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ZOOLOGY

Principles of Ecology

Community: Community characteristics, types of biodiversity,

diversity index, abundance, species richness, vertical and horizontal

stratification: Part IV

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Paper : 12 Principles of Ecology

Module : 20 Community: Community characteristics, types of biodiversity, diversity

index, abundance, species richness, vertical and horizontal stratification: Part IV

Development Team

Paper Coordinator: Prof. D.K. Singh Department of Zoology, University of Delhi

Principal Investigator: Prof. Neeta Sehgal Department of Zoology, University of Delhi

Content Writer: Dr. Haren Ram Chiary and Dr. Kapinder Kirori Mal College, University of Delhi Content Reviewer: Prof. K.S. Rao Department of Botany, University of Delhi

Co-Principal Investigator: Prof. D.K. Singh Department of Zoology, University of Delhi

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Contents

1. Learning Objective

2. Introduction

3. History of study of succession

4. Ecological succession and types: Primary and secondary succession

5. Stages of Primary and secondary succession

6. Process of succession in Hydrosere

7. Process of succession in Lithosere

8. Theories of climax community

9. Summary

Description of Module

Subject Name ZOOLOGY

Paper Name Zool 12, Principles of Ecology

Module Name/Title Community

Module Id M20, Community characteristics, types of biodiversity, diversity index,

abundance, species richness, vertical and horizontal stratification : Part-IV

Keywords Succession, Primary succession, secondary succession, Sera, Climax

community, Hydrosere, Lithosere, theories of climax community

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1. Learning Outcomes

After studying this module, you shall be able to

• Understand the meaning of succession and the history of study of succession.

• Briefly study the types of succession

• Learn about the general pattern of Primary and secondary succession

• Evaluate different stages of Hydrosere succession and Lithosere succession

• Understand different theories of Climax community

2. Introduction

More or less but communities are regularly changing over time and thus, are dynamic

entities. Each community has its own developmental history with its particular prevailing

environment and time. The source of studying the growth and development of ecological

community can be a barren area. The development of the community initiates on arrival of

the propagules or their parent organisms to the area of study. Out of all the immigrating

organisms or popagules only few of them survives, grow and gets established at the new site.

The first developmental stage of the community is called as the pioneering community

which is made up of those few successfully survived and established organisms or their

propagules. As more and more new species immigrates to this site the competition for the

resources becomes more intense between them. Due to the activities of the survived species

and physiography and constant change in the climatic conditions, the prevailing environment

conditions keeps on changing. The nature works on the principle of “survival of the fittest”

and thus, supports only those organisms which adapts and fits best to the changing

environment conditions. Until the community becomes incomplete accordance with the

environment and become a relatively stable community, the process of development and

change in community continues. The changes occurring during the entire process of

community development is ordered and highly directional. This highly directional and

sequences changes in the community are called as succession.

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3. History of the Study of Succession

Europeans have already described the coastal dunes related succession when, in 1899, Henry

Cowles’s got inspired from this study and started his study on sand dunes along Lake

Michigan. He defined the laws essential to the changing patterns of the community and

influenced many ecologists of new generation. He viewed plant communities and

documented that these communities differ in the underlying physical environment and

species composition and are dynamic in nature. The plant communities in sand dunes were

studied at different ages in order to reflect the pattern of changes in the community over time.

Since then, many other ecologists along with Cowles studied the succession process by using

the chrono-sequence approach. This temporal approach to study succession process is

difficult to follow for centuries or decades and thus replaced by another approach that uses

spatial differences between communities to infer the occurrence of succession. Spatial

differences means locations of different ages are compared to understand the process of

succession. Sere is defined as the stage in the succession sequence. Ecologists identified

animals as the factors that affect the rate and direction of succession along with different

patterns of succession. All the taxonomical groups such as rodents, fish, protozoans and

insects and all type of ecosystems such as marine or aquatic etc are being investigated to

study succession.

4. Ecological succession and types: Primary and secondary succession

In ecology, the community structure evolves over time and the process of this progress is

termed as Ecological Succession. Over a period of time, the replacement of one population or

community by another is called as succession. The sequence of change in the type of

community sequentially relates different community types and describes the landform or soil

or total site succession and vegetation succession.

Ecological succession is majorly categorized into primary and secondary succession. Based

on the association between individual with their biotic and abiotic factors, ecologists

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suggested two types of succession process (a) primary and (b) secondary succession.

