Janzen, D.H. 1970. Herbivores and the number of tree species in tropical forests. American
Naturalist 104(940):501-528.
Major question: How do you pack so many species into a forest? It all has to do with predation: as conditions
become more favorable for predators, that habitat will support more species of trees because no one species can
become common enough.
Made this observation of tropical trees: For most spp, adults don’t produce new adults in their immediate vicinitymost adults are more regularly distributed than if the probability of a new adult were proportional to the number of
seeds arrive at that point. Janzen proposes that this is the result of 2 processes:
1) The number of seeds arriving at a point declines with distance from the parent tree (I)
2) Adult tree and its seeds and seedlings are the food source for many host specific plant parasites and
predators, so the negative effect of predators declines with increasing distance from the parent (P).
The resulting theoretical mode shoes these two processes, plus
the PRC: Population Recruitment curve.
The model results from the following processes:
Host specificity: Without host specificity, P would be
horizontal, and seedlings would grow near their parents. Host
specificity is stronger in the tropics than in the temperate zone.
Seed dispersal: Distance responsive predators prey on adults
and on nearby seedlings of the adult. Density responsive
predators prey on high density of seedlings and are thought to be
better at spacing out trees (they’ll hunt until density is so low it’s
not worth it anymore).
Seed immigration: Immigration curves (I) depend on the number of seeds the adult puts out: the higher the curve,
the longer and higher the PRC, and the greater the probability of seedling survival. Predispersal predation reduces
seed distance from tree while post dispersal predation increases it. Increasing the ability of seed predators to move
between seed crops and to eat seeds more rapidly will increase the number of species that can coexist because more
predation results in a lower likelihood the tree species will competitively displace other species.
The method of dispersal also affects I: a steeper curve is expected for
wind dispersal while a broader one is expected for bird and mammal
dispersal. In general, long dispersal distances lead to higher seed
survival, since the animal takes the
seed from the source.
Population recruitment will
be a function then of the dispersal
agents, predispersal seed predators
and parent’s productivity. Habitat
heterogeneity for predators is also an
important factor (see seed shadow
illustration):
Other factors which may also be
causing the high diversity and wide
spacing: competition, interference, edaphic interactions.
See also: Connel, J.H. 1971. On the role of natural enemies in preventing
competitive exclusion in some marine animals and in rain forest trees. In Den
Boer, P.J. and Gradwell, G. (eds). Dynamics of populations. PUDOC. pp298-312.
Dan Janzen: University of Pennsylvania. Tropical conservation biologist, ATBI
coordinator, insect biodiversity, fragmentation