Linking recreational disturbance to population size in a ground-nesting passerine.
Abstract
There is often a perceived conflict between recreational access and wildlife conservation. Although many studies have investigated the potential effects on birds, this is the first study to demonstrate an impact of recreational disturbance on population size. We studied the impact of disturbance on a woodlark Lullula arborea population on 16 heathland sites in southern England. These sites all had historical records of breeding woodlarks and together encompassed a range of visitor-access levels. A logistic regression model of patch use was used to quantify the area of habitat suitable for woodlarks. Woodlarks favoured patches with substantial proportions of bare ground and short vegetation. Across heaths, woodlark density (per hectare of suitable habitat) was lower on sites with higher levels of disturbance. Within heaths with recreational access, the probability of suitable habitat being colonized was lower in those areas with greater disturbance and was reduced to below 50% at around eight disturbance events per hour. There was no relationship between disturbance and daily nest survival rates. Birds on heaths with higher levels of disturbance fledged more chicks (per pair) because of a strong density-dependent increase in reproductive output. A model is presented that predicts the consequences for the woodlark population of a range of access scenarios. The impact on the population depends on both the numbers of people and their spatial distribution. Under current access arrangements, a doubling of visitor numbers has little effect, while the same number of people distributed evenly across all sites leads to a major negative impact on the population. Density-dependent breeding success partially balances the negative effects of disturbance; however, we calculated that there is currently a 17% reduction in productivity compared with that predicted in the absence of disturbance. Synthesis and applications. Many previous studies have stressed the negative effects of recreational disturbance on birds' behaviour, distribution and breeding success. However, from a conservation viewpoint, the impact at the population level is of paramount importance. Modelling the population-scale consequences of alternative access scenarios will help policymakers develop appropriate mitigation measures.