From Tracy Churchill, 2000. Impacts of fire and
grazing on invertebrates. In Managing for healthy country in the
VRD eds. Tropical Savannas CRC. Tracy Churchill is from CSIRO
Wildlife and Ecology.
A project undertaken at Mt Sandford and Kidman Springs has shown
that invertebrates such as spiders, ants, beetles and grasshoppers
can respond quite differently to the effects of grazing and fire.
Variations across seasons within these particular invertebrate
groups were also explained by clear responses at the species
Once data analysis is complete, a summary of responses within
each invertebrate group, and an overall comparison of similar or
contrasting responses across the groups, will be available. This
information will assist in the development of fire and grazing
strategies that minimise impacts on invertebrate biodiversity.
Sites were selected at each location at an increasing distance
from water in order to represent a gradient of grazing pressure.
These sites were associated with 100 metre linear transects used by
another CRC project investigating changes in landscape function
(See completed CRC research project Modelling,
monitoring and managing landscape change).
Sampling was undertaken at six sites at Mt Sandford in April
1998 and at five sites at Kidman Springs in April and October 1998.
At each of these sites, ground active fauna was sampled using
pitfall traps and foliage dwelling fauna was sampled using a sweep
net. The pitfall trap grid was placed parallel to the landscape
function transect at each site, and the sweep netting conducted
adjacent to these. In April 1999 another six sites were selected at
Kidman Springs. At this time, all 11 sites were only sampled.
Invertebrates were preserved and identified so that the abundance
of species in each group was available as a total from each given
Golden orb spider
Grazing impacts on favoured habitats
The work has shown that some invertebrate groups respond quite
differently to changes in grazing pressure, while others respond
quite similarly. This is probably because different aspects of the
environment that change as a result of grazing, such as vegetation
cover and soil compaction, have different levels of importance to
each group, or species within them.
At Mt Sandford, ant and spider numbers determined from
ground-based sampling were high in areas of high grazing intensity
near water. By contrast, grasshopper and spider numbers in foliage
were at their lowest close to water, with the recovery of numbers
increasing with distance from water. A particular species of
grasshopper (Austracris guttalosa) was found to increase in
abundance with decreasing grazing pressure.
Contrasting responses to grazing pressures were shown between
two dominant spider families at Mt Sandford. This demonstrates how
even at the family level, spiders can show changes according to
grazing pressure. Where grazing impacts are high, oxyopids (or lynx
spiders), which move freely across vegetation to feed, are more
prevalent. Where grazing pressure is lower however, araneids (or
orb weavers), which require undisturbed structural supports to hold
their webs in place to feed, occur in greater numbers. Close to
water, the abundance of lynx spiders seemed to coincide with the
amount of grass cover. Decreased grass cover led to a parallel
decrease in the number of lynx spiders. This infers that impacts on
cover due to grazing will affect this type of spider.
The project has also shown that the effects of grazing may vary
across seasons for a given invertebrate group because different
species within the group have different seasonal responses. For
example, seasonal differences were apparent in the overall
abundance of spiders and beetles sampled from the foliage at Kidman
Springs. Although these foliage dwellers responded similarly to
grazing (reduced in numbers in areas of high grazing) on all three
surveys (April and October 1998, April 1999) the response was
affected by season due to the presence of different dominant
species. Beetles were particularly more abundant in April 1998 and
1999 than October 1998. These changes were greatest within the
first few kilometres of impact.
Fire and ants
The impact of various fire regimes on the total number of ants
(abundance) and the number of ant species (richness) was examined
at the Kidman Springs fire plots. This work showed that on black
soil sites the abundance and richness of ants was enhanced after
fire compared to ants on unburnt sites. This effect was not seen on
red loam soils. Comparing all burnt sites, the abundance and
richness of ants on both soil types was generally unaffected by the
fire regime - i.e. by the season and frequency of fire.