The SLOSS debate

I mentioned in my opening lecture that much of the early conservation biology literature was dominated by discussion of genetics. Well, much of the literature that wasn't devoted to genetics was devoted to using the theory of island biogeography to design nature reserves--or to disputing its utility. The most heated of these debates even earned its own acronym: the SLOSS debate.

Suppose you had money to purchase 10,000 hectares of land. Assume for the moment that you can ignore all management problems and that your only concern is the spatial configuration of that 10,000 hectares. Would it be better to have a single large reserve or several small reserves? Would it make more sense to buy a single piece of property 10,000 hectares in extent or 10 pieces of property each of 1,000 hectares?

Early advocates of the use of island biogeography theory, notably Soulé, Wilcox, Terborgh, and Lovejoy, argued that a single large reserve is generally better able to preserve more and larger populations than an equal area divided into a collection of small reserves. They had two reasons for this claim:

• Contiguous areas are better able to preserve intact communities of interdependent species.

• Contiguous areas are better able to maintain viable populations of species that occur at low population densities, especially large vertebrates.

There are at least two problems with these arguments.

• ``Largeness'' is in the eye of the beholder. For annual plants, for vegetative perennials, and for small, sedentary animals a few hectares may encompass all the appropriate habitat, e.g., a peat bog or a serpentine outcrop. Conservationists who want to protect plant diversity, therefore, are probably better off buying several reserves of a few tens to hundreds of hectares in a diversity of habitats, soil types, and geological regimes than buying a single, large reserve of a couple thousand hectares. If you're interested in forest birds, however, small reserves of one or two hundred hectares may not be large enough to support nesting populations.

• As several critics (e.g., Simberloff, Quinn) were quick to point out, island biogeography theory does not require that the species on small islands be a subset of those on large islands. In fact, in a strict interpretation of the equilibrium theory, at least, you'd expect them not to be subsets. As a result, you might actually save more species in a system of small reserves than in a single large one, even though each reserve would contain fewer species (cf. Templeton on genetic diversity).¹⁶

• In addition, there were more fundamental criticisms of the approach because there is little evidence that the differences in species diversity on islands is a colonization-extinction equilibrium. Specifically, there is little evidence for species turnover.

To a large extent, however, 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 to protect the elements of our concern. We are not usually faced with the optimization choice poised in the debate. To the extent we have choices, the choices we face are more like those that Pressey et al. [9] describe, i.e., how small an area can we get away with protecting and which are the most critical parcels?