M. P. Austin

Spatial prediction of species distribution: an interface between ecological theory and statistical modelling

Neglect of ecological knowledge is a limiting factor in the use of statistical modelling to predict species distribution. Three components are needed for statistical modelling, an ecological model concerning the ecological theory to be used or tested, a data model concerning the collection and measurement of the data, and a statistical model concerning the statistical theory and methods used. This component framework is reviewed with emphasis on ecological theory. The expected shape of a species response curv et o an environmental gradient is a central assumption on which agreement has yet to be reached. The nature of the environmental predictors whether indirect variables, e.g. latitude that hav en o physiological impact on plants, or direct variables, e.g. temperature also influence the type of response expected. Straight-line relationships between organisms and environment are often used uncritically. Many users of canonical correlation analysis use linear (straight-line) functions to relate ordination axes to variables such as slope and aspect though this is not a necessary part of the method. Some statisticians have used straight lines for species/environment relationships without testing, when evaluating new statistical procedures. Assumptions used in one component often conflict with those in another component. Statistical models can be used to explore ecological theory. Skewed species response curves predominate contrary to the symmetric unimodal curves assumed by some statistical methods. Improvements in statistical modelling can be achieved based on ecological concepts. Examples include incorporating interspecific competition from dominant species; more proximal predictors based on water balance models and spatial autocorrelation procedures to accommodate non-equilibrium vegetation. # 2002 Elsevier Science B.V. All rights reserved.

Habitat fragmentation and its lasting impact on Earth's ecosystems

Urgent need for conservation and restoration measures to improve landscape connectivity. We conducted an analysis of global forest cover to reveal that 70% of remaining forest is within 1 km of the forest’s edge, subject to the degrading effects of fragmentation. A synthesis of fragmentation experiments spanning multiple biomes and scales, five continents, and 35 years demonstrates that habitat fragmentation reduces biodiversity by 13 to 75% and impairs key ecosystem functions by decreasing biomass and altering nutrient cycles. Effects are greatest in the smallest and most isolated fragments, and they magnify with the passage of time. These findings indicate an urgent need for conservation and restoration measures to improve landscape connectivity, which will reduce extinction rates and help maintain ecosystem services.

Predicting species distributions for conservation decisions.

Species distribution models (SDMs) are increasingly proposed to support conservation decision making. However, evidence of SDMs supporting solutions for on-ground conservation problems is still scarce in the scientific literature. Here, we show that successful examples exist but are still largely hidden in the grey literature, and thus less accessible for analysis and learning. Furthermore, the decision framework within which SDMs are used is rarely made explicit. Using case studies from biological invasions, identification of critical habitats, reserve selection and translocation of endangered species, we propose that SDMs may be tailored to suit a range of decision-making contexts when used within a structured and transparent decision-making process. To construct appropriate SDMs to more effectively guide conservation actions, modellers need to better understand the decision process, and decision makers need to provide feedback to modellers regarding the actual use of SDMs to support conservation decisions. This could be facilitated by individuals or institutions playing the role of ‘translators’ between modellers and decision makers. We encourage species distribution modellers to get involved in real decision-making processes that will benefit from their technical input; this strategy has the potential to better bridge theory and practice, and contribute to improve both scientific knowledge and conservation outcomes.

A new model for the continuum concept

A reformulation of the continuum concept is presented after considering the implications of the community/continuum controversy and current niche theory. Community is a spatial concept dependent on landscape pattern while the continuum is an environmental concept referring to an abstract space. When applying niche theory to plants, the mechanisms of competition are ill-defined and the assumption of bell-shaped response curves for species unrealistic. Eight testable propositions on the pattern of response of vegetation to environmental gradients are presented 1. Environmental gradients are of two types. a) resource gradients or b) direct physiological gradients. 2. The fundamental niche response of species to resource gradients is a series of similar nested response curves. 3. The fundamental niche response of species to direct gradients is a series of separate, independent, overlapping response curves. 4. Species fundamental response curves are such that they have a relative performance advantage in some part of the environmental space. 5. The shape of the realized niche is variable even bimodal but predictable from the fundamental response given the other species present. Propositions 6–8 describe the response shapes of emergent community properties to environmental gradient; species richness is bimodal, dominance trimodal and standing crop unimodal. Detailed comparisons of these propositions are made with the alternative theories of Ellenberg, Gauch and Whittaker, Grime, and Tilman. These theories are incomplete lacking several generally accepted properties of plants and vegetation.

