Interactions With Negative Effect

Organisms And Their Environment of Class 12

Certain interactions between different species result in negative effect on either or both species. Parasitism and predation are interactions when one species gains and the other suffers. But, in the interaction involving competition, both species are harmed.


Interaction between two species, where both suffer adverse effects, is known as competition. Usually, competition occurs when resources, such as space, light and nutrients, etc. are in short supply as a result of competition, the growth and seed production of both species is reduced.

Competition is basically of two types :

  • interspecific
  • intraspecific

Interspecific competition occurs between individuals of two different species occurring in a habitat.

Intraspecific competition occurs between individuals of the same species. Generally, the intraspecific competition is more intense than interspecific competition. The requirements of individuals of the same species are very similar; hence, they compete more fiercely.

Gause’s Hypothesis (Principle of competitive exclusion)—Gause (1934) found that out of two species of Paramecium grown together one is eliminated. This phenomenon is called Gause’s Hypothesis. Sometimes, two or more species & survive this competition. Thus Poa pratensis and Agrostis alba absorb K+ from the medium so rapidly as to eliminate species of Medicago and Brumus nearby.


Interaction between species involving killing and consumption of prey is called predation. It is similar to parasitism in the sense that here also one population is benefitted and the other is harmed. The looser is termed as prey and the gainer as predator. But, unlike parasitism, the gainer is larger than the looser. The predator kills the prey and feeds upon it.

The insectivorous plants such as Drosera, Utricularia, Dionaea, Aldovanda, Pinguicula, Nepenthes, Sarracenia, Cephalotus, Darligtonia, Heliamphora etc. are good examples of predation. These plants feed upon insects and animalcules to meet their nitrogen deficiency. Besides, the herbivores feed not only upon herbs but also on shrubs and trees. The plants are harmed upto different degrees due to grazing and browsing. The annuals are subjected to more extensive grazing than perennials. Due to selective grazing, unpalatable, thorny and shrubby species increase in number. As a result, there is decrease in biodiversity.

This also decreases the humus contents of the soil. However, due to grazing and browsing there is definite increase in the nitrogen contents of the soil by the addition of excreta of grazing animals.

Predation is commonly described by the herbivore–carnivore interaction, grass-deer-tiger food chain. e.g., tiger, the predator, keeps check on deer (prey) population. If the predator population increases, it consumes more prey, thereby reducing the population of prey. The reduction in prey population may reduce the population of predator, resulting from starvation and migration. The reduction in predator population may lead to increase in prey population, since they are not preyed upon. The increased prey population may over-graze the grasses, and the shortage of herbage may eventually lead to reduction in prey population. It may ultimately effect population of predators. Thus these, relationships stabilise the prey and predator populations in a community.


It is a kind of relationship where one component population is benefitted and the other is harmed. The gainer is called parasite and the looser, the host. The host is always larger than the parasite.

On the basis of its position on the host, the parasites are classified into two categories as

  • ectoparasite
  • endoparasite

The endoparasite lives inside the host whereas the ectoparasite lives on the surface.

The parasitism may be total or partial. The former remains a parasite throughout it’s life and is wholly dependent whereas the latter is only partly dependent upon the host showing tendency towards free living.

On the basis of their location on a specific body part, the parasites have been categorised as stem, root or leaf parasites.

The plants of Cuscuta (Dodder) are total stem parasite whereas those of Orobanche, Rafflesia, Balanophora etc. are total root parasites. The partial stem parasites are Dendrophtoe, Viscum whereas Santalum and Thesium are partial root parasites.

Numerous fungi (Albugo, Puccinia, Ustilago etc.) bacteria (Xanthomonas, Pseudomonas, Mycobacterium, Vibrio, Leptospira etc.) and viruses are parasites on plants and animals. In the same way several protozoans (Plasmodium, Entamoeba etc.), flat worms, tape worms, round worms live as endoparasites in other animals. A few insects occur as ectoparasites on other animals. Besides, these ectoparasites may carry fungi or bacteria as hyperparasites.

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