Coupling our current knowledge of V. dubia infestation locations with environmental factors allowed us to create predictive habitat suitability maps that identified areas at high risk of invasion in Gallatin County, Montana, USA. Our modeling approach projected the extent of habitat suitability of V. dubia under different climate scenarios across the road network and the entire county. The model ensemble showed excellent model performance using AUC model evaluation metrics, providing an accurate fit to the current distribution of V. dubia in Gallatin County. The ensemble modeling approach we adopted here avoids predictive variability associated with single models, because ensemble-forecasting generates a consensus prediction by combining the predictions from multiple techniques (Araújo and New 2007). The occurrence of V. dubia is most strongly associated with growing season through fall precipitation (wetter March, May, July and September), early and mid-summer (May and July) potential evapotranspiration, relative humidity and solar radiation, with September also being important for solar radiation; in addition May vapor pressure deficit, July maximum temperature and available soil water holding capacity were important. These variables explained most of the variation in distribution of V. dubia across our study area and relate to the species’ life-history (Wallace et al. 2015). V. dubia is a fall germinating annual species, and in our region precipitation and warmth are important for germination in September. The species then overwinters in a dormant state, often protected from thermal extremes by snow, and starts to grow again in spring when increasing solar gain melts the snow and recharges the soil moisture, and spring and early summer climate variables support growth and reproduction.
The expansion of non-native plant populations with time is primarily associated with climate and other environmental filters (Averett et al. 2016). Understanding which climate variables are most important for individual species provides the capacity to model how a species’ distribution will alter as our climate continues to change. Southwest Montana is projected to increase in average temperature by mid-century (1.5 and 2.5 C for RCP4.5 and RCP8.5, respectively), and get wetter annually (13–19 mm and 19-25mm for RCP4.5 and RCP8.5, respectively) but have drier summers (Whitlock et al. 2017). In our study, the changing climate scenarios both predicted increases in V. dubia, though more so with RCP8.5 than RCP4.5. The projected increases in V. dubia were greater along the road corridor than county-wide. Among the land class types, agriculture land and grassland were highly likely to be infested by V. dubia, and grasslands were much more impacted under the RCP8.5 than RCP4.5 scenario. While road corridors are their own ecosystem due to their construction and maintenance, they are generally vegetated by perennial grasses, with some forbs and low shrubs. Thus, the roadside vegetation is most similar to grasslands in our county, potentially explaining why the roadside increase is similar to adjacent grasslands and greater than the entire county predictions (Gallatin − 46 % and 68% vs Road − 67% and 79% under RCP4.5 and RCP8.5, respectively).
Invasive species can be transported and spread unintentionally to new habitats along road corridors faster than they can spread through primary dispersal (Adhikari et al. 2020; Hulme 2009; Rew et al. 2018). The fact that our models showed 20% of the roadsides are currently suitable for V. dubia and that suitability increases in the future supports the hypothesis that road corridors are strong conduits for dispersal and that roadside management practices provide suitable gaps for new seed establishment. As the number of vehicles in Gallatin County and the entire state of Montana increase (Montana Department of Transportation, 2021), a rapid expansion of V. dubia is highly likely. Because seeds will be spread along the road corridor, they will also then move into adjacent lands where environmental filters are still suitable for V. dubia. Our models suggest that agricultural lands and grasslands will become more suitable as the climate changes, and where these habitats are proximal to busy highways or higher road density, there is likely to be more successful invasion.
Documented impacts of V. dubia are limited but noteworthy and highlight why management practices that prevent the invasion of V. dubia are desirable. Range, pasture, and natural areas impacted by V. dubia result in decreased plant community richness and diversity, low forage production, and potentially increased soil erosion due to the species’ shallow root structure. Prather and Steele (2009) found that pasture, grass-hay and grasslands of north-central Idaho experienced a significant decline in forage production because of V. dubia invasion. In some situations, timing of hay harvest schedules had to be altered to avoid export losses due to V. dubia (Wallace et al. 2015). V. dubia has been associated with a decrease in plant species richness and diversity in the Palouse prairie and Canyon lands (Jones et al. 2020), and a decline in nesting success of insect-eating birds due to a loss of biodiversity in conservation lands in northern Idaho (Jones et al. 2018). Furthermore, V. dubia has displaced Bromus tectorum (cheatgrass or downy brome) in the Snake River Canyon grasslands of Idaho (Wallace et al. 2015). This is particularly alarming because V. dubia appears to be avoided by livestock, possibly due to its high silica content (Prather, Steele 2009). The silica content of V. dubia was found to be about 9% as compared to 3% for cheatgrass and 4% for Pseudoroegneria spicata (bluebunch) wheatgrass (Mangold, unpublished data). Fortunately, increases in V. dubia have not been observed after fire in Pacific Northwest temperate grasslands (Ridder et al. 2021) nor elsewhere. When rangelands become infested by species like V. dubia, however, the invasive, annual grasses create fine fuel for fire, and this fuel dries out quickly due to high surface-to-volume ratio, which in-turn, extends the fire season to earlier in summer (Rottler et al. 2015).
The economic cost of controlling invasive species is increasing every year. Montana alone spends millions of dollars to control invasive species annually. For example, the state spent $12 million on control and management of noxious weeds in 2018 (Burch 2020). Ventenata dubia is a relatively new invader (~ 30 years) in Montana. A guiding principle of integrated weed management is that addressing a species closer to the beginning of an invasion is more effective than waiting until later, when the species is widely distributed and well-established (Hobbs, Humphries 1995). This guiding principle is supported by the Montana Noxious Weed Management Plan (Montana Department of Agriculture 2017) where early detection is emphasized under the broad management goals of prevention, detection, and rapid response. Furthermore, as all habitats are not similar in terms of invasion susceptibility, measurements need to be taken to determine which habitats are more prone to invasion in order to develop the most effective management plan. The current study addresses this by improving our understanding of the potential spatial extent of V. dubia in Gallatin County under current and future climates and evaluating habitat preferences of the species. The fact that the road corridors are likely to become increasingly infested by V. dubia as the climate changes suggests an important first step in the management of this species will be periodic monitoring to 1) evaluate the efficacy of control strategies and b) search for new populations, preferably along the predicted suitability gradient (Rew et al. 2007). In this way the best control strategies can be determined and used, and if control strategies differ along the environmental suitability gradient, a more adaptive approach should be developed. Searching for new populations can be prioritized by starting with the highest risk areas on the environmental suitability map, especially where they coincide with the highest risk land uses (agriculture, grassland), and continuing to lower risk areas as resources allow. Locating new populations along road corridors is vital as these populations will act as source populations, invading into adjacent land, particularly grasslands and agriculture.
Our results identified new habitats along the road corridor and in the adjacent land of Gallatin County that may be at risk of invasion by V. dubia. Montana and counties in adjacent states with high road density or busy highways are most at risk for new invasions along the road corridor, which may then spread into adjacent land. Road corridors in the state are already infested with many noxious weed species, some of which are also predicted to increase as the climate changes (Adhikari et al. 2020). Exactly how the invasive and native species will interact as the climate continues to change is unknown but should be considered (Crossman and Bass 2007; Crossman et al. 2011). However, not all species have equal potential to invade and all habitats are not equally threatened by invasion to the same degree (Lonsdale 1999). The degree of invasion in a particular habitat depends on the traits of invasive species, the environment of recipient habitat, and the propagule pressure with which invasive species are entering into the recipient habitat (Rejmánek et al. 2005). Our study highlights climate variables and land use-types most at risk to V. dubia invasion now and in the future, and these maps can be used to help prioritize monitoring and management of those populations that pose the greatest threat to habitats.