Ecology studies how organisms interact with each other and their surroundings. It is a wide-ranging branch of science. Why? Well, imagine how many organisms there are in a park - including bacteria and fungi that live in the soil. There are probably billions. And compared to the earth's surface, parks are just a speck of land. Think about how many organisms must live on the planet's land surface - not inluding the ocean!
Because ecology is such a vast field, ecologists like subdividing their area to make it easier to conceptualise and research. Let's look closer at the levels of organisations within a population, a community, or an ecosystem, and some examples of these.
Definition of Levels of Organisation
Ecologists categorise the natural world into smaller chunks to make them easier to understand. These chunks are called the levels of organisation.
Levels of organisation are the hierarchical structures of nature.
Hierarchical structure means that nature is 'ranked' in increasing detail.
Levels of organisation can span from a singular cell to the global biosphere.
The hierarchy in ecology involves individuals making up a population, which is made up of one species, whereby various species and their interactions make up a community that then feeds into an ecosystem along with the abiotic factors.
- Fig. 1 - Levels of organisation in nature, species populations form a community and communities along with the abiotic factors form an ecosystem.
In GCSE, you will only cover the level of organisations in populations, communities and ecosystems.
Level of Organisations Within a Population
The term population in biology refers to a group of living organisms that live in a specific geographical location at the same time. As you may notice, there are two main components: the living organisms and the geographical area they inhabit.
A population is a group of individuals of the same species living in an area.
You've probably heard the term population applied to humans. For example, the population of the UK is 68 million. This means that 68 million people are living in the nations of Great Britain and Northern Ireland. But in biology, the term 'population' can also refer to animals and plants. For example, Wales is home to a population of 10 million sheep.
Wildlife populations are not contained within national borders (except in island countries!). Because of this, scientists study populations within their habitat range.
A habitat is a place where an organism lives.
Snow leopards can be found in 12 different Asian countries. They live in mountainous areas, and their population is estimated to be up to 6,500.
Describing Populations
Populations are typically described by their size, density, and dispersion.
Population Size
A population size refers to the number of individuals it has. It is usual for a population to fluctuate over time; many things could cause this. An increase in a population can be brought about by individuals being born or immigration (new individuals entering the population from elsewhere). A population decrease can result from individuals dying or emigration (individuals leaving the population to go elsewhere).
Population Density
Population density is the number of individuals per unit area (or unit volume in aquatic environments).
Population density is often related to the size of the species - small rodents have much higher population densities than large carnivores. But as animals get larger, this change in population density becomes more extreme.
Whales, the world's largest animals, have incredibly low population densities. On average, there is one whale every 37,000 km² in the ocean. For comparison, the average UK population density is 278 people per km². That's over 10 million people per whale.
Population Dispersion
Dispersion is how individuals are spaced out within the population. Individuals can be clumped together in groups, spread out uniformly (in an even pattern), or distributed randomly with no predictable pattern.
Understanding how a population disperses can provide information on how organisms interact.
Clumped: mushrooms aggregate in moist, sheltered conditions, leading them to cluster together and grow from a central point.
Uniform: penguins exhibit uniform distribution due to nesting patterns and territorial behaviours.
Random: dandelion seeds are dispersed by the wind, resulting in these weeds sprouting wherever they have fallen (as long as the conditions are favourable).
Level of Organisations Within a Community
The next group after population in the hierarchical structure of nature is a community. A biological community refers to the collection or gathering of organisms, either from the same or different species, inhabiting a specific geographical location at the same time.
A community comprises all the different species populations living in the same area.
Different species that live within communities interact with each other. Some interspecies interactions are defined in this table.
| Interspecies interaction | What is it? |
| Competition | 2+ species competing for a limited resource |
| Predation | An animal kills and eats another animal |
| Herbivory | An animal eats part of a plant |
| Parasitism | A parasite gains resources from its host, who is harmed |
| Mutualism | Two species benefit from the interaction |
Table 1: Description of the types of interspecies interactions.
One of the major properties of a community is its function. How exactly does a community filled with various species all fit together? A community function refers to the flow or transfer of energy (usually in the form of a food web) and resiliency (how a species survives and creates its own niche) within the community.
Niches
Every species in a community has its own niche. Different niches allow species to coexist in a community.
A niche is the resources (both living and non-living) a species uses to survive.
If two species occupy the same niche in a community, they will compete. Eventually, the more successful species will outcompete the other.
Food Webs
Food chains can represent the transfer of energy within a community and contain the following components:
Producers ⇨ Primary consumers ⇨ Secondary Consumers ⇨ Tertiary consumers
Producers are organisms that make their own food using sunlight, such as plants.
