A multiple-loop model of an urban ecological system might include the following variables: number of people in the city (P), modernization (M), migration into the city (C), sanitation facilities (S), number of diseases (D), bacteria/area (B), and amount of garbage/

area (G), where the symbol for the variable is given in parentheses. The following causal loops are hypothesized:

1. P → G → B → D → P 2. P → M → C → P 3. P → M → S → D → P 4. P → M → S → B → D → P Sketch a signal-flow graph for these causal relationships, using appropriate gain symbols. Indicate whether you believe each gain transmission is positive or negative.

For example, the causal link S to B is negative because improved sanitation facilities lead to reduced bacteria/area. Which of the four loops are positive feedback loops and which are negative feedback loops?