Habitat size modulates the influence of heterogeneity on species richness patterns in a model zooplankton community

Abstract

Habitat heterogeneity is a primary mechanism influencing species richness. Despite the general expectation that increased heterogeneity should increase species richness, there is considerable variation in the observed relationship, including many studies that show negative effects of heterogeneity on species richness. One mechanism that can create such disparate results is the predicted trade-off between habitat area and heterogeneity, sometimes called the area-heterogeneity-trade-off (AHTO) hypothesis. The AHTO hypothesis predicts positive effects of heterogeneity on species richness in large habitats, but negative effects in small habitats. We examined the interplay between habitat size and habitat heterogeneity in experimental mesocosms that mimic freshwater ponds, and measured responses in a species-rich zooplankton community. We used the AHTO hypothesis and related mechanisms to make predictions about how heterogeneity would affect species richness and diversity in large compared to small habitats. We found that heterogeneity had a positive influence on species richness in large, but not small habitats, and that this likely resulted because habitat specialists were able to persist only when habitat size was sufficiently large, consistent with the predictions of the AHTO hypothesis. Our results emphasize the importance of considering context (e.g., habitat size in this case) when investigating the relative importance of ecological drivers of diversity, like heterogeneity.

Dispersal mitigates biologically induced disturbances in heterogeneous metacommunities

“…We added 35 lg/L of phosphorus to each mesocosm weekly to replenish weekly phosphorus losses, which we estimated from a pilot study conducted two weeks prior to this experiment. Similar phosphorus loss rates have been estimated in other mesocosm studies (Hall et al 2005, Schuler et al 2017. We added 18 zebra mussels (0.24 mussels/L) averaging 2.3 (SD AE 0.3) cm in length to each of the disturbed mesocosms five days after the zooplankton were added.…”

“…, Schuler et al. ). We added 18 zebra mussels (0.24 mussels/L) averaging 2.3 (SD ± 0.3) cm in length to each of the disturbed mesocosms five days after the zooplankton were added.…”

An experimental test of the area–heterogeneity tradeoff

“…However, this evidence was criticized as being a result of confounding effects (36) or sampling bias (37). The only experimental evidence for the AHTO of which we are aware is a demonstration of a positive interaction between the effects of heterogeneity and habitat size on species richness (38). Certainly, there is a need for more direct tests of this fundamental tradeoff.…”

Deterministic insights from stochastic interactions

“…In testing the predictions of the AHTO, the study by Ben-Hur and Kadmon (3) leads to an important refinement of the AHTO predictions, and adds to a growing body of empirical studies that demonstrate how stochastic and deterministic forces may both be jointly acting in ecological communities (14)(15)(16). The results of this work are exciting because, if the AHTO is a fundamental characteristic of ecological systems, this refinement of the AHTO will contribute significantly to a theory that will be a significant addition to the ecological toolbox, helping advance the basic ecological goal of predicting patterns of diversity.…”

“…Results from the study by Ben-Hur and Kadmon (3), if they prove to be applicable at larger spatial scales, provide a more informed way to characterize the role of small and large patches for conservation and restoration. For example, small habitat patches may be places where populations are more likely to go extinct due to small size (1,2,16), where they may be less likely to colonize due to dispersal limitation (1,2), and where high edge-to-area ratios increase the potentially deleterious effects of neighboring habitats (17). However, in finding empirical support for the role of small habitats in promoting diversity of inferior competitors, the results of the study by Ben-Hur and Kadmon (3) suggest that small habitats might have the theoretically predicted, but empirically unappreciated, benefits for species with particular characteristics.…”