6

Principles of Ecology:

Self-Sustaining Mechanisms

in Ecosystems

Chapter Outline

Homeostasis: Maintaining the Balance

Homeostasis in Natural Systems

What Factors Contribute to Ecosystem Homeostasis?

The Resilience of Ecosystems

Resisting Changes From Human Activities

Population Control and Sustainability

Species Diversity and Stability

Natural Succession: Establishing Life on Lifeless Ground

Primary Succession

Secondary Succession

Changes During Succession: An Overview

Evolution: Responding to Change

Evolution by Natural Selection

Genetic Variation

Natural Selection

Speciation

Coevolution and Ecosystems Balance

Human Impact on Ecosystems

Tampering with Abiotic Factors

Tampering with Biotic Factors

Simplifying Ecosystems

Why Study Impacts?

Key Terms

Homeostasis growth factors reduction factors

environmental resistance species diversity natural succession

primary succession pioneer communities intermediate communities

climax communities secondary succession evolution

natural selection adaptations gametes

crossing over selective advantage fitness

speciation reproductive isolation allopatric speciation

sympatric speciation coevolution restoration ecology

pathogens monoculture ecosystem simplification

mitigate Environmental Impact Statement

Objectives

1. Define “homeostasis” and discuss how this term relates to natural systems.

2. Discuss the factors that contribute to ecosystem homeostasis.

3. List the mechanisms of population regulation seen in natural systems.

4. Discuss how species diversity can affect ecosystem stability.

5. Define “natural succession,” “primary succession,” and “secondary succession.”

6. Discuss the changes that occur during the process during natural succession.

7. Define “evolution,” “natural selection,” and “adaptations,”

8. List the sources of genetic variation that can be acted on by natural selection.

9. Define “speciation,” “allopatric speciation,” and “sympatric speciation.”

10. Define “coevolution” and give an example of how it works.

11. Define “restoration ecology” and “conservation biology.”

12. Discuss the potential impact caused by simplifying ecosystems.

13. Discuss the significance of the “Environmental Impact Statement” for predicting the effect of human activities.

Lecture Outline

Homeostasis: Maintaining the Balance - Homeostasis in an ecosystem is a steady state or dynamic equilibrium, where internal conditions are held more or less constant despite changes in external conditions.

A. Homeostasis in Natural Systems - Ecosystems have many mechanisms that resist change or help them recover from change. This helps to keep natural systems in balance.

1. Predators - Predators reduce the population size of the prey upon which they feed.

2. Diseases - Disease is also a mortality factor that can stabilize population size.

B. What Factors Contribute to Ecosystem Homeostasis?

1. Growth Factors/Reduction Factors - Ecosystem balance is maintained by the opposing forces of both biotic and abiotic growth and reduction factors.

2. Environmental Resistance - Environmental resistance is a collective term for those factors which have a negative effect on population growth.

3. Resisting Small Changes - Inertia and resilience are characteristics of ecosystems that resist and rapidly recover.

C. The Resilience of Ecosystems - In ecosystems, changes in biotic and abiotic factors may perturb the ecosystem but all ecosystems tend to return to normal over time.

1. Resilience - The ability of an ecosystem to recover from temporary changes in conditions.

D. Resisting Changes from Human Activities

1. The resilience of ecosystems minimizes and sometimes negates human impacts. Human impacts can, however, severely disturb an ecosystem and overcome the recuperative capacity of the system.

E. Population Control and Sustainability - Environmental resistance helps to keep populations within an ecosystem's carrying capacity.

F. Species Diversity and Stability - Species diversity is a rough measure of the number of species living in a community. Species diversity and stability seem to be positively correlated for most ecosystems, though climate uniformity may also play a major role in ecosystem stability.

II. Natural Succession - Succession is a process whereby one biotic community replaces another.

A. Primary Succession - The development of a biotic community where none had existed before is primary succession.

1. Each successional community (pioneer, intermediate, and climax) changes conditions, making them unfavorable for itself and favorable for the next community. The final or longest-lasting stage in succession is the climax community.

B. Secondary Succession - This takes place when an existing community is disturbed or destroyed, either by natural or human-caused events.

