Invasive Species
In 1988, ballast water discharged from a transatlantic cargo ship near the Great Lakes released a small, unremarkable-looking mollusk into fresh water. It had never occupied this water before. Within a decade, the zebra mussel (Dreissena polymorpha) had clogged water intake pipes across the entire Great Lakes basin. It filtered so much plankton from Lake Erie that the water turned clear for the first time in a century. It cost industries and municipalities hundreds of millions of dollars annually. No native predator, no evolved defense, no historical relationship existed between this species and its new neighbors. This is the central puzzle of invasion biology: why do some transplanted organisms merely persist quietly, while others explode into ecological and economic catastrophes? Biological invasions are now recognized as one of the five major drivers of the modern extinction crisis. The other drivers are habitat destruction, overharvesting, pollution, and climate change. Unlike those other drivers, invasions are unusual in being a problem of biogeography gone wrong. Species find themselves, through human agency, on the wrong side of barriers. Evolution never asked them to cross these barriers. A cactus is harmless in the Sonoran Desert and a scourge in the Australian outback. A starling is unremarkable in England and a billion-strong nuisance in North America. The organism has not changed. Its geographic context has. Understanding invasions matters because they are not rare events. Global trade and travel move species across oceans and continents at a rate utterly without precedent in Earth's history. This effectively re-merges continents that plate tectonics separated tens of millions of years ago. Some estimates place the global economic cost of invasive species above US$400 billion per year. Invasions are implicated in a substantial fraction of recorded extinctions on islands. Yet the majority of introduced species fail to establish. Only a small fraction of those that establish become damaging. Ecologists have tried to explain this pattern since Charles Elton's foundational 1958 book on the subject. This article works through the mechanics of invasion. It covers the stages an organism must pass through, and the traits that make some species successful invaders. It examines the theories, and their supporting experiments, explaining why some ecosystems resist invasion while others do not. It also covers the measurable ecological and economic damage invasions cause. It builds on ideas from The Extinction Crisis. It sets up the restoration and management challenges taken up in Restoration Ecology and Sustainability and the Anthropocene.
- Invasion is a multi-stage process — transport, introduction, establishment, spread — and species can fail at any stage; most do.
- Successful invaders often share traits: rapid reproduction, broad tolerance, high dispersal ability, and release from co-evolved natural enemies.
- The "enemy release hypothesis" and "biotic resistance hypothesis" are the two leading, evidence-backed explanations for why invasions succeed or fail.
- Human-mediated pathways (ballast water, horticulture, pet trade, agriculture) now move species across biogeographic barriers far faster than natural dispersal ever did.
- Impacts include competitive displacement, novel predation, hybridization, disease introduction, and ecosystem-level changes to nutrient cycling and fire regimes.
- Classic case studies — zebra mussels, cane toads, brown tree snakes, Nile perch — each illustrate a distinct mechanism of ecological damage, backed by long-term field data.
An invasive species succeeds not because it is inherently "better," but because it has been decoupled — by human transport — from the predators, pathogens, and competitors that limited it in its native range, while landing in a community not evolutionarily prepared to resist it.
The core idea
An invasive species is a non-native organism introduced (usually by human activity) outside its historical range that establishes a self-sustaining population and causes measurable ecological, economic, or health harm. This definition has three necessary parts, and confusing them causes most public misunderstanding. First, the species must be non-native — introduced beyond the range it could reach through its own natural dispersal and evolutionary history. Second, it must establish — form a reproducing, persistent population, not just survive as scattered individuals. Third, it must cause demonstrable harm to native biodiversity, ecosystem function, agriculture, infrastructure, or human health. A non-native species that establishes but causes no measurable harm is typically called merely "introduced" or "naturalized," not invasive — the tomato, introduced to Europe from the Americas, is naturalized but not invasive. Ecologists distinguish this pathway into four sequential stages, borrowed from the transport-establishment-spread framework formalized by Mark Williamson and others in the 1990s: transport (the organism is moved, deliberately or accidentally, across a barrier it could not cross unaided), introduction/release (it is deposited in a novel environment), establishment (it survives, reproduces, and builds a viable population despite Allee effects and demographic stochasticity at low numbers), and spread (it expands its range, often following a lag phase before rapid exponential increase). Each transition is a filter. A commonly cited rule of thumb — sometimes called the "tens rule," proposed by Williamson and Fitter (1996) — suggests roughly 10% of introduced species establish, and roughly 10% of those established species become invasive pests, though the true fractions vary enormously by taxon and context and should be treated as an order-of-magnitude heuristic, not a law. Two complementary bodies of theory explain why this filtering happens. The enemy release hypothesis holds that invaders escape the specialist predators, parasites, and pathogens that regulated their populations in their native range, gaining a demographic advantage in the new range where such enemies have not co-evolved. The biotic resistance hypothesis (sometimes linked to Elton's diversity–invasibility idea) holds that native communities — particularly diverse, functionally saturated ones — resist invasion because competitors and predators already occupy the relevant niches, leaving little "ecological opportunity" for a newcomer. Neither theory is universally correct; both operate, in different strengths, in different systems, and modern invasion biology largely treats them as interacting rather than competing explanations.
