Major threats to terrestrial ecosystems

Forests cover about one third of the Earth’s land surface and are particularly species rich, containing more than two thirds of all terrestrial species (Aerts and Honnay 2011). Globally, deforestation rates from 2000 to 2010 were estimated to be about 130,000 square kilometers per year, an area approximately the size of Mississippi. Africa and South America experienced the highest rates of deforestation (CBD 2010a). More than 1% of total tropical rainforest, which is particularly high in biodiversity, is lost each year (Bradshaw et al. 2009).

Primary forests, which are forests in which human activity has not significantly impacted native species or disturbed ecological processes, account for slightly more than one third of all forests globally. South America has the highest percentage of primary forests globally at about 75%, while less than 15% of Africa’s forests are primary forests (FAO 2010). Between 2000 and 2010, 400,000 square kilometers of primary forest, an area slightly larger than Germany, were lost. Loss of primary forests has a particularly negative impact on biodiversity due to forest species richness (CBD 2010a).

While grasslands and savannas have not been studied to the same detail as forests, there is evidence that these habitats are also being lost at unprecedented rates. For example, it is estimated that only 5% of North America’s original grasslands still stand. In central Brazil, the annual rate of woodland savanna loss is higher than deforestation rates in the Amazon rainforest (CBD 2010a).

The primary threats to terrestrial biodiversity include habitat loss and degradation, unsustainable harvesting of species, climate change, invasive species, and pollution (CBD 2010b). Of these, habitat loss and degradation present the single biggest threat to biodiversity globally, impacting half of all threatened species. In most cases however, species go extinct due to a combination of threats rather than from a single one (Chivian and Bernstein 2008).

Habitat loss and degradation primarily occur due to agricultural expansion, conversion to pasture, and logging for hardwood, paper pulp, fuel and other uses. Other related causes include infrastructure development such as the building of roads, and urban sprawl (CBD 2010b, CBD 2010c). Loss and degradation of habitat result in decreases in population size and genetic variability among species, compromising their long-term viability. A wide range of terrestrial species, including birds, large mammals, insects, bats, and butterflies, are at risk of extinction due to habitat loss (Aerts and Honnay 2011, CBD 2010a). Many of the planet’s most iconic wildlife species, including orangutans, tigers, pandas, and leopards face extinction in the wild if terrestrial habitats continue to be lost at current rates.

The unsustainable harvesting of species through hunting and for activities such as the wildlife trade also poses a major threat to terrestrial biodiversity. Bushmeat hunting (the hunting of wildlife for food) has been taking place globally for millennia but the rates at which it is now conducted are unsustainable in many areas. The recent increase in bushmeat hunting is being driven by higher local demand for food due to population growth as well as by local and international commercial markets for bushmeat. In the Congo Basin region of Africa, almost two thirds of all mammalian species are being hunted in an unsustainable manner; a number of terrestrial species including a range of primates, are threatened due to bushmeat hunting (Chivian and Bernstein 2008, CBD 2010b).

The international wildlife trade, which impacts tens of thousands of animal and plant species, also drives the unsustainable harvesting of species. It is estimated that trafficking in endangered species generates more than US $10 billion annually, exceeded only by trafficking in illegal drugs and weapons (Chivian and Bernstein 2008). Species are harvested for zoos and the pet trade, for use in traditional medicine, and as hunting trophies and decorative items, among other uses. The wildlife trade presents a particularly potent threat to highly valued terrestrial species such as elephants, tigers, and rhinos (Abensperg-Traun 2009).

Climate change is already having an impact on many terrestrial species; for example, scientists have already observed changes in the flowering and migration patterns of some species. While it is true that climate change may also benefit some species, a recent analysis of over 100 European bird species projected that three times more species would be harmed by climate change than would benefit from it. Species that are unable to migrate, and those that require temperature extremes for survival are particularly at risk from climate change (CBD 2010b).

Invasive species, which are species that are intentionally or accidentally introduced into a geographic area that is outside of their natural range, can cause widespread damage to native species by consuming them, competing with them for food or habitat, and introducing disease (CBD 2010c). Island ecosystems are particularly vulnerable to invasive species since native species often have no natural defenses against them and can’t migrate away from the threats they pose. For example, the introduction of rats by humans to many Pacific islands has had devastating impacts on local bird populations due to predation (Pimm et al. 2008).

Pollution, particularly nitrogen deposition from fertilizer use, also presents a serious threat to a variety of terrestrial species (CBD 2010b). Other pollutants that can impact terrestrial species include heavy metals such as lead and mercury, persistent organic pollutants such as dioxins, and pesticides. Exposure to pollutants has been associated with increased rates of infections, reproductive failure, and cancer in vulnerable wildlife species, among other health impacts. For example, exposure to the pesticide DDT contributed to the near extinction of the American bald eagle and the peregrine falcon due to its negative impacts on eggshell formation in these species (Chivian and Bernstein 2008).

Along with the direct threats to terrestrial ecosystems discussed here, a number of indirect threats also drive biodiversity loss. These include increases in the per capita consumption of natural resources, technological advances, migration of human populations, and population growth. Ecosystems that may have been resilient when faced with one or two of these threats now face multiple pressures, resulting in biodiversity loss, ecosystem degradation, and the loss of ecosystem services at unprecedented rates (CBD 2010b).

The main threat to terrestrial ecosystems is the large and diversified number of exotic organism (plant, animals, bacterial, and viral diseases), many of which have become invasive. Most of the introduced organism are competitors, predators, paraites for native species, vectors, or reservoirs of diseases that later spread to organisms. The ecological impact of exotic organisms is worsend, because native species have not usually evolved immunological defenses against recently intoduced diseases or developed behaviors and other life-history strategies to counteract the effects of exotic predators and competitors.