Population Dynamics

Population dynamics is the study of how and why population size changes over time. It examines the characteristics of a group of organisms such as density, natality (birth rate), survival, age structure, migration and form of growth of the population.

The population size — refers to the number of individuals in a species in a population.

1. Birth Rate (Natality Rate)
   The number of live births per 1,000 people in a population per year directly influences the size of the population. Higher birth rates lead to population growth, while lower birth rates result in slower growth or even a decline in population size.
2. Mortality Rate
   The mortality rate refers to the number of deaths in a population per 1,000 people per year. A high mortality rate decreases population size, while lower mortality rates (due to better healthcare, sanitation, etc.) contribute to population growth.
3. Migration
   The movement of people into or out of a region (immigration or emigration) affects population size. High immigration can lead to an increase in population, while high emigration may result in a decrease in the population of an area.

1. Reproduction (Birth Rate/Natality)
   The primary cause of population increase is reproduction. Birth rate refers to the number of offspring produced by females in a population, typically measured per year. A higher birth rate leads to a greater number of individuals in a population over time.
2. Immigration
   Population size also increases through the movement of individuals from neighboring populations into an area. Immigration introduces new individuals, boosting population numbers.
3. Mortality (Death Rate)
   A decrease in population size can occur due to death, which reduces the number of individuals in a population. The mortality rate measures the number of deaths in a population per unit of time, typically expressed per thousand individuals annually.
4. Emigration
   When individuals leave a population to move elsewhere, the population size decreases. Emigration is the outward movement of individuals from an area, contributing to a reduction in population.

1. Reproductive Potential of the Organism
   This refers to the rate of reproduction under ideal conditions, where resources are unlimited. Organisms can reproduce at maximum capacity, leading to exponential population growth when environmental conditions support it.
2. Environmental Resistance
   Environmental resistance includes all the biotic (living) and abiotic (non-living) factors that limit population growth. These factors may include predation, food supply, space, light, temperature, and competition. These resistance factors prevent a population from growing indefinitely by limiting its resources and survival.
3. Intra-specific Competition
   Within a species, individuals compete for resources like food, mates, and territory. As the population increases, competition intensifies, which may limit the resources available for reproduction and survival.
4. Regulatory Mechanisms within Species Affecting Population Growth
   The population growth rate can fluctuate due to changes in birth or death rates. For example, food shortages or increased predation can directly reduce survival rates and increase mortality.

Population growth may change as a result of changes in birth or death rate. Food shortage and increase in predation are two factors which have direct effect on mortality.

• Changes in Birth or Death Rate
  Population growth is influenced by fluctuations in birth and death rates. An increase in the birth rate or a decrease in the death rate leads to population growth, while the opposite results in a decline in population size.

• Food Shortage
  A shortage of food directly affects mortality by reducing the survival rate of individuals. When food is scarce, competition intensifies, and weaker individuals may not survive, leading to higher death rates.

• Increased Predation
  A rise in predation increases the mortality rate of a population. More predators result in a higher number of individuals being killed, which decreases the overall population size.

• Regulatory Mechanisms Affecting Birth Rate

  • Territorial Behavior: In many species, individuals or groups defend their territory to ensure access to resources. This territoriality limits the number of individuals that can occupy a given area, which may impact the birth rate by reducing the available space for reproduction.
  • Physical Effects of Overcrowding: When a population becomes overcrowded, individuals face stress and competition for resources. Overcrowding can lead to a reduction in reproductive success, poor health, and increased mortality, all of which negatively affect population growth.

Many animals exhibit territorial behavior.

A territory

A territory is an area usually fixed in location that individuals defend and from which other members of the same species are usually excluded.

• Territories are typically used for feeding, mating, rearing of young or combination of these activities.
• Territorial animals benefit in several ways they have exclusive access to food supplies and breeding areas within their territories.
• Also familiarity with their areas help them to obtain food there and avoid predators.
• Moreover they can care for their young without interference from other individuals of the same species.

In a number of mammals high population density reduces greatly the birth rate even if there is good supply.

Various hormonal changes occur which affect the reproductive behaviors in a number of ways e.g. Failure to copulate, infertility, number of abortions and eating of the young by the parents all increase and parental care disease.

