Fauna

Fauna refers to all animal life on Earth that currently exists or has existed in the past. Scientists generally study fauna in a particular region at a given time. Many scientists travel the world to study the living animals of the present. Others turn to alternative sources, such as the fossil record, to learn more about animal life of the distant past.

Animals have lived on Earth for billions of years, though for the large majority of this time, they were small and simple organisms. By studying the first fauna, and the development of the larger and more complex life forms that came later, scientists can learn much about the world, its environments, and the behaviors and adaptations of living things.

To make this large-scale study easier to conduct, scientists have divided the extremely broad category of fauna into many smaller subcategories based on criteria such as animal appearance, time period of existence, or regions of habitation. An early innovator of the study of fauna was eighteenth-century Swedish scientist Carl Linnaeus who created the basis of the modern classification systems for animals and other living beings. In 2024, studies published by the World Wildlife Fund and the Zoological Society of London reported that vertebrate wildlife populations had declined 73 percent between 1970 and 2020.

Background

Earth is the only planet scientists currently know of that contains life. However, for millions of years after its formation, the planet was inhospitable to living things. According to the Smithsonian National Museum of Natural History, the first evidence of life can be traced back some 3.7 billion years to microbes, or microscopic creatures, which left lasting traces in rocks. At this time, very few forms of life could exist on the planet due to its low amounts of oxygen and high quantities of methane.

About 2.4 billion years ago, tiny creatures called cyanobacteria developed. These were the first living organism to use photosynthesis, the creation of food using sunlight. This process created oxygen as a by-product. As the cyanobacteria population swelled across the oceans, so too did the amount of oxygen it produced. This world-changing development, over billions of years, created the oxygen-rich atmosphere that most modern living things need to survive.

Even then, most living things were simple, tiny creatures. The first true animals with complex bodies likely did not appear until about 800 million years ago, and then probably took the form of basic sea sponges. From that point, the process of change and development occurred relatively quickly. Increasingly complex animals of all sorts began to populate Earth.

In modern times, trillions of living organisms occupy the planet’s land and water. Some of these living things, like whales and Redwood trees, are enormous, while others are so small they cannot be seen without magnification. Over time, scientists have attempted to study and classify these varied forms of life. By modern times, science has placed living things into three major categories: animals, plants, and fungi. The more formal scientific terms for these categories are fauna (animals), flora (plants), and funga (fungi). Many scientists recognize two additional groups, protists and bacteria.

The word “fauna” is derived from the mythological gods and goddesses of ancient Rome. Fauna was the goddess of fertility, with powers that expanded across all parts of nature. She controlled the health and reproduction of animals in farms and forests. A closely related ancient Roman deity was Faunus, based on the Greek god Pan. Often represented as a mixture between a human and a goat, Faunus promoted the fertility of many kinds of living things, but is most famous for his powers in woodland areas. In the Lupercalia festival, young Romans donned goatskin disguises and raced through the streets cracking whips at one another in a spirited celebration of fertility.

Long after the ancient gods were relegated to children’s stories, the term fauna reappeared with its modern meaning about the eighteenth century. Likely the most influential person in the popularization of this term was Swedish scientist Carl Linnaeus who dedicated much of his career to taxonomy, or the categorization and naming of living things. Dismayed by the bulky classification systems of the day, Linnaeus devised a new system based on taxa, or categories of progressing sizes. Primary among these were genera, orders, classes, and kingdoms.

Followers of this system would thereafter be able to classify living things by comparing them with similar members of the various taxa. Although later scientists would question some of his methods and refine some of his techniques, Linnaeus left an enduring scientific legacy and forever changed the study of living things on Earth.

Overview

The term fauna refers to all of the animals that exist, or once existed, in a given time and place. Scientists most commonly study the fauna of contemporary times, as these can be directly observed. For example, a scientist may travel to the African country of Tanzania to study the unique wildlife of the Serengeti ecosystem. Meanwhile, many other scientists seek to learn about fauna of the past thousands, millions, or even billions of years, generally through traces left in Earth’s extensive fossil record.

Faunistics, or the study of fauna in a given region, is an extremely wide field of investigation. It covers every type of animal known to exist, or to have existed in the past, in all the past or present environments of Earth. To make this field of study more manageable, scientists have divided it into many specialties. Within each specialty, scientists focus on animals of one type, from one area, from one time period, or with some other important characteristic in common. Following are some examples of fauna categories.

