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
- 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.
- 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:
- 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.
- 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.
- 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.
- 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.
- 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.
- Importance percentage values: It is the sum of the relative density, relative dominance, and relative frequency of a species in a community.
× 100
- 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.
- 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.
- 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
- Mortality rate: The mortality rate or death rate refers to the number of deaths in a given period.
- Ecological or realized mortality refers to the loss of individuals under given environmental conditions.
- Minimum mortality refers to the minimum loss under ideal conditions without limiting factors.
- 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:
- Pre-reproductive age (0-14 years)
- Reproductive age (15-44 years)
- 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.
- 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.
- 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.
- 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.
- 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.
- 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.
- 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.
- 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
What does population ecology mean?
Population ecology is the branch of ecology which deals with the study on how the populations of the plants, animals and micro-organism interact with their surrounding environment and change with the course of time. Population is the group of living organisms living with several interactions in a particular geographic at a particular time.
What is an example of population ecology?
Population ecology is related to the study of the interaction of the population with their environment. Study of distribution of tigers in a National Park, abundance and diversity of rodents (small mammals) in a grassland/ hill, growth rate of the human population in a state/city/country, etc. and the factors behind them are the examples of population ecology study.
What are the characteristics of population ecology?
– Population ecologist study how population change over a period of time. They have to investigate one of the central principles of ecology i.e. population growth and the factors driving it.
– Population ecology also deals with the study of fecundity i.e. how many offspring an individual can have in a life time as it can make a huge difference in the size of a population.
– The study of population ecology helps to know how the mosquito population grows so quickly or why a rhino population can hardly recover from a single act of poaching. The answers to the questions like how mosquitoes can have 2,000 offspring in their 2 week life time at early age whereas a rhino can have only around five in forty years can be obtained.
– The population ecology helps to know the interactions among populations of organisms that inhabit the same area.
What are the 5 characteristics of population?
The five characteristics of population are as follows:
Geographical Distribution: It is the range of the population or the area that is inhabited by the population. Migration area of whales, distribution of small mammals in east and south facing slopes of a hill, mold on a piece of bread, etc. are the examples of geographical distribution of a population.
Density: It is the number of individuals living per unit area.
The variation in density depends on the species and the ecosystem.
· Dispersion: It refers to the spatial distribution of individuals within the population.
There are three types of dispersion: Clumped Dispersion, Even Dispersion and Random Dispersion.
a. Clumped Dispersion: In this type, the individuals are clustered together. It occurs when resources (food, water and living spaces) are clumped together and the species have a certain social behavior. For examples, herding animals, flocks of birds, schools of fish, hives of bees, etc.
b. Even Dispersion (Uniform Dispersion): In this type, the individuals are separated by a fairly even distance. It occurs as a result of social interaction. Individuals try to get as far away from each other as possible. Territorial birds like penguins tend to have even distribution. Another example is plants that secrete toxin to inhibit the growth of nearby individuals and the phenomenon is called allelopathy.
c. Random Dispersion: Location of one individual is independent of the location of the other individuals. Example: seed dispersal by the wind or animals.
Growth rate: It is the rate at which the size of a population changes in a given time. It also measures the speed of reproduction.
A population will grow exponentially if there’s some reason that it can’t. Exponential growth means that the population grows at rate proportional to the size of the population. It can be understood by taking the example of the West Nile Virus outbreak struck at Dallas, Texas in the summer of 2012. At the beginning of 2012, there might be only a thousand mosquitoes in Dallas, but after one month it increased (say up to 3000). With three times of reproduction rate of the mosquitoes the population growth became three times faster (3000 to 9000 and likewise infinity) than when there were 1000. The mosquito population exceeded the carrying capacity, but it cannot stay for a long. For example, the humans have been on exponential growth curve since the industrial revolution. But something always knocks down the population size back down which might be a density dependent factor like food scarcity or an epidemic or density independent factor like great catastrophic events that adversely affect the whole countries. Thus, the exponential growth rate can’t go up forever.
