topic 9 classic experiments

Question 1

who first tested predator-prey models? how?

Answer 1

• G. Gause (1934): first empirical tests of predator-prey models
  
  • Predator: Unicellular protist, Didinium nasatum
  • Prey: Unicellular Paramecium caudatum
  • Simple system - Test tubes: oat medium (food for prey)

Question 2

what did gause see at first

Answer 2

• At 1st no predators, prey grew exponentially
• Predators added at day 2 - predators regulate
• By 4 - prey extinct
• By 6 predators extinct
only one cycle, no stable persistence, predator lags behind prey

Question 3

how did cause make it more complex

Answer 3

• Environmental heterogeneity (creating prey refuge) may stabilize populations
• Added sediment (prey refuge)
○ Prey will enter sediment but
predators will not
• Predators consumed visual prey then went extinct - prey then grew exponentially

Question 4

how did gause further complicate it (third try)?

Answer 4

• Next - interference from outside the system necessary? Added an individual every 3 days
•
• Got multiple cycles
• Concluded: stable oscillations not a property of simple predator-prey interaction as L-V
models suggest. Need interference from outside system (immigration

Question 5

why did huffaker question gause?

Answer 5

• Carl Huffaker (UC Berkeley)
  
  • Questioned Gause’s conclusion that predator-prey systems are unstable without outside interference
  • Gause used too simple of habitat (even with sediment)
  • Need to achieve partial prey refuge

Question 6

describe huffaker experiment

Answer 6

• Need to achieve partial prey refuge 
	• Six-spotted mite Eotetranychus
	sexmaculatus as herbivore and
	Typhlodromus occidentalis as predatory
	Mite on oranges 
	• 
	• Results similar to gauses 
	• Then added increasing levels of environmental heterogeneity 

* Still unstable 
* Continued to increase levels of heterogeneity 
* Neither population crashed

Question 7

mechanism behind huffakers 3rd try - where results were similar to LV model predictions

Answer 7

• Mechanism:
○ Increasing habitat complexity provided PARTIAL prey refuge
• Pockets of surviving prey allows population to rebound
• Concluded: partial escape from predation/prey refuge a key component ofmaintaining predator-prey cycles

Question 8

• Factors that stabilize predator prey interactions
• the problem: the simplest predators-prey interactions are not very stable
what is the solytion?

Answer 8

• The solution: other kinds of biology can help stabilize these interactions

1. Interference from outside the system (e.g., immigration)
2. Escape from predation - partial prey refuge
3. ....TBD

Question 9

escape from predation evironment vs traits of organisms

Answer 9

• Characteristics of environment:
-Heterogeneous environments: prey refuges
• Traits of organisms:
– Escape in time (evolution of life cycles)
– Escape in space (dispersal & apparency)
– Behavioral (e.g., intimidation or confusion of predators)
– Physical or chemical defence mechanisms

Question 10

describe escape in time w life cycles

Answer 10

• Prey life cycles evolve to minimize predation
• Minimizing amount of time spent in vulnerable stages, e.g., juveniles, immobile stages
– Increase speed of development
• Synchronize reproduction
– Have many offspring in a short time period
– Predators satiated without a large proportion of prey population being killed
• Unpredictable life cycle:
– Difficult for predators to adapt to

Question 11

look at cicadia ex

Answer 11

ok, escape in time emerging with prime number life cycles - 17 yr or 13 yr
•
• Predator satitation - emerge in mass events - predators satiated immediately

Question 12

describe escape in space

Answer 12

• Dispersal ability
	• Apparency of prey
	– If prey are rare it is difficult for predators to develop a search image
	– Reduces predator efficiency
	– Hard to specialize on rare prey

Question 13

describe 3 types of behavuoral escape

Answer 13

• confusion of predators
– Killdeer drawing attention away from nest using broken wing display
- Schooling of fish: confuses predators who need to focus on an individual to hunt effective
l Intimidation
– Toads fill with air
– Birds ruffling feathers
• Playing dead
– Possums play dead & emit “decay” odour from anus
– Predators do not want decaying prey

Question 14

physical defenses against pred?

Answer 14

• Animals: quills, spines, scales,
	cuticle thickness, slime etc.
	• Plants: thorns, seed coat, waxes
	etc.
	• Size

Question 15

chemical defenses against predators

Answer 15

• Chemical (toxins)
	• Synthesize their own (many
	amphibians)
	• Acquire from food (monarchs
	acquire cardiac glycosides from
	milkweed)

Question 16

coloration defense

Answer 16

• Coloration 
	• Cryptic/camouflage
	• Confusion: false heads
	• Startle patterns: owl spots
	• Warning patterns
	– Aposematic: warns of chemical
	or physical defense
(bright colours, stripes)