Green crabs affect both clams and dunlins


Calidris alpina (also known as dunlins) are one of the migratory birds found wintering in the Pacific and Atlantic coasts of North America as well as coasts in Asia (Japan and China). The study area in the Californian coast showed gradual decline of dunlin populations due to the predation of the invasive Green crabs on clams, which is the dunlin’s primary source of diet.

The global populations of many shorebirds, for whom coastal ecosystems are critical wintering habitats and foraging areas, are on the decline. The quality of food available in the coastal habitats in the non-breeding season affects shorebird survival, migration and breeding success.

Exploring the largely neglected effect that non-native invasive marine invertebrates can have on wintering shorebirds, Veronica Estelle and Edwin Grosholz conducted controlled experiments at Bodega Harbour, central California.

They sequentially placed European green crabs and dunlin (a shorebird) in field enclosures to see what effect the crabs had on the diet of the bird. On measuring the gut content of both the crabs and the birds, and studying the feeding behaviour of the birds, they found that green crabs were associated with a reduction of predation by dunlin on polychaetes and an increase in their predation on clams.

The abundance of clams in Bodega Harbour has declined by 90% since green crabs were introduced in 1994, resulting in an increase in the proportion of juveniles. These increasingly smaller and less energy-rich clams present an unknown energetic cost for dunlin. Green crabs not only selectively consume larger clams, they may render the smaller ones more accessible to dunlin through physically turning and loosening the near-surface sediments.

These changes in prey availability affect fitness of individuals and consequently population dynamics. Authors therefore suggest that effects of introduced marine species be considered when planning shorebird conservation measures.

Further Reading:
Estelle V. and E D Grosholz. 2012. experimental test of the effects of a nonnative invasive species on a wintering shorebird. Conservation Biology, 26: 472–481. doi: 10.1111/j.1523- 1739.2011.01820.x

Sartaj Ghuman is a PhD student at the National Institute of Advanced Studies, India.

Photographs: Changua Coast Conservation Action

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2011 Dec