Biological Diversity

In the broadest sense, biodiversity encompasses all the variety of life forms on Earth. Humans rely upon the earth's biodiversity resources for food, fiber, fuel, building materials, medicine, and technological advances.<!--break--> We do not know exactly how many species live on Earth with us. One educated guess is about 14 million. We only know about the ecological functioning of a small percentage of those. It has also been estimated that around three species per hour go extinct, many due to human activities. In order to conserve biodiversity, we first need to know how to quantify it. In addition, we need a way to compare how biodiversity changes through time and differs from place to place.

There are several ways to look at diversity. We will concentrate on species diversity. Measuring species diversity is based upon two ideas: species richness and species evenness. Species richness refers to the total number of species. Sometimes biologists are simply interested in species richness and use this alone as a measure of biodiversity. Species evenness measures how equally represented the species are, in other words, do all of the species have equal abundances or are they quite skewed with a few being very abundant and others rare?

Why are there so many species? In his classic paper, Hutchinson suggests that diversity might enhance ecosystem stability. He suggested a mechanism to explain this. When more than one species occupies the same ecological niche, it reduces the fluctuation in ecological functioning. he claimed that communities of many species persist better than those containing fewer species. Click here to read: Homage to Santa Rosalia or Why are there so many kinds of animals?

A second influential ecological paper is known simply as HSS. The authors claim that as long as terrestrial herbivores are preyed upon by predators, they will never increase to the extent that they destroy the vegetation. Plants, or producers, are therefore not limited by herbivores, but must therefore be limited by resources. Predators are, however, limited by the supply of herbivores. Producers, carnivores, and decomposers are limited by interspecific competition.Click here to read: Community Structure, Population Control, and Competition

In her 1992 paper, Mary Powers asserts that plants have primacy in food webs.  In particular, she says the primary productivity of plants "is a fundamental control of higher trophic levels."  Read more here: Top-Down and Bottom-Up forces in food webs: Do plants have primacy? 

In another classic paper, MacArthur proposed that there was a common pattern of relative abundance of species in ecosystems. A few species were usually quite abundant, while many other species occurring at the site are more rare. Since species probably use ecological niches in an overlapping manner, their arrangement is likely due to competition. MacArthur proposed that the species abundance arrangement could be described as a broken stick. Click here to read: On the relative abundance of bird species.