Summary

More than 25 000 plant species have been introduced to New Zealand, with 8.5% of those having naturalised so far, and an additional 20 species estimated to naturalise each year. Two-thirds of naturalised species were introduced as garden plants. The number of recognised environmental weeds in New Zealand has almost reached 400 species. Empirical impact data exists for less than 5% of our current environmental weeds, and these are almost all widespread or locally dominant species. The research that does exist has demonstrated a variety of negative impacts, ranging from reduced native species diversity, to altered nutrient regimes. However, management of weeds is more cost effective in the early stages of weed invasion, and there is rarely empirical impact research available at these stages of invasion. While it is not feasible to conduct empirical research on the impacts of all environmental weeds in New Zealand, the development of a comprehensive framework that evaluates known impacts would help inform timely weed management decisions.

Introduction

New Zealand currently has a native vascular flora of 2158 species (de Lange et al. 2006), although human-mediated additions have substantially increased New Zealand’s total flora. Maori transported the first non-native plant species (estimated to be <10 spp.) to New Zealand during their immigration from the Pacific Islands around 1280 ad (Williams & Cameron 2006; Wilmshurst et al. 2008). European colonisation in the nineteenth century resulted in many more plant introductions, currently considered to be approximately 25 049 species (Diez et al. 2009). An estimated 2136 of these introduced species have naturalised (sensu Falk-Petersen et al. 2006; Diez et al. 2009), almost equalling the number of native vascular plant species (Howell 2008). Furthermore, at least 66% of these naturalised plant species were originally introduced to New Zealand as garden plants (Howell 2008). Since 1950, around 20 additional plant species per year have naturalised from the existing pool of introduced species (Howell 2008) and some of these naturalised species then go on to become invasive, expanding their demographic and geographic range (Figure 7.1; Falk-Petersen et al. 2006).