Biology Reference
In-Depth Information
(5) Growth is not limited by one resource, i.e., different species use
different resource ratios;
(6) There is immigration.
Most of the hypotheses were already included in the discussion of
Connell's work. I discuss some of the hypotheses listed above that were
not included in that discussion.
(1) Low population density precludes competition: Wright discusses two experi-
ments testing the hypothesis that low population densities may pre-
clude competition between understorey plants (hypothesis 2 above). In
both experiments, some plants were removed and recovery monitored.
In neither case was there competitive release in the remaining plants.
However, more experiments are needed to confirm these results.
Furthermore, also testing hypothesis 2, herbivores do indeed reduce
understorey plant density. In the absence of almost all mammalian
herbivores, plant density was 230% greater than in a forest with all
of the mammals. Recruitment limitation (hypothesis 2), a hypothesis in
particular promoted by Hubbell, is controversial. It may well limit
competition between rare species, but is unlikely for common species.
(2) Dynamic equilibrium model of Huston: Huston (cited by Wright
2002
)
suggested that slow population dynamics may permit coexistence of
similar species. His dynamic equilibrium model assumes a balance
between disturbances and population dynamics. But such a balance
has not been demonstrated and is unlikely, because disturbances usually
do not reset population sizes, and species diversity is often greatest
where population dynamics are the fastest and where resetting of
population sizes by disturbances is most unlikely.
(3) Community drift model and slow competitive displacement: important theo-
retical considerations based on experimental evidence are presented by
Hubbell and collaborators (Hubbell et al.
1990
). Hubbell's community
drift model was proposed to explain that identical species can coexist
indefinitely, assuming that births and deaths occur by chance. However,
species are not identical, they do not have the same probabilities of death
and reproduction, and it has been shown that the model cannot explain
a long coexistence of species (references in Wright
2002
). Hubbell and
Foster (
1986
), based on circumstantial evidence, concluded that there is
slow competitive displacement in tropical rainforests. Equilibrium
hypotheses may explain some of the results obtained, but overall pair-
wise and predictable interactions between species are not important.
More important are disturbances and the effects of biotic uncertainty.