Primary Succession: The establishment of a community in an area never been habituated by

organisms or where no prior existence of life is evidenced. The presence of complete lifeless

empty area may be because of greater natural disasters such as lava flow, sand dunes or via

glacier newly introduced rocks etc. Over a period of time few living organisms manage to

sustain life in these empty areas and with continuous succession variety of life-forms begins

to grow in the area. Secondary succession: The more common type of succession is

secondary succession where the existed life forms of community are partially destroyed by

small scale disturbances, for example forest wildfire. For a period of time, with remaining

nutrients in environment the area remains empty but gradually life begins to reappear and

communities changes over time.

In both type of succession the sizes, intensities and frequency of change in species vary

within communities while altering the area or habitat leading to mixture of species exploiting

the prevailing conditions of community. The species evolved life histories in a partially

predictable sequential way and leads to gradual progressive changes in species composing the

community. Initially in the unfavourable environmental conditions of this disturbed habitat

only few species from the surrounding areas invade and then they modify the habitat by

alterations such as soil component compositions, amount of sunlight or shades on grounds

etc. The first community established in an originally completely lifeless land is called Pioneer

community and the plants and animals are called Pioneer plants or animals. After these

successive changes in habitat environment a stable community is achieved termed as Climax

dominated by few prominent species. The community with this stable state of equilibrium is

termed as Climax Community. Thus, Climax community is characterized by stable

environment conditions limiting any further admission of other species in the community. At

each successive community the replacing species are more complex. Other than climax

community other communities at the time of succession undergo small scale environmental

disturbances thereby offering species diversity with equal probability of each species to

become dominant.

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5. Stages of Ecological succession

A process of ecological changes where established communities are replaced by more

evolved natural communities is called as succession. Depending on the type of area over

which succession has to take place mainly two types of succession occurred: primary and

secondary succession.

1. Primary succession

When completely lifeless bare area starts succession it is called as Primary succession.

2. Secondary succession

When succession starts from an area which has been once occupied by vegetation but become

vegetation free due to some disturbances, the mechanism called as denudation or secondary

succession. The disturbances can be because of strong winds, fire, rains and cultivation.

The stages of ecological process of primary succession includes following stages:

1. Nudation

Nudation is a stage occurred when greater disturbances leads to formation of a bare land

uninhabited by any plant or animal species. The disturbing factors can be biotic (because of

human activities) or abiotic like climatic (fire, hails, glaciations etc) or topographic (wind

action, soil erosion etc.). This uninhabited land can sustain only autotrophic species

consuming inorganic substrates and offering a new environmental condition for inhabitation

of newer species. For instance, rocks are breakdown to soil with the help of acidic substance

secretion by lichens. These lichens act as a Pioneer species in the community.

2. Migration or Invasion

Migration is a two way direction movement or mechanism of dispersal and a group

characteristic feature of population. The reproductive propagules such as spores, seeds etc.

migrate for the species establishment into a new area. Non-native organisms called invasive

species enter from the neighbouring environment and spread widely in a community. These

invasive species are responsible for community disturbances and thus are threat to the stable

ecosystem but in the succession process they aid in altering the environmental conditions

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such as soil composition, function and texture. R-Strategists with high fecundity rate and high

dispersal rate are the first colonizers.

3. Ecesis

On the basis of soil composition the initial establishment of community is called

colonization. The initial community proliferates abundantly. Allogenic mechanisms of

succession are those which are completely driven by the abiotic factors i.e. environment

mediated and completely regulate the ecesis process. Ecesis happens when different pioneer

species are sustained after dispersal in an area, they have different germination, growth and

reproduction rates and that’s why the pioneer species take much longer time and gradual

replacement of other species. By the establishment of these pioneer species the texture and

composition of soil changes which leads to germination of seeds already present there but

were not able to germinate.

4. Aggregation

The established species in the area increases in population and this increase in population

density is called as Aggregation. As an example in most successions of plant community

small herbs is been replaced by shrubs and further inhabitants are supported by this as a

source of food.

5. Competition

As a result of aggregation, few pioneer and initial species got established and increase in

population which further leads to enhanced intraspecific and interspecific competition among

the species. This stage of competition is for the resource utilization like shelter, nutrients,

water etc. Plants and animal species both exhibit the stage of competition and leads to either

competitive exclusion where no two species utilizing same resources can coexist or resource

partitioning where two competing species share their part of resources and can coexist.

