Organisms in ecological communities interact with one another in four major ways.

  1. One organism, by its activities, may benefit itself while harming the other. For instance, individuals of one species may kill and eat individuals of other species (predator-prey interaction). One type of individuals may obtain their food from the other but may or may not kill it (host-parasite interaction).
  2. Two organisms may mutually harm one another. This type of interaction is common when two organisms use the some resources and the resources are insufficient to supply their combined needs. Such organisms are called competitors, and their interactions constitute competition.
  3. Individuals of two species may develop an intimate, long-term relationship or association (symbiosis). In symbiosis, both participants may benefit from an interaction (multualism); or one participant benefits and the other is unaffected (commenslism); or one participant benefits and the other is harmed (parasitism).

19.6.1 Predation

Consumers that actively hunt other organisms are called predators. The organisms upon which a predator feeds are called prey. A frog that eats an insect is a predator. The insect is the prey. The frog itself is prey to other organisms such as snakes and storks. The predators, in turn, become prey to larger carnivores (Fig 19.20)


Fig. 19.20 predators actively hunt and kill their prey

The size of predator and prey populations is closely linked. A large prey population can support more predators than small prey population. If a prey population grows or shrinks, the number of predators changes as well.

19. 6.2 Symbiosis

The intimate and log-term relationship between two organisms is called symbiosis. It is of three types i.e. parasitism, mutualism and commensalism.

Some organisms do not kill the organism they feed on. Parasitism is a type of symbiotic relationship in which an organism feeds on the tissues or body fluids of another. The organism on which a parasite feeds is called the host. A parasite is harmful to its host and may even be fatal. Most parasites, however, do not kill their hosts. Examples of parasites are fleas, ticks, lice, a variety of worms, protists, many bacteria, and all viruses.

A true parasite is adapted to living on or in the body of its host. A parasite depends on its host for many functions. Many parasites cannot perform functions that the host provides for them.

The population size of a parasite is closely related to the population of its host. A large host population can support more parasites than a small host population. Parasite populations are also affected by the density of the host population. Parasites thrive in crowded host populations because the parasite can find new hosts easily.

bristles-or-tough-spines     obnoxious-chemicals

camouflage     mimicry

Fig. 19.21 As a consequence of predator-prey interactions, prey have evolved a rich variety of adaptations that make them difficult to be captured and eaten. Some of the evolutionary adaptations are tosic hair, bristles or tough spines (a), obnoxious chemicals (b), camouflage (c), and mimicry (d).

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