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6: Species Diversity - Biology

6: Species Diversity - Biology


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Strictly speaking, species diversity is the number of different species in a particular area (species richness) weighted by some measure of abundance such as number of individuals or biomass. However, it is common for conservation biologists to speak of species diversity even when they are actually referring to species richness.

Another measure of species diversity is the species evenness, which is the relative abundance with which each species is represented in an area. An ecosystem where all the species are represented by the same number of individuals has high species evenness. An ecosystem where some species are represented by many individuals, and other species are represented by very few individuals has a low species evenness. Table shows the abundance of species (number of individuals per hectare) in three ecosystems and gives the measures of species richness (S), evenness (E), and the Shannon diversity index (H).

Shannon's diversity index (H=−∑ρ_iln(ρ_i))

  • (ρ_i) is the proportion of the total number of specimens ii expressed as a proportion of the total number of species for all species in the ecosystem. The product of (ρ_iln(ρ_i)) for each species in the ecosystem is summed, and multiplied by (−1) to give (H). The species evenness index ((E)) is calculated as (E=frac{H}{H_{max}}).
  • (H_{max}) is the maximum possible value of (H), and is equivalent to (ln(S)). Thus (E=frac{H}{ln(S)})

See Gibbs et al., 1998: p157 and Beals et al. (2000) for discussion and examples. Magurran (1988) also gives discussion of the methods of quantifying diversity.

In Table, ecosystem A shows the greatest diversity in terms of species richness. However, ecosystem B could be described as being richer insofar as most species present are more evenly represented by numbers of individuals; thus the species evenness (E) value is larger. This example also illustrates a condition that is often seen in tropical ecosystems, where disturbance of the ecosystem causes uncommon species to become even less common, and common species to become even more common. Disturbance of ecosystem B may produce ecosystem C, where the uncommon species 3 has become less common, and the relatively common species 1 has become more common. There may even be an increase in the number of species in some disturbed ecosystems but, as noted above, this may occur with a concomitant reduction in the abundance of individuals or local extinction of the rarer species.

Species richness and species evenness are probably the most frequently used measures of the total biodiversity of a region. Species diversity is also described in terms of the phylogenetic diversity, or evolutionary relatedness, of the species present in an area. For example, some areas may be rich in closely related taxa, having evolved from a common ancestor that was also found in that same area, whereas other areas may have an array of less closely related species descended from different ancestors (see further comments in the section on Species diversity as a surrogate for global biodiversity).

To count the number of species, we must define what constitutes a species. There are several competing theories, or "species concepts" (Mayden, 1997). The most widely accepted are the morphological species concept, the biological species concept, and the phylogenetic species concept.

Although the morphological species concept (MSC) is largely outdated as a theoretical definition, it is still widely used. According to this concept: species are the smallest groups that are consistently and persistently distinct, and distinguishable by ordinary means. (Cronquist, 1978). In other words, morphological species concept states that "a species is a community, or a number of related communities, whose distinctive morphological characters are, in the opinion of a competent systematist, sufficiently definite to entitle it, or them, to a specific name" (Regan, 1926: 75).

The biological species concept (BSC), as described by Mayr and Ashlock (1991), states that "a species is a group of interbreeding natural populations that is reproductively isolated from other such groups".

According to the phylogenetic species concept (PSC), as defined by Cracraft (1983), a species : "is the smallest diagnosable cluster of individual organism [that is, the cluster of organisms are identifiably distinct from other clusters] within which there is a parental pattern of ancestry and descent". These concepts are not congruent, and considerable debate exists about the advantages and disadvantages of all existing species concepts (for further discussion, see the module on Macroevolution: essentials of systematics and taxonomy).

In practice, systematists usually group specimens together according to shared features (genetic, morphological, physiological). When two or more groups show different sets of shared characters, and the shared characters for each group allow all the members of that group to be distinguished relatively easily and consistently from the members of another group, then the groups are considered different species. This approach relies on the objectivity of the phylogenetic species concept (i.e., the use of intrinsic, shared, characters to define or diagnose a species) and applies it to the practicality of the morphological species concept, in terms of sorting specimens into groups (Kottelat, 1995, 1997).

