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How do biologists measure biodiversity?
Because it is impractical or impossible to count every individual
in most populations or communities (groups of populations),
biologists measure biodiversity by first sampling the organisms
and then extrapolating to estimate the total number of
organisms. For example, to compare the number of bird species
in different types of forest, biologists record the number and
species of individual birds encountered at randomly selected
locations within each forest type. Population biologists compare
the average density of the individual species in each forest type.
Community biologists compare the average number of species in
a given area, such as a square meter or square kilometer, or the
diversity index in a given area. The higher the diversity index,
the more species and the more even the distribution of individual
organisms among these species. Biologists interested in genetic
or ecosystem diversity rely on similar sampling procedures and
diversity indices.
Measuring Biodiversity
From the definition alone, which is very broad, it is clear that no single measure of
biodiversity will be adequate. Biodiversity can not be captured in a single number.
Essentially, measurements have two components:
• The number of entities e.g. the number of individuals, the number of species, the
number of different habitats etc.
• The degree of difference (dissimilarity) between those entities.
Species richness (the number of species) describes the number of elements but will
not capture information on the number of individuals of the species. Rarity and
conservation status, dependant on how threatened a species is, can also provide a
measure of biodiversity.
Measures of biodiversity are commonly used as the basis for making conservation
decisions or for planning more generally. Different measures of biodiversity may
support different solutions. Often indicator species are used as a way of measuring
biodiversity. Using this method can be very useful but it introduces an aspect of
how we value different components of biodiversity. For example, we are more
likely to use the abundance of birds or butterflies on a farm as a measure of
biodiversity than the richness of microbes in the soil.
Genetic diversity can be measured directly by looking at genes and chromosomes
or indirectly by looking at physical features of the organisms and assuming they
have a genetic basis. Using the genetic code is arguably the strongest method of
measuring biodiversity as it is looking at the building blocks of life.
Generally, multicellular organisms tend to have more DNA than single-celled
organisms but there are exceptions. Similarly, although there appears to be an
overall trend of increasing amount of DNA with increasing complexity of
organisms, this is not invariant. The species with the greatest amount of DNA has
about 100,000 times as much as that with the least, but the species with the largest
number of genes has only 20 times as many genes as that found in many bacteria.
In other words, much genetic variation is attributable not to differences in the
number of functional genes, but in the amounts of non-coding DNA. One of the
most striking findings is that there are many ‘universal’ gene segments in a wide
range of organisms suggesting the existence of an ancient minimal set of DNA
sequences that all cells must have.
Whilst biodiversity can be measured in a host of ways, in practice it tends to be
measured in terms of species richness; the number of species.
The advantages of this are:
• Practical application: Species richness has proven to be measurable in practice, at
least to the point where different workers will provide similar estimates of species
numbers.
• Existing information: A substantial amount of information already exists on
patterns in species richness, and this has been made available in scientific
literature.
• Surrogacy: Species richness acts as a surrogate measure for many other kinds of
variation in biodiversity. In general, as long as the numbers involved are at least
moderate, greater numbers of species tend to embody more genetic diversity (in the
form of a greater diversity of genes through to populations), more species
diversity, and greater ecological diversity.
• Wide application: The species unit is commonly seen as the unit of practical
management, of legislation and of political discourse. For a wide range of people,
variation in biodiversity is pictured as variation in species richness.
The disadvantages are:
• Definition of species: The lack of agreement as to what constitutes a species. In
major part this results because species can, to a large extent be regarded as
hypotheses, opinions or concepts, as much as real robust entities. Click here to see
the different definitions of species. The vast majority of groups of organisms have
been, and are still being, described based on collections of preserved specimens
using differences in morphological characteristics. Fortunately, this method of
defining a species continues to be relatively effective for most multicelluar species.
It is not an adequate method of defining single cell organisms.
• Different kinds of diversity: An additional limitation of species richness as a
measure of biodiversity has frequently been illustrated with reference to the issue
of whether an assemblage of a small number of closely related species, say two
species of mouse, is more or less biodiverse than an equivalent sized assemblage of
more distantly related species, say a species of mouse and a species of shrimp.
Measuring Biodiversity
Use our database numbers to estimate various measures of biodiversity.
Species Richness (S) - the total number of different organisms present. It does not take into account the
proportion and distribution of each subspecies within a zone.
Simpson Index (D) - a measurement that accounts for the richness and the percent of each
subspecies from a biodiversity sample within a zone. The index assumes that the proportion of
individuals in an area indicate their importance to diversity.
Shannon-Wiener index (H) - Similar to the Simpson's index, this measurement takes into
account subspecies richness and proportion of each subspecies within a zone. The index comes
from information science. It has also been called the Shannon index and the Shannon-Weaver
index in the ecological literature.
More About Measuring Biodiversity
When measuring diversity it is good to remember that what we are trying to describe is the relationship
of individuals of varying subspecies within a zone. In our research, we use the number of individuals of
each subspecies observed (e.g., found in each zone).
There are some underlying assumptions that all the measures of biodiversity have in common:
1. The categories are well known.
In most cases, such as with our study, this assumption is not violated since the classification of
subspecies that we use is accepted world-wide. However, this may be difficult if everyone does
not use the same classification system (e.g., some people may put similar individuals into
several subspecies categories while others use one category).
2. All categories are equally different.
Categories are equally different from each other category. This is not always true since two
subspecies from the same genera are treated the same as two subspecies from different
families.
3. Use a measure of species importance.
Usually, one uses the number of individuals, percent coverage, relative density or biomass. The
choice usually depends on the ease of measurement. In our case, we use the number of
individuals of each subspecies observed.
4. The community under study is well defined.
The relative importance of an individual category will vary greatly depending on the definition of
the extent and makeup of the community. The communities in our studies are the zones.
The most common measures of biodiversity are Species richness, Simpson's index and ShannonWiener index.

