Introduction

There are six kingdoms into which all of Earth’s living things are commonly classified (Figure 3.1). Each kingdom represents a major branch of the tree of life, highlighting the diversity of living organisms on our planet.

Animalia kingdom includes all animals, classified as multicellular, eukaryotic organisms that are typically mobile and obtain energy by consuming other organisms. Plantae kingdom includes all plants, which are also classified as multicellular and eukaryotic organisms, however they produce their own food through photosynthesis. Fungi kingdom includes organisms such as mushrooms, moulds, and yeasts, which are mostly multicellular and obtain their food and nutrients through decomposition of organic matter. Protista kingdom includes single-cell organisms such as amoebas and algae, which can show similar characteristics of the three previously mentioned kingdoms. Archaea kingdom includes single-cell organisms that live in extreme environments such as hot springs and salt lakes. Archaea are similar to bacteria; however, they have a distinctly different genetic and biochemical composition. Bacteria kingdom includes all true bacteria, which are single-celled prokaryotic organisms. Clearly, there is a tremendous variety of life on Earth!

Biological diversity, or biodiversity, refers to the variety of life that exists on Earth. Biodiversity can be described and measured at three different levels: species, genetic, and ecosystem diversity. Species are groups of organisms that share common characteristics and can produce fertile offspring when interbreeding. Genetics is the study of how these common characteristics and traits are passed down from one generation to the next, primarily through studying genes. An ecosystem is the community of all living organisms interacting with one another and the physical environment, such as plants, animals, and microorganisms. 

Species diversity refers to the number of different species in an ecosystem or on Earth as a whole – this is the most common way to measure biodiversity. A high species diversity represents a wide range of different species which contribute to the resilience and stability of the ecosystem. Current estimates for Earth’s total number of living species range from 5 to 30 million different species. 

Genetic diversity therefore refers to the variation in genes within all these species. This includes differences in the DNA code among individuals resulting in variations of traits such as appearance, resistance to diseases, or behaviour. A high genetic diversity indicates that a specific population has a wide range of traits, assisting in adaptation to environmental changes and threats. Ecosystem diversity refers to the variety of ecosystems on Earth. An ecosystem is a system formed by populations of many different species interacting with each other and their environment. This can include a range of habitats, communities, and ecological processes such as forests or oceans. A high ecosystem diversity indicates a wide range of physical environments which supports various species and ecological interactions.

Biodiversity is most often measured by counting species, but what is a species? The answer to that question is not as straightforward as you might think. The formal biological definition of species is a group of actually or potentially interbreeding organisms. This means that members of the same species are similar enough to each other to produce fertile offspring together. By this definition of species, all human beings alive today belong to one species, Homo sapiens. All humans can potentially interbreed with each other but not with members of any other species. In the real world, it isn’t always possible to make the observations needed to determine whether different organisms can interbreed.

For one thing, many species reproduce asexually, so individuals never interbreed even with members of their own species. When studying extinct species represented by fossils, it is usually impossible to know whether different organisms could interbreed. Therefore, in practice, many biologists and virtually all palaeontologists generally define species on the basis of morphology, rather than breeding behaviour and outcome. Morphology refers to the form and structure of organisms. For classification purposes, it generally refers to relatively obvious physical traits. Typically, different organisms which appear more similar to one another have a greater chance to be classified in the same species.

People have been trying to classify the tremendous diversity of life on Earth for more than two thousand years. The science of classifying organisms is called taxonomy. Classification is an important step in understanding the present diversity and past evolutionary history of life on Earth. It helps make sense of the overwhelming diversity of living things.