C. Changes During Succession

a. Complexity and efficiency usually increase as ecosystems mature.

b. Species diversity may peak in either immature or climax stages, depending on the particular ecosystem.

c. Damage to an ecosystem may be so severe as to essentially prevent recovery by natural succession or restoration.

III. Evolution: Responding to Change - Evolution produces species which are structurally, functionally, and behaviorally adapted to their environment.

A. Natural Selection - Natural selection is a process that favors those individuals who are the most fit or reproductively successful. Natural selection is the driving force or evolution.

B. Genetic Variation - Genetic variation refers to naturally occurring differences in the genetic composition of organisms in a population. This is the raw material of evolution. The sources of genetic variation are:

1. Mutation - Mutations are changes in the DNA .

2. Gamete production - Genetic material may be transferred from one chromosome to another (crossing over).

3. Sexual reproduction - Each offspring contains half of each parent’s genetic information and thereby new combinations may arise.

C. Speciation - Speciation, the creation of new species, usually occurs as a result of geographic and reproductive isolation of different groups of a parent species (allopatric speciation). New species may also form without geographical isolation (sympatric speciation), which is common in plants.

D. Coevolution and Ecosystems Balance - Organisms can evolve in concert with each other. Organisms that are closely associated over long periods of time often have adaptations closely related to the other organism. This results from the combined action of selective pressures each species places on the other though evolutionary time.

IV. Human Impact on Ecosystems - Humans may alter the biosphere by affecting either biotic or abiotic ecosystem components. Tampering with abiotic or biotic factors tends to reduce species diversity and thus simplify ecosystems, which makes them considerably more vulnerable to natural forces. Ecologists refer to the loss of species diversity as ecosystem simplification.

A. Tampering with Abiotic Factors - Human activities produce air and water pollution, which can alter the abiotic environment sufficiently to affect ecosystem balance on local, regional, or global scales. Some pollutants biomagnify or accumulate in increasing amounts as they travel up the food web.

B. Tampering with Biotic Factors - Destruction or depletion of resources for human use can have serious adverse effects on other species.

1. Introducing Competitors - Alien species may cause disruption in an ecosystem by out-competing native species.

2. Eliminating or Introducing Predators - Predators play a vital role in regulating ecosystem stability. Their elimination, or their introduction into new habitats, can cause serious ecosystem imbalance.

3. Introducing Disease Organisms - Introduced pathogens can wipe out native species that are not resistant to the alien disease organism.

C. Simplifying Ecosystems - Most human-caused ecosystem changes simplify ecosystems by reducing species diversity. Very simple ecosystems, such as monocultures, tend to be highly unstable. Their protection often causes other environmental problems.

D. Why Study Impacts? Impact models, though imprecise, can serve as frameworks for environmental risk assessment and decision-making. While environmental impact statements are an effort to use modeling to choose preferred options for projects that will affect the environment, they are generally used to find ways to mitigate damage.

1. Environmental Impact Statement (EIS) - An EIS is required by law on all major projects on federal lands. The EIS summarizes the environmental, social, cultural, and archeological impacts of the proposed project.

2. Least Impact Analysis (sustainability analysis) - This analysis would suggest alternative plans and would be preventive in nature, not mitigating.

Suggestions for Presenting the Chapter

· Instructors should strive to provide local examples of primary and secondary succession. Fieldtrips are excellent venues for exploring these concepts.

· Instructors might ask the students to analyze the condition of local predator populations. Does your state promote hunting or trapping of important predators? Stressing the importance of human impact on predator populations is a good starting point for discussion.

· An analysis of attitudes toward predators is also an interesting topic for discussion. Why do humans have problems with predators? Why are many predators threatened or endangered with extinction? How can positive attitudes about predators be formed?

· Examine an Environmental Impact Statement (EIS). Environmental Impact Statements are readily available from the federal government for a variety of proposals. For example, the Forest Service is required to produce an EIS every time a new management plan is proposed for a National Forest. These are available by writing your nearest Forest Service office for a free copy.

· An activism project is always an effective and sustainable project for your classes. Involve your classes in a habitat restoration project on campus or in your area. Local environmental groups always need volunteer help to complete their projects.