biodiversity loss

biodiversity loss, a decrease in biodiversity within a species, an ecosystem, a given geographic area, or Earth as a whole. Biodiversity, or biological diversity, is a term that refers to the number of genes, species, individual organisms within a given species, and biological communities within a defined geographic area, ranging from the smallest ecosystem to the global biosphere. (A biological community is an interacting group of various species in a common location.) Likewise, biodiversity loss describes the decline in the number, genetic variability, and variety of species, and the biological communities in a given area. This loss in the variety of life can lead to a breakdown in the functioning of the ecosystem where decline has happened.

(Read E.O. Wilson’s Britannica essay on mass extinction.)

The idea of biodiversity is most often associated with species richness (the count of species in an area), and thus biodiversity loss is often viewed as species loss from an ecosystem or even the entire biosphere (see also extinction). However, associating biodiversity loss with species loss alone overlooks other subtle phenomena that threaten long-term ecosystem health. Sudden population declines may upset social structures in some species, which may keep surviving males and females from finding mates, which may then produce further population declines. Declines in genetic diversity that accompany rapid falls in population may increase inbreeding (mating between closely related individuals), which could produce a further decline in genetic diversity.

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Even though a species is not eliminated from the ecosystem or from the biosphere, its niche (the role the species play in the ecosystems it inhabits) diminishes as its numbers fall. If the niches filled by a single species or a group of species are critical to the proper functioning of the ecosystem, a sudden decline in numbers may produce significant changes in the ecosystem’s structure. For example, clearing trees from a forest eliminates the shading, temperature and moisture regulation, animal habitat, and nutrient transport services they provide to the ecosystem.

Natural biodiversity loss

An area’s biodiversity increases and decreases with natural cycles. Seasonal changes, such as the onset of spring, create opportunities for feeding and breeding, increasing biodiversity as the populations of many species rise. In contrast, the onset of winter temporarily decreases an area’s biodiversity, as warm-adapted insects die and migrating animals leave. In addition, the seasonal rise and fall of plant and invertebrate populations (such as insects and plankton), which serve as food for other forms of life, also determine an area’s biodiversity.

Biodiversity loss is typically associated with more permanent ecological changes in ecosystems, landscapes, and the global biosphere. Natural ecological disturbances, such as wildfire, floods, and volcanic eruptions, change ecosystems drastically by eliminating local populations of some species and transforming whole biological communities. Such disturbances are temporary, however, because natural disturbances are common and ecosystems have adapted to their challenges (see also ecological succession).

Human-driven biodiversity loss

In contrast, biodiversity losses from disturbances caused by humans tend to be more severe and longer-lasting. Humans (Homo sapiens), their crops, and their food animals take up an increasing share of Earth’s land area. Half of the world’s habitable land (some 51 million square km [19.7 million square miles]) has been converted to agriculture, and some 77 percent of agricultural land (some 40 million square km [15.4 million square miles]) is used for grazing by cattle, sheep, goats, and other livestock. This massive conversion of forests, wetlands, grasslands, and other terrestrial ecosystems has produced a 60 percent decline (on average) in the number of vertebrates worldwide since 1970, with the greatest losses in vertebrate populations occurring in freshwater habitats (83 percent) and in South and Central America (89 percent). Between 1970 and 2014 the human population grew from about 3.7 billion to 7.3 billion people. By 2018 the biomass of humans and their livestock (0.16 gigaton) greatly outweighed the biomass of wild mammals (0.007 gigaton) and wild birds (0.002 gigaton). Researchers estimate that the current rate of species loss varies between 100 and 10,000 times the background extinction rate (which is roughly one to five species per year when the entire fossil record is considered). In addition, a 2019 report by the Intergovernmental Science-Policy Platform on Biodiversity and Ecosystem Services noted that up to one million plant and animal species are facing extinction due to human activities.

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Forest clearing, wetland filling, stream channeling and rerouting, and road and building construction are often part of a systematic effort that produces a substantial change in the ecological trajectory of a landscape or a region. As human populations grow, the terrestrial and aquatic ecosystems they use may be transformed by the efforts of human beings to find and produce food, adapt the landscape to human settlement, and create opportunities for trading with other communities for the purposes of building wealth. Biodiversity losses typically accompany these processes.

Researchers have identified five important drivers of biodiversity loss:

• Habitat loss and degradation—which is any thinning, fragmentation, or destruction of an existing natural habitat—reduces or eliminates the food resources and living space for most species. Species that cannot migrate are often wiped out.
• Invasive species—which are non-native species that significantly modify or disrupt the ecosystems they colonize—may outcompete native species for food and habitat, which triggers population declines in native species. Invasive species may arrive in new areas through natural migration or through human introduction.
• Overexploitation—which is the harvesting of game animals, fish, or other organisms beyond the capacity for surviving populations to replace their losses—results in some species being depleted to very low numbers and others being driven to extinction.
• Pollution—which is the addition of any substance or any form of energy to the environment at a rate faster than it can be dispersed, diluted, decomposed, recycled, or stored in some harmless form—contributes to biodiversity loss by creating health problems in exposed organisms. In some cases, exposure may occur in doses high enough to kill outright or create reproductive problems that threaten the species’s survival.
• Climate change associated with global warming—which is the modification of Earth’s climate caused by the burning of fossil fuels—is caused by industry and other human activities. Fossil fuel combustion produces greenhouse gases that enhance the atmospheric absorption of infrared radiation (heat energy) and trap the heat, influencing temperature and precipitation patterns.

Ecologists emphasize that habitat loss (typically from the conversion of forests, wetlands, grasslands, and other natural areas to urban and agricultural uses) and invasive species are the primary drivers of biodiversity loss, but they acknowledge that climate change could become a primary driver as the 21st century progresses. In an ecosystem, species tolerance limits and nutrient cycling processes are adapted to existing temperature and precipitation patterns. Some species may not able to cope with environmental changes from global warming. These changes may also provide new opportunities for invasive species, which could further add to the stresses on species struggling to adapt to changing environmental conditions. All five drivers are strongly influenced by the continued growth of the human population and its consumption of natural resources.

Interactions between two or more of these drivers increase the pace of biodiversity loss. Fragmented ecosystems are generally not as resilient as contiguous ones, and areas clear-cut for farms, roads, and residences provide avenues for invasions by non-native species, which contribute to further declines in native species. Habitat loss combined with hunting pressure is hastening the decline of several well-known species, such as the Bornean orangutan (Pongo pygmaeus), which could become extinct by the middle of the 21st century. Hunters killed 2,000–3,000 Bornean orangutans every year between 1971 and 2011, and the clearing of large areas of tropical forest in Indonesia and Malaysia for oil palm (Elaeis guineensis) cultivation became an additional obstacle to the species’ survival. Palm oil production increased 900 percent in Indonesia and Malaysia between 1980 and 2010, and, with large areas of Borneo’s tropical forests cut, the Bornean orangutan and hundreds to thousands of other species have been deprived of habitat.