Science Articles

Reconsidering Non-Native Species

Tue, 17th Jan 2012

Ecologists challenge the categories that identify some species as natives and others as invaders.

Rachel Dentinger

In September 2010, the BBC reported an “Urgent call on EU to stop Kudzu, Pueraria lobatabillion-Euro 'alien invasion'”. If you had read no further than the headline, you might have guessed they were raising the alarm on extraterrestrials. More likely, you imagined a vitriolic anti-immigration campaign, a warning about an “invasion” by our fellow humans.

But you would have been wrong on both counts. Though they are not human immigrants, the “aliens” costing us billions of Euros are just as terrestrial as we are. And for all the talk of invasion, we are the ones who brought them across our borders in the first place. These so-called alien invaders are the many species that we have helped to relocate on our voyages around the globe, the birds and shrubs and crabs and vines that have travelled with us to new lands and thrived.

Unfortunately, these non-native species may succeed at the expense of more familiar local species. Concern over the threats posed by non-native species has grown in recent decades, leading governments to invest resources in stopping the introduction and spread of foreign plants and animals.

But what transforms the introduction of a non-native species into an "invasion"? And on a planet rife with biological change and populated by immigrants on every continent, what distinguishes an invader from a trusted native?

These questions are at the core of invasion ecology. The field takes its name from British ecologist Charles Elton’s 1958 tome, The Ecology of Invasions by Animals and Plants, though it only became a defined sub-discipline of ecology in the mid-1990s. Invasion ecologists ask why some species are more likely to succeed and spread in a new ecosystem. By the same token, they also ask why some ecosystems are more susceptible to invasion. Finally, they apply their knowledge in an effort to prevent the spread of species beyond their “natural” ranges, and to mitigate the effects of invasions.

In other words, invasion ecology begins with the premise that a particular species belongs in a particular place and elsewhere it is an unwelcome alien. Based on all of the press that non-native species receive, this premise may seem self-evident. Recently, however, critics from within ecology have questioned this focus on nativeness, arguing that the geographical origin of an organism is a poor indicator of its impact on a new environment, and an even poorer criterion for a costly campaign of eradication. According to a group of 19 such critics, led by ecologist Mark Davis of Macalaster College in Minnesota, reactions to non-native species are too often based on an unexamined bias against biotic outsiders. In a June 2011 issue of Nature, they argue that we should not feel too much allegiance for the historical locale of a species or the historical makeup of an ecosystem: change is one of the few reliable features of the biological world, and we should be open to rearrangement and reassembly, and the emergence of what they call “novel ecosystems”.

Then what's the problem with non-native species, anyway?

How did non-native species get such a bad rap in the first place? In part, some deserve it. Freed from their normal competitors and predators, some organisms truly seem to "invade", providing vivid examples that convince the public and policy makers of the destructive potential of non-native species. Kudzu (Pueraria lobata), an Asian vine that blankets buildings and trees in the southern United States, provides a graphic illustration of the potential of a plant invasion, as it literally chokes out native plant life. Japanese knotweed (Fallopia japonica) is a similarly dramatic Japanese knotweed, Fallopia japonicaUK example, notorious for its ability to regenerate from tiny fragments of cut plant tissue and penetrate weak or cracked concrete and asphalt.

Non-native species especially threaten the biota of islands, where long isolation from mainland organisms can result in the evolution of organisms unable to cope with the weeds and predators that arrive with human exploration and colonisation. For example, many islands were free of predatory mammals until humans arrived, allowing the evolution of flightless birds. Rats, pigs, and humans themselves have all done terrible damage in places like New Zealand, Hawai'i, and the Galápagos, where these birds and other species lack defense mechanisms, thanks to their evolutionary history. Tough, weedy generalist species can run roughshod over specialised, highly interdependent species, and decrease the biodiversity of island ecosystems.

But more often, non-native species do not become invasive, do not represent a risk of extinction to native species, and do not depress the biodiversity of their newly adopted homes. Quite the opposite, claim Davis and his colleagues; in fact, the arrival of non-native species is actually more likely to increase species diversity.

If destructive invasions are rare and migration is the historical biological norm on our planet, the question remains: What is really wrong with non-native species? One key distinction helps us answer this question. While plants and animals and fungi have migrated across seas and mountain ranges for millennia, non-native species cross these geographic obstacles with the help of humans. That is to say, for non-native species, it's guilt by association—with humans.