Despite their differences, all species concepts are based on the understanding that there are parameters that make a species a discrete and identifiable evolutionary entity. If populations of a species become isolated, either through differences in their distribution (i.e., geographic isolation) or through differences in their reproductive biology (i.e., reproductive isolation), they can diverge, ultimately resulting in speciation. During this process, we expect to see distinct populations representing incipient species - species in the process of formation. Some researchers may describe these as subspecies or some other sub-category, according to the species concept used by these researchers. However, it is very difficult to decide when a population is sufficiently different from other populations to merit its ranking as a subspecies. For these reasons, subspecific and infrasubspecific ranks may become extremely subjective decisions of the degree of distinctiveness between groups of organisms (Kottelat, 1997).

An evolutionary significant unit (ESU) is defined, in conservation biology, as a group of organisms that has undergone significant genetic divergence from other groups of the same species. According to Ryder, 1986 identification of ESUs requires the use of natural history information, range and distribution data, and results from analyses of morphometrics, cytogenetics, allozymes and nuclear and mitochondrial DNA. In practice, many ESUs are based on only a subset of these data sources. Nevertheless, it is necessary to compare data from different sources (e.g., analyses of distribution, morphometrics, and DNA) when establishing the status of ESUs. If the ESUs are based on populations that are sympatric or parapatric then it is particularly important to give evidence of significant genetic distance between those populations.

ESUs are important for conservation management because they can be used to identify discrete components of the evolutionary legacy of a species that warrant conservation action. Nevertheless, in evolutionary terms and hence in many systematic studies, species are recognized as the minimum identifiable unit of biodiversity above the level of a single organism (Kottelat, 1997). Thus there is generally more systematic information available for species diversity than for subspecific categories and for ESUs. Consequently, estimates of species diversity are used more frequently as the standard measure of overall biodiversity of a region.

TaxonTaxon Common NameNumber of species described*N as percentage of total number of described species*
Bacteriatrue bacteria90210.5
Archaeaarchaebacteria2590.01
Bryophytamosses150000.9
Lycopodiophytaclubmosses12750.07
Filicophytaferns95000.5
Coniferophytaconifers6010.03
Magnoliophytaflowering plants23388513.4
Fungifungi1008005.8
"Porifera"sponges100000.6
Cnidariacnidarians90000.5
Rotiferarotifers18000.1
Platyhelminthesflatworms137800.8
Molluscamollusks1174956.7
Annelidaannelid worms143600.8
Nematodanematode worms200001.1
Arachnidaarachnids744454.3
Crustaceacrustaceans388392.2
Insectainsects82787547.4
Echinodermataechinoderms60000.3
Chondrichthyescartilaginous fishes8460.05
Actinopterygiiray-finned bony fishes237121.4
Lissamphibialiving amphibians49750.3
Mammaliamammals44960.3
Chelonialiving turtles2900.02
Squamatalizards and snakes68500.4
Avesbirds96720.6
Other19307511.0

Table (PageIndex{1}) : Estimated Numbers of Described Species, Based on Lecointre and Guyader (2001) * The total number of described species is assumed to be 1,747,851. This figure, and the numbers of species for taxa are taken from LeCointre and Guyader (2001).

Glossary

Species diversity
the number of different species in a particular area (i.e., species richness) weighted by some measure of abundance such as number of individuals or biomass.
Species richness
the number of different species in a particular area
Species evenness
the relative abundance with which each species are represented in an area.
Phylogenetic diversity
the evolutionary relatedness of the species present in an area.
Morphological species concept
species are the smallest natural populations permanently separated from each other by a distinct discontinuity in the series of biotype (Du Rietz, 1930; Bisby and Coddington, 1995).
Biological species concept
a species is a group of interbreeding natural populations unable to successfully mate or reproduce with other such groups, and which occupies a specific niche in nature (Mayr, 1982; Bisby and Coddington, 1995).
Phylogenetic species concept
a species is the smallest group of organisms that is diagnosably [that is, identifiably] distinct from other such clusters and within which there is a parental pattern of ancestry and descent (Cracraft, 1983; Bisby and Coddington, 1995).
Evolutionary significant unit
a group of organisms that has undergone significant genetic divergence from other groups of the same species. Identification of ESUs is based on natural history information, range and distribution data, and results from analyses of morphometrics, cytogenetics, allozymes and nuclear and mitochondrial DNA. Concordance of those data, and the indication of significant genetic distance between sympatric groups of organisms, are critical for establishing an ESU.
Ecosystem
a community plus the physical environment that it occupies at a given time.
Sympatric
occupying the same geographic area.
Parapatric
occupying contiguous but not overlapping ranges.

