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Hugh Possingham, Ian Ball, and Sandy Andelman
How Have We Selected Reserves in the Past?
Most areas now thought of as reserves for “conservation” were not chosen to meet
specific biodiversity objectives. Many existing reserves are found in places that
are unsuitable for other purposes (e.g., agriculture or urban development) and
were chosen for cultural or scenic reasons or as favored holiday destinations
(Pressey et al. 1993). Other areas have been selected to protect a few charismatic
flagship or umbrella species (Simberloff 1998; Andelman and Fagan, in review)
without any guarantee that they will adequately conserve regional biota. Consequently, most existing reserve systems have an excess of high mountains, rocky
infertile soil, salt lakes, and inaccessible swamps. Flat, well-drained, fertile land is
rarely conserved. Although a few charismatic species such as the Northern Spotted Owl may be well represented within reserves, in general, existing reserve
systems provide a poor representation of biodiversity.
What does “classical” conservation biology theory tell us about reserve design?
Unfortunately, not much. Early efforts at reserve design were derived from island
biogeography theory (MacArthur and Wilson 1963) in which the emphasis was on
size, shape, and number of reserves. Many conservation biology texts still begin
their discussion of reserve design with diagrams of good and bad reserve design.
For example, long thin reserves with a high edge-to-area ratio are considered poor
in comparison to compact reserves; well-connected reserve networks are considered better than systems made up of isolated reserves; big reserves are considered better than small reserves; and so on. But these prescriptions offer little
explicit guidance for decision makers who are faced with specific choices about
how many, which sites, or which configuration to include in a reserve network.
The SLOSS debate—should we have a single large reserve or several small
reserves—was a product of the island-biogeographic foundation for reserve
design theory and ended in the inconclusive answer, “it depends.” In particular,
close and well-connected patches may be a disadvantage if the arrangement
increases correlations among reserves in environmental variation, by inviting
disease, exotic species, and/or disturbance events to pass from one patch to
another. The disadvantages of such processes may outweigh any advantage to be
gained from elevated dispersal rates and increased recolonization probabilities, at
least for some species.
For single, well-studied species, it is possible to use population viability analysis to find the best arrangement of habitat patches. It may even be possible to plan
a reserve system that will give a particular species an acceptably low extinction
probability. This was the aim of intense debate and planning for the Northern
Spotted Owl. However, the single species approach avoids the issue that a reserve
system designed to be optimal and adequate for a single species is not likely to
satisfy the requirements of all species. Rather than blindly trying to maximize the
number of species based on the very simple ecological abstractions of island
biogeography, the principle of efficiency argues that effective reserve design will
seek a reserve system that achieves defined objectives at minimum expense to
other land uses, based on empirical information on species distributions.
Hugh Possingham, Ian Ball, and Sandy Andelman
How Have We Selected Reserves in the Past?
Most areas now thought of as reserves for “conservation” were not chosen to meet
specific biodiversity objectives. Many existing reserves are found in places that
are unsuitable for other purposes (e.g., agriculture or urban development) and
were chosen for cultural or scenic reasons or as favored holiday destinations
(Pressey et al. 1993). Other areas have been selected to protect a few charismatic
flagship or umbrella species (Simberloff 1998; Andelman and Fagan, in review)
without any guarantee that they will adequately conserve regional biota. Consequently, most existing reserve systems have an excess of high mountains, rocky
infertile soil, salt lakes, and inaccessible swamps. Flat, well-drained, fertile land is
rarely conserved. Although a few charismatic species such as the Northern Spotted Owl may be well represented within reserves, in general, existing reserve
systems provide a poor representation of biodiversity.
What does “classical” conservation biology theory tell us about reserve design?
Unfortunately, not much. Early efforts at reserve design were derived from island
biogeography theory (MacArthur and Wilson 1963) in which the emphasis was on
size, shape, and number of reserves. Many conservation biology texts still begin
their discussion of reserve design with diagrams of good and bad reserve design.
For example, long thin reserves with a high edge-to-area ratio are considered poor
in comparison to compact reserves; well-connected reserve networks are considered better than systems made up of isolated reserves; big reserves are considered better than small reserves; and so on. But these prescriptions offer little
explicit guidance for decision makers who are faced with specific choices about
how many, which sites, or which configuration to include in a reserve network.
The SLOSS debate—should we have a single large reserve or several small
reserves—was a product of the island-biogeographic foundation for reserve
design theory and ended in the inconclusive answer, “it depends.” In particular,
close and well-connected patches may be a disadvantage if the arrangement
increases correlations among reserves in environmental variation, by inviting
disease, exotic species, and/or disturbance events to pass from one patch to
another. The disadvantages of such processes may outweigh any advantage to be
gained from elevated dispersal rates and increased recolonization probabilities, at
least for some species.
For single, well-studied species, it is possible to use population viability analysis to find the best arrangement of habitat patches. It may even be possible to plan
a reserve system that will give a particular species an acceptably low extinction
probability. This was the aim of intense debate and planning for the Northern
Spotted Owl. However, the single species approach avoids the issue that a reserve
system designed to be optimal and adequate for a single species is not likely to
satisfy the requirements of all species. Rather than blindly trying to maximize the
number of species based on the very simple ecological abstractions of island
biogeography, the principle of efficiency argues that effective reserve design will
seek a reserve system that achieves defined objectives at minimum expense to
other land uses, based on empirical information on species distributions.