Chapter 14 Population Ecology • MHR 495The fourth column of Table 14.2 gives
age-specific fecundity — the average number of
offspring born to females of that age. For this grey
squirrel population, fecundity is highest at ages
three and four. Although some life tables record the
average number of offspring for both males and
females, most concentrate only on females (since in
many species males play no role in parental care
and it is thus difficult to determine how many
offspring belong to them). Some ecologists prefer to
include only data on survival and mortality in life
tables. They refer to tables with data on the
numbers of births as fecundity or fertility tables.
Others, however, include both types of data in the
same table. To make comparisons among
populations easier, the size of the cohort is often
standardized to 100 or 1000, regardless of how
many individuals were studied.http://www.mcgrawhill.ca/links/biology12
To find current data on the status of introduced species (such
as the zebra mussel) and other invaders (like the West Nile
virus), visit the web site above, and click on Web Links.WEB LINK
damage by invading and clogging water intake pipes
at water filtration and electrical generating plants (see the
photographs on pages 464 and 465).Nearly all the crops and livestock found over thousands
of square kilometres of Canadian farmland are alien
organisms, brought here originally by immigrants. Wheat
replaced indigenous grasses on the prairies and cattle
replaced the bison.Challenges Ahead
The patterns of plants, animals, micro-organisms, and
diseases that have shaped the North American continent
since the last ice age are changing. As invading species
spread and transform the landscape, other changes
caused by humans threaten to encourage the population
growth of a few species while threatening the survival of
many. A warming climate may give some introduced
insect pests and weedy plants a greater survival rate over
winter. The escape of genetically modified organisms,
farmed fish, and other exotic species will add further
pressures to indigenous organisms and ecosystems.The natural world of tomorrow may mirror the gradual
narrowing of the cultural diversity being experienced in
human society. Some people maintain that large
multinational companies are contributing to the increasing
homogeneity of products found in cities around the
globe. So, too, a relatively few hardy, invasive species
with high population growth rates may, by the end of the
twenty-first century, replace the rich variety of organisms
still living today.Follow-up
1.Fish farms on Canada’s west coast have been raising
increasing numbers of Atlantic salmon, which originated
from the east coast. How do Atlantic salmon differ
from Pacific salmon? What threats are posed by the
potential release of Atlantic fish into Pacific waters?
2.Carry out a case study of an invasive alien species
living in your area or another part of Canada.
Document its origin and the problems caused by
its introduction. What can be done to reduce or
eliminate these problems?
3.In 2000, Canadian authorities tested many dead birds
for the presence of West Nile virus. First detected in
the United States in 1999, it was feared that this virus
would soon spread to Canada. It was not until
August 2001 that this virus was first discovered in
Ontario, and by November of that year there were
125 birds in which the presence of the virus had
been confirmed. Although a virus is not typically
considered a living organism (and therefore might not
be described as an “introduced species” by some),
this pattern of rapid spread is typical of many species
introduced into an area. Do research to learn more
about the West Nile virus. What is its effect on
humans? How did it arrive in North America and why
is it able to spread so quickly? Where in Canada is it
currently found?