L8 - Bottom-up vs. top-down control

Question 1

What controls the population size of species?

Answer 1

Bottom-up and top-down effects

Question 2

Explain how bottom-up and top-down effects the food web

Answer 2

Bottom-up: Resources control the trophic levels above them (e.g. nutrients e.g. Nitrogen (N), phosphorous (P))
Top-down: Consumers control the trophic levels below them (e.g. predator fish)

*if (for example) nutrients/predators increase/decrease then this affects the rest of the food web either above/below it.

Question 3

How does the PEG model combine top-down and bottom-up control?

Answer 3

Zooplankton abundance tracks algal resources at first (bottom-up), but later it controls them and leads to clear-water phase (top down)

Question 4

Explain how context matters for top-down and bottom-up effects

Answer 4

Both effects are important, but their relative strength/importance depends on the specific situation
*For the PEG model, what time of year

Question 5

What are the predictions of bottom-up effects?

Answer 5

Bottom-up effects predict positive correlations between all trophic levels:
More producers → more herbivores → more meso-predators → more top predators

Question 6

What are the predictions of top-down effects?

Answer 6

Top-down effects predict negative correlations between adjacent trophic levels:
More top-predators → fewer meso-predators → more consumers → less producers

Question 7

Explain the difference between what bottom-up and top-down controls

Answer 7

Bottom-up (e.g. the amount of nutrients) controls: 1) the number of trophic levels and 2) the abundance of the top level

Top-down controls the level below the top, and alternating levels after that.

Question 8

What does trophic cascade mean?

Answer 8

When a top-down effect goes down more than one trophic level

Extreme example: orca whales, sea otters, urchins, and kelp in Alaska.
In the late 1990’s, orcas began preying on otters, kelp forests began to disappear from sea urchin overgrazing.

Question 9

What are the 3 types of evidence for testing top-down and bottom-up effects?

Answer 9

Experiments: Manipulate top predators and/or nutrients in isolated habitats or mesocosms ( “medium sized” artificial communities – bigger than the typical lab experiment on individuals, but smaller than nature)
Field studies: Make observations across existing gradients of nutrients or predator presence/absence
“Natural experiments”: Take advantage of fish-kills

Question 10

Summarise bottom-up vs. top-down controls

Answer 10

• Most food webs combine both types of control – and they are especially powerful when combined! (e.g. the mesocosm experiments)
• Comparing different lakes tends to emphasize bottom-up control (i.e. different nutrient/trophic statuses)
• Comparing the same lake over time tends to emphasize top-down control (e.g. after fish kills, clear-water period)
• In general, lower trophic levels are more sensitive to bottom-up control, whilst higher trophic levels are more sensitive to top-down control.

Question 11

Explain how predators/consumers may affect top-down effects

Answer 11

Consumers/predators can affect:
- Behaviour related to habitat use
- Community composition of resource
- Life history traits of resource

Question 12

Explain how fish behaviour affect zooplankton behaviour.

Answer 12

1. Vertical habitat use: zooplankton vertically migrate to avoid fish predation during the day
   (diurnal)
2. Macrophyte refuges: zooplankton hide in macrophytes to avoid fish predation
3. Community structure: crystal lake Connecticut before and after introduction of Alewife fish (Alosa)
4. Traits: daphnia change their reproductive traits and some species grow defensive helmets in the presence of fish kairomones

Question 13

How do zooplankton grazers affect algal community structure?

Answer 13

Copepods prefer big algae → select for smaller algae
Cladocerans eat everything → select for larger algae