Study of populations, Biology tutorial


Organisms don't live solitary lives in isolation from other members of their species. Generally, they live in groups or if normally single, they interact with other members of their species at different times in their lives.

Concept of population:

A group of organisms or individuals of the similar species that live altogether in one geographical region at similar time is termed as population. The individuals might be evenly spaced or arbitrarily dispersed, clumped distribution pattern are the most often in nature. This is so because individuals of plants, animals or microorganisms tend to group in specific preferred local micro-habitats which are not uniformly distributed.

All populations encompass characteristic features such as size, density, dispersion and demography.

Population Attributes:

The attributes to be measured are size, density, growth and structure.

Population size:

Size is a significant feature of a population since it consists of a direct bearing on the capability of a given population to survive. Very small populations are the ones most probable to become extinct. Random events or natural disturbances are more probable to jeopardize a population when it comprises only a few individuals. In- breeding can as well be a negative factor in the survival of the small population. It can lower the vigor by direct genetic effects and can as well decrease the level of variability that is probable to detract from the population's capability to adjust to changing conditions. The whole species which comprises of only one or few small populations is probable to become extinct especially when it takes place in areas which have been or are undergoing radical modifications.

Population density:

We are familiar that how a population can evenly or randomly spread or might encompass a clumped distribution. Density is very significant in a population. When the individuals that make up a population are broadly spaced, they might rarely, if ever, encounter one another.

The reproductive capabilities and thus the future of the population might be much limited even if the absolute numbers of individuals over a broad area are relatively high. Population density is a measurement of the number of individuals per square kilometer of land area.

Population growth:

A significant feature of any population is its ability to grow. Most of the populations tend to remain a relatively constant size in spite of how many offspring are produced. Though under certain situations, a population size can increase quickly. The innate capacity of growth, that is, the biotic potentials for any population is exponential. This can be described by the curve. The rate of increase remains constant however actual raise in number of individuals accelerates fast as the size of the population grows. Real populations hardly ever exhibit the exponential growth rate for lots of generations. No matter how fast populations grow under such conditions, they eventually reach a little environmental limit imposed by shortages of a significant factor like light, space, water or nutrients. The population ultimately stabilizes at a certain size termed as the carrying capacity of the specific place where it lives.

Population Growth Rate:  

The Population growth rate (simply, PGR) is the fractional rate at which the number of individuals in a population rises.

It particularly signifies to the change in population over a unit period of time, often represented as a percentage of the number of individuals in the population at the starting of that period. This can be written by the formula as:     

Population growth rate (PGR) = 

(Population at end of period - Population at the starting of period) / Population at the starting of the period.

Age structure and population estimate:

Age structure is significant for the study of populations all along with knowing the size of the population. The number of individuals is simplest to estimate where the organisms are big or stationary like trees. For small and mobile organisms such as insects and small mammals, then, it will not be possible to count the population precisely.

Beneath such conditions, a process is employed to estimate the populations known as mark release and recapture method. At first, some individuals are captured from the population and marked in such a manner they can be identified again. The marked individuals are discharged back to the population and given time to mix by the unmarked ones. Then more are captured which hopefully will comprise some of the marked individuals. The proportion of marked to unmarked individuals captured this time around can be employed to compute the proportion of individuals in the whole population which were initially marked and therefore the population size. There is a supposition here that the marked individuals discharged after the first capture, mix in with the population after discharge and encompass the similar chance of being recaptured as any unmarked individual. 

Age of several organisms can be find out accurately similar to the annual growth rings in temperate trees, yearly growth rings in the teeth of a few animals, in fish scales, in shells of molluscs and in the otoliths (that is, small calcareous grain found in the inner ear employed for balancing) of mammals. It is hard to find out the exact age of organism studied either since they don't have a structure that records annual cycles or because study of such structure can merely be done by killing or harming the organism. The age of such individuals is generally estimated and only provides approximate ages.

Age structure and population size at each and every age group are important information to build up the survivorship curves or population pyramids.

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