Native, introduced and invasive
A species listed as introduced in a country is not thereby invasive there, and a species that is invasive is not thereby illegal to keep. Reading any of these off another is the commonest error in aquarium writing about distribution, and it runs in both directions.
Short answer
Native, introduced, established, invasive and illegal are five separate claims. Each is a stronger statement than the last, none of them implies the next, and the hobby collapses all five into one word.
The five claims, in order of strength
The last row is the one that catches people. Legal restrictions and ecological harm are correlated and are not the same thing: the zebra mussel is among the most damaging invaders in Europe and is absent from the EU list of invasive alien species of Union concern, because that list is forward-looking and the mussel arrived two centuries ago.
| Claim | What it asserts | What it does not |
|---|---|---|
| Native | It evolved there or arrived without human help | Anything about abundance |
| Introduced | It got there with human help | That it survives, or does harm |
| Established | It is breeding there without help | That it does harm |
| Invasive | It is causing harm there | That keeping it is restricted |
| Restricted | A named instrument regulates it | That it is invasive, or even present |
What a distribution record on this site actually is
Each statement is one checklist saying one thing about where one species occurs. It is not an occurrence record, and it is not AquariumHQ's opinion. Where a checklist coded how the species got somewhere, that code is reproduced exactly; where it did not, the statement says only that the species was recorded present.
That last category is the largest, and treating it as a native record would be the single easiest way to corrupt this dataset. A statement carrying no establishment means is published as exactly that — recorded as present, establishment means not stated.
- Occurrence data is deliberately not used. A cloud of sighting records includes aquarium escapees, and reading it as a range is how an escapee becomes a range extension.
- Introduced statements outnumber native ones several times over. That is a fact about which species this catalogue covers — the ones that have been moved around the world — rather than a bias in the sources.
- A published native range with no checklist statement behind it is recorded as a gap rather than quietly corrected.
Why one jurisdiction tells you nothing about another
Two structurally different regimes are in use and they invert the meaning of silence. Under a prohibited-list regime — the EU, most US federal law — a species absent from the list is not restricted by that instrument. Under a permitted-list regime — Australia, New Zealand — a species absent from the list cannot be imported at all. The same finding of absence means opposite things.
Scope varies as much as structure. The England and Wales crayfish order is drawn at family level and catches a three-centimetre tropical crayfish. The US snakehead listing names two genera and then lists species 'including but not limited to' them, so a species described after the instrument was written is still inside it. The EU Union list names individual species, so a close relative of a listed fish may be unlisted.
The one rule that holds everywhere
Do not release an aquarium animal, its water, or its plants into any watercourse or drain. This does not depend on the species appearing on any list, it is an offence across most of the jurisdictions this catalogue covers, and it is the mechanism behind a large share of the introductions recorded in the distribution data.
The practical extension is that disposal has to be planned before acquisition. For several species in this catalogue — the large predatory catfish especially — rehoming an adult is close to impossible, and that is a reason to think carefully at the point of sale rather than a problem to solve later.