An experimental test of the area–heterogeneity tradeoff

Abstract

A fundamental property of ecosystems is a tradeoff between the number and size of habitats: as the number of habitats within a fixed area increases, the average area per habitat must decrease. This tradeoff is termed the “area–heterogeneity tradeoff.” Theoretical models suggest that the reduction in habitat sizes under high levels of heterogeneity may cause a decline in species richness because it reduces the amount of effective area available for individual species under high levels of heterogeneity, thereby increasing the likelihood of stochastic extinctions. Here, we test this prediction using an experiment that allows us to separate the effect of the area–heterogeneity tradeoff from the total effect of habitat heterogeneity. Surprisingly, despite considerable extinctions, reduction in the amount of effective area available per species facilitated rather than reduced richness in the study communities. Our data suggest that the mechanism behind this positive effect was a decrease in the probability of deterministic competitive exclusion. We conclude that the area–heterogeneity tradeoff may have both negative and positive implications for biodiversity and that its net effect depends on the relative importance of stochastic vs. deterministic drivers of extinction in the relevant system. Our finding that the area–heterogeneity tradeoff may contribute to biodiversity adds a dimension to existing ecological theory and is highly relevant for understanding and predicting biodiversity responses to natural and anthropogenic variations in the environment.

Spatial heterogeneity enhance robustness of large multi-species ecosystems

“…Large ecosystems might include many different types of habitats connected by dispersal, which is mostly found to be positive for biodiversity and robustness [ 25 , 26 , 31 ]. But even in ecosystems with only one type of habitat, the area could be large enough such that external perturbations are not equal, there might be boundaries artificial or natural, or aggregation of species due to conspecific attraction [ 32 , 33 ], leading to patches with equal species interactions connected by dispersal. As we have shown, in such ecosystems the mere possibility of differential species abundances at different locations enhances stability and diversity.…”

Disentangling the mechanisms underlying the species–area relationship: A mesocosm experiment with annual plants

“…Our result that increasing population size had a very small and insignificant contribution to the positive effect of area on island richness (Figure 5a,b) is surprising considering previous theoretical and empirical evidence indicating that population size is a major determinant of extinction rates. We relate this result to the fact that, when species interact with each other in a competitive community, increasing population sizes reduces the impact of demographic stochasticity, thereby increasing the likelihood that superior competitors would exclude inferior competitors from the community (the ecological drift hypothesis, Ben‐Hur & Kadmon, 2020a). This positive effect of population size on extinction probability may offset the negative effect of population size on the likelihood of stochastic extinction (see also Orrock, 2020).…”

“…edge effects, Allee effects, scale-dependent frequency dependence), the effect of population size lies at the core of island biogeography theory (MacArthur & Wilson, 1967) is not restricted to island systems and applies to metacommunity systems as well (Leibold & Chase, 2017). Still, results from a recent mesocosm experiment focusing on the effect of habitat heterogeneity on species richness suggest that, under some circumstances, an increase in population size may reduce richness by facilitating deterministic competitive exclusions (Ben-Hur & Kadmon, 2020a).…”

The spatial configuration of biotic interactions shapes coexistence-area relationships in an annual plant community

“…Likewise, SARs are modulated by the simple fact that larger areas harbor higher species richness and population densities, or larger areas have by definition a higher probability of receiving immigrants by dispersal or stochastic events (Lomolino, 2001). The rela-tive importance of deterministic processes and stochastic/sampling events in driving SARs has been explored in different contexts with contrasting results (Ben-Hur & Kadmon, 2020;Chase et al, 2019;Gooriah & Chase, 2020). Despite these recent efforts, we still poorly understand how SARs emerge in a more general way from integrating the collective effects from multiple deterministic processes and stochastic factors.…”