Community dynamics-
Ecological Succession Dr. Anugrah Tripathi
•Communities are never stable, but keep on changing.
•Series of changes until the formation of successful community is called ecological or
biotic succession.
•Hult gave the term “Ecological Succession”
•The gradual replacement of one community by another in the development of
vegetation towards climax.
Types-
Primary succession- starts from primitive substratum- no already built living matter.
Secondary Succession- starts with already built substratum where there was living
matter earlier.
Autogenic Succession- when succession vegetation itself is responsible for
replacing/ modifying itself by changing existing environmental conditions.
Allogenic Succession- when in succession, any other external condition is
responsible for the replacement of community.
Hydrosere/ Hydrarch- succession beginning in fresh water.
Xerosere/ Xerarch- succession beginning in dry conditions
I. Lithosere- succession on bare rocks
II. Psammosere- succession in sandy conditions
III. Halosere- succession in saline conditions
Mesosere/ mesarch- succession in adequate moisture conditions
•As succession proceeds, changes occur not
only in the biotic community but also in
physical environment and overall
characteristics of ecosystem. (Margalef,1968,
Odum, 1969)
•Odum considered succession as Ecosystem
Development.
Mechanism of succession-
Classical approach-Clementsian Hypothesis-
Clementsian recognized three major phases and six basic processes in succession
Phase(i) Initiation
Nudation
Migration
Ecesis
Phase (ii) Continuation
Competition
Phase (iii) Termination
Stabilization
Clementsian Hypothesis (Clements, 1916) considered climax or terminal community as super
organism.
Initial Floristic Composition (Egler,1954)
•Site may contain the seeds and vegetative parts of pioneer as well as that of several seral
stages
•Group of species which form climax community may be present from the beginning, they
grow slowly and occupy climax later.
Disequilibrium theories of Succession-
•Gleason (1926) Individualistic concept explained that “succession is an
extraordinarily mobile phenomenon whose processes are not to be stated as fixed
laws, but only as general principles of exceedingly broad nature and whose results
don’t occur in any definite predictable way.”
•Disequilibrium theory emphasized that-
•Ecosystems are not super-organisms.
•There are many possible stable vegetations.
•Succession is multi-directional.
•There is no long lasting climatic climax. System is always changing
Resource ratio Hypothesis
•Proposed by Tilman (1980)
•It predicts that inter-specific competition for resources and long term
pattern of supply of limiting resources are the two key elements in
succession.
•Plants that are superior competitors for one resource must be the inferior
competitor for another resource that becomes limiting.
•Because of tradeoff, a good competitor for nutrients can not be the good
competitor for light, as they require contrasting traits.
Characteristics of Pioneer community
•Strong light demander
•Produce large quantity of small seeds.
•Capable to disperse widely and to withstand unpredictable environment.
•Possess prolonged dormancy.
•Have ability to germinate in wide range of environment.
•In late successional stage, environment is more predictable and organisms allocate more
energy to non-reproductive activities.
•So Pioneer species may be called as r strategists and Climax species may be called as k
strategists
“
•Different stages of succession are called seral
stages and all these seral stages constitute a sere.
•First community- Pioneer community
•Last community- Climax Community
Lithosere
or
Xerosere
Hydrosere
Steps of succession
Nudation- formation of bare area. May be biotic, climatic or
topographic.
Invasion- successful establishment of the species in that bare area. It
involves 3 steps-
(i) Migration
(ii)Ecesis or establishment
(iii)Aggregation
Competition and Co-action
Reaction
Stabilization
Climax
Climax concept
▹Final and stabilized community in succession that establishes equilibrium with the
environmental conditions of the area, was termed as “Climax” (Clements,1916)
▹Climax represents a steady state of species composition, community structure and
energy flow.
Characterstics of climax community-
(i) High tolerance
(ii)Mesic for climate
(iii)High degree of biological organization due to complex food webs and stratification.
(iv)High species diversity in maturer community.
(v) Fragile nature due to high resistance and low resilience.
(vi) It is an index of the climate of the area. The life or growth forms indicate the climatic
type.
Mono-climax theory
• According to the mono-climax theory of succession (Clements, 1936), every
region has one climax community toward which all communities are
developing.
• He believed that climate was the determining factor for vegetation and the
climax of any area was solely a function of its climate.
• The character appeared in the climax are due to dominants that control it.
• Various terms such as sub-climax, dis-climax, post-climax, and pre-climax
are used to describe the deviations from the climatically stabilized climax.
• Climatic climax concept of Clements became popular as “Mono-climax
theory”
•According to Cooper, there are various progressive
and retrogressive change which occur and reflect in
the communities.
•So the climax may be stage of minimum change.
•Climax may not be considered as an organism.
Poly climax theory
•This theory was proposed by Tansley (1939) and later supported by
Daubenmire (1966).
•The poly-climax theory of succession holds that many different types of
vegetation as climax communities.
•The stage in succession just preceding the climax was called as sub-climax.
•The community that became stabilized at any seral stage due to microclimate
or soil was called sere-climax.
•Community that is becoming more or less stabilized due to biotic or
anthropogenic disturbances is called Disclimax. It prevents development of
climax community in theta area.
Tansley (1920) recognized the existence of number of climax
communities.
Cimatic Climax
Edaphic Climax
Topographic Climax
Fire Climax
Zootic Climax
Climax Pattern theory
•Whittaker (1953) emphasized that a natural community is adapted to the whole pattern
of environmental factors in which it exists.
•The major factors are:
I. Genetic structure of each species
II. Climate
III. Site
IV. Soil
V. Biotic factors (activity of animals)
VI. Fire, and wind,
VII. Availability of plant and animal species.
VIII. Chances of dispersal
•According to this theory, climax communities are patterns of populations varying
according to the total environment.
