1. Ecological Succession
Biology 11(D)

2. Ecological Succession
Learning Objectives
Describe how events and processes that occur during ecological succession can change populations and species diversity
After this lesson you will be able to describe how events and processes that occur during ecological succession can change populations and species diversity.

3. Ecological Succession
Population – individuals of a given species in a particular area
Community – all the different populations in an area
Pioneer community – first inhabitants in new community
Climax community – established community
Ecological succession – orderly change in makeup of a community over time
Pioneer  Climax
A population is a group of individuals of a given species in a particular area.
A community includes all the different populations in an area. When studying ecological succession, there are two types of communities that need to be considered. A pioneer community includes the first inhabitants of a new community. For example, a pioneer community in a newly formed bog or pond will include algae and certain types of bacteria.
A climax community is an established community that undergoes little change. For example, an oak and hickory forest community includes large trees and many different types of animals.
Ecological succession is an orderly change in the makeup of a community over time. In succession, a climax community gradually replaces a pioneer community.

4. Pioneer vs. Climax Community
Pioneer Community
Harsh environment
Biomass increasing
Energy consumption inefficient
Some nutrient loss
Low species diversity
Fluctuations common
Climax Community
Favorable environment
Biomass stable
Energy consumption efficient
Nutrient cycling
High species diversity
Fluctuations uncommon
A pioneer community usually resides in a harsh environment compared to a climax community’s favorable environment. For instance, a new community succeeding on barren farmland must tolerate dry, unshaded conditions. In contrast, a climax community may contain an abundance of shade trees and moist, rich soil.
A pioneer community’s biomass is increasing as new organisms inhabit the area. The biomass of a climax community remains relatively stable since there is little change in the populations within the established community.
The energy consumption and nutrient cycling in a pioneer community is inefficient because the food chains are not fully established. The flow of energy of must be complete and efficient for successful nutrient cycling to occur. Climax communities involve efficient energy consumption and nutrient cycling due to the high species diversity.
Fluctuations in species and populations are common in pioneer communities, as new “pioneers” move into the community. In contrast, fluctuations are uncommon in climax communities due their state of equilibrium.

5. Ecological Succession
Ecological succession – land and water
Two types:
Primary succession – newly formed area
Secondary succession – area that has been disturbed
How do events and processes that occur during succession change populations and species diversity?
Ecological succession may occur on land and in water. There are two types of ecological succession.
Primary succession occurs in newly formed areas where no organisms existed before. For example, a new pond or bog may be created after a period of heavy rain. Also, a new rock formation may be created after a volcanic eruption or shifts in ground structure.
Secondary succession occurs in an area that once inhabited organisms, but a disturbance has destroyed the community. Natural disasters and disturbances caused by man can lead to secondary succession.
How do events and processes that occur during succession change populations and species diversity? Studying examples of succession will provide answers to this question.

6. Primary Succession in Pond
Primary succession in newly formed pond:
No organisms
Algae and bacteria
Heterotrophic protists and small invertebrates
Floating plants (pondweed)
Larger plants (cattails) grow around edges
Here are the stages of succession, or change, in a newly formed pond. A flood creates a pond. No organisms are in the pond at first. Pioneer species, such as algae and certain types of bacteria, invade the pond and reproduce. Heterotrophic protists and small invertebrates follow the “pioneers”, because now they have sources of nutrients.
Rooted floating plants, such as pondweed, become established. As the pond fills in due to sediment and organic matter, larger plants, such as cattails, grow around edges.

7. Primary Succession in Pond
Primary succession in newly formed pond:
Larger animals
Becomes marsh, fills in
Shrubs and trees
Results:
Pioneer  Climax
Aquatic  Terrestrial
Increased species diversity
Complex food chains
Larger animals, such as water birds and amphibians, move into community because of the favorable changes. Increased plant diversity leads to increased animal diversity due to the vast array of food sources. The pond community may become a marsh community with the increase of plant populations. The marsh may eventually fill in completely, allowing shrubs and trees to grow.
The results of primary succession include the establishment of a climax community from a pioneer community. In this case, a terrestrial ecosystem replaced an aquatic one. This new terrestrial ecosystem has increased species diversity and complex food chains.

