The relationship between possum density and browse damage on kamahi in New Zealand forests

Abstract

To minimize the impacts of introduced pests and to justify and prioritize pest control, managers need to know the relationship between pest density and damage. This relationship can be difficult to quantify because pest impacts can be highly variable. In New Zealand, introduced brushtail possums (Trichosurus vulpecula) browse a wide range of native forest species. However, possum browse is extremely patchy making it difficult to quantify the relationship between density and damage, meaning the benefits of reducing possum densities are poorly understood. We quantified patterns of possum browse on kamahi (Weinmannia racemosa), a common canopy tree species, at 21 forest sites that were repeat-measured over an 8-year period in the North Island, New Zealand, during which time possum densities fluctuated widely. We fitted a multilevel statistical model in order to quantify the relationship between possum density and browse damage while simultaneously quantifying how browse varied among trees, sites and years. Higher possum densities were associated with greater browse damage, but browse was also patchily distributed among trees at the same site, and among sites and years for a given possum density. This heterogeneity meant there was no simple density damage relationship, with the relationship differing from tree to tree and among sites and years. Our results show that at most sites reductions in possum density would have little benefit in reducing the probability of heavy browse on kamahi trees, but at a few sites there would be substantial benefits.This approach provides insights into the pattern and potential causes of variability in possum impacts, and a quantitative basis for prioritizing sites for possum control.

Impacts of an invasive herbivore on indigenous forests

“…Additional data were collected to characterize foraging patterns at a local scale because previous studies found significant site differences in possum impacts (Duncan et al . ). The experimental design included measurements of tree size using diameter at breast height (DBH), local availability of other food sources and possum browse at the patch scale (Fig.…”

Density-impact functions for terrestrial vertebrate pests and indigenous biota: Guidelines for conservation managers

“…Gill and Morgan, 2010;Innes et al, 2010;Ricciardi et al, 2013;Thiele et al, 2010), which can mask important relationships between pest abundance and impact, especially in variable environments. This is exemplified by the high spatial and temporal variability in browsing impacts of invasive brushtail possums in New Zealand forest ecosystems (Duncan et al, 2011;Holland et al, 2013;Sessions and Kelly, 2001). Duncan et al (2011) were able to account for this variation using a hierarchical modelling approach that incorporated heterogeneity at multiple levels (sites, times, individual trees), and Holland et al (2013) used a mechanistic model to generate site-specific DIFs using the type of data routinely collected by management agencies.…”

“…This is exemplified by the high spatial and temporal variability in browsing impacts of invasive brushtail possums in New Zealand forest ecosystems (Duncan et al, 2011;Holland et al, 2013;Sessions and Kelly, 2001). Duncan et al (2011) were able to account for this variation using a hierarchical modelling approach that incorporated heterogeneity at multiple levels (sites, times, individual trees), and Holland et al (2013) used a mechanistic model to generate site-specific DIFs using the type of data routinely collected by management agencies. Managers can therefore mitigate the problem of variability by using local knowledge to their advantage, and deriving DIFs for their system, using adaptive experimental management approaches (e.g.…”

Inferring changes in foliar mass and area from foliage cover: A mechanistic model

“…The brushtail possum (Trichosurus vulpecula) was introduced in the 19th century from Australia, and spread rapidly through forests nationwide. removal and consumption of foliar mass) by possums is highly variable in space and time, even among individual trees of the same species (Duncan et al 2011). As with many herbivores (Bulinski & McArthur 2003;Rafferty et al 2010), browse damage (i.e.…”

“…Sweetapple et al 2004;Forsyth & Parkes 2005;Nugent et al 2010;Duncan et al 2011), and is defined as the proportion of sky occluded by foliage when looking up at the canopy from beneath the tree (Payton et al 1999). Sweetapple et al 2004;Forsyth & Parkes 2005;Nugent et al 2010;Duncan et al 2011), and is defined as the proportion of sky occluded by foliage when looking up at the canopy from beneath the tree (Payton et al 1999).…”