Rene van der Wal á Peter Kunst á Rudi Drent Interactions between hare and brent goose in a salt marsh system: evidence for food competition? Received: 30 March 1998 / Accepted: 6 July 1998 Abstract In this study we accumulate evidence that brown hare competes with brent goose for food re- sources in a temperate salt marsh. We show that both species overlap in habitat use and share food plants. The two herbivores mainly used the common habitat at dif- ferent times of the day, with hares active in the dark and geese during the daylight. During the morning and evening, however, the habitat was exploited simul- taneously. Food availability was manipulated by excluding brent geese on both small-scale (30 m 2 ) and large-scale (0.96 ha) plots, while hares had free access everywhere. Exclusion of brent geese enhanced the level of utilisation by hares in both Festuca and Puccinellia dominated marshes, which are among the most inten- sively grazed parts of the salt marsh. The increase in hare grazing pressure following goose exclusion was stronger, when the adjacent control plots had attracted more goose visitation. When geese were excluded, the decrease in Festuca consumption by geese was com- pletely matched by increased hare grazing, while for Puccinellia only part of the `surplus' was harvested. Enhanced levels of hare utilisation were not due to geese interfering directly with hare, nor due to hares avoiding goose droppings. Considering the interaction from the other perspective, hares were observed to disturb geese eectively in every spring. This might have reduced ex- ploitation by geese of the shared resources. On the basis of our experimental results, we conclude that in this salt- marsh system competition for food with brent geese plays a role in the habitat use of hares, and that hares can reduce goose exploitation of shared habitats. Key words Brent Goose á Hare á Food competition á Direct interference á Salt marsh Introduction Competition between animal species has received much attention from both theoretical and ®eld ecologists. It is evident that competitive interactions between species play an important role in structuring communities (Cody 1974; Tilman 1987; Hairston 1989; reviews in: Schoener 1983; Connell 1983). Competition is consid- ered to induce resource partitioning and can work as a selective force towards specialisation of feeding pro- cesses (Gordon and Illius 1989). Three conditions are generally seen as prerequisite for resource competition to occur (Wiens 1989; De Boer and Prins 1990; Putman 1996). Potentially competing species should have overlap in habitat use, share food plants, and food availability should be limited. In the most extreme case, competition leads to exclusive use of a resource by one species, making coexistence impossible (Schoener 1974; Belovsky 1984). Under these circum- stances, competition is no longer evident in the ®eld, although it may have had an important role in struc- turing the community in the past (Connell 1983; Putman 1996). However, stable equilibria can be found if each species exploits a resource not monopolised by others (Schoener 1974), which will frequently be the case. Despite its importance, studies on competition among herbivores remain scarce (cf. Hairston 1989). In most studies, the occurrence of competition is deduced on the basis of indirect measures such as habitat or dietary overlap. Moreover, studies often focus on in- teractions between livestock, or between livestock and wild herbivores (see Prins 1998 for the African situa- tion). In this paper we present evidence for competitive interactions between two wild herbivorous species. Oecologia (1998) 117:227±234 Ó Springer-Verlag 1998 R. van der Wal (&) 1 á P. Kunst á R. Drent Zoological Laboratory, University of Groningen, P.O. Box 14, 9750 AA Haren, The Netherlands Present address: 1 Institute of Terrestrial Ecology, Banchory Research Station, Hill of Brathens, Glassel, Banchory, Kincardineshire AB31 4BY, Scotland e-mail: RVDW@ITE.AC.UK