Paper No. : 12 Principles of Module : 34 Processes-II; Part-2

Development Team

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

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

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

Content Writer: Dr. Kapinder and Dr. Haren Ram Chiary Kirori Mal College, University of Delhi

Content Reviewer: Prof. K.S. Rao Department of Botany, University of Delhi

1

Principles of Ecology ZOOLOGY Ecosystem Processes-II; Part-2

Description of Module

Subject Name ZOOLOGY

Paper Name Zool 12 Principles of Ecology

Module Name/Title Ecosystem

Module Id M34 Ecosystem Processes-II; Part-2

Keywords , trends in succession, models of succession, examples of succession

Contents: 1. Learning Outcomes 2. Introduction 3. Models of Succession 3.1 Facilitation model 3.2 Tolerance model 3.3 Inhibition model 4. Changes involved in succession 5. Trends in succession 6. Examples of primary succession 5.1 Xerosere or Xerosere 5.2 Hydrarch or 7. Example of Secondary succession 8. Summary

2

Principles of Ecology ZOOLOGY Ecosystem Processes-II; Part-2

1. Learning Outcomes

After studying this module, you shall be able to:  Understand the term Ecosystem.  Know different models of succession.  Understand changes involved in succession.  Learn the example of primary and secondary succession.

2. Introduction

An ecosystem is never static but it is dynamic in nature. The structure and function of an ecosystem changes in a linear direction with time and its changes can be predicted. A.G. Tansley (1935) first proposed the term “Ecosystem” which can be explained as an integration of living organisms in a given area with the physical factors of the environment. All communities of an ecosystem have adapted themselves according to the environmental conditions. The species which are not adapted to the changed environment are replaced by the new species which is more suitable than the previous. This gradual change in species composition and processes of communities over time is known as ecological succession. The term ecological succession was first used by Hult in 1899. Clement in 1916, explained the ecological succession as a natural process in which the fundamental species of biotic components are replaced by other new species. The study of ecological succession plays important role in understanding potential of vegetation and helps in the management of ecosystem. The time needed by process of succession to reach at climax stage varies with the climate and initial quality of soil. The succession process is very slow on a bare rock (primary succession). However, in humid tropics, after deforestation, succession process regains to climax stage within 100 years (secondary succession) after clear cutting.

In the previous module, we had already learned the types of succession, concept and mechanism of succession and different theories of climax. In this module we will learn about the models and examples of succession. 3

Principles of Ecology ZOOLOGY Ecosystem Processes-II; Part-2

3. Models of succession

Connell and Slatyer (1977) proposed three models of succession to accommodate different possible pathways of succession. These models proposed that the vegetation which established early at the site of disturbed area during process of ecological succession, either facilitate favourable condition for new species, does not show any effect on other species or it inhibits the establishment of other species in the area. The species have some special characters which help them to establish in the bare land.

3.1 Facilitation Model: The hypothesis related to facilitation model is based on the Clements ideas of “relay communities” in which the existing species is thought to change the existing . According to this model, the which is formed earlier makes the conditions or the availability of resources (water, nutrients and soil composition) suitable for the new species from other area to establish in the area and thus replace the existing one (figure 1).

A B C D

Figure.1: Facilitation Model. A, B, C and D represent different species and arrow represents replacement of one species by another.

The new species compete for the resources and causes removal of the previous species. The succession process proceeds in a predictable and orderly manner which finally reaches to stable climax stage. Evidence for this model was given from the study of succession of glacial foreland at Glacier Bay, Alaska and sand dunes succession along Lake Michigan.

4

Principles of Ecology ZOOLOGY Ecosystem Processes-II; Part-2

3.2 Tolerance Model : This model is based on the concept of initial floristic composition (IFC) which suggests that the new species which appears in the area does not necessarily cause removal of the pioneer species. The tolerance model suggests that the process of succession proceed in a predictable manner which happens due to the existence of species having different strategies for exploring the resources of the area. The species which invade the area at later can grow to maturity in the presence of the species which was present earlier (figure 2).

A B C D

Figure.2: Tolerance Model. A, B, C and D represent different species and arrow represents replacement of one species by another.

The tolerance model differs from Clement’s idea of relay succession model in the fact that during the process of succession higher complex form of plants are tolerated to join and co- exist, than necessarily swapping the earlier species. During the process of succession, those species which can grow in the presence of other species, would take over the control and finally forms the climax vegetation. This model can be explained by the pattern of old field secondary succession.

