Ecosystem: common doubts, answered
The questions students ask most often about Ecosystem, each with a short answer. For the full chapter, read the Ecosystem notes.
Ecosystem: meaning and types
Read this section in the notes →What is an ecosystem, and what are its types?
An ecosystem is a functional unit of nature in which living organisms interact with one another and with the physical environment around them. Types are terrestrial (forest, grassland, desert), aquatic (pond, lake, wetland, river, estuary) and man-made (crop field, aquarium). There is no clear dividing line, since each type grades into another.
Structure and function
Read this section in the notes →What are the four functional components of an ecosystem?
They are productivity, decomposition, energy flow and nutrient cycling. Structurally an ecosystem is described by its species composition and stratification, for example the vertical layering of trees, shrubs and herbs in a forest. The functional aspects describe how the system works and not what it contains.
The pond ecosystem
Read this section in the notes →Who are the producers, consumers and decomposers in a pond?
Phytoplankton, algae and rooted plants are the producers. Zooplankton, insects and fishes are the consumers, with zooplankton as primary consumers. Fungi, bacteria and flagellates are the decomposers that break down dead matter. A pond is a good example of a complete, self-sustaining aquatic ecosystem.
Productivity
Read this section in the notes →What is the difference between GPP and NPP?
Gross primary productivity (GPP) is the total rate of organic matter produced by photosynthesis in an area, while net primary productivity (NPP) is what remains after the plants use some for respiration. So NPP = GPP − R. NPP is the biomass available to consumers, and is expressed in g m⁻² yr⁻¹ or kcal m⁻² yr⁻¹.
Is primary productivity the same everywhere?
No. It depends on the plant species of the area, photosynthetic capacity, and environmental factors such as nutrient availability and water. Although the oceans cover about 70 per cent of the earth's surface, they contribute only about 55 billion tons of the roughly 170 billion tons of annual NPP, because their productivity per area is low.
What is secondary productivity?
Secondary productivity is the rate of formation of new organic matter by consumers. Primary productivity is the rate of formation by producers through photosynthesis. Consumers do not make their own food but convert the organic matter they eat into their own biomass, so they have secondary productivity.
Decomposition
Read this section in the notes →What are the steps of decomposition?
The steps are fragmentation, leaching, catabolism, humification and mineralisation. Detritivores such as earthworms break detritus into smaller bits (fragmentation). Soluble nutrients move down the soil layers (leaching). Enzymes degrade the detritus (catabolism). Humus forms (humification), and finally inorganic nutrients are released (mineralisation).
What is humus and why does it matter?
Humus is a dark, amorphous substance, rich in nutrients, that results from humification. It is highly resistant to microbial action and decomposes extremely slowly, so it acts as a reservoir of nutrients that is mineralised only gradually. Its presence improves soil fertility and structure.
What factors speed up or slow decomposition?
Detritus rich in nitrogen and water-soluble substances like sugars decomposes quickly. Detritus rich in lignin and chitin decomposes slowly. Warm and moist climates favour decomposition, whereas low temperature and anaerobiosis inhibit it, leading to the build-up of organic matter. Hence decomposition rate depends on both the detritus chemistry and the climate.
Energy flow
Read this section in the notes →Why is energy flow unidirectional?
Energy flows in one direction, from the sun to producers to consumers, because energy lost as heat at each step cannot be reused by producers. Nutrients, by contrast, cycle. The sun is the source of energy for almost all ecosystems, deep-sea hydrothermal ecosystems being the exception.
What is PAR and how much of it do plants capture?
PAR is photosynthetically active radiation, which makes up less than 50 per cent of the incident solar radiation. Plants capture only 2 to 10 per cent of this PAR, and this small fraction sustains the entire living world. The value is 2-10 per cent of PAR, not of total sunlight.
Food chains and food webs
Read this section in the notes →What is the difference between a grazing food chain and a detritus food chain?
The grazing food chain begins with producers, such as grass, that are eaten by herbivores, as in grass, goat, man. The detritus food chain begins with dead organic matter, which is eaten by decomposers and detritivores. In aquatic ecosystems the grazing chain carries most of the energy, but on land more energy flows through the detritus chain.
What is a food web?
A food web is the network of interconnected food chains in an ecosystem. Most species eat more than one kind of food and are eaten by more than one predator, so the single chains link up. The web is more realistic than a single chain.
Trophic levels and the 10 per cent law
Read this section in the notes →What is the 10 per cent law?
The 10 per cent law states that only about 10 per cent of the energy at one trophic level is transferred to the next, and the remaining energy is lost as heat in respiration and other processes. It explains why the number of trophic levels in a food chain is limited.
Can an organism occupy more than one trophic level?
Yes. A trophic level is a functional position, not a fixed label for a species. A sparrow eating seeds is a primary consumer, but when eating insects it is a secondary consumer. Humans similarly behave as primary and secondary consumers depending on the food.
Ecological pyramids
Read this section in the notes →Which ecological pyramids can be inverted?
The pyramid of number can be inverted, as for a single large tree supporting many insects, and the pyramid of biomass can be inverted in the sea, where a small standing crop of phytoplankton supports a large biomass of zooplankton. The pyramid of energy is always upright, because energy is lost at each trophic level.
Why is the pyramid of energy never inverted?
Because energy is lost as heat at every transfer, less energy is available at each higher trophic level than at the one below it. The rate of energy flow at a higher level can never exceed that at a lower level. So the pyramid of energy is always upright, unlike the pyramids of number or biomass.
What is standing crop?
Standing crop is the mass of living organisms, as number or biomass, at each trophic level in a unit area at a given time. Biomass is better measured as dry weight than fresh weight, because fresh weight varies with water content. The standing crop is the quantity plotted in pyramids of number and biomass.
Limits of trophic levels and pyramids
Read this section in the notes →What are the limitations of ecological pyramids?
Pyramids assume a simple food chain and so ignore food webs. They do not accommodate a species operating at more than one trophic level. They also do not include saprophytes, which have an important role in the ecosystem. Despite these, they are a useful way of showing trophic structure.
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