Ecosystem

• German zoologist Ernst Haeckel, who used the term ‘oekologie’ in 1869, became the first person to use the term ‘ecology’. The study of interactions between life forms (biotic) and the physical environment (abiotic) is the science of ecology. 
• Ecology: the scientific study of interactions of organisms with their physical environment and with each other.
• Ecological systems: The interactions of a particular group of organisms with abiotic factors within a particular habitat resulting in clearly defined energy flows and material cycles on land, water and air.
• Ecosystem: It is the functional unit of nature, where living organisms interact amongst themselves and also with the surrounding physical environment.
• Ecological adaption: Process in which various plants and animals adapt themselves to varying ranges of environmental conditions through evolution.

Types of Ecosystems: 

An ecosystem is too complex and big; It is divided into two basic categories:

1. Terrestrial: Further classified into biomes: Land-based plant and animal community that covers a large geographical area. Boundaries of different biomes on land are determined by climate: 
   • Biomes: Can be defined as a total assemblage of plant and animal species interacting within specific conditions.: Rainfall temperature, humidity and soil conditions. Some major biomes are: Forest, Grassland, Desert, and Tundra Biomes.
2. Aquatic: Can be classified into the following aquatic Zones: 

• Marine: includes oceans, costal, estuaries and coral reefs. 
  • Brackish – 5-35 o/oo – Chlika, Pulicat
  • Saline 35-50 o/oo – 
  • Brine >50 o/oo – Sambhar
• Freshwater: Lake, ponds, streams, marshes (relatively drier than bogs) and bogs.
  • Moving water – Lotic ecosystem
  • Stagnant – Lantic ecosystem (Pond)

Ecotone: Junction between two types of ecosystem: Edge effect – Edge species; For example, Mangrove forests between Terrestrial and Aquatic. 

Structure and Function of Ecosystems: 

Involves a description of available plant and animal species. Structural point of view all ecosystem consists of biotic and abiotic factors:

1. Abiotic factors: Rainfall, temperature, sunlight, atmospheric humidity, soil conditions, inorganic substances (CO2, Water, N, Ca, P, K etc.)
2. BioticFactors:  
   • Producers:  include all green plants, which manufacture their own food – photosynthesis.
   • Consumers: (Primary, Secondary and Tertiary)
     • Primary consumers include herbivorous animals:  goats, deer, mice etc.
     • Carnivores: flesh-eating mammals: snakes, tigers and lions. 
     • Top Carnivores: Certain carnivores feed on Carnivores: Hawks and mongooses, man.
     • Scavengers: Vultures and crows.
   • Decomposers: feed on dead organisms: 
     • Decomposing agents: Bacteria and various micro-organisms. Turn into substances such as nutrients, and organic and inorganic salts essential for soil fertility.

Function of Ecosystem: 

• Flow of Energy: The transfer of energy occurs during the process of the food chain from one level to another.
• Nutrient Cycle: We will study it later in the next chapter.
• Ecological Succession: We will study it later in this chapter.

Flow of Energy: 

See Ecosystem (Biology).

• Food Chain: Organisms of an ecosystem are linked together through a food chain – a sequence of eating and being eaten and the resultant transfer of energy from one level to another. For example, Plant – beetle – frog – snake – hawk.
• Food Web: Organisms take more than one type of food: food- chains get interlocked with one another – This interconnecting network of species.
• Generally, two types of food-chains are recognised: 

1. Grazing food-chain: based on autotroph’s energy capture initiated by grazing animals; 

• The first level starts with plants as producers and ends with carnivores as consumers as the last level, with the herbivores being at the intermediate level. 
• There is a loss of energy at each level which may be through respiration, excretion or decomposition. The levels involved in a food-chain range between three to five and energy. 

1. Detritus food-chain: (Detritus: means waste) involves the decomposition or breaking down of organic wastes and dead matter derived from the grazing food-chain. These are then consumed by other organisms. This chain depends less on the sun for energy.

Types of Biomes: 

5 Major types: Forest, desert, grassland, aquatic and altitudinal biomes.

 Difference between Swamp and Marshes

• Swamps have trees, while marshes have grasses.
• Swamps are more nutrient-rich, while marshes have peaty soil.
• Marshes are often flood-dependent, whereas swamps can have standing water year-round.

Both play vital roles in water filtration, flood control, and biodiversity conservation!

Ecosystem services: 

Ecosystem provides humans wih several types of services free, which could have cost us billions of dollars if we had to do ourselves.

