Cambridge International A Level Biology Revision Guide

Chapter 18: Biodiversity, classification and conservation
Pollution is a major threat to many ecosystems.
In many countries, industrial and domestic waste is
processed to reduce its impact on the environment.
For example, sewage is treated before it reaches aquatic
ecosystems, such as rivers and the sea. Much toxic
industrial waste is collected and disposed of so that it
cannot leak into the environment. However, this does not
happen everywhere with the result that many ecosystems
are polluted, often with substances that animals' bodies are
unable to metabolise or excrete. Polychlorinated biphenyls
(PCBs) were used in various manufacturing processes.
Waste from factories used to flow directly into rivers
without any form of treatment. Even though PCBs are no
longer used, the substance persists in the environment and
has entered food chains. Among its effects are weakening
of immune systems and reduction in fertility in birds and
mammals. PCBs were one of the factors contributing to
the deaths of seals in the North Sea from a viral infection.
Non-biodegradable plastic is a major marine pollutant.
Animals, such as dolphins and turtles, get caught in
discarded fishing nets and die. Turtles eat plastic bags
mistaking them for jelly fish.
Much of the world’s farmland is in low-lying land
near coasts. Fertilisers that have not been absorbed by
crop plants drain into rivers and then into the sea. The
extra nutrients that become available to river and marine
ecosystems cause growth of producers, such as algae. This
often occurs faster than herbivorous organisms, such as
fish, can feed on them to keep their growth under control.
Many of these algae produce toxic substances and their
growth often unbalances food webs. Excess growth of
algae has catastrophic effects on coral reefs and hugely
reduces biodiversity.
Pollution of the air leads to problems for aquatic
and terrestrial ecosystems. The combustion of fuel
with a high sulfur content, such as coal, leads to high
concentrations of sulfur dioxide in the atmosphere. This
reacts with water vapour to fall as acid rain. Acid rain has
destroyed vegetation and led to the acidification of aquatic
ecosystems in parts of the world downwind of highly
industrialised areas. Few animals can survive and/or breed
in waters of low pH, so the biodiversity has decreased
markedly. Many ecosystems are still at risk from acid rain.
Industrialisation and the extraction and combustion
of fossil fuels have also led to an increase in the
concentrations of carbon dioxide and methane in the
atmosphere. These are both greenhouse gases. High
emissions of methane are associated with cattle and
rice farming and the breakdown, under anaerobic
conditions, of organic waste in landfill sites. The build-up
of greenhouse gases is leading to climate change. Global
warming is likely to produce changes in the distribution
of terrestrial ecosystems. Organisms are expected to
migrate north or south to cooler latitudes and also to
higher altitudes. There will be competition between
migrating organisms and species in existing communities.
The acidification of the oceans may spell catastrophe for
coral reefs and those species, such as many molluscs, that
make their skeletons and shells from calcium carbonate.
Additionally, corals are very sensitive to temperature
increases. The algae that live inside the polyps tend to leave
the animals if the temperature remains high for a period
of time. This leads to coral turning white. Coral bleaching,
as it is called, can lead to the death of the coral. Coral reefs
are one of the most diverse ecosystems on Earth. We rely
on them to provide protection for many coastlines. A large
proportion of coral reefs have already been destroyed or
degraded by overfishing, mining and fertiliser run-off.
The rise in sea levels associated with global warming
will bring many problems for coastal ecosystems which are
some of the most productive on Earth. Some ecosystems
will become even more restricted in their range than
is currently the case and some will become even more
fragmented. For example, ecosystems associated with high
altitude will retreat higher up mountains. As the world
warms, where will they go then?
The frequency of natural catastrophes, such as
hurricanes, typhoons, severe storms and flooding is
thought to be on the increase. Following typhoons in the
Pacific, flooding increases the concentration of nutrients in
coastal waters. This encourages growth of phytoplankton
which provides food for the larvae of the crown-of-thorns
starfish, Acanthaster planci. Huge numbers of adults then
eat the coral (Figure 18.24). Eventually the numbers of
starfish decrease and the coral regrows. If these population
explosions happen every ten years or so, the coral has time
to recover; if they are more frequent than this, then it
may not.
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