How is a species invasive without any natural barriers?

California Fan Palms, Washingtonia filifera are native in one part of California (Imperial Valley), and introduced and invasive in other parts. This leads to the question: why weren’t they native in the parts of the state where they are invasive? Clearly, they flourish in the climate and environmental conditions, or they would not have established themselves in the wild. And there don’t seem to be any barriers to them dispersing naturally, unlike invasives introduced from another continent. Its natural range includes desert oases, so clearly it is capable of dispersing through that kind of terrain. Are the mountains in that part of the state really so high that they form a barrier? The topo map I’m looking at seems to have a pass between the Imperial Valley and the Los Angeles Basin; and if it could reach the coast, it could disperse along the coastal lowlands. The Coyote is one of the seed dispersers; a coyote dispersing away from its natal range – how far would it travel?

What I am trying to understand is, how can a species be native and invasive in different parts of the same state? Why didn’t it naturally disperse to the parts where it is considered invasive?


I’ve read an interesting hypothesis about the California fan palm that states they might have been native to a small region of Baja before being dispersed by the Cahuilla. The justification for this being that it’s really weird the palms were able to reach extremely remote springs like Corn Spring and Kofa Palm Canyon otherwise. I’ll try to dig up a link.

I will say that I don’t see a whole lot of fan palms just growing on their own on the coastal side of the mountains, and almost never in the big groves you see out in the desert canyons. When I do, I’m not sure if it’s not a different species of palms. It’s possible they just don’t grow as well in the wild on the coastal side. If you look on iNat’s map palms of any kind rapidly disappear as you get out of the urban areas. So I guess there’s something that causes them to not grow very well on the coastal sides vs. inside these desert watered canyons. Maybe competition from native trees that can’t handle the desert?

It is weird to me that the palms don’t grow in the Whitewater river canyon, which has lots of water and is right on the edge of the desert. It’s not far from palm groves in the mountains to the south and east.


Washingtonia filifera (and Washingtonia robusta) apparently have low genetic diversity, something recognized as far back as the mid-80s (McClenaghan et al, 1986), and paleo studies and climate modeling (Minnich et al 2011) indicate that they were much more widely spread during past glacial cycles. In interglacial periods (like now) their range appears to have collapsed, with them only surviving in refugia, and expanding slowly out from there.

The low genetic diversity combined with the current distribution of Washingtonia filifera suggests a very recent and rapid dispersal (Hicks, 1989 p33 & Minnich et al 2011) interpret this be indicative of dispersal by humans.

Apparently they are actually pretty fussy about their habitat preferences, and may have difficulty dispersing the necessary distances and getting to the right habitat. Infrequent long distance dispersal events that bring the seeds to exactly the right location and conditions may be necessary under a dispersal system moderated by non-human elements.

It’s also worth noting that time scales of the organism involved can play a big factor in establishment of populations and characteristics of said populations. Washingtonia filifera has a life expectancy of 80-250 years (which is kind of a big range) with the average being around 150, and cultivated plants take roughly 19 years to reach their first flowering (FEIS USDA) with the time for wild trees to reach their first flower unknown.

This means that if anything happens to a plant in the first 20-25 years of its life (rough estimate for a wild plant) then it has no chance of establishing a new population. Even if it does grow and survive events that to us are infrequent ones can still have a major effect in limiting spread of the tree… let’s say, for example, a major weather event takes place on average every 50 years. For a human that’s a once in a lifetime event, for the average tree that’s 3x in its lifetime (and possibly more). If successful long distance dispersals are rare (let’s say once every 100 years), and extreme weather events that kill trees take place on average every 50 years, then dispersal becomes very unlikely, but an established population would still have enough time between events to reproduce and survive.

These sorts of issues are part of what makes up Reid’s Paradox, the kurtosis of distribution curves and the like.

That doesn’t get to the initial question though, which is one more about how we define “invasive” and what role humans (native populations and colonizing populations) play in that question.


Yeah, are they actually “invasive”, or just “introduced”?

1 Like

Species is not invasive (or better, non-native as not all alien species are invasive) because of way it traveled into a given area. Whether it could do this by itself doesn’t matter. Well, it does when scietists try to know whether a given species traveled by itself (so is native) or with help of human (so is alien). Why does it matter how a species reached a given area? Transport by humans on large distances, even when we don’t see barriers on the way, is something different than natural expansion. First if all, natural expansions are more, more slower. Species usually colonizes the area “in between” old and new grounds - and this takes time - or, if some individual like pregnant female covers large distance, these are very small number of individuals. Human can move both large numbers of individuals and on large distance so the species from the beginning has “better start”. And also there can be barriers on the way but not mountains or other which we see. Maybe on some area there are species which would outcompete or predate a species?

Which would mean that technically, it isn’t even native in its “native” range.

I’m not so sure that iNat’s map is a complete reflection of the situation. But I don’t have the pictures to prove it.

1 Like

The larger a territory is, the more likely a species can be both native and alien at the same time in that territory.

Good question. Indeed in many cases the distribution of certain species appears really peculiar.
In this case, is it possible that these animals defecate before getting to a place where germination could occur?

This topic was automatically closed 60 days after the last reply. New replies are no longer allowed.