Biology Reference
In-Depth Information
adjacent grassland, the canopy was dominated by a few species, juvenile
trees belonging to different species. Tropical rainforests generally contain
some species that have many large trees and few offspring. It is likely that
they will be replaced by different species having more offspring. Connell
interprets this as meaning that high-diversity tropical rainforests are in a
nonequilibrium intermediate stage after a disturbance. Experimental
evidence supports this view. Connell describes an experiment by
Webb, Tracey, and Williams in a Queensland rainforest. In an experi-
mental clearing, over a 12-year period, a uniform pattern of a few early
colonists changed to a patchy pattern of many species of somewhat later
colonists. At least some tropical forests have established a closed equili-
brium. Thus, in the Uganda rainforest discussed earlier, the canopy of
climax stands was dominated by 75-90% ironwood, with much iron-
wood offspring also in the understorey, suggesting closed equilibrium
with a single dominant species.
Compensatory mechanisms
Concerning the compensatory mechanisms permitting coexistence of
species in the absence of environmental disruptions, Connell (
1979
)
evaluates the evidence for frequency-dependent mortality, which could
be responsible for coexistence of many species, and which could, for
instance, be due to greater mortality of seedlings occurring in dense
clumps or near conspecific adult trees. He found no unequivocal evi-
dence for density-dependent mortality. Mortality was either greater in
high-density patches, or it was equal. Concerning the effects of density of
young trees on mortality, Connell examined 49 tree species from sub-
tropical South Queensland and 46 tree species from tropical North
Queensland. The mortality rate was determined by plotting the mortality
rate between 1965 and 1974 against the original number mapped in 1965.
No correlation was found. Also, according to Hubbell et al.(
1990
)
density-dependent effects could not be important in determining diver-
sity since most tropical species are rare. Furthermore, Hubbell (
1980
) and
others demonstrated that differences in per capita recruitment due to
distance or density dependence are minor in comparison with the effects
of larger numbers of seeds falling near trees: experiments have shown that
recruitment was greatest where seed density was greatest.
Concerning the effects of near conspecifics, Connell showed that
seedlings and saplings having the same species as nearest neighbors had
greater mortality than those mixed with other species. Nevertheless,
