Invertebrates in the VRD

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.

Invertebrates responses

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 level.

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.

Methods

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 site.

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.

Seasonal effects

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.