Introduction – humans, nature and human nature - Ecological Applications: Toward a Sustainable World - page 32

Summary Homo sapiens - not just another species

Humans destroy natural ecosystems to make way for urban and industrial develop-
ment and to establish production ecosystems such as forestry and agriculture.
Moreover, the natural ecosystems that remain are also affected by our activities - via
overexploitation of harvested species, the spread of invaders, local pollution and
global climate change. In one sense, we are not so different from many other species
in our effects on other animals and plants. But human impacts are very much more
profound because of the size of our population and the technologies we use.

The biodiversity crisis
To judge t he scale of the human threat to biodiversity we need to know the total
number of species that exist, the rate at which these are going extinct and how this
compares with pre-human times. Roughly speaking, the current rate may be as much
as 100-1000 times the historical rate. Bearing in mind the number of species
believed to be under threat, the future rate of extinction may be more than ten times
higher again.
A reduction in biodiversity can have consequences for the ecosystem as a whole.
Species vary in the contribution they make to overall productivity, nutrient cycling
or decomposition rates in an ecosystem - the loss of some will barely register. Of
particular signifi cance are situations where species are 'complementary' in the way
they contribute to ecosystem function. Where this is the case, lower biodiversity
will generally equate to impaired ecosystem functioning and losses to ecosystem
services - whether provisioning (e.g. fi sh from the sea), cultural (e.g. recreational
opportunities), regulating (e.g. fl ood control) or supporting (e.g. soil formation).

Causes of biodiversity loss
Extinction may be caused by one or a combination of drivers that include habitat
loss, invasive species, overexploitation and habitat degradation (pollution and agri-
cultural intensifi cation). Historically, habitat loss, habitat degradation and overex-
ploitation have been of most signifi cance. In future, climate change and the pollution
associated with agricultural intensifi cation are predicted to become progressively
more important causes of biodiversity loss across all ecosystem types.
Increasing agricultural intensity is associated with increases to soil erosion,
desertifi cation and removal of water for irrigation (so that some major rivers
no longer reach the sea). In addition, excess plant nutrients fi nd their way into
waterways, and chemical pesticides affect nontarget species, often long after they
are fi rst applied. Because greater human population growth is expected in
species-rich tropical areas, increased agricultural activity will place biodiversity at
high risk.
The most far-reaching consequence of our use of fossil fuels has been an increase
in the atmospheric concentration of carbon dioxide, a greenhouse gas. As a result,
air temperature at the land surface is now 0.6 0.2˚C warmer than in pre-industrial

times, and is predicted to continue to rise by a global average of between 2.0˚C and
5.5˚C by 2100. Such changes will lead to a melting of glaciers and icecaps, sea-level
rise, and large changes to global patterns of precipitation, winds, ocean currents
and the timing and scale of storm events. The ecological consequences for biodiver-
sity and ecosystem services will be profound.

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