Biology 100/101
Lecture 3: Ecosystems in Time
(Print Version)

Announcements &

Lecture Objectives

Community Change




Disturbance Regimes


Lecture Syllabus

IB 100/101 Home Page


Text readings in Biology: Concepts and Investigations, 1st edition, by Marielle Hoefnagels

Chapter 40 (Communities and Ecosystems), Pages 809-811

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Ecosystems in Time

The content of today's lecture will help you answer question #3 on this assignment:


Note that exam questions and written assignments will be based on the learner objectives included in this lecture outline. Not all the questions provided at the chapter ends in the text or on the text web pages may be appropriate study aids. Use those that reflect the lecture objectives.

After studying this material you should be able to:

  1. Describe the effects of disturbance, or lack there of, in (natural and managed) ecosystems and explain its relationship to the process of biological succession.

  2. Define the term 'invasive species', give an example of an invasive plant or animal and describe its impact on an ecosystem.

  3. Distinguish between the terms "primary succession"and "secondary succession" and describe some examples of each.

  4. Distinguish between the terms "soil" and "mineral substrate".

  5. Describe how pioneer species in primary and secondary succession change nonliving components of an ecosystem (temperature, light, moisture, humidity, mineral substrate, etc.) during the early stages of succession.

  6. Give an example of a biological community that is maintained in an early stage of succession by repeated disturbance and describe the disturbance regime and its effect on the community.

Key Terms:

succession climax community pioneer species
disturbance primary succession secondary succession
disturbance regime
soil formation
prescribed burning organic matter mineral substrate

Disturbances and Community Change

Community change is initiated by disturbances of all scales:

Ecological Succession - Overview

From the Latin, succedere, to follow after

"Change in the species composition of a community over time." (Hoefnagels glossary, pg G-21)

  • Primary Succession follows the formation of new land surfaces consisting of rock, lava, volcanic ash, sand, clay, or some other exclusively mineral substrate.

    • This means that there is NO SOIL present.

    • Soil is a mixture of mineral material, decaying organic material, and living organisms.

  • Secondary Succession follows the destruction or partial destruction of the vegetation of an area by some sort of disturbance, like a fire, windstorm, or flood that leaves the soil intact.

  • Pioneer species initiate recovery following disturbance in both primary AND secondary successions

  • Pioneers "pave the way" for later colonists by altering the biotic and abiotic environment:

Species composition tends towards a Climax Community through succession resulting in the development of the vegetation common for the biome.

The climax community describes an end product of succession that persists until disturbed by environmental change.

Succession occurs at large scales involving higher plants and animals, but may involve microbial communities on a smaller scale.

Primary Succession

Secondary succession

Disturbance Regimes

  • Many communities are often held at earlier successional stage by repeated but unpredictable disturbances that prevent succession from reaching the climax community that might be expected for the climate of the area.

  • The species are adapted to the dominant modes of disturbance in their communities.  A change in the distubance regime -- whether it be an increase or a decrease -- can result in the loss of biodiversity and species extinction.

  • The original prairies of Illinois are an example of such a community. The early successional grass and perennial plants are fire tolerant because of their underground roots and stems. Repeated fires
    destroy shrubs, young trees, and other plants that would change the environment and result in further successional changes that would eventually result in the establishment of a deciduous forest.

  • Agricultural practices are essentially an artificial form of disturbance. Crops like corn and soybeans as well as the common weeds found in agricultural fields have the characteristics of pioneer species and require repeated soil disturbance.

  • The three factors that characterize a disturbance regime are the
    size, frequency, and intensity of disturbance.

    • frequent or intense disturbances tend to favor early successional species

    • large disturbances tend to result in homogeneous communities

  • Fire supression in western forests has caused a build up in fuel that promotes large infrequent, high-intensity crown fires rather than small, frequent, low-intensity understory fires. In these situations restoration of the natural fire regime can be difficult and costly, but failing to do so is a risk both to humans and nature.

A summary of changes that occur during succession:

  • Pioneer species colonize first.

  • Pioneer species alter the environmental conditions remaining after the disturbance.

  • Eventually new species of plants become established in the conditions altered by the pioneer species and displace the pioneer plants.

  • Animals come in with or after the plants they need to survive.

  • Further environmental change by the new plants and animals result in the establishment of different species.

  • With infrequent disturbance, a stable climax community consisting of plants and animals that can reproduce themselves in the existing conditions will become established.

  • Disturbance of the ecosystem will start the process of succession anew.

  • In a given area there are usually small patches of land in different stages of succession, depending on the time and severity of the last disturbance. This adds diversity in the types of vegetation and animals living in the greater region.

  • Various stages of succession in one area