Heterogeneity

by

David Abson

• LAST REVIEWED: 10 May 2023
• LAST MODIFIED: 26 July 2017
• DOI: 10.1093/obo/9780199830060-0185

Introduction

Heterogeneity can be defined as the difference or diversity in kind or arrangement of component elements or constituents. Such differences can occur across both space and time. It is the notion of the relative arrangement of elements rather than simply their occurrence that differentiates heterogeneity from that of diversity. From an ecological perspective the term is most often associated with land cover, land use, or habitat. The increased awareness of the importance that environmental patterns play in ecological processes has led to an emphasis on conceptualizing and quantifying spatial heterogeneity. The focus on spatial pattern has been described as a paradigm shift in ecology and spatial and temporal heterogeneity are considered a prerequisite to the study of ecological pattern-process relationships. Despite this emphasis, no single accepted definition of heterogeneity exists, in part because of the context dependent nature of the notion—heterogeneity of what and from whose perspective? Heterogeneity can be conceptualized in different ways for different purposes, resulting in multiple definitions and operationalizations. Multiple conceptualizations, definitions, and operationalizations of heterogeneity allow for the investigation of different aspects of ecological patterns on different ecological processes. A key challenge in the quantification of spatial heterogeneity is its relation/response to scale (e.g., extent, grain size, and thematic resolution). Moreover, from an ecological perspective it is necessary to differentiate between functional and structural heterogeneity. Functional heterogeneity is defined with respect to particular ecological processes. In contrast, structural heterogeneity takes an “arbitrary,” or observer-oriented, perspective on the assessment of heterogeneity. Unlike functional heterogeneity measures, structural heterogeneity measures do not need to be “recalibrated” when dealing with each new species. Finally a distinction is required between heterogeneity arising from changes in continuous variables (such as canopy height) and that arising from categorical variables (such as land cover types). These two aspects of heterogeneity are sometimes referred to as either complexity or gradients (based on continuous variables) and variability, or patchiness (based on categorical variables), with both types of heterogeneity often co-occurring at different scales. For example, complex height structures exist within individual patches of grassland and there is variability between the patches (i.e., different species) of grassland that make up the landscape mosaic. Taking all these factors together, heterogeneity is a complex, multifaceted, and context dependent concept that raises considerable challenges in the ecological sciences.

General Overviews and Foundational Texts

Watt 1947 makes an eloquent call for the consideration of pattern and process in plant community ecology based on seven examples ranging from woodlands to grasslands. MacArthur and Wilson 1967 provides early insights into the role of spatial pattern on biological dispersal and biodiversity. Forman and Godron 1986; Forman 1995; and Turner and Gardner 2001 provide foundational texts that tie the notion of heterogeneity into the ecological sciences to the developing field of landscape ecology. Here the focus is on relating spatial patterns to ecological processes at a landscape scale, going beyond the island biogeography of MacArthur and Wilson. Turner and Gardner 2001 builds on the theoretical foundations of Forman and Godron with greater emphasis on methodological approaches to ecological patterns and processes within the fields of landscape ecology and conservation biology. In contrast to these more ecological-oriented approaches, Giles and Trani 1999 and Li and Reynolds 1995 focus on the definition and quantification of heterogeneity. Kolasa and Pickett 1991 provides a broad overview of the theoretical and methodological challenges of using the notion of heterogeneity in ecology, including an extremely useful discussions on terminology, scale issues, and modelling approaches. Fortin and Dale 2005 provides an overview of the wide range of statistical approaches available for the spatial analysis of ecological data. Benton, et al. 2003 gives an overview of the importance of habitat heterogeneity and farmland biodiversity. Together these works highlight the importance of spatial pattern on ecological processes and represent attempts to provide codified approaches to conceptualizing and measuring heterogeneity from an ecological perspective.

• Benton, T. G., J. A. Vickery, and J. D. Wilson. 2003. Farmland biodiversity: Is habitat heterogeneity the key? Trends in Ecology & Evolution 18.4: 182–188.

  DOI: 10.1016/S0169-5347(03)00011-9

  A review and synthesis of the evidence of the impacts of the intensification and spatial homogenization of agriculture practices on farmland habitat heterogeneity. In turn, the authors highlight some of the key consequences for farmland biodiversity resulting from the resulting loss of farmland habitat heterogeneity.

• Forman, R. T. 1995. Land mosaics: The ecology of landscapes and regions. Cambridge, UK: Cambridge Univ. Press.

  This authoritative book takes a multidisciplinary, integrative approach to landscape ecology, covering established and new theories, methods, and the practical implementation of research related to the biophysical interactions that shape and characterize landscapes.

• Forman, R. T., and M. Godron. 1986. Landscape ecology. New York: Wiley.

  A foundational text on landscape ecology, synthesizing understandings drawn from multiple disciplines with a strong focus on the conceptualization of spatial patterns in ecology, including the notions of patches, mosaic patterns, and habitat fragmentation.

• Fortin, M. -J., and M. R. T. Dale. 2005. Spatial analysis: A guide for ecologists. Cambridge, UK: Cambridge Univ. Press.

  This book describes and evaluates methods for detecting and quantifying a number of different facets of spatial patterns. This book provides an overview of the wide range of spatial statistics available for analyzing spatial ecological data. Sampling and experimental design are also discussed.

• Giles, R. H., Jr., and M. K. Trani. 1999. Key elements of landscape pattern measures. Environmental Management 23.4: 477–481.

  DOI: 10.1007/s002679900202

  This paper outlines six key factors (area, land cover classes, proportion of dominant class, number of polygons, polygon size variance, and elevation range) in the description of spatial patterns in categorical maps, arguing that these factors are sufficient to encompass most observed phenomena associated with land patterns.

• Kolasa, J., and S. Pickett. 1991. Ecological heterogeneity. New York: Springer-Verlag.

  DOI: 10.1007/978-1-4612-3062-5

  This important edited book contains fourteen chapters addressing various aspects of ecological heterogeneity. The chapters range from theoretical discussions on the “heterogeneity of heterogeneity” to a description of specific ecological models related to spatial heterogeneity. Somewhat unusually it addresses heterogeneity in both terrestrial and aquatic systems.

• Li, H., and J. F. Reynolds. 1995. On definition and quantification of heterogeneity. Oikos 73.2: 280–284.

  DOI: 10.2307/3545921

  A foundational text on the definition and quantification of spatial heterogeneity in the field of ecology. This paper introduces an operational definition of ecological heterogeneity related to the type of data being used. The authors include examples of how this definition of spatial heterogeneity can be applied in practice.

• MacArthur, R., and E. O. Wilson. 1967. The theory of island biogeography. Princeton, NJ: Princeton Univ. Press.

  Groundbreaking, seminal work on island biogeography that discusses the importance and consequences of distance between resources and habitat extents for population ecology and species distributions. This work is foundational regarding the importance of spatial pattern recognition and assessment in ecology.

• Turner, M. G., and R. H. Gardner. 2001. Landscape ecology in theory and practice: Pattern and process. New York: Springer-Verlag.

  Authoritative book that provides a detailed description of the origins and development of landscape ecology and its relation to scale and spatial pattern and their interactions with multiple ecological processes. Issues addressed include the causes of landscape patterns, the quantification of landscape patterns, and landscape disturbance dynamics.

• Watt, A. S. 1947. Pattern and process in the plant community. Journal of Ecology 35:1–22.

  DOI: 10.2307/2256497

  One of the earliest arguments for the consideration of spatial pattern and its relation to ecological processes. The author uses examples of seven different plant communities ranging from dwarf Callunetum to beechwoods to highlight spatial patterns influences on plant community dynamics.