Long term climate effects are confounded with the biological control programme against the invasive weed Baccharis halimifolia in Australia

Climate has an important influence on the distribution and abundance of invasive species. Habitat suitability for invasive plants could shift with a changing climate and management practices may need to shift in response. Anecdotal evidence suggests that groundsel bush (Baccharis halimifolia) has declined in abundance over the past 50 years in Australia, co-incident with the introduction of a suite of biological control agents. While biological control may be responsible for this decline, here we investigate an alternative hypothesis-that long-term change in the favourability of the climate may have changed growth conditions for groundsel bush throughout its Australian range. We also predict what may happen to the future distribution of this species, using a bioclimatic modelling technique (CLIMEX). We found a significant reduction in the favourability for growth of B. halimifolia over the past 50 years at 29 sites in Australia. Under a likely IPCC future climate scenario prediction (decrease in rainfall and increase in temperature), the favourability for growth of B. halimifolia will continue to decrease in Queensland and its distribution may move further south into New South Wales and Victoria. We conclude that climate alone may have had a significant effect on the distribution and abundance of B. halimifolia and future priorities for management of B. halimifolia should focus on its southern distribution. Determining the success of the biological control programme in isolation from the observed climate effects will be difficult. Given the likelihood of future climate change worldwide evaluation of biological control programmes in general will need to also account for climate effects.