We are the problem with non-native species

Take, for example, the Canada goose, Branta canadensis, a bird that arrived in England in the 1660s. More than 300 years have passed since its arrival, but this fowl is still described as non-native and listed as such by the Great Britain Non-Native Species Secretariat. Canada goose, Branta canadensisThough this goose is a long-distance traveler in its native North American range, its journey across the pond was, like that of many game and ornamental species, facilitated by humans. In fact, the first population of Canada geese is thought to have taken root in London’s St. James's Park, thanks to King Charles II. And every time the Canada goose gets press, this story is recounted, revealing that we humans consider ourselves to be the defining factor in the history of the Canada goose in England.

The Canada goose is considered an interloper for more contemporary reasons as well. Simply put, it’s a pest. Since the mid-twentieth century, its populations have burgeoned, and it has become a terrible nuisance. It nips at fingers, poops in parklands, and collides with aircraft. The Canada goose seems a case in point for the invasiveness of non-native species, who expand their ranges and populations catastrophically when liberated from the predators that keep them in check in their native environment.

But there is at least one problem with this account: The Canada goose is mounting just such an explosive "invasion" of ponds and parks within its home range of North America. In fact, the Canada goose's population explosion has little to do with its geographical origin, and almost everything to do with human cultural practices in both England and North America. There is nothing more appealing to a goose than a carefully manicured putting green or public park, where an endless supply of food awaits, relatively hazard-free. After all, large areas of monotonously trimmed vegetation provide a clear view of any approaching threat. In this way, human land use patterns have created an environment that favours the Canada goose, allowing them to act "invasively"; in fact, it hardly matters whether they are "natives" or "aliens".

We humans are the problem with non-native species in two senses, then.

First, we create the categories that determine who belongs and who does not belong. If humans were instrumental in a species’ migration, intentionally or unintentionally, we define its arrival as “unnatural”.

Second, we create conditions that can allow migrants to act like invaders. And when the conditions we create are truly ideal, the results can even transgress our own categories, enabling a goose to start acting like an invader even within its own native range.

Redefining categories, changing conditions

One part of the solution is to challenge our own categorical condemnation of non-natives, as Davis and his colleagues suggest. Evolutionarily, today’s non-native species are much the same as the opportunistic organisms that found their way to England by hitching a ride on the foot or in the feces of a Jurassic dinosaur. They may be The South American monk parakeet (Myiopsitta monachus)stowaways, like the shellfish that cement themselves to the bottom of your boat. Or they may be former human companions, like the many exotic parakeets that feed on London fruit trees. But in the end, they are also just organisms that have taken advantage of another opportunity (that’s us) to colonise a new land.

While the general category of “non-native species” may not be a useful one, there certainly are invasive species with the potential to upset ecosystems in ways that we humans find unacceptable. Intervening in order to stop their spread could be a worthwhile endeavour, based upon the values we, as a society, choose to promote and the resources we wish to protect. For example, concern has been expressed in the UK over potential crop damage from the ring-necked parakeet (Psittacula krameri), a bird imported to England from Africa and Asia. But no action has been taken, and many people are charmed to see a flock of bright green birds fly overhead. By contrast, the South American monk parakeet (Myiopsitta monachus) has recently become the target of a campaign by the UK Department for Environment, Food and Rural Affairs (DEFRA) to remove or eradicate the species from the wild.

What makes the monk parakeet a target for eradication? Even within its home range, the weight of the monk parakeet’s communal nests threatens power lines and other essential elements of public infrastructure. In other words, while it’s interesting to note the monk parakeet’s status as a non-native species, it’s immaterial to the decision to eradicate it. Instead, the decision demonstrates that our society values protecting infrastructure—a value no more or less noble than that of protecting the historical makeup of an ecosystem or the beauty of public parCanada goose, Branta canadensisks or whatever it is we deem worth protecting. The call by ecologists to stop demonising non-native species is a call to make pragmatic decisions about how we manage our environment, based upon explicitly stated societal values, rather than couching decisions in terms of “nativeness”, which has little relevance to what is really at stake. After all, humans have always promoted the "invasion" of species that we favor, whether they are crop plants or, for that matter, Homo sapiens, a species highly skilled at invading. In discarding the concept of nativeness, we can focus on how best to direct the effects that we inevitably have on the ecosystems of which we are a part.

And as for that park-fouling fowl, B. canadensis, its goose may yet be cooked. Recent press suggests that chefs have set their sights on the Canada goose, and are already considering the sauces that best compliment its meaty - and apparently delicious - flesh. This is not the first time an alien invasion has been transformed into a dinner entrée. The dreaded kudzu has been gathering a following in the States for decades. Locavores may wonder if a non-native species can count simultaneously as a locally grown food. So let's let DEFRA sort out the technicalities and just tuck in!


