The expert gruops of the Norwegian Biodiversity Information Centre chose 1800 AD as a split date for native vs. alien species. You can dowload their assessment at

http://artsdatabanken.no/Pages/201621

http://artsdatabanken.no/Files/13960/Alien_Species_in_Norway_-_with_the_Norwegian_Black_List_2012

Look at p. 11 and search for the term "1800" in the document

(With some avian examples:)

Exotic, introduced, and non-native are used as synonyms to denote a species that occurs in a new biogeographic (or geopolitical) region as a result of human actions. (Example - European Starlings in North America.)

If the animal is of domesticated stock that escaped (or was released from) captivity, then a more accurate term is feral. (I suppose that something like feral llamas in the Andes would be feral but technically not exotic because their ancestral guanaco is native to the same region.) (Domesticated "barnyard" geese living wild on city lakes would be a good example.)

A non-native group of animals is just that - non-native/introduced/exotic/feral - for as long as those individuals are alive. If those individuals begin to breed in the wild and become a self-sustaining population, the species is then naturalized. (There are many parrots living in big cities in the Southern US that live for a while - sometimes many years - but they never become naturalized and self-sustaining.  It is escapees or intentional releases of captive birds that support those occurrences.)

If a naturalized population then expands from its introduced range to the detriment of native species, it is then classified as invasive. (As starlings spread across North America, they outcompeted many native cavity-nesting birds for nesting sites, and populations of Eastern Bluebird, Red-headed Woodpecker, and other native species dropped.)

Some species experience ecological release in their native biogeographic region, vastly increase in population, and expand in their distribution to new areas where they can become just as problematic as an exotic invasive species. Some people will refer to these as native invasives, but most invasive species ecologists will use the term encroaching species. (Brown-headed Cowbird - a brood parasite of other birds - is native to North America.  The problem is not that it's exotic, but that it's superabundant compared to historical abundance.)

I hope this helps!

~tim

Dear Tommy,

Thank you very much for your suggestion to read the work done in Norway. I looked at the book, as you suggested. I was very interested to read that "Allien species.... include species which arrived in the country (Norway) after the year 1800 A.D." For European countries, with long cultural histories and traditions, probably this is "normal" and the information is available for many or most species, but for most countries worldwide this is not the case. Thus, the interpretation of what is a exotic versus naturalized species may have different meanings and not be comparable among countries. Is this correct?

Thanks again!

Gloria

Dear Tim,

Thank you very much for your explanations. You wrote: A non-native group of animals is just that - non-native/introduced/exotic/feral - for as long as those individuals are alive. If those individuals begin to breed in the wild and become a self-sustaining population, the species is then naturalized. That I know, but an introduced species can survive in the wild just with a reduced number of individuals (I can provide examples) and never reach the level of survival or dominance as in its "native" region. Thus, how do name this species? An introduced species that is endangered?

Thank you again,

Gloria

Hi Gloria,

My understanding of naturalized is that it implies an introduced species with a self-sustaining population.  That doesn't mean that the species is invasive. Some naturalized species never expand much beyond the area to which they were introduced. Bird-wise, examples include Eurasian Tree Sparrow in the St. Louis area and Common Skylark on Vancouver Island. These examples sound like what you're describing: a self-sustaining - but perpetually small and non-expanding - population.  I'd call that naturalized.

Dear Tim, 

Thank you very much for your explanations.  I have examples of fishes that were introduced in certain regions of South America at the end of the 1800s, e.g., Cyprinus carpio and Tinca tinca. Although they still survive in certain rivers, the number of individuals is very reduced, not comparable to that of their European ancestors. After the long time that these cyprinids are in certain aquatic systems in South America they are naturalized .... but just, just surviving. The curious thing is that for the public those fishes (and many other examples) are "native" species because of the so many years living in the place. This sometimes is a major problem when a researcher tries to explain the consequences that introduced species may have. 

