Biodiversity




Vertebrate species density. Highest species density is concentrated in the tropics and subtropics.
Source: Jenkins et al., 2013. Data source: BiodiversityMapping.org; with acknowledgement to IUCN, Birdlife International, NatureServe and USGS.


Biodiversity underpins nature’s benefits to people

Biodiversity refers to the variety of plant and animal life in the world. Within the terrestrial environment, it is the tropical and sub-tropical band where the world’s greatest biodiversity is to be found. High biodiversity also tends to correlate with increased rainfall so it is not surprising that the Amazon forest is normally regarded as the area with the highest biodiversity. Despite this, many unique habitats, including arid areas, can have high biodiversity. For instance the Succulent Karoo in South Africa, possibly the arid area with the richest desert biodiversity, has 5 000 plant taxa, of which approximately 40 % are endemic.
The loss of biodiversity itself, can be considered as a form of land degradation.
For instance overhunting in some tropical forests has resulted in the loss of most natural large herbivores, carnivores and primates, leading to what can be referred to as an ‘empty forest’. This can have a cascading effect through different trophic levels. The reintroduction of wolves into the Yellowstone Park USA is a graphic illustration on how the loss of a key predator can have far-reaching impacts. Without wolves, the build-up of elk populations was causing major degradation of plants resulting in erosion along river beds. In some instances human activities cause the targeted loss of specific species. For instance, in South Africa 82 plant species are threatened with extinction as a direct consequence of medicinal plant harvesting. Biodiversity gives ecosystems greater resilience and as a consequence the loss of biodiversity threatens the functions and services delivered by ecosystems. This may be especially important for ecosystems to be able to adapt to climate change.
Land degradation typically results in biodiversity being lost.
As habitats degrade they become less able to support biodiversity, with badly degraded habitats typically having a reduced biodiversity, or a biodiversity that is shifted to early successional species. Heavy overgrazing destroys perennial grass species that are replaced with annual grasses and weedy forbs, including exotic invasive species. This increases the rates of soil erosion and provides less palatable grazing for livestock. From a species loss perspective, it is transformation of natural or semi-natural vegetation to agricultural fields or settlement that has the biggest impact on biodiversity, often resulting in a near total loss of the indigenous diversity.
The Millennium Ecosystem Assessment popularised the concept of ecosystem services and their importance to humans. Although the link between biodiversity and ecosystem services is complex, there is general agreement that maintaining biodiversity is critical for maintaining long -term and sustainable flows of ecosystem services. With climate change likely to result in greater levels of drought conditions in some drylands, the genetic variation in dryland species may be critical for breeding new varieties that can withstand higher temperatures and/or reduced rainfall. The value of biodiversity of drylands is often underestimated because it is less productive than more humid areas. Despite this, these drier areas are home to a disproportionally high proportion of the world’s human population and provide a high proportion of the world’s food and other ecosystem services.
Links between biodiversity use and poverty
“Poverty forces humans to kill the goose that lays the golden eggs.”
There is a destabilising link between poverty and overexploitation of biodiversity in most, if not all dryland ecosystems7. An often cited example is the over-exploitation of woody plants when poverty drives individuals to use cheap biomass-based fuels rather than better technologies for cooking, heating and other forms of energy supply. As a consequence, cities and villages are surrounded by large devastated areas, where trees, shrubs and dwarf shrubs are removed in a radius of up to a dozen kilometres. Charcoal replaces fuelwood as it is more easily transported and the impacts can be experienced over several kilometres. As a consequence, numerous provisioning ecosystem services of the former woody vegetation, including food, medicine, building material, fibres, crafts, fertiliser, etc. are depleted.
Drivers of biodiversity loss
The world is currently losing species at an unprecedented rate. Most of this species loss can be directly attributed to human activities. Land conversion, for instance from natural woodlands to agricultural crop fields, has, over the past few decades, been a major threat to biodiversity. In some vegetation types there are very low levels of formal conservation and in some cases there has been an almost total loss of natural habitat, this despite the global target for conserving a minimum of 17 % of each habitat in protected areas. Fragmenting natural habitats can lead to species loss in what is termed the ‘island biogeographic’ effect. The fragmented habitats prevent free species movement and the remaining patches may become too small to support viable populations of some species. Land degradation of untransformed habitats can also have a profound negative impact on biodiversity. For instance, most large indigenous herbivores are lost from many livestock ranching systems, where overgrazing may also radically change the species mix in the herbaceous layer of vegetation. Invasive alien species further impact indigenous biodiversity, displacing indigenous species, or even altering the entire nature of the habitat. Pollution and (particularly nitrogen deposition) is also impacting biodiversity.
Overlaid on this is the impact of climate change which is anticipated to lead to an unprecedented rate of species extinctions over the coming decades. Climate change will mean that species and entire ecosystems will, in effect, have to migrate to find new areas of suitable climate. As the world heats this typically means moving away from the equator, or moving higher up mountains, to find areas with cooler climates. Land transformation and land degradation will make this species migration even more difficult, potentially exacerbating extinction rates.

Pressures driving global biodiversity loss under a baseline scenario.
Land-use change and encroachment are projected to remain the most important drivers of biodiversity loss, but climate change will also become a significant pressure.
Source: B. ten Brink, Netherlands Environmental Assessment Agency (PBL), 2010.

Links between biodiversity use and poverty.
Source: Mikkelson, G. M. et.al 2007

Links between biodiversity use and poverty.
Source: Mikkelson, G. M. et.al 2007