Food-chain, Trophic levels and Energy Flow in Ecology
How each organism derives its energy determines its place in the food chain and trophic levels. Plants are producers or autotrophs that are independent and produce their food and energy. They form the first trophic level. Then come the animals which are consumers (primary, secondary and tertiary) that feed directly or indirectly on producers. The decomposers are the end of the food-chain.
Food is the source of energy which helps all organisms to function. Based on their food and energy source, organisms find a place in the food-chain, which in turn determines their trophic level. Each food chain has one organisms at each trophic level.
Trophic Level and Food-chain
On the first trophic level are the producers. These organisms can produce their own food and energy and are called producers. On land these are green plants, and in aquatic environments they are phytoplankton and algae (1). They use inorganic elements and energy captured from sun through the green pigment chlorophyll to produce all the organic compounds they need (2). Light is used to combine carbon and water to form simple sugars. The carbon is derived from carbon dioxide from air by leaves, and water is obtained by the roots (3). These are also called the autotrophs and form the first trophic level. There are also some chemo-autotrophs which depend on energy from chemical reactions for survival (1).
Being producers, plants and phytoplankton's form the first link in the food-chain and the first trophic level (2).
All other organisms that cannot produce their own food and which are dependent on producers directly or indirectly for energy and nutrient are called heterotrophs (2). These can be either consumers or decomposers (1).
The primary consumers feed directly on the producers and form the second trophic level. These are the herbivores such as rabbits, cattle, deers on land that eat grass. In marine environments, crabs feeding on seaweed are the primary consumers (4).
Secondary consumers are the carnivores that feed on primary consumers and are the third trophic level. Examples are foxes feeding on rabbits, or barracuda that are predatory fish eating small fishes.
On the fourth trophic level are the tertiary consumers that feed on secondary consumers. For example, foxes are eaten by jackals, wolves or eagles (5). In the seas dolphins which feed on predators fish are the tertiary consumers.
They there are the decomposers which feed on the dead matter of all plants and animals and come last in any food chain. There is usually no trophic level assigned for them separately, because they can eat and decompose producers, and all consumers. So it is hard to assign one level for them. Examples are bacteria, fungi, insects etc., on land (6). In the marine environment it can be bacteria, fungi, sea-slugs and brittle fish (4).
The distribution of different animals among the various trophic level is not rigid, as one animal can feed at more than one level. For example wolves are secondary consumers when they feed on rabbits, but are tertiary consumers when they feed on foxes which feed on rabbits (7). In addition there are omnivores feed on both plants and animals so they can be primary consumers as well as secondary or tertiary consumers like mice.
A food-chain starts with producers, then come the primary, secondary and tertiary consumers, with decomposers at the end. However each organism can be involved in more than one chain. For example the rabbit can also eat many plant types, besides grass. It can in turn be eaten by foxes or wolves. These secondary consumers can also eat other primary consumers like birds etc. So in the natural world one is more likely to find a food web than a chain, with different elements connected in more than one chain.
There are many factors which determine the number of trophic levels in any ecosystem. These are the amount of energy entering an ecosystem, energy loss between trophic levels and the form and physiology of organisms at each trophic level (6).
The amount of energy is dependent on the sunlight entering a system and determines the number of producers that can exist. Since all organisms depend on producers plants and phytoplankton, the total mass of these producers or productivity at the first level determines number of trophic levels (8, 9). Productivity of producers is also called primary productivity of an ecosystem (9). Primary productivity is dependent on soil conditions, and water availability to mention few factors in addition to light. Therefore tropical rainforests have a primary productivity of 2000 grams/square meters/year, while deserts also in tropics have only 100 grams/square meters/year.
Transfer of Energy
At each trophic level a large proportion of energy is lost by functions like respiration, growth, defecation, reproduction, or death of individuals not consumed (6). There is only a 10% transfer of energy from trophic level to the next. So with each trophic level the amount of energy available becomes lesser, with the reminder lost as heat (6). In other words the productivity of each higher trophic level is less (9).
Decomposers that breakdown decaying material circulate nutrients that is used by producers, and are responsible for more energy flow than producers (6). In addition, better quality food provides more energy transfer (6).
With each higher trophic level the size of the animal increases. Larger animals have to feed more, and therefore have larger territories to provide them their feed. So with each trophic level the number of individuals supported becomes less, and there is less energy left for the level. Ultimately no further trophic levels can be supported (6).
Considering all factors terrestrial or land ecosystems can support up to five trophic levels and aquatic systems up to seven.
The decreasing amounts of energy form the trophic pyramid and is the reason why it is more environmentally friendly to eat more vegetables than meat. Any ecosystem produces more at lower trophic levels than higher ones.