TROPIC LEVELS AND PYRAMIDS
ENERGY FLOW THROUGH ECOSYSTEM – TROPIC LEVELS
- The flow of energy from producer to top consumers is called energy flow which is unidirectional.
- The study of Tropic level interaction in an ecosystem gives an idea about the energy flow through the ecosystem.
- Tropic level interaction deals with how the members of an ecosystem are connected based on nutritional needs
- Energy flows through the tropic levels: from producers to subsequent tropic levels.
- This energy always flows from lower (producer) to higher (herbivore, carnivore etc.) tropic level
- There is a loss of some energy in the form of unusable heat at each tropic level so that energy level decreases from the first tropic level upwards.
- As a result there are usually four or five tropic levels and seldom more than six as beyond that very little energy is left to support any organism
- The tropic level interaction involves three concepts
- Food Chain
- Food Web
- Ecological Pyramids
- Organisms in the ecosystem are related to each other through feeding mechanism or tropic levels, i.e. one organism becomes food for the other.
A sequence of organisms that feed on one another, form a food chain.
- A food chain starts with producers and ends with top carnivores.
The sequence of eaten and being eaten, produces transfer of food energy and it is known as food chain.
- The plant converts solar energy into chemical energy by photosynthesis.
- Small herbivores consume the plant matter and convert them into animal matter. These herbivores are eaten by large carnivores
A) TYPES OF FOOD CHAIN
Grazing food chain
- The consumers which start the food chain, utilising the plant or plant part as their food, constitute the grazing food chain.
- For example, in a terrestrial ecosystem, the grass is eaten by a caterpillar, which is eaten by lizard and lizard is eaten by a snake.
- In Aquatic ecosystem phytoplankton (primary producers) are eaten by zooplanktons which are eaten by fishes and fishes are eaten by pelicans.
Detritus food chain
- This type of food chain starts from organic matter of dead and decaying animals and plant bodies from the grazing food chain.
- Dead organic matter or detritus feeding organisms are called detrivores or decomposers.
- The detrivores are eaten by predators.
- In an aquatic ecosystem, the grazing food chain is the major conduit for energy flow.
- As against this, in a terrestrial ecosystem, a much larger fraction of energy flows through the detritus food chain than through the grazing food chain.
B) DIFFERENCE BETWEEN GRAZING AND DETRITUS FOO CHAIN
- The difference between these two food chains is the source of energy for the first level consumers. In the grazing food chain the primary source of energy is living plant biomass while in the detritus food chain the source of energy is dead organic matter or detritus.
- It is made up of multiple interrelated food chains
- It illustrates all possible transfers of energy and nutrients among the organisms in an ecosystem.
- If any of the intermediate food chains is removed, the succeeding links of the chain will be affected largely.
- A major advantage of food web is that it provides more than one alternative for food to most of the organisms in an ecosystem and therefore increases their chance of survival.
- For example, grasses may serve food for rabbit or grasshopper or goat or cow. Similarly a herbivore may be food source for many carnivorous species
C) Types of Biotic Interactions in a Food Web
‘0’ is no effect; ‘–’ is harmful; ‘+’ is beneficial.
- Mutualism: Interaction is favourable to both species
- Example: in pollination mutualisms, the pollinator gets food (pollen, nectar), and the plant has its pollen transferred to other flowers for cross-fertilization (reproduction).
- Competition: both species are harmed by the interaction.
- Example: if two species eat the same food, and there isn’t enough for both, both may have access to less food than they would if alone. They both suffer a shortage of food
- Commensalism: one species benefits, the other is unaffected.
- Example: cow dung provides food and shelter to dung beetles. The beetles have no effect on the cows.
- Amensalism: One species is harmed, the other is unaffected.
- Example: A large tree shades a small plant, retarding the growth of the small plant. The small plant has no effect on the large tree
- Predation and parasitism: one species benefits, the other is harmed.
- Example: predation—one fish kills and eats parasitism: tick gains benefit by sucking blood; host is harmed by losing blood.
- Neutralism: There is no net benefit or harm to either species. Perhaps in some inter-specific interactions, the costs and benefits experienced by each partner are exactly the same so that they sum to zero. It is not clear how often this happens in nature. Neutralism is also sometimes described as the relationship between two species inhabiting the same space and using the same resources, but that have no effect on each other. In this case, one could argue that they aren’t interacting at all.
- An ecological pyramid is a pyramidal representation of the relationship between the different living organisms at different tropic levels
- The pyramid consists of a number of horizontal bars depicting specific tropic levels which are arranged sequentially from primary producer level through herbivore, carnivore onwards.
- The length of each bar represents the total number of individuals at each tropic level in an ecosystem.
- The ecological pyramids are of three categories:
- Pyramid of numbers,
- Pyramid of biomass, and
- Pyramid of energy or productivity.
A) Pyramid of Numbers
- It deals with the relationship between the numbers of primary producers and consumers of different levels
- Depending upon the size, the pyramid of numbersmay not always be upright, and may even be completely inverted.
Pyramid of numbers – upright
- In this pyramid, the number of individuals is decreased from lower level to higher tropic level.
- This type of pyramid can be seen in the grassland ecosystem and pond ecosystem.
