Experimental evidence does not support the Habitat Amount Hypothesis

Most conservationists will agree that the single greatest threat to biodiversity is habitat loss. Against this backdrop, it is no wonder that E.O. Wilson’s aspiration to double down on habitat conservation in his book Half Earth has received so much attention. Conserving half of the Earth must be strategic, including minimizing habitat fragmentation so that landscapes are configured to sustain biodiversity and ecosystems in the long term.

Connecting fragments with corridors is a widely used strategy for slowing biodiversity loss within fragmented landscapes. Is this focus on connecting fragments with corridors well guided? Our group’s recent study says yes.

Our results address the latest critique of corridors and habitat fragmentation, Lenore Fahrig’s “habitat amount hypothesis”. It is framed around a half-century old theory that predicts higher diversity in larger and more connected islands (the theory of island biogeography). Fahrig questions whether the theory is relevant to landscapes and whether effects of habitat configurations (patch size, patch connectivity) can be distinguished from effects of habitat loss.

If the hypothesis is supported, then fragmentation could be ignored and the sole focus of conservation efforts should be preventing habitat loss. Habitat connectivity, including the use of corridors, could then be ignored. This would be a significant concern for conservation projects focused on improving connectivity (e.g. Yellowstone to Yukon initiative).

Given the importance of this issue, we decided to test the habitat amount hypothesis with controlled experiments. Our new study shows that corridors do increase biodiversity, over and above habitat amount.

Our results emerged from two different experiments that control the fragmentation and area of habitat. These studies, which have focused separately on plants and on micro-arthropods, have previously shown that corridors increase dispersal, decrease extinction, and increase biodiversity. In our new analysis, we used these same data to ask whether we would observe changes above and beyond that predicted by changes in the amount of habitat. We concluded that in either experiment, 15-50% more species were lost in isolated fragments.

Our greatest surprise was that the effects of corridor removal in reducing biodiversity were nearly identical to the effects of reducing the area of habitat. Habitat loss and fragmentation acted synergistically to have the greatest impact on species numbers. A second surprise was that after one and a half decades of study in one experiment, habitat fragmentation continues to reduce plant diversity every year. It is unclear how long this loss will continue into the future.

It is reassuring when experimental science supports theoretical predictions. Yet, a singular focus on biodiversity can become a distraction. By isolating habitats and creating edges, fragmentation degrades many aspects of ecological systems, including ecosystem processes and service, and these effects should not be ignored.  Science and conservation must now move to a new plain, with a focus on the synergies that arise when we conserve larger and more connected areas.

Resources

Fahrig, L. 2013. Rethinking patch size and isolation effects: the habitat amount hypothesis. Journal of Biogeography 40(9): 1649-1663.

Haddad, N. M., A. Gonzalez, L. A. Brudvig, M. A. Burt, D. J. Levey, and E. I. Damschen. 2017. Experimental evidence does not support the Habitat Amount Hypothesis. Ecography 40(1): 48-55.

Haddad, Nick M., et al. 2015. Habitat fragmentation and its lasting impact on Earth’s ecosystems. Science Advances 1.2: e1500052.

MacArthur, R. H., & Wilson, E. O. 2015. Theory of Island Biogeography.(MPB-1) (Vol. 1). Princeton University Press.

2017-03-22T10:03:57+00:00 February 21st, 2017|

About the Author:

Nick Haddad

Dr. Nick Haddad is Senior Terrestrial Ecologist in the Department of Integrative Biology at Michigan State University and Kellogg Biological Station. For more than 20 years, he has been studying how plants and animals use corridors. He has worked in the largest and longest-running corridor experiment, the Savannah River Site Corridor Project, and he has studied natural corridors used by rare butterflies.