Two species richness indices try to account for this problem:. Despite the attempt to correct for sample size, both measures remain strongly influenced by sampling effort. Nonetheless they are intuitively meaningful indices and can play a useful role in investigations of biological diversity. The first two indices are based on information theory.
These indices are based on the rationale that the diversity in a natural system can be measured in a similar way to the information contained in a code or message see Appendix A1. The most widely used diversity index in the ecological literature is the Shannon-Wiener diversity index [5] [6]. It assumes that individuals are randomly sampled from a very large community, and that all species are represented in the sample. The Shannon index is given by the expression.
Where the randomness cannot be guaranteed, for example when certain species are preferentially sampled, the Brillouin index [7] [6] is a more appropriate form of the information index.
It is calculated as follows:. The relation between the Brillouin index and the Shannon-Wiener index is explained in appendix A1. This index is used for large sampled communities. Another evenness index was proposed by Pielou [9]. The Hill numbers [10] combine species richness and evenness. Hill defined a set of diversity numbers of different order. If two data-sets have identical numbers of species and equivalent patterns of species abundance, but differ in the diversity of taxa to which the species belong, it seems intuitively appropriate that the most taxonomically varied data-set is the more diverse.
As long as the phylogeny of the data-set of interest is reasonably well resolved, measures of taxonomic diversity are possible. The index has different forms: taxonomic diversity and taxonomic distinctness. The distance can be seen as the length of the path connecting these two organisms through a phylogenetic tree also called dendrogram or a Linnean classification [11].
This index includes aspects of taxonomic relatedness and evenness,. This index is measure of pure taxonomic relatedness,. The positive relationship between ecosystem functioning and species richness is often attributed to the greater number of functional groups found in richer assemblages see also Resilience and resistance.
Petchey and Gaston [13] proposed a method for quantifying functional diversity. It is based on total branch length of a dendrogram , which is constructed from species trait values.
One important consideration is that only those traits linked to the ecosystem process of interest are used. Thus a study focusing on bird-mediated seed dispersal would exclude traits such as plumage color that are not related to this function, but traits such as beak size and shape should be included. With standard clustering algorithms a dendrogram is then constructed.
For example, a community with five species with different traits will have a higher functional diversity than a community of equal richness but where the species are functionally similar. When recording the abundance of different species in a sample, it is invariably found that some species are rare, whereas others are more abundant.
This feature of ecological communities is found independently of the taxonomic group or the area investigated. An important goal of ecology is to describe these consistent patterns in different communities, and explain them in terms of interactions with the biotic and abiotic environment.
Several models have been developed to describe species abundance data Fig. These models are based on species abundance observed in particular ecosystems. It describes the way in which the individuals are divided among the species, which is a measure of diversity. In general diverse communities are believed to have increased stability, increased productivity, and resistance to invasion and other disturbances.
Though seldom acknowledged, there are also disadvantages to high biodiversity:. In the example below, the greatest Beta Diversity is observed between Site A and C with 10 species that differ between them and only 2 species in common. In this example, the gamma diversity is 3 habitats with 12 species total diversity. What is biodiversity? Why is biodiversity measured in plant assessments? What is the difference between alpha, beta, and gamma diversity? Web Design - CTI. Why Measure Biodiversity?
Summarizing values to Describe Biodiversity What is biodiversity? The y-axis of the curve represents the total number of observed species, whereas the x-axis represents the number of quadrats for which species have been enumerated. The total number of species in the first quadrat represents the first point on the graph.
Each successive point represents the number of new species found in each new quadrat sampled, plus all of the species from the previous quadrats. At some point, there will be few or no additional species found in each new quadrat sampled, and the curve will approach an asymptote, which is an estimate of the total number of species present. Even if the asymptote is never reached because of many rare species, biologists can estimate the total number based on this curve.
If comparisons need to be made among different areas or scales, alpha, beta, and gamma diversity measures are used. By using these measures, biologists can get an idea of diversity over space, including both small and large scales. Biodiversity around the world is threatened by pollution, climate change, and invasive species. A main underlying reason for efforts to maintain biodiversity is based on ecosystem functioning. Ecosystems are made up of many working parts, including primary producers, herbivores, carnivores, and detritivores, all of which contribute to ecosystem function.
If species are lost, the ecosystem may collapse. And if the ecosystem collapses, the services that it provides to humans will as well. Tropical coral reefs are a good example of this concept 1. Spikes in water temperatures cause corals to lose their symbiotic algae cells. Without the algae, corals begin to starve, die, then degrade and lose their structure. When corals decay, they no longer provide cover for fish and the abundance of fish species declines, which in turn affects local fishermen, and the people that rely on fish for sustenance.
Over time, dead coral reefs degrade on a larger scale and no longer provide a buffer for adjacent coastlines, eventually eroding the coast and destroying islands. A highly diverse community is less likely to collapse because of functional redundancy 2. For example, corals may vary in their sensitivity to high temperatures. If one coral is extremely sensitive to temperature, another may take its place in the community, but if there are only a few species, it is less likely that such a substitute will be available.
A significant number of medicines that we benefit from are a direct result of the diversity of life. The medicines that we now synthesize were once isolated from animals, plants, fungi, and bacteria. There is a whole industry devoted to the discovery of new potential medicines by scanning various species for the presence of bioactive compounds.
For example, plants produce chemicals for defense against infection and herbivores. Spiders and snakes produce diverse venoms.
Both classes of organisms have been the source of important medicines, like Taxol from yew trees, which treats breast, lung and ovarian cancers, or Ohanin from King Cobra venom, which is a painkiller Each species that becomes extinct may hold the key to curing currently untreatable diseases. The faster we lose those species, the smaller the chance of discovering solutions.
Once a species goes extinct, we will never be able to experience them. This type of thinking has driven the conservation of pandas, sea otters, and other charismatic animals. These species are called flagship species, and their conservation can result in protection of biodiversity. Even though these animals are only a small part of the whole ecosystem, preserving them means preserving the ecosystem they occupy.
Efforts to save the sea otter on the West Coast of North America have resulted in healthy kelp forests housing many thousands of other species 5.
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