Habitat degradation generally refers to impacts that affect many but not all species, and that may be temporary.
Habitat loss usually refers to impacts so severe that all, or nearly all, species are adversely affected, or to cases where the time span needed for recovery is extremely long.
Habitat transformation and habitat conversion are two additional terms often used interchangeably with degradation and loss, and we will use them when referring to processes of change.
Fig. 6.2 Effect of increasing land use intensity on the fraction of species in vertebrate taxa and plants.
Patterns of habitat transformation on land and in the sea
Although humans have transformed their environments for thousands of years, the pace of change has accelerated sharply in the last century, and particularly since 1960, as the world population more than doubled since then.
In the US, only 42% of native vegetation remains, with less than 25% remaining in the midwest (Stein, et al., 2000). (Fig. 6.3)
Fig. 6.3 Natural vegetation has been degraded extensively in the US.
Forest systems and deforestation patterns
Over the past three centuries, about half of the world’s forest cover has been removed to make way for croplands, pastures, and settlements.
In 25 countries, no forest remains, and in an additional 29, more than 90% of forests have been lost.
Europe has the least forest cover of any continent, most of which was lost to agriculture before 1700.
Forests provide many ecosystem services including regulating water supplies by mediating flow rates, controlling erosion, and affecting climate through gas exchange.
The Amazon Basin is the world’s largest expanse of tropical forest, and despite efforts to reduce deforestation, rates of forest clearing remain high.
Loss of and damage to grassland, savanna, and shrubland habitats
Grasslands, savanna, and shrublands cover 52.5 million KM2 or 40.5% of Earth’s surface, and are found predominately in dry subtropical and dry-humid temperate zones.
Temperate grasslands have been the most heavily converted to agriculture and secondarily to urban environments.
Nearly 97% of tallgrass prairies, and over 60% of mixedgrass and shortgrass prairies in North America have been converted since in mid 1800s.
With agricultural and urban expansion, freshwater systems are degraded through water diversions, dams and extensive wetland losses.
Aral Sea, Kazakhstan, once the forth largest lake in the world.
It has been reduced by 60% in extent over just two decades (Fig. 6.6)
Human activities that cause habitat degradation
Agricultural activities (crop and livestock farming, timber plantations, and aquaculture), extraction activities (mining, fisheries, logging, and harvesting) and development (human settlements, industry and associated infrastructure) are the three main proximate causes of habitat degradation and loss.
Fig. 6.9 Bird species that are threatened by habitat degradation and loss.
Pollution as a form of habitat degradation
Air pollution and acid rain
Solid waste and plastics
Habitat degradation due to excessive nitrogen inputs
Not all forms of pollution are derived from toxic chemicals
Human activities have had profound impacts on the global biogeochemical cycles of carbon, nitrogen, and phosphorous,
Humans have approximately doubled the rate of nitrogen input into the terrestrial nitrogen cycle.
There are greater than 80 million metric tones produced annually.
The amount of nitrogen deposited in the world’s major watersheds has increased tremendously.
In the aquatic and marine realms, the most common effects of increased nutrient loads are rapid increases in the abundance of algae and aquatic plants, and reduced water clarity. (eutrophication)
Nitrogen-based agricultural fertilizers, whose use is predicted to double or triple over the next 50 years, are the primary source of nitrogen pollution worldwide.
Protecting what’s left: approaches to global habitat conservation
Globally oriented conservation groups, Conservation International (CI), the Nature Conservancy (TNC), the Wildlife Conservation Society (WCS), and World Wildlife Fund (WWF)
The “Global 200” ecoregions
Conservation of habitats: the how
A number of social and economic factors are the underlying driving force of land use change decisions, which ultimately influence habitat degradation (Fig. 6.20)
Since 1987 more than US $1 billion has been channeled into nature conservation in developing countries, through a mechanism known as “debt-for-nature swaps” (DFNS).
These swaps grew out of two issues that came to the fore in the 1980s- the developing world debt crisis, and increasing concern about the rapid loss of natural ecosystems, particularly tropical forests.
In the 1970s, commercial banks started to see LDCs as potential clients.
However, in 1981, with developed countries in recession following the second oil shock of 1979, interest rates increased as the West tried to combat inflation.
In 1982 Mexico became the first to announce it could not make its payments. Forty two countries followed, and the “debt crisis” (呆帳危機) had begun.
The twin crisis
Because most relied on primary industries – agriculture, forestry and mining – for the bulk of their exports, the debt crisis increased pressure on the natural environment.
Debt was therefore seen as a cause of deforestation and the twin crises of debt and environmental destruction became liked in many people’s minds.
The Debt-for-Nature Swaps idea
In 1984 Thomas Lovejoy of the World Wildlife Fund (WWF) suggested that the crises of debt and deforestation could both be tackled it, “ debtor nations willing to protect natural resources could be made eligible for discounts or credits against their debts.
Three years later, the first “debt-for-nature swap” was concluded by Conservation International (CI), the government of Bolivia and Citicorp bank.
A debt-for-nature swap is an agreement whereby debt in “hard” currency is forgiven (cancelled) for commitments to support conservation on the part of the borrower.
When nature conservation is not in the best interests of poor countries, rich countries will have to pay them to do it and there is evidence to suggest that “generous” bilateral DFNS have occurred.
Case study 6.1 the importance of land use history to conservation biology
Case study 6.2 promoting wildlife conservation in agricultural landscapes