5

Principles of Ecology ZOOLOGY Ecosystem Processes-II; Part-2

3.3 Inhibition Model: According to this model, the success of species in establishing themselves on a new habitat depends upon which species arrived first. It is not necessary that one species has competitive superiority over another and the species which colonizes first in the bare area can hold it against all invaders. Later colonists can invade, grow and mature only when the first occupant die. In general, short lived species are eventually replaced by long lived species (figure 3).

A B

C D Figure.3: Inhibition Model. A, B, C and D represent different species and arrow represents replacement of one species by another.

The inhibition model can be observed in allelopathy which may be the common counter- acting adaptation to inhibit the entry of later species. In such case, relay succession can take the advantages to control only when allelopathic plants die. All models may be correct but applicable in different situations. Figure 4 summarizes various steps involved in different models of ecological succession.

6

Principles of Ecology ZOOLOGY Ecosystem Processes-II; Part-2

Disturbance occurred in large area

Only certain pioneer species Individual of any species in are able to establish in an succession could established and area. exist as adult in prevailing conditions.

Pre-existing species modify the Modification of environment Pre-existing species environment and make the by Pre-existing species has modify the environment suitable condition for late- little or no effect on and make it less suitable successional species. of late- for late-successional

successional species. species.

Early species are eliminated from As long as early species persists the area through for undamaged and/or continue to resources with late successional regenerate vegetatively, they species. exclude or suppress subsequent colonist of all species

The sequence continues until the It external stresses are present, current existing species no longer early established species may be facilitate the invasion and growth damaged or killed and replaced of other species and/or no species by those species which are more exists that can invade and grow in resistant to external stress. the presence of the presently existing species.

Figure 4: Summary of different models of succession.

7

Principles of Ecology ZOOLOGY Ecosystem Processes-II; Part-2

4. Changes involved in succession

As succession process begins, both biotic community and abiotic components of an area changes and hence overall characteristics of ecosystem changes (Margalef 1968, Odum, 1969) which is considered as ecosystem development. The process of succession occurs to achieve: a) Homeostasis with physical environment. b) Stability c) Maximum support of complex structure for sustenance. d) Diversity e) Vigor growth f) Balance between system components g) Absence of disease

The first species formed during succession (pioneer) required high light and produces small but large number of seeds which can withstand the adverse environmental conditions and can be easily dispersed to longer distance by wind or other agencies. These species are also adapted to germinate and grow in wider range of environmental conditions. In the later stage of succession, environmental conditions become more predictable and there would be competition for density-related factors in the species. The individual of the species now allocate more energy for doing non-reproductive activities. As the environment becomes more stable, perennial species of plants appear in the area that can store biomass from one year to another and their production is diverted to storage and maintenance activity.

With the progression of succession the biomass of the area increases due to the presence of late successional species which are long-lived as compared to pioneer stages. At the same time, availability of light intensity decreases due to growth of canopies. Early stages of succession are more explorative than the later stages. The organisms in the early stages are linked through food in the simple and linear manner whereas in later stages, food chains become more complex with increased number of trophic levels. As a result, role of pathway of become more prominent as more complex tissues rich in lignin and

8

Principles of Ecology ZOOLOGY Ecosystem Processes-II; Part-2

cellulose are common. The feedback control between producers and consumers acts against the grazing pathway of energy flow. In the beginning, the individuals of the species are short-lived and most nutrients present in them are returned to soil through . In the later stage of succession, long-lived species lock up the nutrients in their body and only a fraction of it is returned to soil (falling leaves etc). Population interaction such as symbiotic relationship develop which tighten the and stability of an ecosystem. The resilience of early stages of succession is higher than late succession stages. In other words, early formed species have the ability to recover faster from as compared to later stages. Figure 5 summarizes the steps involved in the succession process (ecosystem development).

Figure 5: General model system of Ecosystem Development.

5. Trends in succession

Various trends which occur during the process of ecological succession (figure 6) is as follow:  Continuous change in flora and fauna.  increases.  Organic matter and biomass increases which support available energy flow.  The overall of an ecosystem increases.  The number of increases which indicate more energy transfer.  Complexity of increases.  and biomass reaches to maximum at climax stage.

9

Principles of Ecology ZOOLOGY Ecosystem Processes-II; Part-2

 Concentration of mineralized nutrients (N, S, P, Mg) decreases.  Abiotic factors such as high nutrient content, water retention capacity, high moisture content, pH etc. stabilizes within the ecosystem.

Figure 6: Comparison of diversity during the ecological succession.

