Carrying capacity of a heterogeneous lake for migrating swans

Abstract

One way to express the value of a natural habitat is its capacity to harbour a particular target species. In the case of migratory birds, the cumulative number of birds that can be accommodated at a site for a given period of time (‘bird-days’) became an accepted currency for this carrying capacity. However, there is no general consensus on the factors determining the number of bird-days. By focusing on Bewick’s swans (Cygnus bewickii) foraging on tubers of fennel pondweed (Potamogeton pectinatus) in the Lauwersmeer, the Netherlands, this thesis investigates which factors are relevant in shaping the number of migratory waterbirds using a stopover site. First of all, the results revealed that, in a heterogeneous environment, predictions about consumer numbers cannot rely exclusively on the amount of available food: accessibility, harvest rates and foraging costs should be included. These factors can be combined together in a single measure, the locally achievable net energy intake rate, which proved to be a good proxy for explaining the distribution of animals. This currency also provided evidence that swans exploit human-disturbed parts to a lower extent. On the other hand, the resulting lower swan densities at these parts offer first-winter swans the possibility to compensate for their lower feeding efficiency: they follow a risk-prone behaviour and select these disturbed, but largely unexploited foraging patches. The use of bird-days as a currency assumes that the effects of interference competition on individual intake rates is negligible. Previous work had shown that the effect of interference competition was largely dependent on how food is distributed in the environment. Experiments with ducks revealed that in a spatially autocorrelated, slightly clumped food distribution, resembling the natural situation, differences in intake rate between group or solitary foragers, or between dominants and subordinates are small. In addition, Bewick’s swans avoid forming high competitor densities in the field, thereby minimize the effects of interference competition. Hence, the mean population intake rate at observed competitor densities is only slightly lower than achieved in the absence of interference. Therefore, bird-days may be an appropriate unit for expressing the carrying capacity of the Lauwersmeer for Bewick’s swans. The available tuber density is also shaped by interspecific competition (i.e., with other species). Based on field observations, it was hypothesized that pochards (Aythya ferina) kleptoparasitize Bewick’s swans. However, a controlled experiment revealed that pochards only glean food items that float away from the swans and are not stealing. Hence, swans and diving ducks form a commensalistic relationship, with none suffering from the other species. In contrast, summer herbivores, such as mute swans (Cygnus olor), which consume aboveground vegetation of fennel pondweed, indirectly reduce the tuber stock available for Bewick’s swans in the autumn. In order to predict patterns in complex ecological environments certain simplifications are inevitable. In heterogeneous environments, however, oversimplification lures: the spatial variation in biotic and abiotic factors and the foraging behaviour of the studied species have to be taken into account when forecasting the numbers a site can contain.