Differential effects of fruit availability and habitat cover for frugivore-mediated seed dispersal in a heterogeneous landscape

Abstract

1. We investigated the effect of forest cover and fruit availability on frugivore-mediated seed dispersal of the ornithochorous tree Crataegus monogyna in highly heterogeneous secondary-growth forests of the Cantabrian Range (NW Spain). 2. During 2006 and 2007, we collected dispersed Crataegus seeds from 283 sampling stations in a 400 × 440 m study plot in which forest cover varied from dense to scant. Dispersal kernels were characterized with an extended version of the inverse modelling framework that incorporates the effect of the local environment at the source and the influence of all those environments that seeds potentially encountered during their dispersal path. 3. We found that forest cover and fruit abundance had opposite influences on dispersal patterns. Plants growing in cells with denser cover dispersed more seeds and at larger average distances than those from more sparsely covered cells, while mean dispersal distance and the probability of long-distance dispersal decreased with increasing abundance of fleshy fruits. However, the relative influence of these factors changed between study years, as forest cover had a weak effect on seed dispersal in the second year when fruits were scarcer and more heterogeneously distributed across the landscape. 4. Habitat resistance to seed movement increased with increasing forest cover. Consequently, cells with high forest cover in a matrix of sparse tree density were predicted to intercept a substantial amount of seeds. 5. Synthesis. Our results suggest that the local environment at a seed’s source and, to a lesser extent, all those environments that seeds potentially encounter during their dispersal path can have pervasive effects on frugivore-mediated seed dispersal kernels in heterogeneous landscapes. They also highlight the fact that not just forest cover, but also the underlying fruit-resource distribution, needs to be considered to understand how environmental heterogeneity affects seed dispersal patterns. Our findings could be extended to landscapes subjected to anthropogenic disturbance such as fragmentation. Thus, the consideration of fruit-resource distribution seems essential for establishing the relationship between landscape pattern and the spatial behaviour of frugivores, and in turn, for explaining frugivore-mediated seed dispersal in fragmented landscapes.