Woodland loss differently affects seed dispersal by resident and migratory avian frugivores in olive grove-dominated landscapes

Abstract

Seed dispersal by frugivores is a key ecological process underpinning the functionality of woodland patches and their capacity to maintain biodiversity in agricultural landscapes. Resident and migratory avian frugivores differ in temporal span of their local occurrence, as well as in mobility and territoriality. However, whether landscape homogenization by agriculture affects seed dispersal by avian frugivores differing in migratory strategy remains largely unexplored. We hypothesized a positive role of woodland patches on avian frugivore abundance and seed-dispersal frequency, and a differential impact of woodland loss and landscape simplification on resident and migratory frugivorous birds. We tested these hypotheses in 20 farms distributed along a gradient of woodland loss and olive cropland expansion in south Spain, considering woodland patches and olive fields as two habitats to survey avian frugivore abundance and frequency of seed dispersal. We confirmed that both the abundance and seed-dispersal frequency of avian frugivores were higher in woodland patches than in olive fields, and also that both decayed with decreasing woodland cover. This was largely mediated by a few European (winter) migrants and secondarily by some resident species. However, the woodland patches were unable to intercept sub-Saharan (summer) migrants in sufficient numbers as to make their contribution to dispersal of summer fruits meaningful. Woodland patches with higher seed-dispersal function were located predominantly in landscapes with high woodland cover and/or fruit availability. Moreover, a relatively high woodland cover in the landscape enhanced the seed dispersal function in woodland patches through a balance between high overall frugivore abundance and dispersal frequency, and some increased activity of resident and summer migrants. Our results also inform that the presence of particularly fruit-rich woodland patches can compensate the woodland cover required to maintain the function by enhancing the frequency of seed dispersal of all migratory groups. In contrast, to maintain the function of fruit-poor woodlands their cover at landscape scale should increase. Preserving and restoring woodland patches at landscape scale and enhancing their abundance and diversity of fruits could boost the activity of different disperser groups and their functional complementarity across agricultural landscapes.