An ecosystem is deﬁned as a community of living organisms, together with the
physical environment they occupy at any given time. The diversity of
ecosystems is difﬁcult to estimate as ecosystems grade into one another and
large ecosystems may contain diverse smaller ones. Our planet as a whole is
an eco- system, but it contains many others: forests, deserts and oceans for
instance, which are themselves made up of smaller ecosystems, for example,
coral reefs and shallow seas within the oceans. These in turn are made up of
many yet smaller ecosystems, such as mangrove swamps, which border on and
grade into terrestrial ecosystems. Change in one ecosystem will impact on
the others with which it overlaps and into which it grades.
Species diversity refers to the number of different species in a certain
area. It is, of course, very difﬁcult to count all the species present –
some are too small, live in inaccessible places, only use the area at
certain times of day or year, or are very rare. Despite this, the number of
species present is probably the most common measure of biodiversity used by
conservationists; it is measured in different ways, but most include
weighting for numbers of individuals as well as numbers of species.
A healthy ecosystem can be buffered to some extent, mitigating change. Over
time, the composition of species may change but the ecosystem will still
function to sustain life. The change may be caused by new species moving
into the environment, by existing species increasing or decreasing, or by
evolution over time.
Of course extinctions are not new and it is estimated that the number of
species living at the present time constitute only about 1% of all species
that have ever lived. There have been both gradual and sudden changes to
ecosystems, both in terms of number and composition, over geological time.
Our early human ancestors have been part of the earth’s ecosystem for at
least the last two million years. The biodiversity of the planet has
provided for all our needs: fuels, raw materials for food, clothes and
medicines and ways of dealing with our waste. The requirements of our
species are not going to change – to survive successfully we will always
need clean water, good food, clean air and biological waste-disposal
We have already lost many natural resources over the last two hundred years.
Marine ﬁsheries are collapsing while freshwater ﬁsh are generally in
decline. These are simply two examples; virtually all natural ecosystems are
being diminished at an increasing rate.
Arable soil is a critical resource for agriculture and we are losing it much
faster than it is being formed. Forests are being felled and soils exposed.
Land is cleared and planted with monocultures that do not hold the soil
together. Water and wind loosen the soil so that it is washed and blown
away. A healthy forest acts like a sponge and holds rainwater, releasing it
slowly. By contrast, for instance, the 2010 ﬂoods in Pakistan were
exacerbated by the loss of forests at the headwaters of the Indus.
Humans have not been good at living sustainably within their local
environments, but until recently we have had a whole world to expand into
and exploit; when one geographic area became un-liveable, populations moved,
locally died out or brought resources in from neighbouring areas.
Humans have always been very good at inventing new technologies. These have
brought many beneﬁts (from early agriculture to air travel). On the other
hand, such advances in technology have always consumed more natural
resources and energy than a basic hunter gatherer would have needed. These
progressive technological developments have allowed our species to generate
increasing amounts of material wealth, and since prehistoric times these
developments have supported a gradual accompanying increase in human
All over the earth our need for natural resources to support people and
their tech nologies has pushed other species towards extinction. We take
their food, destroy their habitat, pollute and change the chemical and
physical balance of their environments. A prime example is climate change
caused, or at least exacerbated by human activity, one of the greatest
threats to ecosystems around the world.
In fact, so many species have now vanished forever that many scientists are
referring to the present epoch as “the sixth mass extinction”, on a par with
the one that eradicated the dinosaurs. We do not know how many species we
can afford to lose, nor do we know which endangered species are key to our
own survival, but we do now know many examples of habitats which have been