Top predators suppress rather than facilitate plants in a trait-mediated tri-trophic cascade

Abstract

Classical ecological theory states that in tritrophic systems, predators indirectly facilitate plants by reducing herbivore densities through consumption, while more recent work has revealed that predators can generate the same positive effect on plants non-consumptively by inducing changes in herbivore traits (e.g. feeding rates). Based on observations in US salt marshes dominated by vast monocultures of cordgrass, we hypothesized that sit-and-wait substratedwelling predators (crabs) could actually strengthen per capita impacts of potent grazers (snails), by non-consumptively inducing a vertical habitat shift of snails to their predation refuge high on canopy leaves that are vulnerable to grazing. A two-month field experiment supported this hypothesis, revealing that predators non-consumptively increased the mean climbing height of snails on grasses, increased grazing damage per leaf and ultimately suppressed cordgrass biomass, relative to controls. While seemingly counterintuitive, our results can be explained by (i) the vulnerability of refuge resources to grazing, and (ii) universal traits that drove the vertical habitat shift-i.e. relative habitat domains of predator and prey, and the hunting mode of the top predator. These results underline the fact that not only should we continue to incorporate non-consumptive effects into our understanding of top-down predator impacts, but we should do so in a spatially explicit manner.

Foundation species' overlap enhances biodiversity and multifunctionality from the patch to landscape scale in southeastern United States salt marshes

“…B 282: 20150421 boost individual ecosystem functions through distinct direct and indirect mechanisms. More specifically, from extensive experimental investigations [7,20,31,[43][44][45], we infer that mussels directly contribute to augmenting three ecosystem functions-invertebrate biomass, soil accretion and aboveground primary production-through their own engineering of habitat and deposition of soil-and nutrient-rich pseudofaeces [31,35,45]. Conversely, mussels are very likely stimulating infiltration and decomposition functions indirectly through their facilitation of mud and marsh crabs, species that excavate wide and deep burrows, respectively, through which water can readily infiltrate [7], and facilitation of juvenile fiddler crabs that promote microbial activity as a result of their bioturbation and aeration of marsh soils [36,43].…”

Biodiversity, Species Interactions and Ecological Networks in a Fragmented World

“…Habitat loss in kelp forests reduces biomass and abundance of fish (Deza and Anderson, 2010). The die-off of salt marshes results in changes in the behaviour of key grazers (snails) as they seek shelter from predation by blue crabs (Griffin et al, 2011;Silliman et al, 2005). Finally, increased fragmentation of sea ice habitats results in declines in mating success and searching efficiency of top predators such as polar bears (Molnár et al, 2011) and in changes in phototrophic community structure and relative abundance of dominant marine taxa (Mueller et al, 2006).…”

Independent and combined effects of multiple predators across ontogeny of a dominant grazer

“…Littoraria famously undergo a rhythmic tidal migration, moving with the rising tide from lower Spartina stems and the benthos to elevated Spartina leaves, in a bid to escape nektonic predators (Vaughn and Fisher ). Recent work shows that resident benthic predators cause Littoraria to remain at elevated positions on Spartina leaves, even at low tide (Griffin et al , Kimbro ). This non‐ consumptive effect of resident benthic predators increases the grazing activity and per capita impacts of Littoraria on Spartina (Griffin et al , Kimbro ).…”