Abstract
Invasive herbivores are often managed to limit their negative impact on plant populations, but herbivore density – plant damage relationships are notoriously spatially and temporally variable. Site and species characteristics (both plant and herbivore) must be considered when assessing the potential for herbivore damage, making it difficult to set thresholds for efficient management. Using the invasive brushtail possum Trichosurus vulpecula in New Zealand as a case study, we parameterized a generic model to predict annual probability of browse‐induced mortality of five tree species at 12 sites. We compared predicted and observed tree mortality for each species + site combination to establish herbivore abundance – tree mortality thresholds for each site on a single and combined tree species basis. Model results indicated it is likely that possum browse was the primary cause of all tree mortality at nine of the 12 species‐site combinations, allowing us to estimate site‐specific thresholds below which possum population numbers should be reduced and maintained to keep tree mortality under a predetermined level, for example 0.5% per year. The browse model can be used to set site‐ and species‐specific management action thresholds, and can be adapted easily for other plant or herbivore species. Results for multiple plant or herbivore species at a single site can be combined to create conservative, site‐wide management strategies, and used to: determine which sites will be affected most by changes in herbivore abundance; quantify thresholds for herbivore management; and justify expenditure on herbivore control.
Highlights
Impacts of invasive herbivores on native biodiversity and agriculture are a significant component of ongoing global change (Mack et al 2000)
Using equations 1–6 and the mean and 95% confidence interval parameter values for Is, ys, and hs, we calculated the mean and 95% CI for the annual probability of tree mortality at each site by Foliage Cover Index (FCI) category, and compared these to Gormley et al.’s (2012) empirical model for observed tree mortality
Uncertainty in tree species parameters is likely to mean that the results presented here are a conservative estimate
Summary
Impacts of invasive herbivores on native biodiversity and agriculture are a significant component of ongoing global change (Mack et al 2000). Herbivore management is often required to keep populations below thresholds where their damage is limited to acceptable levels. This can produce idiosyncratic outcomes: relationships between herbivore density and plant damage or mortality (see Hone 2007; Norbury et al 2015) are notoriously variable, both spatially and temporally (Bee et al 2009; Duncan et al 2011; Asner and Levick 2012; Silva et al 2013). The issue is compounded because herbivore control in mixed forests may not produce a proportionate reduction in browse impacts on a single species, or equivalent reduction among different species (Kamler et al 2010). Managers are faced with the difficult question of how to achieve their objectives by distributing limited resources among multiple sites
Sign-in/Register to view highlights & summary Sign-in/Register to access full text options 
Talk to us
Join us for a 30 min session where you can share your feedback and ask us any queries you have
Schedule a callDisclaimer: All third-party content on this website/platform is and will remain the property of their respective owners and is provided on "as is" basis without any warranties, express or implied. Use of third-party content does not indicate any affiliation, sponsorship with or endorsement by them. Any references to third-party content is to identify the corresponding services and shall be considered fair use under The CopyrightLaw.
