470 MHR • Unit 5 Population Dynamics
clumped in specific places, then members of the
population also tend to be clumped (see Figure 14.6).
For example, many types of plants tend to be
clustered in locations where the moisture,
temperature, and soil conditions are optimal
for their growth and reproduction.Figure 14.6These animals show a clumped distribution
because the lakes, rivers, and ponds they rely on are only
found in certain locations in the environment.Clumping is also typical of species in which
individuals gather into groups for protection from
predators or to increase hunting efficiency. For
example, schools of fish and flocks of birds often
form to protect individuals from predators.
Humpback whales (Megaptera novaeangliae) gather
to feed so they can make “bubble nets” around
their prey, and wolves hunt in packs to increase the
probability of catching food. Wolf packs, chimpanzee
groups, and lion prides are also important in
ensuring that young members of the population
are cared for while some adults hunt or engage in
other activities. In general, species in which the
interactions among individuals in a population can
be described as positive(that is, the interactions
draw individuals together) tend to show clumped
distribution patterns.
Finally, clumping is typical of species in which
offspring grow in close proximity to their parents
(Figure 14.7 shows one example). Some species of
plants, for example, tend to be found growing inclumps even if there is suitable habitat elsewhere,
simply because they produce seeds that cannot
travel far from the plant that produced them.Figure 14.7Aspens reproduce asexually, with new
individuals sprouting from the roots of older ones. They
therefore tend to grow in groves of genetically identical
individuals.By contrast, if the resources needed by members
of a population are evenly distributed across an
area and are in short supply, a uniform dispersion
pattern is often seen. Many species of birds and
mammals, for example, are said to be territorial.
They defend a territory that contains the shelter or
food they need for survival, mating, or raising
young. Defending this territory involves keeping
other individuals (or members of other families)
out of the area. Since the amount of the resource
(and thus the area) an individual or pair needs is
relatively constant within a species, the result is
that members of the population are more or less
evenly spread over the habitat.
Certain species of plants, for example the black
walnut tree (Juglans nigra), achieve the same result
by producing chemicals that discourage the growth
of other trees nearby. Secreted into the soil, these
chemicals ensure that an individual plant has its
own territory, containing the water or soil nutrients
it needs. Whether a territory is defended with
behaviours or chemicals, this type of interaction
among the individuals in a population can be