Population ecology is an important area of ecology because it links ecology to population genetics and evolution. A population has certain attributes that an individual organism does not.
An individual may have births and deaths, but a population has birth rates and death rates. In a population these rates refer to per capita births and deaths, respectively. The rates, hence, expressed is change in numbers (increase or decrease) with respect to members of the population.
The characteristics of any population depends on: (i) density of the population, (ii) natality (birth rate), (iii) mortality (death rate), (iv) dispersal, (v) biotic potential (vi) age distribution (vii) dispersion and (viii) growth form.
Characteristics of Population
Populations have a number of attributes. Different populations can be compared by measuring these attributes. A population has following group characteristics:
The size of population is represented by its density. Density is expressed as the total number of individuals present per unit area or volume at given time. Density varies from time to time and from one area to another. For example, greater density of plants during rainy season as compared to dry season.
The increase in the number of individuals in a population under given environmental conditions is called natality. Birth, hatching, germination, vegetative propagation, etc are account for natality. When the increase in individuals is expressed per unit time, it is called natality rate.
The loss of individuals due to death in a population under given environmental conditions is termed mortality. When the number of individual dying is expressed per unit time, it is called mortality rate.
The majority of organisms dispersed at one time or the other during their life cycles. The individuals move into (immigration) and move out of the population (emigration), and these movements affect the size of the population.
5. Age Distribution
Various age groups in a population determine its reproductive status. The three ages referred as ecological ages in a population are:
Distribution of age groups influences the population growth. Populations with more young members grow rapidly, while the declining populations have a large proportion of older individuals. The age structure in a population is also linked to births and deaths.
6. Sex ratio
Sex ratio is an important aspect of population. It refers to the ratio between female and male individuals in a population.
POPULATION GROWTH MODELS
The growth models are classified as:
(i) Exponential growth: Resource (food and space) availability is obviously essential for the unimpeded growth of a population. Ideally, when resources in the habitat are unlimited, each species has the ability to realise fully its innate potential to grow in number, as Darwin observed while developing his theory of natural selection. Then the population grows in an exponential or geometric fashion.
(ii) Logistic growth: No population of any species in nature has at its disposal unlimited resources to permit exponential growth. This leads to competition between individuals for limited resources. Eventually, the ‘fittest’ individual will survive and reproduce. The governments of many countries have also realised this fact and introduced various restraints with a view to limit human population growth. In nature, a given habitat has enough resources to support a maximum possible number, beyond which no further growth is possible. Let us call this limit as nature’s carrying capacity (K) for that species in that habitat. A population growing in a habitat with limited resources show initially a lag phase, followed by phases of acceleration and deceleration and finally an asymptote, when the population density reaches the carrying capacity.
1. Global Tiger Census 2016
• The number of wild tigers has gone up globally by 22 per cent to 3,890, from the earlier 2010 estimate of 3200, according to the World Wildlife Fund and the Global Tiger Forum (GTF).
• The updated minimum figure, compiled from International Union for Conservation of Nature and Natural Resources (IUCN) data and the latest national tiger surveys, can be attributed to multiple factors including increases in tiger populations in India, Russia, Nepal and Bhutan, improved surveys and enhanced protection.
• Despite countries such as India, Nepal, Russia and Bhutan registering a spike in tiger population, the status of the animal remains “endangered”. According to the WWF, hundred years ago there were 100,000 wild tigers. By 2010, there were as few as 3,200. Therefore, in 2010, tiger range governments agreed to act to double wild tigers by the next Chinese Year of the Tiger in 2022. This goal is known as Tx2.
• Trade in tiger products is banned in most of the world, and yet a black market persists. For tigers to assuredly get on more stable ground, demand for their skins and other parts must be curtailed in Asia, particularly China.
• Conservationists have worked with leaders of Traditional Chinese Medicine to reduce the amount of tiger products going into treatments, which scientists say don’t work anyway. At the same time, however, new black markets have emerged in China for shampoos, tonics, and tchotchkes made of tigers, often as status symbols.
• Methodology of Tiger Census
a) Pugmark technique: It has been one of the most popular ways of counting tigers. Each tiger is known to leave a distinct pugmark on the ground and these are different from the others in the big cat family. Photographs or plaster casts of these pugmarks are
b) Installation of Cameras: Cameras could be left in dense forests for several days to capture images of individual tigers. But it is not possible to install cameras at every place that is likely to have tigers, and even in places where they are installed, there is no certainty that the tiger would walk into a camera’s range.
c) Double-sampling Method: This is the new method adopted by Wildlife Institute of India in Tiger Census 2015. The first stage involved ground survey by the forest department. Under this forest department staff collect evidences of tiger presence such as pugmarks, scat, scratches on trees or other such unmistakable signs of tiger presence.
The next stage involved camera trapping. Based on the ground surveys locations were chosen for installing cameras. These cameras are heat and motion sensitive. They lie idle till they detect any motion or a sudden change in temperature which means, they capture just about anything that moves – other animals, even birds. All these get captured by the camera.
Each tiger is known to have a very unique stripe pattern. This is used to differentiate one tiger from the other.
2. Leopard Census
The Census has put the spotted cat population at 7,910 in and around tiger habitats across the country, except the northeast.
The leopards were counted using the methods as getting pictures of animals through camera-trapping and gathering other evidence of their presence, and then extrapolating the numbers to cover the entire forest landscape.
The exercise covered 3,50,000 sq km of forested habitat across the Shivalik hills and Gangetic plains, central India and the Western Ghats landscape.
The study found the species well distributed across the country, indicating that India’s leopard population is “quite healthy”.
With an estimated population of 1,817, Madhya Pradesh has emerged as the top leopard state in the country. It’s followed by Karnataka (1,129), Maharashtra (905), Chhattisgarh (846) and Tamil Nadu (815).
Issues with the census:
• The census did not cover the higher Himalayas.
• The census also did not cover Gujarat, parts of Rajasthan and east India, and the entire northeast.
• Non-tiger states – such as Himachal, Jammu and Kashmir, Haryana or Punjab – were not covered.
3. Lion Census report, 2015
• According to the Lion Census Report, the population of Asiatic lions has been found to have increased considerably in the Gir wildlife sanctuary – from 411 during the last census in 2010 to 523 in 2015.
• The enumerators found 109 adult males, 201 adult females, and 213 cubs and sub-adults distributed across four districts – Junagadh, Gir Somnath, Amreli, and Bhavanagar.
• The Lions are a subspecies of lion which are slightly smaller than their African cousins and have a fold of skin along their bellies. They are a major attraction for tourists to Gujarat.
• The lions face the usual threats of poaching and habitat degradation. Three major roads and a railway track pass through the Gir Protected Area. Also, there are three big temples inside the Protected Area that attract large number of pilgrims, particularly during certain times of the year. This is leading to the devastation of biodiversity in the region.
• On the other hand, the increased population of lions has resulted in their spill over the area. Therefore, at present, the most pressing threat to the lion population of the Gir forest comes from the increasing hostility toward them from the resident human population. Due to the increase in population, about 100 lions stay outside the area and face conflicts with humans.
• Therefore the forest Department has formed a task force under the chairmanship of S C Pant, principal chief conservator of forests (PCCF) and chief wildlife warden of Gujarat to study the phenomenon and form a strategy for conservation of increased population and lion landscape.
• The task force has been asked to study and analyse results of the 2015 lion census and prepare a report based on which a strategy can be formed for future conservation of increased population of endangered lions and management of expanded territory of the species.