Expand or better manage protected areas: A framework for minimizing extinction risk when threats are concentrated near edges

Several international agreements have called for the rapid expansion of protected areas to halt biodiversity declines. However, recent research has shown that expanding protected areas may be less cost-effective than redirecting resources towards threat management in existing reserves. These findings often assume that threats are homogeneously distributed in the landscape. In some cases, threats are more concentrated near the edge of protected areas. As protected areas expand, core habitat in the centre expands more rapidly than its edge, potentially creating a refuge from threats. In this paper, we present a framework linking protected area expansion and threat management to extinction risk, via their impact on population carrying capacity and growth rate within core and edge habitats. We demonstrate the framework using a simple population model where individuals are uniformly distributed in a circular protected area threatened by poachers who penetrate the protected area to a fixed distance. We parameterize the model for Peter's Duiker ( Cephalophus callipygus ) harvested for food in the dense undergrowth of African forests using snares. Expanding protected areas can reduce extinction risk more effectively compared to an equivalent investment in snare removal for larger protected areas that already sustain core unhunted habitat. Our results demonstrate the importance of protected area expansion in buffering susceptible populations from fixed hunting pressure restricted to protected area edges. However, for cases where threats, wildlife, and managers respond to each other strategically in space, the relative importance of expansion versus increased management remains an important open problem.