Abstract
Biodiversity loss is driven by interacting factors operating at different spatial scales. Yet, there remains uncertainty as to how fine-scale environmental conditions mediate biological responses to broad-scale stressors. We surveyed intertidal rocky shore kelp beds situated across a local gradient of wave action and evaluated changes in kelp diversity and abundance after more than two decades of broad scale stressors, most notably the 2013–2016 heat wave. Across all sites, species were less abundant on average in 2017 and 2018 than during 1993–1995 but changes in kelp diversity were dependent on wave exposure, with wave exposed habitats remaining stable and wave sheltered habitats experiencing near complete losses of kelp diversity. In this way, wave exposed sites have acted as refugia, maintaining regional kelp diversity despite widespread local declines. Fucoids, seagrasses and two stress-tolerant kelp species (Saccharina sessilis, Egregia menziesii) did not decline as observed in other kelps, and the invasive species Sargassum muticum increased significantly at wave sheltered sites. Long-term monitoring data from a centrally-located moderate site suggest that kelp communities were negatively impacted by the recent heatwave which may have driven observed losses throughout the region. Wave-sheltered shores, which saw the largest declines, are a very common habitat type in the Northeast Pacific and may be especially sensitive to losses in kelp diversity and abundance, with potential consequences for coastal productivity. Our findings highlight the importance of fine-scale environmental heterogeneity in mediating biological responses and demonstrate how incorporating differences between habitat patches can be essential to capturing scale-dependent biodiversity loss across the landscape.
Highlights
Ongoing biodiversity loss is expected to reduce ecosystem functioning and services [1] but uncertainty remains about the spatial scale at which to investigate the environmental drivers of such loss [2,3,4]
Declines in kelp diversity and abundance were greatest at wave sheltered sites Across the study system, we found widespread declines in kelp species richness and abundance mediated by local variation in wave action (Fig 2)
Habitat-dependent declines in kelp diversity were further demonstrated by changes in the shape of species accumulation curves in 2017 and 2018 relative to 1993–1995 (Fig 4)
Summary
Ongoing biodiversity loss is expected to reduce ecosystem functioning and services [1] but uncertainty remains about the spatial scale at which to investigate the environmental drivers of such loss [2,3,4]. Fine-scale microclimatic differences between sites are often ignored by both climate envelop models–which predict systematic shifts in the latitudinal ranges of species [8]–, and in meta-analyses of local diversity change–which group plots only by habitat-type (e.g. forest, marsh, grassland) or by region [9,10,11] These common approaches, insightful, may miss functionally important trends in community diversity change or local abundance loss if the stresses associated with a habitat patch depend more on local conditions than on regional patterns [12], or if even the most consistent declines occur at only a subset of sites within each habitat type. While much work has focussed on how broad-scale stressors are driving the biological responses of communities [6,8,10,13], fewer studies have examined the role that local, fine-scale conditions play in magnifying or ameliorating them [6]
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