A planning strategy widely used in wildlife management has a hidden flaw that could increase extinctions rather than reduce them, says a UC Davis expert on population dynamics.

The problem lies in a mathematical model -- a set of equations that imitate a natural system -- devised by theoretical ecologist Richard Levins in the late 1960s.

Adopted by myriad resource managers for predicting how habitat disturbances would affect plant and animal populations, the Levins model has influenced decisions worldwide about development and habitat protection. Human activities such as forestry, mining, energy production, commercial fishing, agriculture and outdoor recreation have been affected, and species from Australian marsupials to the dusky seaside sparrow.

In the Sept. 12 issue of the journal Science, UC Davis professor Alan Hastings says that the Levins model is inaccurate because it ignores "patch age." Roughly translated, that is the length of time that a number of patches of habitat have supported the species of interest -- for example, how long all the meadows scattered throughout a particular forest have been home to grizzly bears.

Having recognized this shortcoming, it's not very hard for wildlife managers to mathematically incorporate patch age into their management strategies, Hastings says. "Management approaches that do not consider the role of mean patch age may unnecessarily doom populations to extinction," he concludes.

Writing in the same issue of Science, population biologist Andy Dobson of Princeton University says of Hastings' analysis: "In a world where habitat destruction is boosting the mortality rate of many natural habitats, this is disconcerting news."