Sonoran Desert Knowledge Exchange (SDKE)

Learning About Sonoran Desert Biodiversity

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Biodiversity Information

Biodiversity is the variety of life that exists on our planet. We can think about this variety at many different scales, from the diversity of genetic material within a single species to the diversity of biomes within an entire geographical region.

When biologists set out to study the diversity of a specific place they usually focus on the diversity of one or a few taxa. A taxon (plural: taxa) is simply a group of related organisms. Why don’t biologists study the diversity of all life within their study areas? One reason is simply because species identification requires expert knowledge. A person with expert knowledge in species identification is called a systematist. It often takes a systematist many years of work to develop the skills needed to be able to quickly identify the species they study. Thus most systematists have knowledge of only a few groups of organisms, which limits the scope of most diversity studies. For example, it took a group of scientists studying the diversity of several taxa (birds, butterflies, beetles, ants, termites, and nematodes) in a forest reserve in Cameroon 10,000 hours to identify the 2,000 species that were collected during the study₁.

So how do we decide which taxa we should study? Although arthropods can be difficult to identify, they make good focal taxa because they are essential to many ecosystem processes such as decomposition and pollination. Because humans depend on arthropods to carry out these processes, it is important to monitor the health of arthropod communities in the environment. Another reason that arthropods make good focal taxa is that they often exhibit a high degree of habitat specificity. For example, many herbivorous insect species depend entirely on one or a few species of plant for food. Very few vertebrate species are as specific in their dietary requirements. Thus, by studying the diversity of arthropods we are often able to gain insight into other habitat characteristics, such as the diversity of the plant community.

How do we measure diversity?

Above we discussed why and how scientists select which group of organisms to study. We also have to decide if we want to measure the diversity of this group at the species level or at some higher level such as genus, family, or order. (Remember Kingdom, Phylum, Class, Order, Family, Genus, Species?) Most often scientists describe diversity at the species level. But what exactly is a species and how is a species different from a genus? A species is group of organisms that is genetically distinct from all other groups of organisms. An individual of a given species can only breed with another individual also from the same species. A genus is a group of closely related species. And if we continue up the chain of biological classification we find that families are groups of related genera and so on. We can choose to measure diversity at any level of classification. The diversity of arthropods is often measured at the level of family or order simply because the expertise to identify most arthropods to species does not exist.

When we set out to measure diversity, it is important that we have a clear question in our heads. Sometimes scientists need to inform policy makers of the diversity of a geographical area such as a park or an area targeted for development. Often scientists want to compare the diversity of different habitat types. In order to be able to compare the diversity of one area or habitat to another we have to employ standardized sampling methods. This simply means that we have to sample the diversity of the two habitats in exactly the same manner. Often there are standardized sampling methods used by many scientists across the planet. By conforming to these methods we may be able to compare the results of our study with those of scientists working in different locations.

1. Lawton J.D. et al. 1998. Biodiversity inventories, indicator taxa and effects of habitat modification in tropical forest. Nature 391:72-76.

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