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

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

Do alternate stable community states exist in the Gulf of Maine rocky intertidal zone? – Ecology 83

Abstract. It has recently been hypothesized that intertidal mussel beds and seaweed canopies in the Gulf of Maine are alternate community stable states or disturbance patch mosaics dominated by either seaweed or mussel communities. The community that occurs in a given site is proposed to be stochastic and dependent on the size of the original disturbance and subsequent recruit availability. Large disturbances are postulated to be dominated by mussel beds and barnacles with widely dispersed larvae, whereas smaller disturbances are dominated by seaweeds, with limited dispersal. Positive feedbacks are proposed to maintain these two communities. We tested this hypothesis in a tidal estuary in central Maine. At eight mussel bed and eight seaweed canopy sites, we created 9-m 2 and 1-m 2 clearings and an unmanipulated control area, and in each plot established control, caged, and cage control quadrats. After three years of monitoring, our results do not support the alternate stable state hypothesis. Instead, they suggest that the occurrence of mussel beds and seaweed canopies is highly deterministic. Seaweed canopies dominate habitats with relatively little water movement, whereas mussel beds dominate habitats with high flows; and largely independent of disturbance size, mussel beds and seaweed canopies rapidly returned to their original community type, but only in the absence of consumers (crabs and snails). With consumers present, neither community showed significant signs of recovery, even after three years. In the presence of consumers, community recovery appears to be dependent on cracks and crevices providing refuges from consumers to seaweed and mussel recruits. The idea that natural communities may represent stochastically determined alternate stable states has important implications for understanding and managing natural ecosystems, but the very existence of alternate stable states in nature has been difficult to establish. Our results suggest that intertidal seaweed canopies and mussel beds in tidal rivers in the Gulf of Maine are highly deterministic alternative community states under consumer control. More generally, since all proposed examples of alternate community stable states are based on indirect, inferential evidence, our results imply that stochastically determined alternate community stable states might be an interesting theoretical idea without a definitive empirical example

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

Extracted text; 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

Pictures of representative impact of snail grazing at variable densities at high flow sites.

<p>Pictures of representative impact of snail grazing at variable densities at high flow sites.</p

Pictures of representative impact of snail grazing at variable densities at low flow sites.

<p>Pictures of representative impact of snail grazing at variable densities at low flow sites.</p

Effects of water flow rate and grazer (snail) density on a. Barnacles, b. Fucus, and c.

<p>Mussel recruitment in experimentally generated bare patches on the Damariscotta River, Maine USA. Symbol size depicts the number of data points occuring at that value, lines depict fits to the data using a beta regression, and shaded regions indicate the 95% confidence limits for the fitted line.</p