Just as the people living in your neighborhood can come
and go, ecological communities change over time. One way a community
can change is if external conditions shift. If the weather in a
certain geographical area suddenly gets colder, certain populations
will be better off and will thrive, while others will shrink and
However, change in communities is not always caused by
external factors: populations can change environments simply by
living in them. The success of a particular population in a particular
area will change the environment to the advantage of other populations.
In fact, the originally successful population often changes
the environment to its own detriment. In this way, the populations
within a community change over time, often in predictable ways.
The change in a community caused by the effects of the populations
within it is called ecological succession.
The first population to move into a geographical area
is referred to as a pioneer organism. If this pioneer
population is successful in its new location, it will change the
environment in such a way that new populations can move in. As populations
are replaced, changing plant forms bring with them different types
of animals. Typically, as a community moves through the stages of
succession, it is characterized by an increase in total biomass,
a greater capacity to retain nutrients within the system, increasing
species diversity, and increasing size and life spans of organisms.
Eventually, the community will reach a point where the mixture of
populations creates no new changes in the environment. At this point,
the specific populations in the stable community are said to make
up a climax community. While individuals within a climax
community will come and go, the essential makeup of the populations
within the climax community will stay constant.
Which species are dominant in a particular climax community
is determined by unique factors of that geographical area, such
as temperature, rainfall, and soil acidity. Since a climax community
does not change the environment, it also does not affect its own
dominance; a climax community will remain dominant unless destroyed
by a significant change in climate or some catastrophic event such
as a fire or volcanic eruption.
Succession in Action
Imagine a catastrophic event: a forest fire rages through
the Green Mountains of Vermont. The fires burn everything and leave
behind a barren, rocky expanse.
The population of trees that once lived in this area can’t
grow back because the fire has changed the ground composition. Without
tree roots to act as anchors, rain washes away the soil and the
ground becomes rocky and barren. This rocky ground, however, proves ideal
to lichens, the pioneer population. The lichens colonize the rocks
and thrive. As part of their life process, lichens produce acids
that break down rock into soil. Lichens need solid places to survive:
they are victims of their own success. Mosses and herbs are well suited
to living in the shallow soil environment created by the lichen,
and they replace the lichen as the dominant population.
The mosses and herbs continue to build up the soil. As
the soil deepens, the conditions favor plants with longer roots,
such as grasses. Eventually the land becomes suitable for shrubs
and then for trees. The early dominant trees in the community will
be species like poplar, which thrive in bright, sunlit conditions.
As more trees grow in the area, however, there is less sunlight,
and maples, which grow in shade, supplant the sun-starved poplars. The
maples eventually dominate the community, because they don’t change
the soil composition and thrive in their own shade. The community
has reached its climax community, with maple as the dominant
species. Don’t forget that during all this, the changing vegetation has
brought with it various changes in animal populations.
The SAT II Biology is most likely to test your knowledge
of ecological succession in an originally rocky area, as we just
covered, or in a pond. Succession in a pond follows a similar pattern.
Originally, the pond will contain protozoa, some small fish, and
algae. As individual organisms die and water runs into the pond,
sediment builds up at the bottom and the pond grows shallower. The
shallower pond becomes marshlike and fills with reeds and cattails.
The standing water eventually disappears, and the land is merely
moist: grasses and shrubs dominate. As the land grows even less
moist, it becomes woodland. And as trees come to dominate, the climax
community will arise from a species that can grow in the shade of
Ecological Succession vs. Evolution
For the SAT II Biology, do not get confused between ecological
succession and evolution. In ecological succession, the populations
that make up a community change, but the characteristics of the
individuals within the population will not change over time. Ecological succession
is something that happens to communities, while evolution happens
to populations. Although succession has different rates, it is much
faster overall than evolution.