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Invasive species are a big issue...
Hmmm, species invading human habitat? 
Personally, I've decided to never feed pigeons, but that belief is not universally shared.

I believe that Blackberries are considered an invasive species, but I do like their berries 

The problem is that invasive species (with humanity being one), can tip the balance against native species.  And, of course, the non-native species can overgrow when imported to an environment that lacks natural predators. CliffordK, Tue, 17th Jan 2012

It certainly seems to be a human decision as to what constitutes an invasive species and also appears to human activity which introduces such invaders. Though our definition of an invasive species tends to stop where there is no threat to human activity or infrastructure, we have extended this definition to account for the disruption of nature in some cases.

Japanese Knotweed certainly counts among the invasive species due to the damage it can cause to buildings, roads and other manmade structures and the Canada Geese can certainly make an awful mess of our parks and threaten air traffic. But what of the Rhododendron? This shrub, which can interfere with ‘native’ woodland, still appears to be a popular garden plant, despite the fact that it can escape our gardens and infest natural woodland.

The Japanese Knotweed is not alone in invading the cracks and crevices of tarmac and concrete surfaces. Another imported plant which readily takes hold of such opportunities is the Buddleia, or Butterfly Bush. This plant can also do great damage once it establishes itself and is a common site on many not so well maintained car parks. But is this invader more welcome than others? It is, after all, an excellent food source for Butterflies and Bees. As far as nature is concerned, I would say that this is a welcome invader. As far as man is concerned, I think we should consider it likewise.

Since it’s introduction into Britain in the 19th century,  the Grey Squirrel has been a great favourite of visitors to our parks and woodland. I doubt there are many who have not been taken to the park, or now take their children to the park, to feed the squirrels. A bag of peanuts can provide children with cheap, easy and great amusement , and even teach them a little about our fellow animals, when fed to the squirrels. But this Victorian flight of fancy has proved to be a destructive horror. We have all been made aware of the Grey Squirrel’s impact on the native Red Squirrel, but how many are also aware of damage these cute little creatures do to our woodland and the threat they pose to wild birds? Who is to say that the concern now being aired by some over the Ring-necked Parakeet, won’t turn the same way as the Grey Squirrel problem?

I don’t think we can compare the invasion of species by hitching a ride on a dinosaur with the deliberate movement of species by humans. For one thing, when species hitch a ride on another, they will probably only travel over a relatively short distance in most cases. This more a case of the natural ‘spread’ of a species rather than the long distance transfer from one side of the world to the other. When species spread, in such a way, it will, more often than not, be the case that their predators and diseases will follow them, thus keeping the population in check. With prey, predator and disease spreading at a slow, natural rate, the areas and inhabitants of those areas have the chance to adapt to their new residents. And, of course, this spread will be a multi directional affair, not just species A and B arriving in the habitat of species C and D, but C and D perhaps arriving in the habitat of A and B as well as species E and F.

It is the speed, imbalance and, often, nurture of imported species which is the great problem. When Man has moved species across continents in the past, it has been done without consideration of the impact that that species may have on the existent species of the region. Usually, we have artificially migrated a single species, leaving behind the other species upon which it depends for survival and checking its proliferation. It is the lack of the latter which can cause problems, if there is no such species in the new habitat to take the place of its natural checking mechanism.

It can also be the case that these checking mechanisms need to be updated by nature. Just as we have come to realise that the overuse of penicillin can result in it becoming ineffective, so pathogens carried by some species, which at one time may have been a checking mechanism, can now be tolerated by the carrier. But when the tolerated pathogens of one species come into contact with a similar species which has no resistance, the effect can be devastating, as in the case of the Squirrel. The Grey Squirrel is an asymptomatic carrier of Parapox (Squirrel Pox), but the Red Squirrel, which had probably never been exposed to this virus before the introduction of the Grey, has no defence and with such a rapid introduction of the virus, has been unable to develop a tolerance. Thus Parapox is a killer of Red Squirrels.

Invasive species can cause problems for native species in many ways. The Galapagos Conservancy continues to try to eradicate goats from the islands. These fast grazing and proliferating animals are a serious threat to the slow paced life of the native Giant Tortoises and inadvertently imported rats are a threat to the eggs of the Tortoises and many other species, especially ground nesting birds and the unique Marine Iguana. It is also inadvertently imported bacteria and viruses which pose a threat to the inhabitants of these islands. The same can be said of all island ecosystems, where man has been the invader.