8. Primary Succession on Rocks
Cooled lava rocks form after volcanic eruption
Lichens and mosses appear, dissolve rock, form soil
Dead lichens add organic matter
Small animals
These are the stages of primary succession on cooled lava rocks that formed after a volcanic eruption. Lichens and mosses are the first pioneer species to colonize rocks. Lichens excrete acids that dissolve rock. This dissolved rock forms soil. Dead lichens add organic matter to the developing soil. This richer soil creates a favorable environment for small animals, such as insects.

9. Primary Succession on Rocks
More diverse plants
More diverse, larger animals
Results:
Pioneer  Climax
Barren rock  Terrestrial ecosystem
More diverse species and populations
As the nutrient content of soil increases, more plants grow. The soil can now sustain plants with large root systems. More diverse plant populations result in more diverse animal populations due to the increased variety of food sources.
The results of this primary succession on rocks include the establishment of a climax community from a pioneer community. A barren rock now becomes a terrestrial ecosystem that can sustain diverse species and populations of organisms.

10. Secondary Succession
Secondary succession – sequence of changes after a climax community has been destroyed
Natural disasters
Forest fire
Flood
Secondary succession occurs after a climax community has been destroyed. Natural disasters, such as forest fires and floods, can destroy a community. The picture shows a pine forest community immediately after a forest fire, then a few months later. Weeds have begun to establish themselves as a pioneer species. Succession has begun.

11. Secondary Succession
Secondary succession – sequence of changes after a climax community has been destroyed
Destruction by man
Clearing land for construction or harvest
Abandoned farmland
Secondary succession can also occur after a community has been destroyed by man. The clearing of land for construction or harvest and abandoned farmland can lead to secondary succession.

12. Secondary Succession on Farmland
Secondary succession on abandoned farmland:
Soil is sunlit and dry
Weeds are pioneers species
Here are the stages of secondary succession on abandoned farmland. This photo shows land that was once a forest, but was cleared for farmland. Now the farmland has been abandoned. Due to the lack of shade, the soil in the abandoned field was sunlit and dry.
Weeds, such as crabgrass, can grow in bright, dry conditions. These weeds are the pioneer species of this community.

13. Secondary Succession on Farmland
Secondary succession on abandoned farmland:
Weeds die, adding nutrients to soil
More plant species
Soil shaded by grasses and shrubs
Pine seedlings
Pine forest community (climax)
Results:
Abandoned farmland  Pine forest
More diverse species and populations
Generations of weeds eventually died, adding nutrients to soil. The ground can now support other species of plants, such as grasses and shrubs. These new plant species now shade the soil. This shaded environment is favorable for pine seedlings to sprout up in the field. After about 50 years, a pine forest community now flourishes as the climax community.
The results of this secondary succession on abandoned farmland include more diverse species and populations due to the favorable habitat of the climax community.

14. Ecological Succession
Summary of Ecological Succession
Species, populations, and communities change over time
Unfavorable environment  Favorable environment
Few food sources  Many food sources
Few species  Many species  Diverse populations
Pioneer community  Climax community
Here is a summary of ecological succession. Just as individual organisms grow and change with time, so do species, populations, and communities. Populations in a particular area change the environment by their very presence. A harsh, unfavorable environment may now become a favorable environment to more populations.
Due to the increasingly favorable environment, there is an increase in species diversity which leads to an increase in food sources. This results in nutrient cycling through complex food chains involving many different populations of organisms. The once harsh, pioneer community becomes a thriving climax community.

15. Ecological Succession
Learning Objectives
Describe how events and processes that occur during ecological succession can change populations and species diversity
You should now be able to describe how events and processes that occur during ecological succession can change populations and species diversity.