Communities

Question 1

Whats a community?

Answer 1

• Group of species that occur together in space and time

- Large number of similar species

Question 2

4 basic processes determine species composition:

Answer 2

1. Selection: Species are outcompeted and go extinct
2. Drift: Species go extinct through random chance
3. Dispersal: Species arrive from outside local community
4. Speciation: New species are created (on very long time scale)

Question 3

Whats selection? What does persist mean?

Answer 3

• Each species can persist under a particular range of environmental conditions (niche)
• Persist- having a positive growth rate

Question 4

Whats niche partitioning? Example?

Answer 4

• Many species can coexist because they occupy different niches in heterogeneous environments 
(= niche partitioning)
• Forest composition along elevation/moisture gradients in Great Smoky Mountains

Question 5

Whats a resource utilization niche?

Answer 5

-Niche can be defined based on resource use

Question 6

Whats the principle of competitive exclusion?

Answer 6

-Species with lower fitness 
(in a given environment) 
expected to go extinct

Question 7

Invasibility criterion: do all species have a positive growth rate when rare (and rest of community at equilibrium)?

Answer 7

Given finite resources, this means a rare species must have higher fitness (per-capita growth) than community average

Question 8

How might rarity increase fitness?

Answer 8

Less conspecific interference (e.g., defending territory)
Predators switch to other prey
Attract fewer pests/pathogens

Question 9

Constant environment vs fluctuating environment.

Answer 9

Constant environment

• Niche partitioning
• Frequency-dependent selection

Fluctuating environment

• Non-linearities
• Storage effect

Question 10

What if environment varies so that each species does best at different times?

Answer 10

• This does not necessarily enable co-existence

- The species that does best on average wins 
(unless particular conditions are met)

Question 11

When is coexistence possible in communities?

Answer 11

1) Species A does best when R is constant
; Species B does best when R fluctuates
 (only possible if responses have relative non-linearity)
2) High density of species A causes R to fluctuate

Question 12

Whats the storage effect?

Answer 12

Co-existence is also possible when:

1) Species have different responses to environment
2) Competition covaries with environmental conditions
3) There is buffered population growth (life history stages that are resistant to competition in bad periods)

Question 13

What do environment- competitions interactions happen from?

Answer 13

Life history stages immune to competition

-Arise from buffered population growth

Question 14

Whats buffered population growth?

Answer 14

• competition has a smaller effect in bad environments

- e.g., seed bank, dormancy, long-lived adults

Question 15

How does storage effect work?

Answer 15

When one sp does well its growth is reduced by competition more than the other sp.

Question 16

How does the storage effect give invading sp a fitness advantage?

Answer 16

Invaders do very well in good environments. A little worse in bad environments

Question 17

What conditions are needed for the storage effect?

Answer 17

1) Different environmental responses- Different year-to-year
 environmental responses
2) Competition co-varies with environment- Covariation between competition and environment
3) Buffered pop growth-competition has a greater effect in good times than bad times

Question 18

When does spatial storage effect occur?

Answer 18

When local microhabitats are favourable to different species

Question 19

Conditions producing spatial storage effect

Answer 19

1. Different responses to environment
 - created by local spatial variation (e.g., soil)
2. Covariance between environment and competition
 - higher pop size on good patches
 - competes more strongly with itself than others
3. Buffered population growth
 - automatically produced by dispersal among patches

Question 20

Whats a trade off that enables co-existence across space? How does it work?

Answer 20

Colonization-competition trade-off
1. Each blue square (“colonizer”) has a chance to colonize any empty cell

2. Each red square has a (lower) chance to colonize any non-red cell. If that cell is blue, they take it over (“competitor”)
3. A fraction of all squares are killed off (disturbance)- both species have same mortality rates
4. Repeat

Question 21

Why is the colonization competition trade off stable?

Answer 21

Blue benefits when rare because there are lots of empty cells available

Red benefits when rare because there are lots of non-red cells available

Blue is a “fugitive species” →
persists only by staying on the move

Question 22

How can blue species survive in colonization competition trade off? What would happen if there were no disturbances?

Answer 22

-Be on the move
-red would take over as disturbances cause empty habitats for blue
-

Question 23

Whats the dispersal-fecundity trade-off?

Answer 23

• Requires variation in patch density
• High-fecundity species wins where patches are close together
• Better disperser wins where patches are far apart

Question 24

Whats the tolerance-fecundity trade-off?

Answer 24

• Requires variation in patch quality
• Combines aspects of niche partitioning and colonization-competition trade-off
• Co-existence under a broader range of conditions