When those factors come into action, a population experiences only logistic growth. This means the population is limited to the carrying capacity of its habitat. The graph flattens at the top. The factor that created the plateau is almost always a density dependent limiting factor. For example, the growth of mosquitoes in 2013 lowered down as they ran out of food or space. When the growth level of the number stops it can be understood as the carrying capacity of the mosquito in the particular habitat. There is a simple equation which helps to calculate the population growth of anything we feel like.
How to calculate growth rate?
For example, we have to calculate the growth of mosquito population over a span of two weeks. All we have to do to get the rate of growth (r) is take the number of births minus number of deaths and divide the whole by the initial population size (N). If the initial population size of the mosquitoes is 100, and these mosquito live an average of 2 weeks, so the deaths over a span of 2 weeks will be 100. If the half of these mosquitoes are female and they can produce about 2,000 babies in their life time. So, 50 mother mosquitoes times 2,000 baby mosquitoes per mother result in births equal to 100,000 little baby mosquitoes. So, in two weeks if the population consist of 100,000 baby mosquitoes and only 100 of them died, so this result in a population growth rate of 999. That means for every mosquito, at the beginning of 2 weeks, there will be 999 more at the end of 2 weeks. That is 99,800 % increase, these are hypothetical numbers but it can give a sense of how a population can just go out of control when all the factors that we talked about goes in mosquito’s favor.
· Age structure:
What are the two types of population?
The two types of population are finite or countable population and infinite or uncountable population.
What are the three types of population distribution?
The three types of population distribution are:
Uniform dispersion: population more or less equally spaced apart.
Random dispersion: population dispersed randomly with no predictable pattern.
Clumped dispersion: population clustered in groups.
What are 3 characteristics of population?
The three main characteristics of population are as follows:
Density: Size of population:
Natality: Birth rate of population.
Mortality: Death rate of population.
What are examples of population?
Examples of population are human population in Tokyo city of Japan, tiger population of Bardia National Park, rhino population of Chitwan National Park, etc.
What are the three main properties of a population?
The three main properties of a population are:
Population density: It refers to the total number of individuals in a population of a particular area.
Population Dispersion: It refers to the spatial distribution of population in a geographical area. It can be uniform, random or clumped.
Population age structure distribution: It refers to the distribution of population in terms of age group. From ecological point of view, there are three major types of age structure: pre-reproductive, reproductive and post reproductive age.
What are the four main characteristics of population?
The four main characteristics of population are Distribution, Numbers (size), Age structure and Density.
What is it called when a population levels off?
It is called population level off when the population growth rate plateaus or levels off forming an S-shaped curve. In this state, the maximum population size which a particular environment can support is called its carrying capacity.
What are the features of population change?
The main features of population change are birth, death and migration. Birth increases the population size whereas death decreases the population size. People migrate to more developed region or country than their native ones for better opportunities of infrastructures and employment. Migration can either decrease or increase population of a region or a country.
What are the 4 stages of population growth?
The population growth consists of the following stages:
Pre-industrial stage (Stage 1): This stage shows modest population growth. The birth and death rates both are high.
Transitional stage (Stage 2): This stage shows rapid population growth. The death rates are lowered.
Industrial stage (Stage 3): This stage shows slow population growth. The birth rates decline.
Post-industrial stage (Stage 4): This stage shows very low population growth. The birth and death rates both are low.
What are 4 factors that affect population growth?
The four factors that affect population growth are birth, death, immigration and emigration.
What is the population dispersion?
Population dispersion is the pattern of spacing of individuals within the area the population inhabits. Simply Population dispersion means how the individuals in a population are distributed in a space at a given time. There are three types of dispersion: clumped dispersion, even dispersion and random dispersion.
In clumped dispersion, the individuals of population are clustered in groups like the plants which drop their seeds straight to the ground (oak trees), school of fishes, herd of elephants, etc.
In even dispersion, the individuals of a population are spaces more or more less uniformly like the plants which secrete toxins to inhabit the growth of nearby individuals (allelopathy), animal species who stake out and defend territories, etc.
In random dispersion, the individuals of a population are randomly distributed without a predictable pattern like in dandelions and other plants having wind-dispersed seeds.