Frugivore communities and seed rain in regenerating pastures in Costa Rica
Early secondary succession on degraded, tropical pastures is limited in part by a lack of seed dispersal. Because most Neotropical tree seeds are dispersed by animals, a primary challenge for ecological restoration is to create conditions that attract vertebrate frugivores into degraded environments with few opportunities for food or cover. We used a novel restoration experiment replicated at 13 sites across an agricultural landscape (~100 km2) in southern Costa Rica to study the effects of local restoration interventions, landscape context, and their interaction on the abundance, diversity, and composition of birds, bats, and seed rain 3-8 years after site establishment. Restoration sites were established in 2004-2006, and each site contains three 50 × 50 m plots randomly assigned one of three treatments: not planted (control), 86 seedlings of four species planted in patches (two each of 4 × 4, 8 × 8, 12 × 12 m; nucleation treatment), and 313 trees of the same species planted in rows throughout (plantation). Tree cover in landscapes surrounding restoration sites varies from 11-88% within a 500-m radius. We evaluated how local restoration treatments and landscape context interact to attract vertebrate frugivores into degraded areas.
Local restoration and landscape context interacted to predict bird community composition. Frugivore abundance was greatest in plantations regardless of landscape context - possibly a result of niche complementarity between frugivore communities in forested and agricultural landscapes. In contrast to birds, bat communities were less strongly related to local restoration strategy and landscape context – consistent with some other nocturnal animals. Seed rain abundance in restoration sites was dominated by wind-dispersed individuals (Asteraceae, Heliocarpus), but most species were animal-dispersed, and most animal-dispersed seeds were small (<1 cm diameter; Solanum, Cecropia, Miconia, Piper). Dispersal rates for small-seeded species were similar across local restoration treatments, but compositional similarity to mature forest and dispersal events of larger seeds (>1 cm diameter) were greater in plantations than in nucleation treatments or controls. Our results suggest that a complete lack of vertebrate frugivores is unlikely in our study system, even in the most degraded sites. Regenerating pastures with few vertebrate frugivores are likely to have reduced animal-dispersed seed rain - particularly for large-seeded tree species. Given projections for large-scale forest recovery in post-agricultural lands in Latin America, it seems likely that future forests will initially be dominated by wind-dispersed and small animal-dispersed seeds.