6. Examples of primary succession

6.1: Xerosere or Xerarch:

This type of succession begins in dry area such as bare rock surface where there is severe scarcity of water and nutrient deficiency (figure 7). The original substratum is deficient in organic matter. The minerals are present in disintegrated unweathered state. The different stages are as follows:

10

Principles of Ecology ZOOLOGY Ecosystem Processes-II; Part-2

Figure7: Barren land formed as a result of volcanic eruption.

1) Pioneer or crustose stage: The first species on the bare ground is pioneer plant species. These are often 'opportunist' who rapidly exploits the resources when enter into new area and grow best in the absence of competition for space and resources (figure 8). The bare ground is dry and hard with full exposure to sunlight and rapidly changing moisture conditions. In the absence of soil, the root penetration is not possible and supply of nutrients is also very less. Under these conditions, pioneer species of such as Rinodena, Rhizocarpon, Lecanora and Licidea are found which are brought there in the form of or by fragmentation by wind. These Lichens produces acids which accelerates the process of weathering and when they die, their dead organic matter mixed with rocks to form thin layer of soil. They are replaced by foliose type of lichens.

11

Principles of Ecology ZOOLOGY Ecosystem Processes-II; Part-2

Figure8: Crustose lichen.

2) Foliose lichen stage: The foliose lichen stage appears on the substratum which is built up by previous existing crustose lichens (figure 9). These foliose lichens include Parmelia, Dermatocarpon, and Umbilicaria. The thallus of these lichens shadow the previous crustose lichens and gradually the crustose lichen die and decay. These species are able to absorb large amount of water and accumulate dust particles. The water and are continuously accumulated in the area and evaporation process is decreased.

12

Principles of Ecology ZOOLOGY Ecosystem Processes-II; Part-2

Figure9: Foliose lichen on granite.

3) stage: Accumulation of sufficient amount of soil allow the growth of xerophytic like Grimmia (black moss), Tortula (twisted moss) and Polytrichum (Hair moss). The rhizoids of these mosses compete with lichens for nutrients and other resources and their erect stem overshadows the earlier established communities (foliage lichens). The death of mosses further increase the amount of soil and a thick mat of dead moss is formed over the rocks, which can absorb the large amount of water (figure 10). This change in the structure and quality of soil allows the invasion of new communities of . As a result of low growth and carpeting over ground with little height, many mosses are unable to compete for space amongst taller, dense ground cover. This makes bare ground for the establishment of different moss species. These mosses provide suitable microhabitat for a variety of invertebrates like as mites and .

13

Principles of Ecology ZOOLOGY Ecosystem Processes-II; Part-2

Figure 10: Establishment of moss on a rock.

4) Herbaceous stage: Death and decay of mosses make the ground more fertile and make thick cushiony carpets with more water-holding capacity. These new conditions of the environment make suitable place for the germination and establishment of seeds of other competing plants (figure 11). The soil is now suitable for the germination and growth of seeds of xerophytic herbs. The first established herbs appeared on the ground are annuals and later biennial and perennial grasses appeared. These herbs finally grow into tall, dark thickets which overshadow and displace the moss. Due to death and decay of grassy plants such as Poa and Festuca, further humus increase in the humus takes place. Temperature and evaporation process declined and result in shortened drought period. These changed conditions favoured the growth of shrubs.

14

Principles of Ecology ZOOLOGY Ecosystem Processes-II; Part-2

Figure11: Colonization of herbs and shrubs 5) Shrub stage: As the conditions become favourable for shrubs, seeds or invaders from the adjacent area reach and establish in the area and make the conditions unsuitable for the herbs. Shade of shrubs minimizes evaporation, increases humidity and wind movement is retarded (figure 12). These modified environmental conditions with enriched soil provide ideal conditions for the growth of seedlings. Not only plants, invertebrates that feed on plant species also colonize the area. The presence of also attracts various carnivorous invertebrates which feed on them. A number of small mammals also appear in the area including shrews and mice which in turn attract (owls and foxes) to feed on them. The various interactions of different communities are established in the area. The earlier simple food chains of pioneer stage transforms into more complex food webs.

15

Principles of Ecology ZOOLOGY Ecosystem Processes-II; Part-2

Figure 12: Shrub stage in ecological succession stage.

