Populations | Biology for GCSE/IGCSE - Year 11 PDF Download

Definition of Population

• A population comprises organisms of the same species residing in a particular area simultaneously.

Other important definitions 

• A community encompasses the various populations of different species existing within a specific ecosystem.
• An ecosystem constitutes a unit comprising the interacting community of organisms and their surrounding environment, such as a decomposing log or a lake.

Population Growth

• All living organisms compete for essential resources such as food, water, and living space.
• Organisms that are most suited to their environments tend to thrive and multiply, outcompeting those that are less adapted.
• The growth of populations in most organisms is influenced by three main factors: food availability, predation, and disease.

Factors Influencing Population Growth

•  Food Supply:  Adequate food availability is crucial for the sustenance and growth of populations.
•  Predation:  The act of preying on other organisms impacts population sizes by controlling the number of individuals in a given area.
•  Disease:  Diseases play a significant role in regulating population growth by affecting the health and survival of organisms.

The Population Growth Curve

• When observing the growth of microorganisms in a fermenter over time, the population growth exhibits a specific pattern.

Sigmoid Growth Curve

• The sigmoid growth curve gets its name from its shape, resembling an 'S' curve.
• The curve can be divided into four main phases:
  • Lag Phase: In this phase, organisms adapt to the environment before reproduction begins. Few organisms are present, limiting the production of offspring.
  • Log Phase (Exponential Phase): During this phase, there is an abundance of food supply. The birth rate is high, the death rate is low, and growth is exponential.
  • Stationary Phase: Population levels off due to a limiting factor in the environment, such as a nutrient shortage. Birth and death rates become equal until the nutrient is replenished or severely limited.
  • Death Phase: Population decline occurs as the death rate surpasses the birth rate. This could be due to a scarcity of food or the accumulation of toxic metabolic wastes.

Factors Affecting Population Growth

Organisms in a natural environment do not typically exhibit growth following a sigmoid curve due to various influencing factors, including:

• Changing Temperature or Light: Changes in temperature and light can significantly impact the growth and survival of organisms. For example, a sudden drop in temperature might slow down the reproductive rate of certain species.
• Predators: The presence of predators can exert pressure on a population, leading to a decrease in numbers. An example is the decline in prey species due to increased predation.
• Disease: Disease outbreaks can cause a decline in population size by affecting the health and reproductive capacity of individuals. For instance, a contagious disease can spread quickly within a population, leading to a decrease in numbers.
• Immigration (individuals moving into the area): The influx of individuals from other areas can introduce competition for resources and alter the dynamics of the resident population. An example is the introduction of a new species into an ecosystem, leading to changes in the existing community structure.
• Emigration (individuals moving out of the area): When individuals leave a population, it can impact the gene pool and reduce the overall population size. For instance, emigration of breeding individuals can reduce reproductive potential within a population.