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Influences of climate, landscape and vegetation proximity on the spatial distribution of the Tasmanian Eucalyptus leaf beetle, Paropsisterna bimaculata in Tasmanian Eucalyptus plantations

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Edgar, SC (2011) Influences of climate, landscape and vegetation proximity on the spatial distribution of the Tasmanian Eucalyptus leaf beetle, Paropsisterna bimaculata in Tasmanian Eucalyptus plantations. Honours thesis, University of Tasmania.

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Abstract

The distribution of the beetle Paropsisterna bimaculata was related to 33
environmental covariates distributed across in Tasmania. This beetle is an economic
pest in Eucalyptus plantations, episodically occurring in large aggregations that
damage tree canopies through overgrazing. A number of hypotheses relating to beetle
outbreaks have been proposed by various researchers. Random forest modelling, a
powerful non-parametric statistical approach that has not been frequently applied
before in ecology, was used to assess specific predictions of these hypotheses. The
models were developed using landscape layers and proximity to vegetation layers that
were created using GIS for multiple combinations of climate variables and districts.
The climate combinations assessed included: (1) mean summer maximum
temperature and summer rainfall for survey years, (2) 30 year average for mean
summer maximum and summer rainfall, (3) 30 year average for climate variables, (4)
30 year average for climate variables and mean summer maximum temperature and
summer rainfall for survey years. The district combinations included all districts, Bass
only, and all districts excluding Bass.
Locations less than 10 kilometres from Poa grasslands showed highest beetle
numbers, while increasing elevation also showed a strong positive relationship with P.
bimaculata populations, and mean annual summer rainfall and survey year were
important in the district of Bass. Thus, in terms of prior hypotheses, Poa appears to be
an important overwintering site for P. bimaculata; beetle populations increase with
elevation; beetle populations increase with age of plantation between 2 and 8 years;
and Bass has greater variability than other districts in beetle density. Model outputs
did not support, or only weakly supported, hypotheses that suggest beetle numbers are
highest at highly productive sites, and are affected by the shape and size of forestry
coupes and by plantation species.
The random forests model with least error was based on all districts except Bass for
thirty-year climate average data plus summer mean rainfall and summer mean
maximum temperature data. A cost-benefit analysis nevertheless indicated that
directing the ongoing beetle survey effort on the basis of model outputs would not
reduce costs compared to the current broad-scale monitoring program. The model did,
iv
however, generate state-level maps of value to forestry workers when assessing the
likelihood of beetle outbreaks in their districts.

Item Type: Thesis (Honours)
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Date Deposited: 25 May 2012 04:31
Last Modified: 11 Mar 2016 05:53
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