Biology Reference
In-Depth Information
Eight potentially stabilizing mechanisms were examined. They were:
(1) parasitoid aggregates in response to host density; (2) parasitoid aggre-
gations are independent of host density; (3) there is temporally density-
dependent (possibly delayed) parasitism; (4) the parasitoid sex-ratio is
density-dependent; (5) host-feeding is density-dependent; (6) predation
is density-dependent; (7) there are spatial refuges from parasitism; and (8)
metapopulation dynamics. No evidence for any of the mechanisms was
found except for (7). However, this mechanism (the existence of spatial
refuges from parasitism) was not stabilizing. Additional potentially stabi-
lizing mechanisms based on the responses of individual Aphytis to their
size-structured host were found in modelling and laboratory studies, but
further studies are needed to show whether these or other factors can
indeed explain the remarkable constancy.
Good experimental evidence for disturbance leading to nonequilib-
rium in plant communities comes from the analysis of Tilman (
1982
),
according to whom plant communities at Rothamsted, England, may
not have re-established equilibrium conditions 100 years after resources
had been disturbed, indicative of the large time lag in tracking the
resource change. The Rothamsted Park Grass Experiment is particularly
well documented. It began in 1856. The eight-acre pasture, used for 200
years prior to the commencement of the experiment as grazing land, had
been divided into 20 plots, two used as controls, the others for various
kinds of fertilizer treatments. The first quantitative survey of species
composition was made in 1862, and it has been repeated several times
since. The evaluation of Tilman showed that there was an increasing trend
of dominance of Aleopecurus and Arrhenatherum in two plots since the early
stage of the experiments, but the full effect of fertilizer treatment was not
realized for almost 100 years. In one of the plots, a new equilibrium in the
plant community may not even have been reached after 100 years. The
reason appears to be that most species in the communities are long-lived
perennials, often reproducing clonally by means of underground tissues.
How long it may take to re-establish equilibrium in plant communities,
is indicated by evidence that clones of some plants may reach an age
of thousands of years.
How can equilibrium and nonequilibrium be explained? Levin and
Paine (
1974
), in an important paper, promoted an alternative view to the
then generally accepted view of communities as systems which are in
equilibrium. They view communities as spatial and temporal mosaics of
open and integrated patches. Disturbances interrupt the ''march to equi-
librium,'' and the whole system must be viewed as in balance between
