Oecologia (2009) 159:139–150 DOI 10.1007/s00442-008-1172-1 COMMUNITY ECOLOGY - ORIGINAL PAPER Exotic herbivores directly facilitate the exotic grasses they graze: mechanisms for an unexpected positive feedback between invaders Rebecca J. Best Æ Peter Arcese Received: 8 April 2008 / Accepted: 23 September 2008 / Published online: 22 October 2008 Ó Springer-Verlag 2008 Abstract The ability of an exotic species to establish in a reproductive and competitive strategy that the Eurasian system may depend not only on the invasibility of the native grasses have evolved in the presence of grazers, conferring a community, but also on its interactions with other exotic competitive advantage in a system where the native species species. Though examples of mutually beneficial interac- pool has very few annual grasses and no grazers. Germina- tions between exotic species are known, few studies have tion trials further demonstrated that selective herbivory by quantified these effects or identified specific mechanisms. geese enables their dispersal of exotic grass seed between We used the co-invasion of an endangered island ecosystem heavily invaded feeding areas and the small islands used for by exotic Canada geese (Branta canadensis) and nine exotic nesting. In summary, the exotic geese facilitated both the annual grasses to study the effects of an invading herbivore local increase and the spatial spread of exotic grasses, which on the success of invading grasses. On our study islands in in turn provided the majority of their diet. This unexpected southwestern Canada, we found that geese fed selectively on case of positive feedback between exotic species suggests the exotic grasses and avoided native forbs. Counter to cur- that invasion success may depend on the overall differences rent theory suggesting that the grasses should be limited by a between the evolutionary histories of the invaders and the selective enemy, however, the grasses increased in propor- evolutionary history of the native community they enter. tional abundance under grazing whereas forbs showed declining abundance. Testing potential mechanisms for the Keywords Annual grass Á Branta canadensis Á effects of grazing on grasses, we found that the grasses Poa annua Á Seed dispersal Á Selective herbivory produced more stems per area when grazing reduced vege- tation height and prevented litter accumulation. Forming dense mats of short stems appeared to be an efficient Introduction As the rate of species introductions increases (Levine and Communicated by Miguel Franco. D’Antonio 2003), the success of particular exotic species may increasingly be affected by their interaction with other Electronic supplementary material The online version of this non-indigenous flora and fauna. Simberloff and Von Holle article (doi:10.1007/s00442-008-1172-1) contains supplementary material, which is available to authorized users. (1999) raised the possibility of positive feedbacks between exotic species and reviewed qualitative reports of exotic R. J. Best (&) animals pollinating or dispersing the seeds of exotic plants, Department of Evolution and Ecology, University of California and of one exotic plant or animal benefiting from habitat Davis, Storer Hall, Davis, CA 95616, USA e-mail: [email protected] modification by another. To date, however, there have been relatively few quantitative or mechanistic studies of facil- P. Arcese itation between exotic species (Simberloff 2006; Centre for Applied Conservation Research, Simberloff and Von Holle 1999). To uncover the mecha- University of British Columbia, 3004-2424 Main Mall, Vancouver, BC V6T 1Z4, Canada nisms of interactions between exotic herbivores and exotic e-mail: [email protected] plant species, we studied the ongoing co-invasion of an 123 140 Oecologia (2009) 159:139–150 endangered island ecosystem by an exotic herbivore production, either through direct effects of injury or (Canada geese, Branta canadensis (L.)) and nine exotic through indirect effects of reduced litter accumulation and annual grass species. Counter to current theory, we show increased light availability. that the effect of an exotic herbivore on plant invasion Finally, we determined the potential for seed dispersal success cannot necessarily be predicted from the target of by geese. Because the geese fly between their island herbivore attack. Instead, the outcome of interactions nesting sites and cultivated feeding grounds on larger between exotic herbivores and exotic plants is likely to islands with more heavily invaded plant communities, depend on the individual evolutionary histories of the geese may be a novel dispersal vector for the exotic grasses exotic species and the native community. they consume. We tested the hypothesis that selective Herbivores