•There is thus no discrete number of climax communities and no one factor determines
the structure and stability of a climax community.
Overview
•Community Biomass increases as succession progresses.
•After the stabilization, community structure remains unchanged for a long
period of time.
•Some species gain the control over the composition of other species , those
are called dominants.
•Dominants show physiognomy of community, so the physiognomy of stable
community remains unchanged.
•When dynamic equilibrium is attained between biotic and abiotic factor of
community, stabilization occurs.
Stability of climax community
•For stability of climax community under natural condition
regional climate should remain unchanged.
•An area with climax community is characterized by the
occurrence of dominant species of different stages of
growth and development.
•Physical environment determines the pattern, rate of
change and limits of the stability of a community.
The Climax Community
▹A climax community is a mature, stable community that is the final
stage of ecological succession. In an ecosystem with a climax community,
the conditions continue to be suitable for all the members of the
community.
▹Any particular region has its own set of climax species, which are the
plants that are best adapted for the area and will persist after succession
has finished, until another disturbance clears the area.
▹Two main physical factors determine the nature of the community that develops
in an area. These are temperature and the amount of rainfall.
▹If we place the amount of rainfall on a graph’s “x” axis, from 0-10, 10-20,and 20-
30+ inches and the temperature along the “y” axis from hot, moderate, to cold, the
various types of ecosystems will fit into the graph based on the conditions that
they require.
Temperature
Cold Cold desert Tundra Taiga
Moderate Temperate forest Grassland Deciduous forest
Hot Hot desert Savanna Tropical forest
Rainfall (inches) 0-10 10-20 20-30+
A summary of changes that occur during succession:
• Pioneer species colonize a bare or disturbed site. Soil building.
• Changes in the physical environment occur (e.g., light, moisture).
• New species of plants displace existing plants because their seedlings are better
able to become established in the changed environment.
• Newly arriving species alter the physical conditions, often in ways that enable
other species to become established.
• Animals come in with or after the plants they need to survive.
• Eventually a climax community that is more or less stable will become
established and have the ability to reproduce itself.
• Disturbances will start the process of succession again.
Threats to Succession
▹The grasses that move in as
pioneer species are often thought
of as weeds.
▹The subsequent growth of
shrubs are considered
undesirable "brush".
• Without these intermediate stages, the disturbed
habitat can't return to a natural forest.
• The fragility and stability of the ecological community
are dependent upon several factors.
• For example, in temperate forests, if the shrubs are
not allowed to grow, insect pests begin to feed on young
trees instead. This has happened in many places where
trees are replanted after an area is clear-cut. Large
quantities of pesticides are then brought in, polluting
the soil and water and altering the natural ecosystem
even further.
How Do Humans Affect
Ecological Succession?
▹ Clearing the land for the garden and preparing the soil for
planting represents a major external event that radically re-
structures and disrupts a previously stabilized ecosystem.
▹The disturbed ecosystem will immediately begin a process of
ecological succession.
▹Plant species that adapt to the sunny conditions and the broken
soil will rapidly invade the site and will become quickly and densely
established. These invading plants are what we call "weeds“ in
which we consider a nuisance or invaders.
Paleoecology
• From stand point of time, Ecosystem and
community changes can be divided into
two types:
(i) changes occurring over
geological time scale (Million Years)
(ii)Those occurring over medium
time scale(1-1000 Years)
• Community changes over geological time
scale are called Palaeo-ecological
Changes
• These changes are synthesized on the basis of fossil records i.e. leaves, twigs,
cones, pollens and seeds.
• Paleoecology, the ecology of the past, uses geological and biological evidence from
fossil deposits to investigate the past occurrence, distribution, and abundance of
different ecological units (species, populations, and communities) on a variety of
timescales.
▹There have been major paradigm shifts in conservation biology in the last 20 years
and these shifts are reflected, in part, by the links between palaeoecology and
conservation biology.
▹An essential first stage in conservation was the basic descriptive and evaluation phase,
leading to the recognition of biodiversity hotspots, areas of special scientific interest,
potential nature reserves.
▹Palaecology also tells ‘balance of nature’ paradigm to the ‘dynamic flux of nature’
paradigm.
▹A palaeoecological perspective can identify many potentially important direct drivers
such as Dutch elm disease, chestnut blight, and hemlock defoliation. Land-use changes
and resulting secondary succession are also important direct drivers for ecosystem
dynamics.
Importance
▹landscape and habitat diversity over time
▹determines the development of the mosaic structure of a landscape rich
in flora and fauna
▹ landscapes (including their soils) changes over time
▹ major drivers of ecosystem change at different timescales
▹ establish baseline conditions like studying palaeolimnology using
diatom assemblages at a particular time period.
▹ stablish the natural variability of ecosystems.
▹ thresholds of potential concern’ as benchmarks for management
▹ identify critical thresholds and resilience in ecosystems
▹ Identification of native and non native species.
Also tells about
1. How is the resilience of ecosystems to climate change affected by human activities and
interventions?
2. How will climate change affect the distribution and impacts of climate-dependent
disturbance regimes such as fires and storms?
3. Which aquatic species and communities are most vulnerable to human impacts, and
how would their degradation affect the provision of ecosystem services?
4. What are the likely risks, costs, and benefits of re-introducing and translocating species
as a response to climate change?
5. What time lags can be expected between climate change and ecological change?
6. What was the condition of ecosystems before significant human disruption, and how
can this knowledge be used to improve current and future management?
7. What is the likely relationship between the extent of climate change and the patterns of
species extinctions?
Thanks
