Received: 2 September 2020 | Revised: 17 December 2020 | Accepted: 30 December 2020 DOI: 10.1111/ddi.13232 BIODIVERSITY RESEARCH Modelling species distributions and environmental suitability highlights risk of plant invasions in western United States Devin E. McMahon1,2 | Alexandra K. Urza1 | Jessi L. Brown3 | Conor Phelan4 | Jeanne C. Chambers1 1USDA Forest Service, Rocky Mountain Research Station, Reno, NV, USA Abstract 2USDA Forest Service, Six Rivers National Aim: Non-native invasive plants impact ecosystems globally, and the distributions of Forest, Eureka, CA, USA many species are expanding. The current and potential distributions of many invad- 3Department of Biology, University of Nevada, Reno, NV, USA ers have not been characterized at the broad scales needed for effective manage- 4Department of Natural Resources and ment. We modelled the distributions of 15 non-native invasive grass and forb species Environmental Science, University of of concern in western North America to define their environmental niches and pre- Nevada, Reno, NV, USA dict potential invasion risk. Correspondence Location: Arid and semi-arid western United States. Alexandra K. Urza, USDA Forest Service, Rocky Mountain Research Station, Reno, Methods: We modelled species distribution using presence/absence data NV, USA. from > 148,000 vegetation survey plots to predict the probability of presence of Email: [email protected] each species based on associated climate, soil, topography and disturbance records. Funding information Boosted regression tree models were trained and tested within a buffer distance US Joint Fire Science Program, Grant/Award Number: 19-2-02-11 from presence observations of each species. Buffers were optimized by adding ab- sence observations from increasingly large areas until prediction of presence was Editor: Inés Ibáñez maximized, as defined by the area under the precision-recall curve (AUPRC). We evaluated model AUPRC and other metrics by cross-validation within training areas, then projected final models within focal EPA level III ecoregions. Results: Focal species were present in 2.7% to 21% of plots within model training areas of 40 to 340 km radius. Models conservatively estimated potential species presence, with typically low false positive rates and moderate sensitivity. The most influential predictors of presence included minimum temperature (especially for grasses), climatic water deficit, precipitation seasonality and fire history. Cold desert ecoregions were impacted by the most species, with predicted range expansion into the prairie ecoregions and infilling for Warm Desert species. Main conclusions: Invasive forb and grass species are likely to expand their ranges and continued increases in temperature, aridity and area burned will increase inva- sion risk. Monitoring species presence and absence and mapping known and poten- tial ranges with a focus on presence detection, as in our methodology, will aid in identifying new invasions and prioritizing prevention and control. This is an open access article under the terms of the Creative Commons Attribution License, which permits use, distribution and reproduction in any medium, provided the original work is properly cited. © 2021 The Authors. Diversity and Distributions published by John Wiley & Sons Ltd. Diversity and Distributions. 2021;00:1–19. wileyonlinelibrary.com/journal/ddi | 1 2 | MCMAHON ET AL. KEYWORDS arid, AUPRC, climate suitability, forbs, grasses, invasive plants, semi-arid, species distribution modelling, western USA 1 | INTRODUCTION & Lauenroth, 2006). Precipitation seasonality influences the timing of available water and its storage across soil layers and thus has im- Exotic plant invasions are damaging ecosystems around the world, portant effects on the distribution of plant species based on rooting changing ecosystem structure and function, reducing biodiversity, habits (Lauenroth et al., 2014). At local scales, topography modifies and harming social and economic systems (Mack et al., 2000; Pejchar climate conditions that promote or inhibit invasions (Chambers, & Mooney, 2009; Simberloff et al., 2013). Across the western United Bradley, et al., 2014; Chambers, Miller, et al., 2014; Peeler & States, invasive plant species are compounding the ecosystem Smithwick, 2018). Soil properties influence availability of water and stresses of changing climate and fire regimes. Swaths of the sage- nutrients to invasive species (Brooks et al., 2016), and soil type and brush biome have converted from perennial- and shrub-dominated water availability are useful indicators of ecosystem resistance to ecosystems to invasive annual plant dominance, severely impacting invasion (Chambers et al., 2007; Roundy et al., 2018). sagebrush-dependent species