Oikos 116: 302  312, 2007 doi: 10.1111/j.2007.0030-1299.15289.x, Copyright # Oikos 2007, ISSN 0030-1299 Subject Editor: Dag Hessen, Accepted 25 September 2006 Stream mosses as chemically-defended refugia for freshwater macroinvertebrates John D. Parker, Deron E. Burkepile, Dwight O. Collins, Julia Kubanek and Mark E. Hay J. D. Parker (jdp52@cornell.edu), D. E. Burkepile, D. O. Collins, J. Kubanek and M. E. Hay, School of Biology, Georgia Institute of Technology, Atlanta, GA 30332, USA. Present address for JDP: Dept of Ecology and Evolutionary Biology, Cornell Univ., Ithaca, NY 14853, USA. Present address for DEB: Dept of Ecology and Evolutionary Biology, Yale Univ., New Haven, CT 06520, USA. Marine and terrestrial studies show that small, sedentary herbivores that utilize plants as both food and habitat can gain enemy-free space by living on hosts that are chemically defended from larger, generalist consumers. Although large herbivores are increasingly recognized as important consumers of macrophytes in freshwater communities, the potential indirect effects of herbivory on plant-associated macroinvertebrates have rarely been studied. Here, we show that the large, generalist consumers in a riverine system, Canada geese, Branta canadensis , and crayfish, Procambarus spiculifer , both selectively consumed riverweed, Podostemum ceratophyl- lum , over an aquatic moss, Fontinalis novae-angliae, even though moss comprised 89% of the total plant biomass on riverine rocky shoals. Moss supported twice as many plant-associated macroinvertebrates as riverweed, suggesting that it might provide a spatial refuge from consumption by these larger consumers. Bioassay-guided fractionation of moss extracts led to the isolation of a C 18 acetylenic acid, octadeca-9,12-dien-6- ynoic acid, that deterred crayfish feeding. In contrast to results with Canada geese and crayfish, both the amphipod Crangonyx gracilis and the isopod Asellus aquaticus consumed significant amounts of moss but rejected riverweed in laboratory feeding assays. Moreover, neither amphipod nor isopod feeding was deterred by the crude organic extract of Fontinalis , suggesting that these mesograzers tolerate or circumvent the chemical defenses that deterred larger consumers. Thus, herbivory by large, generalist herbivores may drive freshwater plant community structure towards chemically defended plants and favor the ecological specialization of smaller, less mobile herbivores on unpalatable hosts that represent enemy-free space. Large, generalist herbivores can profoundly alter plant communities by selectively consuming palatable species and avoiding chemically or structurally defended plants (Hay and Fenical 1988, Crawley 1989, Rosenthal and Berenbaum 1992, Lodge et al. 1998). In many cases, however, even the most unpalatable plants are still colonized and consumed by a diverse suite of smaller, often more specialized herbivores that use plants as both habitat and food (e.g. insects) (Strong et al. 1984, Hay 1992). Although the distribution of specialized insects on their hosts can be related to intrinsic differences in plant quality (Ehrlich and Raven 1964), extrinsic factors may also affect host-preference (Jeffries and Lawton 1984, Murphy 2004). In systems with large herbivores, for example, small grazers are at risk of being eaten when living on plants that are palatable to these larger consumers; thus, living on host plants that are well defended from large herbivores can allow smaller grazers to escape incidental predation (Duffy and Hay 1994, Hay 1996). This provision of enemy- free space is hypothesized to have played a large role in the evolutionary radiation of specialist herbivores onto chemically-defended marine and terrestrial plants (Hay et al. 1987, Bernays and Graham 1988, Hay 1992, Singer and Stireman 2005). Herbivores often remove as much or more plant standing stock from freshwater systems as from marine and terrestrial systems (Cyr and Pace 1993, Lodge et al. 1998). Selective feeding by large, generalist herbi- vores, including waterfowl (Sondergaard et al. 1996, Van Donk and Otte 1996, Weisner et al. 1997, Santamaria 2002), crayfish (Lodge and Lorman 1987, 302