Ecology · BEGINNER LESSON
Population
A population is a group of organisms of the same species living in the same area at the same time. In ecology, the term also focuses on how that group’s size, density, and growth change over time and space.
What a population is
In ecology, a population is a group of organisms of the same species living in the same area at the same time. Scientists study populations to understand how many individuals there are, how closely packed they are, and how those numbers change over time and space. A related statistical use of the word can mean the full set being studied, but in this lesson the ecological meaning is the main one.
[1] An Introduction to Population Growth [2] Who and What Is a “Population”? Historical Debates, Current ... [3] Ecology (Stanford Encyclopedia of Philosophy/Fall 2012 Edition)
What scientists measure
Three common measurements are population size, population density, and population growth. Population size is the number of individuals. Population density is how many individuals occupy a particular area. Population growth is the change in population size over time. These measurements help researchers predict future changes and explain why populations increase, decrease, or fluctuate.
[1] An Introduction to Population Growth [3] Ecology (Stanford Encyclopedia of Philosophy/Fall 2012 Edition)
How population size changes
When resources are plentiful and limits are weak, a population may grow exponentially, meaning the increase becomes faster as more individuals are added and reproduce. In a simple model called logistic growth, the population increases quickly at first, then slows and levels off as it approaches carrying capacity, which is the maximum size the environment can support for that situation. Real populations often do not stay perfectly steady; they may rise and fall in cycles or fluctuate for other reasons.
[1] An Introduction to Population Growth [3] Ecology (Stanford Encyclopedia of Philosophy/Fall 2012 Edition)
What limits growth
Ecologists separate limiting factors into density-dependent and density-independent types. Density-dependent factors become stronger as population density rises, such as competition for resources, predation, and infection. Density-independent factors affect populations regardless of density, such as pollution, extreme weather, fires, floods, droughts, and volcanic eruptions. If deaths stay above births for long enough, a population can decline to extinction.
Limits of the idea
Population ecology often relies on models, but real populations are hard to measure perfectly. Basic parameters can be difficult to estimate, and many ecological systems include chance events that make outcomes less predictable than a simple equation suggests. So population size and growth are useful descriptions, but they are not the whole story.
[3] Ecology (Stanford Encyclopedia of Philosophy/Fall 2012 Edition) [1] An Introduction to Population Growth
SEE THE IDEA
How ecologists think about a population
Same species, same area, same time
Count size, density, and growth
Density-dependent and density-independent factors act on it
Growth, leveling off, cycling, decline
Ecologists start by defining the group, then measure it, then study what changes it over time.
A population is not just a count; it is a group whose size and change can be measured and explained.
TRY IT YOURSELF
Test your understanding
Choose an answer, then check the explanation.
PUT IT TOGETHER
Match each concept to its meaning
Drag a label onto its description, or select a label and then select a description. This also works with a keyboard.
4 pairs to match
Sources and further reading
This general lesson was generated from public source material. It explains a concept; it does not interpret a private paper or provide medical advice.