The Millennium Ecosystem Assessment has provided ample evidence that over the past 50 years, humans have changed nature more rapidly and extensively than in any comparable period of time in history. The environmental degradation of dry and sub-humid lands is a case in point; to date 2,311 known dry and sub-humid lands species are endangered or threatened with extinction. The main pressures that impact on dry and sub-humid lands biodiversity are:
- Habitat conversion
The most common transformation is conversion to cropland. Inappropriate conversion or poor soil and water management can lead to degradation. In Mediterranean areas, in particular, conversion for transport, tourism and industrial infrastructure is also very significant.
- Climate change
Long-term changes in temperature and rainfall patterns can have serious impacts on the biological diversity of dry and sub-humid lands. Wetlands in drylands, remnant grasslands, prairies, Mediterranean forests, and desert margins are particularly vulnerable to the impacts of climate change.
- Grazing pressures
Wildlife and livestock impact on dry and sub-humid lands biodiversity through trampling and removal of biomass, alteration of species composition through selective consumption and changed inter-plant competition, and redistribution of nutrients through dropping of urine and faeces. Changes in grazing intensity and selectivity will inevitably change dry and sub-humid lands biodiversity; undergrazing and overgrazing can both have negative effects, but overgrazing by livestock is increasingly problematic.
- Introduced species
Varieties and breeds can radically change dry and sub-humid lands biodiversity. Replacement of traditionally grown crops (such as millet and sorghum) by others (such as maize), and the introduction of improved crop varieties can diminish crop-species and genetic diversity, and limit crop evolution. The introduction of exotic grasses and legumes in pastures and rangelands is particularly significant in this regard. Invasive alien plant and animal species can adversely affect indigenous biodiversity. Introduced feral animals, such as rabbits, can contribute to overgrazing.
- Changes in fire regimes
Fire occurs naturally in many dry and sub-humid lands, but increased frequency or intensity of fire through deliberate or accidental human action can markedly change species composition and often decrease biodiversity.
Since water is a limiting factor in dry and sub-humid lands, changes in water availability, through water abstraction or irrigation, can have disproportionate effects on biodiversity.
Excessive collection of fuel wood, over-harvesting of plants and over-hunting of wildlife can all have direct negative impacts on the components of dry and sub-humid lands biodiversity.
- Soil management
The soils of dry and sub-humid lands are particularly prone to erosion, especially when natural vegetation is removed through inappropriate tillage, grazing or use of fire. Excessive use of artificial fertilizers can change the biotic composition of soils.