Niche modelling the distributions of large Acacia nigrescens and Sclerocarya birrea trees.
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Date
2011
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Abstract
MaxEnt modelling uses only the known locations of a species to predict the overall
distribution of a species. Large trees are important for the functioning of savanna ecosystems,
bringing nutrients to the surface, providing shelter to animals and providing a number of
ecological functions. Large trees have been identified as declining in density in many
southern African reserves, making the conservation of large trees within reserves an issue in
park management, such as in Kruger National Park (KNP) and Hluhluwe iMfolzi Parks
(HiP). Two species of primary concern are Acacia nigrescens and Sclerocarya birrea, which
have similar distributions in Southern Africa. Effective management of large trees requires
understanding their distribution within reserves and any potential distribution changes. By
determining the current locations of a species, and using GIS layers of environmental
variables to predict the extent of habitats that could support the species, niche models can
predict species distribution. Maximum Entropy techniques evaluate the probability of finding
the species in raster squares, with values for environmental factors controlling distribution.
For this study, the locations of A. nigrescens and S. birrea trees higher than 5 m were
recorded in KNP and HiP, and were used in conjunction with MaxEnt to produce distribution
probability maps for both species in each reserve. In HiP, the distribution map was compared
with an independent existing data set to determine if the predicted distributions were
accurate. The factors effecting their distribution were compared between HiP and KNP to
determine why the species were found together in KNP but not in HiP. MaxEnt could predict
the locations of the species within HiP, but predictions were better for A. nigrescens than S.
birrea. In both Reserves, rainfall was the best predictor of tree location, along with elevation.
The niche overlap was higher in KNP, where both species are well within their total species
range, than in HiP where A. nigrescens was at the edge of its distribution. These variables
that are limiting distribution at a reserve scale will have an influence on the overall
distribution of the species. Niche models can be used to inform the establishment of
botanical reserves or other management strategies that can help preserve large trees within
reserves.
Description
Thesis (M.Sc.)-University of KwaZulu-Natal, Westville, 2011.
Keywords
Acacia nigrescens--South Africa., Sclerocarya birrea--South Africa., Conservation of natural resources--South Africa., Kruger National Park (South Africa), Hluhluwe-Imfolozi Park (KwaZulu-Natal), Theses--Botany.