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PLoS One
2016 Jan 01;1112:e0168128. doi: 10.1371/journal.pone.0168128.
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Seed Predation by the Shore Crab Carcinus maenas: A Positive Feedback Preventing Eelgrass Recovery?
Infantes E
,
Crouzy C
,
Moksnes PO
.
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There is an increasing interest to restore the ecosystem services that eelgrass provides, after their continuous worldwide decline. Most attempts to restore eelgrass using seeds are challenged by very high seed losses and the reasons for these losses are not all clear. We assess the impact of predation on seed loss and eelgrass establishment, and explore methods to decrease seed loss during restoration in the Swedish northwest coast. In a laboratory study we identified three previously undescribed seed predators, the shore crab Carcinus maenas, the hermit crab Pagurus bernhardus and the sea urchin Strongylocentrotus droebachiensis, of which shore crabs consumed 2-7 times more seeds than the other two species. The importance of shore crabs as seed predators was supported in field cage experiments where one enclosed crab caused 73% loss of seeds over a 1-week period on average (~ 21 seeds per day). Seedling establishment was significantly higher (14%) in cages that excluded predators over an 8-month period than in uncaged plots and cages that allowed predators but prevented seed-transport (0.5%), suggesting that seed predation constitutes a major source of seed loss in the study area. Burying the seeds 2 cm below the sediment surface prevented seed predation in the laboratory and decreased predation in the field, constituting a way to decrease seed loss during restoration. Shore crabs may act as a key feedback mechanism that prevent the return of eelgrass both by direct consumption of eelgrass seeds and as a predator of algal mesograzers, allowing algal mats to overgrow eelgrass beds. This shore crab feedback mechanism could become self-generating by promoting the growth of its own nursery habitat (algal mats) and by decreasing the nursery habitat (seagrass meadow) of its dominant predator (cod). This double feedback-loop is supported by a strong increase of shore crab abundance in the last decades and may partly explain the regime shift in vegetation observed along the Swedish west coast.
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27977802
???displayArticle.pmcLink???PMC5157998 ???displayArticle.link???PLoS One
Fig 1. Diagram of treatments used in the 1-week field experiment.a) seeds on the sediment surface (Open), b) seeds covered with sand (Open+S), c) cage with seeds and a shore crab (Cage+P), d) cage with seeds covered with sand and a shore crab (Cage P+S) and e) cage with seeds (Cage C). Seed mimics represented in blue. The figure is not drawn to scale.
Fig 2. Size of shore crab (C. maenas) and predation of eelgrass seeds.
Fig 6. Regime shifts and positive feedback mechanism in eelgrass systems involving seed predation by shore crabs.Arrow thickness indicates the strength of the trophic interaction and habitat effects, where dashed lines denote weak effects. Plus and minus signs indicate positive and negative effects, respectively. In the vegetated state (a), with little impact of nutrient pollution and overfishing, eelgrass beds provide nursery habitats for a large cod population that can control the abundance of shore crabs and other small predators. The low abundance of mesopredators allows algal grazers to control ephemeral algae from overgrowing eelgrass and prevents high rates of seed predation from crabs. In the overfished and eutrohicated state (b), overfishing of cod has released shore crabs and other mesopredators from predation control, increasing predation on seeds. The high predation rate on algal grazers releases ephemeral algae from grazer control than can bloom in the nutrient rich water with negative effects on eelgrass, and indirectly in cod recruitment. The high abundance of ephemeral algae increases recruitment of shore crabs by increasing availability of shelter for juvenile crabs.
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