The Manila clam has been on the move for decades. It is native to the western Pacific, but following introduction to other parts of the Pacific, and then to southern Europe, it was brought to Britain in the 1980s as a source of seafood. At one site in Britain, Poole Harbour in Dorset, the clam (Tapes philippinarum) has now become naturalized.

Credit: M. LANE/ALAMY

This could be worrying: when colonizing fresh regions, invasive species may devastate components of the existing flora and fauna. For the Eurasian oystercatcher, however, the advent of the clam at Poole is good news, as Richard Caldow and colleagues report (R. G. W. Caldow et al. Proc. R. Soc. B doi: 10.1098/rspb.2007.0072). This species of bird, Haematopus ostralegus ostralegus (pictured), overwinters in Poole Harbour. From their observations of its feeding habits, and from modelling studies, the authors conclude that the oystercatchers have benefited considerably from the extra source of food.

Their observations show that a large proportion of the overwintering oystercatcher population of around 1,200 feeds on the clams, a habit not previously recorded, and that clam meat constitutes a notable part of the birds' diet.

The simulations were carried out with an 'individuals-based' model of shorebird foraging, with the aim of providing a population-level estimate of the effect of the additional food source. The predicted result is a significant reduction in the mortality of the birds, which face the prospect of starvation in the period between September and March.

As yet, there is no evidence that the Manila clam has affected other species of bivalve at Poole, although it occurs at low densities there compared with populations of the species elsewhere. But the clams' occupation of this northern site was probably made possible by locally warm sea temperatures, and Caldow et al. raise the inevitable question of what consequences a continued warming might have. They envisage a further spread north. That process might exacerbate the retreat of the cold-water species that currently constitute food sources for shorebirds. But if, like the Eurasian oystercatcher, other birds develop a taste for the clam, the results might even be beneficial — at least from the avian point of view.