Theos-Talk Archives (August 2002 Message tt00300)

Community ecologists have long sort the mechanism
whereby communities of organisms organise themselves.
Most of the theory has arisen out of empirical
studies, rather than the empirical studies rising out
of theoretical arguments. So we have Preston's (1948)
lognormal distribution of species richness and
abundance and Fisher et al's (1943) logseries. In
between we have other theories such as MacArthur's
(1957) broken stick hypothesis, which has since been
disregarded.

There has been no one staisfactory answer to this
seemingly enigmatic question until Hubbell (2001)
announced to the world his Unified Neutral Theory of
Biodiversity and Biogeography. Hubbell argues that
ecologists have been alluded from a possible answer
because they have been bunked down and "blinded" by
ecological niche theories. Instead, Hubbell has taken
a leap out of Caswell's (1976) paper and approached
the problem by way of neutrality. Basically,
neutrality states that every individual in a community
of organisms have equal probabilities of being born,
dying, dispersing and speciating. NOTE: every
INDIVIDUAL is equal, NOT species. After stating
assumptions and the like (which I am willing to
divulge to anyone who is further interested) Hubbell
devised a model which generated an expected
distribution of species richness and abundance. This
expected distribution could then be compared to an
observed distribution and a statistical analysis
applied, much like one does with categorical data and
a chi square distribution. 

The results are disturbingly perfect. What is
generated is a new statistical distribution called the
zero-sum multinomial.

Why have I brought this theory to the attention of the
people on this list? Hubbell sort to reconcile his
theory with established niche theories. In this
discussion he talks about how evolutionary constraints
lead to community organisation and the possibilities
of "cheater species" defying the rules and going it
alone. Here is what Hubbell has to say on page 326 of
his 2001 monograph:

"Every now and then, however, a species does manage to
break partially free of the constraints of the
life-history manifold currently governing its
community, and this species will achieve a new level
of fitness with a somewhat different set of
constraints. For a time, such a species will be a
superior competitor, sweeping communities free of, or
at least reducing, populations of its competitors,
depending on the magnitude of differences in relative
fitness....However, this state of affairs cannot go on
for long, ecologically or evolutionarily. The
existence of rule breakers establishes a new, higher
fitness criterion for all other species in the
community, which then come under strong selection for
for life-history adjustments that increase their
fitness to match the rule breaker. Presumably, minor
evolutionary rule breaking goes on all the time but is
largely undetected. However, large breaks in the rules
that are also successful are presumably increasingly
infrequent the larger the break in the rule is. This
is analogous to the argument behind David Raup's
(1991) "kill curve" for the distribution of sizes of
extinction events, but in reverse. Thus, it is only
when a massive rule-breaking episode takes place in
the fossil record that we identify it and label it as
"adaptive radiation"."

References:

Caswell, H. (1976). Community structure: A neutral
model analysis. Ecological Monographs, 46: 327-354.

Fisher R. A., Corbett, A. S. and Williams, C. B.
(1943). The relation between the number of species and
the number of individuals in a random sample of an
animal population. JOurnal of Animal Ecology, 12:
42-58.

Hubbell (2001). The Unified Neutral Theory of
Biodiversity and Biogeography. Monographs in
Population Biology (32). Princeton University,
Princeton and Oxford.

MacArthur, R. H. (1957). On the relative abundance of
species. Proceedings of the National Academy of
Sciences, USA, 43: 293.

Preston, F. W. (1948). The commonness and rarity of
species. Ecology, 41: 611-627.

Raup, D. M. (1991). A kill curve for Phanerozoic
marine species. Paleobiology, 4: 1-15.




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