Mechanism
Step 1 — Transport across a barrier. Natural biogeographic barriers (oceans, mountain ranges, deserts) limit dispersal on ecological timescales, which is why islands, freshwater lakes, and isolated continents evolve unique biotas — a theme explored throughout historical biogeography and central to why extinction risk is so elevated on islands (see The Extinction Crisis). Human transport bypasses these barriers entirely. Major modern pathways include: ballast water (ships take up water containing larvae, plankton, and small organisms in one port and discharge it in another — the vector for zebra mussels, the comb jelly Mnemiopsis leidyi in the Black Sea, and countless marine invertebrates); hull fouling (organisms attached to ship hulls); the horticultural and pet trades (deliberate importation of ornamental plants, aquarium fish, and exotic pets, a fraction of which escape or are released — the origin of Burmese pythons in the Florida Everglades and much of the world's invasive flora); agriculture and forestry (crop and timber species, and their hitchhiking pests and pathogens, such as the fungus causing chestnut blight); and accidental stowaways in cargo, packing material, and soil (the pathway for many invasive insects, including the emerald ash borer).
Step 2 — Introduction and the establishment bottleneck. Newly arrived populations are typically small, and small populations face amplified extinction risk from demographic stochasticity (random variation in births and deaths) and Allee effects (reduced per-capita growth at low density, e.g., difficulty finding mates). Species that are introduced repeatedly (high propagule pressure — the number of individuals and the number of introduction events) are far more likely to establish, because repeated introductions counteract stochastic loss and can replenish genetic diversity lost to founder effects. This is one of the most robust, empirically supported generalizations in invasion biology: propagule pressure is consistently among the best predictors of establishment success across taxa, from birds released in New Zealand and Hawaii historically to modern ballast-water organisms.
Step 3 — Establishment and the invader's toolkit. Species that clear the establishment bottleneck disproportionately share a cluster of traits, though no single trait is universal:
- High reproductive rate / r-selected life history — rapid generation turnover and high fecundity (zebra mussels release up to a million eggs per female per season; many invasive plants produce copious, wind- or animal-dispersed seed).
- Broad environmental tolerance (ecological generalism) — ability to survive a wide range of temperature, salinity, soil, or moisture conditions, letting the species occupy disturbed or marginal habitat unavailable to native specialists.
- Phenotypic plasticity — the capacity to alter morphology, physiology, or behavior in response to local conditions without genetic change, buying time for genetic adaptation to catch up.
- Efficient, long-distance dispersal — wind-dispersed seeds, planktonic larval stages, or strong flight, allowing rapid spread once established.
- Release from co-evolved enemies — arriving without the specialist herbivores, parasites, or pathogens that suppressed population growth at home (enemy release).
- Novel-weapons or resource advantages — some invasive plants produce allelopathic chemicals unfamiliar to native competitors (e.g., catechin exuded by spotted knapweed roots), giving them a competitive edge because native plants have not evolved tolerance.
Step 4 — Spread, often after a lag. Many invasions show a lag phase — years to decades of low abundance and slow range expansion — followed by an abrupt transition to exponential spread. Lags can result from the time needed for local adaptation (evolution in the introduced population, sometimes via hybridization or loss of genetic bottlenecks), for population density to clear Allee thresholds, or for a chance mutation or ecological trigger (a wet year, a disturbance event, a new vector) to remove a limiting factor. This lag is ecologically important and dangerous: it means an invasion can appear "under control" for decades before erupting, which is one reason early detection and rapid response are prioritized in modern management over waiting for confirmed damage.
