During
succession, one species replaces
another. There are at least three
patterns of interaction among earlier and later species in succession:
1.
Facilitation:
One
species can prepare the way for the
next.
2.
Interference:
Early
successional species may in some way prevent
the entrance of later successional species.
3.
Life
History Differences: One species may not affect the time of entrance of another; two species may
appear at different times during
succession because of differences in transport,
germination, growth, and longevity of seeds.
4.
Chronic
patchiness: Succession never occurs and the species that
enters first remains until the next disturbance.
Each
of these processes occurs in nature. Which occurs depends in part on the environmental conditions and in part on the pool of species that are
available to take part in succession.
Facilitation
This
pattern has been found to take place in tropical rain forest. Early
successional species speed the reappearance of the microclimatic conditions
that occur in a mature forest. In tropical forest, temperature, relative
humidity, and light intensity at the soil surface can reach conditions similar
to those of a mature rain forest after only 14 years. Once these conditions are
established, species that are adapted to deep forest shade can germinate and
persist.
Sand
dunes and bogs also illustrate facilitation. Dune grasses anchor the sandy soil
so that seeds of plants that fall on the ground have a chance to germinate
before they are buried too deep or blown away again. Sedges that form floating
mats on the waters of a bog create a substrate where seeds of other species can
lodge, germinate, and grow.
Interference
Examples
of interference can be found in tropical rain forests. When a rain forest is
cleared, used for agriculture, and then abandoned, perennial grasses grow that
form dense mats. For example, in parts of Asia,
these include bamboo and Imperata, as
well as thick-leaved small trees and shrubs. Together, these form stands so
dense that seeds of other, later successional species cannot reach the ground,
germinate, or obtain enough light, water, and nutrients to survive. Imperata either replaces itself or is
replaced by bamboo, which then replaces itself. Once established, Imperata and bamboo appear able to
persist for a long time.
Life
History Differences
An
example of life history differences is illustrated by seed disbursal. When
early successional species produce seeds, the seeds are readily transported by
wind or animals, and so reach a clearing sooner and grow faster than seeds of
late successional species. In many forested areas, birds eat the fruit of
cherries and their droppings contain the seeds that are spread widely. Species
typical of old-growth forest, such as sugar maple, can grow in open areas, but
these seeds take longer to get there. In this case the early succession of
species might be neutral or interfere. Old-growth forest species typically have
seeds with an adequate food supply for a seedling. Therefore, the seeds tend to be heavy and not
transported easily.
Chronic Patchiness
Whether
a change in species occurs during succession depends on the complex interplay
between life and its environment. Life tends to build up, to aggrade, whereas
in harsh environments, disturbances are frequent, the physical, degrading
environments dominates, and succession does not occur. Deserts exemplify the
chronic patchiness that results. Similarly, in highly polluted environments, a
sequence of species replacement may not occur.
Knowledge of the causes of the
succession of species can be useful in the restoration of damaged areas. Plants
that facilitate the presence of others should be planted first, as on sandy
areas, where dune grasses can help hold the soil before attempts are made to
plant larger shrubs or trees.
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