Page 178 - Introduction to Paleobiology and The Fossil Record

MASS EXTINCTIONS AND BIODIVERSITY LOSS 165

families will survive even if most of their con-
tained species disappear. This commonsense Pattern and timing of mass extinctions
observation may be described mathematically Good-quality fossil records indicate a variety
as an example of rarefaction (see also p. 95), of patterns of extinction. Detailed collecting
a useful technique for estimating between of planktonic microfossils based on centime-
scales of observation (Box 7.1). The “inter- ter-by-centimeter sampling up to, and across,
mediate” mass extinctions (Fig. 7.2) are asso- crucial mass extinction boundaries offers the
ciated with losses of 20–30% of families, best evidence of the patterns of mass extinc-
scaling to perhaps 50% of species, while the tions. In detail, some of the patterns reveal a
“minor” mass extinctions experienced perhaps stepped pattern of decline over a time interval
10% family loss and 20–30% species loss. of 0.5–1.5 myr during which 53% of the

Box 7.1 Rarefaction and predicting species numbers from family numbers

Rarefaction is a statistical technique used most commonly by paleontologists to investigate the effect
of sample size on taxon counts. So, a common question might be: “How many specimens should I

collect in this quarry in order to find all the species?” Ecologists have used this concept, sometimes
called the collector curve or accumulation curve, for decades (see p. 535). By plotting cumulative new
species found against the number of specimens collected or observed, you can reconstruct a predictive

pattern (Fig. 7.3a). After collecting one specimen, you will have identified one species. The next
10 specimens probably will not add another 10 new species, perhaps only three or four. The next
100 specimens might add another 10 or 15 species. The more you collect, the more you fi nd, but there
is a law of diminishing returns. At a certain point, as the species versus effort (that is, specimens or
time spent searching) curve approaches an asymptote, it is easy to estimate roughly what the fi nal total
number of species would be if you just kept on collecting doggedly for days and days.
Rarefaction is a procedure to estimate the completeness of a species list if a smaller sample had
been taken. So, if 1000 specimens were collected, it might be of value to know the size of the species
count if only 100 specimens, or 10 specimens had been collected at random. The data in the collec-
tor curve can be culled or sampled randomly by removing 90% or 99% of records, respectively. In
a typical example (Fig. 7.3b), a collection of 750 specimens yielded a species count of 30. If the col-
lection had been half the size, only 20 species would have been identifi ed.
Raup (1979), in a neat example of lateral thinking, applied “reverse rarefaction” to an unknown
question: if we know that 50% of families of marine animals were killed off by the end-Permian
mass extinction, how many species might that represent? Paleontologists are more confi dent of their
raw data on the numbers of families that existed in the past than the number of species because
families are harder to miss (they are bigger, and you only have to find one species to identify the

presence of a family). Raup modeled the distribution of species numbers in families – some families
contain one species, others contain 200. He then culled at random 50% of families from this distri-
bution, and showed that this equates to a loss of as many as 96% of species. McKinney (1995)
criticized Raup’s assumption that the 50% of extinct families would be a random cut from all families
around at the time. McKinney argued, probably correctly, for the “dodo principle”: the extinct
families would include a disproportionate number of those that were vulnerable, especially those
containing small numbers of species. Highly species-rich families would be less vulnerable, and so
the 96% figure might be an overestimate. McKinney (1995) suggested a more likely fi gure of 80%

species loss at the end-Permian event.
Read more about rarefaction in paleobiology in Hammer and Harper (2005) and its use in
ecology in Gotelli and Colwell (2001). Implementations may be found through http://www.
blackwellpublishing.com/paleobiology/.

Continued