Landscape Ecology and Island Biogeography: Fragmentation, Corridors and Edge Effects (Interactive)

Wildlife does not live in neat, isolated boxes. A tiger may move between a national park, a river corridor, a patch of community forest and farmland in a single night. Landscape ecology studies how the arrangement of habitats across large areas, their size, shape and connections, affects species and ecological processes. It is one of the most practical branches of ecology, because the main threat to wildlife worldwide is the loss and breaking up of habitat.

What you will learn

The building blocks of a landscape: patches, corridors and matrix
The species–area relationship and the theory of island biogeography
What habitat fragmentation does to wildlife
Edge effects, connectivity and wildlife corridors
How landscape ecology informs reserve design

Patches, corridors and matrix

Landscape ecologists describe landscapes using three elements:

Patches: relatively uniform areas of habitat, such as a forest fragment, a wetland or a meadow.
Corridors: strips of habitat that connect patches, such as river-bank forest, hedgerows or wildlife bridges.
Matrix: the dominant surrounding land cover, such as farmland, grazing land or towns.

The same landscape looks different to different species. A hedgerow may be a highway for a dormouse but irrelevant to a bird that flies over it. A road may be a barrier to a frog but not to a deer. Ecologists therefore talk about functional connectivity: how easily a particular species can move through a landscape.

The species–area relationship

One of the oldest patterns in ecology is that larger areas contain more species. The relationship is usually described as:

S = cA^z

where S is the number of species, A is area, and c and z are constants. The value of z is typically around 0.2 to 0.35. A useful rule of thumb follows: reducing a habitat to one-tenth of its area eventually leads to the loss of roughly half of its species.

The theory of island biogeography

In 1967, Robert MacArthur and E. O. Wilson proposed that the number of species on an island reflects a balance between two processes:

Immigration of new species, which is higher for islands close to a mainland source.
Extinction of species already present, which is higher on small islands, because populations are small and vulnerable.

The number of species settles where immigration and extinction rates meet. The theory predicts that large, near islands hold the most species, and small, far islands the fewest. Experiment with island size and distance below.

The theory was famously tested by Wilson and Daniel Simberloff in the Florida Keys in the late 1960s. They fumigated tiny mangrove islands to remove all arthropods and watched them recolonise. Within about a year, species numbers had returned to near their original levels, with islands close to the mainland recovering fastest, as predicted.

The theory applies to any isolated habitat, not just oceanic islands: mountaintops, lakes, and, most importantly, fragments of habitat surrounded by farmland or cities.

Habitat fragmentation

When a continuous habitat is broken into smaller, isolated pieces, several things happen:

Less total habitat remains.
Patches become smaller, so they support smaller populations, which are more vulnerable to chance events, inbreeding and disease.
Patches become isolated, so animals cannot move between them to find food, mates or new territories, and populations cannot be rescued by immigration.
More edge is created relative to interior habitat.

The world’s longest-running fragmentation experiment, the Biological Dynamics of Forest Fragments Project near Manaus in Brazil, began in 1979. It found that small Amazon fragments lost many species of birds, primates and large trees within years, and that edges suffered higher tree mortality from wind and drying.

Edge effects

The boundary between two habitats is called an edge, and conditions there differ from the interior. A forest edge next to farmland is typically hotter, drier, windier and brighter, and more exposed to predators, invasive plants and people. In the Amazon, edge effects can extend hundreds of metres into the forest. Some species, such as deer and many songbirds, benefit from edges; others, such as forest-interior birds and large primates, avoid them. Small or irregularly shaped fragments may consist almost entirely of edge.

Corridors and connectivity

Reconnecting habitats is one of conservation’s main tools:

Wildlife overpasses and underpasses, such as the crossings over the Trans-Canada Highway in Banff National Park, which bears, elk, wolves and cougars use thousands of times a year.
River corridors, which provide natural routes for many species.
Transboundary landscapes, such as the Terai Arc Landscape linking a chain of protected areas across Nepal and India for tigers, elephants and rhinos, and the Kavango-Zambezi Transfrontier Conservation Area spanning five southern African countries.
Stepping stones: small patches that allow movement across a landscape without a continuous corridor.

Corridors must be designed with care, because they can also spread fire, disease and invasive species.

Reserve design: SLOSS

In the 1970s and 1980s, ecologists debated whether it was better to protect a Single Large Or Several Small reserves of the same total area, known as the SLOSS debate. The answer depends on the species. Large reserves suit wide-ranging animals such as tigers and elephants; several small reserves can capture more habitat types and spread risk from fire or disease. Most modern conservation plans combine large core areas, buffer zones and connecting corridors.

Case study: the Terai Arc Landscape

The Terai Arc stretches about 800 km along the foothills of the Himalaya, from Nepal’s Bagmati River to India’s Yamuna River. Its protected areas, including Chitwan, Bardiya, Shuklaphanta, Corbett and Dudhwa, were once isolated islands in a sea of farmland. Since the early 2000s, governments and communities have restored forest corridors such as Khata and Basanta, which link Nepalese and Indian parks. Camera traps now record tigers, elephants and rhinos moving through these corridors, and tiger numbers in the landscape have increased strongly. The project shows landscape ecology in action: connecting habitat, involving local communities and managing the matrix as well as the reserves.

Try it yourself

Open a satellite map of an area you know. Identify the patches of natural habitat, the corridors between them and the matrix. Where are the biggest barriers to wildlife movement? Where would you put a new corridor?

Common misconceptions

“Protecting small patches is enough.” Small, isolated patches steadily lose species over time, a delay known as the extinction debt.
“Corridors are always beneficial.” They must suit target species and be managed to avoid spreading threats.
“Island biogeography only applies to islands.” It applies to any isolated habitat, including forest fragments and mountaintops.

Key terms

Patch, corridor and matrix: the building blocks of a landscape.
Species–area relationship: larger areas hold more species.
Island biogeography: species richness as a balance of immigration and extinction.
Fragmentation: breaking up of continuous habitat into smaller, isolated pieces.
Edge effect: altered conditions at habitat boundaries.
Extinction debt: future extinctions caused by past habitat loss.

Quick quiz

Test what you have learned. Scoring 60 percent or more marks this module as complete.

Landscape Ecology and Island Biogeography