New Zealand is one of the world's weediest countries, with over half its flora (>2000 plant species) comprised of naturalised, non-native plants, many of which have become invasive weeds in both managed and natural ecosystems. More than 240 weed species are currently managed on conservation lands nationally and many of these are expanding their range.
To make matters worse, about seven new weed species are added to this list each year. Consequently, although direct weed control costs have risen from $1.56 million in 1994/95 to $9.5 million in 2004/05 for DOC, and are >$11 million per year for regional councils, this effort fails to control all weeds currently on the >700 000 ha of conservation land that is under sustained weed control. Over the coming decades New Zealand will need to know how to manage a dramatically increasing number of weed species at increasing costs unless we can prioritise control to minimise weed impacts. Research on the impacts of invasive animal pests such as deer, possums and rats is covered in other research programmes.
Some weed species have dramatic impacts on ecosystem processes and biodiversity when they invade. For example, most studies internationally have documented increases in soil nutrient contents (particularly of nitrogen, N), plant community composition or plant biomass; most studies have not monitored the impacts of invaders on diversity, and very few have considered the effects on below-ground communities or ecosystem processes. Despite this, the importance of feedbacks involving invaders and soil communities has become increasingly recognised in the ecological literature over the last decade. One of our major research threads is to understand the role of above- and below-ground linkages in mediating the impact of invasive species in ecosystems.
Invasions are managed largely through the removal of invasive organisms (i.e. through spraying or mowing). It is unrealistic to expect that all impacts of an invasive species can be reversed through control; some effects will quickly reverse with removal (e.g. shading by large plants) whereas others may take years or decades (e.g. increased soil fertility under N-fixing plants). Understanding which effects can be reversed quickly or slowly will allow better predictions of what benefits are likely to arise from weed management and over what timescales. In summary, this research will be used to better manage invasive species to mitigate or reverse their impacts on biodiversity and indigenous ecosystems.
What research is needed?
Our research on the ecosystem impacts of weeds is a 12-year research programme that will use a combination of field experiments, modelling and characteristics of current weeds in New Zealand to:
- Determine how biodiversity and ecosystem properties change following invasion or control of existing weeds
- Use characteristics of existing weeds to predict the impacts of new weeds
- Incorporate this knowledge into spatial models of weed spread and impact, and
- Use this information to prioritise weed management by land managers
Impacts of invaders on both above- and below-ground communities, diversity, and ecosystem processes will be carried out at six field sites nationally. This can only be accomplished through the close integration of plant ecology, ecosystem ecology and soil biology skills. Because field experiments are intensive and long-term, both plant traits and spatial models are being used to 'scale up' these impacts across species, and to forecast future impacts. This approach is needed to shed light on what impacts invasive species have in ecosystems currently, what impacts they may have in the future, and how management activities can mitigate these impacts.
How will this information be used?
End-users want to know how to prioritise weed control or management to maximise benefits to indigenous biodiversity and ecosystem processes. They also want to know what the optimal split in effort and resources should be between detecting new weeds vsersus controlling existing weeds to minimise their future impacts. Our research has been developed with input from end-users, and ultimately the results will be used to better manage invasive organisms. Several specific benefits from an ecosystem-level understanding of weed impacts include:
- Management agencies incorporate reversibility criteria in planning weed control
- Effectiveness of weed control increased through early targeting of existing or future high-impact weeds
- Biodiversity gains from weed control maximised by matching management to areas or species having the greatest impacts on diversity
- Certainty of the biodiversity or ecosystem benefits arising from weed control enables managers to reduce risks in planning decisions and maxmise benefits for biodiversity conservation.