Biology Reference
In-Depth Information
steady states over long geological time scales begins with the statement
that only negative feedback variables can produce them. The overall
dynamics, at least for species within one trophic level, are determined
by the following:
(1) speciation rates per species decline with increasing species richness; and
(2) extinction rates per species grow with increasing species richness.
He further concludes that extinction and speciation rates must somehow
be balanced because if extinction rates were greater than speciation rates,
''we wouldn't be here,'' and if speciation rates consistently exceeded
extinction rates ''the Earth would be an ever-expanding collection of
species. However, the data say otherwise.'' Rosenzweig referred to
Whittaker (
1972
) who had claimed that the steady state is eliminated if
more trophic levels are added. According to Whittaker, ''Species are
niches for other species,'' leading to a continuing increase of diversity.
Contradicting Whittaker, Rosenzweig argues that even if positive feedback
mechanisms exist in one part of the system, the system as a whole does not
necessarily ''get out of control'' because the increasing numbers of species
must be supported by the same resource base. Rosenzweig does not rule
out that equilibrium dynamics may exhibit complicated, nonlinear
behaviors that can be expressed, for instance, in limit-cycle oscillations
or almost-periodic trajectories. He also admits that diversity grew over
hundreds of millions of years through colonization of new habitats, such
as the muddy ocean bottom, perhaps by an increase in ''versatility,'' and
possibly some other factors, for which, however, there is no definitive
evidence.
Concerning Rosenzweig's assertion that ''data say otherwise,'' i.e., that
there has been no significant increase in diversity over geological time,
except for that resulting from colonization of large new habitats and some
increase in ''versatility,'' the evaluations of fossil evidence by Benton
(
1995
,
1998
), Jablonski (
1999
) and Jablonski et al.(
2003
) show a very
marked increase in diversity over evolutionary time. Even if the evaluation
of Courtillot and Gaudemer (
1996
) is accepted, who claimed that a steady
state has been reached several times in geologic history, the fact still
remains that after each ''steady state'' a jump to much higher diversity
occurred. Furthermore, there is no reason whatsoever to assume that just
now in history a point of no further increase has been reached (excepting
the possiblity of man-induced mass extinctions). In other words, fossil
evidence alone would suggest that the Earth is indeed an ever-expanding
collection of species.
