Contribution of Organisms to Ecosystem Functioning

The trophic level concept was first given by Lindeman (1942). According to him, a food chain consisted of steps— primary producers, herbivores, carnivores— that he referred to as trophic levels.

Trophic literally means feeding, so trophic levels are the positions or levels at which organisms of an ecosystem feed.

In complex natural communities, organisms obtain their food from plants through one, two, three or four steps and are accordingly said to belong respectively to the first, second, third and fourth trophic level.

(a) Green plants (primary producers) – I trophic level:

Green plants are autotrophs. These are the organisms that literally ‘feed themselves’. They can be divided into two groups— photoautotroph’s or photosynthetic organisms and chemoautotroph’s which are autotrophic but non-photosynthetic {e.g., the prokaryotes).

(b) Herbivores (primary consumers)-II Trophic level:

Herbivores obtain their food directly from producers. Hence, herbivores belong to second trophic level. Herbivores can be called predators if they are larger than their prey and consume most or all of a plant. They are parasites if they live in or on their hosts for long periods of time. Thus, most mammalian herbivores are predators and most insect herbivores are parasites.

(c) Carnivores (secondary consumers) – III trophic level:

Carnivores feed upon herbivores and thus belong to the third trophic level. However, they may be classified as first, second, third level carnivore.

(d) Top carnivores (tertiary consumers) – IV trophic level:

Organisms belonging to fourth trophic level are top carnivores. They are rarely killed or never killed by other organisms for food, e.g., vulture and lion in a forest ecosystem.

It may be noted that trophic complexity increases with the number of trophic levels and vertical connections in the food web. However, decomposers play an important role in all ecosystems. They breakdown the organic waste products and dead remains of producers and consumers into the organic substances needed by the producers. But decomposers are not assigned to any trophic level.

It should be known that classification of organisms by trophic levels is one of function and not of species as such. However, some organisms feed at more than one trophic level. For example, male house flies feed on nectar and plant juices while the females are blood-sucking ectoparasites. Some carnivores also feed at more than one trophic level and as such they cannot be assigned to discrete trophic levels. However, some authors have solved this problem by allocating carnivores to non-integer trophic positions, so that a carnivore might be described as a level 3.2 carnivore, for instance.

Animals of successive trophic levels in a food chain tend to be larger. Of course, there are upper and lower limits to the size of food which a carnivore can eat. Generally large carnivores cannot live on very small food items because they cannot catch enough of them in a given time to satisfy their metabolic needs. The energy flow through a trophic level equals the total assimilation (A) at that level, which in turn equals the production (P) of biomass plus respiration(R).

Trophic Efficiency:

Lindeman observed that less energy was available to each higher trophic level due to the work performed and to the inefficiency of biological energy transformations at the trophic level below. For example, of the light energy impinging on a lake (A₀), plants use only a fraction (A₁) — resulting in the primary production of the system. Herbivores assimilate less energy (A₂) than plants do because plants utilize some of their own production to maintain themselves before herbivores consume, them. The ratio of production on one trophic level to that on the level below it (for example, A₂/A₁) is the biological efficiency of that link in the food chain (Ricklets and Miller, 2000).

It may be defined as the efficiency of energy transfer from a trophic level to the one above. According to Lindeman’s law of trophic efficiency (Lindeman, 1942), the efficiency of energy transfer from one trophic level to the next is about 10%. In other words, about 10% of the net primary productivity of producer’s ends up as herbivores, about 10% of the net primary productivity of herbivores ends up as first-level carnivores, and so on. Thus, Lindeman proposed that trophic efficiencies increase as one ascends a food chain.

However, more recent data support neither Lindeman’s suggestion that trophic efficiency increases as one ascends a food chain, nor the generalisation that the efficiency of energy transfer from one trophic level to the one above is about 10%. It is now known that trophic efficiencies vary from less 0.1% to over 20%. For example, a community of herbivorous small mammals in a grassland ecosystem might have a trophic efficiency of less than 0.1%, herbivorous zooplankton feeding on phytoplankton might have a trophic efficiency of over 20%.

Arrangement of Trophic levels in a Food Pyramid:

In any natural community the distribution of organisms in terms of their total mass and their contribution to the food chain can be compared to the structure of a pyramid. A food pyramid represents several features of a natural community of organisms. The broad base of a food pyramid is always composed of photosynthetic plants. The next higher layer or trophic level in the pyramid is composed of small animals that feed on the many plants of the lowermost pyramid layer.

The smaller animals in turn serve as a food source for the larger animals, which make up the next higher-trophic level in the food pyramid. The successively higher trophic levels in the pyramid are typically constituted of progressively larger but fewer animals.

In general, for terrestrial food pyramids the total mass of protoplasm of any trophic level is always less than the layer below it. The fewest but largest animals usually but not always occupy the peak. By contrast in marine pyramids the microscopic phytoplankton usually consists of smaller total protoplasmic mass than the higher trophic levels because they are unicellular and rapidly consumed. However, their rate of turnover is high. In all cases, whether on land or in the sea, the amount of organic matter produced per unit of time is always greater at the basic trophic level than at the higher trophic levels of the pyramid.