Of seeds and sheep: can poop help plants cope with climate change?

Ungulates perform an important service as long-distance dispersers of seeds. Endozoochory may indeed be the only way for many plant species to travel fast enough to keep pace with climate change but this has been tested only in modelling studies. We used three approaches to consider which ungulate species are likely to move sufficient numbers of seeds sufficiently far or upwards, especially in the context of changing human uses of landscapes. First, we analysed data from 50 studies of endozoochorous dispersal by ungulates, finding that the mean number of seeds per dropping varies by five orders of magnitude and is not simply explained by variation in ungulate body mass or faecal mass. The number of species dispersed is broadly proportional to the number of seeds dispersed, but grazers’ droppings have significantly lower species richness after adjustment for seed number. Secondly, we compared plant species’ current maximum elevations with the viable seed content of faecal pellets from sheep and goats, collected over a steep 800m elevational gradient in the fjord landscapes of western Norway. Of the 40 species we recorded from faecal pellets, only a few species were deposited above their current upper limit. However, we may have been observing species which have already shifted uphill: those species which according to the literature are endozoochorously-dispersed were frequently observed higher up than the elevational range given by the standard flora. This demonstrates the potential importance of outfield grazing for plant elevational range shifts but falls slightly short of direct evidence. Our third approach was to resample historic vegetation plots in parts of the Scandes mountain range which have been subjected to different levels of grazing in recent decades. These data showed that increased grazing has actually decreased the rate at which plant species have moved uphill. It is therefore only under particular circumstances that endozoochorous dispersal is worth the risk associated with being grazed. The lack of information on directed dispersal by ungulates and on seed fate post-dispersal must be addressed in order to understand more completely the role of wild and domestic ungulate grazers in plant distribution shifts.