Biology Reference
In-Depth Information
In other words, meiofaunal richness must be enormous, considering the
fact that ubiquitous, widespread species seem to be rare.
There are many cases of extinctions which are directly man-induced,
for example due to hunting or the introduction of predators, but perhaps
more important is habitat destruction. As pointed out by Freeman and
Herron, most cases of extinctions in the list compiled by Smith et al.(
1993
),
occurred on islands due to the introduction of predators or competitors.
However, whereas all but one of bird extinctions in Australia have been
on islands, some endangered mammals now occur only on islands (Hugh
Ford, personal communication). New introductions are still continuing
(e.g., the cane toad, Indian myna, deer in Australia), but they are apparently
declining on land, where extinctions due to habitat destruction are now
more important. On the other hand, introductions of marine organisms
may even be accelerating because of increasing rates of transport by ships
and increasing volumes of ballast water that are released by ships: we have
discussed the example of the green crab on pp. 46-47. Little is known about
how habitat destruction affects invertebrates, because attention has almost
entirely focused on large species, and in particular large plants and verte-
brates. However, concerning parasites, it is obvious that extinction of a
single vertebrate (or invertebrate) species leads to the extinction of a whole
parasite fauna, because many of the parasites are strictly host specific. For
example, there are well over 100 species of protistan and metazoan parasites
of man, many of them strictly specific to man. Of the cestodes infecting
man, Diphyllothothrium latum is also found in a wide range of fish-eating
mammals, but Taenia saginata and T. solium are only found in humans.
Among the numerous nematodes of man, the pinworm Enterobius
vermicularis, and the large roundworm Ascaris lumbricoides only infect
man (A. suis of pigs is probably a different species). Also, it is not
unlikely that even a reduction in population size of hosts may lead to
the disappearance of parasites, because a certain population density of
hosts may be necessary to safeguard the survival of a parasite species.
However, these aspects have been little studied.
All this means that estimates of extinction are with certainty severe
underestimates (because host-specific parasite species have never been
considered in such estimates), and projections into the future are even
more difficult than commonly thought (because the factors leading to
parasite extinction, and to extinction of small free-living invertebrates, are
poorly understood). Nevertheless, knowledge is sufficient to conclude
that we have indeed entered a period of widespread extinctions at least at
the level of large plants and animals, and that extinctions are likely to