6. Reaction

The established species in the habitat modify the environmental conditions subsequently

leads to gradual replacement of older species by newer ones as the conditions are now

become unfavourable for the survival of older species. This type of succession include

Autogenic mechanism driven by the biotic factors in which existing species themselves

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modify the environmental conditions offering harsh conditions for their own survival. Pioneer

colonizers facilitate the colonization of mature K-selected species.

7. Stabilization

The mechanism of climax community colonization is termed as stabilization. Climax

community is a final sate of equilibrium, stable, self-perpetuating and mature stage of

succession. The entire community is now been supported by the altered abiotic factors

(chemical and physical conditions). Climax community generally made up of K-selected

species adapts according to the area of succession and are well established until any further

external disturbances. The structure, composition and energy flow is very much specific for

climax community and thus represent a well-adapted steady state of equilibrium.

The general pattern of ecological succession occurring in a community is depicted in figure 1.

The process of secondary succession is almost similar to primary succession except some

steps. Also, as secondary succession starts from an initial state already enriched in soil

nutrients and the process occurs at much faster rate than primary succession. The limiting

factor in secondary succession is the disturbances. Both type of succession resulted into

stable and steady state of ecological equilibrium called Climax community.

Figure 1: General patterns of sequential succession

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Different types of succession in Plants

1. Halosere: If the plants succession starts from saline water or salty soil this is called

Halosere.

2. Hydrosere: Hydrarch is when succession of plants starts from aquatic environment and

the subsequent changes in the vegetation is called Hydrosere or Hyrarch.

3. Xeroarch or Xerosere: When the vegetation starts from a dry land, the process of

succession is called as Xerosere or Xeroarch. The two types of xeroarch are:

a. Lithosere: Succession when begins from the rock surfaces.

b. Psammosere: It is the succession that occurs on sandy habitat.

4. Serule: If succession starts from decaying woods, tree barks etc is called as serule. It

includes microorganism’s succession such as fungi or bacteria. All sere ends into mesic

community irrespective of succession types.

6. Process of succession in Hydrosere

When succession of vegetation starts from a water body such as pond it is called as hydarch

or hydrosere. The Stages in Hydrosere/ Hydarch are briefly (fig.2) described below:

1. Plankton stages

The pioneer community consists of cysted spores which germinate first in the new lakes or

ponds. Spores are dispersed there uninhabited places by dispersal agents such as animals or

wind. Population of phytoplanktons includes phytoflagellates, autotrophic diatoms,

cyanobacteria etc. which can withstand the harsh conditions and are regulated/ controlled by

zooplanktons. When these phytoplanktons are dead, they decay into organic substances and

after mixing with silt forms soft mud at the base of the water body.

2. Rooted submerged stage

Utricularis, Hydrilla, Vallisneric etc are the rooted submerged hydrophytes which grows on

the soft mud at the water body bottom when these hydrophytes are dead, their organic matter

along with the sand and silt leads to deposition of extra soil layer at the top of the soft mud

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and this give gradual rise to bottom level of water body eg. ponds. All the decaying matter is

a good source of human for nest successive vegetation.

3. Rooted floating stage

Trapa, Nymphea, Nelumbo, Monochoria etc are the leaved, floating and anchored

hydrophytes which invade this area and make it mineral and organic matter enriched. After

these anchored plants, Lemna, Azolla, Eichornia, Pistia etc are the new free floating

hydrophytes. The decaying matter of these plants will make the water level shallow.

4. Reed swamp stage

Hydrophytes like Sagittaria, Typha, Scripus etc replaces the rooted and free floating stage of

succession and are called as amphibious stage. These plants used transpiration mechanism for

large amount of water loss and are a source of huge amount of organic waste. These waste

materials of organic matter settle down at the ponds bottom and make the environment

unfavourable for these amphibious plant growth and development.

5. Sedge meadow stage

This stage is mainly habituated by hydrophytes like Juncus, Sedge (Carex), Diochanthium

and herbs such as Polygonum, Caitha etc. This stage of succession is also called as “marsh

meadow stage”. These vegetations have a characteristic rhizomatous system which forms a

mat like covering and this develop a mesic condition which is unsuitable for their own

growth.

6. Wood land stage

Shrubby plants like Cephlanthus, Cornus etc invades at the periphery of area. These shrubby

plants can tolerate water logged situation and bright sunlight. Later other invading plants also

grow there such as Alnus, Populus etc. The soil build up, amount of minerals increases and

further falling of these trees into the ponds leads to enhanced soil formation which further

favours next seral community colonization.

