Internal transport of seeds by migratory waders in the Odiel marshes, southwest Spain: consequences for long-distance dispersal.
Waders (Charadriiformes) undergo particularly long migratory flights, making them ideal vectors for long-distance dispersal. We present a study of the dispersal of plant seeds by migratory waders in the Odiel saltworks in southwest Spain (Huelva province) that was conducted from 2001 to 2003. This is the first field study to demonstrate the excretion of viable seeds by waders. Viable seeds of Mesembryanthemum nodiflorum, Sonchus oleraceus and Arthrocnemum macrostachyum were frequent in pellets and faeces of redshank (Tringa totanus), spotted redshank (Tringa erythropus), and black-tailed godwit (Limosa limosa) during spring and autumn migrations, but less frequent during winter. Another 11 seed types were recorded at low densities. More intact M. nodiflorum seeds were present in redshank faeces than in their pellets, but seeds extracted from pellets were more likely to germinate. More S. oleraceus seeds were transported per redshank pellet in the spring, but more redshank migrated through the area in the autumn. The distribution of plants transported are consistent with an important role for long-distance dispersal by waders. M. nodiflorum and S. oleraceus are introduced weeds in the Americas and Australasia, and dispersal by birds may contribute to their rapid spread. Although S. oleraceus is generally thought to be wind-dispersed, birds may be responsible for longer distance dispersal events.