These Products of an ecosystem include healthy air, water, soil, habitat, wildlife, pollinators, carbon sink, as well as aesthetic, cultural and spiritual values.

• Researchers have put an average price tag of $33 trillion a year on these fundamental ecosystem services, which are largely taken for granted because they are free. This is nearly twice the value of the global gross national product GNP which is ($18 trillion).
• Out of the total cost of various ecosystem services, soil formation accounts for about 50%, and contributions of other services like recreation and nutrient cycling, are less than 10% each. The cost of climate regulation and habitat for wildlife are about 6% each.

Ecosystem – Structure and function

Every ecosystem shows a certain degree of Stratification, i.e. Vertical distribution of different species occupying different levels. 

For example, In a freshwater pond ecosystem:

• Sunlight provides energy for phytoplankton, which produce oxygen through photosynthesis. 
• Fish feed on zooplankton, while frogs and birds consume insects and small aquatic creatures. 
• Decomposers like bacteria and fungi break down organic matter, recycling nutrients into the soil. 
• Plants like water lilies and reeds stabilize the pond, providing shelter for amphibians and insects. 

The food chain maintains balance, with predators controlling prey populations. External factors like temperature and rainfall influence water levels, affecting the ecosystem’s stability. Human activities such as pollution can disrupt this delicate biotic and abiotic interaction.

The system performs all the functions of any ecosystem and biosphere as a whole.

Aspects of study:

In this chapter, we will study aspects of an ecosystem, namely: 

• Productivity
• Decomposition
• Energy flow
• Nutrient cycling.

Productivity: 

The rate of biomass production is known as Productivity. In other words, it is the rate at which the nutrients are utilised by the organisms to produce biomass in an ecosystem.

For example, through plants produce proteins, carbohydrate, fats etc. using nutrient from air and soil. This is primary biomass. Herbivores eat this and produce a new type of biomass known as secondary biomass. This includes muscle cells etc.

Similarly, at every trophic level biomass is eaten by the organisms for energy and nutrition, which helps them produce their own biomass.

Based on the type of biomass produced in an ecosystem, productivity can be categorised as the following: 

• Primary productivity: It is the amount of biomass/organic matter produced over the unit area over a time period by plants during photosynthesis. Productivity can be divided into:

• Gross primary productivity (GPP): Rate of production of organic matter through Photosynthesis.
• Net primary productivity (NPP): Gross primary productivity – respiration losses.

GPP – R = NPP

• Primary productivity varies in different types of ecosystems. It depends on plant species inhabiting a particular area, environmental factors, availability of nutrients and the photosynthetic capacity of plants. 
• Secondary productivity: Rate of formation of new organic matter by consumers. 

The net primary productivity of the whole biosphere is approximately 170 billion tons (dry weight) of organic matter. Of this the ocean contributes only 55 billion tonnes. 70% belongs to land.

Decomposition:  

Decomposition includes breaking down of complex organic matter into inorganic substances like CO2, water and nutrients (with the help of decomposers). 

In essence, it is the reverse process of the biomass production. The biosphere largely maintains its equilibrium. The amount of biomass produced through primary and secondary productivity of the ecosystem is equal to the amount of biomass broken down through the process of decomposition.

Decomposition based on the Nature of Detritus (dead matter)

• If detritus is rich in lignin and chitin, decomposition is slower.
• If it is rich in Nitrogen and water-soluble substances like sugars decomposition would be quicker.

Important steps include:

The process of decomposition starts with the Detrivores. These are the Organisms that get nutrition from detritus Such as worms; However, these must be distinguished from other decomposers such as bacteria fungi etc. 

Following steps are involved in the process of breakdown of biomass:

• Fragmentation: detritus material into smaller particles.
• Leaching: water-soluble inorganic nutrients go down into the soil horizon and get precipitated as unavailable salts. 
• Catabolism: Bacterial and fungal enzymes degrade detritus into simpler inorganic substances.
• Humification: accumulation of a dark-coloured amorphous substance called humus.
• Highly resistant to microbial action and 
• Undergoes decomposition at an extremely slow rate)
• Mineralisation: Humus is further degraded by some microbes and releases inorganic nutrients.

Factors Affecting Decomposition: 

• It requires oxygen. 
• It is controlled by the chemical composition of detritus 
• Climatic factors: The most important are temperature and soil moisture. They regulate the activities of microbes.  High Temperature and moisture enhance its rate.

