Chapter 10: Occurrence of Non-Native Invasive Plants: The Role of Anthropogenic Features

The invasion of non-native plants in the Wyoming Basins Ecoregional Assessment (WBEA) area is a major eco- nomic and ecological stress, with invasions thought to be hastened by energy develop- ments. Given the potential impacts of non- native invasive plants and the rapid chang- es in land use in the WBEA, broad-scale assessments and predictive models of non- native invasive plant distribution are need- ed. Using this information, the current ex- tent of populations for targeting treatment and monitoring can be identified, the habi- tat affinities for forecasting where weeds may establish next determined, and the re- sponses to individual human disturbances (such as energy developments) predicted. To address these needs, we conducted veg- etation surveys across the WBEA area at 317 individual survey blocks (five plots per survey block) during the summers of 2005 and 2006. Survey blocks were stratified by both human disturbance and habitat pro- ductivity; in each of five plots per survey block the occurrence of 23 common non- native invasive plants was recorded during early and late season surveys. Here, we report on the four most common invasive plants, crested wheatgrass (Agropyron cristatum), cheatgrass (Bromus tectorum), halogeton (Halogeton glomeratus), and Russian thistle (Salsola spp.). Occurrence models were generated for each species using random-effects logistic regression to account for nesting of plots within sample sites. Predictors of occupancy included lo- cal habitat, abiotic condition, and distance to anthropogenic features. Although oc- currences of all four invasive plants were affected by habitat, abiotic, and anthropo- genic factors, cheatgrass and Russian this- tle were most strongly associated with an- thropogenic disturbance, primarily major roads and energy well sites. We assessed relationships between environmental and anthropogenic predictors and species oc- currences to identify the major factors af- fecting current species distribution, exam- ined shape of the response in occurrence in relation to proximity to individual an- thropogenic disturbances, and provided spatial predictions of the locations where invasive plants are most likely to occur.