Species distribution, or species dispersion, describes how a type of living thing is spread out in different areas. The area where a species can be found is called its range, which is often shown as shaded areas on a map. The way species are spread out can look different depending on the level of detail being studied, such as how individuals are arranged in a small group, patterns within a larger group of the same species, or the overall spread of the entire species. Species distribution should not be confused with dispersal, which refers to the movement of individuals away from where they originally lived or from a place where many of their kind are closely grouped.

Range

In biology, the range of a species is the area where that species lives. Within this range, distribution describes how the population is spread out overall, while dispersion refers to how densely the population is found in different parts of the range.

Range is often described using these qualities:

• A species may have a natural, endemic, indigenous, or native range, which is where it has lived historically. It may also have a new range, such as non-native, naturalized, introduced, transplanted, invasive, or colonized range. Introduced usually means the species was moved by humans, either on purpose or by accident, across a large geographic barrier.
• For species that live in different areas during different seasons, terms like summer range and winter range are used.
• For species that breed in only part of their range, the terms breeding range and non-breeding range are used.
• For animals that move often, the term natural range is used, compared to areas where the species is found only occasionally.
• Qualifiers like geographic or time-related terms are added, such as British range or pre-1950 range. Common geographic ranges include latitudinal range and elevational range.

Disjunct distribution happens when two or more parts of a species' range are far apart from each other.

Factors affecting species distribution

Distribution patterns can change based on the season, human activity, the availability of resources, and other non-living and living factors.

There are three main types of non-living factors:
1. Climatic factors include sunlight, air, humidity, temperature, and salt levels;
2. Soil-related factors involve characteristics of soil, such as how coarse the soil is, the type of rock nearby, soil acidity, and how well air moves through the soil; and
3. Social factors include how land is used and the availability of water.

An example of how non-living factors affect species distribution is seen in dry areas, where most members of a species gather near water sources, forming a clumped pattern.

Researchers from the Arctic Ocean Diversity (ARCOD) project have recorded more warm-water crustaceans in the waters around Norway’s Svalbard Islands. ARCOD is part of the Census of Marine Life, a 10-year project involving scientists from over 80 countries. The project aims to study the variety, spread, and numbers of ocean life. Marine life has been greatly affected by global climate change. This study shows that as ocean temperatures rise, species are moving into colder Arctic waters. For example, snow crabs have expanded their range 500 km farther north.

Living factors, such as predation, disease, and competition for resources like food, water, and mates, also influence species distribution. For example, living factors in a quail’s environment include its food (insects and seeds), competition from other quails, and predators like coyotes. A clumped arrangement, such as in a herd or group, helps a population spot predators earlier and respond better. In areas with limited resources, populations may spread out evenly to reduce competition, as seen in forests where trees grow evenly to share sunlight.

A key factor in determining species distribution is the timing of an organism’s life cycle events, such as when it reproduces or grows. Plants are often used as examples because their life cycle timing can help them survive in changing climates. How an organism’s body functions can influence its distribution because these functions affect movement, exploration, and survival. Individuals that move more often tend to have higher energy levels, better movement abilities, and stronger immune systems.

Humans are a major cause of species distribution due to globalization and the growth of transportation. For example, large ships often take in water at one port and release it at another, spreading aquatic species to new areas.

Patterns on large scales

On a large scale, the way individuals are spread out in a population is often grouped together.

A common example of where birds live is near water, such as along the edges of oceans, rivers, or lakes. These areas are called coastal strips. Another example is when birds depend on water sources like rivers or swamps, or forests near water, and live in areas called river corridors. A river corridor may include the entire area drained by a river, with mountains or higher ground forming the edges of the habitat. In this case, the river itself is only a small part of the corridor, but the corridor exists because of the river.

Another type of bird habitat is a mountain range corridor. In North America, the Sierra Nevada mountains in the west and the Appalachian Mountains in the east are examples of this. These areas are used by different bird species during summer and winter for different reasons.

Birds in these corridors may live in a large, connected area (called a contiguous range) or in a smaller, separated area (called a disjunct range). If birds leave these areas to migrate, they may travel through connected land or over land that is not part of the corridor. In the latter case, they are called passage migrants, stopping briefly on land that is not linked to the wildlife corridor.

Patterns on small scales

On large scales, the way individuals are spread out in a population is often clumped together. On smaller scales, the pattern can be clumped, evenly spaced, or random.

Clumped distribution, also called aggregated distribution, is the most common type of spread found in nature. In clumped distribution, individuals are close to one another. This happens in environments where resources are unevenly spread. Animals need certain resources to survive, and when these resources are scarce, they gather near them. Individuals may group together because of social behaviors, such as family groups or protective herds. Prey animals often form clumped groups in areas where they can hide and spot predators easily.

Another reason for clumped distribution is when young animals cannot move far from their parents. For example, bald eagle eaglets stay in their nest until they can fly. Clumped groups can help individuals survive, but in some cases, like with cows and wildebeests, overgrazing may harm the plants they depend on.