Mr G’s Environmental Systems

During succession Gross Primary Productivity tends to increase through the pioneer and early wooded stages and then decreases as climax community reaches maturity. This increase in productivity is linked to growth and biomass.

Early seral stages are usually marked by rapid growth and biomass biomass accumulation - grasses, herbs and small shrubs. Gross Primary Productivity is low but Net Primary Productivity tends to be be a large proportion of GPP as with little biomass in the early seral stages respiration is low. As the community develops towards woodland and biomass increases so does productivity. But NPP as a percentage of GPP can fall as respiration rates increase with more biomass.

Studies have shown that standing crop (biomass) in succession to deciduous woodland reaches a peak within the first few centuries. Following the establishment of mature climax forest biomass tends to fall as trees age growths slows and an extended canopy crowds out ground cover. Also Older trees become less photosynthetically efficient and more NPP is allocated to none photosynthetic structural biomass such as root systems.

Biomass Accumulation and Successional Stage:

Low GPP but High percetage NPP

Little increase in biomass

Gross Productivity high increased photosynthesis

Increases in biomass as plant forms become bigger

Tree reach their maximum size

Ratio of NPP to R is roughly equal

Biodiversity

Early stages of succession tend to be marked by few species within the community. As the community passes through subsequent seral stages so the number of species found increases. Very few pioneer species are ever totally replaced as succession continues. The result is increasing diversity - more species. This increase tends to continue until a balance is reached between possibilities for new species to establish, existing species to expand their range and local extinction. Evidence following the eruption of the mount St Helens volcano in 1980 has provided ecologists with a natural laboratory to study succession. In the first 10 years after the eruption species diversity increased dramatically but after 20 years very little additional increase in the diversity occurred 1

Disturbance
Early ideas about succession suggested that the Climax community of any area was almost self perpetuating. This is unrealistic as communities are affected by periods of disturbance to greater or lesser extent. Even in large forests trees eventually age, die and fall over leaving a gap. Other communities are affected by flood, fire, land slides earthquakes, hurricanes etc. All of these have an effect of making gaps available that can be colonised by pioneer species within the surrounding community. This adds to both the productivity and diversity of the community.


Species diversity vs. morphological disparity in the light of evolutionary developmental biology

Background: Two indicators of a clade's success are its diversity (number of included species) and its disparity (extent of morphospace occupied by its members). Many large genera show high diversity with low disparity, while others such as Euphorbia and Drosophila are highly diverse but also exhibit high disparity. The largest genera are often characterized by key innovations that often, but not necessarily, coincide with their diagnostic apomorphies. In terms of their contribution to speciation, apomorphies are either permissive (e.g. flightlessness) or generative (e.g. nectariferous spurs).

Scope: Except for Drosophila, virtually no genus among those with the highest diversity or disparity includes species currently studied as model species in developmental genetics or evolutionary developmental biology (evo-devo). An evo-devo approach is, however, potentially important to understand how diversity and disparity could rapidly increase in the largest genera currently accepted by taxonomists. The most promising directions for future research and a set of key questions to be addressed are presented in this review.

Conclusions: From an evo-devo perspective, the evolution of clades with high diversity and/or disparity can be addressed from three main perspectives: (1) evolvability, in terms of release from previous constraints and of the presence of genetic or developmental conditions favouring multiple parallel occurrences of a given evolutionary transition and its reversal (2) phenotypic plasticity as a facilitator of speciation and (3) modularity, heterochrony and a coupling between the complexity of the life cycle and the evolution of diversity and disparity in a clade. This simple preliminary analysis suggests a set of topics that deserve priority for scrutiny, including the possible role of saltational evolution in the origination of high diversity and/or disparity, the predictability of morphological evolution following release from a former constraint, and the extent and the possible causes of a positive correlation between diversity and disparity and the complexity of the life cycle.

Keywords: Phenotypic plasticity evolvability generative key innovation heteroblasty heterochrony large genera life cycle complexity modularity permissive key innovation species diversity species robustness..


