Introduction

Pest animals pose a major threat to wildlife and habitats throughout the Northern Territory. Different pests are a concern in different regions, and not all directly impact on threatened species. Cats, rats, mice, horses and wild dogs are found throughout the Northern Territory; foxes and rabbits in the southern half; and water buffalo in the north. Camels are most common in the south, but occasional animals make their way into the Victoria River District. Pigs are most abundant in the north, but outlier populations occur in the centre. The Cane Toad is still spreading, but in the Northern Territory, is unlikely to establish populations outside the Top End. Mosquitofish are spreading through the waterways of the arid centre. Domestic grazing animals can also be a problem for threatened species when they become feral or occur in unsustainable numbers.

Pests that occur in localised areas or low numbers can also be a threat to some species by altering habitat or competing for food and resources. These include banteng, barbary dove, feral pigeon, house sparrow, sambar deer and spotted turtle dove. These species are not currently considered as problematic as the species discussed above. The European honeybee is also a pest species which competes with native pollinators for nectar and with native animals for tree hollows. They may also promote the growth of weeds as they tend to favour the nectar of weed species over native species. To date, there has been no research on the impact of the European honeybee in the Northern Territory, but the species has the potential to adversely affect native wildlife and threatened species which depend on nectar and hollows.

Wild Horses

Photo: © Gabriel Crowley

Impacts

There are several ways in which pests can be a problem for threatened species. Many introduced animals graze, browse, dig up, trample or push over plants. In doing so, they may also expose, disturb or compact soil, cause erosion and foul wetlands and waterways. Feral grazing animals also impose pressure on pastures over and above that of commercial livestock. Controlling them improves both production values and conservation values. Native grazing animals (such as wallabies and kangaroos) can also reach population sizes where they degrade the ground layer. Restricting their access to water points is the most effective means of controlling numbers of wallabies and kangaroos. Native herbivores have not been identified as posing a risk to threatened species.

Another class of pest that threaten native wildlife are predators, the most problematic of these being cats and foxes. They are certainly implicated in the extinction of several small and medium-sized mammals. Wild Dogs may be a problem in some situations, such as on islands. However, Dingoes have been shown to regulate the populations of smaller predators and goats, and so can be a benefit to threatened species. Cane Toads pose a different kind of threat, by poisoning the animals that eat them.

Invasive ants also threaten biodiversity, and their impact has been highlighted on Christmas Island. There are several exotic ants in the Northern Territory that could cause serious environmental damage. However, ants have not been identified as a current threat to any threatened species in the Northern Territory. Many aquatic species are threatened by invasive fish, and Australia has been identified as one of the top hotspot areas for invasive fish species, but only the mosquitofish has so far been identified as a problem for threatened species in the Northern Territory.

Pest animals can harbour diseases that affect threatened species. Diseases carried by Cats and Rats may be one factor contributing to the collapse of native mammal populations across northern Australia in recent years.

Introduced herbivores are also major contributors to the spread of weeds due to their transportation of weed seeds in dung and fur and the soil disturbance they create that facilitates germination of those seeds.

Control

When controlling feral animals, there are a few principles worth consideration:

The aim of a control program should be to produce a significant reduction in the impact of the species, as a result of the control method. With most pest animals, it is necessary to reduce populations by at least 95% in a single year otherwise the population rapidly recovers to pre-existing numbers. Monitoring the signs of impact is just as important as monitoring the actual number of feral animals in the population. A program controlling Water Buffalo in freshwater springs in Arnhem Land combined Indigenous and Scientific knowledge to develop signs of Buffalo impact for monitoring. The results showed how culling can improve the condition of the springs, and provided evidence to show other land managers about how Buffalo were affecting their country.

Hunting animals as a recreational or commercial exercise rarely results in control, but may be a useful addition to a more strategic control program.

The most effective approaches combine control methods, such as poison baits and warren ripping for Rabbits, or target control to when animals are under environmental stress, such as when dry conditions reduce Pig populations and concentrate them around a few watering places. Control needs to be ongoing to be effective.

Some methods for controlling pest animals can be used against several species, and removal of a single pest may simply allow other pests to take its place. Cat predation is known to increase if foxes are controlled in isolation. Pigs may get into paddocks from which Cattle and Water Buffalo are excluded and do just as much damage. If undertaking control for a single species, consider the outcome you are seeking and whether it might be worth expanding your target species.

Aerial shooting can target a range of large herbivores. Different baits may be required for different species, but the effort used to distribute them may make targeting two species economically viable. Fencing is an approach where it is worth considering all problem species, and weighing the costs and long term benefits of different styles of fencing.

Animal welfare should be considered at all times. It may be necessary to kill animals for conservation, but there is no reason to make them suffer in the process. There are both rules and codes of practices that should be followed.

Finally, make sure that any control program does not adversely affect non-target species. This can be as simple as ensuring farm-dogs are chained up when 1080 baits are laid, or making sure that poisons are delivered in baits that are attractive only to the target species.