Biological Diversity
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. 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. Click here to read: Biodiversity and Litter Decomposition in Terrestrial Ecosystems. 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 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. You can read about this study by clicking here (Tilman, 2001).
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?
Click on the links below to learn more about:
- Measuring biodiversity: Biodiversity Metrics
- Energy Flux in Food Webs
- Elk Game Science Activity - this activity shows students (grades 8-12) how competition, predation, and disease affect populations of elk, cattle, wolves, and hunters in a Rocky Mountain community.
Links to Additional Relevant Journal Articles:
- Trophic Dynamics in Urban Communities
- Resilience and Regime Shifts: Assessing Cascading Effects
- Plant species diversity in managed and natural forests of the Pacific Northwest- 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.