Categorization by Size

Several categories of fauna are distinguished by their varying physical sizes. The smallest of these are microfauna, which are microscopic or extremely small organisms. Animals at this level are generally very simple, and some may tend to show the characteristics of plants. However, only animal-like organisms are included in this category. Animals that are a step larger in size, but still very small, are often grouped within the category of meiofauna. These animals generally live in watery areas, sometimes within sand. Researchers may measure these kinds of tiny animals based on whether they pass through a fine sieve. The next major step up in size is reflected in the category of macrofauna, which includes creatures that are larger than either 0.3 millimeters or 0.5 millimeters, depending on the form of research being conducted.

These levels are all meant to include relatively small animals, from microscopic through the approximate size of a small human. Animals that are larger than may be grouped into the category of megafauna. The exact requirements of megafauna may differ between contexts and forms of research. Some scientists consider megafauna to be any animal roughly human-sized or larger. Meanwhile, other scientists reserve the label of megafauna for exceptionally large animals, such as elephants and whales.

Megafauna may relate to animals of the modern world. However, some of the most striking examples of megafauna belong to the prehistoric world. The most famous examples of long-extinct megafauna were the dinosaurs. One of the best-known of these creatures, the Tyrannosaurus rex, could reach up to 40 feet long and weigh about nine tons—and it was far from the largest. A much larger dinosaur, widely believed to be the largest to ever exist, was the Patagotitan mayorum, a towering creature whose long neck and tail could stretch its body length to a staggering 120 feet. Following the age of dinosaurs, during the Pleistocene Epoch that only ended about ten thousand years ago, giant versions of modern animals roamed Earth. Some of these included mammoths, cave lions, sabre-toothed tigers, and giant sloths, which largely resembled the large animals of modern times, but on an even larger size scale.

Categorization by Habitat

In addition to size, habitat may also be used as a means of categorizing fauna. For example, the category of cryofauna includes animals that inhabit Earth’s coldest locations. A few animals from these regions include the walrus, polar bear, penguin, puffin, and seal. All of these creatures share characteristics that allow them to live in temperatures that would be inhospitable, or even deadly, for other kinds of animals. Meanwhile, cryptofauna refers to animals that live in small, often self-contained habitats that may be difficult to find or observe. Many of these animals are extremely rare and poorly understood. Some may be on the verge of extinction. A few animals that may be considered part of this category include a rare small variety of Asian rhinoceros known as the Sumatran rhinoceros and a type of solitary cold-weather wildcat known as the Amur leopard that only lives in a small area of Russia and China.

Many of the world’s animals live underground, and may be categorized as such. Two of these categories are stygofauna and troglofauna. Animals of the stygofauna group live mainly in groundwater formations. Meanwhile, animals of the troglofauna group also live underground, but in drier areas, such as caves. Animals that developed in subterranean habitats have undergone evolutionary processes to make them more able to adapt to the unique demands of underground life. In particular, these animals must compensate for the total or near-total lack of light under the earth. To do this, animals may develop intensified capabilities in their other senses, such as smell, touch, and hearing. Some animals accustomed to life deep underground have lost their skin pigment (in some cases becoming transparent) and all ability to see. Insects that evolve for life underground may eventually evolve into wingless forms. Members of the stygofauna and troglofauna groups are so accustomed to their natural habitats that many cannot survive on the planet’s surface.

An enormous percentage of the living things on Earth inhabit that planet’s extensive waters. Scientists have created many categories to describe animals that live in different kinds and levels of water and water-based environments. For example, animals that live on the surface of the sea are categorized as epifauna. Meanwhile, animals that live below the surface of the sea, such as in ocean sediments, are categorized as infauna. Although their habitats are in some cases very close, these animals have evolved in different ways. Epifauna often grow much larger and have body parts needed for swimming and other activities. Meanwhile, infauna is generally much smaller, and may even be microscopic. Creatures of the infauna may not be able to swim, but instead have capabilities for burrowing into silt. Some of these creatures include ghost shrimp and various types of sea worms.

Categorization by Type

Other categories of fauna are based on the actual form of the animals. This includes their appearance and behaviors. For example, all fish-like creatures may be grouped into the category of ichthyofauna. Alternately, creatures that exhibit strong bird-like traits may be considered part of the avifauna category. This level of categorization is generally the easiest to observe and understand, although many specialists may disagree on the exact traits of certain animals. For instance, the platypus is an unusual creature that exhibits a wide variety of traits that blur the distinction between land and water animals.

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