Trophic Level, Food Chain, Food Web

 

Introduction

  • In ecology, a trophic level pertains to a position in a food chain or ecological pyramid occupied by a group of organisms with similar feeding modes
  • The trophic level of an organism is the number of steps it is from the start of the chain
  • The concept of trophic level was developed by Raymond Lindeman (1942), based on the terminology of August Thienemann (1926)
  • The trophic levels are shown in a series or a succession to represent the flow of food energy and the feeding relationships between them

 

Categories

  • The trophic levels have two major categories: the autotrophs and the heterotrophs
  • Autotrophs are organisms that can produce organic matter from inorganic matter.
    • Since they can make their own food and do not need to feed on other organisms, they are also referred to as the producers of an ecosystem
  • Heterotrophs are organisms that obtain organic matter directly by consumption
    • Unlike autotrophs, they do not have the ability to manufacture their food from inorganic sources. Thus, they hunt or gather food from other organisms. Hence, these are referred to as consumers
    • Heterotrophs may be further grouped as:
      • Primary Consumers: These comprise the plant-eating organisms called herbivores
      • Secondary Consumers: These feed on the primary consumers
      • Tertiary Consumers: These feed on the secondary consumers and so on
      • Final Consumers: The final group called reducers feeds on dead organic matter. They include the detritivores and the decomposers

 

Trophic Structure

    • Trophic structure refers to the partitioning of biomass between different trophic levels.
      • It is controlled chiefly by the biomass of the primary producers
    • The primary producers affect the transfer efficiency between trophic levels as they essentially provide the energy and the nutrient inputs
    • Apart from them, another important factor is the top-down component.
      • This includes the predators.
      • Their consumption suppresses the lower trophic levels.
      • In a way, the predators help the primary producers by controlling or limiting excessive herbivory by predation. They serve as biological control of the lower trophic levels

 

Trophic Level Pyramid

    • An ecological pyramid is often depicted as a trophic level pyramid.
      • It is a graphical representation in the shape of a pyramid comprised of plants and animals in a certain ecosystem
    • The shape indicates that the bottom trophic level is comprised of organisms that can make their own food through available sources from the environment.
      • They do not feed on other organisms to obtain their nutritional requirements.
      • Thus, they represent the base. This portion of the pyramid is comprised of producers.
    • As the trophic levels go up, it tapers towards the peak.
      • This pyramid shape depicts the biomass in each trophic level.
      • Biomass is the amount of living or organic matter in an organism. The base shows the largest biomass and then diminishes in amount as it moves up to the apex. This is the most common structure in ecosystems
    • However, there are also instances wherein an inverted pyramid occurs.
      • The latter results when the combined weight of producers is smaller than the combined weight of consumers

energy_pyramid

 

Trophic level examples

    • Level 1: producers
      • This level comprises the primary producers, and are found the base of an ecological pyramid
      • They are found at the base of an ecological pyramid
      • The fundamental feature of organisms in trophic level 1 is their ability to produce their own food from abiotic materials
    • Level 2: primary consumers
      • In this level, the organisms occupying this level feed on the primary producers and are called primary consumers.
      • Animals that feed on plant materials are called herbivores
      • They have anatomical and physiological features that make them adapt to a plant diet
      • Examples of herbivores are horses, cattle, and goats
    • Level 3: secondary consumers
      • Secondary consumers are comprised of animals that feed on primary consumers. Organisms that eat other animals are called carnivores (or predators)
      • Predation is an interaction in an ecosystem where a predator hunts or catches, kills and eats prey. Thus, Predators are in turn, adapted anatomically and physiologically for an animal diet
        • Examples of animals with a predator-prey relationship are spiders and flies, lion and zebra, bear and fish, and fox and rabbit
      • Not all predators though have a diet exclusive of meat. Their diet may also include plant materials. Animals that feed on both plants and animals are called omnivores
        • Examples of omnivores are chimpanzees, orangutans, gorillas, pigs, most bears, etc.
      • Other trophic levels
        • The organism that feeds on a secondary consumer is called a tertiary consumer and the one that eats on a tertiary consumer is referred to as a quaternary consumer.
        • The tertiary consumers and the quaternary consumers occupy trophic levels 4 and 5, respectively
      • Decomposers
        • The last of the trophic level is occupied by decomposers, such as detritivores. They feed on dead plants and animal matter.
        • Detritivores are decomposers that specifically fragment to consume their food.
        • Examples of detritivores are worms, millipedes, dung flies, woodlice, and slugs. Other decomposers include fungi and bacteria

