Environmental Engineering Reference
In-Depth Information
A far-reaching consequence of global transport and the spread of human colonists
around the world has been 'homogenization' of the biota. The same set of human
camp followers now occur in widely separate regions - sparrows, cockroaches, rats
and mice, salmonid fi shes and game animals, domestic animals and crop plants
(w it h t heir associated pests and diseases). Native species often do poorly in the face
of this set of invaders so that many parts of North America and the Southern Hemi-
sphere now refl ect a European legacy more closely than their native heritage. A
graphic example of biotic homogenization (involving fi sh, molluscs and crustaceans)
is provided at either end of a trade link between the Great Lakes of North America
and the Baltic Sea. Often spread in the ballast water of the ships that ply their trade
along this route, a third of the 170 invaders in the Great Lakes come from the Baltic
Sea and a third of the 100 invaders in the Baltic Sea come from the Great Lakes
(Millennium Ecosystem Assessment, 2005b).
If native species, endemic to a region, are lost at the expense of a common set of
invaders, local biodiversity can remain high but global biodiversity is diminished.
And invaders can have far-reaching economic as well as ecological consequences.
Table 1.1 breaks down the tens of thousands of exotic invaders in the USA into a
variety of taxonomic groups. Among these, the red fi re ant (
Solenopsis invicta
) from
South America kills lizards, snakes, ground-nesting birds and poultry; in Texas
alone, its estimated damage to wildlife, livestock and public health is $300 million
per year with a further $200 million spent on control. Large populations of zebra
mussel (
Dreissena polymorpha
) from the Caspian Sea threaten native mussels and
other animals by reducing food and oxygen availability and by physically smother-
ing them. The mussels also invade and clog water intake pipes, and millions of
dollars need to be spent clearing them from water fi ltration and hydroelectric gen-
erating plants. The yellow star thistle (
Centaurea solstitalis
) from the Mediterranean
area is a crop weed that now dominates more than 4 million hectares in California,
resulting in the total loss of once productive grassland. Rats destroy $19 billion of
stored grains nationwide per year, cause fi res (by gnawing electric wires), pollute
foodstuffs, spread diseases and prey on native species. Overall, pests of crop plants,
including weeds, insects and pathogens, are the most costly. Imported human
disease organisms, particularly HIV and infl uenza viruses, are also very expensive
to treat and result in 40,000 deaths per year (see Pimentel et al., 2000, for further
details). Ecological knowledge is needed to enable us to predict future invasions that
are likely to have damaging consequences, so that we can confront the 'invaders',
preferably before they arrive (via biosecurity precautions at national borders).
1.2.6
Overexploitation -
too much of a good
thing
The world once had many more large animals (
megafauna
). Toward the end of the
last ice age, for example, Australia was home to giant marsupials, North America
had its mammoths and giant ground sloths, and New Zealand and Madagascar were
home to giant fl ightless birds - the moas (Dinornithidae) and elephant birds (Aepy-
ornithidae), respectively. Much of this megafaunal biodiversity disappeared during
recent millennia (Figure 1.11a), but at different times in different places (Figure
1.11b). The extinctions seem to mirror patterns of human migration - the arrival in
Australia of ancestral aborigines some 50,000 years ago, the appearance of abundant
stone spear points in North America about 12,000 years ago, and the arrival of
humans around 1000 years ago in New Zealand and Madagascar. The demise of the
megafauna may have involved the effects of habitat transformation, particularly by
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