All living organisms are interdependent, otherwise it would be difficult to live together in a population.
Population is a group of similar individuals living in a particular geographical area. Populations of different species of organisms live in the same ecosystem. When organisms encounter one another in their habitats, they can influence each other in a number of ways. Some interactions are harmful to one or both of the organisms. Others are beneficial.
Such relationships can be characterised into different types depending on the interaction and the extent to which they associate.
1. Mutualism: Mutualism in an interaction between individuals belonging to two different species, that benefit both members. Lichen is a complete entity formed by the association of an alga and a fungus. The main body of the lichen is formed by fungus. The alga manufactures food for itself as well as for the fungus, while the fungus provides water, minerals and shelter to the alga.
2. Commensalism: Commensalism is an interspecific interaction where one species benefits and the other is unaffected (neither harmed nor benefitted). Commensal relationships may involve one organism using another for transportation or for housing. For example, hermit crab lives in gastropod shell to protect their body. Sucker fish attaches itself to the under surface of shark and gets a free ride. It is thus protected from its predators and is also widely dispersed in this way.
3. Parasitism: It includes one organism living in or on the body of another living organism from which it derives nourishment and in the process harms its host. For example: tapeworm living in the intestine of man.
4. Symbiosis: A close interaction between two or more different organisms of different species living in close physical association. For example, pollination of flowers where the flowering plants are cross pollinated by the bees which benefit by getting nectar from the plants. Plants pollinated in this manner produce less pollen than do plants that rely on the wind to transfer pollen.
Population is an aggregate of individuals belonging to the same species. Population of any species does not remain static. It undergoes changes.
Population growth is the change in the number of individuals of any species in a population at a given time. The size of the population depends upon the density, natality (birth rate), mortality (death rate), population dispersal, age distribution, and environmental resistance that the population has to face.
The provisions for life in a geographical area where a population lives is limited. Only certain number of organisms can live comfortably in the area. When this number gets exceeded, it is termed as population growth.
Growth rate of a population is the difference between the birth rate and the death rate. When the birth rate is more than the death rate, then the population density increases.
Birth rate or natality: It is defined as the number of live births per thousand per unit time.
Mortality rate: The mortality rate of a population is the number of individuals dying per thousand per unit time.
It is the movement of individuals or groups of living organisms by which they expand the space or range within which they live. Dispersal operates when organisms leave the space that they have previously occupied, or in which they were born and settle in new areas. It affects the size of the population.
Population dispersal can be of two types:
1. Emigration: It is the permanent outward movement of the organisms from a given population. It decreases the size of the local population.
2. Immigration: It is the permanent inward movement of the organisms from outside into a given population. It increases the size of the local population.
It is the resistance presented by the environmental conditions to prevent the species from reproducing at maximum rate and thus limiting a species from growing out of control. Environmental resistance includes both abiotic factors like temperature, space and biotic factors like natural enemies. Environment keeps a check on the rise in the population size.
The physical and biological factors that together prevent a species from reproducing at its maximum rate is called environmental resistance.
Carrying Capacity: It is the maximum population that the environment can sustain indefinitely.
The growth of a population can be expressed in the form of a mathematical expression called the growth curve. If the number of organisms is plotted against time, you get a curve which is called the population growth curve. Population growth curve has a characteristic shape. There are two forms of growth curves - J-shaped growth curve and S-shaped or sigmoidal growth curve.
S-shaped Growth Curve
When a small number of organisms first enter a previously unoccupied area, the growth is slow at first as it adapts to new conditions and establishes itself. Reproduction in these organisms takes place after a certain period of time. This is called the lag phase. During this phase, both natality and mortality remain small and relatively constant.
Gradually, the growth becomes rapid and the population increases rapidly. Now, the natality rate increases while the mortality remains low. This is called growth phase. The rapid rise in population is because of the availability of plenty of food and also because there is no competition between the biotic potential and the natural resources.
But the number of organisms cannot continue to increase at a faster and faster rate because eventually something in the environment will become limiting and cause an increase in the number of deaths. For animals, food, water or resting sites may be in short supply, or predators or disease may kill many individuals. Plants may lack water, soil nutrients or sunlight.
Eventually, the number of individuals entering the population will become equal to the number of individuals leaving it by death or migration and the population size becomes stable. This part of the population growth curve is called stable phase where the natality rate and mortality rate are approximately equal. The graph so obtained is S-shaped and is called the sigmoid curve.
J-shaped Growth Curve
The J-shaped growth curve describes a situation in which the population growth continues in an exponential form until the environmental resistance becomes effective. As the environmental resistance becomes effective, there is a stiff competition for survival and the growth rate stops abruptly. There is a sudden increase in mortality (population crash).