Organism and Population Flashcards
what is ecology and its four levels
Ecology is a subject
which studies the interactions among organisms and between the
organism and its physical (abiotic) environment.
Ecology is basically concerned with four levels of biological organisation – organisms, populations, communities and biomes.
what is meant by a population
live in groups in a well defined geographical area, share or compete
for similar resources, potentially interbreed and thus constitute a
population.
All the cormorants in a wetland, rats in an abandoned dwelling, teakwood
trees in a forest tract, bacteria in a culture plate and lotus plants in a
pond, are some examples of a population.
why is popoulation ecology important
an individual organism is the one that has to cope with a changed environment, it is at the population level that natural
selection operates to evolve the desired traits.
Population ecology is,
therefore, an important area because it links ecology to population genetics
and evolution.
Birth Rates / Biotic Potential / Natility-
Birth rate is defined as total no. of birth in a population with respect to total
no. of individuals of population in a year.
* Birth rate is represented as per capita birth rate.
per capita birth rate= new indivisuals/ old indivisuals = x offspring per lotus per year
Death Rate / Mortality-
- Death rate is defined as total number of death in a population with respect to
total number of individual of the population in a year. - Death rate is represented as per capita death rate.
Per Capita death rate (d) =𝐓𝐨𝐭𝐚𝐥 𝐧𝐮𝐦𝐛𝐞𝐫 𝐨𝐟 𝐝𝐞𝐚𝐭𝐡/𝐈𝐧𝐢𝐭𝐢𝐚𝐥 𝐩𝐨𝐩𝐮𝐥𝐚𝐭𝐢𝐨𝐧
sex ratio
Another attribute characteristic of a population is sex ratio. An
individual is either a male or a female but a population has a sex ratio
(e.g., 60 per cent of the population are females and 40 per cent males).
what are age pyramids
If the age distribution (per cent individuals of a given
age or age group) is plotted for the population, the resulting structure
is called an age pyramid (Figure 11.1). For human population, the
age pyramids generally show age distribution of males and females in
a diagram. The shape of the pyramids reflects the growth status of
the population - (a) whether it is growing, (b) stable or (c) declining.
absolute population density/size
- The size of the population tells us a lot about its status in the habitat.
-Whatever ecological processes we wish to investigate in a population, be it the outcome of competition with another species, the impact of a
predator or the effect of a pesticide application, we always evaluate them in terms of any change in the population size.
-The size, in nature, could be as low as <10 (Siberian cranes at Bharatpur wetlands in any year) or go into millions (Chlamydomonas in a pond).
-Population size, technically
called population density (designated as N), need not necessarily be measured in numbers only. Although total number is generally the most
appropriate measure of population density, it is in some cases either meaningless or difficult to determine.
is measuring population size in numbers the best option?
Although total number is generally the most
appropriate measure of population density, it is in some cases either
meaningless or difficult to determine. In an area, if there are 200 carrot
grass (Parthenium hysterophorus) plants but only a single huge banyan
tree with a large canopy, stating that the population density of banyan is
low relative to that of carrot grass amounts to underestimating the
enormous role of the Banyan in that community. In such cases, the per
cent cover or biomass is a more meaningful measure of the population
size.
relative population density
Total number is again not an easily adoptable measure if the
population is huge and counting is impossible or very time-consuming.
Sometimes, for certain ecological
investigations, there is no need to know the 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 total population
density in the lake. We are mostly obliged to estimate population sizes
indirectly, without actually counting them or seeing them. The tiger census
in our national parks and tiger reserves is often based on pug marks and
fecal pellets.
population growth
The size of a population for any species is not a static parameter. It keeps
changing with time, depending on various factors including food
availability, predation pressure and adverse weather.
The change in population size gives us an understanding of the situation of the population- wheter is declining or flourishing.
Pop density fluctuates due to 4 processes: two of which (natality and immigration) contribute to an increase in
population density and two (mortality and emigration) to a decrease.
4 processes which affect population density
(i) Natality refers to the number of births during a given period in the population that are added to the initial density.
(ii) Mortality is the number of deaths in the population during a given
period.
(iii) Immigration is the number of individuals of the same species that have come into the habitat from elsewhere during the time period
under consideration.
(iv) Emigration is the number of individuals of the population who
left the habitat and gone elsewhere during the time period under
consideration.
formula
So, if N is the population density at time t, then its density at time t +1 is
Nt+1 = Nt+ [(B + I) – (D + E)]
relative contribution of different factors
Under normal conditions, births and deaths are the most
important factors influencing population density, the other two factors
assuming importance only under special conditions. For instance, if a
new habitat is just being colonised, immigration may contribute more
significantly to population growth than birth rates.
