Trophic Levels


Insect ecology is the branch of entomology that focuses on the interrelationships between insects and their environment.  To an ecologist, the concept of “environment” encompasses both the abiotic world (non-living things like climate and geology) as well as the biotic world (all living organisms including plants, animals, microorganisms, etc.).  All of these components interact within a framework called the biocenose (a natural community).

Communities are groups of organisms (populations) that maintain persistent associations with each other.  The members of a typical community include plants, animals, and other organisms that are biologically interdependent through predation, parasitism, and symbiosis.  The structure of a biotic community is largely characterized by the trophic (feeding) relationships among its member species.  These relationships are often represented simplistically as a food chain.  Each link in the food chain represents a trophic level encompassing either producers or consumers.

chain00 In most communities, green plants are the dominant producers.  They represent the first link in a typical food chain.  Plants capture kinetic energy from sunlight and, through the process of photosynthesis, manufacture organic molecules (e.g. simple sugars) from carbon dioxide and water.  The captured energy is “stored” in the chemical bonds of these molecules.  Some of the stored energy is used by plants for their own survival and growth, some is lost as heat, and some passes on to consumers when the plant is eaten, or to decomposers when the plant dies.

Primary consumers occupy the second link of a food chain.  These animals, often called herbivores, survive by feeding exclusively on plants or plant products.  The third link includes primary carnivores, secondary consumers that live as predators or parasites of herbivores.  Any remaining links in the food chain are occupied by secondary or tertiary carnivores (predators or parasites of other carnivores).  Since energy becomes limiting at the uppermost trophic levels, there are seldom more than four or five links in a terrestrial food chain.

Food Webs

foodwebVery few animals have a diet that is restricted to only a single food source, so the concept of a linear food chain is extremely simplistic.  In reality, trophic relationships within a community are more like a food web in which dozens of plant species support a wide variety of herbivores which in turn are consumed by numerous predators and parasites.  If one species within a food chain becomes scarce (perhaps due to bad weather or over-exploitation), there will be serious repercussions on all other species in the chain.  But in a complex food web, changes in individual populations are likely to have a smaller impact because they are buffered by the availability of an alternative prey or host species.

Ecologists who study the structure of natural communities find that plants growing in monocultures (large fields of a single species) tend to be less productive (have a lower yields) than plants of the same species that grow within a more complex community.  This result can be largely explained by the increased numbers of parasites and predators that are able to survive on a wider assortment of hosts and prey.  There is a chain-reaction effect:

  1. Higher plant diversity promotes higher herbivore diversity.
  2. Higher herbivore diversity promotes higher predator and parasite diversity.
  3. Higher rates of predation and parasitism lead to lower population density among herbivores.
  4. Plants are more productive because they are attacked by fewer herbivores.

While it may not be economically practical for most farmers to abandon crop monocultures, there are some cases where use of genetically diverse plants (mixed varieties with similar harvest dates) or compatible species combinations (e.g. alfalfa and orchard grass in pastures) can be grown to obtain higher yields with lower input of pesticides.