Eating their greens: waterfowl effects on plants
Wednesday 16th July 2014
Herbivorous waterfowl, such as swans, geese, ducks and coots, are found on every continent except Antarctica. These plant-eating birds can live as single individuals or in huge flocks containing hundreds of thousands of individuals. A growing number of studies across the globe have reported that waterfowl can reduce the standing crop (i.e. amount of living plant tissues) of aquatic plants through their feeding activities, and in extreme cases can eliminate the plants completely. However, what determines the size of these reductions is not well understood, which hinders our attempts to try to manage waterfowl herbivory and conserve aquatic plants. Aquatic plants fulfil a range of vital roles within ecosystems, not only in providing habitat, shelter and food for other organisms, but also in regulating hydrological, chemical and physical processes. Plants are typically a key component of a healthy aquatic ecosystem. Decreases in plant standing crop, caused by waterfowl, will have negative consequences for that ecosystem and for the plants and animals which inhabit it. The issue of waterfowl herbivory needs to be addressed as populations of most species of herbivorous waterfowl are currently increasing; for example of the 21 goose species for which long-term population trends in Europe are known, 16 are increasing. Secondly, we need to understand when aquatic plants need to be protected from overgrazing by waterfowl, as part of our efforts to meet the European Union’s Water Framework Directive targets and ensure that the widespread efforts to restore aquatic ecosystems are successful.
An obvious factor which could affect the size of the reduction in plant standing crop is the density of waterfowl on a waterbody. Surprisingly, a relationship between waterfowl density and the size of the reduction in plant standing crop has proven hard to demonstrate. However, the problem may lie in the fact that previous researchers have calculated waterfowl density as the number of individuals within an area, ignoring potential differences between species of swan, goose, duck and coot. Many previous studies have considered waterfowl species as identical, despite large differences in body mass, behaviour, and diet. To address this issue we carried out a study of our own in which we reanalysed data from 26 published scientific papers, to estimate the relationship between waterfowl density and the reduction in plant standing crop. When waterfowl density was estimated as individuals per hectare we found no relationship with reduction in plant standing crop. However, when waterfowl density was estimated as kg per hectare a positive, linear relationship with reduction in plant standing crop was found. The greater the biomass density of waterfowl, the greater the reduction in plant biomass that the waterfowl caused. Our results suggest that species body mass is a key determinant of waterfowl impact on plant standing crop.
We were also interested in whether different species groups of waterfowl caused different sized reductions in plant standing crop. For example, would large species such as swans cause greater reductions than small species such as coots or ducks? To answer this question, a measure of plant biomass reduction per bird per hectare was calculated for each waterfowl group from the published studies. Comparison of these values indicated some differences in impact between different waterfowl groups, with swans having a greater per bird impact than smaller-bodied waterfowl groups such as coots. In our paper we present evidence that this difference is linked to disparities in individual body size and associated differences in intake rates, diet composition and energy requirements. For example, the diet of a swan is typically 99 % plant material, consisting of leaves and stems, which has a relatively large effect on plant standing crop. In contrast, the diet of coots and moorhens contains only around 65 % plant material, a large proportion of which is seeds, which has a relatively minor effect on plant standing crop.
We subsequently used the method derived here, i.e. estimating waterfowl effects based on their biomass rather than numbers, in a project which examined the effects of mute swans, together with other environmental factors such as water temperature, on the aquatic plant community of the River Frome in Dorset.
If you would like to read more about this study please see our published paper in the journal ‘Hydrobiologia’:
Wood, K.A., Stillman, R.A., Clarke, R.T., Daunt, F. & O’Hare, M.T. (2012). The impact of waterfowl herbivory on plant standing crop: a meta-analysis. Hydrobiologia, 686: 157-167.