Chapter 2 Populations and feedback systems
Introduction
2-1 Exponential population growth
2-2 Positive feedback
2-3 Negative feedback
2-4 Population regulation
2-5 The practical significance of positive and negative feedback
Introduction
1. Why do environmental problems sometimes appear so suddenly?
2. Feedback: powerful forces, negative and positive
3. Negative feedback: provides stability for system functioning properly
4. Positive feedback: rapid change
2-1 Exponential population growth
1. Water hyacinth: exponential growth
(1) Grow fast: (fig. 2.1, 2.2, 2.3)
(2) Uncontrollable: Lake Victoria in East Africa, similar to people population
2. Carrying capacity: Earth for eight billion people, now six billion
2-2 Positive feedback
1. Water hyacinth story is an example of positive feedback
(1) circular chain of effects that increases sudden change
(2) is a source of instability; is a force for change (fig. 2.4)
2. Exponential population growth
(1) positive feedback: (fig. 2.5)
(2) Endangered species: decline to extinction (positive feedback)
(3) Social system: Cold War for weapon production
3. An example of positive feedback that causes one thing to replace another
(1) Positive feedback can cause one thing to increase and another thing to
decrease. One part replaces the other.
(2) Example: VHS vs. Betamax (fig 2.6)
2-3 Negative feedback
1. Negative feedback is a circular chain of effects that opposes change.
2. Negative feedback is a source of stability; a force against change.
3. Homeostasis: control stable. (fig. 2.7)
2-4 Population regulation
1. Deer in a forest: limited by food supply
(1) Population vs. food supply
(2) Food supply vs. birth and death
(3) Population vs. food supply vs. birth and death
2. Population regulation and carrying capacity
(1) Population regulation: negative feedback to keep populations within limits.
(2) Carrying capacity: survive for a long-term (sustainable) basis
(3) Positive vs. negative feedback (fig. 2.8)
(4) Carrying capacity vs. birth and death (fig. 2.9)
(5) J-shape vs. S-shape (fig. 2.10, 2.11)
(6) Overshoots: population grows rapidly and declines rapidly
2-5 The practical significance of positive and negative feedback
1. Every ecosystem and human social system has numerous positive and negative
feedback loops.
(1) Negative feedback provides stability
(2) Positive feedback provides change
(3) People constantly interact with these forces of change and stability. Negative
feedback “take care of things”; Positive feedback “development or solving
problems”
2. Use the feedbacks to our advantage instead of struggling against system
3. Doing things nature’s way