On the use of three performance measures for fitting species response curves

Abstract. Quantitative response surfaces obtained from three performance measures, density, cover and volume, are compared, using as an example the spatial distribution of Periploca angustifolia (Asclepiadaceae) in SE Spain. Generalized linear models are used to examine relationships between these species performance measures and complex gradients of aspect, slope angle and altitude. All three performance measures showed a skewed response to the environmental gradients, unlike the Gaussian responses commonly assumed in vegetation theory; skewness increased as the number of dimensions of the performance measures increased. Certain asymmetries between the responses are discussed in terms of competition, and problems related to the use of complex gradients are considered.