The fauna is constituted by the whole of the animal species that populate a determined biological environment, a region, a geographic territory, or that has characterized a determined geologic period; it can comprise the autochthonous species and the immigrated species become by now indigenous, as well as the species introduced by the man and gone encounter to indigenation; they do not make part of the fauna the domestic and breeding animals.
The fauna is distinguished, according to biological environments, in marine, inland waters, terrestrial, tropical, temperate zone, glacial, desert. The terrestrial fauna includes animals that live on land and animals that fly (avifauna). The composition of the fauna depends not only on environmental factors, but also on the relationships established between the various species and above all on the flora present, to which it is linked by primary alimentary needs. Man’s intervention, modifying most of the natural environments, has caused and still causes a drastic reduction (up to total disappearance) of the typical faunas and has directly caused the destruction of many animal species (for example bison). Zoogeography is concerned with the distribution of fauna on Earth.
The fauna of a territory, in fact, is not static but dynamic, that is it changes with the passing of time because of the processes of extinction, evolution, speciation and substitution, determined by natural and more and more anthropic factors.
In order to belong to a fauna, a species or a population must be an integral part of the ecosystem that hosts it, it must self-maintain and be perfectly inserted in a food chain and therefore within those energy flows that regulate the balance of that ecosystem. For this reason, only resident species or species of habitual transit (migratory animals that return to that place more or less regularly), that is, those that participate in the mechanisms of the biocenosis, are part of it. The concept of fauna, always understood scientifically, is therefore equivalent to the term “wild fauna”, since, for the reasons just mentioned, there cannot be a “domestic fauna”.
The elements making up the fauna of a territory can thus be divided into two categories: autochthonous species and allochthonous species.
The autochthonous species are those originally present in the region, which have therefore undergone a speciation in the place where they are found starting from elements supplied by the territory itself. Endemic species belong to this category.
Introduced species (allochthonous) are those which originated in other regions and which, subsequently, have immigrated or have been introduced for anthropic reasons in that territory, finding ecosystems suitable for their maintenance and entering the energy flows which regulate their balance, often to the disadvantage of autochthonous species. The latter can be divided into:
- accidental – animals that escape from breeding (e.g. the nutria and the American mink for the Italian fauna)
- intentional – such as the American turtle, the red shrimp, the trout perch and the many fish species imported from North America and other continents to populate the inland waters of other countries for sport and fee fishing, etc..
Subdivisions on the basis of region
- Cryofauna: refers to the animals that live in, or very close to, cold areas.
- Cryptofauna: the fauna that exist in protected or concealed microhabitats.
- Epifauna: also called epibenthos, are aquatic animals that live on the bottom substratum as opposed to within it, that is, the benthic fauna that live on top of the sediment surface at the seafloor.
- Infauna: are benthic organisms that live within the bottom substratum of a water body, especially within the bottom-most oceanic sediments, the layer of small particles at the bottom of a body of water, rather than on its surface. Bacteria and microalgae may also live in the interstices of bottom sediments. In general, infaunal animals become progressively smaller and less abundant with increasing water depth and distance from shore, whereas bacteria show more constancy in abundance, tending toward one million cells per milliliter of interstitial seawater.
- Limnofauna: refers to the animals that live in fresh water.
- Macrofauna: benthic or soil organisms which are retained on a 0.5 mm sieve. Studies in the deep sea define macrofauna as animals retained on a 0.3 mm sieve to account for the small size of many of the taxa.
- Megafauna: is a collective term to describe large animals that exist or have existed. Generally, a “megafauna” is defined as an animal weighing more than 100 lb, often rounded up in the metric system to 40-45 kg.
- Meiofauna: small benthic invertebrates that live in both marine and freshwater environments. The term meiofauna loosely defines a group of organisms by their size, larger than microfauna but smaller than macrofauna, rather than a taxonomic grouping. One environment for meiofauna is between grains of damp sand (see Mystacocarida).
- Mesofauna: macroscopic soil animals such as arthropods or nematodes. Mesofauna are extremely diverse; considering just the springtails (Collembola), as of 1998, approximately 6,500 species had been identified.
- Microfauna: microscopic or very small animals (usually including protozoans and very small animals such as rotifers). To qualify as microfauna, an organism must exhibit animal-like characteristics, as opposed to microflora, which are more plant-like.
- Stygofauna: any fauna that live in groundwater systems or aquifers, such as caves, fissures and vugs. Stygofauna and troglofauna are the two types of subterranean fauna (based on life-history). Both are associated with subterranean environments – stygofauna are associated with water, and troglofauna with caves and spaces above the water table. Stygofauna can live within freshwater aquifers and within the pore spaces of limestone, calcrete or laterite, whilst larger animals can be found in cave waters and wells. Stygofaunal animals, like troglofauna, are divided into three groups based on their life history – stygophiles, stygoxenes, and stygobites.
- Troglofauna: small cave-dwelling animals that have adapted to their dark surroundings. Troglofauna and stygofauna are the two types of subterranean fauna (based on life-history). Both are associated with subterranean environments – troglofauna are associated with caves and spaces above the water table and stygofauna with water. Troglofaunal species include spiders, insects, myriapods and others. Some troglofauna live permanently underground and cannot survive outside the cave environment. Troglofauna adaptations and characteristics include a heightened sense of hearing, touch and smell. Loss of under-used senses is apparent in the lack of pigmentation as well as eyesight in most troglofauna. Troglofauna insects may exhibit a lack of wings and longer appendages.
- Xenofauna: theoretically, are alien organisms that can be described as animal analogues. As of the current day, no alien life forms, animal or otherwise, are known to exist. Despite this, the idea of alien life remains a popular subject of interest in the fields of astronomy, astrobiology, biochemistry, evolutionary biology, science fiction, and philosophy.
Characteristics of fauna
The fundamental characteristics of fauna are dynamism, historicity and interactivity. The fauna of a territory changes with the passage of time due to the processes of extinction, evolution, speciation and substitution, determined by natural factors and today more and more by anthropic factors.
Farm animals and domestic animals are not part of the fauna, which is intended purely wild. In fact, to belong to a fauna a species or an animal population must be an integral part of the ecosystem that hosts it. It must self-maintain and find itself perfectly inserted in a food chain and therefore within the balance of an ecosystem. This includes resident species or species of habitual transit (migratory animals that return to that place more or less regularly).
Dynamics of fauna
All animal species are sensitive to changes in environmental factors. Many of them, however, are necessarily linked to rigid environmental conditions and are defined in zoology as stenocore species. Others instead, more or less able to adapt to changes, populate more extensive territories and are defined as eurychore species. There is therefore a cosmopolitan fauna and as many distinct faunas as there are environments and microenvironments. We recognize a marine, lacustrine, and terrestrial fauna. According to the environments and their climates we recognize a tropical, temperate, glacial, desert, alpine, savanna, tundra, prairie fauna.