Ecosystem

Introduction

• In this chapter we will study about ecology and environment in general; and ecosystem, food chain, food-web, ecological pyramids, succession, nutrition cycling, biomes, population interactions, ecological indicators, importance of ecosystems in particulars.

ECOSYSTEM

• The term ‘ecosystem’ was proposed by a British ecologist A.G. Tansley (1935). It represents the basic fundamental, functional unit of ecology which comprises of the biotic community together with its abiotic (non-living) environment.
• Ecosystem is the functional unit of nature where living organisms interact with each other and with their environment.
• Ecosystems can be recognized as self regulating and self sustaining units of landscapes that may be terrestrial or aquatic. Forests, grasslands and deserts are examples of terrestrial ecosystems. The aquatic ecosystems can be either fresh water (ponds, lakes, streams) or salt water (marine estuaries) type.
• Ecosystem may be natural (forest, sea), if developed under natural conditions or artificial (garden, aquarium, agriculture) if created by man.
• Ecosystem is normally an open system because there is a continuous and variable entry and loss of energy and materials. Ecosystem is known by different terms i.e., biogeocoenosis or geobiocoenosis or microcosm or ecosom or biosystem, etc. the whole earth can be called biosphere or ecosphere.
• Ecosystem is composed of a variety of abiotic (non-living) and biotic (living organisms) components that function in an interrelated fashion.

Kinds of Ecosystem

Ecosystem can be classified as:

Natural ecosystem

• The ecosystem which are completely dependent on solar radiation e.g. forests, oceans, grasslands, lakes, rivers and deserts. This type of ecosystem is a source of food, fuel, fodder and medicines.

Man-made ecosystem

• The ecosystem which are dependent on solar energy, e.g. agricultural fields and aquaculture ponds. Such ecosystems are also dependent on fossil fuels, e.g. urban and industrial ecosystem.

Components of Environment

Structure and Function of Ecosystem

• Ecosystem is self sustained functional units.
• The structure of an ecosystem can be expressed by the following terms –
• Species compositor: Plant and animal species found in an ecosystem
• Stratification: Vertical layers of plants.
• Standing crop: Amount of biomass.
• Standing state: Amount of inorganic substances.

Species composition

• It differs from one ecosystem to another depending upon geography, topography and climate.
• Each ecosystem has a biotic community composed of particular grouping of species.
• Maximum species composition occurs in tropical rainforests and coral reefs. Minimum occurs in deserts and arctic regions.

Stratifications

• Stratification is the occurrence of vertical zonation in the ecosystem & indicates the presence of favorable environmental conditions, for e.g., trees occupy top vertical strata or layer of a forest, shrubs and herbs & grasses occupy the bottom layers.
• Stratification helps in accommodation of large number & types of plants in the same area. It also provide a number of microhabitat & niches for various types of animals.
• It is absent or poor where environment conditions are unfavorable, e.g. desert ecosytems have very few trees & shurbs.

Standing crop

• Standing crop is the amount of living biomass in an ecosystem. It indicates the productivity & luxuriance of growth.
• It is expressed in the form of number or biomass of organisms per unit area.
• A terrestrial ecosystem with high standing crop possesses a forest while the one with low standing crop occurs in grassland followed by arid ecosystem.

Standing state

• The amount of nutrients, e.g. nitrogen, phosphorus & calcium present in the soil at any given time is known as standing state.
• The proper functioning of an ecosystem takes place through the following processes:
• Productivity
• Decomposition
• Relationship of producers and consumers
• Flow of energy through different trophic levels, and
• Cycling of nutrients.

Productivity

• Productivity refers to the rate of biomass production i.e the rate at which sunlight is captured by producers for the synthesis of energy rich organic compounds.
• It is of two types – primary productivity and secondary productivity.
• Primary productivity is the amount of biomass produced per unit area over a time period by plants during photosynthesis.
• “It is expressed in terms of weight (g-2) or energy (kcal m2). It is of two types: GPP and NPP.
• Gross primary productivity (GPP) – It is the rate of production of biomass or accumulation of energy by green plants per unit area per unit time. GPP depends on the chlorophyll content.
• Net primary productivity (NPP) – It is the amount of biomass which has been stored by green plants.
• The net primary productivity results in the accumulation of plant biomass, which serves as the food of herbivores & decomposers.
• NPP is equal to the rate of organic matter created by photosynthesis minus the rate of respiration and other losses.
• Net primary productivity = Gross primary productivity – Respiration losses. (or GPP-R = NPP)

• Secondary Productivity is the amount of biomass synthesized by consumers per unit area per unit time.

