Evolution__3rd_Edition

..
Several models could explain why
extinction rates fit a power law ...
. . . but the facts remain uncertain
CHAPTER 23 / Extinction and Radiation 657
extinct, the herbivores that depend on it will go extinct too, and then the predators that
depend on the herbivore. Thus if one species in an ecologically connected web of
species goes extinct, it will take out a number of other species too. The number taken
out depends on the number of interdependent species.
Over time, the degree of interconnectivity in an ecosystem may change. Sometimes
many species depend on one another. At other times, ecological relations are more diffuse
and few species strongly depend on one another. If one species accidentally goes
extinct at a time of strong, extensive interconnectedness, many species will follow it to
extinction. If one species accidentally goes extinct at a time of weak interconnectedness,
few species will follow it. The same initial cause (the accidental loss of one species) can
trigger a range of extinction rates, depending on the state of the ecosystem.
For this model to produce a fractal pattern of extinction rates, we would need
to assume that the degree of interconnectedness in ecosystems evolves more or less at
random, wandering up and down over time. Then if accidents happen at a steady rate,
the resulting frequency distribution of extinction rates would be fractal in the way
that appears to be observed.
The “ecosystem connectedness” model is not the only one that could explain the
(tentative) observations. Another simple model could propose that almost all extinctions
are caused by asteroid impacts. Asteroids vary in size, and small asteroid impacts
probably cause fewer extinctions than large asteroid impacts. Then if the size distribution
of asteroids fits a power law, the frequency distribution of the resulting extinctions
will also fit a power law. 1
More realistically, various causes of extinction such as asteroids and volcanic eruptions
may interact with the condition of the ecosystem to determine the extinction rate.
A more complex model could be produced. However, the point of the models here is
to show that various factors could explain the observations. What the processes have
in common is that they do not posit a distinct set of causes for mass extinctions as
opposed to extinctions at other times. If extinction rates do fit a power law, we are led to
think of causes for extinction rates that operate in much the same way over time.
However, not all paleobiologists are agreed that extinction rates do fit a power law.
There could be life yet in the search for a distinct set of causes for mass extinctions. This
area of research, like several others in this chapter, will progress along with the quality
of the fossil databases.
23.5 Changes in the quality of the sedimentary record
through time are associated with changes in the
observed extinction rate
So far we have treated changes in extinction rates, and particularly the high extinction
rates at times of mass extinction, as real. Factors such as asteroid impacts and
1 In the next section we look at the possibility that fluctuations in the sedimentary record may explain
changes in the observed extinction rate. If the processes determining sedimentation rates are fractal, this factor
too could produce extinction rates that fit a power law.