Seed dispersal effectiveness in fragmented and defaunated landscapes

Abstract

Landscape fragmentation and defaunation have major impacts on plant dispersal and dynamics. However, whether the impact of such perturbations on seed dispersal and recruitment change in sign and strength across habitats and spatial scales, and whether they amplify or buffer each other, remains largely unknown. To evaluate, for the first time, the joint impact of fragmentation and defaunation on seed dispersal effectiveness (SDE) across spatial scales (e.g., short‐ and long‐distance seed dispersal), we utilized the long‐term field data of a mammal‐dispersed tree (Pyrus bourgaeana) in a spatially explicit individual‐based model. By means of simulation experiments, we evaluated the effects of different levels of landscape fragmentation and defaunation on SDE and tree recruitment. Our simulation results revealed that the direction and magnitude of the fragmentation effect on SDE depended on its strength (mild, severe) and the spatial scale considered. Severe fragmentation decreased SDE for short‐ and intermediate‐distance seed dispersal. Interestingly, mild and severe fragmentation increased SDE of long‐distance seed dispersal, suggesting a positive effect of such perturbations (i.e., an increase in the proportion of successful long‐distance dispersal events). Though defaunation had a consistently negative effect on overall SDE, its magnitude was highly species‐ and spatial–scale‐dependent. The impact of defaunation on seed dispersal distance was also species‐specific: the proportion of long‐distance dispersal increased under total badger (Meles meles) defaunation but decreased under total fox (Vulpes vulpes) defaunation. A pervasive integration of seed dispersal distance into the SDE framework is essential to most comprehensively understand the scale‐dependent nature of human activity impacts on plant dynamics. In the long term, strong perturbations (landscape fragmentation and defaunation) could select either for or against long‐distance seed dispersal, altering in different ways the ability of plants to cope with climate change.

The seed dispersal syndrome hypothesis in ungulate-dominated landscapes

“…Although badgers in Matasgordas may partly compensate for the lack of fox seed dispersal, they are unlikely to fully replace them due to interspecific carnivore differences in patterns of habitat use and fecal marking behavior 63 . Seed dispersal limitation in Matasgordas likely contributes to the low local Iberian pear recruitment and adult aggregated pattern observed 8 . Indeed, field studies have indicated that this population exhibits very limited reproductive and regenerative capacity, featuring a skewed demographic structure with the majority of individuals in older age classes, few juveniles, and even fewer seedlings and saplings 64 .…”

“…Indeed, field studies have indicated that this population exhibits very limited reproductive and regenerative capacity, featuring a skewed demographic structure with the majority of individuals in older age classes, few juveniles, and even fewer seedlings and saplings 64 . Nonetheless, other factors, such as the overabundance of seedling predators (i.e., ungulates) and summer droughts, likely also contribute to this situation 8 .…”

“…), which could increase somewhat their importance as seed dispersers. However, corvids generally disperse seeds over much shorter distances 66 – 68 than do carnivores 8 , 59 . Other smaller bird species visited fruiting trees, although they primarily acted as pulp feeders and seldom removed any fruits.…”

“…Its distribution is highly fragmented, with trees occupying patches of Mediterranean scrubland at low densities (usually < 1 individual ha −1 ), with occupied patches isolated from each other by natural and human barriers 81 . The reproduction and regeneration of this tree is very limited due to factors such as high mortality of seeds and seedlings, and seed dispersal limitation 8 . Each tree produces yearly between ~ 200–1500 fruits.…”

“…Fruits provide frugivores with nutrients and water essential for their reproductive success and survival 4 . Plants benefit from the activity of certain frugivores by having their seeds dispersed which increases both local recruitment (i.e., short-distance dispersal) and also gene flow and the (re)colonization of vacant habitats (i.e., long-distance dispersal; 5 – 8 . Since fruit traits can influence foraging choices by mutualistic seed dispersers and, consequently, the probability of seed dispersal, specific combinations of fruit traits (e.g., color and size) that favor interactions with particular seed dispersers are known as seed dispersal syndromes 9 – 12 .…”