Habitat fragmentation is when an animal's or plant's natural home is broken into separate parts, making it harder for populations to stay connected. This can harm ecosystems and lead to smaller, isolated groups of living things. Natural causes, like slow changes in the Earth's surface over time, can create these breaks. Human actions, such as changing land for farming or buildings, also cause habitat fragmentation quickly and can lead to changes in the number of animals or plants in an area. Specifically, this process divides large, connected habitats into smaller, separate areas where plants and animals live.

Definition

The term habitat fragmentation includes five different aspects:

• A decrease in the total area of the habitat
• A lower ratio of interior habitat to edges
• Separating one habitat area from other nearby habitats
• Dividing one habitat into many smaller areas
• A reduction in the average size of each habitat area

Habitat fragmentation not only reduces the amount of habitat but also changes how the remaining habitat functions (van den Berg et al. 2001). This process affects the entire landscape and the individual patches within it. It includes the size of habitat patches, the effects of edges between habitats, and the complexity of how patches are shaped.

In scientific studies, there is some discussion about whether the term "habitat fragmentation" refers only to splitting habitats into smaller pieces without much loss of total habitat area, or if it also includes the loss of habitat itself. Scientists who use the stricter definition would call habitat loss "habitat loss" and use both terms separately when describing situations where habitats become disconnected and their total area decreases.

Habitat fragmentation is considered a major threat to biodiversity because it affects many species more than other threats like invasive species, overuse of resources, or pollution. The effects of habitat fragmentation harm the ability of species, such as native plants, to adapt to changing environments. This can stop the transfer of genetic traits between generations, especially for species with small populations. Larger populations, however, may have more genetic changes and recombination, which can help them survive in their environments. Overall, habitat fragmentation causes habitat breakdown and habitat loss, both of which contribute to the loss of biodiversity.

Causes

Evidence of habitat destruction caused by natural events like volcanoes, fires, and changes in climate can be found in the fossil record. Studies show how individual species affect entire landscapes. For example, research found that deer eating plants in forests changed plant communities across the Rondeau Provincial Park from 1955 to 1978. Also, habitat fragmentation in tropical rainforests in Euramerica 300 million years ago caused a large loss of amphibian species, but a drier climate led to a rapid increase in reptile diversity.

Habitat fragmentation often happens when humans remove native plants for activities like farming, building homes, or creating reservoirs for hydroelectric power. Habitats that were once connected become broken into separate areas. Because of human actions, many tropical and temperate habitats are already highly fragmented, and fragmentation will likely increase in the future. After large areas of land are cleared, the remaining habitat fragments become small, isolated areas surrounded by farmland, pastures, roads, or empty land. This often happens due to slash-and-burn farming in tropical forests. In central-western New South Wales, Australia, 90% of native plants have been removed, and over 99% of the tall grass prairies in North America have been cleared, causing extreme habitat fragmentation.

Two types of processes can cause habitat fragmentation: endogenous processes and exogenous processes. Endogenous processes are linked to how species live, grow, or interact with each other. These processes can change how species breed or move and are often started by exogenous processes. Exogenous processes are not related to species biology and include events like habitat damage, splitting of habitats, or isolating habitats. These processes can greatly affect endogenous processes by changing how species behave. When habitats are split or isolated, species may move less or change their migration patterns. These changes can reduce the number of species in an area, increase competition, or raise the risk of being hunted or eaten.

Implications

Habitat fragmentation harms biodiversity by reducing the amount of suitable habitat available for living things. This process often involves destroying habitats and splitting large, connected areas into smaller, separate parts. Plants and other organisms that cannot move quickly are especially affected because they cannot easily adapt to these changes. Studies show that habitat fragmentation can reduce biodiversity by 13 to 75% and hurt important ecosystem functions, such as reducing plant growth and changing how nutrients move through the environment. This highlights the serious and lasting effects of habitat fragmentation, which are discussed in sections about its consequences.

