Non-Linear Interactions between Consumers and Flow Determine the Probability of Plant Community Dominance on Maine Rocky Shores

Brian R Silliman,Mark D Bertness,Michael W Mccoy,Patrick J Ewanchuk,Caitlin M Crain,Geoffrey C Trussell,Nathan G Swenson

doi:10.1371/journal.pone.0067625

Brian R Silliman, Mark D Bertness + Show 5 more

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https://doi.org/10.1371/journal.pone.0067625

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Abstract

Although consumers can strongly influence community recovery from disturbance, few studies have explored the effects of consumer identity and density and how they may vary across abiotic gradients. On rocky shores in Maine, recent experiments suggest that recovery of plant- or animal- dominated community states is governed by rates of water movement and consumer pressure. To further elucidate the mechanisms of consumer control, we examined the species-specific and density-dependent effects of rocky shore consumers (crabs and snails) on community recovery under both high (mussel dominated) and low flow (plant dominated) conditions. By partitioning the direct impacts of predators (crabs) and grazers (snails) on community recovery across a flow gradient, we found that grazers, but not predators, are likely the primary agent of consumer control and that their impact is highly non-linear. Manipulating snail densities revealed that herbivorous and bull-dozing snails (Littorina littorea) alone can control recovery of high and low flow communities. After ∼1.5 years of recovery, snail density explained a significant amount of the variation in macroalgal coverage at low flow sites and also mussel recovery at high flow sites. These density-dependent grazer effects were were both non-linear and flow-dependent, with low abundance thresholds needed to suppress plant community recovery, and much higher levels needed to control mussel bed development. Our study suggests that consumer density and identity are key in regulating both plant and animal community recovery and that physical conditions can determine the functional forms of these consumer effects.

Highlights

Understanding factors that regulate the recovery and secondary succession of communities following disturbances is a core focus of ecology and conservation [1,2,3,4,5,6,7]
95% of the area in high flow sites were colonized by barnacles when both snails and crabs were present, but barnacle cover was reduced to 42% at low flow sites under the same consumer treatment (Fig. 1a)
In the consumer exclusion plots, barnacle cover showed the opposite trend: barnacles covered less than 20% of the area at the high flow sites, and achieved nearly 60% coverage, on average, at low flow sites (Fig. 1a)

Summary

Introduction

Understanding factors that regulate the recovery and secondary succession of communities following disturbances is a core focus of ecology and conservation [1,2,3,4,5,6,7]. The recruitment and establishment of plant and animal propagules in local communities can be under strong trophic control because consumers often create unoccupied space for new propagules to exploit (by consuming or disrupting competitors of the settlers) or by consuming or aggravating propagules after they have settled [14,15,16,17,18,19,20] The strength of these top-down consumer effects is often a function of habitat type, consumer density and consumer species [21,22,23]. Much of what we know about how the effects of comsumer density, identity and phsycial factors interact to impact plant communities has been drawn from untested models [23]

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