4 Varieties of Ecological Diversity

The following varieties of ecological diversity are recognized (Singh, 2002, Singh and Kumar 2003).

Species Diversity:

The number of species that occurs in a particular area is called its species richness (Donovan and Welden, 2002). It means the species richness in any habitat and is common currency of the study of biodiversity.

Species richness index is essentially a measure of the number of species in a defined sampling unit (Magurran, 1988).

Species richness is a function of sample size. However, it should not be confused with species abundance. Each natural habitat has a variety of species, which differ in their relative abundance. No community consists of species of equal abundance some species are rare, others are common and still others may be abundant.

Species diversity measures are often more informative than species counts alone. According to Harper, (1977), there is “importance of taking an organism’s eye view of community diversity”. This comment is relevant to structural diversity as it is to species composition.

Resource Diversity:

It means the diversity of resources that an organism (species) utilizes. For example, some fish species in the hill-streams have a wide trophic niche and depend on zooplankton, insects, and algae and diatoms for their food (Singh and Bahuguna 1983). In many cases food resources consumed by an organism differ during different stages of the life cycle, such as fry, fingerling and adult stages in case of fish. Thus, niche width is the measure of the diversity of resources utilized by a species.

The usual approach is to use the Simpson index or the Shannon index to calculate the niche width. The number of resource types observed (e.g., types of food items eaten, varieties of habitat utilized, kinds of behaviour employed) replace number of species in the equation. A separate value must be calculated for each type of resource and measures of abundance will depend on the way in which the index is being used.

If the niche width of a particular species is under consideration then abundance may be measured as the number of individuals either eating each type of food, living in each sort of habitat, or adopting each kind of behaviour. However, if we wish to measure the niche width of an individual, then abundance can be taken as the amount of each food type eaten, the time spent in each habitat or the frequency with which each behaviour is performed (Magurran, 1988).

Habitat Diversity:

It is the number of habitat types in a defined geographical area. This is an index, which measures the structural complexity of the habitat. This structural complexity of environment, in turn, is responsible for the presence of a wide variety of spatial and trophic niches. This means that if any habitat supports more microhabitats its biological diversity will be more as compared to a habitat which has less number of microhabitats. More studies on habitat diversity have been made for terrestrial environments.

The number of substrate types has been related to species diversity for aquatic insects, molluscs and benthic macro-invertebrates. Gorman and Karr (1978) have taken bottom type, depth and current into account to investigate the link between habitat diversity in streams and fish species diversity. The author found that habitat diversity was more in small hill streams and some tributaries than large, snow-fed rivers in the Garhwal Himalayas (Dobriyal and Singh 1988, Kumar 1992).

Differentiation Diversity:

It is also called beta diversity. It means degree of change in species composition between sites or communities or along gradients. A number of studies on faunal diversity of fish and insects have clearly indicated that their distribution and abundance is governed by gradient and altitude, among other factors (Singh et al, 1994, Singh and Nautiyal, 1990). For example, stoneflies and rheophilic fish species in the rhithron parts of hill-streams are characteristic and are absent from their potamon parts.

One study indicated that there is zonation of animals within a river Simulium monticole occurs from source to 12 km, Simulium variegatum from 12 to 35 km., and Simulium equinum from 20 to 50 km. All these studies indicate that greater diversity of species and habitats means greater ecological quality.


Contents

Species diversity in a dataset can be calculated by first taking the weighted average of species proportional abundances in the dataset, and then taking the inverse of this. The equation is: [1] [2] [3]

The denominator equals mean proportional species abundance in the dataset as calculated with the weighted generalized mean with exponent q - 1. In the equation, S is the total number of species (species richness) in the dataset, and the proportional abundance of the ith species is p i > . The proportional abundances themselves are used as weights. The equation is often written in the equivalent form:

Negative values of q are not used, because then the effective number of species (diversity) would exceed the actual number of species (richness). As q approaches negative infinity, the generalized mean approaches the minimum p i > value. In many real datasets, the least abundant species is represented by a single individual, and then the effective number of species would equal the number of individuals in the dataset. [2] [3]

The same equation can be used to calculate the diversity in relation to any classification, not only species. If the individuals are classified into genera or functional types, p i > represents the proportional abundance of the ith genus or functional type, and q D equals genus diversity or functional type diversity, respectively.

