caught less fish. Additional tethering experiments suggested that fish may also avoid bird roosting stakes, as tethered shrimp were recovered more often adjacent to bird roosting stakes than in control habitats. To determine if herons and/or bird roosting stakes caused indirect non-consumptive effects to seagrass communities, the abundances of fish, epifauna, and epiphytes were monitored on artificial seagrass units (ASUs) over several months in response to four treatments: heron models, bird roosting stakes, controls, and bird exclusions. Although fewer fish were collected adjacent to heron models, the evidence for indirect effects to the invertebrate and epiphyte community is mixed and still under investigation.