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Overcoming the pest plant threat

Len Gillman

Biodiversity under threat from exotic plants

Biodiversity is defined as the genetic diversity within species, the diversity among species and among ecosystems. Ecosystem diversity means much more than just a count of species; it reflects the importance of the unique nature of species assemblages in different parts of the world. Therefore, protecting biodiversity is not just about protecting natural systems, it is also about retaining differences among countries.

Unfortunately, ecosystems across the globe, especially those associated with human occupation, are becoming increasingly dominated by introduced plants. This process not only threatens the survival of endemic species, it is leading to more and more homogenization of ecosystems. That is, the unique biotic character relating to location is significantly diminished and replaced by assemblages of species that could represent any part of the world. The distinct character of ecosystems (that is, diversity among ecosystems) is therefore being lost by the introduced species we plant.

The plants and animals that make a particular environment unique are very important for providing a sense of place and belonging for people – more so than the structure and shape of the land, although that is important too. The biota in partnership with the land provide a place to anchor our spirit. Our sense of place, our sense of belonging, our sense of balance and contentedness can all be influenced by the relationship we develop with the biodiversity that is unique to the place where we live. It is therefore important for New Zealanders to experience this biota if they are to develop these relationships with New Zealand.

Not all species are equal when it comes to protecting biodiversity – retaining biodiversity is as much about excluding exotic species as it is about protecting indigenous species. We need to maximize the number of indigenous plants and animals and minimise exotic plants and animals in human dominated environments.

Pest Plants – an accelerating degradation

Plant pests not only dilute our sense of place, they exclude native species and prevent indigenous plants from regenerating. Some species are capable of collapsing entire mature canopies. They have had a particularly high impact around stream margins and on the coast. Unfortunately, pest plant invasion gets progressively worse every year, indicating that current legislation and policies are failing.

I have identified six potential reasons for the accelerating degradation:

1. Insufficient funding

Insufficient funding of pest control at local and central government levels is an obvious reason. This to some extent is a result of a lack of awareness among the public of which plants, and even animals, are indigenous and which are invasive. If people can’t see the problem, they are unlikely to approve of spending money on fixing it. Our obligations under international treaties oblige us to protect biodiversity, but local government has to seek approval from the public to fulfil these higher level agreements, and people tend to vote for reducing rates, not increasing them. The planning system as it is currently designed therefore predetermines a lack of funding for pest control.

2. Lack of data

There is a lack of funding to even effectively monitor the spread of pest plants, let alone control them. The lack of data makes it difficult to raise public awareness.

3. Weak legislation

Legislation is not strong enough to enable councils to compel land owners (with some limited exceptions) to control pest plants on their land and the limited efforts by council to control roadside and reserve pests are frustrated by reinvasion from private land.

4. Lack of the precautionary principle

There is a lack of the precautionary principle in the way legislation is structured. Before a plant can be declared a pest it must be proven to be detrimental and before a control plan can be approved a cost-benefit analysis must be undertaken (National Policy Direction for Pest Management, 2015). This creates large political and financial hurdles to councils wanting to control pests. The precautionary principle if applied would mean the onus rests on those wishing to sell plants to demonstrate that they will not naturalise and for landowners to be compelled to remove all potential pest plants. For example, it took decades before Phoenix Palms were listed as a pest in the Auckland Regional Pest Management Strategy, and landowners are still not compelled to remove them. The regional approach to pest management also enables pest plants that are banned from sale in one region to be freely available in neighbouring regions.

5. Taking a “species approach” to pest management

The regional pest management plans developed separately in each region tend to take a “species approach” rather than an “ecosystem management approach”. An ecosystem approach is desirable because there is little point in spending money on controlling one species if the ecosystem is overwhelmed by another. For example, if Climbing Asparagus is sprayed with herbicide once a year in a reserve, but Tradescantia continues to expand each month without control there is little benefit to the reserve. If Kahali Ginger is sprayed on a roadside, but a myriad of other pest species is left uncontrolled, the money spent is wasted. A “species approach” also makes it difficult for councils (and DOC) to act quickly when a new species to the area is found in the early stages of establishment. The legislation needs to be amended to promote an ecosystem approach to pest plant management.

6. Prioritisation of tourism over environmental protection

A large proportion of Department of Conservation funding provides for tourists, while little attention is paid to preventing pest plant invasions. The recent budget allocated significant extra spending to conservation. However, $68 Million (20%) of this extra spending over the next four years is related to providing for tourists while none of it is targeted at controlling pest plants. The problem on the ground can be illustrated with the following example: two or three Scotch Thistle plants above the tree line in a National Park constitutes an advance front of an invasion that has the potential to devastate the alpine flora. Removing these individuals before they set seed is then crucial to preventing substantial degradation. Typically, a DOC ranger will clip the grasses away from the track edge and walk straight past the thistles, oblivious to the harm they will cause. Other pest plants invade along the tracks, spreading out from where they have been allowed to establish around huts. DOC field staff often have no training in ecology and may lack even basic knowledge about indigenous species versus exotic pests, and their priority is catering for tourists, not protecting the environment. Reprioritisation of DOC towards protecting the environment is important.

In conclusion, there is a need for more public funding of pest plant control and the pest management legislation needs to be reviewed to incorporate the precautionary principle, an ecosystem approach with consistency across regions and greater powers to compel landowners to control pest plants on their land.

Categories: Environment
Len Gillman
About the author

Len Gillman

Len Gillman is a professor of biogeography and Head of Science at AUT. His research interests include forest ecology, global patterns in primary productivity, genetic evolution, polar ecology and conservation. He has published in international journals including; PNAS, Global Ecology and Biogeography, Proceedings of the Royal Society, Ecology, Evolution, and Frontiers in Plant Science. He has used drones for ecological and conservation research in Antarctica, Namibia, and Western Australia, and has field experience in environments from the Arctic to Antarctic and from tropical rainforests to tropical deserts. Prior to joining AUT, Len Gillman worked as a campaign manager for a New Zealand conservation organisation.