Module 8 Lesson 8 - 2

Succession

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Read pages 725 - 728

The changes a community goes through as it ages are known as succession. For every biome, or micro-climate within a biome, a predictable succession of plant and animal species fulfill their roles and then disappear. The process of succession begins with the first pioneer species and continues to the final stable climax species that represents the completed succession. Communities are named by their climax species, such as aspen forest or black spruce forest. Succession is of two types: primary and secondary.

How does primary succession differ from secondary succession?

When a natural disaster such as a rock slide or volcanic eruption destroys all life in the area, the community must start from rocks. This is called a primary succession. Lichen and mosses colonize the rock surfaces and break the rock slowly into finer soil particles. When enough soil can sustain a community, the larger species of plants start to grow. Over time, new species of plants out-compete the initial group of plants for light and space. Due to changing habitat and environment, species that are suited better for the conditions survive, and a stable community forms.

When a natural disaster such as fire or human disturbance such as deforestation disrupts a community partially, a secondary succession starts from the pre-existing soil layer. Secondary succession occurs faster than primary succession does because the process of lengthy soil formation is not required. The first set of pioneer community of small plants appears, and they are replaced subsequently by a larger community of plants, eventually stabilizing into a forest community.

Comparison of Primary Succession and Secondary Succession

	Starting Point (soil or bare rock)	Possible Causes (ecological disturbance)	Pioneer Species
Primary Succession	From rock	Avalanche, rock slide, severe storms, change of sea level, volcanic activity	Lichen growing on rock begins the soil-building process.(Freeze-thaw cycles break off bits of rock to form the first soils.)
Secondary Succession	From soil	Abandoned farmsteads or cropland, clear-cut logging, deforestation	Depends on conditions: In sunny hot areas: grass. In cooler, shady, sandy areas: moss

Why does succession occur?

Succession occurs because species that inhabit the area of disturbance change the habitat and available resources. In the beginning, the species that can tolerate the harsh conditions of barren rocks or bare soil can survive. After the initial community makes the area more hospitable, other plants less tolerant to harsh conditions can invade the area. Sometimes, the new species can out-compete the original species.

Initially, a very small number of species form a pioneer community. As the environment becomes mature, the number of species increases. However, after the climax community stabilizes in the area, the superior species crowd others and plant diversity may decline. Generally, communities evolve to have a greater biomass and biodiversity during succession.

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Watch the following Biologix: Succession and Climax Communities video. It is a summary of the differences between primary and secondary succession.

Self-Check

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Alberta has significant forestation. Every year has a forest fire hazard, and often fires burn out of control. To review the role of fire in the ecosystem and how it affects the succession of communities, try the following self-check.

Fire is a force of nature that has positive and negative roles in a healthy ecological community. In nature, the source of fire is lightning. Research the following questions regarding the role of fire. 

1. What positive and negative roles do forest fires have in nature?


2. What are the advantages and disadvantages of the practice of 'controlled burns' in National Parks? If fire terminates an existing succession and causes secondary succession to restart, is a controlled burn interfering with natural processes?


3. Why, in the past, did First Nations people in Alberta purposely set prairie fires in the spring to begin a secondary succession? (Remember the primary food source.)


4. What are some traditional uses by First Nations peoples of fireweed, which is one of the first species to colonize burned forest?



5. If producer species go through successions, consumer species must as well. As the food source changes, conditions favour the invasion of various animal species. For example, after a fire, the pioneer species of grasses favours the presence of seed eaters such as mice and gophers. The lack of cover suits them in that they can burrow to hide from overhead predators such as hawks. When the soil is too cool and shady to germinate grasses, shrubs start to appear, and rabbits tend to replace mice and gophers.

   Continue this central Alberta plant animal succession on the table below. You may need to do some research on Alberta food chains.

   
   

   Producer succession	Consumer succession
   Pioneer species: grass	Mice, gophers, coyotes
   Low shrubs (berries)
   High shrubs
   Aspen poplar trees

   
   



6. How does primary succession affect genetic biodiversity of communities?
   

   1. What is the effect of time on biodiversity in a  succession? Why does this occur?
      
      
   2. Why is the number of species that can survive at the beginning of the succession so low?
      
      
   3. Why does the number of species level towards the end of the succession?
      
      
   4. How many years were required to complete this succession?