Ecosystem ghosts

Olivia Judson’s blog post in the New York times today talks about the fragility and robustness of ecosystems.  She talks about how we really don’t know what happens to an ecosystem when a single species goes extinct.  Can that species be restored or has another species taken over its niche?  Also, when an invasive species arrives, it can thrive or not thrive.  Mathematical models have found that the perturbation induced by such an event can cause another species to go extinct even if the invasive species also goes extinct.  These transient invaders are called ghosts. She also talks about experimental ecosystems with single-cell organisms.  In these artificial settings, the equilibrium states are generally composed of a small number of organisms and ghosts can cause established species to disappear.

Now, this brings me to something that has always puzzled me, which is why are natural ecosystems so varied and relatively robust when they are at the same time so susceptible to invasive species?  Examples being rabbits and cane toads ravaging Australia, zebra mussels clogging up the North American Great Lakes, and Kudzu taking over the American southwest.  Clearly, these examples show that there were niches in the ecosystems that were not being exploited.  My guess is that if we wait long enough, the these invaded ecosystems will eventually adjust and become varied again.  After all, these invasive species are held in check in their native habitats. Thus, ecosystems may tend to evolve to a state with wide variety but also one that always leaves them vulnerable to attack.  Can we mathematically prove this? The really interesting thing is that this fragile stability seems to require a large number of species since experiments with small numbers tend to evolve to small communities.  Why is that? What is the difference between a large system and a small system?  Is there a bifurcation or phase transition as you increase the size of the ecosystem?  Is there an analogy to economics or the brain?  This is why I’m so interested in large but finite interacting systems.  There seems to be something there that I just don’t understand.

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