Habitat loss, which can happen through fragmentation, is the biggest threat to species. However, the way habitat patches are arranged in a landscape, not just the total amount of habitat, may have a smaller effect. A review of scientific studies found that 76% of reported effects of habitat fragmentation on species were positive, while 24% were negative. Despite this, scientific writing often focuses more on negative effects. Positive effects suggest that many small habitat patches can be better for conservation than one large patch of the same size. This means strategies that share land between habitats might help species more than strategies that keep habitats separate. While habitat loss is usually seen as a bigger problem than fragmentation, the two are closely linked and often cannot be studied separately.

The size of a habitat fragment is the main factor in how many species it can support. The way populations grow and change depends on how habitat is arranged, random environmental changes, and the traits of the species. Small, isolated populations are more vulnerable to problems that would not affect larger populations, such as sudden changes in climate or resources. This makes habitat fragmentation a major cause of species extinction. Populations in fragmented areas often change in different ways over time. In unfragmented areas, a declining population might be saved by animals moving in from nearby areas. In fragmented areas, the distance between habitat patches can stop this from happening. Also, habitat patches that are far from other areas are less likely to be repopulated. Even small species, like the Columbia spotted frog, rely on this movement. Studies show that 25% of young frogs travel more than 200 meters, while only 4% of adults do. Most of these young frogs stay in their new location, showing that this movement is important for survival.

Habitat fragmentation also causes edge effects. Changes in light, temperature, and wind near the edges of habitat patches can alter the environment inside and outside the fragment. These areas are more likely to catch fire because of lower humidity and higher temperatures and wind. In these disturbed areas, non-native species and pests may thrive, and nearby domestic animals can harm natural ecosystems. The climate near the edges of habitat fragments is different from the inside, favoring different species. Small fragments are not good for species that need large, undisturbed areas. Keeping large, connected habitats is important for preserving both genetic diversity and species diversity. Usually, losing 10% of connected habitat can lead to losing 50% of biodiversity.

In many developing countries, habitat fragmentation has happened because of urban growth, such as roads cutting through natural areas. Aquatic species have been affected by dams and water projects that split their habitats. These small, disconnected habitat areas may not be large enough to support species that need big territories to find mates and food. Habitat loss and fragmentation make it hard for migratory species to find places to rest and feed during their journeys.

The effects of current habitat fragmentation will continue for many years. Some species may go extinct even after fragmentation stops, but some benefits from immigration might not happen. Studies show that biodiversity and ecosystem functions continue to decline for decades after fragmentation occurs. Scientists must understand how short-term and long-term changes relate, and experiments on fragmentation will be important for this.

Habitat fragmentation often causes species to become threatened or endangered. Having enough suitable habitat is essential for species survival. Conservation biologists often face tough choices when habitat is limited: should they protect small, isolated patches or buy land to create large, connected areas? In rare cases, some species may be safer from disease if they are spread across different areas. Some studies suggest that more fragmented habitats can support more species, but this idea is debated. Scientists continue to argue about whether one large habitat or several small ones are better for conservation, a debate called SLOSS (Single Large or Several Small). Losing habitat in a biodiversity hotspot can lead to many species going extinct, and this can predict how many species in the area are at risk.

One way to help with habitat fragmentation is to connect habitat patches by preserving or planting corridors of native plants. In some cases, a bridge or tunnel can join two areas. This can reduce isolation but not fix the loss of interior habitat. Wildlife corridors help animals move to find food and mates, increasing genetic diversity. Species that move seasonally can do so more safely if they avoid human-made barriers.

Because cities are growing, scientists are studying green roofs as possible habitat corridors. A recent study found that green roofs help connect habitats for insects like bees and weevils.

Another solution is to expand small habitat areas to increase interior habitat. However, this may be hard because land in developed areas is expensive and hard to restore.

The best solution depends on the species or ecosystem involved. More mobile species, like birds, may not need connected habitats, while smaller animals, like rodents, may be more at risk in open areas. These questions are part of the study of metapopulations.

Forest fragmentation

Forest fragmentation is a type of habitat loss where large forest areas are broken into smaller, separate pieces called forest fragments or remnants. These fragments are separated by areas like open fields, farmland, or developed land. Based on the idea of island biogeography, these forest remnants act like islands surrounded by other land types. Over time, these fragments may experience ecosystem decline.

