Post-dispersal impact on seed fate by livestock trampling - A gap of knowledge

Sheep grazing is an important management tool in threatened sandy grassland of the temperate zone. Besides direct grazing effects, previous studies have shown benefits of seed dispersal, but little is known about post-dispersal processes. We studied the role of sheep trampling for the post-dispersal fate of seeds embedded in sheep and rabbit dung and hypothesized a positive impact for the development of seedlings as a consequence of cracking the dung pellets. Sheep and rabbit dung samples were collected from species-rich sandy grasslands, and their seed potential was assessed in a climate room. In a factorial field experiment we tested the effects of trampling and dung type on seedling emergence and fruiting success. Seedling emergence in the field was only 5% (sheep dung) or 7% (rabbit dung) of the potential without trampling but 18 or 14% with trampling. Plots with trampled sheep or rabbit dung both showed significantly more seedlings (3.6- or 2.1-fold), more species (2.4- or 1.9-fold), more fruiting individuals (3.9- or 2.6-fold) and more fruiting species (2.1- or 1.9-fold) compared to non-trampled dung plots. Both target as well as non-target species profited from trampling, but the proportion of target species is clearly increased by trampling and graminoid competitors did not reach fruiting stage. Sheep play a multifaceted role in dispersal processes: after endozoochoric transport they act as a sort of 'gardener' not only for sheep-dispersed seeds, but also for those dispersed by rabbits.