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PLoS One
2013 Jan 01;811:e78969. doi: 10.1371/journal.pone.0078969.
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Experimental removal and recovery of subtidal grazers highlights the importance of functional redundancy and temporal context.
Elahi R
,
Sebens KP
.
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The extent to which different grazers are functionally redundant has strong implications for the maintenance of community structure and function. Grazing by red urchins (Strongylocentrotus franciscanus) on temperate rocky reefs can initiate a switch from invertebrate or macroalgal dominance to an algal crust state, but can also cause increases in the density of molluscan mesograzers. In this study, we tested the hypothesis that red urchins and lined chitons (Tonicella spp.) are redundant in the maintenance of available space, defined as encrusting algae and bare rock. In a factorial field experiment replicated at three sites, we reduced the densities of urchins and chitons on subtidal rock walls for nine months. The effects of grazers were interpreted in the context of natural temporal variation by monitoring the benthic community one year before, during, and after grazer removal. The removal of each grazer in isolation had no effect on the epilithic community, but the removal of both grazers caused an increase in sessile invertebrates. The increase was due primarily to clonal ascidians, which displayed a large (∼75%) relative increase in response to the removal of both grazers. However, the observed non-additive responses to grazer removal were temporary and smaller than seasonal fluctuations. Our data demonstrate that urchins and chitons can be redundant in the maintenance of available space, and highlight the value of drawing conclusions from experimental manipulations within an extended temporal context.
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24250819
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Figure 1. Densities of urchins (Strongylocentrotus franciscanus) before, during, and after the experiment.Variation in urchin densities (mean±SE; n = 9) during the experimental period was strongly dependent on urchin treatment (Table S1), with a sharp reduction observed on urchin removal transects and higher (but variable) densities on urchin control transects.
Figure 2. Densities of lined chitons (Tonicella spp.) before, during, and after the experiment.During the experimental period (March 2009–March 2010), chiton densities (mean±SE; n = 18) were reduced in the chiton removal treatment relative to the chiton control treatment. The best-fit model included the effects of chiton removal, time, and their interaction (Table S2).
Figure 3. Percent cover of sessile invertebrates and macroalgae before, during, and after the experiment.The percent cover (mean±SE, n = 18) of invertebrates (A) during the experimental period (March 2009– March 2010) was variable, and depended on the interaction between urchin treatment, chiton treatment, and time (Table 1). Note that the percent cover of invertebrates was highest in quadrats subject to both urchin and chiton removal at the end of the experiment, but this effect was transient. After one year of recovery (March 2011), invertebrate cover was indistinguishable among experimental treatments after one year of recovery. In contrast, the percent cover of macroalgae (B) depended only on time. Black squares indicate the dates of consumer removal. The best model (Table 1) is indicated for each panel.
Figure 4. Change in percent cover of clonal ascidians and available space during and after the experiment.The experimental removal of both urchins and chitons caused an increase in the percent cover (mean±SE, n = 18) of clonal ascidians (A), and a decrease in available space (B) in quadrats. After a year of recovery, the percent cover of clonal ascidians (C) decreased in quadrats previously subjected to both urchin and chiton removal. In contrast, the effects of grazer removal did not affect the recovery of space (D). The best model (Table 2) is indicated for each panel, except when no candidate model was better than the null model.
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