The community's side of the story — biotic resistance and the diversity–invasibility relationship. Charles Elton proposed in 1958 that species-rich communities should resist invasion better than species-poor ones, because most ecological niches are already occupied ("niche saturation") and competition for resources is more intense. Evidence is mixed and scale-dependent: at small experimental plot scales, more diverse plant communities often do resist invaders better, consistent with Elton's idea, but at larger landscape scales, species-rich regions frequently also host more invaders — because the same conditions (favorable climate, high resource availability, high habitat heterogeneity) that support high native diversity also support successful invaders. This apparent contradiction, sometimes called the invasion paradox, is now understood as an artifact of comparing effects that operate at different spatial scales, and it remains an active area of research combining field experiments, observational surveys, and increasingly, manipulative removal studies.
The evidence
Elton's foundational synthesis (1958). Charles Elton's The Ecology of Invasions by Animals and Plants is the founding text of invasion biology. Elton compiled case histories — the spread of the muskrat across Europe after escaping fur farms, the collapse of native flora on oceanic islands following rat and goat introductions, the explosive success of the prickly pear cactus in Australia — and from these built the first general argument that biological invasions were a predictable ecological phenomenon driven by identifiable mechanisms (release from natural enemies, disturbance, propagule pressure), not simply bad luck. Crucially, Elton argued this was a global-scale phenomenon of increasing severity as transport technology accelerated, a claim that has been borne out by every subsequent decade of trade growth.
The cane toad in Australia — a controlled natural experiment in enemy release and rapid evolution. In 1935, 101 cane toads (Rhinella marina), native to Central and South America, were released in Queensland to control cane beetle pests in sugar cane — a control effort that failed outright, since the toads could not reach the beetles living high in the cane stalks. Freed from the many specialist predators and parasites that limit toad populations in their native range, and armed with a virulent bufotoxin unfamiliar to Australian predators, the toads spread across more than 1.2 million square kilometers of Australia, at documented invasion-front speeds that have accelerated from roughly 10 km/year in the 1940s to more than 50 km/year in recent decades at the western front. Long-term field studies by Richard Shine and colleagues (Sydney University, running from the 2000s onward) demonstrated evolution of the invasion front itself: toads at the leading edge have longer legs, disperse farther, and show distinct spatial-sorting dynamics compared to toads in the long-established core range, a direct field demonstration that invasions are not static events but ongoing evolutionary processes — a point corroborating theoretical predictions about "spatial sorting" accelerating range expansion. Native predators such as quolls and some snake species have suffered dramatic, documented population crashes from toad poisoning, while other species (some crows, certain snake populations) have shown measurable evolutionary shifts in toxin tolerance or altered attack behavior within just decades — a rare directly observed case of contemporary natural selection driven by an invader.
The Nile perch in Lake Victoria — experimental natural history at ecosystem scale. Nile perch (Lates niloticus) were deliberately introduced to Lake Victoria in the 1950s–1960s to boost fisheries. Long-term ichthyological surveys (notably by Les Kaufman and others through the 1980s–90s) documented that the perch, a large predatory fish with no evolutionary history with the lake's endemic fauna, drove the collapse of an estimated 200+ endemic cichlid fish species through direct predation — one of the fastest, largest vertebrate extinction events ever directly observed and quantified in real time. This case is frequently cited as the clearest evidence that predatory invasive species can cause mass extinction of endemic radiations within a closed system, and it illustrates why island lakes (biogeographically "islands" of water) are exceptionally vulnerable, echoing the island vulnerability patterns discussed under The Extinction Crisis.
The brown tree snake on Guam — a natural experiment via absence of controls. Brown tree snakes (Boiga irregularis), native to parts of Australia, Papua New Guinea, and Indonesia, arrived on Guam via cargo shipments shortly after World War II. With no native snake predators or competitors on the island, and an abundance of naive, ground- and tree-nesting bird prey with no evolved anti-snake behavior, the snake population grew explosively (estimated peak densities of over 1,000–2,000 snakes per square kilometer in some areas — among the highest snake densities recorded anywhere). Systematic bird surveys by Julie Savidge (1987, published in Ecology) provided a landmark test: by comparing bird abundance across Guam's history and against nearby snake-free islands, and by conducting exclusion experiments, Savidge demonstrated statistically that snake predation, not habitat loss or disease, was the primary cause of the near-total extirpation of Guam's native forest bird fauna — nine of eleven native forest bird species were lost. This remains one of the cleanest field demonstrations that a single introduced predator can drive a full island avifauna to collapse.