Though migrating birds have the ability to transfer species over great distances, I rather think that most of these transfers would have taken place slowly and so long ago as to allow nature to adjust. It may even be that where birds have transferred a species over a great distance, that it was also the means by which the checking mechanism would have been transferred at the same time.

The spread of our prehistoric ancestors probably also resulted in species hitching a ride with Homo Sapiens, and perhaps pathogens carried by them were a factor in the demise of the Neanderthals. But it is modern man who is responsible for the greater part of the transfer of species to regions where they pose a threat, albeit that some of those assisted migrations were probably unintentional.

The problem is, that having been the instrument of ‘unnatural’ migration, we must decide which species are beneficial to a region and which pose a threat. This may not be evident at first, in fact, it could be many years before a threat becomes evident and by then, it may be too late to do anything about it.

Who is to decide what constitutes an invasive species and what to do about it? DEFRA have decided that Japanese Knotweed is an undesirable invasive species and, after some research, has decided to import another species to tackle the problem. A Psyllid, Aphalara itadori, will be our ally in the control of this plant pest. We can only hope that Aphalara itadori will not turn it’s attention to some native plant.

Would the RSPB decide that the domestic cat, originally imported from Egypt, is an undesirable alien? After all, even the Cat’s Protection League agrees that there are too many cats being born in the UK, due to the failure of many pet owners having their pets neutered. It is estimated that cats kill 55 million birds a year in the UK (275 million prey in total, such as mice, rats, voles etc.) and over 100 million birds per year in the USA.

Generally, I think it is too late for us to do much about most of the ‘invasive’ species we have moved around the world. Though Man has proven himself quite adept at making some species extinct, dealing with the problems we have created seems to be an altogether different task. Where there is real danger to infrastructure and real danger to ‘native’  and/or beneficial  species, we should do all we can to eradicate the offending invader. But where the invader has settled into the existent ecosystem and perhaps even proved itself a compliment to that system, we must learn to live with them and, if at all possible, help nature to do likewise.
Don_1, Fri, 20th Jan 2012

Rhododendrons are native here, I think.  Although, that is another risk, that the varieties that are planted in gardens aren't the same as those one finds in the woods.

A lot of the "pests" are unwanted hitchhikers, doing what they do best, spreading into new territories.  Of course, there are also those things like Gorse which was specifically imported years ago, and now is no longer wanted. CliffordK, Fri, 20th Jan 2012

"Rhododendrons are native here"
Only if you live in the Himalayas or thereabouts.
The ones in the woods escaped from the gardens.
Bored chemist, Fri, 20th Jan 2012

So, at least here, the risk would be more that the Hybrid Rhodies would invade the areas where the Pacific Rhodies grow.  Although, in some cases, the native plants can out-compete the cultivated ones.  I'm not seeing a lot of spreading of the cultivated plants so I would wonder if they lack some aspect of hardiness that the native plants have.
Perhaps there are native critters preventing the spread of cultivated Rhodies which Britain lacks. CliffordK, Fri, 20th Jan 2012

I will point out that there is moderately poisonous, foul smelling plant called the Tansy Ragweed (or Ragwort)

Hmmm, so that one is native in the UK, and invasive in the USA. 

Until recently it had no natural predators in the USA and could grow quite wild in fields.  As livestock consumed other food sources, the sometimes would start eating the tansy, which could then cause illness.

It has become reasonably well controlled, but not eradicated by importing the Cinnabar moth which feeds on its flowers, and doesn't seem to affect other species of plants or animals. CliffordK, Fri, 20th Jan 2012

Sorry, I meant that the normal rhododendron that is an invasive pest isn't native unless you are in the Himalayas.

Just to confuse the issue, Tansy is a different plant from the tansy ragwort.
There's a close relative that is an alien in the UK

While we are at it, let's hope they find a way to keep kudzu and Japanese knot-weed under control.

Bored chemist, Sat, 21st Jan 2012

As far as the rhododendrons go, it may be a climate thing.  While people in Oregon like to complain about the rain, it is very much a seasonal issue.  It is, in fact, quite dry in the winter.

The domesticated rhodies do best with water supplements in the summer.  So, while they may grow along streams, I'm doubtful that they would take over the local mountain ranges and displace the native rhodies (without first cross-breading with them).

Further east, they may receive the year-around watering they require to get established and to thrive.

Likewise, the hardiest Jatropha strains might be marginal at best to plant in Oregon, but will grow like weeds in the southeast. CliffordK, Sat, 21st Jan 2012

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