Population growth:
- The size of the population changes depending on food availability, predation pressure and reduce weather.
- Population size fluctuated due to changes in four basic processes, two of which (Natality and immigration) contribute an increase in population density and two (mortality and emigration) to a decrease.
- Natality: number of birth in given period in the population.
- Mortality: number of deaths in the population in a given period of time.
- Immigration: is the number of individuals of same species that have come into the habitat from elsewhere during a given period of time.
- Emigration: number of individuals of the population who left the habitat and gone elsewhere during a given time period.
- If ‘N’ is the population density at time ‘t’, then its density at time t + 1 is :
Where B = the number of births
I = the number of immigrants
D = the number of deaths
E = the number of Emigrants.
N = Population Density
r = Intrinsic rate of natural increase
t = Time period
K = Carrying capacity (The maximum population size that an environment can sustain)Exponential growth:
- The Exponential growth equation is Nt = N0ert
- Nt = Population density after time t
- N0 = Population density at time zero
- r = intrinsic rate of natural increase
- e = the base of natural logarithms (2.71828)
Exponential growth (‘J’ shape curve is obtained).
* When resources are not limiting the growth.
* Any species growth exponentially under unlimited resources conditions can reach enormous population densities in a short time.
* Growth is not so realistic.
Logistic growth model
- Verhulst-Pearl Logistic Growth is described by the following equations
- dN/dt = rN (K–N / N)
- Where N = Population density at time t
- r = Intrinsic rate of natural increase
- K = Carrying capacity
Logistic Growth (Sigmoid curve is obtained)
- When responses are limiting the Growth.
- Resources for growth for most animal populations are finite and become limiting.
- The logistic growth model is a more realistic one.
POPULATION INTERACTIONS:
Predation:
- Organism of higher trophic level (predator) feeds on organism of lower trophic level (prey) is called the predation.
- Even the herbivores are not very different from predator.
- Predator acts as a passage for transfer of energy across trophic level.
- Predators keep prey populations under control.
- Exotic species have no natural predator hence they grow very rapidly. (prickly pear cactus introduced in Australia created problem)
- Predators also help in maintaining species diversity in a community, by reducing the intensity of competition among competing prey species. (Pisaster starfish field experiment)
Defense developed by prey against predators:
Animals:
- Insects and frogs are cryptically coloured (camouflaged) to avoid being detected by the predator.
- Some are poisonous and therefore avoided by the predators.
- Monarch butterfly is highly distasteful to its predator (bird) due to presence of special chemical it its body. The chemical acquired by feeding a poisonous weed during caterpillar stage.
Plants:
- Thorns in Acacia, Cactus are morphological means of defense.
- Many plants produce and store some chemical which make the herbivore sick if eaten, inhibit feeding, digestion disrupt reproduction, even kill the predators.
- Calotropis produces poisonous cardiac glycosides against herbivores.
- Nicotine, caffeine, quinine, strychnine, opium etc. are produced by plant actually as defenses against the grazers and browsers.
CBSE Biology (Chapter Wise) Class XII ( By Mr. Hare Krushna Giri )
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