Developing a definition of invasive species
When you think of the term "invasive species", "pest" or "weed", what animals or plants comes to mind? Many Australians think of species that have caused harm to the Australian environment such as cane toads, rabbits or prickly pear. Some people may think of horses or European bees, although this would be strongly opposed by others because of the great benefit that these species have provided. So what defines an "invasive" species? Do we mean the same thing when we call something a "pest" and an "invader".
There are two points of contention when asking these questions. The first point of contention is over the precise definition of terms such as "pest", "weed" and "invader". The second point of contention is over whether a species that brings benefits to some people can be called a pest or weed.
Defining the terms
Defining the terms "invader", "pest", "weed" and "naturalised species" may appear relatively straightforward, but it is a matter of ongoing disagreement amongst invasion ecologists. In this unit, we will adopt the terminology recommended in Reading 1.5: Richardson et al. (2000) (see Table 1 on page 98). When reading through this paper, you should clarify your understanding of what defines a "naturalised species", an "invader", and a "pest" or "weed". Whilst reading this paper, also note that Richardson et al (2000) consider that clearly defined and widely accepted terminology is required for objective management priorities.
So, in this unit we will apply the term "invader" or "invasive alien species" to those species that are naturalised and produce reproductive offspring at considerable distance from the source of invasion, irrespective of whether the species cause negative impacts. We will use the terms "pest" and "weeds" to describe those invaders that have detectable negative environmental, social or economic impacts. Many examples of these are given in Topic 2: The Impacts of Invaders.
A pest/weed, or a matter of opinion?
So, surely if we can agree on a definition of the terms "invasive species", "pests" and "weeds" we will be able to agree on what species fit within the classification. Unfortunately not! Pests and weeds are often different things to different people depending on their livelihood, value system, culture and beliefs, and identifying a species as a pest or weed can be a political issue.
For example, olive trees are invading many natural areas in southern Australia and are considered a major weed to those concerned with conserving these ecosystems, but they are considered a valuable resource to those interested in growing olives commercially. (See reading 1.6: Virtue et al. 2004)
In Queensland, Leucaena (Leucaena leucocephala) and Buffel grass (Cenchrus ciliaris L.) are considered by some livestock graziers to be highly productive and valuable forage plants. However these species are also considered to be serious weeds by people on non-pastoral land. (See reading 1.7: Dyer, 2008)
Can invasive alien species be legally defined or is it just a matter of opinion? Why is it so important to fit these species into classifications? Again, the reason for having clear classifications for species is to ensure objective management priorities. For example, in northern Australia, there have been several cases where pastoral interests have clashed with conservationists over the declaration of species of introduced pasture grasses as environmental weeds. Until these species are declared as environmental weeds, the species can continue to be promoted for use in uninvaded areas and their priority for control is low. This situation changes completely once the species is declared a "weed", and its potential negative impacts are acknowledged. In southern Australia, the olive tree has now been listed as an environmental weed and there are legal restrictions on plantings. A risk assessment must be completed before new olive plantations can be established. The management of this commercially important species has changed as a result of its classification as an environmental weed. (See reading 1.6)
In Topic 2 (The Impacts of Invaders) you will see many examples of the negative impacts caused by invasive species. Many of these species also provide economic and social benefit, so defining them as invasive species will always be an issue for debate and conflict.