19.2 POPULATION INTERACTIONS:

Biological interactions can be categorized into many different classes of interactions.

The interactions between two species vary greatly. They are

- One species eating the other (predation)

- Taking help (commensalism)

- Helping each other (mutualism)

- harming the other (parasitism).

All these interactions are derived through a common underlying need of survival.

To survive the organism has to compete with another or with the environment.

Competition:

Competition can be defined as an interaction between organisms or species, in which the fitness of one overpowers the presence of another.

Limited supply of resources is one of the primary reasons for competition. The organisms compete for resources such as food, water, and territory. They also may fight with other members of the same species for the mate. The survival of fittest or even the competitive exclusion principle states that the species less suited to compete for resources should either adapt or perish. The competition plays a critical role in natural selection.

Survival of Fittest: Charles Darwin's "survival of the fittest" states that in a competition, the organism having suitable variation survives as compared to the other. Gause's "Competitive exclusion principle" states that two closely related species competing for the same resources cannot coexist indefinitely and the competitively inferior one will be eliminated eventually.

The mechanism of competition occurs in various ways. The interference competition occurs directly between individuals, where one individual prevents the survival, reproduction of others, preventing their establishment in a portion of the habitat. The exploitation competition occurs indirectly where organisms compete for a common resource or for space. The apparent competition occurs indirectly between two species which have a common prey.

The competition between individuals of the same species is intra-specific competition. Intraspecific competition occurs when members of the same species fight for the same resources in an ecosystem. For eg. two trees, of the same species, growing close together will compete for light, water and nutrients.

The inter-specific competition may occur when individuals of two separate species share the same resource in the same area. Inter-specific competition has the potential to alter populations, communities and the evolution of interacting species. An example among animals could be the case of leopards and lions, since both species feed on similar prey.

Mutualism:

The term symbiosis (Greek: living together) can be used to describe this relationship, which is between organisms of different species. Mutualism is the way two organisms biologically interact where each individual derives benefit.

Mutualism and symbiosis are sometimes used as if they are synonymous, but this is incorrect. Symbiosis is a broad category, which includes relationships which are mutualistic, or commensals. Mutualism is one of its type.

Mutualism is a kind of positive interspecific interaction in which each member favours the growth and survival of each other and their associtation is obligatory.

A well known example of mutualism is lichen. It is the relationship between alga and fungus.

The other example is ruminants (ungulates such as cows) and bacteria within their intestines. The ungulates benefit from the cellulase produced by the bacteria, which facilitates digestion and the bacteria benefit from having a supply of nutrients.

Another example is mycorrhiza, which is relationship between plants and fungi. More than 48% of land plants rely on mycorrhizal relationships where fungi provide them with inorganic compounds.

Commensalism:

It is a class of relationship between two organisms where one organism benefits but the other is neutral (there is no harm or benefit). The word commensalism means "sharing of food", or "sharing a table".

- The orchid growing as epiphyte on tree is good example. In this relationship orchid gets support but the tree has no harm or benefit.

- The barnacles on the back of whale.

- Cattle egrets get to eat insects as cattles stir insects out of grass, but the cattle appears to have no benefit.

- In India, lone golden jackals (kol-bahl) , trail behind a particular tiger, at a safe distance in order to feed on the tiger's kills. Tigers have been known to tolerate these jackals.

- Sea anemone (Adamsia pallicata) is found attached to snail's shell is used as portable home by hermit crab (Euparagus prideauxi). Sea anemone is carried from place to place by the moving crab and is benefitted by getting more opportunities for the food.

Sucker fish (Echeneis) attaches to the under surface of shark with the help of its dorsal fin (acts as holdfast) and gets a free ride. It detached itself when the latter is feeding to obtain the fallen smaller pieces of the food.

The pilot fish (Remora) always accompanies shark without getting attached to it . It feeds on falling pieces of food.

Parasitism:

Parasitism is a type of symbiotic relationship between organisms of different species where one organism, the parasite, benefits at the expense of the other, the host. The parasites are generally much smaller than their host. They show a high degree of specialization for their mode of life, and reproduce at a faster rate than their hosts.

Classic examples of parasitism include interactions between vertebrate hosts and diverse animals such as tapeworms, flukes, the Plasmodium species, and fleas.

The Greek word parasites means "one who eats at the table of other"

In finer classification, parasitism is differentiated from the parasitoid relationship. The parasitoids generally kill but parasitism need not do so. Parasitoids are organisms whose larval development occurs inside or on the surface of another organism, resulting in the death of the host. This means that the interaction between the parasitoid and the host is fundamentally different from that of a true parasite and shares some of the characteristics of predation.

Types of Parasitism:

Parasites are classified on the basis of their interactions with hosts and on their life cycle.

Ectoparasite

Parasites that live on the surface of the host are called Ectoparasites.

Endoparasites

Parasites that live inside the host are called Endoparasites. Endoparasites can be of two forms: intercellular (inhabiting intercellular spaces) or intracellular (living within the cells in the host's body).

Some parasites, such as bacteria or viruses, depend on a third organism which is generally known as the carrier or vector. The vector does the job of transmitting them to the host.

- An example of this interaction is the transmission of malaria causing protozoan, Plasmodium by an anopheles mosquito.

- Another example is Sacculina, it is a sac like organism which lives a parasitic life on the abdomen of crab.

Hyperparasitism

It is a relationship in which one parasite feeds on another parasite which in turn feeds on its host.

A protozoan (the hyperparasite) living in the digestive tract of a flea living on a dog is example of this relationship.

Social parasites

It take advantage of interactions between members of social organisms such as ants or termites. In kleptoparasitism, parasites steal food gathered by the host.

- An example is the brood parasitism practiced by many species of cuckoo, which do not build nests of their own but rather deposit their eggs in nests of other species. The host behaves as a "babysitter" as they raise the young as their own.

Adelpho-parasite

An adelpho-parasite is a parasite in which the host species is closely related to the parasite, often being a member of the same family or genus.

- An example of this is the citrus blackfly parasitoid, Encarsia perplexa , unmated females of which may lay haploid eggs in the fully developed larvae of their own species. These result in the production of male offsprings. The marine worm Bonellia viridis has a similar reproductive strategy.

Predation:

The energy fixed by autotrophs flows through the food web. The energy flow from one animal to another is due to predation. The carnivore animals eat each other and form various levels of food pyramid. The population of prey and predator species controls each other in the balanced ecosystem.

The herbivore and carnivore population adapt themselves for prey predator relationship. The reproductive cycles of both are also adapted for the same purpose. The herbivores reproduce when the plants are available in plenty and carnivores do so when the herbivore population goes up. The availability of grass, growth in population of deers and then the increase in number of lions and tigers is example of the same. If the predator population is too efficient then the prey might become extinct and then predators will follow.

To survive the prey population has evolved various defences. Some species of frogs are cryptically coloured to take advantage of camouflage.

The monarch butterfly is not preyed upon by birds due to its bad taste resulting from poisonous secretion. The poisonous secretion develops in the larval stage when it feed upon poisonous weeds.

The predation very often refers to the relation between herbivores and carnivores. The relationship between plants and herbivores also fits in the category of predation in the broad ecological sense.