[thedoc (OP)]

In September 2010, the BBC reported an “Urgent call on EU to stop billion-euro 'alien invasion'”.  But for all the talk of "invasion", the "aliens" at issue were none other than the organisms that we humans have taken on our voyages around the globe and relocated.  What makes these species "invaders", rather than migrants?  According to a group of critics from within ecology, it's our own prejudice against biotic outsiders.  On a planet rife with biological change, much of it wrought by ourselves, it's time to reconsider the categories that define some species as "natives" and others as "invaders".

Read the article then tell us what you think...

[CliffordK]

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.

[Don_1]

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.

[CliffordK]

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.

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.

[Bored chemist]

"Rhododendrons are native here"
Only if you live in the Himalayas or thereabouts.
The ones in the woods escaped from the gardens.

[CliffordK]

"Rhododendrons are native here"
Only if you live in the Himalayas or thereabouts.
The ones in the woods escaped from the gardens.

Pacific rhododendron is found along the Pacific Coast from British Columbia to Monterey County in California. It is widely distributed in the Coast and Cascade Mountain Ranges. Pacific rhododendron is less abundant in the Coastal Mountains of Washington and northern Oregon and more common south of the Siuslaw River

There are over 800 species in the genus Rhododendron, some say 1000!  Azaleas are are included in the genus Rhododendron.   Rhododendrons are native to many parts of the world, both tropical and temperate.  The more hardy types are found in China, Japan, and eastern and western North America.   
[...]
     Because many Rhododendron species form spectacular flowers and hybridize easily, many persons, often gardener-hobbists, have and are trying to "improve" rhododendrons.

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]

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

http://en.wikipedia.org/wiki/Jacobaea_vulgaris#Biological_control

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.

[Bored chemist]

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
http://en.wikipedia.org/wiki/Senecio_squalidus

While we are at it, let's hope they find a way to keep kudzu and Japanese knot-weed under control.
http://en.wikipedia.org/wiki/Kudzu#Invasive_species
http://en.wikipedia.org/wiki/Japanese_knotweed#Invasive_species

[CliffordK]

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.