Biology POPULATION ATTRIBUTES

### KEY TOPICS

star Population
star Birth Rate and Death Rate
star Sex Ratio and Age Groups
star Population Size

### POPULATIONS

● In nature, we rarely find color{violet}("isolated, single individuals") of any species; color{violet}("majority ")of them live in groups in a well defined color{violet}("geographical area, share or compete") for similar color{violet}("resources, potentially interbreed") and thus color{violet}("constitute a population.")

● Although the term color{violet}("interbreeding implies sexual reproduction"), a group of individuals resulting from even color{violet}("asexual reproduction") is also generally considered a color{violet}("population") for the purpose of color{violet}("ecological studies.")

● All the cormorants in a wetland, rats in an color{violet}("abandoned dwelling, teakwood trees") in a forest tract, bacteria in a color{violet}("culture plate") and color{violet}("lotus plants") in a color{violet}("pond,") are some examples of a color{violet}("population.")

● Although an color{violet}("individual organism") is the one that has to cope with a color{violet}("changed environment"), it is at the population level that natural selection operates to evolve the color{violet}("desired traits").

● color{violet}("Population ecology") is, therefore, an important area of color{violet}("ecology") because it links ecology to color{violet}("population genetics") and color{violet}("evolution.")

### BIRTH RATE AND DEATH RATE

● A color{violet}("population") has certain attributes that an individual color{violet}("organism") does not.

● An individual may have color{violet}("births") and color{violet}("deaths"), but a population has color{violet}("birth rates") and color{violet}("death rates.")

● In a color{violet}("population") these rates refer to per color{violet}("capita births") and color{violet}("deaths,") respectively.

● The rates, hence, are expressed is change in numbers color{violet}("(increase or decrease)") with respect to members of the population.

● Here is an example:

● If in a pond there are color{violet}("20 lotus plants last year") and through color{violet}("reproduction 8 new plants") are added, taking the color{violet}("current population to 28"), we calculate the color{violet}("birth rate as 8/20 = 0.4") offspring per lotus per year.

● If 4 individuals in a color{violet}("laboratory population") of color{violet}("40 fruitflies died") during a specified time interval, say a week, the color{violet}("death rate") in the color{violet}("population") during that period is color{violet}("4/40 = 0.1") individuals per fruitfly per week.

### SEX RATIO AND AGE GROUPS

● Another color{violet}("attribute characteristic") of a population is color{violet}("sex ratio").

● An individual is either a male or a female but a color{violet}("population") has a color{violet}("sex ratio") (e.g., 60 % of the population are females and 40 % males).

● A color{violet}("population") at any given time is composed of individuals of color{violet}("different ages").

● If the color{violet}("age distribution") ( % individuals of a given age or age group) is plotted for the population, the resulting structure is called an color{violet}("age pyramid").

● For human population, the color{violet}("age pyramids") generally show color{violet}("age distribution") of males and females in a color{violet}("combined diagram").

● The color{violet}("shape ")of the color{violet}("pyramids") reflects the growth status of the population –

(a) color{violet}(" it is growing")

(b) color{violet}("stable")

(c) color{violet}("declining.")

### POPULATION SIZE

● The size of the population tells us a lot about its status in the habitat.

● Whatever color{violet}("ecological processes") we wish to investigate in a color{violet}("population,") be it the outcome of competition with another species, the impact of a color{violet}("predator ")or the effect of a color{violet}("pesticide application,") we always evaluate them in terms of any change in the color{violet}("population size.")

● The size, in nature, could be as low as <10 (color{violet}("Siberian cranes at Bharatpur wetlands in any year")) or go into millions (color{violet}("Chlamydomonas in a pond")).

● Population size, more technically called color{violet}("population density (designated as N)"), need not necessarily be measured in numbers only.

● Although total number is generally the most appropriate measure of color{violet}("population density"), it is in some cases either color{violet}("meaningless or difficult") to determine.

● In an area, if there are color{violet}("200 Parthenium plants") but only a single huge banyan tree with a large canopy, stating that the color{violet}("population density") of banyan is low relative to that of color{violet}("Parthenium") amounts to underestimating the enormous role of the Banyan in that community.

● In such cases, the percent cover or color{violet}("biomass") is a more meaningful measure of the color{violet}("population size.")

● Total number is again not an easily adoptable measure if the color{violet}("population") is huge and counting is impossible or very time-consuming.

● Sometimes, for certain color{violet}("ecological investigations"0, there is no need to know the color{violet}("absolute population densities"); relative densities serve the purpose equally well.

● For instance, the number of fish caught per trap is good enough measure of its color{violet}("total population density") in the lake.

● We are mostly obliged to estimate color{violet}("population sizes indirectly"), without actually counting them or seeing them.

● The color{violet}("tiger census") in our national parks and tiger reserves is often based oncolor{violet}(" pug marks") and color{violet}("fecal pellets.")