• Consumers  utilize already produced food materials in their respiration and convert the food matter to different tissues by an overall process. So secondary productivity is not divided to ‘gross’ & ‘net’ amounts.

• The net productivity of biosphere annually is approx 170 billion tons of organic matter.

Decomposition

• Decomposition is the breakdown of complex organic compounds of dead bodies of plants and animals into simpler inorganic compounds like CO2, water & various nutrients.
• The organisms carrying out decomposition are called decomposers.
• Decomposers include micro-organisms (bacteria and fungi), detritivores (earthworm) and some parasites.

Process of Decomposition

• Decomposition is physical as well as chemical in nature and consists of the following processes:
• Fragmentation: It is the formation of smaller pieces of dead organic matter or detritus by detritivores. Due to fragmentation, the surface area of detritus particles is greatly increased.
• Catabolism: Chemical conversion of detritus into simpler inorganic substances with the help of bacterial and fungal enzymes is called catabolism.
• Leaching: Water soluble substances (formed as a result of decomposition) are leached to deeper layers of soil.
• Humification: If decomposition leads to the formation of colloidal organic matter (humus), the process is called humification. Humus is highly resistant to microbial action and undergoes extremely slow decomposition. It serves as a reservoir of nutrients.
• Mineralisation: Formation of simpler inorganic substances (like CO2, water and minerals) is termed mineralisation.

Factors Affecting Decomposition

• The rate of decomposition is dependent on many factors like chemical nature, temperature, availability of oxygen, moisture, etc.
• Chemical nature of detritus: Decomposition of detritus is slow if contains chitin, tannins and cellulose. It is rapid if detritus possesses more of nitrogenous compounds like proteins, nucleic acids and reserve carbohydrates.

• Temperature: At a temperature more than 25°C,decomposers are very active in soils having good moisture and aeration. In humid tropical regions it may take less than 3-4 months for complete decomposition of detritus. However, under low temperature condition (>10°C) of soil, the rate of decomposition is slow even if moisture & aeration are optimum. Because of this, complete decomposition of detritus can take several years or even decades.

• Moisture: An optimum moisture helps in quicker decomposition. Reduction within moisture reduces the rate for decomposition as in areas with prolonged dryness like tropical deserts where otherwise the temperature is quite high. Excessive moisture also impedes decomposition. It may promote pearl formation. Aeration: It’s required for the activity of decomposers and detrivores. A reduction in aeration will slow down the process of decomposition.

• Soil pH: Detrivores are fewer in acidic soils. Microbial activity is also low in such soils. Detrivores are abundantly available in slightly alkaline soils while decomposers microbes are available in slightly acidic soil.

Energy flow

• Energy is the ability to do work. The main source of energy for an ecosystem is the radiant energy or light energy derived from the sun. 50% of the total solar radiation that falls on earth is photosynthetically active radiation (PAR). The amount of solar radiation reaching the surface of the earth is 2 cals/sq.cm/min. It is more or less constant and is called solar constant or solar flux. About 95 to 99% of the energy is lost by reflection. The light energy is converted into chemical energy in the form of sugar by photosynthesis.
• 6H20 + 6CO2 + Light —> 6C6H12O6+ 6O2
• The rate of energy transfer between elements of an ecological system is called energy flow. The flow of energy is unidirectional in the ecosystem.

• Plants utilize 2-10% of PAR in photosynthesis.

• The energy formulated by the green plants (producers) then flow through different trophic levels of ecosystem i.e., primary, secondary and tertiary consumers.

• Producers including green plants which are capable for manufacturing their own food. These are capable of fixing the energy obtained from sun. Producers are autotrophic, generally chlorophyll bearing organisms.

• Consumers ( phagotrophs ) who cannot make their own food but they are directly or indirectly dependent on producers for obtaining food. Consumers may be:

• Primary consumers or herbivores.

• Secondary consumers or primary carnivores.

• Tertiary consumers or secondary carnivores.

• Primary consumers are those who obtain their food by directly feeding on producers (plants), secondary consumers from primary consumers (herbivores) and tertiary consumers from secondary consumers.

