Evolution__3rd_Edition

668 PART 5 / Macroevolution
Fossil leaf evidence suggests
specialist insects had higher
extinction rates
Table 23.2
Specialist insects were more likely to go extinct in the Cretaceous–Tertiary (KT) mass
extinctions. Insects were divided into three categories, according to whether they had dietically
specialist, generalist, or intermediate relations with plants. Specialists probably fed on only one
plant species, generalists on many plant species. The evidence came from the type of damage
found in fossil leaves. From Labandeira et al. (2002).
Number before Number after Percent
Diet type KT extinction KT extinction extinct
Generalist 12 12 0
Intermediate 16 10 37.5
Specialist 20 6 70
In the Cretaceous–Tertiary mass extinction, Labandeira et al. (2002) found that
ecologically specialist species were more vulnerable than ecologically generalist species.
Specialist insects feed on only one plant species whereas generalists feed on several
plant species. Labandeira et al. used evidence from damage in fossil leaves.
Microanatomic study of leaf damage can suggest whether it was caused by insects.
The different kinds of insect damage can be divided into three categories a those
caused by generalist insects (which cause, for instance, damage to leaf margins or make
holes in the leaf ), those caused by specialist insects (which cause, for example, galls,
or act as leaf-miners), and intermediate cases. Table 23.2 summarizes their results.
None of the generalist species went extinct, but most of the specialist ones did.
Specialist phytophagous insects were more vulnerable. The reason is probably that
plant resources were reduced. Suppose that 70% of plant species disappeared. Then any
insect specialist on those 70% of species would also go extinct. However, the generalists
could survive by eating on the 30% of plant species that survived.
The main point of these examples is that species selection can be studied in mass
extinctions, and that the form of species selection may change during mass extinctions
from other times. However, we can also notice that the results provide an independent
source of evidence that mass extinctions were real rather than artifactual events.
Initially we saw that mass extinctions were inferred as peaks in the graph for extinction
rates through time (see Figure 23.2). We then saw that this evidence was inconclusive,
because changes in the sedimentary record could account for the observations (see
Box 23.2). Here we have seen that the form of extinctions was non-random, and nonrandom
in a pattern that fits with a real mass extinction. Sedimentary sampling (Figure
B23.2) alone would not disproportionately take out specialist insects. However,
a real mass extinction would be expected to remove specialists disproportionately.
The argument in itself is not conclusive, but needs to be weighed in the balance. The
evidence for mass extinctions does not solely come from the total extinction rate.
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