The biological hierarchy of life is the organisation of all living things on earth. As life itself is very complex, scientists use a process called reductionism to simplify it.

Reductionism is just science jargon for reducing complex things into smaller, simpler parts. They do this to better understand how complicated systems work, and how things within those systems relate to one another.

The biological hierarchy organises life into 12 different levels.  Starting with the atom, the biological hierarchy makes it easier to understand how parts work together to create life as we know it.


The atom is the smallest level of all living things on earth.

Atoms are very tiny particles that combine together to create elements. Elements are the simplest form of a substance, and there are a total of 118 that can be found in the Periodic Table. Elements are made of only one type of atom, for example the Oxygen element is two atoms of oxygen bonded together.

When more than two atoms bond together, we reach the next stage of the biological hierarchy – the molecule.


A molecule is the second smallest particle of life, made up of two or more atoms bonded together.

There are many different types of molecules, but biomolecules are the most important molecules of life. This is because they are involved in the maintenance and function of all living organisms.

The four main biomolecules are proteins, lipids, carbohydrates and nucleic acids.

When molecules group together, they form different parts of the cell. This leads us to our next stage – organelles.


An organelle is made up of a combination of molecules, that form essential parts of a cell. These organelles each have their own special function and are crucial to the existence of life.

In plants, an example of an important organelle is the chloroplast. This is where plants generate their food and energy. Other organelles include the nucleus, which is the “control centre” of the cell, and mitochondria, the “powerhouse” of the cell.

Biological hierarchy - the cells


A cell is composed of different organelles, like the nucleus and mitochondria, and each cell performs a specific function. There are many different types of cells, including skin cells and nerve cells, that work together in functional groups.

Many living things are single celled, like bacteria, and they’re called unicellular organisms. Humans, animals, fungi and plants however are multicellular organisms. Multicellular organisms need many different cells working together to survive.

When cells work together in functional groups, they form the next level of life – tissues.


Tissues are a combination of many different cells. These cells that interact and bond together to support living organisms. This is the level of the biological hierarchy that we are finally able to SEE.

Connective tissue, nerve tissue muscular tissue and epithelial tissue (skin) are all examples of tissue in the human body.


The next level is the organ. Organs are groups of tissues working together to form a specific function.

You probably know many of the organs that exist in your body, like your heart and brain. However, plants have organs too, including the roots, leaves, and the stem.

When organs work together to perform a specific function, like the digestion of food in our body, they form an organ system.

Biological hierarchy - the organ systemLEVEL SEVEN: THE ORGAN SYSTEM

In humans and animals, organ systems include the digestive system, the skeletal system, the respiratory system in humans and animals. In plants they include the root system and the shoot system.

Organ systems work in collaboration to perform different functions, and lead us to the last stage of the biological hierarchy in the realm of biology – the organism.


An organism is the basic living system, made up of a variety of organ systems working together. An organism can be an individual animal, plant or even a single-celled (unicellular) life form like bacteria.

It is this point in the hierarchy of life that we ca begin to classify organisms into different species.


This is where we move from biology, into ecology. Moving away from single organisms, this level takes an ecological view of organisms, classifying them into what is known as a population.

A population is a collection of the same species that live and interact in the same area. In science jargon, this is called an “intraspecific” grouping.

The biological hierarchy - communityLEVEL TEN: COMMUNITY

When we think about it logically, one area will never just have one species. There are often multiple populations that co-exist in the same area, which brings us to our next level, the community.

A community has many different interacting populations in one area.  There are many different kinds of communities, where different populations of fungi, plants and animals and unicellular organisms all exist. This type of grouping in known in science as “interspecific.”


The eleventh level of the hierarchy is often debated. Some refer to this level as an ecosystem, and others call it a biome. For the purposes of looking at life in it’s entirety, my belief is that we should refer to this level as an ecosystem.

A biome considers only living things, whereas an ecosystem considers both living (biotic) and non living (abiotic). Non-living components, like minerals, the soil, water and air are important to how life functions. They should not be forgotten when analysing the hierarchy of life.

Major ecosystems include forests, grasslands, savannas and rainforests, yet there are also much smaller, microscopic bacterial ecosystems that are also fundamental to life on earth.

Biological hierarchy - ecosphereLEVEL TWELVE: ECOSPHERE

The all-encompassing and final level of the biological hierarchy is the ecosphere, sometimes referred to as the biosphere. In the same vein as the debate between ecosystem and biome, an ecosphere includes abiotic and biotic life forms, whilst the term biosphere technically only refers to living things.

The ecosphere describes life as one whole unit, a collective of all ecosystems, populations, cells, tissues, communities and organisms. It includes the collaboration and cooperation of each level, as well as the interaction between all living levels and their non-living environments.

Together, these twelve levels of life make up the incredible earth we are so lucky to call home.