DETERMINING SPECIES RESPONSE FUNCTIONS TO AN ENVIRONMENTAL GRADIENT BY MEANS OF A BETA-FUNCTION

The importance of the shape of a species response curve to an environmental gradient is reviewed. The implications for vegetation theory, ordination methods, species as indicators of environmental conditions, predicting species distribution from surveys and simulation models of climatic impact on vegetation are examined. A beta-function V = k (x - a)alpha . (b - x)gamma is used to model species response curves using generalized linear modelling (GLM). Two hypotheses are tested; (1) that response curves differ significantly from a unimodal symmetric (Gaussian) shape and (2) that the direction of skew is a function of species position along the gradient. Nine eucalypt species are modelled using GLM with a beta-function fitted for mean annual temperature. Six other environmental variables and factors are considered in fitting the statistical models; mean annual rainfall, mean monthly solar radiation, topographic position, lithological type, nutrient index and rainfall seasonality. All nine species are significantly skewed in response to temperature. The direction of skew is positive when a species optimum temperature falls below 11.5-degrees-C and negative for species with optimum above. These results indicate that current vegetation analyses which require the modelling of species response curves, or assumptions about the shape of the response, require reassessment.