Pesky possums in New Zealand
Cute and cuddly, and make a great overcoat. Also an ecological disaster in New Zealand - the Brushtail possum. Image: Wikipedia
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Charismatic, but often loathed, Brushtail possums are one of the iconic marsupials from Australia. Naturally, they live in trees and are considered folivorous (eat leaves), however Brushtail possums are often persecuted for their tendencies to nest in the roofs of houses, eat fruit from trees and destroy backyard gardens. They have also been introduced into New Zealand, where they are public enemy #1.
Introduced to establish a fur industry, Brushtail possums have devastated temperate forests in New Zealand. In some parts, they have driven some trees species to local extinction, and fundamentally changed vegetation communities. Strikingly in other places, forests are largely untouched. This has lead to the hypothesis that following release from top-down control on their populations through predation, possums in New Zealand are solely governed by bottom-up processes: namely, nutritional quality of foliage. H. Windley investigated this as part of her PhD. This manuscript has been published in PLoS One. |
Leaf nutritional quality |
Intensive and repeated seasonal sampling of leaves was conducted on 5 key tree species in the Tararuas mountain range, New Zealand over a year. During this time, half the study area was subjected to a possum control program, using aerial baiting for possums. Each tree was scored for the extent of possum browsing activity each quarter, and samples collected to measure nitrogen content within the leaves. Additionally, data was collected on possum abundances throughout the study period.
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Possum browse choice |
Cumulative link modelling of possum browsing damage and leaf nutritional quality revealed that possums are preferentially browsing trees with higher available nitrogen content. Furthermore, possum densities were correlated with the nutritional quality, at the landscape scale. Possum control reduced the impact of possums on individual trees, to the point that the relationship between nutritional quality of foliage and possum browsing damage disappeared. This research highlights that in the absence of top-down processes, landscape-scale nutritional quality may be the regulating factor on possum populations, and may explain the patchy damage inflicted on temperate forests by Brushtail possums.
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Modelled probability of possum browse intensity in relation to leaf available nitrogen content. Dotted line = no browse, short dash = low browse, long dash = moderate browse, solid line = heavy browse.
Windley, H.R., Barron, M.C., Holland, E.P., Starrs, D., Ruscoe, W.A., and W.J. Foley (2016) Foliar nutritional quality explains patchy browsing by an invasive mammal. PLoS One 11(5): e0155216