7. Forest stage

Forest stage is dependent on climatic conditions and leads to climax community (fig.4). For

instance, tropical rain forest within heavy rainfall areas, tropical deciduous forest in moderate

rainfall habitats etc are the forest stages.

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Figure 2: The Stages in Hydrosere/ Hydarch

6. Process of succession in Lithosere

Another example of succession can be studies by understanding the succession occurring in

the dry land. The succession initiating from the dry land is called as Xeroarch. Xeroarch or

Xerosere is further categorized into Lithosere, defined as the succession sequence which

begins from the rock surfaces (fig.3). The succession sequence in the growth and

development of lithosere community includes following stages:

1. Lichen stage

The pioneer community is composed of lichens species such as Rinodina, Rhizocarpon and

Lacanorma. This stage is also called as Crustose and the species of this stage can tolerate

desiccation. Lichens grow on rock surfaces by secreting acidic substrate which will corrode

the rocks and produce minerals and gaps within the rocks. These gaps are filled with organic

matter and soil particles as a result of wind action and initiate formation of soil. This initial

formation of soil in depressions of rocks supports the growth of lichens called foliose lichens

such as Physcia, Parmelia etc.

These newly invaded foliose lichens aggregate more soil particles as well as water leading to

formation of thin soil layer on rocks.

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2. Moss stage

Grimmia and Polytrichum are the moss which invades in an area due to the thin layer of soil

on rock surfaces. After some time, these mosses will accumulate more soil particles, water

and organic matter and leads to colonization of moisture loving mosses such as Bryum,

Hypnum etc. and thus called as moss stage of succession.

3. Herb stage

Moss when decomposed into organic matters forming a mat over fragmented rock and

provide a substratum for the seed germination of annual grasses such as Aristicla, Eleusin,

Poa etc. The death and decay of these grasses leads to replacement of the vegetation by

perennial grasses such as Heteropogon, Cymbopogon etc. Invasion of small animals were

also encounters at his stage of succession.

4. Shrub stage

Colonization of shrub such as Zizyphus, Caparis and Rhus are favourable of rock weathering

and mineralization. Shrubs are larger than herbs in size with deep root penetration and

further leading to soil formation via rock weathering.

5. Forest stage

The environmental is now favourable for the colonization of hard and stunted trees requiring

sunlight for growth. Mesophytic type of vegetation is supported by the environmental factors

and thus leads to an equilibrium, stable and steady state of succession between biotic

community and environment. For example, coniferous forests in temperate region etc are the

climax community (fig.4).

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Figure 3: The Stages in Xerosere/ Lithosere

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Figure 4: Successional stages involved in the succession of Hydrosere and Xerosere/or

Lithosere, both resulting into much stable Climax forest community.

8. Theories of Climax Community

The succession of community structure attains a stable and final equilibrium state called

Climax Community suggested by Shimwell (1971) and Clements (1936). This stable

community is in accordance with the predicting biotic and physically environment and is self-

perpetuating. Birth rate, death rate and growth process induce structural changes in climax

community. The following are the four theories of climax community:

1. Mono-climax theory

In 1936, Clements suggested the mono-climax theory of succession according to which all

communities of different regions to reach climax community. He believed that climate is a

determining functional factor of climax and vegetation is solely influenced by the climatic

conditions of the community. This climate based theory termed various stages of succession

process as sub-climax, dis-, post- and pre-climax with stabilized climax as last stage of

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community. Other than climate controlled community, exceptional cases of edaphic (soil),

topographic and biotic factors influencing communities are also discussed under this theory.

2. Poly-climax theory

In 1939, Tansley proposed Poly-climax theory which was supported by Daubenmire (1966).

According to this theory in an area the climax community may have varied type of vegetation

and these are called as climaxes. These climaxes are influenced not only by the climate rather

by the activities of animals, soil nutrients and moisture etc. Unlike mono-climax theory,

climate is one factor among all other influencing factors regulating the stability and structure

of climax community and allows multiple climaxes and thus called as poly-climax theory.

In 1994, Kreb suggested that it is the time dependent relative stability measurements which

differentiate between the two theories. The factors influencing climax varies with geological

time scale and ecological time scale such as climate. Thus different vegetation develops to

reach different climax leading to a continuous succession. Therefore, the mono-climax theory

and poly-climax theory mainly differ on understanding the functional factor which is

responsible for establishing the steady state climax community.

