Quantifying changes in abundance of food plants for butterfly larvae and farmland birds.
Recent changes in population size of butterflies and farmland birds across north-west European landscapes have been linked to the effects of agricultural intensification. Reduction in food supply has been postulated as one of the proximal causes of these trends but the generality of this explanation has not been tested at a large scale and across animal species. Changes in abundance of food plants for butterfly larvae and lowland farmland birds in the British countryside were analysed for the period 1978-90 using data from fixed plots recorded as part of the countryside surveys of Britain. Associations were sought between these changes and trends in population size of selected butterflies and birds recorded from long-term monitoring schemes. No associations were found between change in abundance of food plants and either increasing, stable or decreasing animal groups. At the level of individual animal species, significantly more decreases than increases in food plant frequency were found for herbivorous bird species irrespective of population trend. The size of the reduction in food plant frequency was also significantly greater for the declining bird group than for stable or increasing groups. Few significant associations were detected between population trend and food plant trend for butterfly species. However, the results suggested a positive relationship between increasing butterflies and increased abundance of Elytrigia repens [Elymus repens] and Urtica dioica. Food plants that increased in abundance comprised species favoured by lack of management and increased nutrient availability. Increases in abundance were also associated with linear landscape features such as hedges and streamsides. Decreasing plant species included arable weeds and stress-tolerant species typical of unimproved grasslands. Based on evidence of local associations between animal and food plant change and the need for viable populations of food plants to exist within the range of each animal species, we support measures designed to enhance and maintain populations of declining food plants. Much evidence now exists to identify practical measures that husband food resources for many bird and butterfly species. The challenge for conservation policy is to manage their implementation at scales necessary to influence national trends.