Populations are groups of individuals of the same species that occupy a given area at the same time. Each population has •unique attributes (characteristics). Two of the most important attributes are population growth and population regulation.
The animal population shows birth, death, and dispersal. Therefore, Animal populations change over time. The death. of individuals is characterized by survivorship curves. The numbers of survivors are plotted on the Y-axis of a survivorship graph. Age is plotted on the X-axis. There are three kinds of survivorship curves:
- Survivorship curve types I: It gives convex curve. Individuals-in type I populations survive to an old age. Then they die rapidly. Environmental
factors are not important. They do not influence mortality.Thus most
individuals live their potential lifespan. Some human populations approach type I survivorship.
- Survivorship curve. types II: It gives diagonal line. Individuals in type II populations have a constant chance of death throughout their lives. The environment has an important influence on death. It has harsh effect on the young than on the old. Populations of birds and rodents have type II survivorship curves.
- Survivorship curve types Ill: It gives concave curve. Individuals in type III population show very high juvenile mortality. There is much lower mortality rate in adulthood. Fishes and many invertebrates show type Ill survivorship curves.
Percentage of maximum fife span
Fig: Survivorship curves
Types of population growth
There are two type of growth: Exponential growth and Logistic growth.
- Exponential population growth
Th increase of population by the same ratio per unit time is called exponential growth. Different populations have different potential to increase the numbers. Not all populations display the same capacity for growth. Many factors influence the reproductive potential. These factors are:
- umber of offspring produced
- he likelihood of survival to reproductive age
- he duration of the reproductive period
- he length of time it takes to reach maturity,
- Logistic population growth
The growth in which population reaches a carrying capacity and does inc :ase further is called logistic population growth. The population size that a particular environment can support is the environment is called caring capacity. It is symbolized by K. Exponential growth cannot occur indefinitely. There are many .environmental resistances. These resistances are climate, food, space. and other environmental. These resistances check the population growth rate. The population reaches the carrying capacity. The growth curves become a sigmoid or flattened S shaped. It is a logistic population growth.
POPULATION REGULATION •
Every species have different conditions for survival But population density and competition affect are common in all the species.
Fig: Logistic population growth
The number of individuals per unit space is called population density. There are two–types of factors:
1. Density independent factors: The factors which are not influenced by density of population are called density independent factors. They influence the number of animals in a population. The example of these factors is weather conditions. A cold winter with little snow cover can destroy a population of lizards. These lizards live beneath the litter of the forest floor. A certain percentage of individuals will freeze to death. Their death is not affected by the size of the population. Similarly, human activities like construction and deforestation affect animal populations.
2.. Density-dependent factors: The factors which are influenced by density of population are called density dependent factors. These factors are more . severe when population density is large. Animals often use territorial behavior song and scent to attract partners for reproduction. These actions become more prominent with the increase of population density. Thus they are density dependent factors. Some other density-dependent factors are competition for . resources, disease, predation. and parasitism.
INTERSPECIFIC COMPETITION •
Competition among members of the same species is called interspecific competition. The animals utilize similar resources. Therefore, they interfere with
each others resources. The resource requirements of individuals of a species
are nearly identical. Therefore, interspecific competition is often intense. There are o types of interspecific competition:
- can occur without coming into direct contact For example a bird gets the worm. But it does not actually see later bird.
- I can occur in which one individual directly affect another. Its examples are territorial behavior and the actions of socially dominant individuals.