• The conversion of radiant energy from sun into chemical energy and then it is subsequently transfered to other organisms occurs in accordance with the laws of thermodynamics.

• First law of thermodynamics: Energy is neither created nor destroyed but can be transferred from one component to another. Eg. sunlight energy can transformed to energy to food & heat.

• Second law of thermodynamics says that at each step of energy transformation, there occurs dissipation of energy and increase in disorderliness.

• Trophic structures of an ecosystem is the type of producer-consumer arrangements in each food-level is called trophic levels.

• All trophic levels are connected to ecosystem by transfer of food or energy.

• There are two aspects with respect to energy flow in ecosystem which are important. First, the energy flows unidirectional i.e. from producers to herbivores the to carnivores: it cannot be transferred in reverse direction. Second, amount of energy which flows decreases with successive trophic level. Producers capture only a small fraction of solar (1 – 5 percent to total solar radiation), and the bulk of initialized energy is dissipated mostly as heat. Part of the energy captured in gross production of producers is used for the maintenance of their standing crop (respiration) and for providing food to herbivores. The unutilized net primary production is ultimately converted to detritus, which serve as energy source to decomposers. Thus, energy actually used by the herbivore trophic level is only a small fraction of energy captured at the producers levels.

• Large amount of energy losses at each trophic level. It is estimated that 90% of the energy is lost when it is transferred from one trophic level to another. Hence, the amount of energy which is  available at one level decreases from one to another level. Only about 10% of the biomass is transferred from one trophic level to the next one in a food chain. And about 10% of chemical energy is retained at every trophic level. When the avaliable food chain is short then final consumers can get a large amount of energy. But when there is long food chain then final consumer can get a less amount of energy.

Food chain

• The ecosystems is characterized by the energy flow and the circulation of material through its members. The different organisms of an ecosystem are linked together by their nutritional requirements. Individual related in this manner constitute a food chain.

• Food chain is an order or sequence of different organisms which are arranged in a way that the food is passed from one type of organism to other organisms such that the organisms of one order or trophic level are the food of the organisms of next order.
• Types of food chains: The food chains are of two types, namely:
• (i) Grazing food chain: This food chain starts from plants, goes throu gh herbivores and ends in carnivores.
• Plant – Herbivores – Primary Carnivores Sec-Carnivores.
• This type of food chain depends on the autotrophs which capture the energy from solar radiation.
• A few chains are given below:
• Grass – Grasshopper – Lizard – Hawk Grass – Mouse – Snake Hawk-Phytoplankton – Zooplankton-Fish Snake.
• The grazing food chain is further divided into two types, namely: (a) Predator (b) Parasitic.
• (ii) Detritus food chain: It starts from dead organic matter and ends in inorganic compounds. There are certain groups of organisms which feed exclusively on the dead bodies of animals and plants. These organisms are called detritivores. The detritivores include algae, bacteria, fungi. protozoans, insects. millipedes, centipedes, crustaceans, mussels, clams. annelid worms, nematodes, ducks, etc.

Difference between Grazing and Detitus food chains

Food web

• Food web represents to a group of inter-related food chains of particular community. Under natural conditions, the linear arrangement of food chain hardly occurs & these remain indeed inter-connected with each other through different types of organisms at different trophic level.

• Simple food chains are very rare in nature. This is because each organism can obtain there food from more then one tropic level of food chain. In other words, one organism forms food for more than one organisms of the higher trophic level.

• Food web is very important to maintain equilibrium (homeostasis) of ecosystem.

• Example: In a grassland ecosystem

• Grass Grasshopper -> Hawk

• Grass → Grasshopper-> Lizard ->Hawk

• Grass -> Rabbit – Hawk

• Grass → Mouse Hawk

• Grass Mouse – Snake Hawk

• Significance of food web: Food web is very important to maintain equilibrium (homeostasis) of ecosystem. e.g, the deleterious growth of grasses usually controlled by the herbivores, when one type of herbivores increases in number to control the vegetation.

• Similarly, when one type of herbivorous animal becomes extinct, the carnivore predating on this type may eat another type of herbivore.

• Similarly, when one type of herbivorous animal becomes extinct, the carnivore predating on this type may eat another type of herbivore.

Diference Between Food Chain and Food Web