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examBoard: Pearson Edexcel
examType: IGCSE
lessonTitle: Population, Community, Habitat
Ecology is the study of how organisms interact with each other and their environment. To understand ecology, we need to look at different levels of organisation in nature, from individual organisms to entire ecosystems.
Key Definitions:
A habitat provides everything an organism needs to survive: food, water, shelter and space. Different species are adapted to different habitats. For example, polar bears are adapted to Arctic habitats with thick fur for insulation, while cacti are adapted to desert habitats with water storage capabilities and spines instead of leaves.
Populations can be measured by their size (number of individuals), density (number per unit area) and distribution (how individuals are spread out). Populations grow through births and immigration and decrease through deaths and emigration. These factors determine whether a population increases, decreases, or remains stable.
Ecologists use various techniques to estimate population sizes, as it's often impossible to count every individual:
Counting organisms in small areas (quadrats) and multiplying to estimate the total population. Useful for plants and slow-moving organisms.
Capturing, marking, releasing and recapturing animals to estimate population size using the Lincoln Index formula: N = (n1 × n2) ÷ m2
Sampling along a line to study how populations change across environmental gradients or boundaries between habitats.
Populations don't grow indefinitely. Their growth is limited by various factors:
Population size is regulated by both density-dependent and density-independent factors:
These factors have a greater effect as population density increases:
These factors affect populations regardless of their density:
Populations typically show one of two growth patterns:
When resources are abundant, populations can grow exponentially, creating a J-shaped curve. Each generation produces more offspring than the previous one. This is unsustainable in the long term as resources eventually become limited.
Most populations follow an S-shaped (sigmoid) curve. Initial exponential growth slows as the population approaches the carrying capacity - the maximum population size that the environment can sustain. At carrying capacity, birth and death rates are equal.
In 1944, 29 reindeer were introduced to St. Matthew Island in Alaska. With abundant food (lichen) and no predators, the population exploded to 6,000 by 1963. However, they had eaten most of the lichen by then. By 1966, only 42 reindeer remained alive. This dramatic boom-and-bust cycle demonstrates what happens when a population exceeds its carrying capacity.
Within a community, different species interact in various ways:
When species compete for the same limited resources. Interspecific competition (between different species) can lead to competitive exclusion, where one species outcompetes another.
When one organism (predator) kills and eats another (prey). Predator-prey relationships often show cyclical patterns, as prey numbers affect predator numbers and vice versa.
When one organism (parasite) lives on or in another (host) and harms it. Examples include tapeworms, fleas and mistletoe.
When both species benefit from the relationship. Examples include pollination, where insects get food while plants get pollinated and lichens (fungi + algae).
When one species benefits while the other is neither helped nor harmed. Examples include birds nesting in trees or barnacles attaching to whales.
Any close, long-term relationship between different species. Includes mutualism, commensalism and parasitism.
Energy flows through communities via feeding relationships:
Organisms in a food chain are grouped by their feeding position:
Only about 10% of energy transfers between each trophic level, with the rest lost as heat or used for life processes. This is why food chains rarely have more than 4-5 links.
When wolves were reintroduced to Yellowstone National Park in 1995, they affected the entire ecosystem. They reduced elk numbers, which allowed trees and shrubs to recover. This provided habitat for birds and beavers. Beaver dams created wetlands, supporting fish and amphibians. This demonstrates how a single species can influence an entire community through cascading effects - known as a trophic cascade.
Biodiversity refers to the variety of life in an area, including:
Biodiversity is threatened by human activities including habitat destruction, pollution, climate change, overexploitation and introducing invasive species. Conservation efforts aim to protect biodiversity through:
Protecting species in their natural habitats through national parks, wildlife reserves and sustainable management practices.
Protecting species outside their natural habitats in zoos, botanical gardens, seed banks and captive breeding programmes.
For your IGCSE exam, you should be able to:
When answering questions about population growth or community interactions, use specific examples to illustrate your points. For calculation questions involving sampling or mark-release-recapture, show all your working clearly. Remember to consider both biotic (living) and abiotic (non-living) factors when discussing what affects populations and communities.
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