Environmental Engineering Reference
In-Depth Information
(a)
(b)
100
100
50
Australia
80
0
100,000
10,000
1000
100
100
60
50
North America
40
0
100,000
10,000
1000
100
100
20
50
Madagascar - New Zealand
0
0
0.01 - 5
5 -100
100 -1000
1000+
100,000
10,000
1000
100
Body mass range (kg)
Years ago
Fig. 1.11
(a) The percentage of genera of large mammalian herbivores that became extinct in the last 130,000 years is
strongly related to size (dat a for North and South America, Europe and Australia combined. (After Owen-Smith, 1987.)
(b) Percentage survival of large animals on two continents and two islands (New Zealand and Madagascar) during the past
100,000 years or so. Signifi cant declines in numbers of large animals (mammals, reptiles, birds) occurred at different times
in different places, mirroring historical evidence about the arrival of effi cient human hunters. (After Martin, 1984.)
fi re, and of diseases introduced by humans and their camp followers. But it seems
likely that the prime cause was the arrival of effi cient human hunters who targeted
the largest and most highly profi table prey. Big animals tend to reproduce at a slow
rate, making them particularly vulnerable to overexploitation and a downward
spiral to extinction.
Prehistoric megafaunal extinctions were particularly dramatic, but in modern
times a host of less conspicuous species have been driven by harvesters to the
brink of extinction or, at least, to such low numbers that it is no longer profi table
to hunt them. The most commonly overexploited species are marine fi sh and inver-
tebrates (e.g. lobsters and shellfi sh) as well as trees and terrestrial animals hunted
for meat.
Three quarters of the world's industrial fi sheries are considered to be fully (50%)
or overexploited (25%) (Millennium Ecosystem Assessment, 1995b). And there is a
parallel here with the prehistoric extinctions - because species with lower reproduc-
tive rates are the most susceptible. Thus, overexploitation of large tuna species is a
recognized problem, whereas smaller fi sh continue to thrive. One consequence of
the size-vulnerability relationship, repeatedly observed around the world, is that
the mean size of fi sh taken for human consumption has been declining. Note that
it is not just harvesting for food that causes problems - overexploitation may involve
plants that provide timber or medicinal products, or live animals and plants col-
lected for the pet and garden trades. The effective regulation of harvesting effort is
a diffi cult business, depending both on a thorough ecological understanding of the
dynamics of exploited populations and an ability to regulate the behavior of
harvesters.
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