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सेमिनार: अंग्रेज़ी सीखने का अवसर (23 सितंबर: दोपहर 3 बजे)
Aug 08, 2015

Ecological communities consist of interacting or potentially interacting species in the same environment. The direct interactions between species are divided into various categories. Sometimes these complex interactions and the flow of energy through the system are diagrammed in food webs, which display the movement of energy from lower to higher trophic, or feeding, levels.

There are six ecological relationships in which two are oppositional and four are symbiotic. The oppositional relationships are :

⇒ Predation and

⇒ Competition. 

The symbiotic relationships are:

  • Mutualism
  • Commensalism
  • Amensalism
  • Parasitism.

Symbiotic relationships:The word symbiosis literally means 'living together,' but when we use the word symbiosis in biology, what we're really talking about is a close, long-term interaction between two different species. There are many different types of symbiotic relationships that occur in nature.

In many cases, both species benefit from the interaction. This type of symbiosis is called mutualism. An example of mutualism is the relationship between bullhorn acacia trees and certain species of ants. Each bullhorn acacia tree is home to a colony of stinging ants. True to its name, the tree has very large thorns that look like bull's horns. The ants hollow out the thorns and use them as shelter. In addition to providing shelter, the acacia tree also provides the ants with two food sources. One food source is a very sweet nectar that oozes from the tree at specialized structures called nectaries. The second food source is in the form of food nodules called beltian bodies that grow on the tips of the leaves. Between the nectar and the beltian bodies, the ants have all of the food they need.

So, the ants get food and shelter, but what does the tree get? Quite a lot actually, you see the ants are very territorial and aggressive. They will attack anything and everything that touches the tree - from grasshoppers and caterpillars to deer and humans. They will even climb onto neighboring trees that touch their tree and kill the whole branch and clear all vegetation in a perimeter around their tree's trunk, as well. The ants protect the tree from herbivores and remove competing vegetation, so the acacia gains a big advantage from the relationship. In this case, the acacia is considered ahost because it is the larger organism in a symbiotic relationship upon or inside of which the smaller organism lives, and the ant is considered to be a symbiont, which is the term for the smaller organism in a symbiotic relationship that lives in or on the host.

a) Mutualism

MutualismMutualism is an interaction characterized by mutual benefit, so both species benefit from the relationship. A flowering plant producing nectar to attract an animal, such as a bee, is one example. The bee benefits by feeding on the nectar, while the plant benefits because the bee goes on to disperse the plant's pollen. Mutualism can also be thought of as "mutual exploitation."

Other well-known example of mutualism is the relationship between ungulates (such as Bovines) and bacteria within their intestines. The ungulates benefit from the cellulase produced by the bacteria, which facilitates digestion; the bacteria benefit from having a stable supply of nutrients in the host environment.

Mutualism plays a key part in ecology. For example, mutualistic interactions are vital for terrestrial ecosystem function as more than 48% of land plants rely on mycorrhizal relationships with fungi to provide them with inorganic compounds and trace elements. In addition, mutualism is thought to have driven the evolution of much of the biological diversity we see, such as flower forms (important for pollinationmutualisms) and co-evolution between groups of species. However mutualism has historically received less attention than other interactions such as predation and parasitism.

Mutualistic transversals can be thought of as a form of "biological barter" in mycorrhizal associations between plant roots and fungi, with the plant providing carbohydrates to the fungus in return for primarily phosphate but also nitrogenous compounds. Other examples include rhizobia bacteria that fix nitrogen for leguminous plants in return for energy-containing carbohydrates.

b) Commensalism is an association between two different species where one species enjoys a benefit, and the other is not significantly affected. CommensalismCommensalism is sometimes hard to prove because in any symbiotic relationship, the likelihood that a very closely associated organism has no effect whatsoever on the other organism is pretty unlikely. But, there are a few examples  where commensalism does appear to exist. For example, the cattle egret follows cattle, water buffalo, and other large herbivores as they graze. The herbivores flush insects from the vegetation as they move, and the egrets catch and eat the insects when they leave the safety of the vegetation. In this relationship the egret benefits greatly, but there is no apparent effect on the herbivore.

