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Proc Biol Sci
2022 Jan 26;2891967:20211697. doi: 10.1098/rspb.2021.1697.
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Southeast Alaskan kelp forests: inferences of process from large-scale patterns of variation in space and time.
Gorra TR, Garcia SCR, Langhans MR, Hoshijima U, Estes JA, Raimondi PT, Tinker MT, Kenner MC, Kroeker KJ.
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Humans were considered external drivers in much foundational ecological research. A recognition that humans are embedded in the complex interaction networks we study can provide new insight into our ecological paradigms. Here, we use time-series data spanning three decades to explore the effects of human harvesting on otter-urchin-kelp trophic cascades in southeast Alaska. These effects were inferred from variation in sea urchin and kelp abundance following the post fur trade repatriation of otters and a subsequent localized reduction of otters by human harvest in one location. In an example of a classic trophic cascade, otter repatriation was followed by a 99% reduction in urchin biomass density and a greater than 99% increase in kelp density region wide. Recent spatially concentrated harvesting of otters was associated with a localized 70% decline in otter abundance in one location, with urchins increasing and kelps declining in accordance with the spatial pattern of otter occupancy within that region. While the otter-urchin-kelp trophic cascade has been associated with alternative community states at the regional scale, this research highlights how small-scale variability in otter occupancy, ostensibly due to spatial variability in harvesting or the risk landscape for otters, can result in within-region patchiness in these community states.
Figure 1. . Map of study area showing location of Torch Bay (northern site, yellow) and Sitka Sound (southern site, red) (a), as well as sites sampled across the latter two time periods (2003/2009 versus 2018/2019, respectively) in each location (b and c). The maps only display sites from the latter two sampling periods. (Online version in colour.)
Figure 2. . Patterns of variation with distance over time from the town of Sitka for sea otter abundance index, urchin biomass density and kelp density. The line indicates a significant linear (2018 otters) or nonlinear relationship with distance from Sitka. With the exception of the relationship between the distance to the town of Sitka and the probability of otter presence, all analyses shown here were performed on square-transformed data. (Online version in colour.)
Figure 3. . Variation in abundance of sea urchins and kelp in Sitka Sound and Torch Bay among the years. Green = Strongylocentrotus droebachiensis, Red = Mesocentrotus franciscanus, Black = total urchin biomass density, Brown = Laminariales. Inset year comparison is from ANOVA/TUKEY analyses. (Online version in colour.)
Figure 4. . Functional relationships between urchin biomass density (a) and kelp density (b) as a function of the probability of seeing an otter in Sitka Sound in 2018. (Online version in colour.)
Figure 5. . State space plots of total urchin biomass density versus kelp density by sample sites for Sitka Sound and Torch Bay among the years these areas were sampled. (Online version in colour.)
Figure 6. . Non-metric multidimensional scaling plot of the centroids and 80% confidence ellipses for benthic community structure for Sitka Sound and Torch Bay based on the broad taxonomic and functional groups. Black arrows depict the temporal trajectory of the communities at each location (1988 to 2003 (TB) or 2009 (SS) to 2018). Blue vectors indicate the region of the graph with greater abundances of indicated species. (Online version in colour.)
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