Geography Reference
In-Depth Information
harvested product and therefore there is less organic matter and
nutrients in the soil. Nutrients and organic matter are therefore
added to the system by adding fertilisers or by having crop rota-
tions with fallow years. Agricultural systems tend to have low bio-
diversity and are fairly simple. Selective herbicide, reinforced by
the use of selective fertilising or manual or mechanised weeding,
reduces diversity. Further impacts of agriculture include increases
in soil erosion rates (see Chapter 3) and leaching of pesticides and
fertilisers into watercourses causing aquatic ecosystems to be
altered.
Humans have encouraged species evolution through agricultural
processes. Normally evolution is slow. Variant individuals of a
species only give rise to a new strain under conditions that favour
their survival and allow them to maintain their variance by avoid-
ing cross-fertilisation with the 'normal' members of the same
species. This is quite rare. Also mixing two species to get a hybrid
is also quite rare. This is because of cross-fertilisation back with the
parent species dampens any variance. However, humans can create
habitats that give variants a competitive advantage and indeed
humans deliberately select plants and animals for domestication.
The selection, planting and propagation of favoured plants, their
variants and hybrids to suit human needs means that these plants
grow well in the conditions provided for them, but many of them
would not survive in the face of competition in the wild (e.g. they
have lost their thorns, hairiness, toughness and so on which is good
for human consumption but bad when it comes to surviving in the
wild). In fact now, most crop plants lack the ability to reproduce or
maintain themselves independently. Many food crops are entirely
dependent on humans for their propagation. The banana is a sterile
hybrid which produces attractive fruit but is unable to develop
seeds necessary for its own propagation. It should also be noted that
around 85 per cent of food supplied to humans today is derived
from less than 20 plant species and there are concerns that these
species may no longer be tough enough to withstand a new disease
and so we should be diversifying our food sources.
Genetic mod-
iication
is another step along the route of evolutionary change
that humans have fostered in their search for more productive food
crops. Instead of cross-breeding or selective breeding of crops,
genetic modification (often called GM) speeds up the process of
