How does fragmentation affect spatial patterns of animal-mediated seed dispersal?

Background/Question/Methods

The ecosystem service of seed dispersal in tropical forests, where over 80% of tropical trees rely on animal vectors to disperse their seeds, is important in maintaining forest biodiversity and can facilitate regeneration of forest habitat in degraded landscapes. Previous research in animal-mediated seed dispersal has focused primarily on continuous forest habitat. However, little is known about how fragmentation of forests, which is often a barrier to animal movements, affects spatial patterns of seed dispersal. We hypothesized that increasing fragmentation results in more clumped spatial patterns of seed dispersal relative to continuous habitat; however, we hypothesized that this pattern is somewhat mitigated by a fatter tail in movement distances when seed retention times are high. To test these hypotheses, we created a theoretical cellular automata model in Matlab consisting of a spatial matrix of 25% suitable habitat and 75% unsuitable habitat in eight configurations representing zero to high fragmentation of suitable habitat. We simulated an animal disperser moving through suitable habitat in a random walk and dispersing seeds. We varied the distance the animal traveled per movement and the seed retention time. 

Results/Conclusions

Small movements and short seed retention times resulted in increased spatial clumping as fragmentation increased, supporting our first hypothesis. The animal was often unable to leave the starting patch and thus, dispersed seeds in a limited area. However, long distance movements and high seed retention times allowed the animal and its dispersed seeds to escape starting patches and move more freely within a fragmented matrix. In general, this decreased clumping patterns in space, despite increasing fragmentation and supported our second hypothesis. In highly fragmented configurations, long distance movements and high seed retention times substantially increased the fatness of tails on the seed dispersal distribution relative to the unfragmented configuration. Animals that move long distances and exhibit long retention times should be most effective in dispersing seeds in fragmented habitats, which could facilitate forest regeneration. However, tests with empirical data will be required to validate our results.