Weeds

The Conservation Threat of Weeds in Savanna Woodlands

"Weeds pose a great threat to the wildlife and economy of northern Australia."

Fig 1 Gamba is a big grass

Weeds in Savanna Woodlands The Northern Territory (NT) is now host to almost 400 weed species, and a large proportion of these are from savanna woodlands. They have come from a variety of sources: some escaped from gardens, some were accidentally imported, but others were deliberately introduced as animal fodder. The list includes only species that cause environmental problems because they invade native vegetation. They are a small proportion of the several thousand exotic plants that have been planted somewhere in the NT. The worst weeds can invade and replace an entire storey of vegetation, unassisted and over a whole landscape. Weeds pose a huge environmental and economic threat to northern Australia.

The Threat

Weeds can threaten wildlife in many ways. The most obvious impact is by replacing the native vegetation, so that native plant species are eliminated from the environment. Although animals eat many weeds, by replacing native plants, weeds simplify the vegetation, and so reduce the options for animals. Variety is very important for animals in our highly seasonal climate. Some weeds alter the structure of the vegetation, for example choking waterways, or becoming so dense that many animals cannot move through them. Many species are poisonous to native animals and to cattle. Lastly, some species can influence the bushfire regime and will indirectly change the vegetation community. The level of threat posed by any weed species depends on the kinds of effects they have, and on how well and quickly they spread. Some of the weeds causing particular problems at the moment are described below.

Fig 2 Probability of occurrence of mission grasses in Litchfield Shire (grey = not surveyed).

Mission grass and Gamba grass

Mission grasses (Pennisetum polystachion and P. pedicellatum) were introduced from Africa as pasture grasses in the 1970's, and were soon recognised as having no agricultural value. They have both spread very rapidly around Darwin and some other towns, to be present in about 75% of rural properties near Darwin. The history of Gamba grass (Andropogon gayanensis) is similar, although it is still planted as a favoured pasture grass, and it has spread somewhat more slowly than the mission grasses. The perennial mission grass (P. polystachion) and gamba grass will invade most environments, even if undisturbed, and form dense stands replacing native species. In comparison to native grasses, the biomass of these grasses is very high, and they dry out later in the dry season. This provides ideal conditions for very hot bushfires that may eventually eliminate trees and shrubs from the infested areas leaving only the exotic grasses. We fear that these species will gradually transform vast areas of savanna into grassland. The fires also increase the risks to the lives and property of affected residents.

Other species

Some of the most serious weeds in savanna woodlands are listed in Table 1. Several of these are usually found in riverside or rainforest vegetation where the climbers choke out the canopy. Many form dense, thorny patches that impede movement. Some are also poisonous. All of them replace native species, and most of them will invade undisturbed areas.

Fig 1 Gamba is a big grass. View larger image

What can we do?

In recognition of the impact that weeds have on the environment and on peoples livelihood, there are strict rules governing the spread of some weed species and the entry of new species. Most of the species discussed here are on the Noxious weeds register in the NT, which means that landholders have a legal responsibility to stop them spreading beyond their property. Selling contaminated hay or moving seed on earth-moving equipment are illegal activities. Please check with the authorities if you need advice. There are hundreds of weedy species waiting to arrive in Australia from all over the world, and the Australian Quarantine Service tries to keep a vigil to keep them out.

There are many methods available to control weeds, and the best method to use varies from species to species. For the worst weeds, that cause economic losses, very costly pesticide programs, and research projects to identify the best methods are carried out. Millions of dollars are spent fighting weeds in the NT every year. Biological control can be particularly effective. This is where a disease of insect that attacks only the weed species is released into the wild. Rigorous research is undertaken to ensure the control species will not also become a problem. There may be some weeds that are so widespread, or hard to kill, that there is no cost effective control. In this case, the best strategy is to contain it where it is. There are some simple principles for weed control:

The best course of action is not to invite trouble by not introducing new species.
Control weeds early, before they have spread to many places or large patches.
Treat the source: where new infestations will come from.
Co-operation among landholders is needed to eliminate a weed from an area.
Obey the law: minimise the risk of spreading seeds.

Fig 3 Perennial mission grass

Fig 4 The leaf of coffee bush

Threats to Wildlife