Plant Species Invasions Along the Latitudinal Gradient in the United States

Abstract

It has been long established that the richness of vascular plant species and many animal taxa decreases with increasing latitude, a pattern that very generally follows declines in actual and potential evapotranspiration, solar radiation, temperature, and thus, total productivity. Using county‐level data on vascular plants from the United States (3000 counties in the conterminous 48 states), we used the Akaike Information Criterion (AIC) to evaluate competing models predicting native and nonnative plant species density (number of species per square kilometer in a county) from various combinations of biotic variables (e.g., native bird species density, vegetation carbon, normalized difference vegetation index), environmental/topographic variables (elevation, variation in elevation, the number of land cover classes in the county, radiation, mean precipitation, actual evapotranspiration, and potential evapotranspiration), and human variables (human population density, cropland, and percentage of disturbed lands in a county). We found no evidence of a latitudinal gradient for the density of native plant species and a significant, slightly positive latitudinal gradient for the density of nonnative plant species. We found stronger evidence of a significant, positive productivity gradient (vegetation carbon) for the density of native plant species and nonnative plant species. We found much stronger significant relationships when biotic, environmental/topographic, and human variables were used to predict native plant species density and nonnative plant species density. Biotic variables generally had far greater influence in multivariate models than human or environmental/topographic variables. Later, we found that the best, single, positive predictor of the density of nonnative plant species in a county was the density of native plant species in a county. While further study is needed, it may be that, while humans facilitate the initial establishment invasions of nonnative plant species, the spread and subsequent distributions of nonnative species are controlled largely by biotic and environmental factors.

Factors Governing the Naturalization-to-Invasion Transition of Exotic Plants in Shenzhen, China

“…The lack of significant associations between the socio-economic factors and the invasive/naturalized ratio was initially surprising. This unexpected finding, in combination with other results of the present study, instead suggests that although human activities facilitate the initial establishment of exotic plants, biotic and environmental factors are more likely to be the major drivers of their further spread and impacts (Stohlgren et al, 2005). However, the effects of human activities on plant invasion may vary greatly at different spatial scales.…”

Non-native plant invasions of United States National Parks

“…Climatic and geographical data from Hof et al (2004) were obtained on each park unit by following the methods of Stohlgren et al (2005a). Longitude and latitude of each park were overlaid onto averaged county data to identify the county in which each park was located and to obtain elevation, precipitation, minimum and average temperature, vegetative carbon (an estimate of productivity), human population size, and distance to coast, which were then added to the database of potential non-native plant species predictors.…”

“…Invasion by non-native plant species appears to increase with greater latitude (Lonsdale 1999; but see Rejmánek and Randell (1994) and Stohlgren et al (2005a)). Sites that are further from the equator may have an abundance of potential niches, but at the greatest latitudes, species limitations exist due to extreme climatic conditions (Sax 2001).…”

“…In addition, elevation was a primary predictor in an invasion model designed for the Cape Peninsula of South Africa (Higgins et al 1999). Range in elevation, which can characterize habitat heterogeneity and extreme climatic environments, might also have a positive relationship with non-native plant species richness (Stohlgren et al 2005a). …”

Phosphorus-mediated changes in life history traits of the invasive New Zealand mudsnail (Potamopyrgus antipodarum)

“…Characteristics of the invaded community might Communicated by Joel Trexler. also affect the success of a new species. Escape from coevolved predators or parasites in the new environment, the level of species diversity and high ecosystem productivity can all contribute to invasion success (e.g., Keane and Crawley 2002;Kennedy et al 2002;Stohlgren et al 2005). A more detailed understanding of how environmental factors, such as nutrient availability and food quality, interact with life history traits of introduced species is critical to increasing the predictive ability of control measures (Simberloff 2003).…”