A Spatial Depletion Model of the Responses of Grazing Wildfowl to the Availability of Intertidal Vegetation

Effect of grazing by Canada geese Branta canadensis on an intertidal eelgrass Zostera marina meadow

“…These transects had the greatest initial eelgrass biomass, and geese likely spent the greatest amount of time feeding at these areas. At a European Zostera noltii meadow, brant spent the majority of their time within 100 m of the low tide line, as shown in a spatial depletion model developed by Percival et al (1996Percival et al ( , 1998. At the Fishing Island meadow, Transects B and C are closest to the low tide line, and Canada geese may have similarly concentrated their feeding efforts at these locations.…”

“…The rate of depletion of vegetation by grazing waterfowl is primarily influenced by the number of birds present at a site and accessibility of the plants (Baldwin & Lovvorn 1994, Percival et al 1996, Clausen 2000. Seagrass is accessible to birds in shallow water systems or in intertidal areas (Ganter 2000); changes in water levels due to the tidal cycle can limit the amount of time that food resources are obtainable (Fox 1996, Clausen 2000.…”

“…Observations of brant that congregate in intertidal areas have shown that the birds feed whenever they have access to seagrass, day or night, and rest when plants are inaccessible due to high water levels (Percival & Evans 1997). Over time, the total seagrass biomass available at a given site declines as the plants are consumed, causing birds to spend a greater amount of time feeding in order to achieve an adequate food intake (Percival et al 1996(Percival et al , 1998.…”

The feeding ecology and distribution of Common CootsFulica atraare affected by hunting taking place in adjacent areas

“…We found the average biomass of the food stock in the lagoon to be 155 g dry weight/m 2 compared to 107 g dry weight/m 2 within 400 m of the hunting area, which both should be well above the threshold that enables Coots to attain their daily energy requirements. Verhoeven (1980) found that a Coot in autumn requires 52.3 g dry weight of Ruppia cirrhosa per day and that Coots stop exploiting a feeding area when the available biomass falls below 10 g/m 2 , a giving-up threshold that is supposedly dictated by the birds inability to cover their daily energy requirements while feeding at lower food densities, and comparable to other herbivorous species such as Tundra (Bewick's) Swans Cygnus columbianus bewickii with 10-20 g/m 2 (Nolet & Drent 1998) and Brent Geese Branta bernicla with 4.5 g/m 2 (Percival et al 1996). Thus the food biomass should not be a limiting factor for the number of Coots in this part of the lagoon, showing that the Coots left the area well before the food resources were exhausted.…”

“…To correct for this, an estimate of the biomass/m 2 of each plant species at each sample station was calculated by multiplying the average dry weight of the six samples with the percentage of the average leaf cover of each plant species for the sample stations. This estimation method is based on the assumptions that (1) vegetation covered areas had a uniform plant distribution, and (2) that proportional coverage has a linear relationship with biomass density, as found by Percival et al (1996). A t-test was used to analyse changes in vegetation dry weight between the two sampling dates.…”