In the study of populations of organisms different patterns are used to comprehend their characteristic features. The analysis of acquired data over time gives opportunities for understanding and interpreting a few of the characteristic features.
Presentation of demographic data:
The numerical data collected throughout a population study can be presented in different ways for the purpose of clarity. Some of the ways of examining the data will be considered as follows.
Population Life Tables:
Numerical data collected throughout a population study and presented as a table of figures is termed as a Life Table. Life tables generally represent data for a cohort however can as well be produced by using age structure data. A cohort is a group of organisms from one population that are roughly of the similar age. A life table data for a cohort follows one group of individuals from the starting of their life throughout to their death. The number alive at a given time is shown to the death of the last individual. Cohort studies are thus not appropriate for very long-lived species as they would take too long to complete. The mixture of age structure and cohort is best for longevity species.
The other way of representing population data is as population pyramids. These are frequently employed in human demography as they encompass the benefit of showing females and males separately on the similar graph. The pyramid is really two bar graphs back to back having males conventionally represented to the left and females to the right of a central vertical age axis. The population pyramid exhibits the composition of a population by age and sex. For humans, where population pyramids generally represent numbers for a whole country, the shape of the pyramid reflects death and birth rates. Bottom-heavy pyramids exhibit increasing annual birth rates or high birth rates and high infant mortality.
Population Survivorship Curves:
One of the methods to express the characteristic of populations by means of respect to age distribution is the survivorship curve. This is a graph exhibiting the number of individuals which survive per thousand of population via each and every phase of life. Even where the population is less than a thousand, it is standardized to a thousand in such a way that life tables can simply be compared. Moreover using numbers, survivorship curves can as well be presented by employing a semi-log plot.
The general forms of survivorship curves can point out what evolutionary strategy a species consists and how population numbers are maintained. The individuals in populations having type 1 curves generally encompass some offspring. These offspring are well cared for so that their chance of endurance is high. Such strategies take place in birds and large animals, comprising humans. Species with type III curves generally encompass large numbers of offspring, most of which die before they reach maturity. Most of the fungi, plants, fish, amphibian and invertebrates encompass this strategy.
r- and k - strategies:
No matter how fast populations grow under particular circumstances, they finally reach some environmental limit imposed by shortage of a significant factor like light, space, water or nutrients. A population finally stabilizes at a certain size termed as the carrying capacity of the specific place where it lives. The growth curve of a specific population that is always limited by one or more factors in the environment can be estimated by the given equation.
Growth rate (R) = dN/dt = r N (K - N/K)
In another words, the growth rate of the population under consideration (dN) equivalents its rate of increase (r) multiplied by N the number of individuals present at any one time and then multiplied by the expression equivalent to K, the carrying capacity of the environment, minus N divided by K. As N rises (the population grows in size), the fraction by which r is multiplied becomes lesser and lesser, and the rate of increase of the population refuses. It follows thus that as N approaches K, the rate of population growth dN/dt starts to slow till it reaches O, when N = K.
Graphically the relationship provides an S shaped curve termed as the Sigmoid growth curve. The curve is termed as sigmoid as it resembles the Greek letter sigma (s). As the size of population stabilizes, its rate of growth slows down and finally it doesn't increase further.
r- and k- strategies are the other way of categorizing evolutionary strategies proposed by MacArthur and Wilson (1967). The initials r- and k- come from the logistic equation for explaining the actual rate of growth of population R or dN/dt. From the equation it can be observed that an r- selected population is one in which the maximum rate of increase (r) is significant. An r- chosen population can take benefit of a favorable situation by having the capability to increase the population size rapidly. This means having lots of offspring which beneath normal circumstances die before reaching maturity however which might survive when circumstances changed.
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