Biological DiversityEssential Knowledge for Naturalists
The Variety of Life on Earth
Biological Diversity is the variability among life forms, including land and water-based organisms and the ecological systems of which they are part; this includes diversity within species, between species, and of habitats and ecosystems. So, when a naturalist talks about biological diversity, or biodiversity, what they mean is all of the organisms that live in a particular area. Areas with a greater mix of species, or variations within a species, are considered more biologically diverse. Conversely, those areas with fewer species are considered less diverse.
Biological diversity is an important concept to grasp, because the health of an ecosystem is directly tied to its diversity. Each species in an ecosystem plays a role in that system. For example, the toothwort, a wildflower, is the host plant for the West Virginia White butterfly. If toothwort were lost from an ecosystem, one of its functions (being the host to this butterfly) would be lost to the system. The cascading results of such losses weaken the natural community in many indirect ways that humans sometimes have a difficult time grasping.
Maintaining biological diversity is a key to keeping the planet healthy. We, as humans, depend on the services of the ecosystem to sustain us. Clean air, healthy water supplies, beautiful scenery, and our food supplies, all depend to some degree on biological diversity. Each component of the natural system is important in its own way.
Everything in Nature has its niche. Certain birds are bug-eaters. If poisons kill off those birds, bug populations may increase, which in turn may harm the plant communities that those bugs use for nourishment. That in turn could change other dynamics.
Everything has its niche, and even with our advanced science, we humans still do not have the capability of really predicting, of really knowing, what the long term cascading impacts of apparently small changes, such as the loss of one species in an ecosystem, really are.
Measurements of diversityOn a simplistic level, we can simply count species. This method (species richness) can give a gross assessment of how healthy an ecosystem is, if you have baseline statistics to compare to current counts. However, this method fails to take into account the relative abundances of the species present. Species richness is the same for a site with 33 species represented by 1 individual per species, or a site with 33 species, each species with 10 individuals.
Other measurements of diversity take this into account. One widely used measurement of biological diversity is known as the Shannon Diversity Index. The Shannon Diversity Index takes into account not only species richness (the number of species present), but also the proportion of each species present, as well as evenness (the equitability of the various species populations. By taking into account all three of these factors, a meaningful picture of the biological diversity of a natural community emerges.
When inferring the health of a community from impressions of diversity, the concept of native diversity should be considered. What if there are two communities, each with 100 individuals, representing 10 species, with an evenness of 1 (which means that each species has the same number of individuals). The first community is entirely made up of native species, whereas the second community is half-composed of non-native species that only could have migrated to the area with the interference of mankind. Which community is healthier? Which is more likely to have been the result of past disturbance?
Natural succession also plays a role in the level of biological diversity of an ecosystem. In transitional areas, where more light and moisture are available, such as old fields and forest edges, you may find more species, more abundance, and a more even distribution. In an old growth woods, the system may appear less diverse. This doesn't mean old growth woods are less healthy. It is simply a different stage of succession, with its own attributes.