Unit 2 - The Forest Ecosystem

Lesson 7: Communities


Any given ecosystem contains many populations of different species.  Together, these plant and animal populations form a  biological community.  For example, a forest of trees and undergrowth plants, inhabited by animals and rooted in soil containing bacteria and fungi, constitutes a biological community. The biological community of an ecosystem is essentially the sum total of its living elements.

It is important to realize that biological communities do not stay exactly the same over time. In fact they change in rather predictable patterns; this process of change is known as ecological succession.

Basically there are two different types of succession: primary succession and secondary succession.



Primary succession is the natural pattern of growth that begins with bare rock and ends with the formation of a climax forest.

  • This process, which spans years, begins when pioneer species, such as mosses and lichens, begin to grow on the barren terrain left behind after a lava flow or glacial retreat, for example.

  • These pioneer organisms help to build up the soil layer, which is necessary for the weeds and grasses to thrive.

  • Once weeds and grasses have been established, the shrubs begin to gain a foothold. The shrubs, in turn, give way to the early trees rapidly growing, sun-loving species such as aspen and birch.

  • As they become established, these trees will begin to produce shade on the forest floor - a condition that their seedlings cannot tolerate. Over time then, sun-loving species are replaced by shade-tolerant species. These late trees require the most soil support and are the slowest to grow, but once they are established, they will form the climax community.

  • The climax community remains relatively stable over time and is often referred to as a mature or old growth forest.

  • It is important to realize, however, that even a climax community experiences some degree of change over time, but on a relatively small scale.



Secondary succession occurs on pre-existing soil and occurs when a disturbance of some kind affects the biological community in a previously established forest.

  • Disturbances can include events such as a forest fire, a flood, climate change, or human interference. Typically the disturbance is not severe enough to kill all the pre-existing plants. Rather, the disturbance will make conditions less ideal for some species and more ideal for other species.

  • As a result, populations within the community change. Some species populations may increase, some may decrease, and some may disappear altogether.

  • The process whereby a forest recovers from a disturbance is known as secondary succession. For example, a harvested forest returning to its original state is an example of secondary succession.





Secondary ecological succession can also be the result of interactions between biotic (living) elements.

For example, the introduction of non-native (exotic) plants, animals, and microorganisms can have a huge impact on forest ecosystems. This is because the introduced species can sometimes out-compete the native species for the basic needs of life, causing the native species to decline.

In the case of introduced animals, harm may also occur if the introduced species causes disease or excessive predation of a native species. We will examine this concept further when we discuss forest health.

 

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