Evolution__3rd_Edition

670 PART 5 / Macroevolution
One group of Bryozoa replaced
another group
We have direct evidence of
competition
Diversity
350
300
250
200
150
100
50
0
also arise without competition. For instance, environmental change alone might produce
a simultaneous decline of one group and radiation of the other. However, the test
still has some interest. If the earlier group does go extinct clearly before the rise of the
later one, it is difficult to explain the replacement by competition. And if the takeover
is correlated in time and the two groups are ecological analogous, competition is at
least suggested. Other kinds of evidence of competition can also be brought in, as we
shall see.
23.7.2 Two bryozoan groups are a possible example of a competitive
replacement
Cyclostomata
J. E. Cret. L. Cret. Paleogene Neogene
150 100 50 0
Age (Myr)
One of the most plausible examples of competitive replacement concerns two groups
of Bryozoa. (Perhaps I should say “least implausible” a any conclusion on the
influence of competition in the past will be uncertain. It takes hard work to show that
competition is at work in a modern ecosystem, and the evidence for fossils is much
more limited.) Bryozoans are sessile, aquatic invertebrate animals that live attached
to rocks or other surfaces. The two main taxa of bryozoans are called Cyclostomata
and Cheilostomata. Between approximately 150 and 50 million years ago, the
Cheilostomata steadily replaced the Cyclostomata (Figure 23.12). The pattern in the
figure itself suggests competitive replacement: one taxon (Cheilostomata) rises as
the other (Cyclostomata) falls.
In this case, we also have other evidence of competition and of a competitive
advantage for Cheilostomata. Bryozoans compete with one another by “overgrowth”:
one bryozoan grows over the top of another. The animal that does the growing over
expands in size and can feed over a larger area. The overgrown animal is prevented
from feeding, and is killed. Overgrowth can be seen in nature today, and is also seen in
fossils (Figure 23.13). In the majority of cases, when members of the two main taxa are
involved, a Cheilostome is overgrowing a Cyclostome. Cheilostomes therefore seem
to have a competitive advantage over Cyclostomes and are more aggressive growers.
This competitive superiority is probably part of the explanation for the taxonomic
replacement over time.
Cheilostomata
Figure 23.12
Diversity (measured as number of genera) of two bryozoan taxa
through time. Cheilostomata have replaced Cyclostomata as the
main group. From Sepkoski et al. (2000).
..