Lecture 20 - Community Dynamics II

Question 1

What are 3 aspects of an equilibrium community?

Answer 1

1. No disturbance → no change
2. Disturbance → community will return to the equilibrium state
3. Effect of history is erased because the species composition ends the same way and remains there

Question 2

Explain the Paradox of the plankton.

Answer 2

Gause’s hypothesis: Complete compe.tors cannot coexist.

However, Paradox of the plankton: A limited range of resources supports an unexpectedly wide range of phytoplankton species

Question 3

Define disturbance

Answer 3

A discrete event that disrupts community structure, and changes resources, substrate availability, or physical environment.

Question 4

What did we learn from the coral reef communities?

Answer 4

1) Reef responding to disturbance in different ways
2) Recruitment rates differed across sites (exposed and protected crest) and years.

Conclusion:
Disturbance rate > recruitment rate (# of year 0 individuals in a year (different than reproduction))

At reef scale → continuously changing due to disturbances.

Question 5

Does each tropical fish has its own ecological niche?

Name 2 competing hypotheses for maintenance of coral fish diversity

Answer 5

1. Niche diversification: community assemble is outcome of competition. Different niche.
2. Variable recruitment: same niche and community assembly is outcome of unpredictable recruitment, a random sample of the regional species pool. Competition is not a factor.
   From table, all the same niche.

Question 6

Is recruitment variable?

Explain the experiment to show this.

Answer 6

Experiment: setting fake coral reefs at the bottom of the ocean

Found out that composition of each species that came to visit the reefs were different from reef to reef.

Fish similarity between reefs = 32%, which means the rest is variable

20-40% change/ month in which species were there.

So same niche, but different fish.

Question 7

Does recruitment affect population size? Give two hypotheses

Answer 7

1. Doesn’t matter how many recruits there are, you always see the same number of adult fish
2. The # of recruits you have increases the # of adult fish you have (supply-side ecology).

Question 8

What are 4 ways to be in non-equilibrium:

Answer 8

1. Fluctuating environment
   ex: seasonal succession)
2. A process that is not density dependent is important (the more things you have the slower it keeps growing)
   disturbance occurs frequently, or recruits come from elsewhere
3. Directionally changing environments: climate change
4. Slow competitive displacement:
   competitive abilities are nearly equal, so no time trend or succession.

Question 9

Define intrinsic non-equilibrium

Answer 9

Intrinsic fluctuations, oscillations or cycles can maintain a non-equilibrium state

Question 10

Define 3 measures of community stability

Answer 10

1. Resilience: Speed at which a community returns to its form state after a disturbance balance.
   (more resilience = fast return to original state)
2. Resistance: Ability of a community to avoid displacement from its current state
   (ability to not change at all)
3. Persistence: avoidance of extirpation
   (how long it can exist before being driven to extinction)

Question 11

Explain how predation can be on form of disturbance that maintains disequilibrium

Answer 11

Removal of sea otters lead to kelp forest decline through trophic cascade.

Question 12

Define Trophic Cascade.

Where can it be observed?

Does it always occur?

Answer 12

Trophic Cascade: Direct and indirect effects of predation in food webs.

Lots of evidence in food chain in lakes.

Mostly occur in aquatic communities but can also occur in terrestrial communities.

Don’t always occur → happen when you have top-down control.

Doesn’t occur when you have bottom-up control.
For example, lake filled with plankton. If add top predator doesn’t work, then can reduce plant nutrients.

Question 13

Define Applied trophic control

Answer 13

Applied trophic control - The idea of biomanipulation in lakes

Adding or removing predators to reach particular states.

Question 14

Describe the effect of high and low environmental stress on the importance of predation and physical factors for plants.

What about low recruitement?

Answer 14

High stress → physical factors are important (temperature, nutrients)
Low stress → predation off those plants is importance

When recruitment is low, competition can’t be important.