Ah, so let me see if I'm capturing the nuance of the situation.

It sounds to me like you've got naturalized populations of introduced species. They aren't invasive in that they haven't spread much beyond where they were introduced, and the populations are small and not especially harmful to native species.

Efforts to eliminate those introduced populations are stalling, however, because people do not appreciate that they are non-native species. They've always known their rivers to have Cyprinus carpio, for example, so they consider them to be native.

Is this an accurate assessment?

Introduced populations that remain relatively small in number and concentrated in a specific location for years, are often thought not to pose any threat, because they remain stable in size. Then, sometimes they balloon in size, presumably due to behavioral or genetic change. At this point, after being tolerated, accepted, and sometimes even embraced, it becomes invasive, and then controlling or eliminating the population is no longer possible.

Thank you very much for your replies. But in the special case that I gave as example there is something interesting concerning Cyprinus carpio. At the beginning of its introduction, Cyprinus carpio expanded and kept a good number of individuals reaching similar sizes as those in European rivers. Then, it began a slow invasion of others rivers, occupying an extensive geographic area. Nevertheless, something happened because Cyprinus began to reduce its populations in all rivers and is now reduced to certain rivers, in a small number of individuals. Those individuals are smaller than their European ancestors. Thus, there are not only changes throughout time, but there are also changes in body size and mass and probably there are other changes that have not been studied. The fish, in the example that I am given, is in decline. Someone could consider that the species is actually endangered, but it is not a native species....

Fish size (length, weight) and fish population size (number in population) often varies depending on where and when the population is there. These parameters also depend on such factors as competitors, predators, food availability, parasites, diseases, weather, etc.. It's good you documented your population size changes and population movements. It would be ideal if you also documented all related information about all the other species. Then you would have the perfect dataset, from which you could mine all the answers.

Sorry to keep tossing bird examples into the mix, but there's a somewhat similar example to be provided from game management. Here in the US, we've imported, intentionally introduced, and continually breed and restock Ring-necked Pheasant (native to Asia) as a wildlife resource for hunters of upland game birds. They're often not that successful breeding on their own so without regular restocking their numbers dwindle.

Despite their non-native origin, Ring-necked Pheasant does not appear to become invasive here, even in populations that have become naturalized.  That might be due to the fact that they do well in areas developed for cereal grain agriculture that are generally incompatible with our native grouse and quail. Thus, the pheasants provide a game bird to hunt in landscapes that often don't support other game birds anyway.

A strict application of a "no non-native species" policy applied to Ring-necked Pheasant would face serious opposition in the US.  Pheasants are fun to hunt, delicious to eat, and extremely popular with the general public.  Not only do state wildlife programs support them, but there is at least one well-funded NGO dedicated to keeping pheasants on American landscapes: https://www.pheasantsforever.org/

Most people don't realize that pheasants aren't native and they don't care. When you consider intentional introductions to provide some specific consumptive resource, there are many examples, e.g., Red Deer, Chamois, and Tahr in New Zealand; Rainbow Trout in the eastern US, etc.

Dear Tim,

No problem about bringing birds to this exchange of ideas and information, because different examples can help us to elucidate aspects that affect different species, either plants and/or animals as part of an ecosystem. In the case of the carp, it does well in pools or sections of rivers where the water accumulated, the water speed is low, and the depth increases in comparison to other sections of torrential rivers. Since Andean torrential rivers are not the most friendly ones for fishes such as Cyprinus and Tinca, and are under significants annual changes depending on environmental conditions as melting of snow, rains, etc., the survival of those introduced species is very difficult. The important thing is that after 100 years of their introduction, still they are surviving, even in small populations, only in some places. A comparison (either morphological, genetic, molecular, others)  with their European ancestors has never been done. Probably, it is time to do and to see what the results are after a separation of over 100 years. The carp was introduced as an additional food in the diet of European people living in other continents. However, it never became part of the diet of the native people.