POPULATION INTERACTIONS

POPULATION INTERACTIONS:

Predation:

  • Organism of higher trophic level (predator) feeds on organism of lower trophic level (prey) is called the predation.
  • Even the herbivores are not very different from predator.
  • Predator acts as a passage for transfer of energy across trophic level.
  • Predators keep prey populations under control.
  • Exotic species have no natural predator hence they grow very rapidly. (prickly pear cactus introduced in Australia created problem)
  • Predators also help in maintaining species diversity in a community, by reducing the intensity of competition among competing prey species. (Pisaster starfish field experiment)

Defense developed by prey against predators:

Animals:

  • Insects and frogs are cryptically coloured (camouflaged) to avoid being detected by the predator.
  • Some are poisonous and therefore avoided by the predators.
  • Monarch butterfly is highly distasteful to its predator (bird) due to presence of special chemical it its body. The chemical acquired by feeding a poisonous weed during caterpillar stage.

Plants:

  • Thorns in Acacia, Cactus are morphological means of defense.
  • Many plants produce and store some chemical which make the herbivore sick if eaten, inhibit feeding, digestion disrupt reproduction, even kill the predators.
  • Calotropis produces poisonous cardiac glycosides against herbivores.
  •  Nicotine, caffeine, quinine, strychnine, opium etc. are produced by plant actually as defenses against the grazers and browsers.

 Competition:

  • Interspecific competition is a potent force in organic evolution.
  • Competition generally occurs when closely related species compete for the same resources that are limiting, but this not entirely true:
  • Firstly: totally unrelated species could also compete for the same resources.
    • American lakes visiting flamingoes and resident fishes have their common food, zooplanktons.
  • Secondly: resources need not be limiting for competition to occur.
    • Abingdon tortoise in Galapagos Islands became extinct within a decade after goats were introduced on the island, due to greater browsing ability.

 

  • Competitive release: A species, whose distribution is restricted to a small geographical area because of the presence of a competitively superior species, is found to expand its distributional range dramatically when the competing species is experimentally removed.
    • Connell’s elegant field experiment showed that superior barnacle Balanus dominates the intertidal area and excludes the smaller barnacle Chathamalus from that zone.
  • Gause’s competitive Exclusion Principle’: two closely related species competing for the same resources cannot co-exist indefinitely and the competitively inferior will be eliminated eventually.
  • Resource partitioning: If two species compete for the same resource, they could avoid competition by choosing, for instance, different times for feeding or different foraging pattern.
    • MacArthur showed five closely related species of warblers living on the same tree were able to avoid competition and co-exist due to behavioral differences in their foraging activities.

Parasitism:

  • Parasitic mode of life ensures free lodging and meals.
  • Some parasites are host-specific (one parasite has a single host) in such a way that both host and parasite tend to co-evolve.

Parasitic adaptation

  • Loss of unnecessary sense organs.
  • Presence of adhesive organs or suckers to cling on to the host.
  • Loss of digestive system.
  • High reproductive capacity
  • Parasites having one or more intermediate host or vectors to facilitate parasitisation of its primary host.
  • Liver fluke has two intermediate hosts (snail and a fish) to complete its live cycle.

 Effects on the host:

  • Parasite always harms the host.
  • They reduce the survival, growth and reproduction of the host.
  • Reduce its population density.
  • They make the host more vulnerable to the predators, by making it physically weak.
  •  Ectoparasite: feeds on the external surface of the host.
    • Lice on human
    • Ticks on dog
    • Marine fish infested with copepods
    • Cuscuta a parasitic plant grow on hedge plants.

 

  • Endoparasites: are those that live inside the host body at different sites.
    • Life cycle is more complex.
    • Morphological and anatomical features are greatly simplified.
    • Highly developed reproductive system.
  • Brood parasitism:
    • Special type of parasitism found in birds.
    • The parasitic birds lay its eggs in the nest of its host and let the host incubate them.
    • The egg of the host is very similar with the egg of the host.
    • Cuckoo lays eggs in the nest of the crow.

Commensalism: 

This is the interaction in which one species benefits and the other is neither benefited nor harmed.

  • Orchids growing as an epiphyte on a mango branch.
  • Clown fish living among tentacles of sea anemone.
  • Barnacles on back of whales.
  • Cattle Egret and grazing cattle.

Mutualism:

It is the interaction between two living organism, both are equally benefited, no one is harmed.

  • Lichen: a mycobiont and a Phycobiont.
  • Mycorrhiza: relationship between fungi and root of higher plant.
  • Pollinating insects and flowering plants.
  • Fig trees and its pollinating agent wasp.

Sexual deceit

  • Mediterranean orchid Ophrys employs ‘sexual deceit’.
  • Petal of the flower resembles the female bee.
  • The male bee attracted to what it perceives as a female, ‘pseudocopulates’ with the flower but does not get any benefits.

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