Biological Diversity

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 (Homage to Santa Rosalia or Why are there so many kinds of animals?), 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. 

A second influential ecological paper (Community Structure, Population Control, and Competition) 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.

In her 1992 paper (Top-Down and Bottom-Up forces in food webs: Do plants have primacy?), 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."  

In another classic paper (On the relative abundance of bird species), 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. 

Often, biologists use a combination of species richness and species evenness to calculate what is called a diversity index. The Shannon Index uses relative abundance data to incorporate species evenness and species richness into a single measure of diversity, represented by H'. As H' increases, diversity increases. The Simpson Index, represented as D, gives more weight to the relative abundance of species in a community in that it is based on the probability that two individuals drawn from an infinitely large community are the same species. As D increases, diversity decreases, which is rather counter-intuitive. For the value to be more intuitive, the Simpson Index is usually expressed as 1-D or 1/D, so that the value actually rises as the community becomes more diverse.

What affects species diversity? This question has occupied ecologists for a long time. In the paper Biodiversity and Litter Decomposition in Terrestrial Ecosystems, Hallenschwiler and colleagues explore the role of decomposer species in nutrient and carbon turnover to help explain the tremendous diversity of decomposer species in soil litter layer. You might expect that if you artificially added nutrients to the soil where they were naturally limited, you would consequently increase diversity. For example, one experiment, published in the paper Human-caused environmental change: impacts on plant diversity and evolution, investigating this question has been conducted in Rothamsted, England over the past 150 years. In fact, when compared to the control plots, which have not been fertilized, the species diversity in plots that were fertilized has progressively declined.

Some additional ecological questions are as follows. Why do some ecological communities have patterns of greater species richness than others? What explains the differences in these patterns? What are the relationships between these species? How does the physical environment (such as the patchy occurrence of different types of environments within an area) help shape which species occur where?

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Additional Relevant Journal Articles:

  • Trophic Dynamics in Urban Communities by Faeth, S. H., Warren, P. S., Shochat, E. and Marussich, W. A.
  • Resilience and Regime Shifts: Assessing Cascading Effects by Kinzig, A. P., Ryan, P., Etienne, M, Allison, H., Elmqvist, T. and Walker, B. H.
  • Plant Species Diversity in Managed and Natural Forests of the Pacific Northwest by Halpern, C. B. and Spies, T. A. - This paper shows that vascular plant diversity in the Pacific Northwest tends to increase over time, peaking in old growth forests. Many species differ in their abundance in different stages.