Protected Areas Can Create Ecological Traps

Protected Areas have gained increasing attention as mechanisms for sustaining heavily harvested fish populations. Much of the theory regarding the impacts of such reserves on populations assumes that they function mainly as sources of larvae or young fish, which disperse randomly between protected and unprotected areas. However, adult movement often occurs, and is likely to be determined by the impacts such movement has on adult fitness.  Models of simple food-webs in spatially heterogeneous systems are used to compare the dynamics of harvested fish stocks when one or more other patches are protected areas.  One or more species are assumed to have adaptive movement towards patches that confer higher fitness.  The mortality imposed by harvesting is assumed to be undetectable by the harvested species.  Both assumptions are commonly met. Increases in the mortality factor in the harvested area can produce an abrupt collapse of either the population in the refuge area or of the entire population of the harvested species when the mortality rate exceeds a threshold level. This is due to the attraction of the harvested patch which has abundant food due to its high mortality.  Population collapse is often associated with alternative outcomes in which the harvested species is present or absent from the entire system of patches.. In such cases, a large reduction in mortality may be needed to avoid extinction in a collapsing population, or to re-establish an extinct population. Similar dynamics can occur with spatially restricted mortality applied to a single species with density dependent growth. The scenario described by these models qualitatively matches some of the ‘Catch per Unit Effort’ data available for populations that have decreased rapidly in size. More generally, the analysis argues for increased consideration of adaptive adult movement in designing protected areas.