Environmental Engineering Reference
In-Depth Information
Box 7.3
Mass Killings: Bison and Elephants
Bison were once present from the Atlantic coast to California and from northern parts
of Mexico to northern Saskatchewan and Alberta, their numbers kept well below the
carrying capacity maximum by native hunting. As the imported epidemics reduced the
native populations, bison numbers rose after 1700, and the continental total reached
as many as 50 million animals during the late eighteenth century. A hundred years later,
only a few hundred buffaloes escaped the mass slaughter that began during the 1840s
and culminated in the 1870s (Branch 1929; Isenberg 2000). Even if we assume, rather
conservatively, an average body mass of 500 kg/animal, the killing of some 40 to 50
million animals during the nineteenth century would have amounted to 20-25 Mt of
(fresh-weight) zoomass.
The maximum carrying capacity of African elephants was about 27 million animals
at the beginning of the nineteenth century, and the actual number of elephants might
have been as high as 75% of that value (Milner-Gulland and Beddington 1993). In
contrast, the latest continent-wide summary put their dei nite number at about 470,000
in 2006, with another 160,000 animal in probable and possible categories and 50,000
in a speculative class (Blanc et al. 2007). The best available reconstructions of the ivory
trade (Parker 1979; Luxmoore et al. 1989) show a relatively steady l ow of around
100 t/year until about 1860, followed by a rise to more than 500 t/year just after 1900;
the rapid plunge induced by World War I was followed by a brief rise, then another
(World War II) slump, but the annual rate then rose steadily, to peak at more than
900 t by the late 1980s.
Integration of these l uctuating harvests yields about 55,000 t of ivory harvested
during the nineteenth century and at least 40,000 t removed during the twentieth
century. Assuming an average of 1.8 tusks per elephant is an easy part of the conver-
sion, but selecting an average weight of tusks is associated with a much larger error.
The largest and the most sought-after tusks used to weigh commonly more than 50 kg
(102 kg is the record), but with the progressing slaughter, the average mass of tusks
has been declining (Milner-Gulland and Mace 1991). Assuming, conservatively, 7 kg
for the nineteenth century and 6 kg for the twentieth century results in cumulative
slaughters of, respectively, about 14 and 12 million elephants. Assuming further
that average body mass is 75% of the adult female weight of about 3.5 t brings the
twentieth-century total of slaughtered elephant zoomass to around 35 Mt of live
weight.
