MEASUREMENT OF THE REALISED QUALITATIVE NICHE: ENVIRONMENTAL NICHES OF FIVE EUCALYPTUS SPECIES
P. AUSTIN, A. 0. NICHOLLS, AND C. R. MARGULES
Commonwealth Scientific and Industrial Research Organisation Division of Wildlife and Ecology, P.O. Box 84, Lyneham, Australian Capital Territories, 2602, Australia
Abstract. A method is described, using Generalised Linear Modelling, for determining the qualitative environmental realised niche (QERN) of plant species. Five species of Eucalyptus growing in south-eastern Australia are used as examples. The frequency of each species occurrence in a data set comprising 6080 observations over an area of 40,000 km2 is modelled as a function of four environmental variables: mean annual rainfall, mean annual temperature, a solar radiation index, and rock type. The position and shape of the response of a species to these four variables is the species' QERN. The effect of rock type on the distribution of each species was found to be highly significant. To reduce model complexity, the data were stratified on rock type, and models for each species were generated for the two common rock types, Granites and soft sediments. Whilst the model-fitting procedure generally is the same, each species' response demanded some modifications of the procedure. The continuous variates, mean annual temperature, mean annual rainfall, and solar radiation index, were categorised so that the magnitude of the model coefficients from each category would indicate the shape of each species' response. Continuous functions were then fitted to the categorical response shapes. Of the 10 models (5 species and 2 rock types) 5 revealed a skewed response to one or more of the environmental variables, one was linear, and two were bell shaped. Two more models revealed complex response shapes, suggesting that the environmental variables were inappropriate or insufficient to model those species' realised niches.
The results provide evidence for asymmetric responses of species to environmental variables contrary to the bell-shaped symmetrical responses commonly assumed in ecological theory. Models of species' realised niches are necessary in community ecology to describe the role of environment, prior to studies on the role of competition in determining community composition. Limitations of the models, both statistical and ecological, are considered, with suggestions for improvements.