436 MHR • Unit 5 Population Dynamics
barren-ground caribou in Canada migrate hundreds
of kilometres annually from tundra in summer to
taiga in winter. The woodland caribou are larger
and darker and may only move a few kilometres
each year, from forested mountain valleys in winter
to alpine tundra (tundra-like habitat found at the
top of a mountain) in summer.
The rangeof a population or species is defined
as the geographical area where that population or
species is found. The limit of a species’ range is
generally determined by its habitat requirements.
The species will only be found where its habitat is
present, which is determined by environmental
variables, including both abiotic factors (such as
temperature and rainfall) and biotic factors (such
as type of food).Figure 13.12This figure shows the range of the eastern
massasauga rattlesnake (Sistrurus catenatus). What are
some of the reasons why this rattlesnake is not found
outside this range?Figure 13.13The Canada warbler (Wilsonia canadensis) has
what are referred to as disjunct summer and winter ranges.
Why do you think this species is called the Canada warbler?
Why does this bird, like many others, fly such a long way to
spend the winter elsewhere? Why does it fly back to Canada
each summer?Some species, such as the caribou, have large
ranges. The species we call caribou also live in
northern Europe and Asia, where they are known
as reindeer. Other species, such as the eastern
massasauga rattlesnake, have relatively small ranges
(see Figure 13.12). Still others, including many
types of birds, live in different locations in summer
and winter and migrate long distances between
these separate ranges each year (see Figure 13.13).Ecological Niche
Members of different species can share the same
range and even the same habitat, or at least show
considerable overlap in the type of habitats they
prefer. This is possible because they have different
ecological niches. The ecological nicheof a
population is the role its members play in a
community. This includes the resources (such as
food and living space) they need and how they
interact with the other members of the community
(what they compete with, and perhaps what they
get eaten by).
Many ecologists describe a population’s habitat
as its street address, and its ecological niche as its
job in the community. It is common for two species
to share the same habitat and be able to survive
and reproduce because they have different
ecological niches. Some species, such as the giant
panda and the koala, have very narrow ecological
niches. They eat a very limited range of food types,
and are therefore restricted to a specific habitat. For
other species, such factors as a strong preference
for a particular breeding site, a low tolerance for
temperature extremes, or specific moisture
requirements may restrict their habitat and niche.
Species like this are called specialistsbecause
they have very narrow, specialized preferences
or tolerances.
Other species are very flexible in their
requirements and can be found in a wide variety of
habitats, often across large expanses of geographical
terrain. These types of organisms are referred to as
generalistsbecause of their more general (less
specific) requirements. Cockroaches, mice, rats,
and especially humans are examples of ecological
generalists.
In Investigation 13-A, you will perform a
simulation investigation to determine the
effectiveness of being a generalist or specialist
when foraging for food.
In the next section, you will explore how
communities are structured and what specific roles
individuals have within a community.