Population Ecology

Population ecology is the study of how populations of organisms change over time and how they are affected by their environment. It is a subfield of ecology that focuses on the factors that influence the size, growth, and distribution of populations, as well as the interactions between populations and their environment.

Population ecologists may study a wide range of topics, including the factors that affect population growth and decline, the impacts of habitat loss and other environmental changes on population dynamics, and the ways in which populations interact with each other and their environment. They may also study the impacts of human activities on population dynamics and the ways in which these impacts can be managed or mitigated.

To study population dynamics, population ecologists may use a variety of methods, including field observations, experiments, and data analysis. They may also use mathematical models and computer simulations to understand and predict the impacts of different factors on population dynamics.

Population ecology is an important field for understanding the impacts of human activities on the environment and for informing conservation efforts to protect and preserve species and their habitats. It is also important for understanding the impacts of climate change on populations and ecosystems.

Population means the group of the same species living in a particular area. Population function as a part of a biotic community. A biotic community comprises a group of populations living together and undergoing metabolic transformations in a specific area of physical habitat. A population has various properties that can be best expressed statistically, and these properties are not of the individuals but the group as a whole. The properties are: 

i. Density         

ii. Natality (birth rate)    

iii. Mortality (Death rate)

iv. Age distribution  

v. Biotic potential         

vi. Dispersion

vii. r-selected growth form 

viii. K-selected growth form

1. Density: Density is known as the number of individuals per unit area. Population density is the size of the population occurring in a definite space. It is expressed as the number of individuals or the population biomass per unit area or volume. The indices of density are crude density and ecological density. The crude density is defined as the number of biomass per unit of total space. Ecological density is defined as the number of biomass per unit of habitat space (available area or volume that the population can colonize). In a trophic level, at the lower level, the density would be higher, and within a given group, when the number of individuals is large, the biomass is high. Basal area is a measure of tree density used by forest ecologists. The basal area is the total cross-section area of tree trunks. The variation in density depends upon the species and the ecosystem.  

There are many techniques and methodologies for estimating the population densities like Lincoln index, Minimum known alive (MKA) method, total counts, quadrat or transect sampling, removal sampling, plotless methods, and importance percentage values. Firstly, it is essential to learn the sampling methodologies to be applied in the field to estimate population densities. 

1. Lincoln index: A common mark-recapture method is applied to estimate the total population density in a specified area. This method involves live trapping, marking captured small mammals, recaptures, and finding population estimates. The following formula calculates it:

1. Minimum known alive (MKA) method: It is another mark-recapture method applied to estimate the population densities over an extended period. This method involves the ‘calendar-of-catches’ method using a capture history (calendar) kept for each individual, followed by a period of intensive removal to update the calendar at the end of the study. 
2. Total counts: They are applied for a sampling of large organisms like bison on open plains and whales in the area of the sea or with organisms that form a group in large breeding colonies.
3. Quadrat or transect sampling: This method is applied for counting organisms of a single species in plots or transects of appropriate size and number to estimate the density in the area sampled. Transect sampling is done in both terrestrial and aquatic habitats.
4. Removal sampling: In this method, the number of organisms removed from an area in successive samples are plotted on the Y-axis of a graph, whereas the total number previously removed is plotted on the X-axis. If the probability of capture remains constant, the points will lie in a straight line which can be extended to the point on the X-axis, which indicates the 100 % removal from the area and gives the estimate of population density.
5. Plotless methods: This method is applied to non-motile organisms like trees. The point-quarter method is based on a series of random points. The distance to the nearest individual is measured in each of the four quarters at each end. The density per unit area can be estimated from the mean distance. 
6. Importance percentage values: It is the sum of the relative density, relative dominance, and relative frequency of a species in a community. 

 × 100

1. Natality (Birth rate): Birth rate refers to the number of births occurring in a given period. Natality is the capacity of a population to increase by reproduction. 
2. Maximum or absolute or physiological natality: The theoretical maximum production of new individuals under ideal conditions without limiting factors. It is constant for a given population. 
3. Ecological or realized natality refers to the population increase under an actual or specific environmental field condition. It is not constant for a given population. 

Natality can be expressed as:

Or

1. Mortality rate: The mortality rate or death rate refers to the number of deaths in a given period. 
2. Ecological or realized mortality refers to the loss of individuals under given environmental conditions.
3. Minimum mortality refers to the minimum loss under ideal conditions without limiting factors. 

1. Age distribution is an important characteristic of populations that influences both natality and mortality. Generally, a rapidly increasing population will contain a large proportion of young individuals, a stable population will consist of a more even distribution of age classes, and a decreasing population will have a large proportion of old individuals. Age structure: Age structure refers to the number and proportion of people at each age. The people are categorized into three types on the basis of age:
2. Pre-reproductive age (0-14 years)
3. Reproductive age (15-44 years)
4. Post-reproductive age (45 years and older)

‘Age-structure diagrams’ are used to predict the future growth of a population. A population’s development depends on how many people make up the given population. Around 29 % of the human population is under the age of 15 years. 

