BIODIVERSITY RESEARCH: Native‐exotic species richness relationships across spatial scales and biotic homogenization in wetland plant communities of Illinois, USA

Abstract

Aim To examine native‐exotic species richness relationships across spatial scales and corresponding biotic homogenization in wetland plant communities. Location Illinois, USA. Methods We analysed the native‐exotic species richness relationship for vascular plants at three spatial scales (small, 0.25 m2 of sample area; medium, 1 m2 of sample area; large, 5 m2 of sample area) in 103 wetlands across Illinois. At each scale, Spearman’s correlation coefficient between native and exotic richness was calculated. We also investigated the potential for biotic homogenization by comparing all species surveyed in a wetland community (from the large sample area) with the species composition in all other wetlands using paired comparisons of their Jaccard’s and Simpson’s similarity indices. Results At large and medium scales, native richness was positively correlated with exotic richness, with the strength of the correlation decreasing from the large to the medium scale; at the smallest scale, the native‐exotic richness correlation was negative. The average value for homogenization indices was 0.096 and 0.168, using Jaccard’s and Simpson’s indices, respectively, indicating that these wetland plant communities have been homogenized because of invasion by exotic species. Main Conclusions Our study demonstrated a clear shift from a positive to a negative native‐exotic species richness relationship from larger to smaller spatial scales. The negative native‐exotic richness relationship that we found is suggested to result from direct biotic interactions (competitive exclusion) between native and exotic species, whereas positive correlations likely reflect the more prominent influence of habitat heterogeneity on richness at larger scales. Our finding of homogenization at the community level extends conclusions from previous studies having found this pattern at much larger spatial scales. Furthermore, these results suggest that even while exhibiting a positive native‐exotic richness relationship, community level biotas can/are still being homogenized because of exotic species invasion.

Positive diversity–invasibility relationships across multiple scales in Finnish agricultural habitats

“…Beta diversity of native and alien plant species results from environmental heterogeneity in space, time, resources, disturbance regime and niche differences among species (Loreau 2000;Gabriel et al 2006). In addition, higher beta diversity of both native and alien species can ensue from greater spatial heterogeneity in resource availability and disturbance regime (Davies et al 2005;Belote et al 2008;Chen et al 2010).…”

“…Generally, researchers have been concerned with cumulative diversity (gamma diversity) when studying diversity-invasibility relationships (e.g. Stohlgren et al 1999;Deutschewitz et al 2003;Gilbert and Lechowicz 2005;Davies et al 2007;Belote et al 2008;Chen et al 2010), whereas the model of Shea and Chesson (2002) only accounted for patterns of mean diversity (alpha diversity).…”

The new flora of northeastern USA: quantifying introduced plant species occupancy in forest ecosystems

“…In their review of the effects of the composition and configuration of the landscape on the distribution of introduced species, Vilà and Ibàñez (2011) confirmed that there are more alien species at habitat edges than in the interior of fragments, and in numerous localized studies, proximities to other land uses are important factors that contribute to higher incidence of introduced species invasions (Chen et al 2010;Meekins and McCarthy 2001;Moser et al 2009;Ohlemüller et al 2006;Schulte et al 2011;Yates et al 2004). In Europe, those habitats with the greatest proportion of alien species were either man-made or those subject to frequent disturbance by flooding (Chytrý et al 2008).…”

“…These authors analyzed the relationship between archaeophytes and neophytes rather than natives and found that archaeophyte richness was the single best predictor for neophyte richness across the 33 habitat types they examined. Others have found no evidence of a relationship between native and introduced species (Ohlemüller et al 2006) or found that it does not hold true at the smallest scales (Chen et al 2010). Most studies surmise that positive correlations between native and introduced species richness indicate more about resource availability to all plant species, especially pulses of resources or short-term changes in resources-flooding, light (canopy gaps), temperature regimes, and events that alter soil nutrients (Chytrý et al 2008) Our analyses showed a positive, significant relationship between native and introduced species richness over the entire population of forested lands and for most intact forests over the various ecological provinces.…”

“…However, this relationship between native and introduced species richness has been debated in recent years. Especially at larger spatial scales, a number of investigators have found that the best predictor for alien species richness has been native species richness (Chen et al 2010;Huebner and Tobin 2006;Stohlgren et al 1999Stohlgren et al , 2005. The long European history of phytosociological work allowed Chytrý et al (2008) to classify species as natives, archaeophytes (species introduced over 1,500 years before present), and neophytes (species introduced in the past 1,500 years).…”