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examBoard: Cambridge
examType: IGCSE
lessonTitle: Population Growth Curves
Population growth curves show how the number of individuals in a population changes over time. Understanding these patterns helps us predict future population sizes and manage resources sustainably.
Key Definitions:
Population growth affects everything from food security to pollution levels. By understanding how populations grow, we can better plan for the future and manage our impact on the environment. For conservationists, these patterns help predict whether endangered species will recover or decline further.
Population growth can be calculated using a simple equation:
Population change = (Births + Immigration) - (Deaths + Emigration)
When births and immigration exceed deaths and emigration, the population increases. When the opposite occurs, the population decreases.
There are two main types of population growth curves that describe how populations change over time: J-shaped curves and S-shaped curves.
J-shaped growth occurs when a population grows exponentially without any limiting factors. The curve looks like the letter "J" because the population starts small but then increases rapidly.
In J-shaped growth, the population eventually reaches a point where it can't be sustained. This often leads to a population crash due to resource depletion, disease, or other limiting factors.
S-shaped growth occurs when a population initially grows exponentially but then slows down as it approaches the environment's carrying capacity. The curve resembles the letter "S".
Initial slow growth as the population adapts to its environment. Numbers increase gradually.
Rapid population growth as resources are plentiful and reproduction rates are high.
Growth slows and eventually stabilises as the population reaches carrying capacity.
S-shaped growth is more sustainable than J-shaped growth because the population eventually stabilises rather than crashing. Most natural populations follow S-shaped growth patterns in stable environments.
In 1910, 25 reindeer were introduced to St. Paul Island in Alaska. With no predators and plenty of food, the population showed perfect J-shaped growth, reaching 2,000 reindeer by 1938. However, this exceeded the island's carrying capacity. The reindeer destroyed their food source (lichen) and by 1950, only 8 reindeer remained on the island. This real-world example dramatically illustrates the consequences of exceeding carrying capacity.
Several factors determine whether a population follows J-shaped or S-shaped growth patterns:
These affect populations regardless of their size:
These have a greater impact as population density increases:
Carrying capacity is a crucial concept in understanding population growth curves. It represents the maximum population size that can be sustained indefinitely in a particular environment.
Environmental resistance refers to all the factors that prevent a population from growing indefinitely. As a population approaches carrying capacity, environmental resistance increases. This creates the levelling off seen in S-shaped growth curves.
Human population growth provides a fascinating real-world example of population growth curves. For most of human history, our population grew very slowly, but it has exploded in the last few centuries.
Human population growth has largely followed a J-shaped curve, but there are signs it may be transitioning to an S-shaped curve as growth rates slow in many countries. Unlike other species, humans can:
However, there are still ultimate limits to Earth's carrying capacity for humans, related to:
As human populations grow, they place increasing pressure on natural resources and ecosystems:
Managing population growth sustainably requires:
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