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Diverse ecosystems are important for the health of the planet and our survival as humans; it is therefore incredibly important for us to understand and measure biodiversity, which is defined as the variability among living organisms in an ecosystem. Biodiversity can be measured at many different levels including genetic, species, community, and ecosystem. One way to measure biodiversity is to assess species richness of an ecosystem, which is the total number of distinct species within a local community. While having many species generally coincides with having a diverse and healthy ecosystem, the evenness also needs to be considered. Evenness refers to the equality of the proportion of each species within an area or community. For instance, when one species dominates the area while the others are very rare, the biodiversity in this area is lower than in an area with species of equal abundance. Therefore, areas with many species that are relatively equal in abundance have the highest values of biodiversity.
The differences in richness and evenness between two communities can be visualized by rank-abundance curves. If the number of species is equal, the shape of the line can tell us which community is more diverse. If the line is flat, there is high evenness among species. However, if the line quickly dips, the evenness is low. If richness and evenness are both different between two communities, biologists must use equations to calculate diversity. These equations weight the importance of each component differently, and a consensus on which equation is the best at calculating diversity is still debated.
Sometimes there are too many species in an area that it is unrealistic to count every single species. For example, a single tree in the Amazon Rainforest may contain hundreds of species of beetles. To circumvent this problem, ecologists use sampling tools called quadrats. A quadrat is simply a frame with a known internal area. For example, to measure the species richness of a one-acre field of grass, ecologists randomly place the quadrat in the field and count the species within the quadrat, instead of counting all of the species within the acre. They may also systematically sample by using transect tapes. Transects are stretched across the field, and quadrats are then placed along the transect at regular intervals. This method is semi-random and ensures ample coverage of sampling across the entire field to estimate its biodiversity.
While quadrats and transects may pick up most of the species, some rare species may go unnoticed. In this case, ecologists may use a species accumulation curve, which represents the cumulative number of species seen in a series of quadrats. The y-axis of the curve represents the total number of observed species, whereas the x-axis represents the number of quadrats for which species have been enumerated. The total number of species in the first quadrat represents the first point on the graph. Each successive point represents the number of new species found in each new quadrat sampled, plus all of the species from the previous quadrats. At some point, there will be few or no additional species found in each new quadrat sampled, and the curve will approach an asymptote, which is an estimate of the total number of species present. Even if the asymptote is never reached because of many rare species, biologists can estimate the total number based on this curve.
If comparisons need to be made among different areas or scales, alpha, beta, and gamma diversity measures are used. Alpha-diversity (α) refers to the number of species in an area. Beta-diversity (β) compares two different areas and is the sum of species unique to each area. Gamma-diversity (γ) is the number of species in many areas combined into a region. By using these measures, biologists can get an idea of diversity over space, including both small and large scales.
Threats to Biodiversity and their Implications
Biodiversity around the world is threatened by pollution, climate change, and invasive species. A main underlying reason for efforts to maintain biodiversity is based on ecosystem functioning. Ecosystems are made up of many working parts, including primary producers, herbivores, carnivores, and detritivores, all of which contribute to ecosystem function. If species are lost, the ecosystem may collapse. And if the ecosystem collapses, the services that it provides to humans will as well. Tropical coral reefs are a good example of this concept1. Spikes in water temperatures cause corals to lose their symbiotic algae cells. Without the algae, corals begin to starve, die, then degrade and lose their structure. When corals decay, they no longer provide cover for fish and the abundance of fish species declines, which in turn affects local fishermen, and the people that rely on fish for sustenance. Over time, dead coral reefs degrade on a larger scale and no longer provide a buffer for adjacent coastlines, eventually eroding the coast and destroying islands. A highly diverse community is less likely to collapse because of functional redundancy2. For example, corals may vary in their sensitivity to high temperatures. If one coral is extremely sensitive to temperature, another may take its place in the community, but if there are only a few species, it is less likely that such a substitute will be available.
A significant number of medicines that we benefit from are a direct result of the diversity of life. The medicines that we now synthesize were once isolated from animals, plants, fungi, and bacteria. There is a whole industry devoted to the discovery of new potential medicines by scanning various species for the presence of bioactive compounds. For example, plants produce chemicals for defense against infection and herbivores. Spiders and snakes produce diverse venoms. Both classes of organisms have been the source of important medicines, like Taxol from yew trees, which treats breast, lung and ovarian cancers, or Ohanin from King Cobra venom, which is a painkiller3-4. Each species that becomes extinct may hold the key to curing currently untreatable diseases. The faster we lose those species, the smaller the chance of discovering solutions.
Once a species goes extinct, we will never be able to experience them. This type of thinking has driven the conservation of pandas, sea otters, and other charismatic animals. These species are called flagship species, and their conservation can result in protection of biodiversity. Even though these animals are only a small part of the whole ecosystem, preserving them means preserving the ecosystem they occupy. Efforts to save the sea otter on the West Coast of North America have resulted in healthy kelp forests housing many thousands of other species5. Without protection of the sea otters, herbivores like sea urchins, which are usually eaten by the otters, are capable of completely devouring kelp forests leaving barren rocks where very few species could survive.
- Knowlton, Nancy. The future of coral reefs. PNAS. 2001, Vol. 98 , (10) 5419-5425.
- Andrea S. Downing, Egbert H. van Nes, Wolf M. Mooij, Marten Scheffer. The Resilience and Resistance of an Ecosystem to a Collapse of Diversity. PLoS One. . 2012; , Vol. 7(9): e46135.
- Wall, Monroe E. Camptothecin and taxol: Discovery to clinic. Med Res Rev. 1998, Vol. 18, 5 (299-314).
- Yuh Fen Pung, Peter T. H. Wong, Prakash P. Kumar, Wayne C. Hodgson, R. Manjunatha Kini. Ohanin, a Novel Protein from King Cobra Venom, Induces Hypolocomotion and Hyperalgesia in Mice. J Biol Chem. 2005, 280, 13137-13147.
- Estes, J.A., et al. Complex Trophic Interactions in Kelp Forest Ecosystems. Bulletin of Marine Science, Volume . 2004, Vol. 7, 3: 621-638.