Biodiversity Defined
Biodiversity is the term used to describe the full variety of life on Earth, encompassing all living organisms and the ecological systems they form. As our scientific understanding develops, so too does the way in which life is classified. Since 2015, many biologists have recognised seven major biological kingdoms, as proposed by Thomas Cavalier-Smith. Ongoing advances in molecular biology and genetics continue to reveal new organisms and metabolic pathways, meaning that further refinements and reclassifications are likely in the future.
The Plant Kingdom
Plants are predominantly photosynthetic, multicellular eukaryotic organisms, meaning their genetic material (DNA) is contained within a membrane-bound nucleus. Photosynthesis takes place in specialised cell structures called chloroplasts, which capture energy from sunlight to produce nutrients using water and carbon dioxide, releasing oxygen as a by-product.
Although most plants obtain their energy through photosynthesis, some have evolved additional strategies, such as carnivory. Examples include sundews (Drosera spp.) and pitcher plants such as Nepenthes spp., which supplement nutrient intake by trapping insects.
Plant cells are further distinguished by the presence of a rigid cell wall containing cellulose, providing structural support. In modern plants, reproduction is commonly sexual, occurring through the production of seeds (as in flowering plants) or spores (as in ferns, mosses, and liverworts). Many species can also reproduce vegetatively through asexual means, producing stolons—above-ground horizontal stems that generate new plants at their tips—or rhizomes, which are underground stems capable of producing new growth.
From an ecological perspective, stolons and rhizomes may be viewed not as independent new plants, but as extensions of an existing organism seeking to exploit new resources. For example, a mature plant growing in increasing shade may extend growth into a brighter area to improve photosynthesis. In such cases, resources such as water and nutrients can be shared, with older plant tissues supporting newer growth while benefiting from improved energy capture.
The Animal Kingdom
Animals are multicellular eukaryotic organisms whose cells contain a nucleus and other organelles, but lack both cell walls and chloroplasts. Most animals are capable of movement at some stage in their life cycle and typically reproduce sexually. This kingdom includes organisms ranging from large mammals to microscopic invertebrates and insects.
Animals are heterotrophic, meaning they obtain their nutrition from complex organic substances, whether living or dead. This contrasts with plants, which are generally autotrophic and able to manufacture their own food from simple inorganic substances such as carbon dioxide.
Most animals possess some form of nervous system, ranging from simple nerve networks to complex systems involving a brain, spinal cord, and extensive peripheral nerves. These systems allow animals to respond rapidly to their environment and to coordinate movement and behaviour.
The Fungi Kingdom
Fungi—including mushrooms, yeasts, and moulds—share characteristics with both plants and animals. Their cell walls contain chitin, a strong fibrous polysaccharide also found in the exoskeletons of arthropods. Unlike plants, fungi lack chloroplasts and cannot photosynthesise.
Many fungi reproduce by spores, a trait they share with certain plant groups. Multicellular fungi are generally immobile and grow as fine filaments known as hyphae, which collectively form a mycelial network. This network spreads through soil, wood, or organic matter, absorbing dissolved nutrients.
Fungi play a crucial ecological role as decomposers and are also believed to have been instrumental in enabling early plants to colonise land from aquatic environments, through ancient symbiotic relationships similar to modern mycorrhizal associations.
The Bacteria Kingdom
Bacteria, sometimes referred to as eubacteria, are single-celled organisms with cell walls but no membrane-bound nucleus or organelles such as mitochondria. Their genetic material is free within the cell, making them prokaryotes. Bacteria are among the earliest forms of life on Earth, having evolved approximately four billion years ago.
They are extraordinarily abundant and occupy almost every habitat on the planet. In ecosystems, bacteria play vital roles as decomposers, nutrient recyclers, and symbiotic partners, although some species are also responsible for disease in plants and animals.
The Archaea Kingdom
Archaea, sometimes called archaebacteria, are superficially similar to bacteria in that they are single-celled and lack a nucleus. However, they differ significantly in their molecular structure and metabolic processes. Archaea are thought to represent an ancient lineage distinct from both bacteria and eukaryotes.
They are often found in extreme environments, such as very high or low temperatures, highly saline waters, or acidic conditions. Despite this, many archaea also occur in more moderate environments, including soils and the digestive systems of animals.
The Protozoa Kingdom
Protozoa are microscopic, single-celled eukaryotic organisms whose DNA is enclosed within a membrane-bound nucleus. They are often described as animal-like due to their heterotrophic feeding behaviour. Examples include amoebae, ciliates, sporozoans, and flagellates.
Historically, protozoa were grouped within a broad category known as Protista. More recent classifications tend to separate these organisms into Protozoa and Chromista, reflecting differences in their cellular structure and nutritional strategies.
The Chromista Kingdom
Chromista includes single-celled and multicellular eukaryotic organisms that possess plastids enabling photosynthesis. This kingdom includes groups such as diatoms, brown algae, and certain red and green algae. Many chromists are primary producers in aquatic ecosystems and play a fundamental role in global carbon cycling and oxygen production.
Biodiversity and the Garden Ecosystem
A glance at the kingdoms described above reveals that our knowledge tends to diminish as organisms become smaller and less visible. Plants and animals, being more conspicuous and aesthetically appealing, receive the most attention. Fungi are often noticed through their striking and sometimes colourful fruiting bodies. The remaining kingdoms, made up largely of microscopic organisms, are less understood and frequently overlooked.
Yet these less visible forms of life are essential to the functioning of healthy ecosystems. In wildlife-friendly gardens, bacteria, fungi, protozoa, and archaea underpin soil health, nutrient cycling, and plant resilience. An appreciation of biodiversity therefore extends beyond what we can easily see, reminding us that thoughtful gardening practices support a vast and interconnected web of life.