- The grasses occupy the lowest tropic level (base) because of their abundance.
- The next higher tropic level is primary consumer – herbivores like a grasshopper.
- The individual number of grasshoppers is less than that of grass.
- The next energy level is a primary carnivore like rats.
- The number of rats is less than grasshoppers, because, they feed on grasshoppers.
- The next higher tropic level is secondary carnivore like snakes. They feed on rats.
- The next higher tropic level is the top carnivore like Hawk.
- With each higher tropic level, the number of individual decreases
Pyramid of numbers – inverted
- In this pyramid, the number of individuals is increased from lower level to higher tropic level.
- A count in a forest would have a small number of large producers, for e.g. few number of big trees
- This is because the tree (primary producer) being few in number and would represent the base of the pyramid and the dependent herbivores (Example – Birds) in the next higher tropic level and it is followed by parasites in the next tropic level. Hyper parasites being at higher tropic level represents higher in number
- Note-It is very difficult to count all the organisms, in a pyramidof numbers and so the pyramid of number does not completely define the tropic structure for an ecosystem
B) Pyramid of Biomass
- Pyramid of biomass indicates decrease of biomass in each tropical level from base to apex.
- In this approach individuals in each tropic level are weighed instead of being counted. This gives us a pyramid of biomass, i.e., the total dry weight of all organisms at each tropic level at a particular time.
- Pyramid of biomass is usually determined by collecting all organisms occupying each tropic level separately and measuring their dry weight.
- This overcomes the size difference problem because all kinds of organisms at a tropic level are weighed.
- This can also be upright and inverted
Pyramid of Biomass – upright
- For most ecosystems on land, the pyramid of biomass has a large base of primary producers with a smaller trophic level perched on top.
- The biomass of producers (autotrophs) is at the maximum. The biomass of next trophic level i.e. primary consumers is less than the producers. The biomass of next higher trophic level i.e. secondary consumers is less than the primary consumers. The top, high trophic level has very less amount of biomass.
Pyramid of Biomass – Inverted
- In many aquatic ecosystems, the pyramid of biomass may assume an inverted form. (In contrast, a pyramid of numbers for the aquatic ecosystem is upright)
- This is because the producers are tiny phytoplankton that grows and reproduces rapidly.
- Here, the pyramid of biomass has a small base, with the consumer biomass at any instant exceeding the producer biomass and the pyramid assumes an inverted shape.
C) Pyramid of Energy
- It is used to compare the functional roles of the tropic levels in the ecosystem.
- An energy pyramid represents the amount of energy at each tropic level and loss of energy at each transfer to another tropic level. Hence the pyramid is always upward, with a large energy base at the bottom.
- Suppose an ecosystem receives 1000 calories of light energy in a given day. Most of the energy is not absorbed; some is reflected to space; of the energy absorbed only a small portion is utilized by green plants, out of which the plant uses up some for respiration and of the 1000 calories; therefore only 100 calories are stored as energy-rich materials.
- Now suppose an animal, say a deer, eats the plant containing 100 calories of food energy. The deer use some of it for its metabolism and stores only 10 calories as food energy. A lion that eats the deer gets an even smaller amount of energy. Thus, usable energy decreases from sunlight to producer to herbivore to carnivore. Therefore, the energy pyramid will always be upright.
- Energy pyramid concept helps to explain the phenomenon of biological magnification – the tendency for toxic substances to increase in concentration progressively with higher tropic levels.
D) Ecological Efficiency
- It describes the efficiency with which energy is transferred from one tropic level to the next.
- When energy transformations take place in living organisms the process is never 100% efficient
- Most of the energy is lost to the organism – either used in respiration, released as heat, excreted in faeces or unconsumed
- Typically energy transformations are ~10% efficient, with about 90% of available energy lost between tropic levels
- The amount of energy transferred depends on how efficiently organisms can capture and use energy (usually between 5 – 20%)
E) Limitations of Ecological Pyramids
- The ecological pyramids do not take into account the same species belonging to more than one tropic level.
- It assumes a simple food chain that almost never exists in nature.
- It does not explain food webs.
- Saprophytes are not given a place in ecological pyramids even though they play a vital role in ecosystem.
POLLUTANTS AND TROPIC LEVEL
- Pollutants especially the non-degradable move through the various trophic levels in an ecosystem.
- Non-degradable pollutants (persistent pollutants), which cannot be broken down by detrivores, not only move through the various tropic levels but also remain in that tropic level for a very long duration.’
- Example- Chlorinated Hydrocarbons
- Movement of these pollutants involves two main processes:
- It refers to how pollutants enter a food chain.
- In bioaccumulation there is an increase in concentration of a pollutant from the environment to the first organism in a food chain.
- Biomagnifications refers to the tendency of pollutants to concentrate as they move from one tropic level to the next
- In order for biomagnification to occur, the pollutant must have a long biological half-life (long-lived), must not be soluble in water but must be soluble in fats. E.g. DDT.
- If the pollutant is soluble in water, it will be excreted by the organism.
- Pollutants that dissolve in fats are retained for a long time. Hence it is traditional to measure the amount of pollutants in fatty tissues of organisms such as fish.
- In mammals, milk produced by females is tested for pollutants since the milk has a lot of fat in.