Climate change rarely acts alone. As temperatures rise and rainfall patterns shift, native species are squeezed out of habitats they have occupied for millennia, while many invasive species find the warming world increasingly to their liking. Conservation scientists have long treated these two threats separately, mapping climate refugia on one hand and invasion risk on the other. A new study published in the journal Web Ecology argues that this separation is a dangerous oversight, and it offers a practical solution: a single metric that identifies places where a threatened native species can find both a livable climate and freedom from its worst invasive enemies.
The research, led by Finnbar Lee of the Cawthron Institute in Nelson, New Zealand, together with Ian Kusabs, George Perry, and Calum MacNeil, introduces what the team calls the refugia habitat metric, or RHM. The idea is elegantly simple. Standard ecological niche models predict where conditions are suitable for a species based on environmental variables such as temperature, rainfall, and habitat structure. The RHM takes those native-species suitability scores and then down-weights them wherever an invasive species is also predicted to thrive. The result is a map that highlights habitat which is simultaneously good for the native species and bad for the invader — precisely the kind of ground where a vulnerable population has the best chance of holding on through the coming decades.
Mathematically, the metric works by calculating a weighting parameter from the habitat suitability scores of all invasive species considered, each scaled by a coefficient reflecting the strength of its interaction with the native species. The native suitability score is then reduced by a negative exponential function of that combined invasion pressure. The exponential form reflects a deliberate ecological assumption: as invasion pressure mounts, each additional increment of invader suitability has a diminishing further effect, because the harm to the native species eventually saturates. The metric is deliberately flexible. It can incorporate any number of invasive species, works with any niche-modelling algorithm that produces suitability scores between zero and one, and includes tunable parameters that let experts adjust how strongly a particular invader penalizes the native score. When the invader is absent or harmless, the metric simply collapses back to the ordinary habitat suitability value.
To demonstrate the tool, the researchers turned to a case study with deep cultural resonance. The northern kōura (Paranephrops planifrons) is a freshwater crayfish endemic to Aotearoa New Zealand, treasured as a taonga species by Māori and central to customary food-gathering practices known as mahinga kai. Kōura prefer cool, slow-moving water with woody debris and coarse substrates, and their biology is tightly governed by temperature: their optimum mean daily temperature is around 19 degrees Celsius, with a critical upper limit near 32 degrees. The species is already classified as At Risk: Declining, and it faces a formidable adversary in the brown bullhead catfish (Ameiurus nebulosus), a North American invader deliberately introduced to New Zealand in 1877 for sport fishing.
The bullhead catfish is a textbook climate-change winner. It tolerates water temperatures up to 37.5 degrees, spawns across a wide thermal range of 14 to 29 degrees, survives low oxygen and poor water quality, and eats almost anything. Until 1985 it was confined to the lower Waikato River, but it has since spread through the river system, reached Lake Taupō, and by 2016 and 2018 had arrived in the Rotorua lakes. Its impact on kōura can be severe: a study of catfish stomach contents from the rocky shorelines of Lake Taupō found that 64 percent of large catfish contained kōura, marking the catfish as a major predator. Where catfish establish, kōura decline.
Lee and colleagues built maximum entropy (MaxEnt) niche models for both species using thousands of cleaned occurrence records from the New Zealand Freshwater Fish Database, combined with river-reach variables from the Freshwater Environments of New Zealand database and climate variables from national projection datasets. The models were carefully tuned, cross-validated, and assessed with multiple performance statistics; the catfish model achieved an area under the curve of 0.92, while the kōura model reached 0.81. The team also quantified how far their projections extrapolated beyond the training data using a novel shape metric, flagging where predictions were least certain. Future projections used the mean of six global climate models downscaled to New Zealand under the SSP3-7.0 emissions scenario for the period 2080 to 2100.
The projections tell a story of converging pressures. By the end of the century, suitable habitat for bullhead catfish is predicted to remain stable or expand, with significant increases across the northern and western North Island. Kōura habitat, by contrast, is expected to decline across much of its current North Island range and shift southwards and towards more mountainous terrain, while parts of the South Island outside the current range become more suitable. In total, the amount of the country favorable to kōura may remain relatively stable, but its location will move — and the catfish will be following the warmth into exactly the kind of slow, lowland waters the crayfish favors.
Applying the RHM to these suitability surfaces produced maps of potential refugia with immediate management value. Under current conditions, high-priority refugia exist within the kōura’s present range, particularly in the western and southern North Island and across the north and west of the South Island, with additional significant refugia beyond the current range in the eastern South Island. Under the future scenario, refugia contract in the northern North Island but persist in the western North Island and the South Island. Because kōura have limited dispersal ability — one distributional gap in the eastern North Island is thought to date from local extirpation by volcanic eruptions — the authors argue that natural range shifts are unlikely without help. Translocation, a practice with a long history in kōura fisheries management, could bridge that gap, and the RHM offers a principled way to choose destination sites.
The authors are candid about the metric’s limits. It assumes that high invader suitability implies high ecological risk, a relationship that varies with context: catfish predation on kōura, for example, is moderated where native eels are abundant and dominate the catfish, and the same Lake Taupō study found kōura in only 15 percent of large catfish from weedy shorelines compared with 64 percent from rocky ones, showing how habitat structure shapes interaction strength. The metric also inherits the limitations of its underlying niche models, ignores dispersal barriers and fine-scale microrefugia such as spring-fed headwater streams, and treats invasion pressure as static rather than dynamic. The researchers recommend using the RHM alongside mechanistic models, physiological data, and connectivity analyses rather than in isolation, and they call for empirical validation against long-term monitoring data and invasion chronosequences.
Even with those caveats, the tool arrives at a pressing moment. The 2023 invasion of New Zealand’s freshwaters by the gold or Asian clam (Corbicula fluminea), an ecosystem engineer capable of displacing culturally valued native bivalves, has pushed conservation translocations — moving native species out of the path of spreading invaders — into active policy debate. The RHM’s flexibility means it can be applied to terrestrial, freshwater, and marine systems, to plants as readily as to animals, and at scales from single reserves to continental assessments. For conservation managers forced to allocate scarce resources, the promise is concrete: instead of choosing between climate protection and invasion defense, they can now identify the places where a single investment serves both, giving vulnerable natives their best remaining ground on which to make a stand.
Subject of Research: A refugia habitat metric integrating ecological niche models of native and invasive species to identify climate refugia, demonstrated on New Zealand's kōura crayfish and invasive brown bullhead catfish
Article Title: Identifying refugia from the synergistic threats of climate change and invasive species
Article References: Lee, F., Kusabs, I. A. K., Perry, G. L. W., & MacNeil, C. (2025). Identifying refugia from the synergistic threats of climate change and invasive species. Web Ecology, 25(2), 221-239. https://doi.org/10.5194/we-25-221-2025
Image Credits: AI Generated
Keywords: climate change, invasive species, refugia, ecological niche modelling, MaxEnt, kōura, freshwater crayfish, brown bullhead catfish, New Zealand, conservation, biodiversity, translocation
News Source: Sloane Callahan. (October 9, 2026). New Mapping Tool Finds Safe Havens Where Native Species Can Outrun Climate Change and Invaders. Scienmag.
