The Importance of Shorebirds to Energy Fluxes in a Food Web of a South European Estuary

Abstract

This paper presents the results of a study on the diets and energetic needs of shorebird populations in the Tagus Estuary (Portugal). The average biomass removed by the bird populations during 1 year was estimated as 1755 kg ash-free dry weight day−1, equivalent to a predation pressure of 103 kJ m−2year−1. Gulls ingested more than half of this total, with waders and ducks ingesting 15–20% each. Cormorants, egrets and flamingos each took less than 7% of the total ingested biomass, thus constituting secondary groups for energy transfers in the food web. Assuming an invertebrate production estimate of 853 kJ m−2year−1, the consumption efficiency of birds on the invertebrate populations was 12%.Twelve bird species took 90% of the total biomass ingested during winter by the bird populations, each specializing on one or two preferred prey species that constituted more than 75% of their ingested biomass.During winter, the Tagus bird community is characterized by its great dependence on one prey species, the bivalveScrobicularia plana. This prey, taken either in the form of siphons or intact individuals, represented a significant percentage of the biomass taken by the bird populations (45%), and it is important for three reasons. Firstly, other bivalves taken by birds in North European estuaries are absent or occur in low densities in the Tagus. Secondly,Scrobiculariadensities in the Tagus are high compared with values observed further north. Thirdly, although high densities also occur in Northern Europe, these density peaks are occasional as there is a trend for the occurrence of irregular recruitment, whereas further south, there is regular recruitment. Gulls and waders took significant percentages of bivalve siphons. This is an interesting finding, as the predation of siphons is a form of grazing with important ecological implications for energy fluxes across the estuarine ecosystem and for the prey population.