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Sci Rep
2020 Oct 22;101:18079. doi: 10.1038/s41598-020-75117-2.
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Trophic downgrading reduces spatial variability on rocky reefs.
Edwards MS
,
Konar B
.
Abstract
Trophic downgrading in coastal waters has occurred globally during recent decades. On temperate rocky reefs, this has resulted in widespread kelp deforestation and the formation of sea urchin barrens. We hypothesize that the intact kelp forest communities are more spatially variable than the downgraded urchin barren communities, and that these differences are greatest at small spatial scales where the influence of competitive and trophic interactions is strongest. To address this, benthic community surveys were done in kelp forests and urchin barrens at nine islands spanning 1230 km of the Aleutian Archipelago where the loss of predatory sea otters has resulted in the trophic downgrading of the region's kelp forests. We found more species and greater total spatial variation in community composition within the kelp forests than in the urchin barrens. Further, the kelp forest communities were most variable at small spatial scales (within each forest) and least variable at large spatial scales (among forests on different islands), while the urchin barren communities followed the opposite pattern. This trend was consistent for different trophic guilds (primary producers, grazers, filter feeders, predators). Together, this suggests that Aleutian kelp forests create variable habitats within their boundaries, but that the communities within these forests are generally similar across the archipelago. In contrast, urchin barrens exhibit relatively low variability within their boundaries, but these communities vary substantially among different barrens across the archipelago. We propose this represents a shift from small-scale biological control to large-scale oceanographic control of these communities.
Figure 1. Map of study area showing the nine islands sampled across the Aleutian Archipelago (inset shows portion of archipelago where islands are located). Coordinates in decimal degrees for approximate sampling locations are: Attu: 52.92°, 173.20°; Nizki/Alaid: 52.74°, 174.00°; Kiska: 51.97°, 177.58°; Amchitka: 51.41°, 179.28°; Tanaga: 51.81°, â 177.94°; Adak: 51.87°, â 176.66°; Atka: 52.10°, â 174.69°; Yunaska: 52.66°, â 170.74°; and Chuginadak: 52.84°, â 169.75°. Image from Metzger et al. 2019.
Figure 2. Bar graphs showing variance components (Ï2) associated with each spatial scale in the PERMANOVA models for both density-based and biomass-based data. Gray bars on the right show total amount of variation in each model.
Figure 3. Bar graphs showing magnitude of effects (% of total variation) that is associated with each spatial scale in each PERMAOVA model for both density-based and biomass-based data.
Figure 4. Density data-based nMDS plots comparing relative similarities in barren and kelp forests communities at different spatial scales; (A) among quadrats within each site, (B) among sites within each island, and (C) among islands. Data were square root transformed prior to analyses, and each resemblance matrix was based on zero inflated BrayâCurtis similarities. Shaded areas represent two-dimensional convex hulls representing areas connecting exterior data points.
Figure 5. Biomass data-based nMDS plots comparing relative similarities in barren and kelp forests communities at different spatial scales; (A) among quadrats within each site, (B) among sites within each island, and (C) among islands. Data were square root transformed prior to analyses, and each resemblance matrix was based on zero inflated BrayâCurtis similarities. Shaded areas represent two-dimensional convex hulls representing areas connecting exterior data points.
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