Impact of habitat management on grey partridge populations: assessing wildlife cover using a multisite BACI experiment.
The grey partridge is a species of conservation concern, in common with many farmland birds. Its widespread decline in western Europe has been attributed to agricultural intensification. Attempts to restore populations have concentrated upon habitat management. In France, a mosaic of strips planted with maize- or kale-based mixtures is widely used to benefit the grey partridge on intensively cultivated farmlands. The rationale for planting these summer-to-winter cover strips is to increase the overwinter survival rate and hence breeding density. Although this policy is costly to apply, there is little information on its effectiveness. We tested effectiveness of this management scheme as a practical way of improving or restoring partridge populations. We present data from a 6-year before-after control-impact (BACI) experiment replicated across four study sites. We monitored partridge populations to estimate breeding density, reproductive success and overwinter mortality. During the course of the study, no partridge population increase occurred on managed areas compared with control areas. Wildlife cover strips did not improve reproductive success but were associated with higher overwinter 'apparent mortality' rates. Some field data suggested that there was a predation risk at cover strip-field edges. Cover strips may concentrate a number of species within small isolated areas and may act, under some circumstances, as an ecological trap for prey species such as the grey partridge. Due to these complex and unforeseen interactions, this habitat management measure proved unsuitable for partridge restoration. Synthesis and applications. This study has demonstrated the necessity of field experiments at a farm-scale to test the effectiveness of habitat management schemes. A priori assumptions based on smaller scale studies, even if they are supported by some field evidence, can be misleading because they fail to capture the emergent properties of larger scale systems. This study is a specific illustration of how the BACI approach is a powerful tool for addressing wildlife management problems at large spatial scales.