The zebra mussel and quagga mussel invasion — a hydrological and economic case study. First detected in Lake St. Clair in 1988, Dreissena polymorpha spread through the Great Lakes via ballast water and then overland via recreational boats carrying larvae in bilge water. Long-term water-quality monitoring documented measurable, quantified ecosystem engineering: individual mussels filter roughly 1 liter of water per day, and dense mussel beds (locally exceeding 10,000–100,000 individuals per square meter) filtered enough phytoplankton from Lake Erie to increase water clarity several-fold and shift the entire food web's base of production toward the benthos — a documented "benthification" of a historically pelagic lake ecosystem. U.S. Fish and Wildlife Service and Army Corps of Engineers economic assessments through the 1990s–2000s estimated costs to power plants, water treatment facilities, and industry in the hundreds of millions to over a billion dollars, providing quantitative economic evidence to accompany the ecological data.
Worked examples
Example 1 — Propagule pressure and establishment probability. Suppose a monitoring program tracks intentional and accidental introductions of an ornamental fish species into freshwater systems across a region over 20 years. Records show:
| Number of introduction events at a site | Sites where the fish established | Total sites in category |
|---|---|---|
| 1 introduction event | 3 | 60 |
| 2–5 introduction events | 11 | 40 |
| 6–20 introduction events | 22 | 30 |
| >20 introduction events | 18 | 20 |
Calculating establishment probability per category: 3/60 = 5%, 11/40 = 27.5%, 22/30 ≈ 73%, 18/20 = 90%. The pattern is unambiguous and monotonic: establishment probability rises sharply with propagule pressure, consistent with the theoretical expectation that repeated introductions overcome Allee effects and demographic stochasticity at low population size. Notice the biologically important implication for management: a policy that reduces introduction frequency (e.g., ballast water treatment reducing viable propagules per discharge event by 90%) will have a disproportionately large effect on establishment probability, because the relationship is nonlinear — cutting propagule number from "6–20" events down to "1" event drops success probability from ~73% to ~5%, a fourteen-fold reduction from what looks like a modest reduction in shipping traffic. This is the quantitative logic behind ballast-water treatment regulations adopted internationally (IMO Ballast Water Management Convention, entered into force 2017).
Example 2 — Reading a lag-phase spread curve and estimating time-to-detection risk. An invasive shrub is discovered covering 4 hectares in a nature reserve. Herbarium and aerial-photo records show it was first collected in the region 35 years earlier, but remained under 1 hectare in extent for the first 25 years (the lag phase), then began expanding. Suppose post-lag spread has been well-fit by exponential growth: after the lag ended (year 25), cover grew from 1 ha to 4 ha over the subsequent 10 years. Using the exponential growth formula, N(t) = N₀ · e^(rt): 4 = 1 · e^(r·10) ln(4) = 10r r = ln(4)/10 ≈ 1.386/10 ≈ 0.139 per year This corresponds to a doubling time of ln(2)/r ≈ 0.693/0.139 ≈ 5 years. If this rate continues unchecked, projecting forward 15 more years: N = 4 · e^(0.139 × 15) ≈ 4 · e^2.08 ≈ 4 × 8.0 ≈ 32 hectares. This kind of calculation — routine in invasive-plant management plans — demonstrates why the lag phase is dangerous: a manager observing "only 1 hectare, low priority" for 25 years would have badly underestimated the threat, and the same math shows why control budgets allocated early, during the lag or the earliest exponential years, achieve vastly more cost-effective suppression than the same budget deployed after the population has multiplied several doubling times. This is the ecological and economic rationale behind "early detection and rapid response" (EDRR) programs used by agencies such as the US National Invasive Species Council.
