The role of fruit crop size, diversity of avian frugivores and long-term temporal variation over seed dispersal effectiveness in a neotropical savanna

Abstract

Fruit crop size may be a key-trait for attracting an abundant and diverse assemblage of dispersers, possibly leading to positive outcomes for seed dispersal effectiveness (SDE). Temporal changes in resources or frugivore abundance can also influence SDE, but it is not yet clear how seed dispersal dynamics work when we consider broad time scales. Current scenarios are marked by increasing anthropogenic impacts leading to population decline and loss of frugivores (including key-dispersers), and thus it is fundamental for us to understand seed dispersal dynamics. We chose two abundant zoochoric plant species native from Cerrado, a Neotropical savanna hotspot, the sugar-rich Miconia rubiginosa and the lipid-rich Xylopia aromatica. We evaluated how fruit crop size affected the diversity of visitors, the quantity component of SDE (quantitative effectiveness), frequency of visits by specialist and generalist birds, and how diversity of visitors affected quantitative effectiveness (QE). Also, we compared QE provided by disperser assemblages (birds and ants) temporally separated by 15 years. We found that fruit crop size may have a role in attracting diverse assemblages of dispersers, with positive outcomes for QE. Also, seed dispersal services may be maintained over time by a core of species. There are also some fluctuations in seed dispersers contributions to QE over time, causing systems to vary in time in their vulnerability to species loss. Seed dispersal is a key ecosystem function for plant community diversity and regeneration, influencing the provision of ecosystem services. Accessing the factors influencing seed dispersal, as well as how seed dispersal systems vary along temporal scales, reveal how fragile dispersal systems could be, how problematic are population declines and species loss to plant regeneration and conservation, and how crucial it is to preserve this mutualism.