Equilibrium or not? Modelling potential distribution of invasive species in different stages of invasion

Abstract

Aim The assumption of equilibrium between organisms and their environment is a standard working postulate in species distribution models (SDMs). However, this assumption is typically violated in models of biological invasions where range expansions are highly constrained by dispersal and colonization processes. Here, we examined how stage of invasion affects the extent to which occurrence data represent the ecological niche of organisms and, in turn, influences spatial prediction of species’ potential distributions. Location Six ecoregions in western Oregon, USA. Methods We compiled occurrence data from 697 field plots collected over a 9‐year period (2001–09) of monitoring the spread of invasive forest pathogen Phytophthora ramorum. Using these data, we applied ecological‐niche factor analysis to calibrate models of potential distribution across different years of colonization. We accounted for natural variation and uncertainties in model evaluation by further investigating three hypothetical scenarios of varying equilibrium in a simulated virtual species, for which the ‘true’ potential distribution was known. Results We confirm our hypothesis that SDMs calibrated in early stages of invasion are less accurate than models calibrated under scenarios closer to equilibrium. SDMs that are developed in early stages of invasion tend to underpredict the potential range compared to models that are built in later stages of invasion. Main conclusions A full environmental niche of invasive species cannot be effectively captured with data from a realized distribution that is restricted by processes preventing full occupancy of suitable habitats. If SDMs are to be used effectively in conservation and management, stage of invasion needs to be considered to avoid underestimation of habitats at risk of invasion.

Emerging Opportunities for Landscape Ecological Modelling

“…However, a fundamental assumption of these approaches is that the environmental factors can be measured throughout the landscape and that their relationship with species presence/occurrence remains constant in space and time [162]. This can be particularly problematic when studying invasive species [163]. Despite the limitations, SDM approaches have been used for species delimitation [164], to study the past distributions of species [165], to identify potential new areas of occurrence [166], and to project environmental niches into future conditions [167].…”

Tracking Invasive Alien Species (TrIAS): Building a data-driven framework to inform policy

“…Second, the use of different data selection strategies for characterizing species' niches combined with an ensemble modelling approach allows quantification of the uncertainties associated with species' potential distributions under current and future climates, rather than trying to identify a single 'best' model (Buisson et al 2010). Lastly, in contrast to the static way in which SDM-based invasion risk predictions are commonly presented, the proposed modelling framework flexibly allows updating of the models when new species occurrence data become available (Franklin 2010, Václavík andMeentemeyer 2011).…”

Climate-induced range shifts of the American jackknife clam Ensis directus in Europe

“…However, this kind of approach remains scarcely applied because mechanistic models require substantial knowledge on the physiology and life history traits of organisms (Sarà et al 2013a). The availability of such data for parameters estimation remains often limited to a few species (Václavík and Meentemeyer 2012;Beaugrand et al 2013). However, ENMs and mechanistic models are complementary approaches and rely on different assumptions.…”

“…Moreover, in this study we assessed the ecological niche of E. directus using only occurrence data from its native range (America) and projected it into the introduced area (Europe). This precaution was taken to reduce the risk of violating the assumption of equilibrium since the introduction of the clam in Europe remains relatively recent (Václavík and Meentemeyer 2012).…”

“…As in any modelling approach, it is important to consider carefully hypothesis behind procedures being used. The first hypotheses made when using ENMs is that species are in equilibrium with their environment (Václavík and Meentemeyer 2012;Araújo and Pearson 2005;Elith et al 2010). This assumption is easily violated for species with low demographic turn-over and limited dispersal capacities (Pearson and Dawson 2003).…”