Communities Flashcards
Whats a community?
- Group of species that occur together in space and time
- Large number of similar species
4 basic processes determine species composition:
- Selection: Species are outcompeted and go extinct
- Drift: Species go extinct through random chance
- Dispersal: Species arrive from outside local community
- Speciation: New species are created (on very long time scale)
Whats selection? What does persist mean?
- Each species can persist under a particular range of environmental conditions (niche)
- Persist- having a positive growth rate
Whats niche partitioning? Example?
- Many species can coexist because they occupy different niches in heterogeneous environments (= niche partitioning)
- Forest composition along elevation/moisture gradients in Great Smoky Mountains
Whats a resource utilization niche?
-Niche can be defined based on resource use
Whats the principle of competitive exclusion?
-Species with lower fitness (in a given environment) expected to go extinct
Invasibility criterion: do all species have a positive growth rate when rare (and rest of community at equilibrium)?
Given finite resources, this means a rare species must have higher fitness (per-capita growth) than community average
How might rarity increase fitness?
Less conspecific interference (e.g., defending territory)
Predators switch to other prey
Attract fewer pests/pathogens
Constant environment vs fluctuating environment.
Constant environment
- Niche partitioning
- Frequency-dependent selection
Fluctuating environment
- Non-linearities
- Storage effect
What if environment varies so that each species does best at different times?
- This does not necessarily enable co-existence
- The species that does best on average wins (unless particular conditions are met)
When is coexistence possible in communities?
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
Whats the storage effect?
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)
What do environment- competitions interactions happen from?
Life history stages immune to competition
-Arise from buffered population growth
Whats buffered population growth?
- competition has a smaller effect in bad environments
- e.g., seed bank, dormancy, long-lived adults
How does storage effect work?
When one sp does well its growth is reduced by competition more than the other sp.
How does the storage effect give invading sp a fitness advantage?
Invaders do very well in good environments. A little worse in bad environments
What conditions are needed for the storage effect?
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
When does spatial storage effect occur?
When local microhabitats are favourable to different species
Conditions producing spatial storage effect
- Different responses to environment - created by local spatial variation (e.g., soil)
- Covariance between environment and competition - higher pop size on good patches - competes more strongly with itself than others
- Buffered population growth - automatically produced by dispersal among patches
Whats a trade off that enables co-existence across space? How does it work?
Colonization-competition trade-off
1. Each blue square (“colonizer”) has a chance to colonize any empty cell
- Each red square has a (lower) chance to colonize any non-red cell. If that cell is blue, they take it over (“competitor”)
- A fraction of all squares are killed off (disturbance)- both species have same mortality rates
- Repeat
Why is the colonization competition trade off stable?
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
How can blue species survive in colonization competition trade off? What would happen if there were no disturbances?
-Be on the move
-red would take over as disturbances cause empty habitats for blue
-
Whats the dispersal-fecundity trade-off?
- Requires variation in patch density
- High-fecundity species wins where patches are close together
- Better disperser wins where patches are far apart
Whats the tolerance-fecundity trade-off?
- Requires variation in patch quality
- Combines aspects of niche partitioning and colonization-competition trade-off
- Co-existence under a broader range of conditions
