Indirect Interactions Mediated by Changing Plant Chemistry: Beaver Browsing Benefits Beetles

We documented an indirect interaction between beavers (Castor canadensis) and leaf beetles (Chrysomela confluens), mediated by changing plant chemistry of their cottonwood hosts (Populus fremontii × P. angustifolia). Resprout growth arising from the stumps and roots of beaver-cut trees contained twice the level of defensive chemicals as normal juvenile growth. However, rather than being repelled by these defenses, leaf beetles were attracted to resprout growth, resulting in a strong positive association between beavers and beetles. Why? Cottonwoods contain phenolic glycosides, chemicals that are defensive against mammalian herbivores, but are sequestered and used by the beetles for their own defense. Experiments showed that beetles fed resprout growth were better defended against their predators than those fed nonresprout growth. There also may have been a nutritional benefit, because the conversion of the plant’s defense, salicin and other phenolic glycosides, to salicylaldehyde releases glucose. Also, resprout growth contained more total nitrogen than did nonresprout growth. Transfer experiments showed that, in apparent response to these increased nutritional benefits, beetles fed resprout growth developed faster and weighed more at maturity. Although indirect interactions are much less studied than direct interactions, our work suggests that the indirect interactions resulting from beaver cutting of cottonwoods have important consequences for other organisms and could represent an important component of community structure. The habitat mosaics created by beaver herbivory increase arthropod biodiversity and may benefit other organisms such as birds and mammals. Furthermore, by stimulating the production of resprout growth, beavers may play an important role in the regeneration of a habitat type that is rapidly vanishing in the West.