Common misconceptions & traps
| Misconception | Why it's wrong |
|---|---|
| "Non-native = invasive" | Most introduced species never establish, and most that establish cause no measurable harm; "invasive" requires demonstrated ecological or economic damage, not just foreign origin. |
| "Invasive species are simply superior organisms" | Success reflects ecological context — release from co-evolved enemies, novel niche opportunity, high propagule pressure — not intrinsic biological superiority; the same species is often unremarkable at home. |
| "Invasions are a modern, purely human phenomenon" | Natural range expansions and biogeographic exchanges (e.g., the Great American Biotic Interchange) have occurred throughout Earth's history; what is modern is the rate and scale, driven by global trade bypassing natural barriers almost instantaneously. |
| "Eradication is always the right goal" | For well-established, widespread invaders (e.g., feral cats across a continent), eradication is often biologically or economically impossible; management frequently shifts to containment, biocontrol, or mitigating impact rather than elimination. |
| "If it's not causing visible damage yet, it's not a threat" | The lag phase means populations can remain small and inconspicuous for decades before erupting into exponential spread — absence of current damage is not evidence of future safety. |
| "Biological control agents are risk-free once approved" | Historical biocontrol releases (e.g., the cane toad itself, released as a biocontrol agent) show that introducing one non-native species to control another can itself become an invasion; modern biocontrol requires rigorous host-specificity testing precisely because of such failures. |
The scientist behind it
Origins · 1 minCharles Sutherland Elton and how this was discovered
Charles Elton was a British zoologist and one of the founders of modern population and community ecology, working for most of his career at Oxford University's Bureau of Animal Population, which he founded in 1932 and directed for over three decades. Elton is best known for two foundational contributions: his 1927 book Animal Ecology, which introduced (or crystallized) core concepts including the ecological niche and the food chain/pyramid as organizing frameworks for community ecology; and his 1958 book The Ecology of Invasions by Animals and Plants, which — drawing on decades of natural-history case studies from around the world, from muskrats in Europe to introduced predators on oceanic islands — established biological invasion as a distinct, general, and predictable subfield of ecology rather than a collection of isolated anecdotes. Elton's central intellectual move was to treat invasions as natural experiments revealing the rules governing community assembly and stability: if diverse, "saturated" communities resist invaders while simplified, disturbed ones do not, then invasions are a diagnostic probe of what makes ecosystems stable in the first place. This framing — invasions as a lens onto community ecology broadly, not just a pest-management problem — set the research agenda that invasion biology has followed, with modifications and corrections, for over sixty years. Elton was also an early and forceful voice connecting invasions to what would later be called global environmental change, arguing in 1958 that accelerating transport technology made invasions an urgent, worsening global problem — a prediction that subsequent decades of trade globalization have thoroughly confirmed.
Origins · 1 minhow this was discovered
Where this matters today
Biosecurity and trade policy. International agreements such as the International Maritime Organization's Ballast Water Management Convention (in force since 2017) mandate ballast-water treatment on cargo ships specifically to reduce the propagule pressure of aquatic invaders — a policy directly derived from the quantitative propagule-pressure research described above. National biosecurity agencies (e.g., APHIS in the US, DAFF in Australia) run risk-screening protocols for imported plants, animals, and agricultural products, prioritizing high-risk pathways identified through invasion-pathway analysis.
Agriculture and food security. Invasive pests and pathogens — the fall armyworm moth spreading across Africa and Asia, wheat rust fungal strains, the Asian citrus psyllid vectoring citrus greening disease — cause billions of dollars in crop losses annually and directly threaten food security in developing economies, making early-warning surveillance networks a major agricultural priority.
Public and animal health. Invasive mosquito species (Aedes albopictus, the Asian tiger mosquito) have expanded across the Americas and Europe carrying dengue, chikungunya, and Zika viruses into regions with no prior exposure, making mosquito invasion tracking a core component of vector-borne disease surveillance.
Conservation and island restoration. Because island endemics evolved without mammalian predators, introduced rats, cats, goats, and pigs remain the single largest identifiable driver of historical island bird and reptile extinctions. Eradication programs on islands such as South Georgia (rat eradication, completed 2018) and numerous New Zealand offshore islands have successfully restored native bird populations, providing direct proof-of-concept for invasive-species removal as a conservation tool — work that feeds directly into the practices developed in Restoration Ecology.
Biological control and its risks. Modern biocontrol programs (e.g., using host-specific parasitoid wasps against invasive scale insects) are now subject to rigorous pre-release host-specificity testing precisely because of historical failures like the cane toad, illustrating how invasion biology's own painful case studies have reshaped biosecurity practice.
