Geoscience Reference
In-Depth Information
(b) Light-demanding pioneers
(a) Gap phase
(c) Gap closed
(d) Secondary forest
Figure 20.12
Theory of cyclical succession as illustrated from an area of tropical rain forest. Clearings are caused by natural
disturbances, old age or human activities, and are invaded by species different from those originally growing there.
not be able to thrive on the same site.
A biological mechanism involving animals has been
reported by the US ecologist Janzen from his studies of
tropical forests in Costa Rica. In explaining why individ-
uals of a species are scattered in the forest, with no near
neighbours, he came to the conclusion that intense
predation of tree fruits when they fall to the ground by
insects, especially ants, means that a tree can reproduce
only when its seed is carried far from the parent by birds
or monkeys. Seed mortality is always 100 per cent, so that
regeneration
in situ
is never possible.
After the gap has opened, light and temperature increase
on the forest floor, killing the shade-tolerant understorey.
Young seedlings of light-demanding species become
established, and the gap becomes closed by the upward
growth of these pioneers and by the crowns of trees
surrounding the gap growing into the open. When the gap
is closed again, no more light-demanding seedlings
become established, but adults may persist. Seedlings of
forest species will become established, and understorey
plants adapted to low light and temperatures will thrive
again.
In addition to the tropics, cyclical development has
been reported from the species-rich forests of eastern
Europe. Here, as in the tropics, it is noted that trees which
die are replaced not by the same species but by different
species from the forest. There may be several causes. One
is competition for light. Beneath trees that give heavy
shade (e.g. common beech,
Fagus sylvatica
) the young
plants of light-requiring trees like beech are unable to
mature in the shade. Another reason to explain the failure
of a plant to thrive on the same site as its parents may be
the fact that different species do not have identical
nutrient requirements. If a particular species has removed
nutrients from a site for a long time the same species may
CONCLUSION
The British ecologist Sir Arthur Tansley revolutionized the
study of natural systems of vegetation and soil in the 1930s
when he introduced the concept of the ecosystem. Since
then the Canadian ecologist Stan Rowe and the US
ecologist Odum have both placed the ecosystem within a
hierarchy of ecological units. The distribution of plants is
governed by a range of environmental factors. There
are also strongly competitive relations between species
(inter-specific competition) and between individual plants
