Corridors

The theory of island biogeography was the primary ecological theory from which conservation lessons were drawn in the 1980's. In the 1990s, island biogeography theory was replaced by metapopulation dynamics.⁵. Metapopulations, recall, are sets of interacting populations, populations connected by migration, extinction, recolonization. Not surprisingly, the degree of connectedness among populations within a metapopulation play an important role⁶ in determining whether and how long a metapopulation is likely to persist. This has led to the widespread belief that it is important and desirable to provide corridors that connect remaining fragments. There are, however, several questions to be addressed (see [1] for a recent review and critique):

• Will the species of concern actually use the corridors? The evidence on this point is mixed.

• Will the presence of corridors enhance the transmission of disease among populations?

• Will invasive exotics use corridors to extend their influence more deeply into core areas than they otherwise could?

Unfortunately, it is very difficult to address these questions experimentally.

• It would be nice to have several replicates with the same (or at least similar)
  • distances among patches,

  • type of matrix habitat,⁷ and

  • number and type of corridors.

  Moreover, you'd want to have a different set of replicates for each combination of distances, types of matrix habitat, and corridor arrangements that you thought you needed to investigate. In other words, you're going to need a very large experiment, assuming it's even possible to find a place where you can do that much manipulation.⁸

• In addition, models that suggest the utility of corridors typically assume that
  • the characteristics of each habitat patch are identical, with the possible exception of the size of the populations they will support,

  • the matrix is uniform, and

  • the fragments are not influenced by any large ``mainland''-type populations, i.e., ones not subject to an extinction-recolonization dynamic.

  It's not obvious whether the theoretical advantages claimed for corridors will also apply in situations where one or more of these assumptions are violated. Of course, it's not obvious that the advantage won't apply either. We simply have to admit that the a priori evidence isn't particularly strong in either direction.

What to do? Well, I think that we don't trust our biological intuition as much as we should. Neither do we remember how severe are the constraints affecting those involved in conservation ``on the ground'', nor how different each situation is from the next. I suspect that in most circumstances it will be fairly easy to determine whether corridors are worthwhile based on the elements about which we are concerned in a particular place and what we know about their biology. And if we adopt an adaptive management approach, we can learn from our mistakes.