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SPECIES AND COMMUNITIES 1: LAND COVER AND BIOTIC IMPACTS


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OVERVIEW

Before the eighteenth century, humans adapted to the landscapes in which they lived. During the
Great Transformation of the last two centuries (p. 5), they radically altered many of those
landscapes, and especially the plant communities that they supported, converting forest and
grassland to cropland and built-up areas. Habitat fragmentation is a major side effect of land cover
change.

Broad tracts of natural and semi-natural habitats have been broken into smaller and
isolated blocks. The fragmentation of habitats occurs locally, but has cumulative effects that are
regional in scale. A knock-on effect of this habitat fragmentation is a reduction in biodiversity. As
with habitat fragmentation, so with the loss of biodiversity: it occurs locally but cumulates over
regional and global scales. The Great Transformation also saw humans exploiting the
natural world to an unprecedented extent. Conversion of land to agriculture and silviculture
has promoted the expansion of some species that formerly had smaller populations, making
them newly abundant. Bourgeoning trade between far-flung places encouraged both the
accidental and deliberate spreads of exotic species, some of which have become invasive
and cause enormous management problems.


LEARNING OUTCOMES

This chapter should help you to understand:

• What biodiversity is and how it varies across the planet
• How humans alter biodiversity by destroying habitats and breaking them into smaller and
  more isolated blocks
• How humans alter biodiversity by encouraging the rise of newly abundant species
• How humans alter biodiversity by aiding and abetting the spread of exotic invasive species.
  

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SUMMARY

Biodiversity takes in the diversity of genetic material, the diversity of species and the diversity of habitats. It is unevenly distributed around the world, with a couple of dozen or so, mainly tropical and Mediterranean-type climate, diversity hot-spots superimposed upon a tropical high to polar low biodiversity gradient. Biodiversity is always changing, but the recent biodiversity nose-dive is worrying. The chief drivers of current biodiversity change are changes in land cover, species
exploitation and exchange, climatic change and changes in environmental chemistry. Habitat fragmentation is a large part of land cover change. It leads to habitats being lost, shrinking and becoming increasingly isolated.

The size, shape and degree of connection between habitat fragments affects the distribution and abundance of many species, as well as community processes. Particularly important is the proportion of habitat edges to habitat interiors, with edge effects more noticeable in smaller and irregularly shaped habitat fragments. Corridors of various kinds – road, railways, trails, powerlines, hedgerows, rivers and so on – act at once as conduits, as barriers and as filters. Linkages between habitat fragments appear to help in maintaining species diversity, and a network of well-connected fragments seems to fare better than a network of ill-connected fragments. ‘Landscape context’ as measured, for example, as landscape diversity, also affects biodiversity. Human activities have altered the species composition of many places: witness the emergence of newly abundant species, such as the white-tailed deer, and the introduction of invasive exotic species, such as efficient predators on formerly prey-friendly islands.

QUESTIONS

1. Why does biodiversity vary from place to     place? 4. Try to identify the traits of a successive     invasive species. 2. Why does habitat fragmentation affect    some animal and plant species more than    others? 5. Examine the range of steps taken to    overcome the overexploitation of species. 3. How important are wildlife ‘corridors’ to     species conservation?

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FURTHER READING

• Bennett, A. F. (1999) Linkages in the Landscape: The Role of Corridors and Connectivity in Wildlife Conservation. Gland, Switzerland and Cambridge, UK, IUCN.
  An excellent book looking at the detrimental effects of habitat fragmentation on biodiversity and ways of combating it through landscape linkages. Contains many examples and a discussion of conservation strategy.
  
• Elton, C. S. (1958) The Ecology of Invasions by Animals and Plants. London: Chapman and Hall.
  Old, but a classic and well worth having a look at.
  
• Huggett, R. J. and Cheesman, J. E. (2002) Topography and the Environment. Harlow: Prentice Hall.
  Chapter 6 contains many examples of the effects of landscape elements (patches, corridors and matrices) and landscape structures (networks) on species and communities.
  
• Hunter, M. L. (2001) Fundamentals of Conservation Biology, 2nd edn. Oxford: Blackwell
  Science.
  Covers issues of habitat loss and overexploitation.
  
• Jeffries, M. L. (1997) Biodiversity and Conservation. London and New York: Routledge.
  An excellent basic text.
  
• Quammen, D. (1996) The Song of the Dodo: Island Biogeography in an Age of Extinctions.
  London: Hutchinson.
  An excellent, readable and entertaining introduction to biogeographical ideas.