Consider the Knight Anole: native to Cuba, introduced and invasive in Florida.
Now consider how island biota originate. Island endemic lizards are thought to originate when rafts of drifting vegetation float from one island to another with lizards or their eggs on board; isolated from their place of origin, the lizards become the ancestors of an endemic species (or radiation).
Cuba is near enough to Peninsular Florida that it is surely possible for such a vegetation mat to cover the distance. If human beings make that crossing on dangerous rafts, surely lizards can make it on vegetational mats. In fact, I suspect that the ancestors of North America’s native Green Anole arrived from the Caribbean in this way.
Suppose that a natural raft of vegetation, drifting from Cuba with Knight Anoles on board, washes ashore in Florida. Those Knight Anoles have colonized naturally, by the normal process. Should the Knight Anole, from that point, no longer be considered an invasive species? Should we (can we?) distinguish the naturally colonizing population from the introduced population? Should we discount the natural colonization and continue to treat them all as invasive?
Historically, what I call the “Frozen in Time” approach to conservation has been prevalent – the idea that an ecosystem should be preserved forever just as it was at some time point identified as its “natural” state. More recently, conservation as a movement has begun to see the fallacy and ultimately the futility of this paradigm. How does the scenario I described fit into all this?
Species that have undergone or that are undergoing natural colonization or range expansion events within recorded history are not treated equally. As with most things, humans are biased.
Nine-banded Armadillos arrived and spread in the U.S. starting around 1850. While translocation likely played a small role, this was largely a natural occurrence. And yet, many people discuss them as if they were an introduced/invasive species. Western Cattle Egrets arrived in North America much more recently (1941), also via natural means, and yet I have never heard anyone refer to them in a negative light.
I think it’s important to remember, for the most part, it’s not isolated species that really matter; it’s the rate at which these things happen. Extinction and climate change both occur naturally, the problem arises when humans greatly increase the rate/speed at which these occur. Colonization / range expansions also occur naturally, but only at a fraction of the rate we see today with human-induced globalization.
“Frozen in Time” is not possible. The most it can be is a goal that will never be reached. The reason for choosing this strategy on small parcels of land is… pragmatism. It’s hard to know which cosmopolitan plants and animals to “fit” into the parcel to best serve downstream organisms. Might as well choose from whatever has historically served the organisms living here that are being threatened by exponential growth in new invasives. Besides, anything relying on invasives has plenty available. I don’t know anyone who is planting native plants that thinks it’s possible to go back to a past era and freeze what was here then. Heck, it’s not even possible on a really small plot of land. Mostly they’re thinking… how can I not make things worse.
In my opinion, natural colonizers can still become invasive if they outcompete native species or disrupt ecosystem functions. I don’t think the “Frozen in Time” approach works because ecosystems are dynamic and constantly changing due to various factors, including natural dispersal and climate change.
If I remember correctly, there are parts of the US which did not have House Finches, but which now have them both from expansion of the natural West Coast population, and from expansion of the introduced East Coast population. I vaguely remember some population genetics studies on the subject. In both cases, human habitat alteration aided the expansion.
There is a number of cases of species that in the same area are both native and introduced, at least plants here where I live. It is conceivable that the native and older populations are characterized by a more complex genetic pattern than those that have been introduced recently. The first should be “protected” from the latter in order to avoid hybridization and substitution.
In Hong Kong, several species of babblers and laughingthrushes are thought to be first of all descended from escaped cagebirds, but it is also possible that they were supplemented later by wild flocks of them settling here from further north.
The general definition I have seen is something like: Invasive = Introduced + harmful.
And in the bird world, at least, people seem to have some kind of handle on this. Species are not necessarily vilified just because they came from somewhere else originally.
For example, Cattle Egrets are thought to have introduced themselves to the Americas, where they don’t seem to be very harmful, but as far as I can tell they are only really treated as invasive in Hawaii (where they were brought on purpose, but also they cause clear harm).
Another example in North America is Eurasian Collared Doves. They were brought to the hemisphere on purpose and then dispersed on their own. So far they don’t seem to cause much harm, either, though there is research being done to figure out what effect they have on certain native dove populations.
On the other hand, people hate House Sparrows and European Starlings, both of which definitely do cause harm (and it’s very easy to observe it). Neither Cattle Egrets nor Collared Doves attract that kind of animosity. People have much stronger feelings about cowbirds (definitely native). So, it’s not as simple as just hating everything that is introduced.
In general, at this point I think it is less about how introduced organisms got there and more about what effect they have. Unless we know that people brought the organism there on purpose, how do we even tell which species were introduced artificially and which ones “introduced themselves” like the Cattle Egrets? How can we make decisions on that basis?
I don’t have any thoughts, but I have some relevant information: a NATIVE species can be considered invasive
The Mountain Pine Beetle is probably the most common example that people would actually associate with the word “invasive,” but White-Tail Deer are also invasive in much of their native range
Another thing I think is worth considering is how an invasive species, naturally occurring or otherwise, affects local species that are threatened due to human activity. If our actions bring a species to imperilment and then another species naturally expands its range and begins outcompeting or otherwise undermining the niche of the the local, threatened species, can we really say that that’s a natural process?
If all the populations in a given area are native the answer is NO.
Sometimess people who are not into this topic erroneously define invasive a native species that maybe is a weed. But in these cases it is just a wrong anthropocentric view. Of course speciees tend to occupy the niches that are made free, especially by human disturbance like cultivated fields or gardens.
Yes, I think if human actions result in a native species expanding its range in a way that imperils other natives — e.g., range expansion through extirpation of a competitor/predator and/or human changes in habitat — then that native can be just as bad as a nonnative. Coyote and Barred Owl in some places are possible examples.
Added: I was trying to remember the term: competitive release. I think there’s evidence for that with coyotes following the demise of wolves in most of North America.
I am not confusing noxious and invasive; I am repeating what I learned while I was studying to become an invasive species biologist. I am not an invasive species biologist and I’m not in school anymore, so maybe the terminology has changed meaning without me knowing, but I was explicitly taught that invasive status is independent of native status. The Mountain Pine Beetle was the example my professor gave to showcase it
There is, at least for plants but not just one. Some years ago they were defined as a subgroup of naturalized species that are able to spread over vast distances. More recently, an alternative approach relied only on the possibility to exert ecological - health - economic impacts.
Both have their pros and their cons. Personally I would be for a sort of synthesis, that means that alien species should be defined casual or naturalized or invasive and, in the two latter cases, impacting or not.
Yes I’d thought of that.
After we (humans) have made changes to the environment, all sorts of species that didn’t used to be able to live there now just sort of arrive naturally.
So we really say naturally?