Biology 12

Chapter 13 Ecological Principles • MHR 431

Organizing Interactions

Ecology as a whole is a very large area of study.
Ecologists thus often specialize, focussing their
research efforts on specific types of interactions.
For the most part, these specialties relate to four
increasingly general categories, which ecologists
use to organize their thinking — individual
organism, population, community, and ecosystem
(see Figure 13.2).
For example, many ecologists study how
individual organisms interact with the abiotic
components of their environment. These
researchers are interested in how the behaviour or
physical features of an organism allow it to cope
with such things as the temperature and moisture
conditions typically found in its environment. This
understanding helps explain why some types of
organisms are only found in specific areas (that
is, it helps us to understand the distribution of
species), or why they are more abundant in some
places than in others.

Populations

As you saw in Chapter 11, a populationis a group
of individuals of the same species living in the
same geographic area. Figure 13.3 shows a
population of monarch butterflies. The size of the
geographical area defining the population can vary.
The geographical area can depend on how fast or
how far an organism can travel, or on how the
abiotic conditions vary from place to place.
Differing conditions may explain why different
populations of the same species display variations
in behaviour, physiology, and physical features.
The variations evolve over time, as populations
become adapted to their local environments.
Population ecologists focus mainly on factors
that affect the size and composition of populations.
They are interested in what causes populations to

grow or decline, the rate at which populations

change in size, and what determines the relative

numbers of males and females (or young and old)

in populations.

Figure 13.3Each monarch butterfly is part of a population.

How would you define this population — what are its edges

or boundaries?

Communities

In nature, populations are rarely isolated. Instead,

they interact with each other to form the next level

of organization: a community. A community

consists of all the organisms in all the interacting

populations in a given area. Community ecologists

typically study how interactions among the

organism population community ecosystem

Figure 13.2The study of ecology occurs at several levels, from individual
organisms, to populations and communities, to ecosystems.