Farmland biodiversity and the footprint of agriculture.

Sustainable development requires the reconciliation of demands for biodiversity conservation and increased agricultural production. Assessing the impact of novel farming practices on biodiversity and ecosystem services is fundamental to this process. Using farmland birds as a model system, we present a generic risk assessment framework that accurately predicts each species' current conservation status and population growth rate associated with past changes in agriculture. We demonstrate its value by assessing the potential impact on biodiversity of two controversial land uses, genetically modified herbicide-tolerant crops and agri-environment schemes. This framework can be used to guide policy and land management decisions and to assess progress toward sustainability targets.

The depletion of algal beds by geese: a predictive model and test.

The short-term impact of herbivores on plant productivity has been well studied. Demonstrating long-term effects of grazing on plant populations is much more difficult, but knowledge of such long-term effects is vital in understanding plant-herbivore interactions. We address this issue in a relatively simple plant herbivore system involving the dark-bellied brent goose Branta bernicla bernicla and two marine macroalgae, Enteromorpha spp. and Ulva lactuca, on which the geese graze. In 3 years of sampling, goose grazing was responsible for depleting between 23% and 60% of the algae in the autumn, while wave damage caused between 15% and 70% depletion. The degree of depletion in autumn had no effect on the biomass of algae present the following summer, suggesting no long-term consequences of grazing for the population dynamics of the algae. A model simulating the change in mean algal biomass over the autumn and winter, incorporating changes due to depletion by geese, wave action and productivity, successfully described the date at which geese abandoned the algal bed in six different years. These years varied in numbers of geese, strength and timing of storms and initial biomass available. The most important factor determining the date of abandonment of the algal bed was a tradeoff between the timing of storms and the numbers of geese. When storms occur early, most depletion is due to wave action and geese must abandon the bed early, regardless of the numbers grazing there. As the algae become depleted, the geese switch to feeding on saltmarsh, pastures or arable crops. The rate of depletion of algae affects the timing of this movement, and the model presented here enables the switch to be predicted.

Habitat switching by dark-bellied brent geese Branta b. bernicla (L.) in relation to food depletion.

Seasonal changes in the distribution and feeding behaviour of dark-bellied brent geese Branta b. bernicla (L.) and the biomass of their food plants were studied in three successive winters on the Norfolk coast. The data was used, in conjunction with published information, to show how depletion, productivity and mortality of food plants drive the pattern of habitat switching in this species. It is then possible to explain the habitat shifts observed over the last 35 years and predict future changes. On arrival, geese fed first on algal beds and then on salt marsh, grass and arable fields before returning to feed entirely on the salt marsh in spring. The biomass of green algae, and subsequently the salt marsh vegetation, declined during the autumn and this could be attributed to depletion through goose grazing and natural mortality. As depletion occurred the geese fed more intensively, for a greater percentage of time and with an increasing pace rate, the net result, however, was a declining intake rate (as measured by defaecation rate). The algal biomass at which the geese switched from the algal beds to salt marsh was consistent between years, with heavy storm-induced loss of algae in one year resulting in an earlier switch. That the timing of habitat switches may be explained by depletion of food plants was further supported by historical data: the number of brent geese wintering at the site has increased dramatically over the last 30-35 years and the time of switching from algal beds to salt marsh and from salt marsh to salt marsh and fields has become progressively earlier, as expected from the increased depletion. The expected further increase in brent goose numbers will increase the rate of depletion of intertidal vegetation so that the switches between habitats will be more rapid and the geese will move inland earlier and remain inland longer. The expected increase in the brent goose population will thus result in a disproportionate increase in the levels of conflict between brent geese and agriculture.

Egg mass and shell thickness in dippers Cinclus cinclus in relation to stream acidity in Wales and Scotland.

The shell thickness and mass of eggs of the dipper (Cinclus cinclus) collected on streams of different pH in Wales and Scotland were measured. The aluminium, phosphorus and calcium content of their invertebrate prey were also measured. In a regression analysis, significant at p<0.05, stream pH accounted for up to 7% of the variance in shell thickness, with shells decreasing by 2.5% of the overall mean with each unit of pH decline. In the Welsh sample, differences in shell thickness due to pH were small compared with differences between years across all sites. In data pooled between Scotland and Wales, pH accounted for 17% of the variance in egg mass, but a greater proportion (25%) in Scottish eggs alone. Aluminium concentrations in invertebrates showed no relationship with stream pH, but calcium levels in two insect orders increased significantly with pH. Calcium rich prey, such as Gammarus, were found only in circumneutral streams. The importance of calcium in the diet of dippers before and during egg formation is discussed. No evidence that aluminium in prey adversely affected dipper eggs was found.