Ecosystem and its Components Ecosystem Functions Ecological Pyramids
An ecosystem is defined as a structural and functional unit of biosphere consisting of community of living beings and physical environment, both interacting and exchanging materials between them. Ecosystem is a self-contained, dynamic system composed of a natural community along with its physical environment.
Components of an ecosystem
The components of the ecosystem are divided as:
• Abiotic Components
Abiotic components are non-living chemical and physical factors on an ecosystem. The non-living factors are either resources or conditions. Important abiotic components can be listed as follows:
1. Physical factors: They sustain and limit the growth of organisms in an ecosystem.
a) Light: Light energy (sunlight) is the primary source of energy in nearly all ecosystems. It is the energy that is used by green plants (which contain chlorophyll) during the process of photosynthesis; a process during which plants manufacture organic substances by combining inorganic substances. The intensity of the light that reaches the earth varies according to the latitude and season of the year and also effect geographical and seasonal vegetation distribution. Many species of small plants (herbs and shrubs) growing in forests are adapted to photosynthesis optimally under very low light conditions because they are constantly overshadowed by tall, canopied trees. Many plants are also dependent on sunlight to meet their photoperiodic requirement for flowering.
b) Temperature: The distribution of plants and animals is greatly influenced by extremes in temperature.
c) Water: The life on earth originated in water and is unsustainable without water.
d) Atmospheric gases: The most important gases used by plants and animals are oxygen, carbon dioxide and nitrogen. Oxygen is used by all living organisms during respiration. Carbon dioxide is used by green plants during photosynthesis. Nitrogen is made available to plants by certain bacteria and through the action of lightning.
e) Soil: Various characteristics of the soil such as soil composition, grain size and aggregation determine the percolation and water holding capacity of the soils. These characteristics along with parameters such as pH, mineral composition and topography determine to a large extent the vegetation in any area. This in turn dictates the type of animals that can be supported.
2. Organic compounds: They are the building blocks of living systems and therefore, make a link between the biotic and abiotic components. Examples are: Carbohydrates, proteins, lipids and humic substances.
• Biotic components
The biotic components in an ecosystem include the living organisms. They are grouped in to 3 classes based on the organism’s role in the flow of material and energy within the ecosystem:
1. Producers (autotrophs): Autotrophs produce organic compounds from carbon dioxide as a carbon source. They take energy from the sun (or from inorganic sources in some cases) to convert it into organic molecules or food, e.g., plants, algae, bacteria, etc. A portion of food synthesized, is used by autotrophs for their growth and other biological functions and remaining is stored for future use. This stored food in autotrophs is utilized as food by other organisms (called heterotrophs).
2. Consumers (heterotrophs): They are called heterotrophs and they consume food synthesized by the autotrophs. Based on food preferences they can be grouped into three broad categories. Herbivores (e.g. cow, deer and rabbit etc.) feed directly on plants, carnivores are animals which eat other animals (eg. lion, cat, dog etc.) and omnivores organisms feeding upon both plants and animals e.g. human, pigs and sparrow.
3. Decomposers: Decomposers are organisms (often fungi or bacteria) that break down organic materials to gain nutrients and energy. Decomposition is a natural process but decomposers accelerate it. The role that decomposers perform in an ecosystem is extremely important. When an organism dies, it leaves behind nutrients that are locked together. Decomposers unlock these nutrient and release as raw nutrients (such as nitrogen, phosphorus, and magnesium) in a form which are usable for plants. Decomposers also convert organic carbon into Carbon dioxide, which can be trapped by photosynthesizers.
Other terms associated with Ecosystem
• Primary production is defined as the amount of biomass or organic matter produced per unit area over a time period by plants during photosynthesis. It is expressed in terms of weight (g –2 ) or energy (kcal m–2). The rate of biomass production is called productivity.
• Gross primary productivity of an ecosystem is the rate of production of organic matter during photosynthesis. Gross primary productivity minus respiration losses (R), is the net primary productivity (NPP).
• Secondary productivity is defined as the rate of formation of new organic matter by consumers.
• Detritivores are heterotrophs that obtain nutrients by consuming detritus (decomposing plant and animal parts as well as feces).Detritivores (e.g., earthworm) break down detritus into smaller particles.
• Catabolism: When Bacterial and fungal enzymes degrade detritus into simpler inorganic substances, this process is called as catabolism.
• Humification leads to accumulation of a dark coloured amorphous substance called humus that is highly resistant to microbial action and undergoes decomposition at an extremely slow rate. Being colloidal in nature it serves as a reservoir of nutrients. The humus is further degraded by some microbes and releases of inorganic nutrients occur by the process known as mineralisation.
Food chains, food web and energy flow are the functional properties of ecosystems which make them dynamic.
The unidirectional transfer of food energy from the producers, through a series of organisms (herbivores to carnivores to decomposers) with repeated eating and being eaten, is known as food chain. It is the movement of organic matter and energy from the producer level through various consumer levels.
The various steps in a food chain are called trophic levels and transfer of energy in term of food from one trophic level to another is called energy flow. Energy flow is always unidirectional.