Often researchers have used the values given by one or more diversity indices to quantify species diversity. Such indices include species richness, the Shannon index, the Simpson index, and the complement of the Simpson index (also known as the Gini-Simpson index). [5] [6] [7]

When interpreted in ecological terms, each one of these indices corresponds to a different thing, and their values are therefore not directly comparable. Species richness quantifies the actual rather than effective number of species. The Shannon index equals log( 1 D), that is, q approaching 1, and in practice quantifies the uncertainty in the species identity of an individual that is taken at random from the dataset. The Simpson index equals 1/ 2 D, q = 2, and quantifies the probability that two individuals taken at random from the dataset (with replacement of the first individual before taking the second) represent the same species. The Gini-Simpson index equals 1 - 1/ 2 D and quantifies the probability that the two randomly taken individuals represent different species. [1] [2] [3] [7] [8]

Depending on the purposes of quantifying species diversity, the data set used for the calculations can be obtained in different ways. Although species diversity can be calculated for any data-set where individuals have been identified to species, meaningful ecological interpretations require that the dataset is appropriate for the questions at hand. In practice, the interest is usually in the species diversity of areas so large that not all individuals in them can be observed and identified to species, but a sample of the relevant individuals has to be obtained. Extrapolation from the sample to the underlying population of interest is not straightforward, because the species diversity of the available sample generally gives an underestimation of the species diversity in the entire population. Applying different sampling methods will lead to different sets of individuals being observed for the same area of interest, and the species diversity of each set may be different. When a new individual is added to a dataset, it may introduce a species that was not yet represented. How much this increases species diversity depends on the value of q: when q = 0, each new actual species causes species diversity to increase by one effective species, but when q is large, adding a rare species to a dataset has little effect on its species diversity. [9]

In general, sets with many individuals can be expected to have higher species diversity than sets with fewer individuals. When species diversity values are compared among sets, sampling efforts need to be standardised in an appropriate way for the comparisons to yield ecologically meaningful results. Resampling methods can be used to bring samples of different sizes to a common footing. [10] Species discovery curves and the number of species only represented by one or a few individuals can be used to help in estimating how representative the available sample is of the population from which it was drawn. [11] [12]

The observed species diversity is affected not only by the number of individuals but also by the heterogeneity of the sample. If individuals are drawn from different environmental conditions (or different habitats), the species diversity of the resulting set can be expected to be higher than if all individuals are drawn from a similar environment. Increasing the area sampled increases observed species diversity both because more individuals get included in the sample and because large areas are environmentally more heterogeneous than small areas.


Trampling

In grasslands trampling by people and animals can affect both species richness and the distribution of each species. The morphology of individual plants within the same species may also be influenced by trampling.

Impact of trampling on species richness

  • in most trampled areas where the chance of physical damage is high and the soil is very compacted
  • in non-trampled areas where the vegetation is tall and there is strong competition for light

It is at the boundary between the two extremes that the greatest number of species is usually found. At the edge of a trampled areas species are least affected by trampling but avoid too much competition with the more vigorously growing species.

most trampled area in center of path number of species = 6

boundary between trampled and least trampled area number of species = 12

least trampled area number of species = 3

Impact of trampling on the distribution of a particular species

Two species of plantain, ribwort plantain (Plantago lanceolata) and greater plantain (Plantago major), are very common in grassy areas in Britain

If you carry out sampling on trampled footpath (i.e. where there is more than just bare soil in the center of the path), you are likely to find that there is a higher abundance of greater plantain in the more trampled center of the path, and a higher abundance of ribwort plantain in the less trampled edges of the path.

Ribwort plantain is less well adapted to heavily trampled sites. It is less tolerant to physical damage and is less likely to grow in waterlogged soil.

By contrast, greater plantain is most abundant on heavily trampled ground. It is very tolerant of waterlogging and physical damage due to trampling. Its seeds which germinate best on open ground.