Forest fragmentation also includes less obvious changes, such as utility right-of-ways (ROWs). Utility ROWs are areas cleared for power lines, gas pipelines, or communication towers. These ROWs are common in many forests and can cover large areas, such as 5 million acres in the United States. Some studies show that these ROWs may support more plant species than nearby forests because they create different microclimates. These areas can also provide homes for native bees and grassland species, as they remain in an early stage of plant growth.

Forest fragmentation reduces food and shelter for animals, separating species and making them more vulnerable to predators. It also makes it harder for animals to mate, which lowers genetic diversity.

This fragmentation also affects plant species by splitting large groups into smaller ones. Smaller groups are more likely to face genetic issues and reduced survival. It can also disrupt relationships between animals and plants, such as seed dispersal or pollination.

Forest fragmentation is a major threat to biodiversity, especially in tropical regions. The problem is worsened by:
– Small forest fragments struggling to support large animals.
– Species going extinct if no fragment can support their survival.
– Edge effects, which change conditions near the edges of forest fragments, reducing interior forest habitat.

The impact of fragmentation depends on the size of the forest fragment and how isolated it is. Isolation depends on how far the fragment is from similar patches and how different the surrounding area is. For example, if cleared land is allowed to regrow, it can reduce isolation. However, if land is permanently turned into farmland or developed areas, fragments become more isolated.

Smaller or more isolated forest fragments lose species faster than larger or less isolated ones. Many small forest "islands" cannot support the same level of biodiversity as one large, connected forest, even if their total area is larger. However, in rural areas, forest fragments can increase biodiversity. In Chile’s Maulino forest, some studies found that plant diversity in fragments is not much lower than in large forests, and tree diversity is actually higher in fragments.

A study by McGill University in Canada found that 70% of the world’s remaining forests are within one kilometer of a forest edge, putting biodiversity at risk.

Changes in fragment size, isolation, and edges affect all ecosystems. Habitat fragmentation can lead to unexpected outcomes, such as some species becoming more common or requiring long timeframes to see major changes.

Forest fragments can influence nearby agricultural areas. For example, research in Quebec, Canada, studied how forest fragments affect soybean fields by looking at factors like crop production, soil health, and water quality. Sustainable forest management can help protect forests through methods like managing ecosystems for services beyond just resources, using government support, and strong laws. The only realistic way to protect forests is through sustainable management to avoid further loss.

There is high demand for wood, paper, and other forest resources, leading to more deforestation. The Rainforest Alliance has promoted sustainable forest management since the 1980s, helping save nearly half a billion acres of land worldwide.

To conserve forests, methods include reducing erosion, proper waste disposal, protecting native tree species, and setting aside land for wildlife. Forest fires can also be prevented through management. For example, in Guatemala’s Petén region, forests managed under FSC certification had lower deforestation and fire rates than nearby protected areas. However, short-term decisions about logging and employment can harm long-term biodiversity. Planted forests are becoming more important, as they provide about a quarter of global wood production and are expected to supply half within two decades. While challenges remain, forest certification has helped raise awareness about balancing economic, environmental, and social issues globally. It also helps track carbon storage and other applications.

Approaches to understanding habitat fragmentation

Two methods are commonly used to study habitat fragmentation and its effects on nature.

The species-oriented method examines how individual species react to changes in their environment and habitat. However, this method has a limitation because it focuses only on one species at a time and does not consider how habitat changes affect many species together.

The pattern-oriented method looks at how land cover patterns relate to where species live. A model used to study these patterns is the patch-matrix-corridor model, created by Richard Forman. This method studies land cover shaped by human activities. It is based on island biogeography and tries to find connections between landscape patterns and the presence of species. However, this method has a limitation because it makes general assumptions that may not apply to all species or landscapes.

Another model is the variegation model. In variegated landscapes, much of the natural vegetation remains, but some areas are changed by human activities like farming. This model is often used to describe landscapes altered by agriculture. Unlike the fragmentation model, which describes isolated habitat areas surrounded by unsuitable land, the variegation model describes landscapes where small habitat areas remain near original natural areas. Between these areas is a mix of grasslands that are often modified versions of the original habitat. These areas are less likely to block the movement of native species.