Scale‐dependent effects of marine subsidies on the island biogeographic patterns of plants

Abstract

Although species richness can be determined by different mechanisms at different spatial scales, the role of scale in the effects of marine inputs on island biogeography has not been studied explicitly. Here, we evaluated the potential influence of island characteristics and marine inputs (seaweed wrack biomass and marine‐derived nitrogen in the soil) on plant species richness at both a local (plot) and regional (island) scale on 92 islands in British Columbia, Canada. We found that the effects of subsidies on species richness depend strongly on spatial scale. Despite detecting no effects of marine subsidies at the island scale, we found that as plot level subsidies increased, species richness decreased; plots with more marine‐derived nitrogen in the soil hosted fewer plant species. We found no effect of seaweed wrack at either scale. To identify potential mechanisms underlying the decrease in diversity, we fit a spatially explicit joint species distribution model to evaluate species level responses to marine subsidies and effects of biotic interactions among species. We found mixed evidence for competition for both light and nutrients, and cannot rule out an alternative mechanism; the observed decrease in species richness may be due to disturbances associated with animal‐mediated nutrient deposits, particularly those from North American river otters (Lontra canadensis). By evaluating the scale‐dependent effects of marine subsidies on island biogeographic patterns of plants and revealing likely mechanisms that act on community composition, we provide novel insights on the scale dependence of a fundamental ecological theory, and on the rarely examined links between marine and terrestrial ecosystems often bridged by animal vectors.

Equivalent roles of marine subsidies and island characteristics in shaping island bird communities

“…Marine subsidies may affect aspects of species communities that are not captured if only alpha diversity is evaluated, but the underlying mechanisms may be revealed through the evaluation of species level responses. Understanding species level responses to environmental conditions and biotic interactions provides insights on broader patterns of biodiversity, but these responses also depend on the spatial scale of investigation (Obrist, Fitzpatrick, et al, 2022). At larger scales, dispersal limitations and environmental filters may have more influence than biotic interactions in structuring species communities.…”

Corrigendum

“…In the recent article by Obrist et al ( 2022 ), the labeling of the y‐axes showing species richness in Figure 3 was unclear. The correct figure is shown below:…”