Lecture 9: Janzen-Connell Flashcards
Problem of explaining diversity:
- competitive exclusion
- limiting similarity
- not enough niches for all species
when is competition highest in the tropics:
when a gap opens
in tropical forests: insect species richness is correlated with
tree species richness –> implies that increase in plant species richness drives evolution of insect species
___ enemies
(especially fungi) are a major cause of mortality in seeds and seedlings
– may play role in maintaining diversity
=Janzen cone hypothesis
Janzen-connel hypothesis
- all plants are attacked by natural enemies
- many natural enemies are specialists
- specialists will aggregated on high densities of their hosts
- if a spp. is v common, will attract a lot of natural enemies
- rar will attract fewer
- hence rare app. should increase, common become rarer
“death Zone”
is an area beneath a parent tree into which conspecifics cannot recruit
- species cannot ‘self replace’
- as increased density causes death of seedlings/seeds
- must be a different species
Prediction of J-C: Distance dependence:
- Distance dependence
- -arises because of relationship between dispersal and distance, & modulated by local density dependance
- but accumulation of enemies (esp. soil pathogens) may yield other distance effects
predictions of J-C:
- Distance dependence
- rare species advantage
prediction on J-C: rare species advantage
- species that become locally abundant are at a disadvantage
- rare species attract fewer enemies hence have an advantage
- ‘Density dependance’
assumptions of J-C hypothesis
- overcompensating density dependance
- specialist natural enemies
models with generalist natural enemies yield
no rare species advantage
neutral/J-C favoured at the minute?
J-C
