Navigating Complex Systems: Insights from a Rat Infestation Study

1. Thinking SystemsClass 10Matt Cohen, PhD

2. - - + A Rat Infestation • Gainesville home built in 1928 • No rats when we moved in • Lived there for just under 2 years • “Massive” control efforts by the end • Owners of 2 large dogs • Exceedingly poor hunters • Neighborhood of cat owners • Every direction (E, W, N, S) had one or more felines • Drove the dogs crazy…ever-vigilant border patrols

3. Elements of Systems • Boundary (the yard, canine patrolled) • Inputs and outputs (cats, dead rats) • Internal components (rats, dogs, cats) • Interactions • Positive interactions (rats breeding) • Negative interactions (cats on rats, dogs on cats)

4. Why Systems? • Interactions create complexity • Emergent behavior • Water is “wet” • Traffic snarls (even without accidents) • The Rise of Fall of Pet Rocks • Thresholds (tipping points) exist • Predicting these is enormously important • Global climate change, business cycles, disease epidemics • Epileptic seizures, landslides, fisheries collapse • Systems aren’t more complex than we think, they are more complex than we can think. • But…we have to try! $3.95 each (!)

5. Key Attributes of Systems I. • Mutual causality • Components affect each other, obscuring linear cause-effect • Popularity → sales → popularity • Poverty → soil erosion → poverty • Chicken → Egg → Chicken • Indirect effects • Component A exerts control over Component B via its action on Component C A B C A B

6. Indirect Effects - Aleutian Islands • Nutrients are essential for plant and animal production • Phosphorus (P) is often limiting nutrient • Essential for ribosomes and metabolism • Limited geologic source in the region • Amount of P controls the productivity of the ecosystem • Grassland production of Aleutian islands is P limited • Sea bird guano is a rich P source • Was mined for fertilizer for years Abundant P Depleted P Croll et al. (2005) - Science

7. Nutrients and Sea Birds • Seabirds eat fish from the sea but poop on land • Major flow of P from sea to land that supports productive grasslands + Fish Marine Birds + Soil P Grassland Production +

8. Predator Control of Ecosystems Arctic Foxes • Introduce Arctic Foxes • Top-predator • Seabirds never had a terrestrial predator • Decimated the sea-bird populations - + Fish Marine Birds + Soil P Grassland Production + Roughly 300% more soil P AND biomass on fox-free islands than on fox-infested islands

9. Key Attributes of Systems II. • Consist of processes at different space/time scales • Fast and slow variables • Humans and viruses • Evolution and extinction • Supply and demand • Systems are historically contingent • Deep dependence on what happened in the past • The Great Unfolding • Beta-max, Bacteria, Base 10 A B B A C

10. Fast and Slow: Beer and the Business Cycle • There exists a cycle of boom (bull) and bust (bear) periods in economic systems…WHY?

11. A Systems View of Boom and Bust • The structure of a system influences behavior. Systems cause their own problems, not external forces or individual errors. • Distribution chains (and economies) contain fast and slow moving parts • Communication between parts is LAGGED 2. Human systems include the way in which people make decisions. 3. People tend to focus on local optimization NOT global optimization.

12. Consider a Typical Supply Chain • Retailer: Sells products, varying consumer demand, orders to wholesalers for next weeks delivery • Wholesalers/Distributors: Distribute beer to multiple retailers, orders to brewery for two weeks in the future • Brewery: Make beer, adjust production to demand • ALL • Avoid the costs of excess and insufficient inventory J. Sterman at MIT http://web.mit.edu/jsterman/www/SDG/beergame.html

13. Beer Game Simulator Oscillation Team 1 Team 2 Team 3 Team 4 Brewery Wholesaler Distributor Retailer ORDERS Lag Amplification EXCESS/ BACKLOG Changing Demand

14. Dependence on History: Algae, Nutrients, and Shallow Lakes • Shallow lakes (< 10 m deep) • Two alternative “states” • Rooted vegetation (macrophytes) • Algae • Shifts between the two occur catastrophically, and BOTH can occur under the same environmental conditions • Where you are depends on where you’ve been

15. Self-Reinforcing Feedbacks in Shallow Lakes • Rooted Plant State • Plants require clear water • Plants stabilize sediments • Stable sediments keep water P concentrations low AND limit stirring • Low P limits algae and high clarity favors rooted plants • Algae State • Algae makes ooze • Ooze is easily stirred up, making the water turbid and recycling P • More P makes algae grow faster AND sediments looser via loss of plants • Regime shifts due to combined effects: • Too much P (human pollution) • Disturbances (pollution affects vulnerability)

16. Environmental Change and Ecosystem “State” Shifts Typical Models of Nature Emerging Model of Many Complex Systems Scheffer et al. (2001) - Nature

17. Thinking for Managing Complex Systems • The “state” of a system is controlled by external forces AND internal interactions • Indirect effects lead to surprising behavior • Fast and slow variables interact to create instability • Spatial variability (local vs. global variable) also • Managing for ONE THING often creates bigger problems later (discussion section)

18. The End Matt Cohen mjc@ufl.edu