janzen-connell hypothesis (lecture 8)

Question 1

How do gap dynamics relate to biodiversity?

Answer 1

• key driver individual and species turnover in tropical forests
• death of tree + creation of gap in canopy is point where forest competition is most intense
• processes affecting composition at this stage very important

Question 2

What are natural enemies?

Answer 2

• extremely prevalent
• insect species richness correlated with tree species richness
• implies plant species richness drives evolution of insect species
• other natural enemies, especially fungi, major cause of mortality/seedlings
• postulated natural enemies play role in maintaining tropical forest diversity
• Janzen-Connell

Question 3

What is the Janzen-Connell hypothesis?

Answer 3

• all plants attacked by natural enemies
• many natural enemies are specialists
• specialists aggregate where hosts are in high density
• v common species attract high number of enemies
• rare species attract fewer enemies
• number rare species should increase, common species should become rarer

Question 4

What are the principles of the Janzen-Connell hypothesis?

Answer 4

• dispersal shadows
• aggregation of hosts
• local density dependence

Question 5

What is a dispersal shadow?

Answer 5

• seed density reduces with distance from parent tree

- some species disperse seeds further than others

Question 6

What is aggregation of hosts?

Answer 6

• because seeds are dispersed close to parent tree hosts aggregate

Question 7

What is local density dependence?

Answer 7

• probability of death increases with seed density

- number of survivors and seed density = left skewed humpbacked curve

Question 8

How do seed density and survival vary with distance from parent tree?

Answer 8

short distances from parent tree:

• high seed density
• high density attracts lots of natural enemies
• low survival
• death zone beneath parent tree, essentially no survivors

further distances from parent tree:

• low seed density
• few natural enemies
• high survival

intermediate distance

• total number of survivors maximised
• number of seeds times survival
• precise distance depends of strength of density dependence

Question 9

How does Janzen-Connel theory fit together to explain diversity?

Answer 9

• death zone in area beneath parent tree
• net effect on recruitment is simple: species can’t self-replace bc intense density dependence kills seedlings adjacent to parent tree
• when gap created, canot be reoccupied by same species
• must be different species
• diversity enhances

Question 10

What does Janzen -Connell predict about distance dependence?

Answer 10

• distant dependence is emergent property of J-C model
• arises bc of dispersal and distance
• modulated by local density dependence
• accumulation of enemies e.g. soil pathogens may yield other distance effects

Question 11

What does Janzen -Connell predict about rare species advantage?

Answer 11

• locally abundant are at a species disadvantage
• rare species attract fewer enemies hence have an advantage
• density dependence

Question 12

What are the assumptions of the Janzen-Connell hypothesis?

Answer 12

• density-dependence must be overcompensating (hump-backed not plateau)
• must have specialist natural enemies
• rare species have no advantage in models with generalists