Seed dispersal by birds in the Llao-Llao forest, southern Argentina

AMICO, GC; AIZEN, MA

San Pedro, San Pablo, Brasil

Simposio; 3rd International Symposium-Workshop on Frugivores and Seed Dispersal; 2000

Despite an impoverished vertebrate fauna, seed-dispersal by birds is an important mutualism in the temperate forest that extent from 35° S to 55° S along the Pacific rim of southern South America. We studied the fruit-bird interaction web of the Llao-llao forest (41°S-71° 30´W), Argentina, near the eastern, drier boundary of this temperate biome. We surveyed bird populations by mist-net and sight/sound censuses during two consecutive fruiting seasons. We also recorded the phenology of trees and shrubs. Fruit consumption and plant species identity was determined by analysis of gut content. We captured a total of 296 birds belonging to 12 species. We found evidence of fruit consumption by only two species, the migratory Elaenia albiceps and resident Turdus facklandii, which consumed fruits from eight of the nine fleshy-fruited plant species locally present.  This two bird species were common in the forest, however, accounting for 52 and 9% of all captures and 40 and 4% of all point counts, respectively. A similar study carried out in Chiloé (42°30´S- 73°46´W), at the western, wetter extreme of this temperate forest biome by Sabag (1993) also found that this two bird species were the main seed dispersers despite the somewhat different and richer plant assemblage present there. These two studies stress the asymmetric nature of plant-disperser interaction webs in the temperate forest of southern South America.  Although these two bird species seem to be still relatively abundant, the present low redundancy in alternative mutualistic partners may easily shift the outcome of relationships that involve many plants but only a few animal species.