Species Richness
This is the simplest of all the measures of subspecies diversity. All you do is count of the number
of subspecies found in a community (e.g., the number of the subspecies found in the Meadow
Fields zone).
However, this does not indicate how the diversity of the population is distributed or
organized among those particular subspecies. For example, if there were 4 different
subspecies observed in Zone 1 and Zone 2 the richness would be equal. This does not
indicate what percentage of the abundance there were of each subspecies. In Zone 1, 80%
of the abundance could have been Blue Jay while at Zone 2 there could have been an
even 25% of each subspecies.

Simpson's Index
A measure that accounts for both richness and proportion (percent) of each species is the
Simpson's diversity index. It has been a useful tool to terrestrial and aquatic ecologists for many
years and will help us understand the profile of biodiversity across our Zones.
The index, first developed by Simpson in 1949, has been defined three different ways in
published ecological research. The first step for all three is to calculate Pi, which is the
abundance of a given subspecies in a zone divided by the total number of subspecies
observed in that zone.
1. Simpson's index: D = sum(Pi2)
The probability that two randomly selected individuals in the zone belong to the same
subspecies.
2. Simpson's index of diversity: 1 - D
The probability that two randomly selected individuals in a zone belong to different
subspecies.
3. Simpson's reciprocal index: 1/D
The number of equally common subspecies that will produce the observed Simpson's
index.
D is influenced by two parameters - the equitability of percent of each species present
and richness. For a given species richness, D will decrease as the percent of the species
becomes more equitable. The researcher must observe the species patterns carefully to
interpret the values effectively.

Shannon-Wiener Index
This diversity measure came from information theory and measures the order (or disorder)
observed within a particular system. In ecological studies, this order is characterized by the
number of individuals observed for each subspecies in the sample plot (e.g., zone on our site). It
has also been called the Shannon index and the Shannon-Weaver index.
Similar to the Simpson index, the first step is to calculate Pi for each category subspecies.
You then multiply this number by the log of the number. While you may use any base,
the natural log is commonly used. The index is computed from the negative sum of these
numbers. In other words, the Shannon-Wiener index is defined as:
H = -sum(Pilog[Pi])
Using species richness (S) and the Shannon-Wiener index (H), you can also compute a
measure of evenness:
E = H/log(S)
Evenness (E) is a measure of how similar the abundances of different species are. When
there are similar proportions of all subspecies then evenness is one, but when the
abundances are very dissimilar (some rare and some common species) then the value
increases.