What is habitat fragmentation and how does it occur?

Habitat fragmentation creates small and isolated subpopulations that have fewer opportunities to find food, water, shelter, and mates and occurs due to human activities such as agriculture, deforestation, infrastructure, and road development.

As governments and industries implement measures to accelerate economic growth, ecosystems that once covered large areas continue of land are being increasingly divided into smaller plots by roads, farm fields, cities, and other human constructions.

A recent study estimated that roads divided the African continent into more than 50,000 individual land units; the median size of units was an alarming 6.75 km2. The process, known as habitat fragmentation, divides abundant and widespread wildlife populations, already numerous suffering from habitat loss, into several increasingly smaller subpopulations.

How habitat loss affects biodiversity?

Habitat fragmentation as well accelerates extinctions, because each of these fragmented subpopulations is more exposed, and has a number of deleterious genetic effects compared to the previously large and connected population. As if they were victims of a double danger, even the fragmentation of the habitat prevents the smaller subpopulations’ dispersal and colonization abilities.

Most species, especially those found in low densities, have large home ranges and/or live in ephemeral environments habitat and it must be able to move freely across the landscape to find shelter, food, water, and mates.

A recent global study found that habitat fragmentation has already happened and reduced the average distance animals travel by two-thirds, from 22 km to7 km: in recent decades. If they cannot move freely, these individuals cannot support themselves and are threatened with extinction.

Physical barriers that prevent wildlife from moving freely through the landscape also represent a form of habitat fragmentation. Dispersal prevented by artificial barriers, such as railway tracks; dams; ditches filled with water; streets; and fences, can have disastrous consequences for biodiversity.

Consider, for example, the seasonal arid zones of Africa. These areas are historically characterized by large herds of migrating herbivores constantly moving from area to area after fresh pastures. But when land management systems have changed over time, the construction of roads and the erection of fences to delimit the boundaries of the property have prevented the possibility of these herds to move freely after the resources they needed to stay alive.

Limited to only small parts of the range, these once migratory animals were forced to overgraze areas they already had exploited, leading to a sharp decline in the population. Through this process, Africa has already lost seven mass migrations, each involving millions of animals. With diligent conservation efforts, all of Africa’s migratory herbivores have succeed persist in small, scattered populations throughout their range.

In one of the few studios to visit this problem in Africa, researchers found that valuable timber trees in eastern Tanzania Usambara Mountains are uprooted as the forest fragments grow too small to support viable populations of fruiting birds. The loss of these important seed dispersers will then have ripple effects on plants that depend on them for their survival. In the end, if enough seed dispersers or maybe even a single keystone species are disappearing due to the fragmentation of the entire habitat ecosystems could eventually collapse.

Marginal effects are closely associated and compound the negative effects of habitat loss and fragmentation changing the environmental conditions of the habitat. Dense woods, thickets, and forests are particularly vulnerable to marginal effects.

Imagine a rainforest, especially its tall trees that form a continuous leafy canopy. These continuous canopies regulate the microclimate of the undergrowth of a forest blocking sunlight and wind maintaining humidity during the day but also trapping heat rising from the forest.

When the trees in the forest are felled, the continuous canopy breaks up, which in turn compromises the ability of the canopy to regulate the forest microclimate. Reclaimed areas, as well as forested areas directly adjacent to cleared areas, will therefore be sunnier, warmer, windier, and drier during the day and cooler at night; these climate changes also disrupt nutrient cycles and biomass balances.

All these changes further reduce the size of the forest area, smaller than the remaining canopy indicates as new conditions prevent forestry specialists such as seedling shade mosses from moisture-sensitive trees and amphibians living at the edge of the forest, leaving them with less internal forest habitat for them to compete for.

It is important to note that these microclimate changes can penetrate a forest patch over much greater distances than would be expected. For example, some forest birds in Uganda are sensitive to edge effects up to 500 m from deforested areas.

Edge effects also create several additional threats to the forest species already suffering from altered microclimates. Notably, disturbing edge conditions present a favorable environment for colonization by fast-growing and fast-reproducing invasive species.

Those forest species that are not displaced by the altered microclimates and invasive species also face elevated predation risk. That is because trees that have died due to altered edge conditions provide suitable perches with clear views from which predatory birds can hunt.

The degraded forest edge, sometimes resembling a savannah structure, also provides opportunities for woodland species such as snakes to enter the forests, pushing the remaining forest species even deeper into the forest. For this reason, forest edge communities generally consist of widespread generalist species and invasive species, while specialist species that can hang on are, literally and figuratively, living on the edge.

The most devastating impact of edge effects is that edge effects beget further edge effects in a positive feedback loop leading to a rapidly disappearing ecosystem. First, expanding invasive (and generalist) species populations habitat edges can easily overwhelm more sensitive habitat specialists.

