This is the first part of a two part series on the conservation implications of the Habitat Amount Hypothesis. (View Part 2)
In the forty years since the negative effects of habitat fragmentation were identified as an extension of well-established ecological theory, there has been a robust scientific exchange about the merits of this idea. Most recently, the discussion has centered on the Habitat Amount Hypothesis, which suggests that the number of species in a region depends only on habitat area, and not on habitat fragmentation.
(Much of the exchange is centered around issues of definitions, assumptions, and methods, topics that will no doubt be addressed in the scientific literature [Fletcher, et al 2018 and Fahrig, et al. 2018].)
What is the relevance of this hypothesis for biodiversity conservation? I was part of a critique in which we expressed alarm about the paper’s conclusion that “there is no justification for assigning lower conservation value to small patches than to an equivalent area within a large patch,” and about other strident language such as “negative effects [of habitat fragmentation] on ecological responses qualifies as a zombie idea.”
In that paper, and in an ensuing response, everyone agreed that habitat loss is the primary cause of biodiversity loss. The key issue is how to conserve what is left and how to expand conservation areas.
Should the focus be on area alone, as Fahrig (2017) would contend based on a review that showed 76% of significant effects of fragmentation on biodiversity were positive, relegating fragmentation to a minor or insignificant factor? Or should it be on fragmentation, since Fletcher, et al. (2018) clarified that “positive” was a number that might be irrelevant to conservation, elevating fragmentation to an important consideration along with area. For example, small areas have a high amount of edges, and these can support a high diversity of species that often are not the targets of conservation. Fahrig, et al. (2019) expressed their core conservation issue: “what we find ‘alarming’ for conservation is the nearly complete lack of protection for habitat that is divided into small patches.”
Here is what never comes up in either paper: isn’t the conservation landscape we live in already scattered with small protected areas?
Fully 70% of the world’s forests are less than a kilometer from the edge of the forest. Two-thirds of the protected areas of the United States are less than one kilometer wide. State natural heritage programs target significant natural history areas that support threatened taxa, even in tiny areas. Conservation NGOs target purchase of land parcels that are necessarily small. For the very rarest plants and animals, I find that only the smallest areas receive protection (my topic for next month).
As we already live in a world of fragmented nature, any land conservation increases habitat amount, and increases habitat connectivity (bringing fragments closer together). Such is the inevitable correlation between habitat loss and fragmentation in a fragmented world. For now, Fletcher, et al. (2018) argue that the best available science supports reconnecting nature.
Fahrig, L., 2017. Ecological responses to habitat fragmentation per se. Annu. Rev. Ecol. Evol. Syst. 48: 1–23.
Fahrig, L., V. Arroyo-Rodríguez, J. R. Bennett, V. Boucher-Lalonde, E. Cazetta, D. J. Currie, F. Eigenbrod, A. T. Ford, S. P. Harrison, J. A. G. Jaeger, N. Koper, A. E. Martin, J.-L. Martin, J. P. Metzger, P. Morrison, J. R. Rhodes, D. A. Saunders, D. Simberloff, A. C. Smith, L. Tischendorf, M. Vellend, and J. I. Watling. 2019. Is habitat fragmentation bad for biodiversity? Biological Conservation 230:179-186.
Fletcher, R.J., Didham, R.K., Banks-Leite, C., Barlow, J., Ewers, R.M., Rosindell, J., Holt, R.D., Gonzalez, A., Pardini, R., Damschen, E.I., Melo, F.P., Ries, L., Prevedello, J.A., Tscharntke, T., Laurance, W.F., Lovejoy, T., and Haddad, N. M. 2018. Is habitat fragmentation good for biodiversity?. Biological Conservation 226:9-15.
Haddad, N.M., Gonzalez, A., Brudvig, L.A., Burt, M.A., Levey, D.J., Damschen, E.I., 2017. Experimental evidence does not support the habitat amount hypothesis. Ecography 40: 48–55.
Simberloff, D. S. and Abele, L. G. 1976. Island biogeography theory and conservation practice. – Science 191: 285–286.
Wilson, E.O. and Willis, E.O. 1975. Applied biogeography. In M. L. Cody and J. M. Diamond [Eds.], Ecology and evolution of communities, pp.522-36. Harvard University Press, Cambridge MA.