Environment : Surroundings or conditions in which an organism lives or operates.
Ecosystem : An ecosystem is a functional unit of nature encompassing complex interaction between its biotic (living) and abiotic (non-living) components.
I. Abiotic components :
They can be grouped into following three categories :
1. Physical factors: Sun light, temperature, rainfall, humidity and pressure. They sustain
and limit the growth of organisms in an ecosystem.
and limit the growth of organisms in an ecosystem.
- Light – Sunlight provides energy. Green plants utilize it for synthesizing food for themselves & other living organisms, through photosynthesis
- Rainfall –Water is essential for all living beings. Majority of biochemical reactions take place in an aqueous medium. Water helps to regulate body temperature. Water bodies form the habitat for many aquatic plants & animals.
- Temperature– Temperature is a critical factor of the environment which greatly influences survival of organisms. Organisms can tolerate only a certain range of temperature & humidity
2. Inorganic substances: Carbon dioxide, nitrogen, oxygen, phosphorus, sulphur, water,
rock, soil and other minerals.
- Atmosphere - The earth’s atmosphere is made of 21% oxygen, 78% nitrogen & 0.038% carbon dioxide. Rest are inert gases (0.93% Argon, Neon etc).
- Substratum- Organisms may be terrestrial or aquatic. Land is covered by soil & a wide variety of microbes, protozoa, fungi & small animals (invertebrates) thrive in it. Roots of plants pierce through the soil to tap water & nutrients. Terrestrial animals live on land. Aquatic plants, animals & microbes live in fresh water as well as in the sea. Some microbes live even in hot water vents under the sea.
3. Organic compounds: Carbohydrates, proteins, lipids and humic substances. They are the building blocks of living systems and therefore, make a link between the biotic and abiotic components.
II. Biotic components
1. Producers: The green plants manufacture food for the entire ecosystem and are called autotrophs, as they absorb water and nutrients from the soil, carbon dioxide from the air, and capture solar energy for this process
2. Consumers: They are heterotrophs and they consume food synthesized by the autotrophs.
Based on food preferences they can be grouped into Herbivores (feed directly on plants), carnivores (animals which eat other animals) and omnivores (organisms feeding upon both plants and animals)
3. Decomposers: Also called saprotrophs. These are mostly bacteria and fungi that feed on dead decomposed and the dead organic matter of plants and animals by secreting enzymes outside their body on the decaying matter. They play a very important role in recycling of nutrients. They are also called detrivores or detritus feeders
Living organisms, therefore, need both abiotic & biotic components of the environment for survival. A delicately balanced relationship between living organisms & their environment is critically important for their survival.
FUNCTIONS OF ECOSYSTEM
Ecosystems are complex dynamic system. They perform certain functions
- Energy flow through food chain
- Nutrient cycling (biogeochemical cycles)
- Ecological succession or ecosystem development
- Homeostasis (or cybernetic) or feedback control mechanisms
FUNCTION 1 : Energy flow through food chain
Food Chain : Transfer of food energy from green plants (producers) through a series of organisms with repeated eating and being eaten.
Each step in the food chain is called trophic level. During this transfer of energy, some energy is lost into the system as heat energy and is not available to the next trophic level. So the number of steps are limited in a chain to 4 or 5.
Following trophic levels can be identified in a food chain :
Autotrophs : They are largely green plants and convert inorganic material in the presence of solar energy by the process of photosynthesis into the chemical energy (food).
The total rate at which the radiant energy is stored by the process of photosynthesis in the green plants is called Gross Primary Production (GPP) or total photosynthesis or total assimilation. From the gross primary productivity a part is utilized by the plants for its own metabolism. The remaining amount is stored by the plant as Net Primary Production (NPP) which is available to consumers.
Herbivores : The animals which eat the plants directly are primary consumers or herbivores e.g. insects, birds, rodents and ruminants.
Carnivores: They are secondary consumers if they feed on herbivores and tertiary consumers if they use carnivores as their food. e.g. frog, dog, cat and tiger.
Omnivores: Animals that eat both plant and animals e.g. pig, bear and man
Decomposers: They take care of the dead remains of organisms at each trophic level and help in recycling of the nutrients e.g. bacteria and fungi.
There are two types of food chains: Grazing food chain & Detritus food chain
Food web: A network interconnected food chains existing in an ecosystem. Trophic levels in an ecosystem are not linear rather they are interconnected and make a food web. One animal may be a member of several different food chains.
The quantity of energy flowing through the successive trophic levels decreases as shown by the reduced sizes of boxes in the figure here. At every step in a food chain or web the energy received by the organism is used to sustain itself and the left over is passed on to the next trophic level.
Graphic representations of trophic levels in an ecosystem. They are pyramidal in shape; the producers make the base of the pyramid and the subsequent tiers of the pyramid represent herbivore, carnivore and top carnivore levels. They are of three types:
Pyramid of number: This represents the number of organisms at each trophic level. It may be upright or inverted.
Pyramid of biomass: This represents the total standing crop biomass at each trophic level. Standing crop biomass is the amount of the living matter at any given time. It is expressed as gm/unit area or kilo cal/unit area.
In most of the terrestrial ecosystems the pyramid of biomass is upright. However, in case of aquatic ecosystems the pyramid of biomass may be inverted e.g. in a pond phytoplankton are the main producers, they have very short life cycles and a rapid turn over rate (i.e. they are rapidly replaced by new plants). Therefore, their total biomass at any given time is less than the biomass of herbivores supported by them.
Pyramid of energy: This pyramid represents the total amount of energy at each trophic level. Energy pyramids are never inverted.
ECOLOGICAL EFFICIENCY: Ratio between the amount of energy acquired from the lower trophic level and the amount of energy transferred from higher trophic level
10% rule : Proposed by Lindman. If autotrophs produce 100 cal, herbivores will be able to store 10 cal. (10% of 100 cal.) and carnivores 1 cal. (10% of 10 cal.)
There may be slight variations in different ecosystems and ecological efficiencies may range from 5 to 35%. Ecological efficiency (also called Lindman’s efficiency) can be represented as
Increase in concentration of a pollutant from the environment to an organism in a food chain (every organism is part of atleast one food chain). It occurs within a trophic level, and is the increase in concentration of a substance in certain tissues of organisms' bodies due to absorption from food and the environment.
The affected organism has a higher concentration of the substance than the concentration in the organism's surrounding environment. The toxic substances are very slowly metabolized or excreted so if the organism keeps on consuming prey or food contaminated with toxic substances, the concentration of the substance will further increase in its body, hence, bioaccumulation results.
Bio-concentration : Occurs when uptake from the water is greater than excretion
Increase in concentration of a pollutant from one link in a food chain to another. It is the increase in concentration of a substance in a food chain, not an organism.
Thus, biomagnification is similar to bioaccumulation but is descriptive of higher level biological processes, not individual.
Bioconcentration and bioaccumulation occur within an organism, and biomagnification occurs across trophic (food chain) levels.
Bio-dilution : It is also a process that occurs to all trophic levels in an aquatic environment; it is the opposite of biomagnification, thus a pollutant gets smaller in concentration as it progresses up a food web.
1. NOS material on Environment and Biodiversity