Novelty does not Undermine Niche-Based Invasion Projections for an Invasive Annual Grass

Invasive annual grasses are recognized as important ecosystem-altering species and their distributions are expected to shift under future climate conditions, underscoring the importance of assessing future invasion risk. Species distribution models (SDMs) are commonly used to predict areas at risk of invasion. However, the reliability of predictions is affected by uncertainty, which can arise from the modeling process in addition to the variability of natural systems. Climate novelty is an important source of uncertainty and can cause unreliable predictions when SDM models are applied across new spatial and temporal domains. Therefore, it is critical to assess climate novelty when predicting a species' distribution. Uncertainty in future predictions can also arise from the SDM models or from variability among Global Climate Models (GCM). Our objective was to explore the impact of climate novelty and uncertainty on predictions for an invasive annual grass (Ventenata dubia) in the western United States. We developed a SDM and examined how 1) current and future climate novelty may impact predicted suitability, and 2) GCM and SDM derived uncertainty may impact our understanding of future predictions.

Climate novelty did not suggest predictions were unreliable in cases where the species-environment relationship was unimodal and the novel climate conditions fell well outside the observed relationship. GCM-derived uncertainty exceeded SDM uncertainty and was highest along the Rocky Mountain region of the interior western US, as the 30 GCMs varied widely in future projected cool season precipitation and temperature seasonality (Figure 1). Our novelty and uncertainty assessments suggest that ventenata suitability in the current species range will likely persist and areas outside the current range and at higher elevations will likely experience a substantial increase in invasion risk. Because many of these mountain areas are forested, disturbances that remove forest canopies could catalyze the rapid expansion of ventenata in the western US given available seed sources.