Habitat fragmentation

Habitat fragmentation is patchiness in the landscape caused by human land use and land management. 

Before humans started chopping down trees to build ships and make space for agriculture, vegetation was as a kind of blanket over the landscape — mixtures of trees, shrubs, herbs and grasses covered the land. Bare earth was unknown.

Clearing for agriculture, forestry, fuel and urban development broke up this continuity. In a short time the landscape became more like a patchwork quilt. 

Google Earth image of an agricultural landscape north-west of Goodawindi in southern Queensland, Australia — the red line represents 10 km

In the parts of the world where agriculture dominates, natural habitats exist in patches of different sizes with most of them isolated from similar habitat types.

This has happened because a world population growth has required half the global net primary production. People now grow food wherever the soil will readily support cropping or managed pastures. 

Some 14 million km2 of land is cropped each year and another 1.5 million km2 are permanent orchards, vineyards and plantations. Then there is 33.5 million km2 of pasture and meadow used for livestock. All up over a third of the global landscape has been altered for agriculture.

And this transformation of the landscape is recent. 

For a very long time farms were like islands in a sea of forest, woodland or natural grassland. In most industrial countries this has been reversed by the spread of industrial agriculture. Now it is the forests that are patches in a sea of fields.

Google Earth image of an agricultural landscape in northern NSW, Australia. The dark areas are forests, habitat that would have covered the landscape prior to clearing for agriculture. The red line represents 10 km

Fragmentation has happened because plants that grew naturally were cleared and replaced with more useful species that provided food, fibre, fuel, and various saleable goods from tobacco to medicines. 

Now there was no point going to all the effort of cultivation unless it was worth it. Before tractors and fertilizers productive land was limited. There was limited ability to till the soil and no easy nutrient subsidies to be had. Small–scale farming with animal traction and dung as fertilizer was about the limit. 

Thanks to the serendipitous combination of fossil fuels and the industrial revolution agriculture scaled up. In the last 100 years fields have become massive, as have yields. And most of this production we have converted to more people.  

Now the importance of agriculture is pivotal to our future

Only land difficult to clear or on unproductive soils have escaped — wilderness is usually so because it is remote, inaccessible or inhospitable. Spend a couple of hours in the heat and humidity of the Pilbara region of northern Australia and it easy to see why most of the land is left to the cattle.

In the book Awkward News for Greenies there is a chapter that with the help of Google Earth takes you on a virtual tour and shows how this habitat fragmentation plays out in different landscapes. 

the consequences of habitat fragmentation 

There are significant consequences of habitat fragmentation that has altered vegetation from blanket to patchwork quilt. 

  • Habitat loss 

The best soils for agriculture are not found everywhere. They are patchy in the landscape and are also the most productive places for natural vegetation often evolving unique species combinations. The same applies to unusual areas, places with rare soil types, a peculiar drainage pattern, unique topography and the like. Plants respond to these locations too.

The nature of the vegetation blanket is that clearing can easily erase whole habitat types.

If some habitat survives it becomes isolated in the landscape.

Fragmentation means the loss of species and populations too. Cleating results in direct biodiversity loss as plants are removed by taking away habitats and food sources for birds, mammals, insects and microbes. 

Species and populations can be lost even if some habitat fragments remain for some species need large areas of continuous habitat to survive.  

Over 150 years ago when Alfred Russel Wallace was exploring the Malay Archipelago he already knew that many bird species were restricted to forest habitat. His detailed observations frequently report species that had not crossed narrow straits between islands even though they appeared big enough and powerful enough to do so. 

  • Weakened resilience

Biodiversity loss has its own consequences for the resilience of ecosystem services including NPP. 

When there are fewer species present in systems the reliability of production tends to decline especially in unusual or extreme conditions because there are fewer species available to respond to unusual conditions.

This secondary effect is hard to quantify and is often overlooked or masked by nutrient and energy inputs. 

just part of the story

Habitat fragmentation has been a popular topic for conservationists. When faced with landscape scale conversion of vegetation and species loss the preservation of individual species has become challenging. 

The solution has been to focus more on the quality of the landscape by improving the condition of remaining natural vegetation patches and attempting to link them together with ‘corridors’ of vegetation that some of the larger animals can disperse along.

What we know is that the footprint of agriculture is heavy. It is inevitable though because food security trumps most other environmental values. 

Much of the world now has fragmented landscapes that are very different to what they were even a few hundred years ago. It is truly a global change and a rapid one.

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some habitats escape fragmentation like this eucalyptus forest retained to protect the watershed around a dam near Sydney, Australia

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