NSF PR 02-01- January 4, 2002
Evolutionary "Speed Limit" Governs How Quickly
Life Bounces Back After Extinction
Biodiversity recovers more slowly than thought
The 500-million-year history of life on Earth is a
series of booms and busts. But while the busts, or
extinctions, can be either sudden or gradual, the
booms, or diversifications, of new organisms rarely
occur quickly, according to a new study by a National
Science Foundation (NSF)-funded scientist at the University
of California at Berkeley. A paper on the subject
appears in this week's issue of the journal Nature.
"This research has profound implications for our ongoing
impact on Earth's fragile biotic communities and ecosystems,"
says Rich Lane, program director in NSF's division
of earth sciences, which funded the research.
A statistical analysis of the rates of extinction
and origination in the fossil record shows that life
seldom rebounds rapidly from an extinction. The results
imply that the diversification of life obeys "speed
limits" set by evolutionary processes, said study
author James Kirchner of UC-Berkeley. "There seem
to be biological mechanisms that limit diversification
of new organisms and control which ones become successful
enough to persist," he said. "Biodiversity is slow
to recover after an extinction."
This apparent speed limit on the rate at which surviving
organisms evolve and diversify has major implications
for present-day extinctions.
"If we substantially diminish biodiversity on Earth,
we can't expect the biosphere to just bounce back.
It doesn't do that. The process of diversification
is too slow," Kirchner said. "The planet would be
biologically depleted for millions of years, with
consequences extending not only beyond the lives of
our children's children, but beyond the likely lifespan
of the entire human species."
Kirchner has been mining a fossil database created
by the late University of Chicago paleontologist Jack
Sepkoski, who catalogued the genera and families of
fossil marine animals over the past 530 million years,
from the Cambrian to the present. Using a technique
called spectral analysis, Kirchner has looked for
patterns in the rates at which new organisms appear
Last year Kirchner and colleague Anne Weil reported
that the Earth needs, on average, about 10 million
years to recover from global extinctions -- whether
they involve the loss of most life on Earth or wipe
out far fewer species. This was much longer than most
scientists thought. The new results come from asking
a related question: How do rates of extinction and
diversification vary, and how are they related? This
is important because, if rapid diversification is
possible, biodiversity might be able to rebound quickly
from a global extinction. Kirchner's analysis found
that extinction rates and diversification rates are
about equally variable over long spans of geological
time. Over shorter periods, however, diversification
rates vary much less than extinction rates do. That
means that evolution doesn't accelerate quickly in
response to rapid bursts of extinction.
One possible explanation for why diversification takes
so long to rev up after an extinction is that extinction
doesn't just eliminate species or groups of species,
but takes away ecological niches. It eliminates both
organisms and the roles those organisms played in
the ecosystem. Recovery thus becomes more complicated.
"This shows that extinction is not like knocking chess
pieces off a chessboard, with the empty squares ready
for you to plunk down new pieces," Kirchner said.
"Extinction is more like knocking down a house of
cards. You only have places to put new cards as you
rebuild the structure of the house."