Abiotic and biotic resistance to grass invasion in serpentine annual plant communities

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, H 2 O, all resources together, and a no-addition control) and two levels of competition (with resident community present or removed). Resource limitation and competition had strong eVects 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 20-fold larger and sixfold to 20-fold 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 signiWcantly aVected native or invader performance, suggesting that N was the key limiting resource during our experiment. We found a signiWcant 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 conWrm 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 resistance.

Combined effects of plant competition and insect herbivory hinder invasiveness of an introduced thistle

“…Thus, as predicted under the biotic resistance hypothesis, interspecific competition and native insect herbivory significantly reduced both survival and subsequent reproduction for the introduced species C. vulgare, and did so more than for the ecologically similar and co-occurring native congener C. altissimum. These results add to the accumulating body of experimental work demonstrating that both interspecific competition (e.g., Kuijper et al 2004;Seastedt and Suding 2007;Going et al 2009) and insect herbivory (e.g., Louda and Rand 2002;Jacobs et al 2006) often significantly reduce the survival, growth, and reproduction of introduced plant species. We found that interspecific competition from the ambient native plant community was a stronger component of biotic resistance to establishment, growth (biomass) and reproduction for the introduced C. vulgare than for the native C. altissimum.…”

Species specific plant-soil interactions influence plant distribution on serpentine soils

“…Our results indicate that adding mineral N or Ca to barrens might assist at least one metal tolerant species (Ceanothus cuneatus) to establish more quickly. If this is attempted in the field, it should be done incrementally at a small scale and monitored well, as Ca did increase Bromus root growth on barren soils, and because N fertilization has favored non-natives over natives in some cases in previous studies (e.g., Huenneke et al 1990;Going et al 2009). In addition, Harrison et al (2006) found that predictors of exotic species cover on serpentine soils throughout California included higher soil Ca:Mg.…”

Resource colimitation governs plant community responses to altered precipitation

“…S1). Previous work showed that water addition alone had little effect on grassland communities on the infertile soils (25,32), whereas fertilization alone had stronger effects (40)(41)(42). In this study, we examined a gradient comprising three habitats: "harsh serpentine" grassland with coarse rocky soils, high native diversity, and very low biomass; "lush serpentine" grassland with fine-textured alluvial soils and intermediate biomass and species composition; and "nonserpentine" grassland with sedimentary soils and high biomass of mainly exotic species (Table S1).…”