Habitat Loss, Deforestation, and Fragmentation

Habitat loss is the destruction or degradation of the natural environment in which a species lives, so that it can no longer support the species; it includes deforestation (the permanent clearing of forest) and fragmentation (the breaking of continuous habitat into small, isolated patches). In brief: habitat loss is considered the leading cause of biodiversity decline on land. The species-area relationship (S = cA^z) predicts how many species are lost as area shrinks, and fragmentation adds further harm through edge effects and isolation.

Habitat is the place with the resources and conditions that a species needs. When the habitat is cleared for agriculture, cities or roads, the species that depend on it decline. This guide describes the causes of habitat loss and deforestation, explains fragmentation and edge effects, introduces the species-area relationship and extinction debt, compares loss and fragmentation in a table, works through calculations of species loss and core area and corrects common mistakes.

What causes habitat loss?

• Agriculture: the conversion of forests, grasslands and wetlands to cropland and pasture. It is the largest cause globally.
• Urbanization and infrastructure: cities, roads, dams and mining.
• Logging: unsustainable cutting of trees for timber and fuel.
• Draining and filling of wetlands, and damage to coral reefs and mangroves by coastal development, trawling and pollution.
• Degradation: habitat that remains but is reduced in quality by pollution, invasive species, overgrazing, altered fire patterns or climate change.

What is deforestation and why does it matter?

Forests cover about 31 percent of the land area of the world, according to the FAO. Deforestation is the conversion of forest to another use. The FAO estimated that around 10 million hectares of forest were lost each year in 2015 to 2020, though the net loss is lower after the area of new forest is included. Tropical forests are cleared mostly for cattle pasture, soy, palm oil and small-scale farming, and are the most species-rich ecosystems on land, so the loss of species is especially high.

• Biodiversity: forests hold the majority of terrestrial species.
• Climate: trees store carbon, and clearing, often by burning, releases carbon dioxide. Deforestation and land-use change contribute roughly 10 percent of human greenhouse gas emissions, according to some estimates.
• Water and soil: roots hold soil, so clearing causes erosion, landslides and silting of rivers. Forests also recycle moisture through transpiration, and large-scale clearing of the Amazon may reduce rainfall.
• People: forests supply food, fuel, medicines and livelihoods to indigenous and local communities.

What is habitat fragmentation?

Fragmentation occurs when a large continuous area of habitat is divided into smaller patches separated by other land uses such as farms or roads. It has several effects:

• Smaller populations: each patch holds fewer individuals, which makes them more vulnerable to chance events, inbreeding and loss of genetic diversity.
• Isolation: animals cannot move between patches to find mates, food or new territories, and recolonization after local extinction is less likely. Wildlife corridors can reduce this.
• Edge effects: the margins of a patch have a different microclimate (more light, wind and heat, less humidity) and are more exposed to predators, parasites and invasive species from the surrounding land. Forest interior species decline within some distance of the edge, which is often hundreds of meters in tropical forests.
• Roads: cause collisions, noise, and pollution, and act as barriers.

The Biological Dynamics of Forest Fragments Project in the Brazilian Amazon, started by Thomas Lovejoy in 1979, has experimentally studied isolated forest plots of 1, 10 and 100 hectares, and found that small fragments lose species faster and that edge effects penetrate far into the plots.

What is the species-area relationship?

The number of species S in a habitat grows with the area A according to the power law S = cA^z, where c is a constant and z is usually between about 0.15 and 0.35. A rough rule is that a tenfold increase in area increases the number of species by a factor of about 2 (when z ≈ 0.3). The relationship can be used to estimate the species lost when habitat is cleared: if the area falls to a fraction f of the original area, the number of species falls to the fraction f^z.

Species do not disappear immediately. The delay between habitat loss and the extinction of the species that cannot survive in the remaining habitat is called the extinction debt. Estimates from species-area curves are approximate, and may over- or underestimate losses, because they depend on whether the species can also live in the new habitat.

Habitat loss and fragmentation compared

How can the effects be reduced?

• Protected areas that are large and well connected, and that protect biodiversity hotspots.
• Corridors, stepping-stone patches and wildlife bridges over roads.
• Sustainable forestry and certification, and agroforestry.
• Restoration of degraded land, and reforestation with native species.
• Higher yields on existing farmland, reduced food waste and changes in diet, so that less land is needed.
• Land-use planning and supply chains that avoid products from newly cleared land.

Worked example: species loss and core area

Part A: species lost. A forest with 200 bird species has z = 0.25. Habitat is reduced to 10 percent of the original area.

1. Fraction of area remaining: f = 0.10.
2. Fraction of species remaining: f^z = 0.10^0.25 = 0.562.
3. Species remaining: 0.562 × 200 ≈ 112, so about 88 species (44 percent) are predicted to be lost in the long term.
4. Other reductions: losing half the area (f = 0.5) gives 0.5^0.25 = 0.841, so about 16 percent of species are lost (32 of 200). Losing 75 percent of the area (f = 0.25) gives 0.25^0.25 = 0.707, so about 29 percent are lost.

The first losses of area remove proportionally fewer species than later ones, but the loss is substantial even for modest area loss.

Part B: core area. Take square forest patches. Edge effects penetrate 100 m (0.1 km) from every side, so the core is a square with sides reduced by 0.2 km.

1. 10 km patch: core side 9.8 km. Core fraction = (9.8 ÷ 10)² = 0.960, or 96 percent.
2. 1 km patch: core side 0.8 km. Core fraction = (0.8 ÷ 1)² = 0.64, or 64 percent.
3. 0.5 km patch: core side 0.3 km. Core fraction = (0.3 ÷ 0.5)² = 0.36, or 36 percent.

Small patches are mainly edge, which is why interior species disappear from them first.

How was habitat loss studied?

The species-area relationship was described by Olof Arrhenius in 1921, and Robert MacArthur and E. O. Wilson linked it to island biogeography in 1967, with species richness resulting from the balance of immigration and extinction. Conservationists applied the theory to reserve design in the 1970s in the “SLOSS” debate (a single large or several small reserves). The Biological Dynamics of Forest Fragments Project began in 1979, and satellite imaging, including the Global Forest Watch data, now allows forest loss to be monitored.

Which mistakes are common?

Mistake 1: Treating forest loss as equal to tree cover loss

Tree cover loss includes plantations and temporary loss from fire or harvest. Deforestation is permanent conversion to another use.

Mistake 2: Expecting all species to disappear immediately

There is an extinction debt: species continue to decline for years or decades after the habitat is lost.

Mistake 3: Using the species-area formula as an exact prediction

It is an average pattern. The value of z differs among regions and groups of species.

Mistake 4: Ignoring the quality of habitat

Habitat degradation can harm species as much as clearing, even if the land cover appears unchanged.