Patterns of plant species richness in relation to different environments: An appraisal

We review patterns of plant species richness with respect to variables related to resource availability and vari- ables that have direct physiological impact on plant growth or resource availability. This review suggests that there are a variety of patterns of species richness along environmental gradients reported in the literature. However, part of this diversity may be explained by the different types and lengths of gradients studied, and by the limited analysis applied to the data. To advance in understanding species richness pat- terns along environmental gradients, we emphasise the im- portance of: (1) using variables that are related to the growth of plants (latitudinal and altitudinal gradients have no direct process impact on plant growth); (2) using multivariate gra- dients, not single variables; (3) comparing patterns for dif- ferent life forms; and (4) testing for different shapes in the species richness response (not only linear) and for interaction between variables.

Searching for a Model for Use in Vegetation Analysis

Current methods of indirect vegetation analysis either explicitly or implicitly assume a certain ecological model of how vegetation responds to environment. Indirect or vegetational ordination methods; (Whittaker 1978) including the more recent methods; of reciprocal averaging (Hill 1973), multidimensional scaling (Fasham 1977, Prentice 1977), PARAMAP (Noy-Meir 1974), and Gaussian ordination (Gauch et al. 1974; Ihm & van Groenewoud 1975) appear sensitive to small changes in the generating model (Austin 1976a, b). These indirect methods are multivariate exploratory data analysis techniques (Noy-Meir 1971, see also Tukey 1977) whose purpose is to expose the unknown ecological dimensions associated with floristic variation; their efficacy depends on the relevance of their model of the vegetation/environment relationship (Austin 1976a, Whittaker 1978). Testing their effectiveness with artificial data sets is entirely dependent on the appropriateness of the model used to generate the artificial data.

Models for the analysis of species’ response to environmental gradients

A procedural model for vegetation analysis is presented. Suggestions are made that analysis methods can test theory as well as examine vegetation-environment correlations. Gauch and Whittaker’s propositions regarding species behavioural properties expected for an individualistic continuum are tested on a eucalypt forest data set. The data set is carefully stratified to control environmental heterogeneity. The shape and distribution of species response curves are then examined along a temperature gradient using 750 sites. The conclusions are: (1) Bell shaped response curves to environmental gradients are not universal (2) Positive-skewed curves are characteristic of major canopy species in eucalypt forest in southern New South Wales (3) Species richness increases with temperature along the gradient (4) Tests of other propositions regarding species modes and ranges are confounded by the change in species richness along the gradients (5) More rigorous statistical analysis and analyses on other vegetation types are needed.

Vegetation survey design for conservation: Gradsect sampling of forests in North-eastern New South Wales

Abstract A method is described which obtains a representative sample of the floristic variation in a forested area of c. 20 000 km 2 . Using the climatic, topographic and lithological characteristics of the study area, a series of gradsects (transects incorporating significant environmental gradients) were selected to represent the environmental variability present in the area. Rules for a field sampling strategy are outlined which ensure that the widest possible range of environments are sampled with reasonable constraints on travelling time and costs. Gradsects in combination with a set of explicit sampling rules are shown to be an effective technique for obtaining a representative data set.

On non-linear species response models in ordination

Ordination techniques are plagued by the non-linearity of vegetation data. The purpose of ordination is discussed and considered to be the process of arranging samples (or species) in relation to one or more environmental gradients or abstract axes that may represent such gradients, the arrangement to be ecologically significant. The appropriateness of various models of vegetation to current ordination techniques is considered, particularly the gaussian species response curve. Two alternatives are suggested based on β-functions and an ecological response curve model incorporating competition between species.

Use of ordination and other multivariate descriptive methods to study succession

Multivariate techniques can be applied to both the static approach to succession (determining trends from data collected at one time) and the dynamic approach (observing actual change following perturbation). Such applications, which are few in number, are reviewed; and two studies are described in more detail: a numerical analysis of Australian rain-forest succession by Williams et al. (1969b), and a study of lawn succession as influenced by shading and trampling effects, by the author. Complex data embodying threefold relationships (sites × times × species) are shown to be amenable to multivariate analyses, and to representation of successional change by trajectories in an ordination field. Multivariate approaches have advantage over classificatory approaches for the description and understanding of interactions between spatial pattern, and change through time. Problems of experimental design and modelling for such studies are discussed.