432 MHR • Unit 5 Population Dynamics
members of different populations affect community
structure. For example, they may be interested in
why some communities are made up of many
different species, while others contain only a few.
Interactions that might influence community
structure include competition between individuals
in different populations and the relationship
between who gets eaten and who does the eating
(for example, prey and predator populations).
Environmental factors (such as how much moisture
is present or the number of hours of sunlight) also
have a strong influence on which and how many
organisms live in a community.
Communities are dynamic; they do not remain
the same. The number and types of organisms that
communities contain change over time. As the
populations in a community interact with each
other, they often change the abiotic environment
such that it becomes more suitable for other
species. These other species then gradually take
over and form a new community. This process of
successive change in species composition is called
ecological succession.
Succession can be clearly witnessed after some
type of disturbance (such as a fire) has removed
some or all of the members of an existing
community. After a fire like the one shown in
Figure 13.4, an open area emerges. The first
organisms to establish themselves in this open area
are members of species that do well in open,
disturbed habitats. Such species are referred to as
colonizers, not just because they are the first to
arrive after the disturbance, but also because their
growth and reproduction processes change thehabitat (just as the activities of the human colonists
on Easter Island did). As colonizers fill up the area,
they create shade, alter the soil, and in various
other ways make the habitat less suitable for
species like themselves and more suitable for other
species. In some cases, this process may continue
with new populations replacing old ones, until a
climax community is formed. A climax community
is a self-perpetuating community in which
populations remain stable and exist in balance
with each other and the environment, indicating
that succession has come to an end.Figure 13.4Wildfires may be destructive in the short term,
but because they occur frequently in the natural environment,
some species have adapted to them. What effects might
long-term suppression of fire have on ecosystems? What
other types of disturbances could affect succession?Figure 13.5Mount St. Helens, located in the state of
Washington, erupted violently on May 18, 1980. It sent ash
into Canada and significantly disturbed an area of over
600 km^2 around the volcano. (A) shows the mountain fourmonths after the eruption. Five years later, (B) shows various
kinds of shrubs starting to grow alongside the smaller plants
that had first colonized the bare areas.