6) Climax forest stage: The first species of the which established in the area are relatively xeric having stunted growth and distributed widely. Process of soil formation continues and moisture in soil increases gradually. Humus concentration further increases due to death and decaying of plant species which increase the fertility of soil. As a result, number of tree species further increase in the area. The xeric species is replaced by mesophytic species and give rise to new vegetation indicating a more humid atmosphere. Finally the succession stage reaches into forest which is self-sustainable and interacts with environment. This stage is called as in which rich fauna develops in the area having several species of invertebrates and vertebrates. These include arthropods, mollusks, amphibians, birds, reptiles, small rodents and mammals (like fox etc).

16

Principles of Ecology ZOOLOGY Ecosystem Processes-II; Part-2

Figure13: Climax forest stage.

6.2: Hydrarch or hydrosere:

Hydrarch succession starts in ponds, lakes or elsewhere in a water body (figure 14). The succeeding changes occur in plants and animal life but changes are more noticeable in plants as compare to animals. The various stages of hydrarch are as follows:

Figure 14: Open Freshwater

17

Principles of Ecology ZOOLOGY Ecosystem Processes-II; Part-2

1) Pioneer stage: The pioneer species include the phytoplankton such as , bacteria and animals like Paramecium, Amoeba and Vorticella (figure 15). When these phytoplanktons die, they get mixed with silt brought by rain water in the pond and forms soft mud at the bottom of the pond. Due to continuous accumulation of the mud, the pond become shallow which allows the light to penetrate up to the depth of pond and make a suitable environment for rooted submerged hydrophytes.

Figure15: Pioneer community of Hydrosere.

2) Submerged stage: The depth of pond gradually decreases (20 feet) due to accumulation of large amount of sediments carried from surroundings which allow rooted, submerged plants (starwort and pondweed) to grow (figure 16). Water lilies with floating leaves may also appear in the ponds. The submerged stage includes Vallisneria (eel grass), Potamogeton (pond weed), Cerratophyllum (hornwort), Ranunculus (butter cups), Elodea (water weed), Hydrilla, Chara etc. The animal appears in the pond may include nymphal stages of Dragonflies and Mayflies, Daphnia, Cyclops, Cypris and Gammarus. After death, these plants sink at the bottom and forms mass of humus which binds with the muddy soil. Gradually, the depth of water body decreases further and the conditions become unfavorable to the present community making it suitable for the new invaders. 18

Principles of Ecology ZOOLOGY Ecosystem Processes-II; Part-2

Figure16: Submerged plants.

3) Floating stage: The depth of water decreases up to 6-8 feet which allow different types of rooted hydrophytes with large floating leaves to invade and established in the area. The main species of plants include Nymphaea (water lily) and Castalia with broad floating leaves. The pond weeds include Potamogeton (pond weed), Eichhornia (Water Hyacinth) (figure 17) and Polygonum (knotweed). Free floating plant species include Wolffia (water meal), Pistia (water cabbage), Lemna (Duckweed) and Salvinia. The animal life includes Hydra, gill breathing snail, frog, salamander, diving beetles and various insects’ larval forms.

Fig.21. Floating stage-Duckweed and water hyacinth Figure 17: Floating plants.

19

Principles of Ecology ZOOLOGY Ecosystem Processes-II; Part-2

4) Reed-swamp stage: The depth of water becomes shallower (about 1-4 feet) which support fully submerged or floating plants. Instead, emergent plants such as Branched Bur-reed, Yellow Iris and reed mace grow in the water. These plants have tall, flexible and spear-shaped leaves which allow the plants to overcome the fluctuations in water level. These plants always hold some part of leaves above the water for effective (figure 18). The main plants include Scirpus (the sedges), Typha (cattail, bulrush), Saggitaria (arrowheads), Phargmites (reed grass), Rumex etc. By the activities of these plants, the conditions become more suitable for higher forms. The animals life includes Lymnea, nymphal stages of may fly and dragon fly, Physa, mud pickerel and bullheads. Birds such as kingfishers, red winged black birds, swamp sparrow and ducks also arrive in the area. Mammals like beavers, muskrats may also be present. The swamp area provides an ideal place for spawning of frogs.

Figure18: Reed swamp stage

5) Marsh-meadow stage: As the marsh floor rises above the water level, plants which are adapted to grow in partially submerged conditions of swamp, gradually die out except for few such as Yellow Iris, which grow equally well in swamp or marsh conditions, continues to grow, along with marsh plants. These damp areas provide a suitable habitat for young amphibians like frogs and toads (figure 19). The abundant wet vegetation provides an ideal hiding place for Grass Snakes. Trees such as willow become well-established in the wet soil conditions. Willow exhibit high transpiration rate due to which, large amounts of water is lost from the sediment into the

20

Principles of Ecology ZOOLOGY Ecosystem Processes-II; Part-2

atmosphere. The substratum at this stage is covered with 1-2 inches of water and the soil become marshy. Numerous species of Sedges like Juncus (rushes) Carex (carices), Eleocharis (spike rushes) and forbes like Mentha (mint), Colha (marsh marigold) and Campanula (bell flower) invade the habitat. They make the soil more suitable for the mesophytes and terrestrial plants.

