Potential for invasive mammalian herbivore control to result in measurable carbon gains

Invasive mammalian herbivores (e.g. deer, feral goats and brushtail possums; hereafter ‘herbivores’) are widespread throughout New Zealand and their control is important for conservation. In addition to known biodiversity benefits, it has recently been suggested that herbivore control could lead to measureable carbon gains when aggregated across a large area of conservation land. However, a significant amount of uncertainty exists regarding the potential effects of herbivore control on carbon, and the practicalities of successfully implementing such projects. This paper provides a general basis for managers and ecologists to design scientifically robust herbivore control projects for carbon gain in New Zealand. Although there are few direct data on changes in carbon sequestration rates following herbivore control, the data that are available suggest that effect sizes are likely to be small in magnitude, variable in direction, and to occur primarily though complex indirect mechanisms. The largest positive effects of herbivore control (carbon sequestration rate of 1–2 Mg C ha^–1 year^–1) are likely to occur in localised areas of highly palatable early-successional vegetation and high herbivore densities where control initiates rapid development of woody vegetation. Project location is therefore critical in determining the potential for carbon gain in herbivore control projects. A power analysis reveals that the ability to monitor changes in carbon stock using plot-based methods is limited to effect sizes of > 0.5 Mg C ha^–1 year^–1, as smaller effect sizes would require an impractically large number of plots (i.e. >100), and the financial and carbon costs of implementing the control and quantifying the effects are likely to outweigh any potential carbon gains. Although more research is urgently required to quantify potential gains, and the mechanisms that underlie them, our findings suggest that with careful site selection, implementation, and monitoring, control of invasive mammalian herbivores could sometimes provide carbon gains in certain areas of New Zealand’s indigenous vegetation.