From Alan Andersen, 2000. Invertebrate
biogeography. In Managing for healthy country in the VRD eds.
Tropical Savannas CRC. Alan Andersen is from CSIRO Wildlife and
Ecology.
The distribution of invertebrates within northern Australia is
poorly known. There has been no comprehensive survey of any
invertebrate group in the VRD and the findings presented below are
therefore only preliminary and based on limited information. What
has been determined, however, is that the VRD supports an extremely
rich but largely unstudied array of invertebrate animals.
The invertebrate biodiversity and bio-indicators project has
collected sufficient information for biogeographical analyses of
three major invertebrate groups — ants, grasshoppers and
spiders.
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Recent studies suggest that savannas support 1500 species of ants
from 65 genera
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A recent study across the whole of Australia's savannas suggests
they support more than 1500 species of ants from 65 genera. About
44% of these species belong to species-groups that occur primarily
in northern Australia or elsewhere in the Indomalayan region. These
tropical species are described as having "Torresian affinities".
Another 41% of the species-groups occur primarily in the Australian
arid zone and are known as having "Eyrean affinities".
Australian savannas therefore support an approximately equal
mixture of "tropical" and "arid" ants. About 50 of the Torresian
species can be considered rainforest specialists and within the
savanna they are restricted to patches of monsoon rainforest.
Virtually all of these rainforest specialists also occur in the
humid rainforests of northeastern Queensland. Their occurrence
diminishes as one moves westward from the Top End to the Kimberley,
and, more particularly, southward from the coast with decreasing
rainfall.
Sampling of ants in the VRD has been very limited, and only 110
species from 23 genera have been recorded. Compared with the
savanna zone in general, there is a slightly higher representation
of arid ants (45% of species) and lower representation of tropical
ants (35% of species). This is consistent with the location of the
VRD within the southern, semi-arid part of the savanna zone.
Typically, the number of ant species within the Australian arid
zone is extremely high, with more than 100 species
characteristically occurring within a hectare. This is also true
for the tropical savannas of semi-arid central Queensland. Ant
species richness is, however, far lower than this in the few sites
sampled in the VRD.
A comprehensive biogeographical study of the 161 grasshopper
species known from Kakadu National Park recently found that about
40% of these species are unique to the Top End, and a similar
proportion belong to genera that are unique to the Australian
tropics. Preliminary data suggest that the number of grasshopper
species is substantially higher in the VRD than in the Top End and
that there is a much greater representation of Eyrean or arid
species.
Preliminary data suggest that the overall number of spider
species remains relatively constant as one moves down the rainfall
gradient from the Top End to the VRD. However, the type of spiders
found changes markedly. For example, wolf spiders (family
Lycosidae) occur predominantly in the Top End, whereas the
arid-adapted Zodariidae occurs primarily in drier regions of the
VRD.
The limited information that is available about invertebrates in
the VRD indicates two major biogeographical trends. Firstly, both
tropical (Torresian) and arid (Eyrean) elements of Australian
invertebrate faunas are strongly represented in the VRD. Secondly,
compared with the savanna zone more generally, the VRD has fewer
Torresian and more Eyrean species, which reflects its semi-arid
location.
