Biology Reference
In-Depth Information
parasite species for all vertebrates is probably a vast underestimate. Also, a
great majority (millions) of animal species are likely to be plant-parasitic
insects which share many ecological characteristics with parasites of animals
(e.g., Rohde
1991
). It is likely that they live in loose assemblages and not in
densely packed interactive communities, and that many niches are empty
(Lawton and Strong
1981
,Lawton
1982
). For example, Lowman (
1985
)
concluded that canopy insects never appeared to compete, their survival
rather ''was a matter of chance in terms of locating a branch with leaves of
palatable age, texture, and chemistry.'' The packing rule may therefore not
apply to them, that is, to the vast majority of animal species, although further
studies (especially of herbivorous insects) are needed to confirm this. It
wouldalsobeusefultotestwhetherspatialscalinglawsapplytopara-
sites living in supposedly interactive communities. (e.g., Haukisalmi and
Henttonen
1993a
,
b
). It may well be that interactions occur or are even
common in nonsaturated communities to which spatial scaling laws do not
apply, indicating that occasional interactive effects are without evolutionary
consequence.
Convincing evidence for species nonsaturation comes from introduced
species, although such cases are abnormal in the sense that they are man-
induced and fairly recent events, and that a return to an equilibrium state
in the near future cannot be excluded. The best examples again are
parasites because their hosts represent small habitats that have been
examined in large numbers from many areas. Torchin et al .(
2003
) and
which show that, as a rule, hosts introduced into new regions lose most
if not all of their parasites, and, as a consequence, may become dangerous
pests (for an exception see Poulin et al.
2003
). Attempts are being made to
introduce parasites to control these hosts. Torchin et al.(
2003
) compared
the parasite faunas of 26 host species of molluscs, crustaceans, fishes, birds,
mammals, amphibians, and reptiles in their native and introduced ranges.
16 parasite species were recorded, on average, from native host popula-
tions. Only three of these, again on average, moved with their hosts into
the new habitats, and a further four were newly acquired. A very well-
examined marine example is that of the European green crab, Carcinus
maenas, which has been introduced into several regions around the world,
including the west coast of North America, and Australia. In its native
habitats, it is infested by a variety of parasites; in particular by parasitic
rhizocephalan barnacles (Sacculina carcini ), which control the population
size of the crab very effectively by castrating infected crabs. Rhizocephalans
were not introduced into any of the newly colonized habitats. The reason is
