Macroalgae maintain growth outside their observed distributions: Implications for biodiversity-ecosystem functioning at landscape scales

1. The spatial insurance hypothesis states that at landscape scales with environmental variability between different places, biodiversity increases ecosystem functioning if species respond asynchronously to environmental variation, and they are highest functioning in places where they dominate the relative abundance. 2. Under this hypothesis, observed species turnover between places in a landscape with environmental variation might suggest that species diversity is an important driver of landscape-scale ecosystem functioning. However, the spatial insurance implicitly assumes that species found in one place in the landscape will not be able to maintain high functioning in other places. 3. Given this assumption of the spatial insurance hypothesis, we would predict that species' functioning in monoculture should decline if transplanted to a different place on an environmental gradient, away from where they naturally dominate. If this is the case, we would expect that the loss of one species in one place of the environmental gradient could not be compensated through the establishment of another species in that place. 4. We tested this prediction using a model system of marine macroalgae on intertidal rocky shores on the Swedish west coast. We performed a reciprocal transplant experiment with adult individuals of four fucoid seaweeds that dominate the standing stock biomass at different depths on these shores and monitored their relative growth rate over 2 months. 5. Counter to the assumptions made by the spatial insurance hypothesis, growth rates for three of the four species showed limited responses to being transplanted to different depth zones. Spatial insurance may, hence, play a minor role in sustaining landscape ecosystem functioning in this and other systems. 6. Synthesis. The functional consequences of species loss at landscape scales may not be as obvious as observational studies and ecological models suggest. Consequently, natural patterns of species turnover should not be used directly to argue for the role of biodiversity at landscape scales. Instead, how species may or may not be able to compensate for the loss of other species is a critical aspect if we are to understand how changes in biodiversity at the landscape scale affect ecosystem functioning.

Automated summary

The spatial insurance hypothesis posits that biodiversity increases ecosystem functioning in landscapes with environmental variability, assuming that species maintain high functioning only in places where they dominate. We tested this prediction in a marine macroalgae system and found limited responses to transplanting species, suggesting that spatial insurance may play a minor role in sustaining landscape ecosystem functioning.