Global Ecology
Lecture 13
Ecological Succession
Ecological succession is the process that leads to mature
vegetation.
-can be a recovery process that leads to revegetation
of an area.
Old view of succession –
Frederick Clements (1916) – viewed plant community as a superorganism, with succession leading to a climax
community determined by climate.
Clements believed that all successions under the same
climate type would lead to the same climax community.
Example: he predicted that in the sand dunes around
Gleason (1926) argued:
that species are geographically
distributed according to their individual environmental tolerances
that succession is not a predetermined regional pattern of development, but is influenced by local growing conditions and chance arrival of seeds.
Modern view -
Disturbance events are common; change is “normal”.
Modern ecologists don’t use climax because it implies
stasis.
Instead, we call well-developed vegetation a mature
community.
Primary succession – occurs on a new surface such as lava
flows, sediments from retreating glacier
Secondary succession – occurs where vegetation is removed,
but soil remains
Primary succession has been observed over 100 years. (Fig.
15.3)
Fig. 15.4 shows the successional
sequence of vegetation forms that have occurred on
Fig. 15.6 shows the accumulation of plants species; it may
take 1000 years to attain species diversity of mainland.
Secondary succession
Disturbance provides a shock to ecosystem functions, such as
nutrient cycling, carbon storage, soil water retention
Example: intact forests have little surface run-off
disturbed forests can have huge run-off, causing erosion
Soil Temperature –
can
increase greatly with forest canopy removal, killing fungi and bacteria.
Without decomposers, nutrient cycling slows. (Fig. 15.7)
In level forests with high rainfall, removing trees (and
transpiration) can cause waterlogged soils.
-microbes
use up oxygen
-in
anaerobic conditions, chemical reduction causes some nutrients to be leached
from soil or become unavailable to plants.
Lack of tree roots can allow rapid erosion.
Increased wind near ground dries soils à
soil erosion
Large openings in forest can expose trees to blowdown.
When disturbance alters ecosystem functions, the available
niches change.
Secondary succession on an abandoned field in
Surface soil water and temperature conditions (microclimate)
change throughout the plant succession.
Soil accumulates nutrients.
Pines overtop shrubs.
Hardwoods establish in the cool, moist shade beneath oaks.
As pines senesce and die after 50 years, they are replaced
by oaks.
Secondary succession
Seed bank – seeds accumulated in soil
Early successional species – shade
intolerant; small seeds; opportunist species
Late-successional species – shade
tolerant; large seeds; good competitors (Fig. 15.10)
Succession and animals
-important
roles as herbivores, seed eaters, seed dispersers
-some
animals are associated with a certain stage of plant succession
northern spotted owl of
-needs
mature forest habitat with some dead trees for nests
Fig. 15.11 shows birds associated with successional
vegetation types in piedmont region of
The intermediate disturbance hypothesis: (Fig. 15.12)
-highest species diversity occurs with moderate disturbance
-provides a diversity of patch age in a forest
-more niches à more species
Fire and succession
Fire is frequent in some ecosystems.
Where fires are frequent, they have low intensity.
Here plants are fire-adapted to resprout
or germinate after fire.
Ash is fertile in potassium and phosphate, but low in
nitrogen
Large intense fires can kill all plants and roots, burning
soil organic matter and allowing erosion.
Recovery from intense fire takes much longer.
Fire suppression alters fire regime from frequent,
low-intensity to infrequent, high-intensity fires.
Controlled burns can re-establish natural fire frequency.
If managing for timber production, forests are even-aged
stands of young, fast –growing trees.
If managing for biological diversity, forests are mature,
containing all ages of live trees and large standing dead trees for nesting
holes.
Foresters and conservationists remain at odds over forest
management.
Equilibrium or Nonequilibrium?
If disturbed, will a forest repeatedly return via succession
to its previous structure and species composition?
In most cases no; the chance arrival of species can affect
composition of mature vegetation.
Disturbance events are frequent enough that most vegetation
is currently in some successional stage.
Succession has no absolute endpoint.
Most ecosystems are not equilibrial.