Biology Reference
In-Depth Information
in 1862. Over the next 60 years it took over the southern half of
Australia, and it is now found in most regions of the country; more
than 84% of New South Wales alone is affected. Reflecting the rabbit's
origin in the Mediterranean region, populations have the greatest
potential for rapid increase in Mediterranean-type temperate zones.
In the more arid areas of Australia, population crashes may occur in
periods of drought. Rabbits are territorial, and form linear dominance
hierarchies. They have native predators such as eagles, and introduced
predators such as foxes and cats; up to 70% of the young may be taken by
predators. Rabbits are an important pest competing with kangaroos,
sheep, and cattle for food resources, although competition between
these herbivores is minimal at high biomasses (see pp. 104-108).
Consequently, they have an enormous economic impact, causing an
annual loss of around A$ 600 million (with which we are not concerned
here). But they also cause much ecological damage through habitat
degradation and competition (for details see Dickman et al. 1993 ).
Rabbits feed on more than 60 plant species, preferring Gramineae
and dicotyledons. They prevent regeneration of cassuarine species
(Allocasuarina verticillata). Coman ( 1996 ) recorded 17 new plant species
in an area from which rabbits had been excluded, that did not occur
in the rabbit-infested control area. Food competition has led to the
reduction of native animals, e.g., the Bilby (Macrotis lagotis)(Coman
1996 ), and the Long-billed Corella (Cacatua tenuirostris)(Emisonet al.
1994 ). But importantly, competition for a limiting resource is not
the only factor responsible for species impoverishment. Rabbits may
also disrupt the predator-prey cycle and thus cause damage to the native
fauna. In the absence of rabbits, predator numbers decline when native
prey numbers decline, and it takes some time to rebuild the predator
population after the prey population is re-established. In their presence,
predators simply switch to the easily available food source, i.e. rabbit, thus
maintaining high densities, and maintaining the pressure on native
animals as well (for further details and additional references see Nichols
1996 , on whom much of this account is based).
All this means is that introduction of a species can lead, at least locally
but also on a larger scale, to reduction in diversity and thus to non-
equilibrium. A very long (evolutionary) time may be necessary for
re-establishment of the original diversity, if it can be re-established at
all, i.e., nonequilibrium conditions are very long-lasting indeed.
Our general conclusion for this section is that abiotic and biotic
disturbances are important factors leading to nonequilibrium.
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