Biology Reference
In-Depth Information
although interactions between species and an Allee effect (a reduced
survival chance of species occurring in very small densities) may influence
the shape of the curve as well. Kennedy (
1985
,
1992
) has shown that some
species of eel parasites do indeed interact negatively. Such interactions
may modulate the basic shape of the curvilinear curves showing the
relationship between infra- and component community richness, but
probably in a relatively minor way. The supposition that an Allee effect
and not only interactions may be involved, is suggested by the demon-
stration by Rohde and Hobbs (
1988
) that simulation of an Allee effect has
the same consequence for parasite distributions in host populations as has
mortality. Differential colonization rates and life spans are a more parsi-
monious explanation of curvilinearity than competition and/or Allee
effect, because it does not make assumptions about processes at the
infracommunity level: the probability of different parasite species all
having very high and similar likelihoods of being encountered in com-
munities is infinitesimally small; curvilinearity of graphs representing the
relationship between infra- and component community richnesses is
therefore expected. Norton et al.(
2003
) also concluded that higher
infracommunity richness in eels is possible, and they suggested that
infracommunity richness may be a stochastic reflection of component
community richness.
Dispersal of eels in geologic time may explain the differences in
species richness between Europe and North America on the one hand,
and Australia on the other. Aoyama and Tsukamoto (
1997
),
Tsukamoto and Aoyama (
1998
), Aoyama et al.
2001
), and
Tsukamoto et al.(
2002
) constructed a molecular phylogeny of species
of Anguilla, and, based on this phylogeny, concluded that ancestral
eels originated in the western Pacific (in what is now Indonesia)
(Figure
9.1
). The species A. borneensis in Borneo is the most likely
basal eel in the tree. Anguilla mayhaveoriginatedca.50-60Ma
(during the Cretaceous-Eocene) and, from there, one group dispersed
through the Tethys Sea before its closure in the Oligocene (ca. 20-30
Ma),thelong-livedlarvaeprobablytransportedbytheglobalcircum-
equatorial current. It then split into two species, one colonizing
eastern Africa (A. mossambica), and one colonizing the Atlantic, and
further splitting into the European A. anguilla and the North American
A. rostrata. A. reinhardti, A. dieffenbachi,andA. australis, among others,
dispersed southward over much shorter distance. Snails, the inter-
mediate hosts of trematodes, could not disperse with the eels via the
Tethys Sea, explaining the smaller number of trematodes and in