Climate change interactions. Warming and changing precipitation patterns are shifting the climatic suitability of regions for both native and invasive species, and many invasive species — being broad generalists — are predicted to track and exploit climate change faster than native specialists, linking invasion biology directly to the broader themes of Sustainability and the Anthropocene.
Ask the tutor: How do ecologists decide when an invasive species is a good candidate for eradication versus long-term containment or biocontrol?
Are all non-native species harmful?
No. The overwhelming majority of non-native species either fail to establish at all, or establish and persist ("naturalize") without causing measurable ecological or economic harm — think of most ornamental garden plants or many introduced earthworm species in already-disturbed soils. The label "invasive" is reserved for the smaller subset that demonstrably damages native biodiversity, ecosystem processes, agriculture, infrastructure, or human health. Rules of thumb like the "tens rule" suggest only about 1 in 10 introduced species establishes, and only about 1 in 10 of those becomes a damaging invader — though these fractions vary widely across taxa and should be treated as rough heuristics rather than fixed laws.
Why do islands suffer so disproportionately from invasive species?
Island species evolve in isolation, often losing defenses that are costly but unnecessary in the absence of certain enemies — flightlessness in birds, lack of chemical or behavioral anti-predator responses, and small population sizes with limited genetic variation. When mammalian predators, competitors, or novel pathogens arrive via human transport, island endemics have no evolutionary history of dealing with them, and small isolated populations have nowhere to retreat to and little standing genetic variation to draw on for rapid adaptation. This vulnerability is the same reason islands dominate historical extinction records, a pattern detailed under The Extinction Crisis.
Can an invasive species ever become "naturalized" and stop being a problem?
Sometimes ecological and evolutionary adjustment reduces impact over time — native predators or parasites can evolve tolerance or new attack strategies (as documented in some Australian snake and bird responses to cane toads), and invader populations themselves can decline from resource depletion, disease, or evolved reductions in virulence. However, this is not guaranteed, is highly system-specific, and can take many decades to centuries; it should never be assumed or used to justify inaction, since irreversible extinctions of native species can occur long before any such equilibration.
How is biological control different from just adding another invasive species?
Classical biological control deliberately introduces a natural enemy (often a specialist predator, parasitoid, or pathogen) from the pest's native range to suppress it in the new range. The critical difference from an accidental invasion is supposed to be rigorous pre-release testing for host specificity, ensuring the control agent attacks only the target pest and not native species. Historical failures — like the cane toad, itself introduced as a biocontrol agent that ignored its intended target and became a major invader itself — show that when this screening is inadequate, biocontrol can simply add a second invasive species. Modern regulatory frameworks require extensive host-range testing precisely because of such cautionary cases.
What is "biotic resistance" and does the evidence actually support it?
Biotic resistance is the idea that established native communities — through competition, predation, and full niche occupation — limit the ability of new species to invade, with diverse communities resisting best. Small-scale field and experimental studies (manipulated plant plots, for instance) often support this: more diverse plots resist invader establishment better. But observational studies across larger landscapes often find the opposite pattern, with species-rich regions also hosting more invaders, because favorable environmental conditions boost both native diversity and invasion success simultaneously. This scale-dependent contradiction, the "invasion paradox," means biotic resistance is real but context- and scale-dependent, not a universal rule.
Why does propagule pressure matter so much for policy like ballast water treaties?
Establishment from a new location is inherently stochastic when population sizes are small — chance events (a storm, a bad year, failure to find a mate) can wipe out a founding population that would otherwise have thrived. Repeated introductions (high propagule pressure) buffer against this bad luck, and empirically, establishment probability rises steeply and nonlinearly with the number of individuals and introduction events. This means policies that reduce the number of viable individuals per discharge — such as mandatory ballast water treatment — can produce disproportionately large drops in establishment risk, making it one of the most cost-effective points to intervene in the entire invasion pathway.
Is climate change going to make invasions worse?
Most evidence suggests yes, though effects will be uneven. Many successful invaders are broad ecological generalists with high phenotypic plasticity, traits that also favor rapid range tracking as climate zones shift, potentially giving invasive species an advantage over more specialized native competitors in rapidly changing environments. Climate change can also increase disturbance frequency (fires, floods, storms), and disturbed habitats are consistently more invasible than intact ones. This intersection of invasion biology with climate change is a growing research and policy focus, connecting directly to themes explored in Sustainability and the Anthropocene.