Clumped distribution can also help animals avoid predators or catch prey. African wild dogs hunt together in groups, which increases their chances of catching food. During dry seasons in Africa, animals like lions, hyenas, and gazelles gather near limited water sources. Studies show that species at risk of extinction are often found in clumped groups because they share traits that make them vulnerable to threats in similar habitats.

A contiguous distribution is a type of clumped distribution where all individuals are in one group.

Uniform distribution, which is less common, happens when individuals are evenly spaced apart. This occurs when animals or plants need to keep distance from each other, such as when competing for resources like water or nutrients. For example, penguins defend their space aggressively, and gerbil burrows are regularly spaced. Some plants, like creosote bushes, grow in uniform patterns because they release chemicals that stop other plants from growing nearby. This process is called allelopathy. Allelopathy can help or harm other plants, and some plants use it to limit competition. Farmers sometimes use similar methods to arrange crops in rows.

Random distribution, the least common type, happens when individuals are spread out without a clear pattern. This occurs when environmental conditions are consistent and individuals do not influence each other’s positions. For example, dandelion seeds blown by the wind may land randomly, and oyster larvae carried by ocean currents may spread out over large areas. Random distribution can sometimes form small, accidental clusters.

Statistical determination of distribution patterns

There are several ways to study how species are spread out in an area. One method is the Clark–Evans nearest neighbor method, which helps scientists determine if a species is clumped together, evenly spaced, or randomly scattered. To use this method, researchers study a group of the same species. They measure the distance from each individual to its closest neighbor. If two individuals are each other's nearest neighbor, the distance is recorded twice, once for each individual. For accurate results, scientists should measure the distance for at least 50 individuals. The average distance between neighbors is compared to the expected distance if the distribution were random. This comparison creates a ratio:

If the ratio (R) equals 1, the species is randomly spread out. If R is much greater than 1, the species is evenly spaced. If R is much less than 1, the species is clumped together. Scientists use statistical tests, such as t-tests or chi-squared tests, to determine if R is significantly different from 1.

Another method is the variance/mean ratio. This method mainly checks if a species is randomly spaced but can also suggest if it is evenly spaced or clumped. To use this method, scientists collect data from many random samples of a population. At least 50 sample plots should be studied. The number of individuals in each sample is compared to the expected number if the distribution were random. The expected number can be calculated using a Poisson distribution. If the variance/mean ratio equals 1, the species is randomly distributed. If the ratio is much greater than 1, the species is clumped. If the ratio is much less than 1, the species is evenly spaced. Scientists use tests like the t-test or chi-squared test to check if the ratio is significantly different from 1.

Some scientists believe that models based only on statistical analysis, without including ecological theories, may not be complete enough for predicting species distribution. Instead of using simple presence-absence data, models that show the probability of a species being in a certain area are preferred. These models include confidence levels about the likelihood of the species being present or absent. They are more useful than simple presence-absence data because they allow scientists to create maps showing where a species is likely to be found. These maps help compare similar areas and show how habitat suitability relates to where a species is likely to occur.

Species distribution models

Species distribution can be predicted by looking at how biodiversity changes across different areas. A general model can combine factors like disturbances, how species move, and how populations grow. By considering how species spread, disturbances, limited resources, and other species, scientists can create a bio-climate range, which shows the conditions where a species can live. This range can be studied at a local or global level and can depend on how many individuals of a species are present. The model considers the needs of species, how they affect their environment, and how disturbances, such as fires or floods, cause local extinctions. These models combine three types: a model for how species move, a model for disturbances, and a model for how many individuals of a species are present. Species distribution models (SDMs) help scientists understand how climate change affects species and how to protect them. These models include: models that show where a species is present or absent, models that show how species move, models that show how disturbances affect species, and models that show how many individuals of a species are present. A common way to create maps showing where species might live is to use land cover data and decide if a species is likely to live in each type of land cover. This simple model is often improved by using extra information, such as elevation or distance to water.

Recent studies show that the size of the grid used in models can affect the results. For example, a standard grid size of 50×50 km can cover up to 2.89 times more area than a 1×1 km grid for the same species. This can influence how scientists plan to protect species under climate change. Many climate models used in SDMs use grids that are 50–100 km in size. This may cause predictions to show larger future ranges for species than is accurate. This could lead to mistakes in identifying areas that should be protected for a species’ future habitat.

Species Distribution Grids Project

The Species Distribution Grids Project is a project started by the University of Columbia to make maps and databases showing where different animal species live. This work focuses on stopping deforestation and identifying areas with many types of species. As of April 2009, data is available for the global locations of amphibians, as well as birds and mammals in the Americas. The map gallery called Gridded Species Distribution includes example maps from the Species Grids data set. These maps do not show all data but instead display a sample of the types of information available for download:

• Species Richness Map (Amphibians)
• Species Richness Map (Birds)
• Species Richness Map (Mammals)