Native Plant Diversity Resists Invasion at Both Low and High Resource Levels

Abstract

Human modification of the environment is causing both loss of species and changes in resource availability. While studies have examined how species loss at the local level can influence invasion resistance, interactions between species loss and other components of environmental change remain poorly studied. In particular, the manner in which native diversity interacts with resource availability to influence invasion resistance is not well understood. We created experimental plant assemblages that varied in native species (1-16 species) and/or functional richness (defined by rooting morphology and phenology; one to five functional groups). We crossed these diversity treatments with resource (water) addition to determine their interactive effects on invasion resistance to spotted knapweed (Centaurea maculosa), a potent exotic invader in the intermountain West of the United States. We also determined how native diversity and resource addition influenced plant-available soil nitrogen, soil moisture, and light. Assemblages with lower species and functional diversity were more heavily invaded than assemblages with greater species and functional diversity. In uninvaded assemblages, experimental addition of water increased soil moisture and plant-available nitrogen and decreased light availability. The availability of these resources generally declined with increasing native plant diversity. Although water addition increased susceptibility to invasion, it did not fundamentally change the negative relationship between diversity and invasibility. Thus, native diversity provided strong invasion resistance even under high resource availability. These results suggest that the effects of local diversity can remain robust despite enhanced resource levels that are predicted under scenarios of global change.

Origin Matters: Diversity Affects the Performance of Alien Invasive Species but Not of Native Species

“…We also examined how native species diversity influenced the performance of Centaurea stoebe L., which is a potent invader in North America but is native to Europe, where our experiment was conducted. In a similar diversityinvasibility experiment previously conducted in North America, Marler (2007, 2008) found that C. stoebe heavily invaded low-diversity assemblages but performed more poorly in high-diversity plots. Thus, our goal was to determine whether C. stoebe responded similarly to diversity in its native range.…”

“…To facilitate comparison of the data from our experiment with those reported by Marler (2007, 2008), we used the same methods for selecting native plant species and constructing experimental plant assemblages as those described in Maron and Marler (2008). Plant assemblages varied in both species (two to 16 species) and functional groups richness (one to six functional groups; table 1).…”

“…As the functional diversity of resident plant communities is likely to affect the amount of untapped resources remaining for invaders (Fargione and Tilman 2005), we selected native plant species that varied in the timing and depth of resource uptake. As in Maron and Marler (2008), the 16 species that we used belong to the following six functional groups: grasses, early flowering forbs that branch with short rhizomes or stolons or at the root crown, midseason forbs with spreading rhizomes, midseason forbs with woody root crowns, early flowering ephemeral forbs with a shallow taproot, and late-season forbs with deep taproots (see table 1 for species names). Species from a specific functional group were randomly assigned when the native community contained several species from that functional group.…”

“…For plants, much work has focused on the competitive effects of resident native diversity in resisting invasion (Levine and D'Antonio 1999;Stachowicz et al 1999;Jiang and Morin 2004;Fridley et al 2007;Maron and Marler 2008). Experimental studies have generally found a negative relationship between resident diversity and invasibility (cf.…”

“…reviews by Levine and D'Antonio [1999], Hooper et al [2005], and Fridley et al [2007]). The mechanism driving this pattern is most often found to be resource use complementarity, where fewer free resources are available for an invading species in assemblages with high versus low numbers of resident species (Loreau 1998;Spehn et al 2000;Maron and Marler 2007). Based on a meta-analysis of biotic resistance to plant invasions, Levine et al (2004) proposed that high richness of resident plant species primarily constrains the performance of alien plant species once they have established, rather than completely preventing invasions.…”