Factors between species which affect population growth

A number of well recognized types of interactions may occur between populations of different species. These are termed as interspecific interaction, populations from different trophic levels may also interact as for example in case of predator-prey relationship and host parasite relationship.

• Population growth curves
• Two basic forms of growth curves can be identified.
• The J-shaped growth curves
• The S-shaped growth curves or sigmoid growth curve.

The S-Shaped or sigmoid growth curve

This describes a situation in which in a new environment, the population density of an organism increases slowly initially, as it adapts to new conditions and establish itself, then increases rapidly approaching an exponential growth rate. It then shows a rate of increase until zero population growth rate is achieved where rate of reproduction (natality) equals rate of death (mortality).

The slowing rate of population growth results from increasing competition for essential resources such as food or nesting material.

The decline in growth rate continues until eventually feedback in terms of increased mortality and reproduction fails (fewer mating, stress induced abortion) reduces population growth rate to zero.

This type of population growth is said to be density dependent since for a given set of resources, growth rate depends on the numbers present in the population.

J-Shaped growth curve

• This type of growth curve describes a situation in which after initial establishment phase (lag phase) population growth continues in an exponential form until stopped abruptly as the environmental resistance becomes suddenly effective.
• Growth is said to be density independent since regulation of growth rate is not tied to the population density until the final crash.
• The crash may be triggered by factors such as seasonality or the end of a breeding phase, either of the organism itself or of an important prey species. It may also be associated with a particular stage in a life cycle such as seed production or it may be induced by human intervention.

A biome is a large region in the biosphere that possesses characteristic physical conditions that support organisms which show adaptations to those conditions found there.

OR

Is a collection of similar ecosystems in a particular region of the earth, it could be a habitat or zone such as desert, forest, grassland or water bodies such as ocean, lakes etc.

OR

Is the largest terrestrial communication or the largest ecological unit.

A biome has a specific kind of plants and annual species within a geographical area that has distinct climatic conditions. A biome is a product of physical factors that influence the rainfall, temperature and light.

A major biomes includes

1. Tundra
2. Northern coniferous forest or Taiga
3. Deciduous forest
4. The Mediterranean shrub
5. Tropical savannah
6. Grassland
7. Desert

Biomes that involve aquatic ecosystems

• Marine environment
• Fresh water environment
• Human population is the most abundant (about 7 billion) which is next to certain species of fishes and insects.
• Human population is widely distributed on earth in almost all climatic conditions from Arctic to Antarctica.
• The population differs economically, socially, culturally and color and height in different parts of the world.
• The study of trends in human population growth and the prediction of future development is known as demography.
• Demography involves the study of various parameters along with number and proportion of different age group, educational requirements training and employment.

The human population growth patterns differ in different regions of the world due to various factors some of these are discussed below;

1. Climatic and Edaphic factor
2. The region of extreme heat like deserts or extreme cold like Arctic and Antarctica are less populated.
3. Location of natural resources
4. Transport facilities.
5. Industrial development and education
6. Demography factor

The birth rate and death rate of a population determine an overall population growth. It varies in different countries.

• Rate of growth of human population is not uniform in all the countries, the developed countries have stable or negative growth rate (both the birth and death rate are low).
• The developing countries have high birth rate and the population is increasing at an enormous rate.
• Rate of growth differ among different group of people within a country.
• The population of the urban areas is leading to overcrowding and is causing adverse environmental implications.

The human population as a whole is growing exponentially having doubled three times in the last three centuries, it now stands at 6 billion and may reach 8 billion by the year 2020.

Most of the increase is due to improved health and technology which have decreased the death rate. The human population faces uncertain future, although some are optimistic about our ability to expand earth's carrying capacity, others are concerned that our increase in numbers may damage the biosphere beyond repair.

Human impact on the ecosystem

Since the development of agriculture and technology an increasing human impact on the environment has occurred. In last two centuries especially widespread industrialization has led to potentially damaging environmental pollution.

Is the release into the environment of substances or energy in such quantities and for such duration that they cause harm to the people or other organisms or their environment.

• The harmful substances are collectively known as pollutants
• Pollutants destroy the natural quality of the environment.

Example of pollutants

1. Agricultural chemicals such as fertilizers, pesticides and herbicides
2. Industrial by products or emissions such as sulphur-dioxide, mercury, carbon monoxide, CFCs, lead,
3. Sewage
4. Smog
5. Oils
6. Radiations such as gamma rays, x-ray
7. Dusts

Natural resources - Materials from the natural environment or ecosystem that man can use.

