Direct and indirect effects of fragmentation on seed dispersal traits in a fragmented agricultural landscape

Abstract

Habitat fragmentation entails major effects on many ecological processes. Theory offers two contradicting hypotheses for the expected effects of fragmentation on seed dispersal strategy. On the one hand, fragmentation may select for increased dispersal by purging poor dispersers that are incapable of moving between patches (spatial sorting). On the other hand, fragmentation may select for reduced dispersal due to the increased cost of leaving a colonized patch; especially whenever the surrounding matrix is hostile. Furthermore, fragmentation can have additional indirect effects on dispersal through its' effects on abiotic and biotic environmental characteristics, but these pathways are rarely accounted for. Empirical evidence testing these hypotheses in plants comes mainly from distinctly fragmented landscapes at either very large scales or very small scales, whereas studies at intermediate scales, such as those that characterize many agricultural landscapes are lacking. Here, we quantified direct and indirect effects of fragmentation-related characteristics, such as patch size and isolation level, on multiple dispersal-related traits of a heterocarpic annual plant in a fragmented agricultural landscape. Using a structural equation modelling approach, we examined whether fragmentation had positive or negative effects on seed dispersal potential and seed dispersal-related traits. Overall, we found more support for the spatial sorting hypothesis. However, the patterns were not simple, nor consistent across all traits and paths. The positive association between fragmentation and dispersal was mainly via a negative effect of connectivity on seed dispersal potential, but the effect of patch area on dispersal traits was less clear. We identified multiple direct and indirect effects of fragmentation-related patch characteristics on dispersal-related traits. Furthermore, the relative importance of direct and indirect effects was qualitatively different between overall effects. Our findings show that different characteristics of fragmented habitats may differentially affect different dispersal-related traits, and that these effects may take place via multiple paths, often leading to a complex relationship between fragmentation and dispersal.