James Cook University: Completed
Greg Calvert
Summary | Grazing and plant
diversity | Results | Experiments |
Future directions | Linkages |
There has been considerable debate over the role of biodiversity
in the environment, however, there has been very little research
into what impact cattle grazing has on plant diversity, and even
less in the semi-arid rangelands of north Queensland. This
uncertainty about the relationship between cattle and diversity
reduces the ability to predict the long-term consequences of
grazing patterns on pasture composition, stability and
sustainability.
This project focused on clarifying the way in which grazing
patterns affect the composition of grazing pastures. The
information gained in this project should provide a tool for the
grazing industry to determine ecologically sustainable levels of
grazing so that this industry can continue for generations to come.
Most of this research was carried out in the Dalrymple Shire in
north Queensland.
The results are not easily predictable. Grazing can reduce the
competitive exclusion of a dominant grass, allowing a much higher
diversity of plant life to exist in that area. Because of this
competitive exclusion, there are levels of grazing that would
enhance species diversity. At the same time, few plants species are
specialised to cope with extreme disturbance levels, so there is a
point at which cattle grazing would be expected to cause a decline
in diversity. What that level of grazing is, what effects can be
observed occurring, and how significant any of those effects might
be is the essence of this project.
My research has revealed a broad range of impacts of cattle
grazing on plant communities. Conclusions about cattle grazing
impacts include:
- The diversity and composition of a pasture is determined
primarily by soil and climate, and secondarily by grazing pressure
and timing.
- Changes to species diversity are dependant on the dominant
grass species. If the dominant is a native perennial tussock grass,
moderate grazing will increase diversity, however, if the dominant
grass is a less palatable exotic species, then grazing will result
in a decrease in diversity.
- Native perennial tussock grasses are often the first to decline
with increasing grazing pressure, and are generally replaced by
exotic grasses and unpalatable species such as forbs and woody
plants.
- Many native legumes increase in abundance, while exotic pasture
legumes usually decline.
- Grazing reinforces domination of a site by Indian couch or
Buffel grass.
- Buffel grass can dominate pastures, causing a decline in
pasture diversity.
- Grazing results in a decline in ground cover, except where
Indian couch dominates, since this species forms a low creeping
stoloniferous habit.
- Highest diversity is reached under intermediate grazing
regimes; conditions being more favourable than either total
herbivore exclosure or unrestricted grazing. Macropod grazing was
responsible for this level of disturbance.
- There is a correlation between grazing and increased rate of
tree dieback during droughts, especially of larger trees, however,
dieback will still occur to some degree independently of grazing
pressure.
- All levels of cattle grazing caused some deleterious effects on
vegetation communities.
Many sites examined were in a transition to a more degraded
condition, but would be easily rehabilitated through the strategic
manipulation of grazing intensity. Other sites were functionally
degraded irreversibly and would require a high input of money and
labour for their rehabilitation.
Sites were chosen by either examining locations of fence-lines
and water points on topographic maps, or through examination of
satellite imagery provided generously by the NT Dept. Lands,
Planning & Environment. Experiments included:
- Examination of several plots set up by the Queensland
Department of Primary Industries 12 years ago, and which
selectively exclude cattle and kangaroos.
- Comparing and contrasting species composition on fence lines
which represent the border between two different grazing
levels.
- Sampling at different points at increasing distances from
watering points. Sources of water create "piospheres", circular
zones of grazing effect around watering points that diminish with
distance.
At each site, plant species and total cover were recorded in
quadrats laid along several transects. The plants were identified
and recorded using the BOTANAL system, with voucher specimens
taken. Many plants needed to be identified later using the James
Cook University herbarium and local experts, or sent to Brisbane
for confirmation. The data was analysed using the CANOCA
multi-variate statistics program to identify trends and patterns in
the plant communities.
It is strongly hoped that the results of this research will be
of significant assistance in allowing the cattle grazing industry
to achieve sustainability. Graziers, extension officers and other
researchers are expected to benefit from the conclusions and
recommendations made in this thesis.
The project was overseen by the Tropical Savannas CRC, the Dept.
of Tropical Plant Sciences at James Cook University and the
Queensland Dept. Primary Industries. Numerous graziers agreed to
allow access to their properties and expressed interest and
enthusiasm in the research. Similarly, Normandy allowed access to
some of its mine sites which have areas grazed and excluded from
grazing.
The project drew upon research experience at QDPI and CSIRO,
and, in turn, research officers from those institutions developed
an interest in this research project.