Types

1. Renewable resources - Are those which can be replaced within a short period of time after being used e.g. water, wildlife, forest etc.
2. Non-renewable resources - Cannot be easily replaced within a short time e.g. fossil fuels, minerals.

Conservation - retaining the status quo through carefully management i.e. preservation of species, habitats and living system from harmful influence of man thus avoiding decline and extinction of species and permanent detrimental or degradation to the environment.

1. We conserve to maintain world ecosystems by conserving the terrestrial, aquatic and aerial habitats.
2. Ethical reasons - cultural, tradition, religious beliefs, political persuasion, shape our attitude toward nature.
3. Nature does not exist simply for humans to transform and modify because all living species have a right to coexist with us on earth. Man has no right to cause the extinction or to diminish the quality of life of any organism.
4. Custodianship — Man has to pass on to the future generations all the diversity of life and quality of environment that we inherited.
5. Aesthetic reasons - Humans derive pleasant environment and the presence of other living organisms.
6. Local national and international organization exist worldwide to promote wildlife contribute to our immediate needs e.g. in forestry many plant species have important medical uses.
7. Fisheries from water bodies as direct source of food
8. Agriculture - We have to conserve food stocks and soil quality for crops.
9. Usage of pollinating insects and beneficial predators in pest control.

1. Maintenance of balanced biochemical cycles to avoid pollution and regulation of the earth's climatic conditions.
2. To avoid deforestation and desertification.
3. To avoid loss of vegetation cover this may affect soil erosion leading to accumulation of mud on river beds and coasts.
4. Reduce extinction of species to retain diversity gene pool (retaining biodiversity)
5. To preserve rare species that may have generation potential.
6. To minimize effects of mining, drilling, urbanization.

1. People should be educated on the importance of our environment by:
2. Encouraging them to plant trees
3. Encouraging the use of organic manure and good agricultural practices in agriculture such as terrace farming on hill sides, crop rotation.
4. Discouraging harmful traditions such as burning or slash method of farming to clear the land.
5. Legislation
6. Laws that govern the protection of national parks and grass kinds and punishments for those who start bush fires
7. Laws that protect the endangered species as a means of preventing extinction e.g. rhino
8. Some animal species must be regulated to avoid over-population e.g. wild beasts.
9. Reduction of pollution e.g. by using lead free petrol, alternate energy source (solar power, ethanol cars) use of biodegradable products, recycle waste such as paper to avoid deforestation, glass and cans water as resource and its sources must be preserved. Government should increase biodiversity by increasing botanical gardens, seed banks and field gene banks.
10. Sustainability - exploitation of natural resources and conserving them for future generations.

1. Avoid excessive use of chemicals such as pesticides and herbicides.
2. Encouraging biological control.

Is the artificial control of pests and parasites those which compete with human for food or damage the health of humans or livestock by the use of organisms or their products and therefore it is based on predator-prey relationship. The control agent either control the pest by feeding on it (lady bird or aphids, caterpillars larvae of butterflies and moths) are parasitized by bacillus species or by causing disease to the pests e.g. Virus sprays on army worm caterpillar.

Aim of biological control

Is to bring the population of the pest down to a tolerable level and not to eradicate it by its natural enemy.

Advantages

1. It doesn't pollute the environment
2. It's easy to apply as it doesn't involve the use of sophisticated machine
3. It's effective through generations as the pest will not develop resistance to the predator as it is with the pesticides and herbicides. Thus time to time application is not important.
4. The predator eliminates or feeds on the intended organisms or i.e. it is selective.
5. It is cheap.

Disadvantages:

1. The beneficial organisms may also be eliminated due to lack of food if it dominates the harmful organisms or pest which may cause or lead to disruption of food chain.
2. The predator (agent) may turn into a pest by feeding on other useful organisms instead of feeding on the intended organisms e.g. after controlling the pest population it may become a pest itself.
3. It's expensive i.e. involves studying the biology of the pest and the predator and this may take a long time.
4. If important agents or predators may fail to survive when the environment of the new country changes.
5. Biological control may not be effective if the pest outbreak is high because the agent or predators are fairly slow to react or to eat large numbers of pests. The population of pest species left may still be great enough to continue causing damage.