Biology Reference
In-Depth Information
them for more than 30 years, beginning in Britain and later extending the
studies to other countries. Lawton (
1999
) briefly summarizes his results,
and Lawton's (
2000
) review gives greater detail. In Britain, the fern is
extremely common, growing among other plants or in large monocultures,
some of which extend over hundreds of hectares. The fern has an under-
ground rhizome and fairly evenly spaced fronds of uniform size, which
appear first in April/May and reach a height of more than 1m. They die
with the first frost in autumn. The fronds are a source of food to a variety of
insects. At an open site, which was sampled at two- to three-week intervals
throughout the growing season in most years between 1972 and 1990, 24
species of insects were recorded, and 27 species were found to regularly
feed on bracken foliage in Britain, although some of them were not
recorded every year, possibly due to a failure in sampling. A patch at a
woodland site was sampled for 8 years between 1980 and 1987. Other
patches in Britain were sampled sporadically. The open site had 15-19
(mean 17.6) species of feeding stages per bracken frond, the woodland
patch had 3-16 (mean 14.6). There were often unoccupied fronds. Several
major questions were asked. The first question relates to the role of
density-dependent processes in species dynamics. Statistical tests and con-
sideration of the rank order of species abundances in an assemblage were
used to answer the question. Statistical tests revealed direct or delayed
density dependence in about three quarters of the species in the open site
and in about one third in the woodland site, although the rates may be
somewhat lower because of statistical error. Using rank orders of species
abundances, i.e., whether common species remained common and rare
species rare over considerable periods, tests revealed that rank orders are in
fact conserved over some time, although there is a steady decline in W
(Kendall's coefficient of concordance). All this means that the community
has a regular structure which is slowly changing over time.
A second question relates to the importance of top-down and bottom-up
effects in structuring the communities. An attempt was made to answer this
question by combining field observations with mathematical models and
field experiments. However, a problem arose because experiments on insect
communities of bracken were very difficult: larvae of many insect species
live in galls, mines or silk-woven refugia that could not be manipulated. It
wasdifficulttomakeadultslayeggsinfield cages, and many species occurred
in low population densities leading to logistical difficulties. Nevertheless,
some results on the relative contributions of top down effects (by predators,
parasitoids and disease) and of bottom up effects (due to resources, especially
food) were obtained. They revealed effects of the latter on population
