You can't always get what you want: conservation planning with feedback effects.

W ithout careful thought, policies to promote a goal can end up doing less good than intended and, in extreme cases, can do more harm than good. Actions lead to reactions, namely, changes in human behavior or in biological or physical processes. Such reactions often cause unintended consequences that can render policies inefficient and ineffective. Some unintended consequences are hard to anticipate and incorporate into planning. Others are almost entirely predictable, although recognition of unintended consequences may require a shift in perspective, either from new insights or from the combination of knowledge from separate disciplines. In this issue of PNAS, Armsworth et al. (1) combine economic analysis of land markets with conservation planning to show how the unintended consequences of buying land for conservation lead to negative feedbacks on conservation objectives. Buying land for conservation increases land prices. With the consequent changes in behavior of other land buyers and landowners, development may not be forestalled as much as shifted. In the extreme, these feedbacks may lead to the paradox of less conservation being accomplished with conservation land purchases than without. Unintended consequences frustrating good intentions are not uncommon in conservation and environmental policy. When logging was curtailed on public lands in the Pacific Northwest to conserve old growth habitat for the northern spotted owl, there was a consequent increase in logging on private forests in the region and increases in timber production outside the region (2). Another example of unintended consequences is furnished by the Endangered Species Act. Section 9 of the Endangered Species Act prohibits causing harm to listed species, which may prevent otherwise legal development, logging, or other activities on private lands. Landowners who want to pursue such activities have an incentive to engage in preemptive habitat destruction that prevents listed species from becoming established on their property (3). These examples illustrate the possibility that policies meant to conserve species may end up harming them instead.