- On the use of different performance measures - 57 Journal of Vegetation Science 6: 57-62, 1995 © IAVS; Opulus Press Uppsala. Printed in Sweden response models: density (stem counts per unit area), cover, biomass - or some alternative non-destructive measures such as volume -, etc. However, most of the available information for examining the species response shape is based on qualitative data (presence/absence), and little attention has been paid to ascertaining whether this shape is independent (or not) of the response meas- ures considered. This paper examines quantitative response surfaces obtained from several species performance measures, i.e. density, cover and volume, of Periploca angustifolia (Asclepiadaceae), in southeastern Spain. The main aim is to compare the shape of the responses, which were determined with the help of generalized linear models (GLM; McCullagh & Nelder 1989). This method was suggested by Austin et al. (1984) as an approach that allows an explicit rigorous analysis to be made of the shape of species response to a number of environmental factors considered simultaneously (cf. Austin et al. 1990). The response variables were related to complex gra- dients (sensu Whittaker 1956, 1967) of aspect, slope angle and elevation as their use assumes that many single factors that change together in space - incoming solar radiation, temperature, humidity, wind velocity - may be important for plant distribution. Therefore, meas- ures of aspect, slope angle and altitude are used here as crude indices of relative position along complex envi- ronmental gradients. Methods Data collection 43 10 × 10 m plots were located on limestone and dolomite substrate around two hills of the Sierra de Cartagena (Murcia Region, SE Spain), with every expo- sition (N, NE, E, SE, etc.) represented. Topographical variation was represented in the sample as well (see Ferrer Castán 1994 for sampling details). On the other On the use of three performance measures for fitting species response curves Ferrer-Castán, Dolores, Calvo, José F., Esteve-Selma, Miguel A., Torres-Martínez, Antonio & Ramírez-Díaz, Luis Departamento de Ecología y Medio Ambiente, Universidad de Murcia, Campus Universitario de Espinardo, E-30100 Espinardo - Murcia, Spain; Tel. +34 68 307100 ext. 2373; Fax +34 68 363963 Abstract. Quantitative response surfaces obtained from three performance measures, density, cover and volume, are com- pared, using as an example the spatial distribution of Periploca angustifolia (Asclepiadaceae) in SE Spain. Generalized linear models are used to examine relationships between these spe- cies performance measures and complex gradients of aspect, slope angle and altitude. All three performance measures showed a skewed re- sponse 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. Keywords: Complex gradient; Cover; Density; Direct gradi- ent analysis; Generalized linear model; Periploca angustifo- lia; Regression diagnostic; Shrub volume; Spain. Abbreviations: GLM = Generalized linear model. Nomenclature: Peinado et al. (1992). Introduction The shape of plant species response along environ- mental gradients has been considered of central interest in vegetation science since the concept of relating spe- cies distribution to environmental gradients was formal- ized in the 1950s (Whittaker 1956, 1967). From a theo- retical point of view, both the species niche concept and the use of multivariate ordination techniques are de- pendent on assumptions regarding species response to environmental gradients (Austin 1985; Austin et al. 1990). Most theoretical studies assume the shape of species response to an environmental gradient to be Gaussian, although there is little evidence for this (Austin 1985; Austin et al. 1990; Austin & Gaywood 1994). Various performance measures have been used to build species