can have top–down effects on plant inva- herbivory by geese may result in their dispersal of exotic sions both by opening spaces in the native vegetation for propagules by collecting goose feces in the field and ger- exotic plant species to germinate and spread (Hobbs and minating viable seeds in a growth chamber. Huenneke 1992) and by failing to attack the exotic species In this paper, we provide quantitative evidence that the themselves. The latter mechanism underlies the enemy presence of a novel herbivore produced an overall demo- release hypothesis, which predicts that exotic plants should graphic benefit to the exotic grasses even though they were benefit from the absence of their co-evolved enemies selectively grazed. In addition, expanding populations of (Keane and Crawley 2002; Williamson 1996). In some nesting geese are likely to benefit from their ability to circumstances, however, the opposite can be true. Exotic increase both the abundance and the potential dispersal of grasses benefit from the presence of their co-evolved their preferred food source. To explain this unexpected grazers when those grazers do greater damage to grazing- positive feedback between exotic species, we identify the intolerant native grasses (Crosby 1986; DiTomaso 2000; underlying mechanisms and show how they may arise from Hobbs 2001; Mack and Thompson 1982). To explain this similarities in the evolutionary history of the grasses and observation, Colautti et al. (2004) formulated the ‘‘enemy the grazer, and from the absence of equivalent species in of my enemy’’ hypothesis, which predicts that exotic plants the native community. may benefit from their exotic generalist enemies if they are more tolerant of these enemies than the native species are. Using the same logic, Mitchell et al. (2006) point out the Materials and methods opposite; plant invasions should be limited by specialist herbivores that consume only the invading species. Study area In our study system, grazing-tolerance may be a particu- larly important trait because the islands lack native We conducted our study in the southern Gulf Islands of vertebrate herbivores. The exotic Branta canadensis has British Columbia, which comprise more than 100 smaller been nesting in the islands only since the 1980s. These geese islands between the southwest corner of mainland Canada originate from the Giant and Great Basin subspecies, which and the southern end of Vancouver Island (approximately were introduced to the lower Fraser Valley of southwestern 48°410N, 123°230W). The endangered Garry oak meadow British Columbia from Minnesota, Ontario, Saskatchewan, ecosystem, also classified as Oregon white oak woodland Alberta, and the grasslands of south central British Columbia or savannah, stretches north along the Pacific coast from in the 1960s (Banks et al. 2004; Hanson 1997; Smith 2000). southern California and covers the margins or the entirety We therefore hypothesized that if geese are generalist her- of these islands (Fuchs 2001; Griffin 1977). Located in the bivores, their feeding should reduce the abundance of native rain-shadow of the Coastal Mountain Range, the ecosystem plant species not adapted to grazing, and indirectly benefit has a sub-Mediterranean climate, with mild wet winters the exotic grasses. Conversely, if geese selectively feed on and dry summers (di Castri 1973; MacDougall et al. 2006; the grasses this should limit their invasion success. Roemer 1972). The islands have shallow sandy to sandy Using paired open and exclosed plots spread over island loam soils that have accumulated in depressions and cre- sites varying in grazing intensity, we tested (1) grazing vices sheltered from the wind (Ward et al. 1998). selectivity, (2) the effects of grazing on the proportional Germination begins in October or November and continues abundance of native and exotic grasses and forbs, and (3) until April or May, and flowering begins as early as Jan- demographic mechanisms for these effects. We hypothe- uary but is concentrated between March and May. Severe sized two possible demographic mechanisms for the drought stress leads to death of the annual species and increase in the abundance of exotic annual grasses that we dieback of the perennials in July and August. Native her- observed under grazing. First, grazing might reduce flower baceous vegetation is dominated by perennial and annual production by removing inflorescences or preventing their forbs, with a single native perennial grass species and no production. Second, grazing might increase grass stem commonly found native annual grasses. 123 Oecologia (2009) 159:139–150 141 Although the exotic plant species in this system have to be enclosed by a 75-cm-high fence of 2.5-cm-square certainly been present in the general