New limitations of the habitat amount hypothesis

Atlantic Forest

The Brazilian Atlantic forest is one tropical region under threat from habitat loss and disrupted connectivity, where the amount of habitat is rapidly decreasing as fragmentation accelerates. Credit: SPOT

A recent question that has bedeviled landscape ecology is: what is the role of the area of habitat versus the role of habitat fragmentation?

All agree that the loss of habitat has been the major cause of species extinction and the major threat to biodiversity. Whereas conventional wisdom, supported by decades of research and conservation, has supported the effects of habitat loss AND fragmentation in reducing biodiversity, some scientists have staked out the polar extreme that only habitat amount affects biodiversity. As evidence in the academic literature supports the role of both habitat loss and fragmentation in reducing biodiversity, an important question remains: when is habitat fragmentation of importance in biodiversity conservation?

A new study provides an answer. Rybicki and colleagues created a model of spatial dynamics in fragmented landscapes. The scientific literature is replete with such models at the population level. The novel aspect of this research was to add a dimension of real world landscapes: different species that interact to affect each others’ abundances are affected differently by habitat loss and fragmentation. These species interactions then affect the persistence of individual species and ultimately biodiversity across landscapes.

The authors focus on the competitive interactions between species. This focus has a long-standing tradition in ecology, most famously in the demonstration of a time lag between fragmentation and extinction (known as the extinction debt). Landscapes varied in habitat amount and fragmentation. Spatial variation was imposed through variation in resources that then affect species survival and reproduction. Species also varied in their traits, especially dispersal.

Forest border

Species dispersal abilities can have an affect on the magnitude of the effect of habitat fragmentation.

This variability produced results whereby fragmentation could have positive or negative effects on biodiversity. Of clearest relevance for conservation, fragmentation could have positive effects on biodiversity if the total area of habitat in the landscape was high. Conversely, negative effects appeared when total area of habitat in the landscape was low. The magnitude of the effect of fragmentation was affected particularly by species’ dispersal abilities.

By addressing the mechanisms underlying fragmentation’s effects, this work overcomes a recognized limitation of the habitat amount hypothesis that is devoid of mechanism. The work is consistent with empirical studies that also show species loss is particular affected by habitat fragmentation when habitat area declines below thresholds.

Rybicki and colleagues best summed up their paper’s relevance to the ecological and conservation communities: “[W]e conclude that it may be time to move on from debating whether fragmentation matters or not, onto developing a comprehensive and fine‐grained understanding of when and how fragmentation matters.”

Resources

Rybicki, J., Abrego, N. and Ovaskainen, O. 2020. Habitat fragmentation and species diversity in competitive communities. Ecology Letters 23: 506-517.

To preserve species density, habitat amount may matter more than patch size or isolation (April 2020)

Banks-Leite, C., Pardini, R., Tambosi, L.R., Pearse, W.D., Bueno, A.A., Bruscagin, R.T., Condez, T.H., Dixo, M., Igari, A.T., Martensen, A.C. and Metzger, J.P. 2014. Using ecological thresholds to evaluate the costs and benefits of set-asides in a biodiversity hotspot. Science 345: 1041-1045.

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 Jr, 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. and Melo, F.P. 2018. Is habitat fragmentation good for biodiversity?. Biological Conservation 226: 9-15.

Haddad, N. M., L. A. Brudvig, J. Clobert, K. F. Davies, A. Gonzalez, R. D. Holt, T. E. Lovejoy, J. O. Sexton, M. P. Austin, C. D. Collins, W. M. Cook, E. I. Damschen, R. M. Ewers, B. L. Foster, C. N. Jenkins, A. J. King, W. F. Laurance, D. J. Levey, C. R. Margules, B. A. Melbourne, A. O. Nicholls, J. L. Orrock, D.-X. Song, and J. R. Townshend. 2015. Habitat fragmentation and its lasting impact on Earth’s ecosystems. Science Advances 1(2): e1500052.

Tilman, D., May, R.M., Lehman, C.L. and Nowak, M.A. 1994. Habitat destruction and the extinction debt. Nature 371: 65-66.

2020-04-14T20:46:12-04:00 May 11th, 2020|

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. His latest book, The Last Butterflies, is currently available from Princeton University Press.