Ecosystem Structure

On the basis of trophic structure, ecosystem is two layered:

“trophe = nourishment”

1. Autotrophic Stratum (Green Belt):
   • Upper, self-nourishing layer of chlorophyll-containing plants.
   • Fix the light energy, convert inorganic substances into complex organic compounds which predominate in the stratum.
   • The most intensive autotrophic metabolism occurs in this layer where light energy is available.
2. Heterotrophic Stratum (Brown Belt):
   • Lower, ‘other’ nourished layer of soils & sediments, decaying matter, roots etc.
   • Predominately use, rearrange & decompose complex materials.
   • The most intensive heterotrophic metabolism occurs in this layer where organic matter accumulates in soils & sediments.

Components of Ecosystem:

1. Abiotic (Non-living) Components:
   • Inorganic Substances such as Carbon, Nitrogen, Hydrogen, Phosphorus, Sulphur etc.
   • Organic Compounds such as proteins, carbohydrates, lipids, humic substances etc.
   • Climate including air, water, substrate environment & other physical factors.
2. Biotic (Living) Components:
   • Producers (Autotrophic organisms): Manufacture own food from simple inorganic substances. Includes mostly photosynthetic organisms besides chemosynthetic organisms. For more refer “Autotrophic stratum” mentioned above.
   • Hetertrophs (Heterotrophic organisms): Consumes other organisms, living or dead, as food source. Heterotrophs are of following types under two categories:-

A) Mode of nutrition:

(i) Phagotrophs: (Phago = to eat)

• Ingest other organisms or particulate matter.
• They are chiefly animals.

(ii) Saprotrophs: (Sapro = to decompose) (= decomposers)

• Obtain food by either breakdown of dead tissues or by absorbing exuded or extracted dissolved organic matter from plants or other organisms.
• Saprophages are organisms feeding on dead organic matter.
• Decomposition activity – (a) Releases inorganic nutrients which can be further utilized by producers. (b) Provide food for macroconsumers. (c) Excrete substances which inhibit or stimulate other biotic components.

For more refer “Heterotrophic stratum” mentioned above.

B) Size of Organisms:

i. Microconsumers: (Example: Bacteria, fungi etc.)

• Very small, some are microscopic.
• Feed by breakdown of complex organic compounds, releasing inorganic substances in environment besides absorbing some of the breakdown products.
• Have high rate of metabolism & turnover.
• Relatively immobile.
• Usually embedded in the material being decomposed.
• Their functional specialization is more evident biochemically than through their morphology, like, by simple visual observation or counting their numbers etc.
• Also known as decomposers, but according to Odum (Fundamentals of Ecology), it seems to be preferable to consider “decomposition” as a process involving all biotic & abiotic processes instead of designating any particular organisms as decomposers.

ii. Macroconsumers: (Example: Herbivores, Carnivores and Omnivores etc.)

• Obtain their food (energy) through heterotrophic ingestion of particulate matter.
• Morphologically adapted for active food seeking.
• Their sensory-neuromotor, digestive, respiratory, circulatory systems etc. also tend to be adapted for the same.

Ecosystem Related Terms:

Standing State

• Amount of inorganic matter present in ecosystem.
• Represents part of non-living matter.
• Circulates between living & non-living ecosystem components.

Standing Crop

• Amount of whole biomass present in an ecosystem.
• Represent living matter.
• Circulation absent.

Ecotype

• Subspecies or population of a widespread species adapted to local particular set of environment i.e. a locally adapted population of a species.
• Show minor changes in morphology and/or physiology w.r.t. their habitat.
• Changes are genetically induced.
• Able to reproduce with other ecotypes of same species.

Ecomorph

• A population of a species having small differences in morphology.
• Represents adaptations to very local environmental variations
• Became reproductively isolated from others very recently.

Ecotone

• Narrow but fairly defined transition zone between two or more different communities or ecosystems.
• Arise naturally.
• Species rich
• Example: Transition zone between a grassland & a forest.

Some more points to consider for Ecosystem structure:

• Universal feature of all ecosystems: Interaction between autotrophic & heterotrophic components is present.
• Delay in using products of autotrophs by heterotrophs: Example – Photosynthesis predominates in forest ecosystem. Only a small part of photosynthate is immediately & directly used by plants themselves & by herbivores + parasites feeding on them. But most of the synthesized material (leaves, wood, stored food etc.) reaches litter & soil on land or sediments in water bodies. After weeks, months or even years all the accumulated matter might be used e.g. Fossil Fuels, which we are still consuming.

• Most inorganic substances like carbon, nitrogen, phosphorus etc. are found both inside & outside living organisms. Additionally, they are also found in constant state of flux/turnover between living & non-living states.
• Some substances appear to be unique to living & non-living states: Example: ATP (in Living state) whereas Humic substances (Non-living state).

• Complexes like DNA, Chlorophyll etc. are present in both living & non-living states but they are functional in former & non-functional in latter.