Exotic Plant Species Invade Hot Spots of Native Plant Diversity

Abstract

Some theories and experimental studies suggest that areas of low plant species richness may be invaded more easily than areas of high plant species richness. We gathered nested-scale vegetation data on plant species richness, foliar cover, and frequency from 200 1-m 2 subplots (20 1000-m 2 modified-Whittaker plots) in the Colorado Rockies (USA), and 160 1-m 2 subplots (16 1000-m 2 plots) in the Central Grasslands in Colorado, Wyoming, South Dakota, and Minnesota (USA) to test the generality of this paradigm.At the 1-m 2 scale, the paradigm was supported in four prairie types in the Central Grasslands, where exotic species richness declined with increasing plant species richness and cover. At the 1-m 2 scale, five forest and meadow vegetation types in the Colorado Rockies contradicted the paradigm; exotic species richness increased with native-plant species richness and foliar cover. At the 1000-m 2 plot scale (among vegetation types), 83% of the variance in exotic species richness in the Central Grasslands was explained by the total percentage of nitrogen in the soil and the cover of native plant species. In the Colorado Rockies, 69% of the variance in exotic species richness in 1000-m 2 plots was explained by the number of native plant species and the total percentage of soil carbon.At landscape and biome scales, exotic species primarily invaded areas of high species richness in the four Central Grasslands sites and in the five Colorado Rockies vegetation types. For the nine vegetation types in both biomes, exotic species cover was positively correlated with mean foliar cover, mean soil percentage N, and the total number of exotic species. These patterns of invasibility depend on spatial scale, biome and vegetation type, spatial autocorrelation effects, availability of resources, and species-specific responses to grazing and other disturbances. We conclude that: (1) sites high in herbaceous foliar cover and soil fertility, and hot spots of plant diversity (and biodiversity), are invasible in many landscapes; and (2) this pattern may be more closely related to the degree resources are available in native plant communities, independent of species richness. Exotic plant invasions in rare habitats and distinctive plant communities pose a significant challenge to land managers and conservation biologists.

Biotic homogenization and changes in species diversity across human-modified ecosystems

“…Thus, increases in richness have mainly occurred in communities that are (or were) still on the rising part of the unimodal biodiversity-fertility/disturbance curves or, more locally, where reduced land-use intensity impacted communities on the left of the mode. Our results therefore lead to the further prediction that initial abiotic conditions help control the extent to which species with losing and winning trait syndromes can coexist, albeit temporarily, and hence whether a-diversity initially increases or decreases in response to the same human-induced driving forces (Stohlgren et al 1999;Wright & Jones 2004). Dependence upon starting conditions is therefore a further obvious reason why a consistent relationship between species diversity changes and biotic homogenization is unlikely.…”

Biodiversity as a Bulwark Against Invasion: Conceptual Threads Since Elton

“…Experimental studies conducted at small scales tend to fi nd support for DIH, largely through limiting similarity mechanisms, but observational studies tend to fi nd the opposite pattern (Fridley et al 2007 ). Many researchers have suggested that such confl icting patterns indicate that the role of diversity itself in invasibility is weak compared to other factors like propagule pressure (Brown & Peet 2003 ), variation in resource availability (Stohlgren et al 1999(Stohlgren et al , 2003 or disturbance levels (Byers & Noonburg 2003 ; see also Shea & Chesson 2002 ;Huston 2004 ;Tilman 2004 ;Fridley et al 2007 ;Davis 2009 ). The view, referred to as niche -driven community assembly (Hubbell 2001 ) or species packing (Slobodkin 2001 ), asserts that patterns of biodiversity, such as species richness, are determined by differences between species in how effectively they acquire particular resources in an environment where the quality of one or more resources varies (e.g.…”

An experimental study of the relative humidity preference and survival of the Argentine ant, Linepithema humile (Hymenoptera, Formicidae): comparisons with a native Iridomyrmex species in South Australia

“…For instance, Lyons and Schwartz (2001) concluded that the removal of rare native plant species was more likely to facilitate the invasion of the exotic grass, Lolium temulentum, in comparison with areas that sustained higher levels of diversity. However, other research has provided mixed support for this hypothesis (Robinson et al, 1995), with one study concluding that highly diverse communities are fundamentally unstable, resulting in a regular turnover of species (Stohlgren et al, 1999).…”