THE IMPORTANCE OF SHOREBIRDS TO ENERGY FLUXES IN A FOOD-WEB OF A SOUTH EUROPEAN ESTUARY

This paper presents the results of a study on the diets and energetic needs of shorebird populations in the Tagus Estuary (Portugal). The av erage biomass removed by the bird populations during 1 year was estima ted as 1755 kg ash-free dry weight day(-1), equivalent to a predation pressure of 103 kJ m(-2) year(-1). Gulls ingested more than half of th is total, with waders and ducks ingesting 15-20% each. Cormorants, egr ets 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(-2) year (-1), the consumption efficiency of birds on the invertebrate populati ons was 12%. Twelve bird species took 90% of the total biomass ingeste d during winter by the bird populations, each specializing on one or t wo preferred prey species that constituted more than 75% of their inge sted biomass. During winter, the Tagus bird community is characterized by its great dependence on one prey species, the bivalve Scrobiculari a plana. This prey, taken either in the form of siphons or intact indi viduals, represented a significant percentage of the biomass taken by the bird populations (45%), and it is important for three reasons. Fir stly, other bivalves taken by birds in North European estuaries are ab sent or occur in low densities in the Tagus. Secondly, Scrobicularia d ensities in the Tagus are high compared with values observed further n orth. Thirdly, although high densities also occur in Northern Europe, these density peaks are occasional as there is a trend for the occurre nce 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 i s a form of grazing with important ecological implications for energy fluxes across the estuarine ecosystem and for the prey population. (C) 1997 Academic Press Limited.