DO SVALBARD REINDEER REGULATE STANDING CROP IN THE ABSENCE OF PREDATORS - A TEST OF THE EXPLOITATION ECOSYSTEMS MODEL

The Svalbard reindeer is the only mammalian herbivore in Adventdalen ( 78 degrees N), Svalbard, where it has no natural predators. To test if herbivores in the absence of predators regulate standing crop to a le vel independent of productivity, which is one of the predictions of th e ''exploitation ecosystems'' model, herbivore exclosures were set up in 1992 in Salix heath, Luzula heath, Cassiope heath, and Alopecurus m eadow in Adventdalen. Standing crop of vascular plants was harvested a nd measured inside and outside the exclosures in 1994, when the reinde er population was at peak density (ca 5.4 animals km(-2)), and in 1996 , when the reindeer density was about 30% lower (ca 3.7 animals km(-2) ). Standing dead material was reduced by grazing in the Luzula heath i n 1994. However, we found no effect of grazing, year, or interactions between grazing and year on live standing crop. Also contrary to the p redictions from the model, differences in standing crop between vegeta tion types were highly significant. Mean biomass of plant material was lowest in the Alopecurus meadow (36 g m(-2)), two fold higher in the Luzula heath, and about threefold higher in the Salix heath and Cassio pe heath, indicating that reindeer do not regulate standing crop to th e same level on a local scale. The predictive power of the ''exploitat ion ecosystems'' model is low due to lack of recognition of the import ance of plant chemistry, plant compensation ability, variation in fora ge availability during the year, parasites functioning as predators: a nd adverse weather conditions. which may cause density-independent var iations in fecundity and mortality of reindeer.