and ecosystem functions (Chambers Disturbances can facilitate the arrival (Gill et al., 2018) or in- et al., 2019; Coates et al., 2016; Germino et al., 2016). The annual crease the abundance of non-native plant species (Jauni et al., 2015). grass Bromus tectorum has been extensively studied due to its ability Successful invaders often share traits that allow exploitation of to form monocultures, increase fire severity and thwart restoration habitat openings, such as short generation times, high fecundity (Balch et al., 2013; Bishop et al., 2019). However, much less is known and capacity for long-distance dispersal (Van Kleunen et al., 2010). about other non-native invaders in the region, which also have the Wildland fires suppress native species and create pulses of avail- capacity to impact ecological processes. Grasses including Bromus able resources, which can release invasive plants from competition rubens, Schismus barbatus and Taeniatherum caput-medusae also alter (Gill et al., 2018; Hanna & Fulgham, 2015; Steers & Allen, 2010). fire behaviour, contributing to the development of grass-fire cy- Anthropogenic land use and infrastructure can also remove com- cles that can lead to progressive loss of the native plant community peting vegetation and increase resource availability, and the asso- (Fusco et al., 2019). Annual forbs such as Lactuca serriola, Tragopogon ciated movement of equipment and animals provides a vector for dubius, Sisymbrium altissimum and Salsola tragus can displace native the spread of seeds (Gelbard & Belnap, 2003; González-Moreno vegetation, limit forage and habitat for animals, and facilitate an- et al., 2014; Leu et al., 2008). nual grass establishment (Antill et al., 2012; Piemeisel, 1951; Prevéy Our primary objective was to assess the current distributions and et al., 2010). environmental suitability for non-native invasive plants and the rel- Mitigating the impacts of plant invasions requires rapidly iden- ative risk of invasion across the arid and semi-arid western USA. We tifying and controlling local populations before they spread farther leveraged presence and absence observations from extensive, pub- (Reaser et al., 2020), yet monitoring efforts may not detect non-na- licly available vegetation survey and monitoring data (BLM, 2020b; tive species until invasion is well underway. To anticipate future in- Pyke et al., 2011; USGS, 2012). We defined a model training region vasions, models of invasion risk based on the current distribution for each species using threshold distances from observed presences, of non-native species have become increasingly common (Abella in which absences were more likely to represent environments where et al., 2012; Ibáñez et al., 2009). However, modelling suitable hab- species may have dispersed but did not establish and inclusion of itat for invading species poses unique challenges (Bradley, 2013). absence data improved model performance (Barve et al., 2011). We Newly arrived species have not yet explored the range of available projected the model predictions across ecoregions to identify areas environmental conditions and may have patchy and sparse distribu- at risk of future invasion. tions that reflect dispersal limitations (Jarnevich & Reynolds, 2011; Welk, 2004). Because species currently undergoing range expansion are not yet at climate equilibrium, presence data may not represent 2 | METHODS the full range of suitable habitat and absences may not necessarily represent unsuitable conditions (Uden et al., 2015). Describing hab- 2.1 | Study area itat suitability thus requires a flexible modelling approach that uses broad-scale, comprehensive data on species occurrences and can Our study focused on six-level II EPA ecoregions in the arid and semi-arid distinguish informative and uninformative absences. western USA: Cold and Warm Deserts, South-Central and West-Central Multiple factors define an invasive species’ niche and deter- Semi-Arid Prairies, Western Cordillera and Upper Gila Mountains (US mine whether it establishes and threatens native ecosystems once Environmental Protection Agency, 2011). EPA ecoregions are based on it arrives. Temperature restricts species’ ranges directly based on abiotic and biotic factors at nested scales for consistent analysis across physiological thermal tolerances and interacts with precipitation to the continent (Omernik & Griffith, 2014). The level II ecoregions outline determine ecosystem water inputs (Allington et al., 2013; Bradford the arid and semi-arid western USA, and level III ecoregions within them MCMAHON ET AL. | 3 represent units of soils, climate and vegetation patterns within which 2020) of the Landfire Reference Database (USGS, 2012) and Bureau species may have similar invasion potential.