The SLOSS Debate

Two Burchell's Zebras in morning light Two
Mike Hill/Getty Images

One of the most heated controversies in conservation history is known as the SLOSS Debate. SLOSS stands for "Single Large or Several Small" and refers to two different approaches to land conservation in order to protect biodiversity in a given region.

The "single large" approach favors one sizeable, contiguous land reserve.

The "several small" approach favors multiple smaller reserves of land whose total areas equal that of a large reserve.

Area determination of either is based on the type of habitat and species involved.

New Concept Spurs Controversy

In 1975, an American scientist named Jared Diamond proposed the landmark idea that a single large land reserve would be more beneficial in terms of species richness and diversity than several smaller reserves. His claim was based on his study of a book called The Theory of Island Biogeography by Robert MacArthur and E.O. Wilson.

Diamond's assertion was challenged by ecologist Daniel Simberloff, a former student of E. O. Wilson, who noted that if several smaller reserves each contained unique species, then it would be possible for smaller reserves to harbor even more species than a single large reserve.

Habitat Debate Heats Up

In The American Naturalist journal, scientists Bruce A. Wilcox and Dennis D. Murphy responded to an article by Simberloff by arguing that habitat fragmentation (caused by human activity or environmental changes) poses the most critical threat to global biodiversity.

Contiguous areas, the researchers asserted, are not only beneficial to communities of interdependent species, they are also more likely to support populations of species that occur at low population densities, particularly large vertebrates.

Harmful Effects of Habitat Fragmentation

According to the National Wildlife Federation, terrestrial or aquatic habitat fragmented by roads, logging, dams, and other human developments "may not be large or connected enough to support species that need a large territory in which to find mates and food. The loss and fragmentation of habitats make it difficult for migratory species to find places to rest and feed along their migration routes."

When habitat is fragmented, mobile species that retreat into smaller reserves of habitat can end up crowded, increasing competition for resources and disease transmission.

The Edge Effect

In addition to interrupting contiguity and decreasing the total area of available habitat, fragmentation also magnifies the edge effect, resulting from an increase in the edge-to-interior ratio. This effect negatively impacts species that are adapted to interior habitats because they become more vulnerable to predation and disturbance.

No Simple Solution

The SLOSS Debate spurred aggressive research into the effects of habitat fragmentation, leading to conclusions that the viability of either approach may depend on the circumstances.

Several small reserves may, in some cases, be beneficial when many species are bound together on small patches of habitat. Fragmentation may in fact be beneficial in such cases, allowing species the space needed to separate. But the debate is far from resolved, according to many papers.

Reality Check

Kent Holsinger, Professor of Ecology and Evolutionary Biology at the University of Connecticut, contends, "This whole debate seems to have missed the point. After all, we put reserves where we find species or communities that we want to save. We make them as large as we can, or as large as we need to protect the elements of our concern. We are not usually faced with the optimization choice poised in the [SLOSS] debate. To the extent we have choices, the choices we face are more like … how small an area can we get away with protecting and which are the most critical parcels?"

View Article Sources
  1. Diamond, Jared. "The Island Dilemma: Lessons of Modern Biogeographic Studies for the Design of Natural Reserves," Biological Conservation, vol. 7, no. 2, 1975, pp. 129−146, doi:10.1016/0006-3207(75)90052-X

  2. Simberloff, Daniel and Lawrence G. Abele. "Refuge Design and Island Biogeographic Theory: Effects of Fragmentation." The American Naturalist, vol. 120, no. 1, 1982, pp. 41-50, doi:10.1086/283968

  3. Wilcox, Bruce A., and Dennis D. Murphy. "Conservation Strategy: The Effects of Fragmentation on Extinction." The American Naturalist, vol. 125, no. 6, 1985, pp. 879-887., doi:10.1086/284386

  4. Laurance, William F., et al. "Habitat Fragmentation, Variable Edge Effects, and the Landscape-Divergence Hypothesis." PLoS ONE, vol. 2, no. 10, 2007, p. e1017., doi:10.1371/journal.pone.001017

  5. Rybicki, Joel et al. “Habitat Fragmentation and Species Diversity in Competitive Communities.” Ecology Letters, vol. 23, no. 3, 2020, pp. 506-517., doi:10.1111/ele.13450

  6. May, Felix et al. “The Geometry of Habitat Fragmentation: Effects of Species Distribution Patterns on Extinction Risk Due to Habitat Conversion.” Ecology and Evolution, vol. 9, no. 5, 2019, pp. 2775-2790., doi:10.1002/ece3.4951

  7. Tjørve, Even. “How to Resolve the SLOSS Debate: Lessons from Species-Diversity Models.” Journal of Theoretical Biology, vol. 264, no. 2, 2010, pp. 604-12., doi:10.1016/j.jtbi.2010.02.009

  8. Holsinger, Kent. "Theory and Design of Nature Reserves." EEB Articles, vol. 41, 2012.