TS-CRC Student project - Effects of cattle grazing on plant species diversity in the semi-arid rangelands of north Queensland

James Cook University: Completed

Greg Calvert

Summary | Grazing and plant diversity | Results | Experiments | Future directions | Linkages |

Summary

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.

Grazing and plant diversity

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.

Results

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.

Experiments

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:

  1. Examination of several plots set up by the Queensland Department of Primary Industries 12 years ago, and which selectively exclude cattle and kangaroos.
  2. Comparing and contrasting species composition on fence lines which represent the border between two different grazing levels.
  3. 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.

Future directions

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.

Linkages

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.

Contacts

Dr Greg Calvert
Consultant
ERISS
Tel: 08 8979 9789

Mobile: 0416 360 155

3 Melaleuca Close
JABIRU , NT 0886