Invasive plant species:
I pointed out before that invasive species of plants can overgrow native species.  More information is now available.  (Invasive Plant Paradox.  Marnie E. Rout and Ragan M. Callaway SCIENCE volume 432 issue 5928, page 734) 

Not only can an invasive species overwhelm the local mix of native plants.  It can become the only plant in the area.  The total amount of productivity can increase.  Far from depleting the soil with this vigorous growth, it can fix more nitrogen and other nutrients in the soil.  And it can produce more area of leas per square foot of earth covered so that shade is deeper and more of the available sunlight is exploited. 

The paradox is that usually the more different plant species the more the total productivity.  Apparently individual species exploit slightly different niches and maximize productivity, but there is a limit to this.  A single species can substantially exceed the usual limit.  The exotics on average seem to increase productivity by 80%.  The highest productivity of all comes from systems in which a few of the native forms survive.  So apparently there is something in the niche argument. 

I put this down to the fact that the invasive species may have escaped some predator that limits it in its own environment.  Since it is now able to dispense with genetically based safeguards against such a predator, it can turn its DNA to exploiting the new environment.  That is based on the principle that genomes have been optimized, so when some portion of the DNA is unneeded, now genetic material can be exploited.  Indeed they have found that the invaders do undergo rapid evolution. 

The second advantage the invader has is that the founding population presumably had a lower gene pool size in the new environment than in the old.  The exotic has the benefit of lower gene pool size and thus greater fertility, just as a rural village has more offspring per couple than does a city. 

It has happened before in people.  One of the factors, perhaps the most important factor, in the westward expansion of Europeans across the American continent was a much higher reproductive rate than the Indians had.  A small group of settlers would start a village, have many children, and then a group of those children would move out west with the frontier, carrying with them a fraction of the village gene pool and thus maintaining their fertility in a process that lasted for generations.  The Indians, old local tribes that found themselves under pressure and forced to make alliances, faced the opposite effect.  More of this later.

Those who take an interest in these invasive plants suspect that it has something to do with the interaction between the plants and the microorganisms at their roots.  And that can include predators.  The idea of excessive gene pool size is foreign to them. 

Of course the situation is unstable.  Eventually the productivity of the invaded ecosystem will return to that of the original system.  The authors do not say as much, but one suspects they expect that eventually a specialized predator will emerge that will end their glory days.  One looks forward to the arrival of the kudzu beetle.  And that may happen. 

But it may eventually happen that they become victims of their own success.  Their density and their monopoly of landscape will increase their gene pool size to the point where something, perhaps their fertility, will collapse. 

There has recently been expressed the sentiment that invasive species are no big deal.  (End of the Invasion? Emma Marris.  NATURE volume 459 nomber 7245 May 21, 2009 page 327 reviewing INVASION BIOLOGY Mark A. Davis Oxford University Press 2009)  The idea is that a plant does not know whether it is exotic or local and the local plants don’t know either.  So it is exotic.  So what?  Well I say so you know what a local plant is likely to do.  You don’t know what an exotic is likely to do.  Since an ecosystem is an irreplaceable treasure, offering any number of ecological services and holding resources we will never completely understand, you do not run the risk of wrecking it unless you are stark mad, stone stupid or evil beyond ken.  

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