3. Climax-pattern theory

In 1953, Whittaker proposed climax pattern hypothesis. The climax pattern theory of

succession holds that the existing ecological community is affected by the entire pattern of

environmental factors. The environmental factors influencing the community can be climate

site, activity of animals (biotic), soil, species genetic structure, plant and animal species,

dispersal, wind and fire etc. This theory emphasizes on the total environmental conditions

regulating the pattern of population constituting the climax community. There is no single

functional factor influencing the climax community and thus, there is no mono- or poly-

climax.

Unlike mono-climax hypothesis resulting into one climatic climax in an area and several

climaxes in poly-climax theory, the climate pattern theory allow continuous inseparable

climax types under the influence of gradually changing gradient of environmental conditions.

4. Climax as Vegetation

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In 1954, Egler hypothesized in a broad sense that the whole pattern of vegetation represents

the climaxes. He favours the vegetation study of an area as by the careful observation of the

vegetation we can predict the community conditions.

Thus from these entire hypothesis we can conclude that the ultimate goal of succession is to

achieve a stable form of community structure called climax. The climax community can be

disturbed and incompletely stable. The vegetation of an area is totally controlled by the

existing climatic conditions but the broad climatic zones leads to many climax populations

under the influence of animal activities, topographic and soil compositions.

Figure 5: Flow chart depicting different theories of Climax community.

7. Summary

Communities are regularly changing or dynamic entities. The development of the community

initiates on arrival of the propagules or their parent organisms to the area of study. The first

developmental stage of the community is called as the pioneering community which is made

up of those few successfully survived and established organisms or their propagules. The

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nature supports only those organisms which adapts and fits best to the changing environment

conditions. The changes occurring during the entire process of community development is

ordered and highly directional. Until the community becomes in complete accordance with

the environment and become a relatively stable community, the process of development and

change in community continues. This highly directional and sequences changes in the

community are called as succession. Many ecologists along with Cowles studied the

succession process by using the chrono-sequence approach. This temporal approach is later

replaced by another approach that uses spatial differences between communities. Spatial

differences means locations of different ages are compared to understand the process of

succession. Sere is defined as the stage in the succession sequence.

Based on the association between individual with their biotic and abiotic factors, ecologists

suggested two types of succession process (a) primary and (b) secondary succession.

Primary Succession: The establishment of a community in an area never been habituated by

organisms or where no prior existence of life is evidenced. The more common type of

succession is secondary succession where the existed life forms of community are partially

destroyed by small scale disturbances, for example forest wildfire. Initially in the

unfavourable environmental conditions of this disturbed habitat only few species from the

surrounding areas invade and then they modify the habitat by alterations such as soil

component compositions, amount of sunlight or shades on grounds etc. The first community

established in an originally completely lifeless land is called Pioneer community and the

plants and animals are called Pioneer plants or animals. After these successive changes in

habitat environment a stable community is achieved termed as Climax dominated by few

prominent species. The community with this stable state of equilibrium is termed as Climax

Community. Thus, Climax community is characterized by stable environment conditions

limiting any further admission of other species in the community. The stages of ecological

process of primary succession includes following stages: Nudation, Migration or invasion,

ecesis, aggregation, competition, reaction and stabilization. The process of secondary

succession is almost similar to primary succession except some steps. The limiting factor in

secondary succession is the disturbances. Both type of succession resulted into stable and

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steady state of ecological equilibrium called Climax community. Halosere, Hydrosere,

Xeroarch or Xerosere (Lithosere and Psammosere) and Serule are the different types of

ecological succession occurring in plant communities. The Stages in Hydrosere/ Hydarch

succession includes initial plankton stages followed by secondary stages like rooted

submerged stage, rooted floating stage, reed swamp stage, sedge meadow stage, wood land

stage and forest stage ultimately leading to climax community. The succession initiating from

the dry land is called as Xeroarch. Xeroarch or Xerosere is further categorized into Lithosere,

defined as the succession sequence which begins from the rock surfaces. The succession

sequence in the growth and development of lithosere community includes stages such as

lichen stage, moss stage, herb stage, shrubs stage and forest stage. Birth rate, death rate and

growth process induce structural changes in climax community. The following are the four

theories of climax community: Mono-climax theory, Poly-climax theory, climax pattern

theory and climax as vegetation theory.

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