Seed dispersal by birds in a temperate forest of southern South America: Who disperses to whom?

Abstract

Many plants depend on animals for seed dispersal, a task carried out mainly by birds that ingest fruits and defecate or regurgitate seeds undamaged. Unlike other temperate forest biomes, the Temperate Forest of Southern South America (TFSA) is characterized by a large proportion of woody plants (~ 60% of genera) producing fleshy fruits. We studied the interactions between plants and bird dispersers in a mesic forest dominated by Nothofagus dombeyi at the Llao-Llao Forest Reserve, Argentina (41°00’S; 71° 30 ́W), close to the eastern boundary of the TFSA. We sampled birds using eight mist-nets and five sight/sound census stations during two consecutive fruiting seasons (1999 - 2000). We also recorded the phenology and fruit availability of six tree and shrub species. Gut content analysis was used to determine which fruits were consumed by the dispersers. Through the two sampling methods, we surveyed a total of 23 bird species occurring in the study area. We captured a total of 296 birds belonging to 12 species. We found evidence of fleshy fruit consumption only in two species, Elaenia albiceps and Turdus falcklandii, that consumed fruits and defecated seeds of eight from the nine fleshy-fruited plant species locally present. These two bird species were common in the forest during the fruiting season accounting for 52% and 9% of all mist-net captures and 35% and 4% of all sight/ sound censuses, respectively. Other birds consumed fruit or seeds (without dispersing them), insects, and/or small vertebrates. Phenological records indicated that most plant species bear ripe fruit during January and February. The most important fleshy-fruited plants in the area were Aristotelia maqui and Tristerix corymbosus; the latter dispersed by a marsupial. Results presented here, together with a similar study carried out in Chiloé, stress the importance of E. albiceps and T. falcklandii as key mutualists throughout the TFSA.