As habitat specialists are displaced at the contact zones, microclimatic conditions change, which allows for invasions even deeper into the fragmented habitat patch. In this way, invasions systematically penetrate deeper and deeper into the forest as microclimates are disturbed, habitat specialists are displaced, and new contact zones are created.

The forest plants that die in the process also increase fuel loads, which, combined with drier and windier edge conditions, create an environment increasingly favorable for fire disturbance. Whether from lightning strikes or human activities, subsequent fires burn hotter and over a larger area, disturbing and destroying more and more habitats each time.

Through these mechanisms, edge effects can degrade entire ecosystems over time, harming both the native species and human livelihoods that depend on those areas. 1

At present, Africa’s biggest driver of habitat loss is agriculture. African farmers have always reclaimed land to meet their subsistence needs. Much of this reclamation has traditionally and historically been done in the form of slash-and-burn agriculture, also called mobile agriculture.

To prepare the land for crops, smallholder farmers would first cut down trees to clear the land for firewood. The remaining vegetation would then be burned to release carbon and other nutrients, increasing the fertility of the land.

Farmers would grow crops in these cleared areas for two or three seasons. So soil fertility would decrease, agricultural production would decrease and farmers would abandon the area and free land, giving the natural ecosystem on the abandoned land time to regenerate.

Medical and technological advances and the arrival of settlers have seen Africa’s human population grow dramatically since the 1800s. Feeding and accommodating the activities of this growing human population has seen an increasing number of natural ecosystems replaced by agricultural land, and less land having time to regenerate.

An increasing number of people also started abandoning their rural subsistence lifestyles for cities seeking work, financial freedom, and an easier life. With increasing urbanization (eg. more people have moved to cities) and competition for jobs has intensified, increasing numbers of city dwellers became dependent on charcoal harvesting for cooking and growing lucrative crops, such as sweet potatoes and cassava.

This has seen even more natural ecosystems converted, especially in city suburbs. Meanwhile, the remaining rural population has become increasingly sedentary due to land change property systems, which forced them into unsustainable agricultural practices with increased competition for land.

These factors have not only increased rates of habitat loss but have also altered soil nutrient content which, in turn, reduces the ability of the land to regenerate and produce food, which, in turn, leads to further deforestation for agriculture.

While deforestation for smallholder agricultural needs continues to be a major driver of habitat loss, its impact is increasingly overshadowed by the needs of commercial interests. The impact of land grabbing is of particular concern.

Foreign companies from Asia and other parts of the world have acquired millions of hectares of land across Africa claiming the continent’s rich natural resources and producing food and biofuels for their people. Foreign interested parties, who often enter into these real estate contracts through government-level loan agreements (eg. with little or no local input), generally prioritize their own needs and profits over local interests with little concern for the environment.

These deals therefore often end with a country burdened with debts that struggle to repay and environmental damage that will take generations to reverse. Additionally, foreign companies often employ migrant workers with fewer protections and rights than local people.

In this process, while a small number of local people can benefit from job creation, investment in technology and infrastructure development, large numbers of local people are deprived of their civil rights and displaced from lands that previously served as their livelihoods.

These foreign investments are a type of neo-colonialism because of their resemblance to the previous colonial era of Africa. Not only do they cause large-scale habitat loss, but in many cases, they also leave local populations impoverished and desolate.

To understand the impact of land grabbing on Africa’s natural environment, one need only consider its scale. For example, Chinese bioenergy producers have recently secured more than 48,000 km2 of land in the Democratic Republic of Congo and Zambia.

Another deal, between the Ethiopian government and Indian and Saudi companies, saw 5,000 km2 of land (including sections of Gambella National Park) set aside for commercial agriculture. At the time, this Ethiopian deal threatened the second-largest mammalian migration to Earth, and the livelihood of the local Anuak pastoral community.

Fortunately, the Ethiopian government and developers have responded to concerns raised by environmentalists and human rights activists and agreed to set aside certain areas for conservation, also implementing measures to maintain the free movement of animals and shepherds.

Infrastructure development is also becoming an important driver of habitat loss. By providing access to previously unexploited areas, roads are perhaps the biggest contributor to habitat loss in the last remaining wilderness areas in Africa. A growing body of literature from Africa supports these claims.

For example, research in the Congo Basin has shown how deforestation typically occurs less than 2 km from the roads: more roads, therefore, mean more deforestation.

Roads also facilitate other factors that cause forest loss, including the spread of invasive species, human settlements, fires, and pollution. By providing access points for hunters, roads also facilitate unsustainable hunting; a recent study found that reductions in wildlife due to hunting could be detected up to 40 km from the nearest road.