Environmental Engineering Reference
In-Depth Information
1.1.1
Homo sapiens
- just another
species?
Feces, urine and dead bodies of animals are sometimes sources of pollution in
their environments. Thus, cattle avoid grass near their waste for several weeks,
burrow-dwelling animals defecate outside their burrows, sometimes in special
latrine sites, and many birds carry away the fecal sacs of their nestlings. Humans
are not unique, either, in regarding corpses as pollutants to be removed. The 'under-
taker' caste of honeybee, for example, recognizes dead bodies and removes them
from the hive.
And just like humans, many species make profound physical changes to their
habitats. These 'ecological engineers' include beavers that build dams (changing a
stream into a pond), prairie dogs that build underground towns and freshwater
crayfi sh that clear sediment from the bed. In each case other species in the com-
munity are affected. The impact may be positive (for pond dwellers in the beaver
ponds, for species that share the prairie dog town, for insects whose gills are sensi-
tive to clogging by sediment) or negative (stream species, plants displaced by bur-
rowing, insects that feed on sediment).
Overexploitation, where individuals of a population are consumed faster than
they can replenish themselves, is also a common feature in natural ecosystems.
Sometimes overexploitation is subtle, with preferred prey species less common
in the presence of their consumers - as compared to their less tasty or harder-
to-catch counterparts. But overexploitation may be more dramatically dem-
onstrated when the disappearance of top predators (such as wolves) allows herbivores
(such as moose) to multiply to such an extent that the vegetation is virtually
destroyed. And the appalling loss of fi sh species in Lake Victoria after the arrival of
the 'invader' Nile perch provides a graphic example of overexploitation by one fi sh
of others.
Invaders have always been a fact of nature, when by chance some individuals
breach a dispersal barrier such as a mountain range or a stretch of ocean. But some
species that migrate or disperse over large distances can carry their own invaders
with them - just as humans do along transport routes. Examples include diseases
carried by dispersing fruits and seeds and migrating mammals and birds. The
animals may also have parasites and small hitchhikers in their fur and feathers.
Finally, there are species that, like farmers, increase plant nutrient concentrations
in their habitats, and even some that produce 'pesticides'. Leguminous plants have
root nodules containing symbiotic bacteria that fi x atmospheric nitrogen into a form
readily available to plants. The soil in their vicinity, and the water draining into
neighboring streams, are both likely to contain higher concentrations of nitrate. And
certain plants produce chemicals (allelochemicals) whose function appears to be the
inhibition of growth of neighboring plants, giving the producer a competitive
advantage.
So humans are hardly unique in their ecological impacts. When population
density was low, and before the advent of our ability to harness nonfood energy,
human populations probably had no greater impact than many other species that
shared our habitats. But now the scale of human effects is proportional to our huge
numbers and the advanced technologies we employ.
1.1.2
Human
population density
and technology
underlie
environmental impact
The expanding human population (Figure 1.1) is the primary cause of a wide variety
of environmental problems. Someone has calculated that the total mass of humans
is now about 100 million tonnes, in comparison to a paltry 10 million tonnes for all
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