Concept · Ch 17
Biological invasion (they don't add — they overwrite)
Introduced species don't simply lengthen the list of what lives here. They overwrite it — imported predators, grazers and weeds take the place of native players in country that had no defences ready — and a weed-forest of lantana or camphor laurel is disturbance made visible.
First, meet: Species dependence (a life is a bundle of needs)
There is a comforting way to think about an introduced species, and it is wrong. The comforting version says the newcomer simply gets added to the list — the country’s ecology, plus one more. What actually happens, again and again, is that the newcomer does not join the list; it crosses names off it. Invasion is overwriting, not addition, and the reason sits in an idea we have already met: every native plant and animal is a bundle of needs and defences shaped by the particular players it evolved alongside, and it has no answer ready for a player it never met.
That absent answer is the whole story. The small mammals had no behaviour tuned to a fox or a cat, so they vanished. The goannas and quolls had no tolerance for a cane toad’s poison, so the ones that ate it died and the toad went on unchecked. The native ground plants had no strategy against a lantana thriving on cleared, disturbed soil, so the lantana took the ground outright. In each case the import does the native’s job badly or not at all, while sitting in its place.
Which gives the reader a useful rule. A weed-forest of lantana or camphor laurel where rainforest should be is not bad luck; it is disturbance made visible — a sign that the ground was cleared or broken first, and colonised by an import second. And it is a warning about repair: because the invader overwrote a whole working system, you rarely fix things by pulling the weed alone. You have to restore the conditions the native cover needed, or the overwriting simply happens again.
In depth
It is tempting to picture an introduced species as one more name added to a region's list of inhabitants — the country's ecology plus a newcomer. That is almost never what happens. A successful invader replaces rather than joins, and the reason lies in the concept beneath this one: every native species is a bundle of dependencies evolved against the particular players it grew up with (species-dependence), and it carries no answer to a player it never met.
That missing answer is what makes invasion so one-sided. Native prey have no behaviour tuned to a fox or a cat that hunts by night and kills beyond hunger, so the small and medium mammals are simply removed. Native predators have no tolerance for a cane toad's toxin, so the ones that try to eat it die, and the toad spreads unchecked. Native plants competing for light and space on their own poor soils have no strategy against a lantana or a camphor laurel subsidised by disturbed, enriched, cleared ground — so the weed takes the site outright. In each case the imported player does the native one's job worse, or not at all, while occupying its place: the fisheries nursery, the seed-disperser's role, the ground-layer's light.
Two honest points keep the concept sharp. First, invasion rides on disturbance: a wall of lantana or a stand of camphor laurel where rainforest should stand is not a random misfortune but a reading — it marks ground that was cleared, drained or broken, colonised by an import because the native cover was removed first. The weed is disturbance made visible. Second, the damage is rarely reversible by removing the invader alone; because the invader overwrote a system, undoing it means restoring the conditions — the intact edge, the unenriched soil, the missing predator — and not merely pulling the weed. That is why the work of repair is so patient, and why reading correctly matters: you cannot mend an overwriting you have mistaken for the country's own.
Primary sources & further reading — the doorway
- Leiper, G. et al. (2022). Mangroves to Mountains (3rd ed.). Native Plants Queensland. — Regional flora doorway — telling native cover from the exotic weed-forests that replace it on disturbed ground.