Succession and management - Ecological Applications: Toward a Sustainable World - page 216

Box 8.1 The theory of
ecological succession

Allogenic succession
Sometimes community composition shifts because of slow physical change, such as when silt
gradually builds up on the bed of a small lake or in a coastal salt marsh - leading to the eventual
replacement of the aquatic community by a forest. The changing abiotic conditions are mainly
responsible for shifts in community composition as the niche requirements of a sequence of
species are met in turn. Such successions are called allogenic (caused by outside infl uence). The
shifting species composition depends on the slowly changing physical environment, while interac-
tions among the species themselves are of secondary importance.

Autogenic succession
In many other cases successions begin as a result of disturbance (an external physical factor) but
the sequence in community composition results primarily from interactions among the species.
These successions are known as autogenic (caused by internal infl uence). Gaps, both large and
small, are opened up in forests by disturbances caused by high winds, fi res, earthquakes, lumber-
jacks or simply by the death of a tree through disease or old age. Disturbances in grassland are
caused by fi res, frost, burrowing or grazing animals, or by the ploughs of farmers. On rocky shores
or coral reefs, gaps are opened by severe wave action during hurricanes, tidal waves, battering by
logs or the fi ns of careless snorkellers. Predictable sequences in the appearance and disappearance
of species during autogenic successions result primarily because different species have different
strategies for exploiting resources - early species colonize effi ciently and grow fast, whereas later
species can tolerate lower resource levels and grow to maturity in the presence of early species,
eventually outcompeting them. Sometimes the early species actually enhance the success of the
later ones, perhaps by increasing the nutrient status or water-holding capacity of the soil.

Primary and secondary successions
Autogenic successions can be divided into primary and secondary successions. A primary succes-
sion occurs when a newly exposed landform has not been infl uenced by a previous community -
primary successions occur on volcanic lava fl ows (Figure 8.1), in craters left by meteors, on
substrate exposed when glaciers retreat and on freshly formed sand dunes.
Secondary successions, on the other hand, occur when the vegetation of an area has been par-
tially or completely removed, but where well-developed soil and seeds remain. The loss of trees
through disease, high winds, fi re or felling starts a secondary succession, as does the abandonment
by a farmer of a previously cultivated fi eld (or of a suburban garden when its gardener loses inter-
est). In secondary successions on abandoned farmland the typical sequence starts with annual
weeds, proceeding to herbaceous perennials, shrubs, early-successional trees and fi nally late-
successional trees. In some parts of the world, however, successions on abandoned fi elds culmi-
nate in a grassland community (Figure 8.2).
Successions commonly take decades, centuries or even millennia to run their course from
'pioneer' to 'climax' community compositions. But exactly the same ecological processes are
responsible for the succession of seaweeds on a boulder overturned by a wave, and this may be
complete within a few years.

Species traits determine the course of succession
Early-successional plants have a series of correlated traits, including high fecundity, small seeds,
effective dispersal, rapid growth when resources are abundant, and poor growth and survival when
resources are scarce. Late-successional species usually have the opposite traits, including an
ability to grow, survive and compete when resources are scarce. Note that early-successional
species have the general characteristics of r -species, and late-successional species those of K -
species (Box 3.1). In the absence of further disturbance, late-successional species eventually out-
compete early species by reducing resources (light and nutrients) beneath the levels required by
the early-successional species. Early species persist for two reasons: (i) because their dispersal
ability and high fecundity permits them to colonize and establish in recently disturbed sites before
late-successional species can arrive; or (ii) because rapid growth in unshaded conditions allows
them to temporarily outcompete late-successional species even if these arrive at the same time.
The fi rst mechanism is known as a competition-colonization trade-off and the second as a suc-
cessional niche (conditions suit particular species because of their niche requirements) (Rees