Ecosystems have unique physical structures resulting from the interaction between living (biotic) and non-living (abiotic) components.
The species composition of an ecosystem is determined by identifying and counting its plant and animal species.
Stratification refers to the vertical distribution of species within an ecosystem, with trees at the top, shrubs in the middle, and herbs and grasses at the bottom.
Ecosystems function as units through productivity, decomposition, energy flow, and nutrient cycling.
An aquatic ecosystem, like a pond, demonstrates these ecosystem components and functions effectively.
In a pond, the abiotic component includes water with dissolved inorganic and organic substances, along with soil at the pond’s bottom.
Solar input, temperature, day-length, and other climatic conditions influence the pond’s functioning.
Autotrophic components, such as phytoplankton and algae, convert inorganic materials into organic matter using solar energy.
Consumers, including zooplankton and various aquatic organisms, feed on autotrophs.
Decomposers like fungi, bacteria, and flagellates break down dead matter, releasing nutrients for autotrophs.
Energy flows unidirectionally through trophic levels, with some energy dissipated as heat to the environment.
Productivity
Solar energy is a fundamental requirement for ecosystem sustainability.
Primary production measures the biomass or organic matter produced by plants through photosynthesis, expressed as weight (gm^-2) or energy (kcal m^-2).
Productivity, the rate of biomass production, is expressed in terms of gm^-2 yr^-1 or (kcal m^-2) yr^-1 for comparing different ecosystems.
Primary productivity includes gross primary productivity (GPP) and net primary productivity (NPP).
GPP is the rate of organic matter production during photosynthesis; a portion is used by plants for respiration.
NPP is the biomass available for consumption by heterotrophs (herbivores and decomposers) and is calculated as GPP minus respiration losses (R).
Secondary productivity refers to the rate at which consumers create new organic matter.
Primary productivity varies among ecosystems due to plant species, environmental factors, nutrient availability, and plant photosynthetic capacity.
The global annual net primary productivity of the entire biosphere is around 170 billion tons (dry weight) of organic matter.
Oceans, despite covering 70% of the Earth’s surface, contribute only 55 billion tons to this productivity, with the rest occurring on land.
Decomposers
Earthworms are often called the farmer’s “friend” because they aid in breaking down complex organic matter and improving soil structure.
Decomposers play a crucial role in breaking down complex organic matter into simpler inorganic substances, such as carbon dioxide, water, and nutrients, through a process called decomposition.
Detritus, which includes dead plant material like leaves, bark, and flowers, as well as animal remains and fecal matter, serves as the raw material for decomposition.
Decomposition involves several steps, including fragmentation (breaking detritus into smaller particles), leaching (where water-soluble inorganic nutrients move into the soil and precipitate as salts), catabolism (enzymatic degradation of detritus by bacteria and fungi), humification (formation of dark, resistant humus), and mineralization (release of inorganic nutrients).
These steps in decomposition operate simultaneously on detritus.
Humus, a dark amorphous substance formed during humification, is resistant to microbial action and serves as a nutrient reservoir.
Some microbes further degrade humus in a process known as mineralization, releasing inorganic nutrients.
Decomposition is an oxygen-dependent process, and its rate is influenced by the chemical composition of detritus and climatic factors.
Decomposition is slower in detritus rich in lignin and chitin, while it’s faster in detritus rich in nitrogen and water-soluble substances like sugars.
Temperature and soil moisture are critical climatic factors that regulate decomposition; warm and moist conditions favor decomposition, while low temperatures and anaerobic conditions inhibit it, leading to the accumulation of organic materials.
Energy Flow
Solar energy is the primary energy source for ecosystems, except for deep-sea hydrothermal ecosystems.
Less than 50% of incident solar radiation is photosynthetically active radiation (PAR).
Plants capture 2-10% of PAR, sustaining the entire ecosystem.
Energy flows unidirectionally from the sun to producers (plants and photosynthetic bacteria) and then to consumers in ecosystems.
Ecosystems must constantly receive energy to counteract increasing disorderliness, following the Second Law of Thermodynamics.
Producers, such as green plants, are essential for energy capture and are the basis of terrestrial and aquatic ecosystems.
Food chains and webs form as organisms feed on each other, illustrating interdependency.
Energy is not retained within organisms indefinitely; it is passed on or released upon an organism’s death, starting the detritus food chain/web.
Consumers depend on producers directly or indirectly; they are called heterotrophs.
Primary consumers (herbivores) feed on producers, while secondary consumers (carnivores) eat other animals.
Detritus food chains begin with dead organic matter and involve decomposers like fungi and bacteria.
In aquatic ecosystems, grazing food chains are more significant, while terrestrial ecosystems have a larger fraction of energy flow through the detritus food chain.
Natural interconnections among food chains create food webs.
Organisms are classified based on their trophic level in the food chain: producers (first trophic level), herbivores (second trophic level), and carnivores (third trophic level).
Energy decreases as it moves up trophic levels; only 10% of energy transfers between trophic levels, following the 10% law.
The number of trophic levels in a grazing food chain can vary, with producer, herbivore, primary carnivore, and secondary carnivore levels possible.
Ecological Pyramid
Ecological pyramids represent the food or energy relationships between trophic levels in ecosystems, resembling the shape of a pyramid.
Three common types of ecological pyramids are: (a) pyramid of number, (b) pyramid of biomass, and (c) pyramid of energy.
These pyramids encompass all organisms at each trophic level, and a species can occupy multiple trophic levels simultaneously.
In most ecosystems, the pyramids are upright, with producers having more numbers and biomass than herbivores, and herbivores having more than carnivores. Energy is also greater at lower trophic levels.
Exceptions include situations like counting insects on a big tree, where the pyramid shape may change due to the interdependence of species.
The pyramid of biomass in the sea is often inverted because fish biomass exceeds that of phytoplankton, illustrating a paradox.
The pyramid of energy is always upright because energy is lost as heat at each trophic level due to the Second Law of Thermodynamics.
Ecological pyramids have limitations, including not accounting for species that belong to multiple trophic levels, assuming simple food chains that rarely exist in nature, and neglecting saprophytes despite their vital ecosystem role.
