Australia already pays a high price for introducing hundreds of exotic plant species for livestock pasture production. Many of these species have become weeds with major environmental, social and economic impacts. Some introduced grasses increase fire intensity, transforming natural savanna woodlands to exotic-dominated grasslands. Gamba grass, introduced to northern Australia, has increased flammable biomass from six to ten tonnes per hectare, with the cost of fire management now nine times higher.
Many new pasture plants have a high risk of becoming invasive weeds. One of the most reliable predictors of a species’ invasiveness is whether or not it is invasive elsewhere. Globally, over 90% of plants developed for pasture are regarded as weeds, and one third are classed as weeds in the country in which they are sold.
If new varieties of existing weeds are introduced, the weed problem could escalate. New varieties are bred to increase pasture production, but also possess characteristics that increase invasion risk. Some are inoculated with bacteria or fungi that increase reproduction and growth, or can interact with the soil to increase nutrient availability, which could exacerbate weed invasion. Pasture varieties that resist disease, tolerate drought or grow in poor soils can become more successful than their less tolerant relatives, increasing the threat to ecosystems with high conservation value. Introducing additional genetic variation into existing weed populations can enable weeds to invade new habitats, and to take advantage of changing climates.
Although proposals to introduce new species are scrutinized for potential environmental and economic impacts, the likely impacts of new varieties of permitted exotic species are not considered. Without regulations that take into account the environmental impacts of new pasture varieties, existing weeds will likely become more invasive.