1. The biotic potential is the intrinsic or inherent property of an organism to reproduce and survive. Mathematically, it can be expressed as the algebraic sum of the number of young produced at each birth, the number of reproductions in a given period of time, the sex ratio, and their ability to survive under given physical conditions. 
2. Dispersion: It is known as the spatial distribution of the individuals within the population. There are three types of dispersion: clumped dispersion, even dispersion, and random dispersion. 
3. Clumped Dispersion: In this type, the individuals are clustered together. It occurs when the resources like food, water, and living spaces are clumped together. The species display a specific behavior. Examples are herding animals, flocks of birds, schools of fishes, hives, etc. 
4. Even dispersion: It is also known as Uniform Dispersion. In this type, the individuals are separated by a fairly even distance. It occurs due to social interaction. The individuals try to get far away from each other as possible. There are limited resources in this dispersion, and competition occurs due to this. 
5. Random dispersion: In this type, the location of one individual is independent of the location of the other individuals. Examples: seeds dispersed by the wind or animals.
6. R-selected growth form: The population of species having an r-selected growth form is usually far from the carrying capacity. Generally, r-strategists species are those with a high reproduction rate, i.e., they produce many offspring even in a short time. The density-independent factors influence them, and their environment is unstable. They exhibit a J-shaped curved growth rate, i.e., high growth rate or exponential growth. They are highly affected by abiotic factors like weather. They have small body size and short life span. The main characteristics of the species having r-selected growth form are small body size, early maturity, short life span, large broods, little or no parental care, and low probability of long-term survival. Examples of r-strategist species are mosquitoes and dandelions.

K-selected growth form: The population of species having a k-selected growth form is usually near the carrying capacity. They exhibit an S-shaped curved growth rate, i.e., logical growth. Biotic factors like competition highly influence them. They typically have large body sizes. They have few offspring but live a long time. The density-dependent factors influence them, and their environment is unstable. The main characteristics of the species having a k-selected growth form are large body size, long life span, small broods, slow development, and low reproductive rate. 

1. Examples of k-strategist species are kangaroo, redwood trees, and human beings.

Population Growth Rate:

 It is the rate by which the size of the population changes in a given time. It is a measure of the speed of reproduction.

The following formula can calculate the population change:

r = (b – d) + (i – e)

where, r = growth rate, b= birth rate, d= death rate, i= immigration rate, e= emigration rate 

Birth, death, immigration, and emigration are calculated per 100 people. 

The factors affecting population size or rate of population growth are the number of births, deaths, and migration (immigration and emigration).

Population Dynamics

The population changes its size and composition over time, which means the population is dynamic.

• The population size increases if the birth rate is greater than the death rate. 
• The population size remains stable if the birth rate is equal to the death rate. 
• The population size decreases if the birth rate is less than the death rate. 

Life expectancy is the duration of the time an individual is expected to live.

Carrying Capacity:

 It refers to the maximum number of organisms a given area can support.

• If the actual population size is lesser than the carrying capacity, then the birth rate increases.
• If the actual population size is greater than the carrying capacity, then the death rate increases.

Overshooting carrying capacity, i.e., the increased ecological footprint, can lead to a sudden sharp reduction in population size due to starvation, resource scarcity, and competition. 

Human Population

Demography refers to the science that deals with the study of population structure and growth. Demography means the scientific study of human populations. The growth rate curve of the human population since 1980 is J-shaped which signifies a high population growth rate. The human population statistics show the world’s total population in the year 1800, 1930, 1960, 1975, 1987, 1999, and 2006 as 1 billion, 2 billion, 3 billion, 4 billion, 5 billion, 6 billion, and 6.6 billion, respectively. There were less than 1 billion people on the earth at a time (until the time of Napoleon Bonapart). Since the Second World War, there has been adding of a billion population of human beings on the earth every 12-15 years. The human population is more than double at the present what it was in 1970. At present, the world population is estimated to be more than 7 billion (nearly 8 billion). Every two years, the United Nations makes projections for future population growth. Its latest median projection is a population of 9.7 billion in 2050 and 10.9 billion in 2100. According to the report of the United Nations (2019), nine countries (India, Nigeria, Pakistan, the Democratic Republic of Congo, Ethiopia, Tanzania, Indonesia, Egypt, and the USA) will make up over half of the projected total population increase by 2050. It is expected that around 2027, India will overtake China and becomes the most populous county in the world. By 2050, around 55 countries are projected to experience population reduction. 

The major effects of the rapid population growth in the global scenario are seen to be a hindrance in the achievement of Sustainable Development Goals (eradicating hunger and poverty, achieving gender equality, and improving health and education). 

FAQs on Population Ecology