Geography Reference
In-Depth Information
Humans have disturbed the phosphorus and nitrogen cycles not
only by developing fertilisers but also by changing the properties of
soils, accelerating soil erosion and also impacting the ability of
phosphorus and nitrogen to become available to plants in soil
water. Furthermore, humans have also produced more nitrous
oxide from industrial activity and vehicle emissions. This has
resulted in enhanced nitrogen deposition from the atmosphere in
rainwater, for example.
Box 5.2 Bioaccumulation
Through the cycling processes of the ecosystem there may be locations
where some chemicals become concentrated and are available for
uptake by organisms which are not desirable. When producers and con-
sumers take up nutrients they may also take up other materials. As a
result, toxins may accumulate in specific parts of an ecosystem, usually
in the higher levels of food chains. For example, a contaminant such as
mercury can build up in the sediment of the seabed and then be taken
up by mussels. Each mussel may only contain a small amount of
mercury. However, as small fish eat lots of mussels the mercury
becomes more concentrated within the small fish. As predators eat the
small fish then the mercury can accumulate to high levels within them.
This can have health consequences for humans who may eat the larger
fish such as tuna and swordfish.
Ecosystems are dynamic and are constantly adjusting to chang-
ing environmental conditions or disturbance.
Succession
occurs
when older groups of plants and animals are replaced by more
complex groups. Primary succession may begin on a bare rock or
disturbed site. Lichens, mosses and ferns are often part of the early
colonisers. Through inputs of organic matter and continued
weathering and/or shelter, these plants help alter the site con-
ditions and thereby make them more suitable for other plants and
animals to colonise. These secondary colonisers in turn build up
the complexity of the site and alter conditions further and so the
ecosystem changes through time. Succession on ponds and lakes
can result in the eventual infilling of the lake by organic matter
and other debris.
