Geography Reference
In-Depth Information
accidental (rats on ships colonising islands being visited) or deliberate
(introduction of new crops to an area), and the evolution of species
through domestication of crops and animals, soil erosion and envir-
onmental pollution. Many ecosystems are complex and it is normal
for complex systems to be more robust than simple systems when
faced with change. Given the vast changes in climate experienced
over the past 2.4 million years alone (see Chapter 2), it seems that
the biosphere is able to accommodate vast fluctuations in climatic
and environmental conditions and changes in the numbers and
density of the species which it supports. However, humans have a
large capacity to alter the ability of the ecosystem to cope with such
stresses. Furthermore, if the system is highly interconnected then the
removal of one key species could have major impacts. Species that
are highly connected with the rest of the food web are those whose
elimination is likely to be the worst for the ecosystem. These are
known as
keystone species
. Prairie dogs are keystone species in
temperate grasslands, which act as grazers, predators, prey and
provide habitats for other species through burrowing.
Biodiversity
Biodiversity
has various measures but essentially it is a term that
describes the number and variety of species within an ecosystem.
Global areas of high biodiversity usually result from lack of distur-
bance and lack of isolation. Regional and local patterns may result
from short-term disturbances (fires which maintain overall ecosys-
tem diversity) or habitat diversity. There is concern that human
activities, including accelerated climate change, are causing a
decline in the number of species. While 1.5 million species are cur-
rently known there may be twice this number yet to be discovered.
Since the tropical forests hold around half of the world's species,
their deforestation is of major concern. Over the past 400 years
around 500 plants and 600 animals have become extinct that we
know of, mainly due to human action. Natural extinction rates are
normally around one mammal per 400 years and so the current rate
of 36 mammals in the last 400 years far exceeds this. There may be
many undiscovered species that are being driven to extinction as
you read this. That is a cause for concern not only because it
suggests we are damaging the ecosystems around us but also
