Succession and management - Ecological Applications: Toward a Sustainable World - page 237

of disturbance (an external physical factor) but the sequence in community composi-
tion results primarily from interactions among the species.
Primary and secondary successions
Autogenic successions can be either primary or secondary. A primary succession
occurs where a newly exposed landform is uninfl uenced by any previous community
- examples occur on volcanic lava fl ows and on freshly formed sand dunes. Second-
ary successions, on the other hand, occur where the vegetation has been partially
or completely removed, but where well-developed soil and seeds remain. Examples
include the loss of trees through disease, high winds, fi re or felling. The disturbances
responsible for starting a succession vary in both spatial and temporal scale, often
producing a mosaic of patches at different successional stages.
Underlying mechanisms
Early-successional plants have traits such as high fecundity, small seeds, effective
dispersal and rapid growth when resources are abundant. Late-successional species
have the opposite traits, including an ability to grow, survive and compete when
resources are scarce. Early species persist either because their dispersal ability and
high fecundity permits them to establish in recently disturbed sites before
late-successional species arrive (competition-colonization trade off) or because
rapid growth in unshaded conditions allows them to temporarily outcompete late-
successional species even if these arrive at the same time (successional niche theory).
Some early species change the abiotic environment in ways that make it easier for
later species to establish and thrive (facilitation theory). On the other hand, her-
bivory can slow or stop a successional sequence in its tracks (enemy-interaction
theory).
Managing succession for restoration
When planning the restoration of natural communities, managers need to be aware
of the spatial and temporal scale of disturbance and the timetables of natural recov-
ery of plant and animal communities. They also need to understand which underly-
ing mechanism applies in their particular case.
In many parts of the world fi res are now less frequent but more intense, affecting
larger areas than was the case before human infl uence. By taking active steps to
reduce the likelihood of forest or grassland fi res, highly fl ammable 'fuels' have been
allowed to build up so that when a fi re is started it burns long and hard. Effective
management requires a return to more natural fi re patterns. In other cases, the aim
is to restore land previously under agriculture and managers can wait for natural
succession to run its course. But recovery can often be speeded up by sowing a
species-rich mixture of seeds of desirable plants adapted to the prevailing condi-
tions. The same is true for restoration of tidal salt marshes or of communities on
old mine sites.
Managing succession for harvesting
To m a i nt a in the characteristics of an early-successional stage - growing an annual
crop - farmers resist the natural succession to herbaceous perennials, shrubs and
trees. In other cultures, though, agricultural 'gardening' is more in tune with suc-
cessional processes. The aboriginal managers of remote landscapes in Australia and