The group of members of a species in a well defined geographical area is called population.
- Birth Rate: Number of live births per thousand members is generally taken as birth rate.
- Death Rate: Number of deaths per thousand members is generally taken as death rate.
- Sex Ratio: The ratio of females to males in a population is called sex ratio.
When the age distribution of population is plotted, the resulting structure on graph is called age pyramid.
- Growing: When the population of pre-reproductive age group is highest, followed by reproductive and post-reproductive age group in that order, the age pyramid represents a growing population.
- Stable: When the population of pre-reproductive age group is almost same as that of reproductive age group; followed by population of post-reproductive age group, the age pyramid represents a stable population.
- Declining: When the population of pre-reproductive age group is lower than that of reproductive age group, the age pyramid represents a declining population.
Measurement of Population Size: Population size is generally measured in terms of population density, i.e. number per unit geographical area. But it is difficult to determine the absolute numbers in some cases. For example; there can be a solitary banyan tree in the midst of a huge lawn full of grass. Per cent cover or biomass is a more meaningful measure of the population size in this case. In some cases, indirect measurement is more practical way of assessing the population size, e.g. population of bacteria in a petri dish, or population of tigers.
Following factors affect the population growth:
- Natality: The number of births during a given period in the population is called natality.
- Mortality: The number of deaths in the population during a given period is called mortality.
- Immigration: The number of individuals of the same species that have come into the habitat from elsewhere during the given time period is called immigration.
- Emigration: The number of individuals of the population who left the habitat and gone elsewhere during the given time period is called emigration.
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)]
This equation shows that population density will increase if the number of births plus the number of immigrants (B + I) is more than the number of deaths plus the number of emigrants (D + E), otherwise it will decrease.
Ideally, when resources in the habitat are unlimited, each species has the ability to realize fully its innate potential to grow in number. In this case, the population grows in an exponential or geometric fashion. If in a population of size N, the birth rates are represented as b and death rates as d, then the change in N during a unit time period t (dN/dt) will be
dN/dt = (b – d) × N
Let (b–d) = r, then
dN/dt = rN
The r in this equation is called the ‘intrinsic rate of natural increase’ and is a very important parameter chosen for assessing impacts of any biotic or abiotic factor on population growth.
Resources are limited and there is competition for resources. So, exponential growth is not possible in practical situations. A given habitat has enough resources to support a maximum possible number. No further growth is possible beyond that number. This number is called the carrying capacity (K) of nature, for a particular species.
In a habitat with limited resources, the population growth initially shows a lag phase. This is followed by phases of acceleration and deceleration, and finally an asymptote.
A plot of N in relation to time (t) results in a sigmoid curve. This type of population growth is called Verhulst-Pearl Logistic Growth and is described by the following equation:
N = Population density at time t
r = Intrinsic rate of natural increase
K = Carrying capacity
The logistic growth model is considered a more realistic growth model.
Life History Variation
Populations evolve to maximize their reproductive fitness in the habitat in which they live. This is also called Darwinian fitness. Under a particular set of selection pressures, organisms evolve towards the most efficient reproductive strategy. Some organisms breed only once in their lifetime, e.g. Pacific salmon fish, bamboo, etc. Some organisms breed many times during their lifetime. Some organisms produce a large number of small-sized offspring, while some others produce a small number of large-sized offspring. The life history traits of organisms have evolved in relation to the constraints imposed by abiotic and biotic components of the habitat.
Interactions of populations of two different species are called interspecific interactions. Following are the various interspecific interactions:
Predaton is nature’s way of transferring the energy (fixed by plants) to higher trophic levels. Apart from acting as conduits for energy transfer across trophic levels, predators play other important roles as well. Predators keep the population of prey under control. This helps ion preventing ecosystem instability. Predators also help in maintaining species diversity in a community. Prey species have evolved various defense mechanisms to lessen the impact of predation. Camouflage, thorns, poisoned armory, etc. are examples of such defenses.
Competition for resources can be between closely related species, or between entirely unrelated species. Competition is best defined as a process in which the fitness of one species (measured in terms of r) is significantly lower in the presence of another species. Recent studies suggest that species facing competition might evolve mechanisms that promote co-existence rather than exclusion. One such mechanism is ‘resource partitioning. Species may choose different times for feeding or different foraging patterns.
Many parasites have evolved to be host-specific in such a way that both host and the parasite tend to co-evolve. If the host evolves special mechanism for rejecting or resisting the parasite, the parasite also evolves special adaptations to counter that mechanism.
This is the interaction in which one species benefits and the other is neither harmed nor benefited. An orchid growing as an epiphyte on a mango branch, and barnacles growing on the back of a whale benefit while neither the mango tree nor the whale derives any apparent benefit. The cattle egret and grazing cattle in close association, is a classic example of commensalism. When the cattle move, they stir up and flush out the insects from vegetation. Such insects would otherwise be difficult for egrets to find and catch.
This interaction confers benefits on both the interacting species. Lichens are classic examples of mutualism. Micorrhizae too are examples of mutualism. The relationship between plants and animals show fascinating examples of mutualism. Plants need the help of animals for cross pollination. In lieu of that, an animal gets nectar or fruit as food.