Time-scale effects in the interaction between a large and a small herbivore

In the short term, grazing will mainly affect plant biomass and forage quality. However, grazing can affect plant species composition by accelerating or retarding succession at longer time-scales. Few studies concerning interactions among herbivores have taken the change in plant species composition into account. In a salt-marsh system, the long-term effects of exclusion of a large herbivore (cattle) on the abundance of a small herbivore (hare) were studied. Excluding cattle grazing for 30 years resulted in large changes in vegetation composition. In general, the cover of tall-growing species increased in the absence of cattle grazing. These long-term changes negatively affected hare grazing intensity. Hares preferentially fed on Festuca rubra and negatively selected tall growing plants, such as Elymus athericus, both in cattle-grazed and long-term ungrazed areas. However, the intensity of hare grazing was not related to the cover of F. rubra. The cover of tall-growing plants (E. athericus, Atriplex prostrata and Juncus maritimus) appeared to be the best predictor and hare grazing intensity decreased sharply with an increase of the cover of tall plants. When cover of tall plants did not increase, hare grazing intensity was not affected. The study shows that the time-scale of the experiment is of prime importance in studying interactions between herbivores. Species that do not seem to influence the abundance of one another or are competing for the same resources on a short time-scale might well be facilitating each other when looking at larger time-scales while taking plant species replacement into account.