Abiotic and biotic resistance to grass invasion in

Authors: Barbara Marie Going Janneke Hillerislambers Jonathan M Levine

Publish Date: 2009/01/13

Volume: 159, Issue: 4, Pages: 839-847

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Abstract

Biological invasions severely impact native plant communities causing dramatic shifts in species composition and the restriction of native species to spatially isolated refuges Competition from resident species and the interaction between resource limitation and competition have been overlooked as mechanisms of community resistance in refugia habitats We examined the importance of these factors in determining the resistance of California serpentine plant communities to invasion by three common European grasses Avena barbata Bromus diandrus and Hordeum murinum We added seeds of each of these grasses to plots subjected to six levels of resource addition N P Ca H2O all resources together and a noaddition control and two levels of competition with resident community present or removed Resource limitation and competition had strong effects on the biomass and reproduction of the three invaders The addition of all resources together combined with the removal of the resident community yielded individual plants that were fourfold to 20fold larger and sixfold to 20fold more fecund than plants from control plots Competitor removal alone yielded invaders that were twofold to sevenfold larger and twofold to ninefold more fecund N addition alone or in combination with other resources led to a twofold to ninefold increase in the biomass and fecundity of the invaders No other resource alone significantly affected native or invader performance suggesting that N was the key limiting resource during our experiment We found a significant interaction between abiotic and biotic resistance for Bromus which experienced increased competitive suppression in fertilized plots The threefold increase in resident biomass with N addition was likely responsible for this result Our results confirm that serpentine plant communities are severely N limited which in combination with competition from resident species promotes the resistance of these systems to invasions Our work suggests that better understanding the relative sensitivities of invaders and residents to the physical environment is critical to predicting how abiotic and biotic factors interact to determine community resistanceThe ecological and economic consequences of biological invasions have motivated tremendous interest in the factors enabling communities to resist these invasions While the overwhelming focus of past work has been biotic resistance the reduction in invasion success caused by resident consumers competitors or disease Elton 1958 Alpert et al 2000 Levine et al 2004 Mitchell et al 2006 there is a growing awareness of the importance of abiotic factors that contribute to community resistance These factors include low resource availability and other environmental stresses Davis et al 2000 D’Antonio et al 2001 Davis and Pelsor 2003 Barger et al 2003 Richardson and Pyšek 2006 However the effect of the interactions among abiotic and biotic sources of community resistance on the success of invaders is only beginning to be understood Moreover the few studies on these interactions that exist come primarily from animal systems eg Byers 2002 Holway et al 2002Abiotic factors can control the success of an invader directly Stressful environments eg deserts generally have fewer numbers and a lower abundance of invasive species at broad spatial scales Randall et al 1998 Lonsdale 1999 Alpert et al 2000 A number of experimental studies have also demonstrated that stressful environments become more invasible when limiting resources are added Burke and Grime 1996 Barger et al 2003 Leishman and Thomson 2005 For example the addition of N increased the density and abundance of exotic annual grasses in the Mojave Desert Brooks 2003 and promoted exotic tree invasion in a Texas coastal prairie Siemann and Rogers 2007 The addition of limiting resources may also interact with a decrease in resource competition eg disturbance to even further increase invader success Burke and Grime 1996 Barger et al 2003The abiotic environment can also affect invasions by influencing the outcome of biotic interactions Byers 2002 Holway et al 2002 generating an interaction between these two sources of resistance Such interactions can amplify or dampen overall community resistance Invaders that are already physiologically stressed by the abiotic environment may be particularly vulnerable to negative interactions with the resident community D’Antonio 1993 Alpert et al 2000 Levine et al 2004 Alternatively abiotic stress may reduce the ability of the resident community to achieve the necessary abundance and biomass to repel invaders through competitive effects The nature of interactions between biotic and abiotic factors depends on the relative tolerances of the residents and the invader to the stresses imposed by their environmentRecently several hypotheses have been proposed to integrate the factors that influence community resistance to invasion eg Davis et al 2000 D’Antonio et al 2001 Huston 2004 For example the fluctuating resource hypothesis suggests that communities become more vulnerable to invasions when resources are made more available either because resource supply is increased or competition for resources is decreased Davis et al 2000 If however resource addition disproportionately favors resident competitors an interaction between abiotic and biotic resistance then increased resource supply can increase invasion resistanceIn this paper we examine the roles of abiotic and biotic factors and their interaction in controlling the success of invasive species in a serpentine grassland These systems are relatively stressful for plants because of the chemical composition of their soils and are dominated by native species Thus they are an ideal environment in which to examine the relative roles and interaction of biotic and abiotic resistance In addition the role of resource limitation in constraining grass invasion of these habitats is established Huenneke et al 1990 setting the stage to examine interactions between abiotic and biotic resistance In general serpentine soil is high in Mg Fe minerals and heavy metals and low in Ca N and P Kruckeberg 2006 These chemical conditions are compounded by the shallow rocky nature of many serpentine soils which results in low waterholding capacity The environmental stresses imposed by serpentine soils vary across the landscape ranging from harsh shallow soils on rocky outcrops to deeper more fertile soil with a higher organic matter content in the intervening matrix Serpentine grasslands like nearly all grasslands in California have been invaded by European annual grasses particularly species within the genera Avena Bromus and Hordeum Invasions in serpentine grasslands are remarkably patchy Invaders are most successful in deeper more resourcerich soils but invader density is low on serpentine outcrops which remain dominated by native forbs and grasses despite a potentially large pool of nonnative propagules Harrison 1999 Gram et al 2004We examined how resource limitation and the resident community influenced the growth and seed production of three invasive European annual grasses in serpentine communities Although metal toxicity may be an important factor limiting productivity on serpentine soils we focus here on nutrient limitation and competition because these factors are likely to play crucial roles in community resistance to invasion and are amenable to manipulation Competition from the resident community has been largely overlooked as a mechanism of community resistance in serpentine grasslands because shortstatured native species are generally considered to be poor competitors on more fertile nonserpentine soils Brady et al 2005 However native species are adapted to the multiple stresses imposed by serpentine soils and in these habitats might effectively compete with the more poorly adapted invaders

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