Energy flow: 

Photosynthetic and chemosynthetic bacteria fix the sun’s radiant energy to make food from simple inorganic materials. Except for deep-sea hydro-thermal ecosystems, the sun is the only source of energy for all ecosystems on Earth.

This process is highly inefficient. Only about 50% of solar radiation is photosynthetically active radiation (PAR), i.e. chlorophyll has the ability to absorb the radiation. The following figure shows the particular wavelength that can be absorbed by the chlorophyll. We can see that chlorophyll is unable to absorb yellow and nearby colours.  

Further, Plants capture only 2-10% of the PAR and this small amount of energy sustains the entire living world. 

Categorisation of Species based on their trophic level

• Producers (autotrophs or first trophic level): phytoplankton, algae, and green plants in the ecosystem. Major producers are herbaceous and woody plants. 
• Consumers(heterotrophs): depend (directly or indirectly) on plants for their food needs.
  • Primary consumers (herbivores or second trophic level): Feed on autotrophs. Many Insects, birds and mammals in the terrestrial ecosystems; and Molluscs in the aquatic ecosystem act as primary consumers. 
  • Secondary consumers (Carnivores or third trophic level): animals eat other animals.
    1. Primary Carnivores:  feed on herbivores.
    2. Secondary Carnivores: depend on primary carnivores.

Food chain: 

Food chains are formed because of the interdependency of the animals on the producers or other consumers for food. No energy that is trapped in an organism remains in it forever.

• The grazing food chain (GFC): It is named so because it generally begins with Grass. It includes autotrophs (which take sun’s energy to produce primary biomass), and hetrotrophs, as seen earlier.
• Detritus food chain (DFC): begins with dead organic matter. It is made up of decomposers which are heterotrophic organisms, mainly fungi and bacteria.  
• Saprotrophs: [sapro: decompose/degrade] They meet their energy and nutrient requirements by degrading dead organic matter or detritus.
• They secrete digestive enzymes that break down dead and waste materials into simple, inorganic materials which are subsequently absorbed by them.

In terms of Energy flow

• For Aquatic ecosystem, GFC is major conduit for energy flow. 
• For Terrestrial ecosystem: A much larger fraction of energy flows through DFC than GFC.

• Food web: DFC may be connected with GFC at some levels:  some of organisms of DFC are prey to GFC animals, and in a natural ecosystem, [animals like cockroaches, crows, etc., are omnivores] These natural interconnections make food web.
• Trophic level: Based on source of nutrition or food, organisms occupy a specific place in the food chain.
• Tertiary consumers (top Carnivore or Fourth trophic level): No one feeds on them.
• Standing crop: Each trophic level has a certain mass of living material at a particular time called as the standing crop; measured as the mass of living organisms (biomass) or the number in unit area.  Can be measured in dry mass or fresh mass.

  

Ecological Pyramids: 

Relationships between number, biomass or energy is represented in the form of Pyramids.

• Pyramid of Number
• Pyramid of biomass
• Pyramid of energy

Pyramid of Number and Biomass

In most ecosystems, all the pyramids, of number, of energy and biomass are upright, i.e., producers are more in number and biomass than the herbivores, and herbivores are more in number and biomass than the carnivores.

Exceptions: 

1. If you were to count the number of insects feeding on a big tree. [of Number]
2. [Of Biomass] in terms of g/m2

Pyramid of Energy

10% law: Only 10% of the energy is transferred to each trophic level from the lower trophic level. Thus, Pyramid of energy is always upright, i.e. Energy at lower trophic level is always more than higher level. Some energy is always lost at each step.  

Limitations of Ecological pyramids: 

1. Doesn’t consider that a given species may occupy more than one trophic level in the same ecosystem at the same time; For example, a sparrow is a primary consumer when it eats seeds, fruits, peas, and a secondary consumer when it eats insects and worms.
2. Can’t accommodate food web: It assumes a simple food chain, that almost never exists. 
3. Saprophytes (Decomposers) are not given any place in ecological pyramids; Though have a vital role.

Ecological succession: 

Composition and structure of all communities (or population) change in response to changing environmental conditions. This change is orderly and sequential, parallel with the changes in the physical environment. Succession and evolution are a parallel process. 

Climax Community

Ecological succession is Fairly predictable change in species composition of a given area. It always results in the formation of a climax community.

A Climax community is a community which lives in near equilibrium with the environment. 

Sere and Serals

Sere(s) refers to the entire sequence of communities that successively change in a given area.

Stages or Seral refers to the Individual transitional communities during the stages of succession.

