For those of us who are fascinated with the biodiversity of life on Earth, we constantly marvel at the overwhelming diversity of life that has come from such a small portion of the Earth. Below us only some five hundred meters from the planets surface bacteria thrive. The varieties of life increase as the Earth's surface approaches. On the surface is where most of the life exists in a plethora of shapes and sizes that include microorganisms, plants, and animals. And among this great diversity of life there is a constant struggle for existence. The Earth's atmosphere acts like a partially shaded window that lets in only about a tenth of the sun's energy that strikes the Earth. Divide that by a factor of ten and that is the removed from the food web by herbivores; ten percent of that is consumed by spiders and other similar carnivores; some species of birds consume the next ten percent in the form of the spiders, etc., as the process continues until the top carnivores such as lions, tigers, sharks, eagles, etc. exist at the top of the chain only to be consumed themselves by parasites and other scavengers.
On closer observation it becomes apparent that all species are part of two hierarchies. An energy hierarchy, or pyramid consists of plants at the bottom that receive the greatest share of the sun's energy with the largest carnivores located at the very tip of the pyramid.
Biomass makes up the second hierarchy as the plants are once again located at the bottom. The next level consists of scavengers and other decomposers, including bacteria and fungi. Again, the top tip of the pyramid is populated by the largest carnivores. There is a completely different situation when the hierarchy of the oceans biomass is studied. The bottom, or largest portion of that pyramid is occupied by organisms that eat the photosynthetic life. The reason for this is that the photosynthetic organisms are not similar to the plants found on land because they are microscopic single-celled phytoplankton which despite their small size are able to fix more energy from the sun and they have a rapid life cycle. Zooplankton subsist on the phytoplankton but are unable to completely eradicate the phytoplankton. From the zooplankton, the pyramid grows smaller until the very tip is reached and is occupied by the ocean's largest carnivores.
One of the first points of interest drawn from studying the hierarchies is that the largest carnivores are completely dependent on biological diversity.
And the theoretical and experimental assumption taken away from studying diversity is that complex systems can be broken down into increasingly simpler systems. This means that the concept of understanding species and their interdependence is key to the study of biodiversity. And from this realization it can be inferred that a species is a population that consists of members that can interbreed freely under normal conditions. The caveat, under normal conditions is added to exclude consideration of hybrids created under unnatural conditions. This also means that each species is a closed gene pool and is a part of the reality that few if many hybrids occur naturally.
The biological species method of classification is not unambiguous and problem free. The inclusion of the concept of evolution creates exceptions and ambiguities that enter into the biological species method of classification. It is evolution that makes the same reproductively isolated species different from another group of the same species in another location (biogeography-the study of the distribution of biodiversity over space and time)
thus making geography a factor. Another difficulty arises when semispecies are considered because they include aspects of partial interbreeding that create hybrids under natural conditions and is a problem seen mostly among plants. A third difficulty for the biological species method of classification involves hermaphrodites where the idea of a closed gene pool holds no meaning. Continuing with the closed gene pool idea is the existence of chronospecies which simply relates to the different stages of evolution of the same species over the passage of time.
The great range of biological diversity found throughout the globe is a product of the divergence of species that express actualization of the biological species concept.
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