Botkin Environmental Science Earth as Living Planet 8th txtbk

264 CHAPTER 13 Wildlife, Fisheries, and Endangered Species
A below-average 2009 production year in Canada
with 22 fledged chicks from 62 nests was half the
production of the previous summer and is expected to
result in a break-even year for the AWBP. Threats to
the flock including land and water development in
Texas, the spread of black mangrove on the wintering
grounds, and wind farm construction in the migration
corridor all remained unabated in 2009. 7
Even with this limitation, this method provides invaluable
information. Unfortunately, at present, mathematical
estimates of the probability of extinction have
been done for just a handful of species. The good news is
that the wild whooping cranes on the main flyway have
continued to increase and in 2008 numbered 274. The
total wild population (all flyways) was 382, and there were
162 in captive flocks, for a total of 534. 8, 5
Age Structure as Useful Information
An additional key to successful wildlife management is
monitoring of the population’s age structure (see Chapter
4), which can provide many different kinds of information.
For example, the age structures of the catch of salmon from
the Columbia River in Washington for two different periods,
1941–1943 and 1961–1963, were quite different. In
the first period, most of the catch (60%) consisted of fouryear-olds;
the three-year-olds and 5-year-olds each made up
about 15% of the population. Twenty years later, in 1961
and 1962, half the catch consisted of 3-year-olds, the number
of 5-year-olds had declined to about 8%, and the total
catch had declined considerably. During the period 1941–
1943, 1.9 million fish were caught. During the second period,
1961–1963, the total catch dropped to 849,000, just
49% of the total caught in the earlier period. The shift in
catch toward younger ages, along with an overall decline in
catch, suggests that the fish were being exploited to a point
at which they were not reaching older ages. Such a shift in
the age structure of a harvested population is an early sign
of overexploitation and of a need to alter allowable catches.
Harvests as an Estimate of Numbers
Another way to estimate animal populations is to use
the number harvested. Records of the number of buffalo
killed were neither organized nor all that well kept,
but they were sufficient to give us some idea of the number
taken. In 1870, about 2 million buffalo were killed.
In 1872, one company in Dodge City, Kansas, handled
200,000 hides. Estimates based on the sum of reports
from such companies, together with guesses at how many
Migratory
route of
whooping crane
(a)
240
(b)
FIGURE 13.4 The whooping crane
(a) is one of many species that appear
to have always been rare. Rarity does
not necessarily lead to extinction, but
a rare species, especially one that has
undergone a rapid and large decrease
in abundance, needs careful attention
and assessment as to threatened or
endangered status; (b) migration route;
and (c) change in population from
1940 to 2000.
Number of whooping cranes
200
160
120
80
40
0
1940
1950 1960 1970 1980 1990
Year
2000 (c)