Testing the assumption of environmental equilibrium in an invasive plant species over a 130 year history

Abstract

Invasive plants are an increasing threat to global biodiversity. Effective management depends on accurate predictions of their spread. However, modelling the geographic distribution of invasive species, particularly with correlative species distribution models (SDMs), is challenging. SDMs assume that species are in equilibrium with their environment (i.e. they occur in all suitable environments); this assumption is likely to be violated for a species that is actively invading new environments. This assumption is rarely assessed, and when violated can have consequences for model reliability. Using the invasive vine Vincetoxicum rossicum, we tested the hypotheses that: 1) invasive species' distribution in environmental and geographic space increase to a plateau over time; 2) this plateau is a useful proxy for equilibrium distribution, a key assumption underlying SDMs. We compare V. rossicum's expansion in environmental and geographic space between historical and current time periods and infer equilibrium when its distribution has remained stable for an extended period. We also compare the performance of SDMs from historical time periods in predicting the current geographic distribution of V. rossicum. We found that V. rossicum has reached equilibrium in environmental space, but is still expanding its geographic distribution. SDM performance was poor in the first 30 years following introduction, but improved as V. rossicum approached environmental equilibrium. Our findings demonstrate the power of including temporal dynamics and the need to consider environmental and geographic equilibrium separately when modelling the distribution of invasive species. In light of our findings, we address shortcomings of the current approach to defining an equilibrium distribution and present a new perspective for reconciling the potentially confounding influence of dispersal limitation when assessing equilibrium distribution.

Biology of Invasive Plants 3.Vincetoxicum nigrum(L.) Moench andVincetoxicum rossicum(Kleopow) Barbarich

“…Any discrepancies between our conclusions and those of Foster et al (2022) could reflect the different goals, methods, and spatiotemporal scales of the two analyses. As different approaches provide different insights, it is useful to construct diverse models to develop a consensus picture of invasion risk.…”

“…Foster et al (2022) developed MaxEnt and spatial dispersal models for V. rossicum across its invasion history in North America (10-yr intervals, 1890 to 2020). This analysis suggested that the North American distribution of V. rossicum was largely a function of dispersal in the early stages of invasion.…”

“…Environmental constraints played a more important role in later stages. Foster et al (2022) concluded that V. rossicum may have reached environmental equilibrium and that its geographic distribution may be stabilizing, but noted caveats related to the period of reference, spatial scale, and potential for dispersal limitation or local adaptation.…”

The cumulative niche approach: A framework to assess the performance of ecological niche model projections

“…ENMs assume that the species currently occurs in all suitable environments (i.e. is at a species‐environment equilibrium; Foster et al., 2022 ). However, the current occupancies of many species do not reflect all the environments that are suitable, and in these cases, the ENM model will underestimate the species' possible realised niche will not accurately project its suitability to new areas or future climatic projections (Liu et al., 2020 ; Ludwig et al., 2023 ; Manzoor et al., 2018 ; Parravicini et al., 2015 ; Werkowska et al., 2017 ).…”

“…For example, some studies take advantage of the temporal sequence of invasions to compare the performance of ENMs trained only on native occurrences to those trained on occurrences from the native range and early stages of invasion on projecting the environmental suitability for the later stages of invasion (e.g. Barbet‐Massin et al., 2018 ; Foster et al., 2022 ). Other studies incorporate occurrence data for closely related species when modelling species' fundamental niches, as a tool to include a broader sampling of environments the species is likely to find suitable and improve model projections (Jiménez & Soberón, 2022 ).…”