Introduction

  • Food chain is a feeding hierarchy in which organisms in an ecosystem are grouped into trophic (nutritional) levels and are shown in a succession to represent the flow of food energy and the feeding relationships between them
  • A food web is the natural interconnection of food chains and a graphical representation of what-eats-what in an ecological community
  • Food chains intertwine locally into a food web because most organisms consume more than one type of animal or plant
  • Major parts of Food Chain
    • Sun: This is the initial source of energy, which provides energy for everything on the planet
    • Producers: This is the first stage of Food Chain. These are any plant or other organisms that produce their own nutrients through photosynthesis
    • Consumers: These are all organisms that are dependent on plants or other organisms for food. This is the largest part of a food web, as it contains almost all living organisms
    • Decomposers: These are organisms that get energy from dead or waste organic material. This is the last stage in a food chain, and they convert organic waste materials into inorganic materials like nutrient-rich soil or land
  • Because energy, in the form of heat, is lost at each step, or trophic level, chains do not normally encompass more than four or five trophic levels

 

Types of Food Chains

  • Two types of food chains are present in ecosystems:
  1. Grazing food chain
    • Grazing animals play an important role in the transfer of energy to the carnivores in this type of food chain, hence the name grazing food chain.
    • Green plants in the terrestrial ecosystems and phytoplankton in the aquatic ecosystems are the producers.
    • The primary consumers are the cattle, sheep, rabbits, deer, insects, and snails which feed on the green plants in the terrestrial ecosystems and the zooplankton, fishes and animals which feed on phytoplankton in the aquatic ecosystems.
    • In the soil the unconsumed dead organisms and biological wastes become the food for the detritivores of the detritus food chain.
    • Herbivores (the primary consumers) are eaten by the secondary consumers or primary carnivores.
      • Similarly secondary consumers are eaten by the tertiary consumers or secondary carnivores.
    • The grazing food chains are linear and are usually with 4 to 5 trophic levels in the chain
    • Examples
      • In terrestrial Ecosystem
        • Grass
        • Grass
      • In aquatic Ecosystem
        • Phytoplankton
  1. Detritus Food Chain
    • It starts from dead organic matter of decaying animals and plant bodies consumed by the micro-organisms and then to detritus feeding organism called Detrivores or Decomposers and to other predators
    • Examples
      • Litter → Earthworms→ Chicken → Hawak
  • The distinction 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
  • The two food chains are linked as well
    • The initial energy source for detritus food chain is the waste materials and dead organic matter from the grazing food chain

 

Food Web

  • A food web is a graphical model depicting the many food chains linked together to show the feeding relationships of organisms in an ecosystem
  • It differs from a food chain in a way that the latter is a linear system showing a succession of organisms whereby each species is eaten in turn by another species
    • While Food web is a more complex network of what-eats-what in a particular ecosystem
  • The diagram below shows an example of a food web

  • In food webs, arrows point from an organism that is eaten to the organism that eats it. As the food web above shows, some species can eat organisms from more than one trophic level

Concept Check!

Differences: Food Chain vs Food Web

Food ChainFood Web
It is a pathway in which organisms of an ecosystem are grouped in trophic levels, which are shown in succession to represent a linear flow of food energyIt is a graphical model showing the interconnecting food chains in an ecological community
It represents a single linear pathway of energy flowIt has a number of interconnected pathways, through which the energy glows within an ecosystem
It generally consists of 4-6 trophic levelsIt consists of many number of trophic levels
It does not improve the adaptability and competition among the organismsIt improves the adaptability and competitiveness of organisms
The whole food chain can be disturbed, with an interference in a single trophic levelThe whole food web won’t be disturbed if a disturbance occurs in a single trophic level
Energy transfer efficiency limits food chain lengths

  • Energy is transferred between trophic levels when one organism eats another and gets the energy-rich molecules from its prey’s body. However, these transfers are inefficient, and this inefficiency limits the length of food chains
  • When energy enters a trophic level, some of it is stored as biomass, as part of organisms’ bodies.
    • This is the energy that’s available to the next trophic level since only energy storied as biomass can get eaten.
    • As a rule of thumb, only about 10% of the energy that’s stored as biomass in one trophic level—per unit time—ends up stored as biomass in the next trophic level—per the same unit time
    • This pattern of fractional transfer limits the length of food chains; after a certain number of trophic levels—generally three to six, there is too little energy flow to support a population at a higher level