Figure19: Marsh stage

6) Woodland stages: The marsh becomes almost dry but soil still remains wet. The willow dominates the area which supports wide varieties of invertebrates (more than 450 different species) which in turn attracts large number of insectivorous birds. Different species of fungi which are adapted to wet conditions also become established in the area. Another tree, Alder also flourishes in wet conditions. The marshy plants are replaced by different species of woodland floor plants such as sedges, ferns, rushes, and small flowering herbs. They are adapted to low light and wet conditions. The wood land stage also consists of shrubs (Alnus and Populus), trees and those plants which can tolerate water logged soil such as Salix, Cornus and Cephalanthus. These plants interact with the environment by producing shades and decreasing the water table. Transpiration process of plants continuously dries out the soil to make the place suitable for climax communities such as , or Ash.

21

Principles of Ecology ZOOLOGY Ecosystem Processes-II; Part-2

Figure20: Woodland stage. 7). Climax forest stage:

The area which was previously covered by deep water has now changed into a forest. The climax community consists of different tree species such as Oak, Ash and Beech. They are slow-growing but they are tall and long-lived which dominate an area. The particular climax tree species which eventually dominates the woodland will depend on soil type and other environmental conditions (figure 21).

The presence of different species of plants particularly flowering plants and grasses attracts large number of woodland butterflies. A different range of fungi will also inhabit the climax community which grows on rotting wood and provides microhabitats for many different invertebrates. The trees also provides habitat for different variety of mycorrhizal fungi, such as the Fly Agaric, which is associated with birch. Various mammals also inhabit the woodland like squirrel, fox and badger. The biodiversity increases gradually in a climax oak woodland community.

22

Principles of Ecology ZOOLOGY Ecosystem Processes-II; Part-2

Figure21: Climax forest stage.

7. Example of secondary succession

Trees of climax community mature and eventually die. When they fall to the ground, they initiate new process of succession to begin all over again. But the communities which colonize in the new opening are different from the species which established on the bare ground due to different environmental conditions. As starting point is different, this cycle of succession is known as a secondary succession (figure 22). Thus, secondary succession is a series of community changes which take place on a previously colonized, but disturbed or damaged habitat. The disturbance could be due to deforestation or forest fires etc. Secondary succession usually occurs much faster as compare to primary succession because:  or seed bank of suitable plants are already present in the soil.  Regeneration of previously existing plants occurred from undisturbed root systems.  The fertility of the soil is higher in the soil as compared to barren land due to significant modification done by previously existing communities making it more suitable for growth and colonization.

23

Principles of Ecology ZOOLOGY Ecosystem Processes-II; Part-2

Figure22. Secondary succession in climax stage forest

24

Principles of Ecology ZOOLOGY Ecosystem Processes-II; Part-2

8. Summary

• The gradual change in species composition and processes of communities over time is known as ecological succession.

• Understanding the process, rates and pattern of ecological succession is important for the management of ecosystem and for understanding vegetation potential and dynamic changes in the landscape.

• The time required for succession to proceed varies with the nature of the climate and initial quality of soil.

• The stabilized community in ecosystem development in the final phase is able to form equilibrium with the environment and results in climax community.

• Facilitation model of succession states that new community which is formed prepares suitable ground to replace its own by some other better adapted community.

• Tolerance model suggests that appearance of new higher form of species does not necessarily eliminate the pioneer species.

• Inhibition model suggests the early arrived species in a new habitat may develop counter-mechanism to normal replacement process.

• Xerosere on rock: Barren rockCrustose lichen stageFoliose lichen stageMoss stageShrub stage Climax forest.

• Hydrosere: Fresh waterPlanktonic stageSubmerged stageFloating stageMarsh meadow stageReed swamp stageWoodland stageclimax forest stage.

• Secondary succession: Stable deciduous forestForest fireForest incompletely destroyed  Grass  Herb  Shrub Evergreen forest

25

Principles of Ecology ZOOLOGY Ecosystem Processes-II; Part-2