Some biologists maintain that algae and barnacles growing on turtles and whales have a commesalistic relationship with their hosts. Others maintain that the presence of hitchhikers causes drag on the host as it moves through the water and therefore the host is being harmed, albeit slightly. In either case, it is unlikely that the fitness of the host is really affected by the hitchhikers, so commensa lism is probably the best way to describe these relationships as well.

c) Parasitism: It is relationship between two species of plants or animals in which one benefits at the expense of the other, sometimes without killing the host organism. ParasitismUnlike predators, parasites typically do not kill their host, are generally much smaller than their host, and will often live in or on their host for an extended period. Both are special cases of consumer-resource interactions.

Parasites may be characterized as ectoparasites—including ticks, fleas, leeches, and lice—which live on the body surface of the host and do not themselves commonly cause disease in the host; or endoparasites, which may be either intercellular (inhabiting spaces in the host’s body) orintracellular (inhabiting cells in the host’s body). Intracellular parasites—such as bacteria or viruses—often rely on a third organism, known as the carrier, or vector, to transmit them to the host. Malaria, which is caused by a protozoan of the genus Plasmodium transmitted to humans by the bite of an anopheline mosquito, is an example of this interaction. The plantailment known as Dutch elm disease (caused by the fungus Ceratocystis ulmi) can be spread by the European elm bark beetle.

Parasites reduce host biological fitness by general or specialized pathology, such as parasitic castration and impairment of secondary sex characteristics, to the modification of host behavior. Parasites increase their fitness by exploiting hosts for resources necessary for their survival, e.g. food, water, heat, habitat, and transmission. Although parasitism applies unambiguously to many cases, it is part of a continuum of types of interactions between species, rather than an exclusive category. In many cases, it is difficult to demonstrate that the host is harmed. In others, there may be no apparent specialization on the part of the parasite, or the interaction between the organisms may remain short-lived.

d) Amensalism: It is an interaction where an organism inflicts harm to another organism without any costs or benefits received by the other. A clear case of amensalism is where sheep or cattle trample grass. Whilst the presence of the grass causes negligible detrimental effects to the animal's hoof, the grass suffers from being crushed. Amensalism is often used to describe strongly asymmetrical competitive interactions, such as has been observed between the Spanish ibex and weevils of the genus Timarcha which feed upon the same type of shrub. Whilst the presence of the weevil has almost no influence on food availability, the presence of ibex has an enormous detrimental effect on weevil numbers, as they consume significant quantities of plant matter and incidentally ingest the weevils upon it.

Oppositional relationships 

i) Predation: Predation describes one species, the predator, feeding on and typically killing another organism, the prey species. Predators use various methods to capture prey, just as their prey use various methods to avoid capture. Herbivory is comparable to predation, but herbivores feed on plants rather than animals. Herbivores do not necessarily kill a plant they feed on but sometimes put pressure on the plant species.Predation

The best-known examples of predation involve carnivorous interactions, in which one animal consumes another. Think of wolves hunting moose, owls hunting mice, or shrews hunting worms and insects. Less obvious carnivorous interactions involve many  small individuals consuming a larger one. Such group predation is common among social carnivores such as lions, hyenas, and wolves. Group predation also occurs with ants and social spiders. This is, however, only part of the picture. Seed consumption can sometimes constitute predation. Seeds are considered organisms. Under ideal circumstances, seeds grow to become plants. However, consumption of a seed kills the plant before it can grow, making seed consumption an example of predation. 

ii) Competition: It describes multiple organisms fighting for the same resources. Interspecies competition is competition between different species; intraspecies competition is competition between organisms of the same species.The competition may or may not involve active interference. Squirrels and deer may both eat acorns in a site but do not directly fight for the acorns and instead make fewer acorns available for the other. Alternatively, competition may involve direct interference, like when a plant secretes chemicals from its roots to keep other plants from growing around it. The more similar two species in a community are, the more competitive they are with each other, fighting for limited resour  ces.

Intraspecies competition: Organisms competing can be from within the same species for example, two male elk fighting for a female mate. Elephants also fight each other so that the dominant elephant will get to breedIntraspecies competition with the female.

Another species that shows great competition between each other are the dolphins. Dolphins go along together and play with each other, but when it is time to eat; all dolphins have to compete for a meal.

Interspecies competition: Competition can be also found in two different species. A lizard and a frog can compete for a similar food they eat such as a small insect. This type of competition is only found when two different species share an ecological niche that they must compete over.

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