Primary consumers are organisms that eat producers, such as rabbits.
Secondary consumers (also known as predators) are organisms that eat primary consumers, such as foxes.
Tertiary consumers (also known as large predators) are organisms that eat secondary consumers, such as a wolf.
An example of a simple food chain: Grass ⇨ Cricket ⇨ Frog ⇨ Snake
Consumers that kill and eat other animals are known as predators. The animals that get eaten are known as prey. Predator and prey populations are linked, rising and falling in cycles.
Food chains are linked together to form complex food webs. These can be used to understand the interdependence of species within an ecosystem.
To find out more about food webs, check out our article Interdependence and Competition!
Level of Organisation Within Ecosystems
We've learned that a community comprises all the different species populations living in an area. So, what is the next level up?
An ecosystem is a community of organisms and the abiotic (non-living) components of the environment.
Organisms interact with their physical environment and the rest of their community. Ecologists focus on energy flow and how nutrients recycle within the ecosystem. Think of food chains feeding into larger food webs of a habitat. The predator-prey relationship was discussed in the section above. However, it also involves decomposers when talking about ecosystems.
Decomposers are fungi or bacteria that break down dead organisms, usually in a process known as rotting or decomposition.
These decomposing organisms release enzymes that break down the dead matter and are eventually consumed by the decomposers. They are essential in the recycling process. Plants also absorb these decomposed nutrients through their roots.
Abiotic components of an ecosystem include temperature, water, oxygen levels, light availability, and geology.
Examples of Levels of Organisation
Now that we've covered populations, communities and ecosystems, let's see how they all fit together.
Individuals from one species live together in a population. The population is found within a broader community. Think about what other species might be present in that community. This information is tabulated below, with the wider ecosystems in which the species live.
| Population | Other Species in the Community | Ecosystem |
| Squirrels | Trees, grass, invertebrates, birds | Woodland |
| Damselfish | Algae, corals, molluscs, fish | Coral Reef |
| Cacti | Camels, desert foxes, geckos, scorpions | Desert |
| Penguins | Krill, seals, fish, orcas | Icecap |
Table 2: Some examples of levels of organization, where a population forms part of a community, and the community forms part of an ecosystem.
Human Impact on the Different Levels of Organisation
Human activities have disrupted the natural environment. Burning fossil fuels has increased the concentration of carbon dioxide in the atmosphere, causing climate change. Furthermore, deforestation and land use change has led to habitat loss.
How have these impacted the different levels of organisation?
| Level of Organisation | Climate Change Impacts | Habitat Loss Impacts |
| Population | - Species decline
- Changes to the timing of growth and reproduction
- Changes in distribution
| |
| Community | - Mismatches between consumers and their food sources
- Altered community structure
- Altered food webs
| - Altered community structure
- Altered food webs
|
| Ecosystem | - Reduced species diversity
- Impacts on photosynthesis
| - Ecosystem fragmentation
- Reduced species diversity
|
Table 3: Examples of climate change and habitat loss impacts in the nature level of organisations due to human activities.
Levels of Organisation - Key takeaways
- Levels of organisation are the hierarchical structure of nature.
- A population is a group of individuals of the same species living in an area. Populations vary by size, density, and dispersion.
- A community comprises all the different species populations living in the same area. Every species has its own niche, and the different populations interact with each other.
- An ecosystem is made up of the community of organisms and the abiotic factors that affect that area.
- Human activities that cause climate change and habitat loss can impact populations, communities, and ecosystems.
1. CIA, United Kingdom, The World Factbook, 2022
2. CGP, AQA A-Level Biology, 2015
3. Luca Santini, Population density estimates for terrestrial mammal species, Global Ecology and Biogeography, 2022
4. Mark A. McDonald, Passive acoustic methods applied to fin whale population density estimation, The Journal of the Acoustical Society of America, 1999
5. Markus I. Eronen, Levels of Organization in Biology, Stanford Encyclopedia of Philosophy, 2018
6. Neil Campbell, Biology: A Global Approach Eleventh Edition, 2018
7. USGS, The Impacts of Climate Change on Phenology: A Synthesis and Path Forward for Adaptive Management in the Pacific Northwest, Climate Adaptation Science Centers, 2016
8. Welsh Government, Sheep, Business Wales, 2022
9. The World Bank, Population density - United Kingdom, 2021
10. WWF, Snow Leopard, 2022
Explanations 
Exams 
Magazine 
