Increased dispersal explains increasing local diversity with global biodiversity declines

The narrative of biodiversity decline in response to human impacts is overly simplistic because different biodiversity metrics show different trajectories at different spatial scales. It is also debated whether human-caused biodiversity changes lead to subsequent, accelerating change (cascades) in ecological communities, or alternatively build increasingly robust community networks with decreasing extinction rates and reduced invasibility. Mechanistic approaches are needed that simultaneously reconcile different metrics of biodiversity change, and explore the robustness of communities to further change. We develop a trophically-structured, mainland-archipelago metacommunity model of community assembly. Varying the parameters across model simulations shows that local alpha diversity (the number of species per island) and regional gamma diversity (the total number of species in the archipelago) depend on both the rate of extirpation per island and on the rate of dispersal between islands within the archipelago. In particular, local diversity increases with increased dispersal and heterogeneity between islands, but regional diversity declines because the islands become biotically similar and local one-island and few-island species are excluded (homogenisation, or reduced beta diversity). This mirrors changes observed empirically: real islands have gained species (increased local and island-scale community diversity) with increased human-assisted transfers of species, but global diversity has declined with the loss of endemic species. However, biological invasions may be self-limiting. High-dispersal, high local-diversity model communities become resistant to subsequent invasions, generating robust species-community networks unless dispersal is extremely high. A mixed-up world is likely to lose many species, but the resulting ecological communities may nonetheless be relatively robust. Significance Statement Biodiversity is commonly regarded as threatened due to human impacts, but biodiversity metrics at different scales produce contradictory results. A framework is needed that can reproduce and connect these results across scales and address whether biodiversity change will inexorably accelerate following perturbation or become self-limiting as new ecological communities form. We address this challenge by constructing size-structured model communities using a mainland/island paradigm and tracking diversity at different scales. Our simulations reproduce the literature’s discrepancy across scales and provide new insight. Ecological communities (islands) gain species with increasing (human-assisted) dispersal, but global diversity declines with the consequent loss of endemic species. Communities also become less invasible as dispersal increases, suggesting that human-mediated dispersal favours robust communities that resist subsequent change.