PARENT SESSION
Contributed Oral Session 18: Avian Communities and Populations
Monday, August 8, 8:00 AM - 11:30 AM, Meeting Room 524 C, Level 5, Palais des congrès de Montréal

Towards predictive carrying capacity models.

Nolet, Bart^*,1, ¹ Netherlands Institute of Ecology (NIOO-KNAW), Nieuwersluis, Netherlands

ABSTRACT- For staging birds, the carrying capacity of a site can be expressed as the maximum number of bird-days (i.e., the product of the number of birds and the number of days spent at the site). According to current theory, this maximum is determined by food depletion and interference among birds. We studied swans feeding on below-ground tubers in a 20 ha fennel pondweed stand in the Lauwersmeer, and tested to what extent the number of swan-days can be understood from food depletion alone (the swans switch to feeding at beet fields after a few weeks). We had seven years of data on tuber (biomass) density before and after swan grazing and six years of data on swan-days. By plotting final on initial tuber densities for the seven years we found that the giving-up density (GUD) used by the swans was 8.6 g/m² (= intercept) and the proportion of tubers out of reach (i.e. buried too deep) was 0.14 (= slope). The critical density below which we expected no grazing to occur was 8.6/(1-0.14) = 10.0 g/m². This is lower than the density (14.0 g/m²) at which pondweed feeding and (daytime) beet-feeding would yield the same daily net energy gain (apparently, the swans stick to pondweed feeding for other reasons than energy). If we used the critical density and the non-accessible proportion to predict the number of swan-days observed, we ended up with a good correlation, but a considerable overestimate. Because interference would increase feeding time, inclusion of interference would lead to an even higher overestimate. The most likely explanation of the discrepancy between observed and predicted swan-days is that we did not include water depth, which would greatly affect the (non-)accessible proportion of tubers. In our system the year-to-year variation in the accessible density of the food seems to be the most important variable in determining annual variation in bird-days. Using exclosures we found that the overall food density increases, but that the accessible proportion decreases with grazing pressure in winter. In contrast, summer grazing on aboveground parts by other birds decreases overall food density but increases the accessible proportion of tubers. Thus, there are complex feedback mechanisms operating between accessible food density and bird-days.

Key words: bird-days, depletion, giving-up density, food accessibility