Population fragmentation is a type of population segregation. It happens when habitats are broken into smaller pieces, causing a population to split into smaller, separate groups. This separation can lead to changes in genetic traits and increased mating among closely related individuals within these groups.

Causes

Population fragmentation happens when habitats are lost or damaged, which reduces the number of animals and makes it harder for them to move between groups. This damage can come from natural events or, especially today, human activities. Common reasons for fragmentation include:

• Building roads or fences near protected areas, even small changes like adding one road lane or a fence line.
• Capturing or killing animals that help connect different groups of the same species.
• Animals moving away from others of their kind, such as wolves and moose on Isle Royale.
• Natural events like landslides or volcanoes that split habitats.
• Rising sea levels causing islands to become separated from the land they were once part of.
• Global warming, especially when mountains block movement between habitats.

Genetic effects

Population fragmentation can cause serious genetic problems in species. One major issue is inbreeding depression, which happens when closely related animals breed. This reduces genetic diversity within small, isolated groups. Lower genetic diversity harms a population in two ways. First, inbreeding increases competition between related individuals, making the whole species less healthy. Second, it raises the chance of harmful traits, like those that reduce litter size, appearing more often. These traits can shrink the population further.

Small populations are also more affected by genetic drift, which causes random changes in gene frequency. This leads to more individuals having the same genes, which weakens their health. Because small populations often have related members, inbreeding is more common. Natural selection may also be less effective in these groups, allowing harmful mutations to stay in the population. Over time, this reduces a species' ability to adapt to changes like climate shifts. Limited gene flow between groups makes it harder for species to survive.

The severity of genetic problems depends on how fragmented a population is. If a population splits into many equal-sized groups, gene flow might be similar across all groups. In other cases, gene movement may only happen between nearby groups, creating uneven genetic patterns. More complex setups might have a larger group acting as a source of genes for smaller groups. If populations are very isolated with little to no gene flow, they are more likely to go extinct. Genetic effects depend on how easily individuals can move between groups and how regularly genes are shared.

Although population bottlenecks from fragmentation usually lower genetic diversity, some species can still keep high diversity. Splitting into smaller groups, especially with low gene flow, can preserve the number of different genes present. However, this often means fewer differences between genes in individuals.

Habitat fragmentation also increases genetic differences between subpopulations. Physical barriers reduce gene flow, making groups more genetically distinct.

Proposed conservation solutions

When populations are broken into smaller groups, gene flow decreases. This can lead to problems like inbreeding, which weakens the health of species and increases the risk of extinction. However, more research is needed to fully understand how population fragmentation affects conservation efforts. Gene flow is the sharing of genes between groups, which helps create similar traits in organisms and can increase biodiversity by bringing new genetic traits into a population.

To reduce or prevent population fragmentation, scientists suggest several solutions related to human activities. One idea is removing barriers, such as fences or highways, which can block movement between habitats. This is especially important in cities, where roads and buildings can stop animals from moving freely between areas, forcing them to move to new places or form separate groups. However, removing barriers may not always be possible, especially in areas where cities are growing quickly.

Another solution is keeping habitats connected and maintaining high-quality environments, such as areas with good climate conditions and low pollution. These ideas were mainly for water-based animals, but they might not apply the same way to land animals. Climate change could make it harder to achieve these goals without laws to protect habitats. Scientists from another study also suggested opposing policies that harm habitats and limiting human access to prevent disturbances. These steps may require new laws to be effective.

Finally, more research is needed to understand how climate change and differences in species’ roles in their environments affect population fragmentation and conservation efforts.

Wildlife corridors

A major way to help reduce the problems caused by broken-up animal habitats is to create wildlife corridors. These corridors are special paths that connect areas of land that have been separated. They allow animals to move between places, find food and water, keep their genes healthy, and act naturally. How well these corridors work depends on several things, such as where they are built, how wide they are, and how they are designed. Studies of animals that move long distances in the United States show that corridors built along paths animals already use, and that look like natural areas, help animals cross roads more often than enclosed structures. Research also shows that corridors need to be wide enough for animals to move safely and avoid problems near the edges of the corridor. Keeping fences and other guides in good condition is also important to make sure the corridors work for a long time.

Examples from different areas show how wildlife corridors can help reduce the harm caused by broken-up animal populations.

• Banff National Park: Along the Trans-Canada Highway in Banff National Park, a system of wildlife overpasses and underpasses has helped animals move safely across the highway. Since these structures were built, accidents between cars and animals in the park have dropped by about 80%, and deaths of many species have decreased by 50–100%. Studies also show that these crossings help animals like grizzly bears and black bears move, find mates, and share genes across the highway.

• Florida Panther Project: One of the first examples of building wildlife crossings in the United States was along I-75 in Florida. This project helped protect panthers by creating 23 tunnels under the highway. These tunnels allowed panthers to move more freely, which helped them breed successfully and keep their population alive.