Types of Food Chains
• Grazing Food Chain
Grazing food chain (GFC) is the most common food chain. It is also called predator food chain as predation occurs at every step. This food chain is consists of producers, consumers and decomposers. Consumers are often of 3 to 5 types: first order (primary), second order (secondary), third order (tertiary), fourth order (quarternary) consumers.
Grasses Grasshopper Frog Snake Hawk/Eagle
• Detritus Food Chain
Detritus food chain (DFC) begins with detritus or dead organic matter. The food energy present in detritus passes into detrivores and decomposers who feed over it. Detrivores and decomposers are consumed by smaller carnivores which in turn become food for larger carnivores and so on. A common detritus food chain with earthworm as detrivores is:
Detritus _____ Earthworm _____ Sparrow _____ Falcon
• Parasitic Food Chain
Parasitic food chain also called auxiliary food chain; it begins with host and usually ends in parasite.
Simple food chains are very rare in nature because each organism may obtain food from more than one trophic level. Thus in an ecosystem, the various food chains are interconnected to each other to form a network called food web. A food web illustrates all possible transfers of energy and nutrients among the organisms in an ecosystem, whereas a food chain traces only one pathway of the food. Food webs are very important in maintaining the stability of an ecosystem.
It is a network of food chains which interconnect various trophic levels and form a number of feeding connections amongst the different organisms of a biotic community. Food web increases the stability of an ecosystem by providing alternate sources of food and allowing the endangered population to grow in size.
Composition of Food Web
A food web operates according to food preferences of the organisms at each trophic level. However, availability of food source and other compulsions are equally important. In Sunderbans, the tigers eat fish and crab in the absence of their natural preys. Some organism normally operates at more than one trophic level. Thus human beings are not only herbivores but also carnivores of various levels. Jackals are both carnivores and scavengers. Snakes feed on mice (herbivores) as well as frogs (carnivores). Wild cats prey upon mice as well as birds and squirrels. A wolf eats fox as well as rabbit and deer.
A herbivore like rabbit does not get starved if its preferred plant species is reduced in quantity due to some calamity. It begins feeding on alternate plant species. The preferred one gets chance to recover from the loss. Similarly, rabbits are preyed upon by foxes, wild dogs, wild cats, jackals, etc. In case the population of rabbit decreases, the predators begin to eat mice, shrews, squirrels, etc. Meanwhile rabbits increase their population and restore the balance.
Difference between Food Chain and Food Web
Ecological pyramid Ecological pyramids are the graphic representations of trophic levels in an ecosystem. They are pyramidal in shape and they are of three types: The producers make the base of the pyramid and the subsequent tiers of the pyramid represent herbivore, carnivore and top carnivore levels.
There are three types of Ecological Pyramids
a) Pyramid of energy
The pyramid of energy or the energy pyramid describes the overall nature of the ecosystem. During the flow of energy from organism to other, there is considerable loss of energy in the form of heat. The primary producers like the autotrophs there is more amount of energy available. The least energy is available in the tertiary consumers. Thus, shorter food chain has more amount of energy available even at the highest trophic level.
• The energy pyramid always upright and vertical.
• This pyramid shows the flow of energy at different trophic levels.
• It depicts the energy is minimum as the highest trophic level and is maximum at the lowest trophic level.
• At each trophic level, there is successive loss of energy in the form of heat and respiration, etc.
b) Pyramid of Numbers
It is a graphic representation of the number of individuals per unit area of various trophic levels stepwise with producers being kept at the base and top carnivores kept at the tip. In most cases, the pyramid of number is upright with members of successive higher trophic level being less than the previous one. The maximum number of individuals occurs at the producer level. The producers support comparatively fewer numbers of herbivores, the latter fewer number of primary carnivores and so on. Top carnivores are very few in number.
In a grassland, a larger number of grass plants or herbs support a fewer number of grasshoppers that support a still smaller number of frogs, the latter still smaller number of snakes and the snakes support very few peacocks or falcons.
A similar case is found in a pond ecosystem where a large number phytoplankton support comparatively smaller number of zooplanktons the latter fewer number of small-sized fishes, the small-sized fishes become food of still fewer larger-sized fishes or water birds.
The number of pyramids in a higher trophic level is generally smaller than that of the lower trophic level because the organisms of the higher trophic level are dependent for their food and energy on the organisms of the lower trophic level.
c) Pyramid of biomass
The pyramid of biomass is more fundamental, they represent the quantitative relationships of the standing crops. In this pyramid there is a gradual decrease in the biomass from the producers to the higher trophic levels. The biomass here the net organisms collected from each feeding level and are then dried and weighed. This dry weight is the biomass and it represents the amount of energy available in the form of organic matter of the organisms. In this pyramid the net dry weight is plotted to that of the producers, herbivores, carnivores, etc.
Upright Pyramid of Biomass occurs when the larger net biomass of producers support a smaller weight of consumers.
Inverted Pyramid of Biomass happens when the smaller weight of producers support consumers of larger weight.
Limitations of Ecological Pyramids
1. Ecological pyramids assume that food chains are simple. Simple food chains do not occur in nature. Instead food webs are present.
2. A simple species may operate at two or more tropical levels. Ecological pyramids have no method of accommodating such cases.
3. Ecological pyramids have no place for detrivoves and decomposers though they play a vital role in ecosystem.