PARENT SESSION
Oral Session #14: Theoretical Ecology. Presiding: C. Klausmeier.
Monday, August 6, 2001. 1:00 PM to 4:30 PM. Hall of Ideas E.
Continuum theory revisited: what shape are species responses along ecological gradients?
OKSANEN, JARI1, MINCHIN, PETER2, 1 2
ABSTRACT- The shape of species responses along ecological gradients has important implications for both continuum theory and community analysis. Most current theories and analytical models in community ecology assume that responses are unimodal and symmetric, however, interactions between species and extreme environmental stress may cause skewed or non-unimodal responses. To date, statistical tools for evaluating response shapes have been either inappropriate, inefficient or biased. Using data sets on diatom distributions along a pH gradient and vascular plant distributions on an elevation gradient, we show that Huisman-Olff-Fresco (HOF) models are an effective method for this purpose, allowing models of various forms to be tested for adequacy. HOF modelling was compared with alternative methods for response fitting, including Gaussian GLM's, Generalized Additive Models (GAM) and Beta Functions. GAM's cannot provide clear tests of skewness or kurtosis of response curves, though we show that GAM's, in general, confirm the shapes chosen by HOF modelling. In our data sets, skewed and flat (plateau) responses are rare. Many species have skewed responses that can not be adequately modelled by Gaussian models. The modes of skewed unimodal responses are not concentrated towards the gradient extremes, refuting some currently popular community models that were based on Beta Functions. We show that Beta Function models are biased, confounding skewness and the location of the mode, and should not be used to test response shapes.
KEY WORDS: response curve, HOF models, skewed responses, beta function models