Environmental Engineering Reference
In-Depth Information
ler fires and sometimes set their own. Fires, once seen as aberrations, are the new normal. Once seen as
destroyers of ecosystems, they are now seen as renewers and enablers, as part of what forests do.
Other leading ecologists have been busy debunking the old orthodoxies. Steve Hubbell of the Smith-
sonian Institution and UCLA studied the rain forests of Panama for two decades. He began with pre-
sumptions about the forests as climax ecosystems but eventually concluded that they have no settled
arrangement of species. He saw little sign that the current crop of species had coevolved together. Rath-
er, there was a constant turnover of new species. Even Darwinian forces weren't much in evidence. The
forest seemed to renew itself almost at random.
The orthodoxy is that when a large tree falls in the forest and a clearing appears, there is a grand
battle between species to fill the gap, with the best-adapted winning. Not so, says Hubbell. He never saw
it happen. “The gaps were being occupied largely at random.” Rather than the best adapted driving out
the weaker species, those that moved in were usually just those that happen to be closest at the time.
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Hubbell has since argued his “unified neutral theory” at topic length. He calls for rethinking not just how
forest ecosystems work but how biodiversity happens. The conventional view is that the rich variety of
species we see around us is a result of species evolving to occupy myriad different ecological niches.
In the hothouse of evolution, specialism requires lots of different species. In fact, he argues, it happens
as a result of species being generally so competitively identical, or “neutral,” that many can prosper.
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Other researchers have argued that the old idea that “survival of the fittest” means only one species will
survive in any niche does not stand up to scrutiny. In fact species often benefit by sharing niches.
The law of Hubbell's jungle is not so much “survival of the fittest” as “live and let live.” Most
change is random. The result is not optimum—certainly not some preordained perfection—but a work-
able mishmash of species, constantly reorganized by a throw of the dice. Some arrangements do not
work, but many do. There is room for all in this liberal, nonjudg-mental jungle. Hubbell no doubt ex-
aggerates. His coauthor on a number of articles, James Rosindell of Imperial College London, told me:
“The world isn't literally neutral in that way, but we can find out interesting things from its study.” And
it does show that, in real ecosystems, there is rather less ruthless elimination of the weakest than is often
assumed, and that ecosystems are generally open—including to aliens.
That may explain new findings about the Amazon rain forest. Hans ter Steege of the Naturalis Biod-
iversity Center in the Netherlands reported in 2013 that there are more than eleven thousand tree species
in the forest. Could they all have a natural niche or vital role in the rain forest system, as conventional
ecology suggests? Or did they just hang in there, according to the Hubbell neutral theory? Some tree
species did a great deal better than others, of course. More than half of the forest was made up of just
227 “hyperdominant” tree species. One species, a type of native palm called
Euterpe precatoria
, had an
estimated five billion members in the forest. Yet, said ter Steege, such hyperdominants “do not have any
particular ecological features that stand out.” If Hubbell is right, maybe they just got lucky.
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Whatever precisely is going on, this pattern doesn't look like a highly tuned rain forest ecosystem
created by coevolution or even a very rigorous process of ecological fitting. If the pressure to outperform
each other is as strong as commonly assumed, you would expect the most successful species to have ob-
viously superior attributes and for the rest to fade away. It looks like Hubbell may be right that hanging
in there is just fine. No pressure at all.
The emerging picture, whether from the neutral theory or ecological fitting or the growing body of evid-
ence of what actually happens inside ecosystems, is that they are more adaptive and random than is as-
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