| Summary: | Roberts Bank on the Fraser River Estuary, British Columbia, is a globally important site for migrating shorebirds such as the western sandpiper (Calidris mauri) and the Pacific dunlin (Calidris alpina pacifica). Some 800,000 individuals of these two species alone pass through the site northward bound each spring. Here they feed intensively to fuel migration. Westerns sandpipers consume very small prey, with >60% of their diet at Roberts Bank coming from biofilm, and most of the remainder comprised of meiofauna. Dunlins have a more diverse diet, also consuming macrofauna and terrestrial prey obtained nocturnally in uplands. The objective of this study was to assess the carrying capacity of Roberts Bank for shorebird migration. We ran computer simulations combining empirical shorebird passage counts (a 23 yr dataset collected by the Canadian Wildlife Service), biofilm distribution and abundance (based on hyperspectral imagery), the energy requirements of western sandpiper and dunlin, biofilm consumption rates, assimilation efficiencies (based on primary literature), the regeneration rate of biofilm after grazing by shorebirds (measured in the field), and the proportion of biofilm that can be harvested profitably by shorebirds (assumed 50%). Results indicated that there was an excess of biofilm under all but the very highest passage rates, attained on fewer than 0.2% of days for all scenarios, when populations numbered 1.3 million shorebirds or more on a single day. While modelling excluded potential biofilm consumption by other organisms (e.g., meiofauna and macrofauna), which could reduce the carrying capacity of the system, it also did not account for additional capacity resulting from other food sources such as small invertebrate prey. Also, altering the proportion of biofilm consumable by shorebirds was not investigated; however, overall results indicate a large capacity of Roberts Bank to support migrating shorebirds.
|
|---|