Consumers mediate natural variation between prey richness and resource use in a benthic marine community

Abstract

Space is the limiting resource for sessile organisms on marine rocky substrata, and the availability of space is decreased by recruitment and growth but increased through senescence, physical disturbance and consumption. In the present study, we examined whether consumers mediate variation in the relationship between prey richness and resource (space) use in subtidal epifaunal communities. First, we used surveys to identify relationships between prey richness, consumer richness, consumer identity and consumer abundance with available space. As predicted, available space was inversely correlated with sessile prey richness and positively correlated with consumer richness. However, a model selection approach identified the abundance of sea urchins and chitons specifically as the best predictors of available space, suggesting that the proportion of available space is a reasonable indicator of recent disturbance. Next, we manipulated urchin density in the field to test the hypothesis that urchins control the structure of this community by grazing sessile taxa and facilitating smaller consumers. Diet analyses and structural equation models together indicate that urchins generate available space directly by consuming macroscopic sessile prey, and indirectly by facilitating chitons, which maintain patches of space free of microscopic algae and recruits of larger sessile taxa. The significant interaction between prey richness and experimental urchin density on available space suggests that prey richness may buffer the impacts of urchin grazing. More generally, we highlight the need to study the effects of species richness on the structure of communities in the context of relevant ecological processes.KEY WORDS: Space · Diversity · Strongylocentrotus · Tonicella · Subtidal · EpifaunaResale or republication not permitted without written consent of the publisher

Sedentary urchins influence benthic community composition below the macroalgal zone

“…The amount of bare space was distinctly increased and the presence of fragile organisms (including hydroids and unidentifiable fuzz) decreased in the presence of urchins. These results support the findings of previous studies showing that urchins create space and change sessile communities (Sammarco ; Elahi & Sebens ). Interestingly, the influence of urchins was often limited to the diameter of the spine canopy, such that adjacent areas were seemingly undisturbed.…”

“…No correlations were observed among the quantified sessile and mobile organisms (Mantel, P = 0.764). Available space, defined as bare bedrock and crustose coralline algae (Elahi & Sebens 2012), was significantly higher in urchin plots (21.2 versus 8.6% control, P < 0.0001; Table 2). This difference was greater at 23 m, yet significant at both 23 and 30 m ( Table 2).…”

Not all space is created equal: Distribution of free space and its influence on heat-stress and the limpet Patelloida latistrigata

“…Free space is usually created by physical disturbances or biotic processes such as predation and senescence and is reduced by recruitment and growth of sessile organisms (Elahi and Sebens, 2012;Sousa, 1984). The amount of free space has been shown to be a leading factor governing rates of larval settlement, recruitment and food supply within marine benthic habitats (Bertness, 1989;Gaines and Roughgarden, 1985;Minchinton, 1997;Minchinton and Scheibling, 1993).…”

Habitat modification in tidepools by bioeroding sea urchins and implications for fine‐scale community structure

“…We hypothesize that the differences in community patterns between microhabitats for primary space holders and mobile taxa are likely related to interactions between biotic forces (such as urchin disturbance and herbivory; Fewkes ; Benedetti‐Cecchi & Cinelli ; Elahi & Sebens ) and abiotic factors (such as wave force; Dayton ). Biotic disturbance and herbivory by urchins likely resulted in the relatively low abundances, diversity, and richness of tidepool flora and fauna we observed in urchin‐inhabited pits.…”