BRANTA – Abel Gyimesi
Carrying capacity of a heterogeneous lake for migrating swans
Institution: Netherlands Institute of Ecology
Supervisors: Bart Nolet, Marcel Klaassen, Theunis Piersma
Details: PhD 2010 (Completed)
Address: Bureau Waardenburg, Postbus 365, 4100 AJ Culemborg, the Netherlands (Jan 2013) Email
Subject Keywords: carrying capacity, Bewick's swan, Cygnus columbianus bewickii, fennel pondweed, Potamogeton pectinatus, interspecific food competition, interference competition, feeding efficiency, giving-up net energy intake rate, human disturbance
Species Keywords: Bewick's swan, Cygnus columbianus bewickii
Due to climate change, human disturbance, habitat degradation and fragmentation, natural areas are under increasing pressure and there is growing need to set conservation priorities. One way to express the value of a site is its capacity to harbour a particular target species. In the case of migratory birds, it became accepted to use as a currency for this carrying capacity the cumulative number of birds ('bird-days') that can be accommodated at a site for a given period of time. However, there is no general consensus on the factors determining the number of bird-days, and whether bird-days is a correct unit for expressing carrying-capacity.
The most basic models assume that the number of bird-days simply depends on the amount of food present. More advanced models consider that not all the food might be accessible and harvestable to the foragers, while further extensions suppose that foragers only deplete food sources until a certain giving-up density (i.e. the food density at the moment the animals leave the patch). The giving-up density is often assumed to be the same everywhere on the site, and equivalent to the critical food density (i.e. the density that provides an intake rate just sufficient to keep the energy budget in balance). In heterogeneous environments, however, both the spatially varying accessible food density and the costs of foraging (metabolic, predation and the missed opportunity costs) shape giving-up densities. Therefore, calculations that rely on a single fixed giving-up density may overestimate or underestimate a site's actual carrying capacity.
In this project, my aim was to accomplish a good understanding of the relevant factors shaping the number of migratory waterbirds making use of a stopover site. By focusing on Bewick's swans (Cygnus columbianus bewickii) foraging on tubers of fennel pondweed (Potamogeton pectinatus) in the Lauwersmeer, the Netherlands, we tested which of the above factors should be taken into account to predict the number of birds the site can support.
First of all, I show that in a heterogeneous environment, such as my study system, predictions about consumer numbers cannot rely exclusively on the amount of available food: accessibility, harvest rates and foraging costs cannot be ignored. These factors can be combined together with food supply 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 competitor densities at these parts seemed to offer first-winter swans the chance to compensate for their lower feeding efficiency, by following a risk-prone behaviour and selecting these disturbed foraging patches that are largely unexploited.
Our results revealed that the outcome of interference competition is largely dependent on how food is distributed in the environment and that in spatially autocorrelated, slightly clumped food distributions, differences between dominants and subordinates are less prominent due to difficulties with monopolizing the resources. Interference competition seemed to generally regulate the maximum group density of Bewick's swans in the field: in order to minimize the effects of interference swans avoid high competitor density. Hence, the mean population intake rate at field-observed competitor densities is only slightly lower than achieved in the absence of interference. Therefore, using bird-days, which assumes that the effects of interference competition on individual intake rates is negligible (i.e. the food consumption of ten birds in one day is equal to that of one bird in ten days), may be an appropriate unit for expressing the carrying capacity of the Lauwersmeer for Bewick's swans.
Furthermore, I discuss how the available tuber density to Bewick's swans is shaped by competition with other species. Based on field observations, it was hypothesized that pochards (Aythya ferina) kleptoparasitize the swans. However, through a controlled experiment I show that pochards are only gleaning food items that float away from the swans and are not stealing: the two species form a commensalistic relationship. On the other hand, we demonstrate that through time-staggered interspecific competition, avian herbivores, such as mute swans (Cygnus olor), consuming aboveground vegetation of fennel pondweed in the summer, do reduce tuber density in the autumn available for Bewick's swans.
I realize that for describing complex ecological processes certain simplifications are inevitable. In heterogeneous environments, however, oversimplification lures. Based on my results, I am convinced that 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.
Gyimesi, A., Franken, M., Feige, N. & Nolet, B.A. 2012. Human disturbance of Bewick's Swans is reflected in giving-up net energy intake rate, but not in giving-up food density. Ibis, 154: 781-790.
Gyimesi, A., van Lith, B. & Nolet, B.A. 2012. Commensal foraging with Bewick's Swans Cygnus bewickii doubles instantaneous intake rate of Common Pochards Aythya ferina. Ardea, 100: 55-62.
Gyimesi, A., Varghese, S., De Leeuw, J. & Nolet, B.A. 2012. Net energy intake rate as a common currency to explain swan spatial distribution in a shallow lake. Wetlands, 32: 119-127.
Gyimesi, A., de Vries, P.P., de Boer, T. & Nolet, B.A. 2011. Reduced tuber banks of fennel pondweed due to summer grazing by waterfowl. Aquatic Botany, 94: 24-28.
Gyimesi, A., Stillman, R.A. & Nolet, B.A. 2010. Cryptic interference competition in swans foraging on cryptic prey. Animal Behaviour, 80: 791-797.
Gyimesi, A., van Rooij, E. & Nolet, B.A. 2010. Nonlinear effects of food aggregation on interference competition in mallards. Behavioral Ecology and Sociobiology, 64: 1897-1904.