What determines species richness in a habitat patch? Island biogeography theory argues that it is a combination of patch size and the distance of that patch to others – i.e. the smaller the patch, and the further from other patches, the fewer species. This theory has decades of support in the literature.
In contrast, the habitat amount hypothesis argues that it is not the size or isolation of individual habitat patches, but instead the simple amount of habitat available in a landscape that determines richness. The more habitat in the surrounding landscape, the higher the species density will be in a given patch, and this will have a bigger influence on richness than the size and isolation of those surrounding patches. This hypothesis serves as a null model to test island biogeography against: is it just that larger patches contribute more to habitat area, or is there something more that larger patches contribute compared to smaller patches?
The differences between these two hypotheses lead to important management implications. Island biogeography would argue we should focus our efforts on preserving the largest patches, and not worry as much about protecting smaller patches. In contrast, the habitat amount hypothesis would argue small patches be given just as much priority in management as the large patches, making them a focus for conservation. Simply put, does fragmentation matter? Given an equal area preserved, are a bunch of fragmented, small patches the same as fewer, larger patches?
A new study aimed to be a comprehensive test of the habitat amount hypothesis and considered the effects of patch size, patch isolation, patch density (# patches/area), and habitat amount on species density (species/area). The authors surveyed 35 studies and 8 taxonomic groups (plants, fungi, gastropods, insects, amphibians, reptiles, birds, mammals) to test their predictions.
Across the studies they found that habitat amount, patch size, and patch isolation were all drivers of species density in most studies, supporting both the island biogeography and habitat amount hypotheses. However, consistent with the habitat amount hypothesis, they found that habitat amount was a stronger determinant of density than both size and isolation combined. They conclude that in most studies habitat fragmentation did not have negative effects on species density; however, in 6 studies patch size and isolation were necessary to explain species density patterns. For management, the authors argue that to maintain biodiversity, it is important to preserve as much habitat as possible, and to not count out small, isolated patches in our efforts.
Watling, J.I., et al. 2020. Support for the habitat amount hypothesis from a global synthesis of species density studies. Ecology Letters. DOI: 10.1111/ele.13471
Fahrig, L., 2013. Rethinking patch size and isolation effects: the habitat amount hypothesis. Journal of Biogeography, 40(9), pp.1649-1663.