ECOLOGYFriday Sep 18 03:03 PMAllen Press•DTProecol 79 118 Mp 192 v File # 18sc192Ecology,79(1), 1998, pp. 192–200q1998 by the Ecological Society of AmericaINDIRECT INTERACTIONS MEDIATED BY CHANGING PLANTCHEMISTRY: BEAVER BROWSING BENEFITS BEETLESGREGORYD. MARTINSEN,ELIZABETHM. DRIEBE,ANDTHOMASG. WHITHAMDepartment of Biological Sciences, Northern Arizona University, Box 5640,Flagstaff, Arizona 86011 USAAbstract.We documented an indirect interaction between beavers (Castor canadensis)and leaf beetles (Chrysomela confluens), mediated by changing plant chemistry of theircottonwood hosts (Populus fremontii3P. angustifolia). Resprout growth arising from thestumps and roots of beaver-cut trees contained twice the level of defensive chemicals asnormal juvenile growth. However, rather than being repelled by these defenses, leaf beetleswere attracted to resprout growth, resulting in a strong positive association between beaversand beetles. Why? Cottonwoods contain phenolic glycosides, chemicals that are defensiveagainst mammalian herbivores, but are sequestered and used by the beetles for their owndefense. Experiments showed that beetles fed resprout growth were better defended againsttheir predators than those fed nonresprout growth. There also may have been a nutritionalbenefit, because the conversion of the plant’s defense, salicin and other phenolic glycosides,to salicylaldehyde releases glucose. Also, resprout growth contained more total nitrogenthan did nonresprout growth. Transfer experiments showed that, in apparent response tothese increased nutritional benefits, beetles fed resprout growth developed faster andweighed more at maturity.Although indirect interactions are much less studied than direct interactions, our worksuggests that the indirect interactions resulting from beaver cutting of cottonwoods haveimportant consequences for other organisms and could represent an important componentof community structure. The habitat mosaics created by beaver herbivory increase arthropodbiodiversity and may benefit other organisms such as birds and mammals. Furthermore, bystimulating the production of resprout growth, beavers may play an important role in theregeneration of a habitat type that is rapidly vanishing in the West.Key words: beavers; cottonwoods; defensive chemistry; indirect interactions; leaf beetles; phe-nolic glycosides; sequestration.INTRODUCTIONThrough its feeding and dam-building activities, thebeaver,Castor canadensis,acts as a keystone speciesand has disproportionate impacts on the community(Naiman et al. 1986, Johnston and Naiman 1990, Joneset al. 1994, Naiman et al. 1994). Once a major herbi-vore of riparian habitats, beavers were heavily trappedand extirpated from most of North America by the turnof the century (Jenkins and Busher 1979). In recentyears, however, they have returned to much of theirformer range. The removal of dominant herbivoressuch as beavers can have profound impacts on the land-scape. For example, the near-elimination of beaversfrom Yellowstone National Park in the 1800s is thoughtto have resulted in the conversion of beaver ponds andriparian habitat to dry grasslands (Chadde and Kay1991). Here, we examine how beaver feeding impactsan abundant insect herbivore and affects a habitat typethat is designated as threatened due to human activity(Noss et al. 1995).Few studies have examined indirect interactions be-Manuscript received 7 October 1996; revised 28 February1997; accepted 1 March 1997.tween distantly related taxa. More common are studiesof distant relatives that interact directly, and these in-teractions are often negative, e.g., competition (Brownand Davidson 1977, Hay and Taylor 1985) and pre-dation (Paine 1966, Estes and Palmisano 1974). How-ever, indirect interactions in which one organism ben-efits from the actions of another may be common; suchinteractions are not well documented because they areprobably more difficult to detect. For example, in theshort term, desert ants, birds, and mammals competefor seeds (Brown et al. 1986), but in the long term,rodents facilitate both ants (Davidson et al. 1984) andbirds (Thompson et al. 1991).Many studies have shown that herbivores affectplants and plant communities (reviewed by Huntley1991), but few have examined the effects of one her-bivore on other herbivore species. Mammalian herbi-vores are notable because they impact plant commu-nities in two major ways. First, their browsing impactsvegetation and can change competitive interactionsamong dominant species (McNaughton 1976, Cantorand Whitham 1989, McInnes et al. 1992). Second, thedisturbances they create greatly alter landscapes, e.g.,beaver dams (Naiman et al. 1986), mima moundsJanuary 1998 193BEAVER BROWSING BENEFITS BEETLESECOLOGYFriday Sep 18 03:03 PMAllen Press•DTProecol 79 118 Mp 193File # 18sc(Mielke 1977), and other soil disturbances created byfossorial mammals (Platt 1975, Reichman and Smith1985, Huntley and Inouye 1988, Whicker and Detling1988, Martinsen et al. 1990). Beavers are especiallyimportant because they radically alter communities inboth ways. The changes in plant communities thatbeavers create almost certainly affect other herbivoresand other trophic levels.At the individual plant level, it is well known thatherbivory can induce changes in defensive chemistry(e.g., Tallamy and Raupp 1991). Such changes are like-ly when beavers cut down cottonwoods (Populussp.),removing nearly all aboveground biomass. When felledby beavers, these trees often vigorously resprout fromtheir stumps and sucker from their roots. We predictedthat resprout growth resulting from beaver herbivorywould contain higher levels of defensive chemicals(Basey et al. 1990). Phenolic glycosides are a group ofdefensive chemicals found in many members of theSalicaceae (Palo 1984). Although higher levels of phe-nolic glycosides in resprout growth could repel gen-eralist herbivores, here we specifically examine howthese plant defenses might attract a specialist insectherbivore, the leaf beetleChrysomela confluens,whichis known to sequester them for its own defense (Kear-sley and Whitham 1992). If beaver herbivory causesan increase in plant defensive chemicals, then leaf bee-tles benefit.Chrysomela confluens(Coleoptera Chrysomelidae)is a chemically defended leaf beetle that feeds predom-inately on juvenile cottonwood trees (Kearsley andWhitham 1989, Floate et al. 1993). We studied thesebeetles and their cottonwood hosts (Populus fremontii,P.
View Full Document
Unlocking...