Impact of trampling on the morphology of a particular species

Trampling can also affect the morphology of certain species that show phenotypic plasticity. Ribwort plantain can readily vary its growth form in response to environmental conditions. In shorter grass, it grows in the rosette form with short leaves held flat to the ground, while in longer grass its leaves are longer and more angled off the ground. Its seeds can germinate amongst other plants. These factors help it to grow in less trampled areas with taller vegetation, where there is more competition to reach the light.

Light

Investigation comparing different woodland areas e.g type of woodland or woodland management, are usually a comparison of the effect of light available to the ground layer plants. Therefore this impacts on the type or abundance of plants in the two sample areas.

Impact of light availability on distribution of an individual woodland species

The bluebell (Hyacinthoides non-scripta) needs a lot of light to grow and flower successfully, but does not compete well with other species. It avoids competition by growing under the dense canopies of deciduous trees such as oak and beech where most species are excluded by lack of light. The bluebell is able to tolerate these conditions by using food reserves stored in its bulb. It grows rapidly in spring and is in flower by early summer before the trees come into leaf and lack of light becomes a problem.

In recently cleared or coppiced areas where light levels are high throughout the summer bluebells may initially do particularly well but eventually succumb to competition as other species invade.

The bluebell grows best on slightly acidic soils. On alkaline soils it may be replaced by species such as dog's mercury which occupy the same niche.


Importance of Biodiversity

All these diversities help in maintaining the correct balance of nature. But, gradually over the years, there has been a major loss in the biodiversity across the globe. The loss of biodiversity could adversely affect our environment as the balance is lost and the natural food web is disturbed.

Thus, due to its major role in our survival, conservation of biodiversity has now become a matter of high priority. Everybody is paying high attention to it. We still have not identified all the species living on the earth but of all the ones identified till now, many have already been marked as extinct.

Recently, the rate of extinction has gone high and this is causing direct impact on our earth like overuse of resources in some parts, the overpopulation of some species, etc. This has created a great imbalance in nature. Thus, we have to understand the importance of biodiversity.

Also, we must take necessary actions to maintain all the three diversities. Without the proper conservation of this diversity, we could end up in different precarious situations.


What is the difference between species diversity and species richness?

Species diversity is a measurement of species richness and species evenness. Species richness is the number of species.

Explanation:

Species richness is the number of species found in a community or ecosystem.

Species diversity is a measurement of species richness combined with evenness, meaning it takes into account not only how many species are present but also how evenly distributed the numbers of each species are.

For example, if two communities both have five species, species richness would be five for both communities. If the first community had 100 individuals and 80 of them were all one species, this would not be a community with a very even distribution. If the second community had 100 individuals, with 20 individuals belonging to each of the five species, this community would be more evenly distributed. Because it was more evenly distributed, community two would have a greater species diversity.

In the image below, community one would have a greater species diversity because the spread of species is more even.


Top 6 Ecological Role of Biodiversity

Ecological diversity is the intricate network of different species present in different ecosystems and the dynamic interaction between them.

An ecosystem consists of interacting organisms of many different species living together in a region that are connected by the flow of energy and nutrients.

The radiant energy of the Sun provides the ultimate source of energy in nearly all the ecosystems.

The Sun’s radiant energy is converted to chemical energy by the green plants through the process of photosynthesis. This energy flows from the producers to the consumers and lastly to the decomposers.

The following joints characterize the ecological role of biodiversity:

Biodiversity reflects the natural assemblage of large number of plant and animal species in a given area. It is part of a larger ecosystem in which biotic and abiotic components interact and bring about circulation, transformation and accumulation of energy and matter. A distinct area with uniform habitat conditions and supporting characteristic type of flora and fauna is termed biotope. Each species of a community has got a definite range of tolerance towards the physical and biological environmental conditions of the habitat. The range of environment that a species can tolerate is called its ecological amplitude.

The species occurring in a particular habitat do not live in isolation but coexist with mutual adjustment. The coexisting populations are interrelated and they show some sort of interaction.

The interactions between two coexisting species are of the following types:

Where one species lives at the expense of another.

In this, the coexisting species benefit from the relationship.

In this, the coexisting species compete for the same resources.

In this, the coexisting species are independent of one another.

