Ecological Interdependence of Organisms

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).

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)

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.

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.

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).

19.6.3 Competition: Competition occurs when two or more individuals attempt to use essential common resource such as food, water, shelter, living space of sunlight. Because resources are often in limited supply in the environment, their use by one or more individuals or populations decreases the amount available to others.

Competition can occur among individuals within a population, the intraspecific competition, or between different species, the interspecific competition.

Intraspecific competition may result in:

1. Reduced growth and reproductive rates for some individuals,
2. Exclude some individuals from better environments,
3. Cause the death of some individuals due to non-availability of resources.

Interspecific competition affects individuals in the some way, but in addition, an entire species amy be excluded from habitats where it cannot compete successfully. In extreme cases, a competitor may cause extinction of another species.

Mutualism and Commensalism

A relationship where two species live together closely is called symbiosis and the type of symbiosis that benefits one species and neither helps nor harms the other is called commensalism (Fig. 19.22). Lichens and other epiphytes growing on trees are examples of commensalism. Similarly, barnacles attached to the backs of whales and turtles get a free ride to better feeding places.

Fig. 19.22 Commensalism

Fig. 19.23 Mutualism

A symbiotic relationship in which both species benefit is called mutualism. An example is the relationship between flowers and the insects that pollinate them. The flower provides the insect with food in the form of nectar, and in return, the insect fertilizes the flower. Zooflagellates living in the guts of termites is another example of mutualism. The zooflagellates secrete enzymes to digest cellulose, which termite cannot do. The zooflagellates in return absorb prepared food from the gut. Mycorrhizae, lichens and nitrogen-fixing bacteria in the root nodules of leguminous plants (Fig. 19.23) are other examples of mutualism.

Key points

1. An environment is an interplay of all living (biotic) and non-living (biotic) factors. Organisms and are affected by the changes in the environment, and in turn bring changes in the environment.
2. Interactions among organisms and between organisms and their physical environment of an organized ecological system or ecosystem. All ecosystems are self-sustaining and self-regulatory. So, ecosystems are naturally balanced and possess equilibrium.
3. Changes in one part of the ecosystem lead to changes in some other part (s) of the ecosystem. If disruptions are not catastrophic, they are overcome and the ecosystem returns to its original state. An ecosystem may not be able to bear and absorb large disruption. New ecosystems will replace the old ones.
4. Materials and energy flow through the ecosystem as expressed by food chains, food webs and natural cycles. Flow of material in an ecosystem is cyclic; very few materials enter or leave the ecosystem. One the other hand flow of energy is in one direction only at the producers level.
5. The relative amount of energy in an ecosystem is represented by ecological pyraminds. Less energy reaches a particular trophic level than the level below it. Some energy is lost in the form of heat at each trophic level.
6. Ecological interdependence of organisms results in predator-prey, host-parasite, competitive, and symbiotic relationships.
7. Unpleasant changes in the biosphere are occurring at an unprecedented and faster rate than ever before due to human activity: leading to environmental degration, pollution and species extinction.