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Until when an invasive species is considered an "invasive species" and when it is starting to be considered a "native" species, i.e. a species that is an integral part of the local fauna/flora?

I ask this because the expansion of species to new territories also occurs in nature and without human involvement. And in some cases, a newly-arrived species drive out the local species and causes its extinction. However, today it is regarded as native.

Many of today's fauna are the result of different species coming from different areas. Maybe in the next century, species that were considered invasive today will be considered native or an integral part of the fauna/flora.

The criterion may be time passed since that species was introduced or the time when the invasive species reached an equilibrium with the ecosystem.

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  • $\begingroup$ On a human timescale or a geologic timescale, on a human timescale never. $\endgroup$
    – John
    Jun 26, 2023 at 17:39
  • $\begingroup$ A time in years. It may be a century (probably a human timescale) or a millennium (human or geologic timescale?) or at least 50,000 years or even million of years (the last is surely a geologic timescale). $\endgroup$ Jun 26, 2023 at 17:43
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    $\begingroup$ It seems to me that this is simply a problem of some people allowing themselves to extend the label "invasive species" to include anything not transported (directly or indirectly) by humans, as the term ordinarily indicates. $\endgroup$
    – Bryan Krause
    Jun 26, 2023 at 21:18
  • $\begingroup$ 2 issues. 1) Inconsistencies/disagreement about how to define "invasive" and 2) well, time, and the [narrow] subjectivity in which we typically observe/analyze it. I don't think this question can easily be answered, and - quite frankly - I think it more of a philosophical question more than a biological one. $\endgroup$ Jun 28, 2023 at 3:01

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Invasive species is defined by the USDA as:

  1. non-native (or alien) to the ecosystem under consideration and,
  2. whose introduction causes or is likely to cause economic or environmental harm or harm to human health.

Under this definition, a species that was labeled 'invasive' will lose the label when it stops causing harm. Although I can think of no examples, in theory an ecosystem could stabilize into a new state after an invasive species causes significant disruptions, which would cause the invasive species to lose the label.

Personally, I really like this definition because it differentiates between non-native species that integrate into an established ecosystem (e.g. coyotes in Eastern US) and non-native species that disrupt an established ecosystem (e.g. rodents in New Zealand). While the former is a natural and healthy biological process, the study of which dates back to island biogeography in the 1800's, the latter is a damaging process almost always caused by humans.

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There are two uses of invasive,

  1. species introduced to new territory by humans, theses are considered invasive essentially forever. So to answer your question, they don't.

  2. species that enter a new territory via a natural pathway like a landbridge or storm drift and destabilizes the local ecosystem. there is no specific cut off for these as it is more about the context than anything else. Bison are considered native today but paleontologically they were originally invasive. This is more about how they got to north America. Often once the biosphere restabilizes with the new species they often stop being referred to invasive but this is not a hard and fast rule and could take millions of year or a lot less. On a long enough timeline all linages could be considered invasive.

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Essentially nothing since the start of the anthropocene: any invasive organism that owes its transportation and dissemination to human intervention remains forever precluded from acquiring the status of endemism.

There are over 40 million flights per year and 50,000 cargo ships with 11 billion tonnes of goods affecting places like the Galapagos, New Zealand, Madagascar, Hawaii, not only places that were joined during ice ages, so biologists use chronology to label endemism.

From microscopic fungi that decimate forests, tics and mosquitos, the black death (yersinia pestis), covid19, to hippos, pythons and trees, ecosystem breaches are going on at a massive rate of 1000's per century, causing extinctions and ecosystem degradation which is of particular interest to biologists.

Biologists specify times when a species migrates to a new ecosystem, so invasive vs native is a simplified concept of recency. Bears in America 3 million years ago, Dingos in australia 5000 years ago. *quote: The brown bear appeared in eastern Beringia only 50–70,000 years ago.

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In any field, some terms may be defined within the context of a particular research study. The same goes for conservation biology, where definitions may reflect the specific goals of a conservation plan.

NPS defines "native species" as, "All organisms that have occurred, now occur, or may occur as a result of natural processes." (Source). So if something is introduced by humans or because of human activity, it's never considered native by NPS. Also, Non-native species are not necessarily considered invasive (most are probably not).

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