Janzen-Connell Theory

Question 1

Are interactions more specific in the tropics?

Answer 1

In tropical forests there are not necessarily more insect species per plant than in temperate forests but insect species richness is correlated with tree species richness
Implies that increase in plant species richness drives evolution of insect species (Novotny et al. 2006)
Also other natural enemies (especially fungi) are a major cause of mortality in seeds and seedlings.
-> Postulated that natural enemies might play a role in maintaining diversity in tropical forests (and other hyperdiverse systems) == “Janzen-Connell hypothesis”

Question 2

Janzen-Connell Hypothesis

Answer 2

All plants are attacked by natural enemies

Many natural enemies are specialists

Specialists will aggregate on high densities of their hosts

If a species becomes very common, it will attract high numbers of enemies

Rare species will attract fewer enemies

Hence rare species should increase, common species become rarer

Question 3

Principles of J-C

Answer 3

1. Dispersal shadows
2. Aggregation of hosts
3. Local density-dependence

Question 4

density and survival vary with distance from the parent tree

Answer 4

at short distances the density of seeds is high, and this high density attracts lots of natural enemies. Consequently the survival is low. In contrast at larger distances, the density of seeds is low and there are few natural enemies and hence survival is high. The total number of survivors (# of seeds x survival) is maximised at an intermediate distance, the precise distance depending on the strength of density-dependence. But, importantly, immediately beneath the parent there is a death zone in which there are essentially no survivors.

Varying the strength of DD and the extent of dispersal affect the precise locations of the death zone and peak of recruitment. But the overall pattern remains, given some assumptions (e.g. leptokurtic dispersal and overcompensating density-dependence).

Question 5

Population Recruitment Curve

Answer 5

= seeds * survival

Question 6

How JC works

Answer 6

“Death Zone” in an area beneath a parent tree into which conspecifics cannot recruit

Net effect on recruitment is simple: species cannot ‘self-replace’

Because intense density dependence kills all seeds/seedlings immediately adjacent to parent tree in the death zone

i.e. when a gap is created, that gap cannot be re-occupied by the same species.

Must be a different species -> this enhances diversity

Question 7

Predictions of J-C

Answer 7

1. Distance dependence
   Distance dependence is an emergent property of the Janzen-Connell model
   It arises because of the relationship between dispersal and distance, and modulated by local density dependence
   But accumulation of enemies (esp. soil pathogens) may yield other distance effects
2. Rare species advantage:
   Species that become locally abundant are at a disadvantage
   Rare species attract fewer enemies hence have an advantage
   ‘Density-dependence’

Question 8

Assumptions of JC

Answer 8

1. Needs overcompensating DD

2. Specialist natural enemies

Question 9

Remove natural enemies – does this affect density-dependence?

Answer 9

Yes, JC mechanisms and NDD present only when pathogens are present.

Question 10

Is there overcompensating DD?

Answer 10

Yes, when fungicide was removed from Bagchi et al’s study (2012) it led to overcompensating DD, with fungicide, there is no DD.

Question 11

Do you need host specialisation?

Answer 11

Yes, generalists do not increase species richness but specialists do according to models.

Question 12

How to test for host specialisation?

Answer 12

Bagchi et at 2009
Parent tree was Parashorea melaanonan: measured survival of other species in plots
Plot survival of other species against evolutionary distance from P. melaanonan
Evolutionary distance should measure likelihood that other species share enemies with P. melaanonan

They found evolutionary distance was positively related with survival.

Question 13

Importance of Community level Density-Dependence

Answer 13

Community level density-dependence is tendency for rare species to increase in abundance and common species to become rarer.

Specifically J-C hypothesis generates this effect as a consequence of natural enemies.

Neutral theory has no density-dependence.

Also more general theory: if density-dependence is strong and inter-specific competition is weak, conditions for coexistence are fulfilled (Lotka-Volterra).

Question 14

Prevalence of Density Dependence?

Answer 14

Simple experiment: compare density of seedling recruits with density of seeds….. recruitment density decreases with seed density

Question 15

Pairwise interactions in a Tropical Forest

Answer 15

Looked at pairwise interactions – i.e. effectively measuring pairwise interaction outcomes from something like the Lotka-Volterra model

DD is strong, but interspecific competition is weak

Question 16

Density-dependence and Abundance

Answer 16

In both of these studies, strength of density-dependence (resulting from natural enemies) correlates with abundance

Question 17

There are more species in tropical forests: but are these organised in the same way as in other systems?

• communities in tropical forests appear to be more complex with a greater diversity of interactions
• trophic networks in modified habitats are more simplified

Answer 17

Or are communities organised differently in hyperdiverse tropical forests?

• in modified habitats there was a higher ratio of parasitoid to host species and increased parasitism rates
• the most abundant parasitoid species was more specialized in human modified habitats