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Unit 9: Biodiversity Decline // Section 2: Defining Biodiversity

The basic currency of biodiversity is species richness—the number of species in a given habitat, or worldwide. By analyzing fossilized life forms, which date back as far as 3.5 billion years, scientists can estimate how many species were present during past eras and compare those numbers to the present range of life. As we will see in Section 3, "Counting Species," species biodiversity is at a peak today compared to past levels, but at the same time many scientists believe that the current rate of extinction is also alarmingly high.

Another important biodiversity indicator is the level of genetic diversity within a species, which may influence the species' future trajectory (Fig. 2).

Color variation in the Oldfield mouse (Peromyscus polionotus)

Figure 2. Color variation in the Oldfield mouse (Peromyscus polionotus)
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Source: Hopi Hoekstra, Harvard University.

Species with low genetic diversity may be less likely to survive environmental stresses because they have fewer genetic options when problems arise. Conversely, populations with high levels of genetic diversity may be more likely to survive environmental and other stresses.

A historic example comes from Harvard Forest in Petersham, Massachusetts, where researchers have documented that an outbreak of an insect called the eastern hemlock looper caused an abrupt decline in the population of hemlock trees across the northeastern United States about 4,800 years ago. Hemlock numbers remained low for nearly 1,000 years after the blight struck but then recovered, possibly because the remaining individuals developed a resistance to the looper (footnote 3). If this theory is correct, it indicates that some hemlocks were genetically less susceptible to looper infestations than others and that natural selection slowly favored those trees in the years following the blight.

Ecosystem diversity is a third type of biodiversity. Life on Earth is distributed among many types of habitats, each of which provides a suitable living environment for specific kinds of organisms. These ecosystems range from tropical rainforests to hydrothermal vents on the ocean floor, where super-heated water bursts through cracks in the planet's crust (for more details, see Unit 4, "Ecosystems"). Many ecosystems are made up of species that have adapted to life under unusual conditions, such as Arctic sea ice communities (Box 1). The loss of these unique ecosystems can wipe out the many species that are highly specialized and unable to shift to other areas.

Arctic marine food web

Arctic marine food web
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Source: 2004. Arctic Council and the International Arctic Science Committee. Arctic Climate Impact Assessment, graphics set 3, slide 12.

Arctic sea ice communities

The ice that is created when Arctic ocean water freezes is different from ice in a freshwater lake. Tiny channels, usually less than one millimeter in diameter, form between sea ice crystals and fill with a salty fluid called brine. Hundreds of species of single-celled algae carry out primary production in the brine channels, using the energy from sunlight that penetrates the ice. Small protozoans feed on the algae and serve as prey for tiny crustaceans and zooplankton. Larger predators like Arctic cod raise their young in these areas, attracting marine mammals and birds.

The communities that form the base of Arctic marine food chains are uniquely adapted to conditions under the ice, including wide fluctuations in light, temperature, and salinity levels, and constant change in the extent and thickness of ice cover. Much remains unknown about how the structure and formation of sea ice influences Arctic food webs, extending to and including native Eskimo communities. However, it is clear that many organisms living in, on, and under Arctic sea ice have adapted very effectively to extreme conditions.

Warming associated with global climate change is shrinking Arctic sea ice coverage. Researchers predict that this trend may accelerate so drastically that the Arctic ocean could be virtually ice-free in summer by 2040. Melting on this scale poses a serious threat not only to the microorganisms that live in Arctic sea ice, but also to the many larger species that are part of Arctic marine ecosystems. For example, sea ice is the preferred habitat both for polar bears and for ringed and bearded seals, which are polar bears' main prey. Phytoplankton that grow on sea ice are food for amphipods and copepods—small crustaceans that in turn are preyed on by Arctic cod and bowhead whales. An Arctic Ocean without summer sea ice will still have significant biological activity, but many of the remarkable crustaceans, fish, birds, and mammals that currently exist on and around sea ice will no longer be viable in such a habitat.

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