Biology 12

Chapter 13 Ecological Principles • MHR 433

The combination of species making up a climax
community varies widely from one area to another,
limited in part by climate and other abiotic
features. The species living in a climax community
tend to dominate the area — until the next
disturbance occurs. In reality, most communities
are routinely disturbed at various stages of
succession and never achieve what might be
considered a typical “climax” for the area. Because
the size, frequency, and severity of disturbances
vary, many communities actually consist of a
mosaic of patches. These patches are at different
stages of succession, and the particular species that
replace others in a particular area may differ from
one successional series to the next. Therefore, it is
often difficult to predict the future composition of
species in a particular community.
The process described above is known as
secondary succession, and is the redevelopment of
a previously existing community after a disturbance
(see Figure 13.5). Since disturbances are common
(over an area as large as Ontario, for example,
many disturbance events occur in various locations
each year), secondary succession happens
repeatedly. Secondary succession can always be
found occurring somewhere.
Although less common, succession can involve
the development of a first-time community in an
area. This process, called primary succession, occurs
on newly formed volcanic islands, as shown in
Figure 13.6, or in areas left bare by retreating
glaciers. In these situations, soil has not yet formed
and the only organisms present may be prokaryotes
(bacteria and archebacteria). The first eukaryotic
species (protists, fungi, plants, or animals) to arrive

Figure 13.6When volcanoes erupt beneath the floor of an
ocean, the result is often the formation of an island. Over
time, the island may be colonized by organisms that arrive
accidentally.

in these new areas are lichens, which are carried in

by the wind and can grow on bare rock (see Figure

13.7). Their growth begins the breakdown of the

rock, eventually leading to the production of soil.

Figure 13.7Several species of lichen have formed a

community on this rock. What other types of organisms

might be living on the rock as part of this community?

Organisms (such as lichens) that are the very

first to arrive in a barren landscape, and whose

activity changes the landscape dramatically, are

referred to as pioneer species. Primary succession

is generally a much slower process than secondary

succession. Producing enough soil to support the

first grasses, for example, can take up to 1000 years,

depending upon abiotic conditions. Once soil is

present, small plants are able to colonize the area.

The process of succession then continues with

other larger plants, animals, and other organisms

arriving and replacing the pioneers. Later these

are replaced in their turn, until either a climax

community is reached or a disturbance occurs and

the process starts all over again.

Ecosystems

A community of living organisms, together with

the abiotic factors that surround and affect it, is

called an ecosystem. An ecosystem includes all of

the non-living parts of the environment and the

living organisms in a particular area, as well as the

interactions among them.

Although we often think of ecosystems as being

quite large, they can be very small. In fact, the size

of an ecosystem can depend on what you are

studying. There are small ecosystems within large

ones, which are within even larger ones. For