A data-driven simulation of endozoochory by ungulates illustrates directed dispersal

Large herbivorous mammals act as vectors of dispersal for many forbs and grasses through ingestion and excretion of seeds (endozoochory). Attributes from the plant, animal and landscape need to be integrated in order to study patterns of dispersal in the field, for instance, with respect to the spatial deposition of single plant species in heterogeneous landscapes (‘seed shadows’). Drawing on a set of empirical data, we investigated the dispersal of 25 plant species by cattle in a coastal dune area by means of a spatially explicit simulation model. Model output for the number of dispersed seeds matched observations from field-collected dung pats well. We then applied the model to quantify the seed shadow among the landscape's main vegetation types, focusing on whether the transfer of plants is directed towards suitable habitat. We herewith examined how changing habitat availability in the face of vegetation succession and management affects deposition patterns. A simple wind dispersal model was included for comparison. Simulation showed endozoochorous deposition to be most likely in grassland, but not when this habitat's surface area was low. Since the probability to end up in grassland was higher than expected from random dispersal, grassland plants were always provided with directed dispersal (but not always as strong). Our model illustrates the potential of grazers to contribute in directed seed transfer, apart from their role in long-distance dispersal.