Types of Successions: 

1. Primary Succession: When succession starts from scratch. Life occurring in an area which was devoid of life. It means that no living organism ever existed before.  In case of a new biotic community.
2. Secondary succession: Life occurring where, it existed before but somehow lost. Where natural biotic community have been destroyed such as abandoned farm lands, burned or cut forests, flooded land.

Examples Secondary succession Depends on condition of soil, availability of water, environment and also seeds or other propagates present.

Succession of Plants:  

Based on nature of habitat – wet or dry; towards mesic habitat (well-balanced moisture content)

• Hydrarch Succession: In Wetter areas and successional series progress from hydric(saturated) to mesic(well-balanced) conditions.
• Xerarch Succession: Takes place in dry areas and series progresses from xeric(dry) to mesic.

Xerarch Succession:

1. Pioneer species: Lichens and other small plants that can survive with little soil are the first to colonize the area. These species are called pioneer species because they require minimal resources to survive. 
2. Soil formation: As the pioneer species decompose, they create the first layers of soil. 
3. Grasses and shrubs: Grasses and shrubs replace the pioneer species, shading them out and altering the soil. 
4. Trees: Large trees and more shade-tolerant species replace the grasses and shrubs. 
5. Climax community: This is the final stage of primary succession, where the ecosystem is stable and has high species diversity.

Hydrarch succession

If we leave any terrestrial waterbody such as lakes undisturbed, it would eventually grow to be a lush forest.

1. Stage 1: Phytoplankton Stage: First, phytoplankton, microscopic algae and cyanobacteria that colonize the water body. These organisms play a crucial role in oxygen production and form the base of the aquatic food chain. Over time, this initiates sediment deposition, making the habitat suitable for larger aquatic plants.
2. Stage 2: Submerged Plant Stage: As sediment increases, rooted submerged plants such as Hydrilla, Vallisneria, and Ceratophyllum begin to grow. These plants help in stabilizing the sediment, reducing water currents, and further accumulating organic matter, creating shallow vegetation.
3. Stage 3: Submerged Free Floating Plant Stage: Floating plants like Pistia, Lemna (duckweed), and Eichhornia (water hyacinth) dominate this stage. These plants shade the bottom, restricting light penetration and causing submerged plants to decline. Their decomposed biomass increases siltation.
4. Stage 4: Reed Swamp Stage: This stage is characterized by the growth of emergent plants like Typha, Phragmites, and Scirpus, which establish root systems in shallow water. These plants trap more sediment, and the open water area shrinks further, transforming the habitat into a swampy marshland.
5. Stage 5: Marsh Meadow Stage: The water body is now almost completely filled with soil, allowing grasses, sedges, and herbaceous plants like Cyperus and Juncus to take over. Water becomes scarce, and the wetland slowly transitions into moist meadows.
6. Stage 6: Scrub Stage: With the ground stabilized, shrubs and small trees such as Salix (willow) and Alnus start colonizing the area. These plants absorb excess moisture, further preparing the land for larger vegetation.
7. Forest Stage: Eventually, the area develops into a climax forest, dominated by large trees like oak, maple, and pine. This marks the end of hydrarch succession, transforming the once aquatic habitat into a terrestrial ecosystem.

Important species Playing role in Succession

• Keystone species: Which has a big effect on its environment relative to its number. For example Corals. The ecosystem depends on them and would be much changed if they were not there.
• Indicator Species: no species will replace it after it gets extinct. For example, Great Indian Bustard or Godawan is known as the indicator species.
• Engineer species: Organism that significantly modifies, maintains or destroys a habitat. For example, Corals, Beavers of North America
• Pioneer Species: That invade a bare area. Examples of pioneer species in primary succession. For example: 

• Lichens are generally able to secrete acids to dissolve rock and help soil formation by weathering; They later pave way for other small plants like bryophytes (mosses), which are able to hold soil. Taken over by bigger plants; Later Climax forest environment. Thus Xerophytic habitat (low water) is changed to mesophytic(medium water).
• [In Marshy area] Phytoplanktons replaced by free floating angiosperms then by rooted hydrophytes, sedges, grasses and then trees. Climax forest.

Ecological Balance: 

It is a state of dynamic equilibrium within a community of organisms.

• Disturbance in plants: competition where the secondary forest species such as grasses, bamboos or pines overtakes the native species changing the original forest structure is called succession. [Read ecological succession in bio]
• Equilibrium in animals: between predators and prey. 
• There is a very close relationship between the plant and animal communities within particular habitats. Diversity of life in a particular area can be employed as an indicator of the habitat factor.