Ecological Consequences of Habitat Fragmentation

Abstract

Habitat fragmentation is the process by which habitat loss results in the division of large, continuous habitats into smaller, more isolated remnants. Thousands of scientific studies now show unequivocal evidence for the impacts of patch area, edge effects, patch shape complexity, isolation and landscape matrix contrast on community structure and ecosystem functioning. However, striking disparities in the results of these studies have raised considerable debate about the relative importance of different mechanisms underlying fragmentation effects, and even about the utility of the ‘fragmentation’ concept in general. Resolution of this debate lies in clear discrimination of direct versus indirect causal relationships among patch and landscape variables. The most important recent advances in our understanding of fragmentation effects all stem from recognition of strong context‐dependence in ecosystem responses, including spatial context‐dependence at multiple scales, time‐lagged population declines, trait‐dependent species responses and synergistic interactions between fragmentation and other components of global environmental change.

Key Concepts:

  • Habitat fragmentation is an umbrella term describing the complete process by which habitat loss results in the division of large, continuous habitats into a greater number of smaller patches of lower total area, isolated from each other by a matrix of dissimilar habitats, and is not just the pattern of spatial arrangement of remaining habitat.

  • Habitat loss and habitat fragmentation are not independent drivers of ecological change – habitat loss acts via the change in habitat arrangement, not independently of it.

  • Habitat fragmentation is a landscape‐level phenomenon, and patch‐level processes (patch area, edge effects and patch shape complexity) can only be understood within a landscape context (isolation and matrix structure).

  • A dominant effect of increasing habitat loss is a reduction in patch area, with resulting declines in population density and species richness, and significant alterations to community composition, species interactions and ecosystem functioning.

  • The habitat edge is not a discrete boundary line around a patch, it is a fuzzy three‐dimensional zone that straddles both sides of the patch‐matrix boundary, and the intensity of edge influence may be variable and asymmetrical around the physical vegetation boundary.

  • Quantification of edge impact requires explicit discrimination of two distinct components of edge influence: edge extent (i.e. the distance over which a statistical difference in response can be detected between the matrix and the patch) and edge magnitude (i.e. the degree of difference in response between the patch interior and the matrix interior).

  • Increasing patch shape complexity substantially reduces the availability of ‘core’ habitat area unaffected by edge effects.

  • Patch isolation reduces population connectivity and reduces the probability of population persistence, but geographic isolation is not an absolute quantity and can only be interpreted in the light of matrix permeability, the dispersal traits of the species in question and the time‐scale over which effects might become apparent.

  • Matrix quality and surrounding landscape composition have a dominant influence on population dynamics, species diversity and ecosystem processes in habitat patches.

  • Habitat fragmentation interacts strongly with other components of global environmental change, including species invasions, land‐use intensification and climate change.

Keywords: connectivity; context‐dependence; edge effects; habitat area; habitat fragmentation; habitat loss; isolation; landscape structure; matrix contrast; patch shape

Figure 1.

Land‐use change in a fragmented tropical forest landscape in the highlands near Sapa, Vietnam, showing the scale of habitat loss, the altered spatial arrangement of remaining rainforest habitats and the mosaic of human land‐use intensification in the surrounding landscape matrix. Copyright © Ashley Whitworth, reproduced with permission from www.iStockPhoto.com, file no. 3280280 (http://www.istockphoto.com).

Figure 2.

Defining habitat fragmentation. There is strong debate about whether the term ‘habitat fragmentation’ should be used to describe: (a) the entire spatio‐temporal process by which habitat loss leads to the subdivision of large, continuous habitats into a greater number of smaller patches of lower total area, isolated from each other by a matrix of dissimilar habitats; or (b) solely the differences that occur due to the differing pattern of spatial arrangement of remaining habitat after the amount of habitat remaining in the landscape has been taken into account. The satellite images in (a) show typical correlated changes in habitat loss and habitat fragmentation in the same landscape east of Santa Cruz, Bolivia, over three time intervals (images reproduced courtesy of NASA/Goddard Space Flight Center Scientific Visualization Studio; http://visibleearth.nasa.gov). The satellite images in (b) show three different landscapes in southern Mato Grosso, Brazil, with approximately the same amount of habitat loss, but very differing spatial arrangement (images reproduced courtesy of Jacques Descloitres, MODIS Land Rapid Response Team, NASA/GSFC, 29 May 2001; http://visibleearth.nasa.gov).

Figure 3.

A schematic representation of the problem of attributing causality to ‘habitat loss’ versus ‘habitat fragmentation’. (a) In modified landscapes, all measures of spatial habitat configuration are strongly intercorrelated with the amount of remaining habitat, making separation of ‘independent’ effects impossible. (b) The reason for the strong intercorrelation is that the effects of habitat amount do not only operate directly and separately from the effects of habitat fragmentation, they predominantly operate through indirect pathways mediated by altered spatial configuration. For clarity, not all possible indirect pathways are shown in (b).

Figure 4.

Widely held generalisations about community responses to habitat fragmentation. Predictions of how species richness typically changes as the five main components of the spatial context of habitat fragments are altered. Reproduced with permission from Ewers and Didham .

Figure 5.

Edge effects can only be quantified by measuring ecological responses on both sides of the physical vegetation boundary. The ‘edge’ itself is not a simple boundary line around a patch, but is a fuzzy three‐dimensional zone of edge influence, that can be measured in terms of both edge extent (DEI, the depth of edge influence) and edge magnitude (MEI, the magnitude of edge influence). The edge zone is frequently asymmetrical around the physical vegetation boundary, and studies that take a ‘one‐sided’ approach in measuring edge effects can obtain quite different estimates of edge extent and edge magnitude than studies taking a two‐sided approach (DEIpatch vs DEImatrix and MEIpatch vs MEImatrix).

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Didham, Raphael K(Nov 2010) Ecological Consequences of Habitat Fragmentation. In: eLS. John Wiley & Sons Ltd, Chichester. http://www.els.net [doi: 10.1002/9780470015902.a0021904]