Here, the different species within a community live under similar environmental conditions and are interdependent on each other,

Not all the species of a community are found in abundance. Only a few species are abundant, either in number or in biomass, while the majority of the species are rare. The species that are most common and abundant and contain maximum biomass are known as dominants,

In a plant community, the different plant species are represented by trees, shrubs, herbs, etc. These plants form, more or less, distinct strata or layers on vertical as well as horizontal plains known as stratification,

The interacting species within an ecosystem are characterized by death and replacement, which are continuous processes. In this way, the composition and shape of an ecosystem remains dynamic. This is known as succession. The changes go on taking place until a complete balance is established between the species and the environment.


6: Species Diversity - Biology

Article Summary:

No communities are having all the species in equal abundance. Ecologists have devised numerous indices and ecological models as indicators of diversity, yet diversity remains hard to be defined specifically and constrained within such definitions.

The earlier conservation approaches in biodiversity were based on the principle that if the communities have more diversity, the ecosystem is more stable. But this is not the case always and conservation approaches has now replaced this simple approach with more complex mathematical models integrating and analyzing the species diversity indices. These indices are also used as indicators for changes in the community.

A good conservation approach consists of identifying the different indices which when put together gives the most fitting model that explains the stability of ecosystem. Alternatively a simple comparison of the different diversity indices or application of parametric statistical tests can be done to gain more reliable and significant results regarding the characteristics of ecosystem under observation.

Competitive exclusion reduces the species diversity in uniform and unstructured ecosystems. However, the species diversity would increase if the intra and interspecies interactions are more.

Diversity is measured by counting the number of species, assessing the relative abundance of the species, or by using an index which cumulatively analyzes the species richness and abundance.

The most common indices of species diversity are dominance index, Simpson's index and Shannon's index. Apart from these basic indices, there are more modified approaches and newer indices used in ecological studies. Some of them are listed below.

a. Pielou's evenness index

The index measures equitability and allows comparison of Shannon Weaver index with the distribution of individuals in the observed species that would have the maximum diversity. It is calculated as
J= H'/ log (S)
H- Shannon Weaver index and S- total number of observed species in the community

H can take a maximum value which is equal to log (S). Hmax is the theoretical maximum value for H(s) when all the species in the sample were equally abundant. The index measures how equal a community is numerically. The index can have values ranging from 0 to 1. When there are frequent variations in the community, the index has higher values.

b. Brillouin index
The index is more sensitive to species abundance.
It is calculated as:

Where HB = the Brillouin index,
N is the total number of individuals in the sample,
ni is number of individual of species i,
ln(x) refers to natural logarithm of x

c. Fisher's alpha index

It is a tool to measure the diversity within a population. It is a parametric diversity index which assumes that species abundance follows log distribution. It is a scale independent indicator of diversity, but can be underestimated in communities where clustered distribution of species is found.
It is calculated by the formula S=a*ln(1+n/a)
where S- number of taxa, n- number of individuals and a- the Fisher's alpha.

d. Menhinick's index
It is the ratio of the number of taxa to the square root of sample size.

e. Margalef's index
It is given by (S-1)/ln(n)
Where S is the number of taxa, and n represents the number of individuals.

f. Shannon's Equitability index
It is Shannon diversity divided by the logarithm of number of taxa. This measures the evenness with which individuals of the community are divided among the taxa present. For a given equitability, the Simpson's index increases as the species richness increases.
Similarly for a given species richness, the Simpson's index increases as the species diversity increases.
This can also be calculated by expressing the Simpson's index as a proportion of the maximum value. Equitability value of 0 refers to complete evenness.

g. Berger- Parker Dominance Index: It is a simple mathematical expression relating the species richness and abundance
It takes into account only the commonest species in the sample and is calculated as
d = Nmax/N
where Nmax is the number of individuals in the most abundant species and N is the total number of species.

h. Sorensen's coefficient of community
It is used to measure similarities between communities. It is calculated as
CC = 2c / (s1+s2)
Where c is the number of species common to both communities s1 number of species of community 1 and s2 is the number of species in community 2.

i. Percent Similarity (PS)

To calculate PS, add the lowest percentage for each species that the communities have in common. Percentage similarity is based on relative abundance of the species.

j. Buzas and Gibson's evenness measure is given by eH/S
where H' is the Shannon Weaver index and S is the species number

These diversity indices are now being used as indicators of pollution, for biomonitoring, for deciphering conservation models and to simply observe the trends of changes in ecosystems.

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