Measuring and Evaluating Biodiversity

In 1982, Terry Erwin came up with a provocative estimate that there were 30 million arthropod species world-wide [Erwin 1982] .

• How can you estimate such figures for global species numbers?

• Why should anyone even bother to do this?

There are various ways that biodiversity be measured and evaluated, and also monitored.

To all intents and purposes, most measures of biodiversity are concerned with species diversity, though many other factors may be used in the assessment.

Canopy fogging

Erwin's Calculations

Canopy insects, from a seasonal tropical rainforest in Panama, were collected byfogging.

[Erwin 1982] estimated there to be over 41,000 arthropod species per hectare in the rainforest.

Furtherextrapolations gave an estimate of 30 million species.

Previous estimates had suggested there were only 2 - 3 million species, so Erwins calculations were quite a revelation.

Many authors, e.g. [Stork 1988] , have challengedErwins's assumptions and figures. [Stork 1988] suggested that a figure of 10 million insect species would be a better estimate.

• Howreliable are such estimates?

Estimating Species Numbers

Methods for evaluating global species numbers include:

• predictions from current species description rates;
• comparisons of known to unknown floras and faunas;
• extrapolations from samples of biodiversity;
• estimates inferred from theoretical and empirical patterns.

Why Estimate Species Numbers?

• Biodiversity is under threat from many sources.

• Biodiversity provides invaluable resources for mankind.

• The topics of biodiversity, sustainability and conservation are key areas for both political debate and scientific research.

So, solving the question "how many species are there?" should not only lead to an improved understanding of the distribution of biodiversity but should also lead to improved predictions as to its fate [Stork 1993] .

Measuring Biodiversity

Speciesrichness is only one way of measuring biological diversity. Other methods include the use ofspecies diversity indices, which not only include the number of species present in their calculation, but are also"weighted" to take into account other measures of relevance.

The use of modernmolecular techniques has revolutionised the evaluation and assessment of the genetic element of biodiversity within and amongst populations. Karyotipic studies are also useful in assessing genetic diversity between closely related species.

Ecological diversity is often evaluated in terms of the richness and abundance of the component species in any particular area, coupled with a description of the physical characteristics of that area. Ecological diversity can be compared at spatial, temporal, trophic and taxonomic levels, amongst others.

Insect species are often used to monitor biodiversity

Describing and providing inventories of global species diversity is the essential work of systematic biologists and ecologists.

Biodiversity Inventories and Monitoring

Large-scale biodiversity assessments can be provided by global initiatives to discover, describe and classify the world's species, such as Systematics Agenda 2000 [SA 2000] , which has a mission to:

• discover, describe and inventory global species diversity;
• analyse and synthesise this information into a predictive classification system that reflects the history of life;
• and organise this information in an efficiently retrievable form that best meets the needs of science and society.

More local information often comes from using rapid biodiversity assessment techniques (RBAs), which can provide quick and cost-effective biodiversity assessments.

Rapid biodiversity assessments may provide information on:

• the status of aparticular taxon;
• the functioning of a particular ecological system;
• the overall biodiversity in an area designated for conservation or resource exploitation.

Quantitative and Qualitative Measures of Biodiversity

Although most measures of biodiversity are concerned withspecies diversity, many other factors may be used in the assessment.

Even if these factors cannot bequantified, they should be considered qualitatively in a full biodiversity assessment.