7: Alpha, Beta, and Gamma Diversity - Biology

7: Alpha, Beta, and Gamma Diversity - Biology

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Whittaker (1972) described three terms for measuring biodiversity over spatial scales: alpha, beta, and gamma diversity. Alpha diversity refers to the diversity within a particular area or ecosystem, and is usually expressed by the number of species (i.e., species richness) in that ecosystem. We can walk a transect in each of these three ecosystems and count the number of species we see; this gives us the alpha diversity for each ecosystem; see Table (this example is based on the hypothetical example given by Meffe et al., 2002; Table 6.1).

If we examine the change in species diversity between these ecosystems then we are measuring the beta diversity. We are counting the total number of species that are unique to each of the ecosystems being compared. For example, the beta diversity between the woodland and the hedgerow habitats is 7 (representing the 5 species found in the woodland but not the hedgerow, plus the 2 species found in the hedgerow but not the woodland). Thus, beta diversity allows us to compare diversity between ecosystems.

Gamma diversity is a measure of the overall diversity for the different ecosystems within a region. Hunter (2002: 448) defines gamma diversity as "geographic-scale species diversity". In the example in Table, the total number of species for the three ecosystems 14, which represent the gamma diversity.

Hypothetical speciesWoodland habitatHedgerow habitatOpen field habitat
Alpha diversity1073
Beta diversityWoodland vs. hedgerow: 7Hedgerow vs. open field: 8Woodland vs. open field: 13
Gamma diversity14

Table (PageIndex{1}) Alpha, beta and gamma diversity for hypothetical species of birds in three different ecosystems


a community plus the physical environment that it occupies at a given time

Alpha diversity

the diversity within a particular area or ecosystem; usually expressed by the number of species (i.e., species richness) in that ecosystem
Beta diversity
a comparison of of diversity between ecosystems, usually measured as the amount of species change between the ecosystems
Gamma diversity
a measure of the overall diversity within a large region. Geographic-scale species diversity according to Hunter (2002: 448)

What are alpha, beta, and gamma diversity?

Alpha diversity refers to the average species diversity in a habitat or specific area. Alpha diversity is a local measure.

Beta diversity refers to the ratio between local or alpha diversity and regional diversity. This is the diversity of species between two habitats or regions. It is calculated by the following equation:

(number species in habitat 1- number of species habitat 2&1 have in common)+(number of sp in H2- number of sp H1&2 have in common)

Gamma diversity is the total diversity of a landscape and is a combination of both alpha and beta diversity.

In the image below, assume the different colors represent different species of fish. The alpha diversity of Site A is 3 because there are three different species at Site A. The beta diversity of sites A and B is 2 because there are two species (the red one at site B and the teal one at site A) that are distinct from one another. The gamma diversity for the whole ecosystem is 5 because there are five different species of fish (black, orange, green, red, and purple with black stripes).

What are Alpha Particles

An alpha particle is a chemical species that is identical to the Helium nucleus and is given the symbol α. Alpha particles are composed of two protons and two neutrons. These alpha particles can be released from the nucleus of a radioactive atom. Alpha particles are emitted in the alpha decay process.

Alpha particle emission occurs in “proton rich” atoms. After the emission of one alpha particle from the nucleus of an atom of a particular element, that nucleus is changed, and it becomes a different chemical element. This is because two protons are removed from the nucleus in the alpha emission, resulting in a reduced atomic number. (The atomic number is the key to identify a chemical element. A change in the atomic number indicates the conversion of one element into another).

Figure 1: Alpha Decay

Since there are no electrons in the alpha particle, the alpha particle is a charged particle. The two protons give +2 electrical charge to the alpha particle. The mass of the alpha particle is about 4 amu. Therefore, alpha particles are the biggest particles that are emitted from a nucleus.

However, the penetration power of alpha particles is considerably poor. Even a thin paper can stop alpha particles or alpha radiation. But the ionizing power of alpha particles is very high. Since alpha particles are positively charged, they can easily take electrons from other atoms. This removal of electrons from other atoms causes those atoms to get ionized. Since these alpha particles are charged particles, they are easily attracted by electric fields and magnetic fields.

Variation in Species and Habitats in Ecology and Its Measurement

Great variation is observed in the natural world and our earth holds an immense variety of habitats and ecosystem. These habitats and ecosystems shelter a vast group of living organisms in the form of innumerable number of species.

All of this variation in species and habitats is referred to as bio-diversity. Bio-diversity is related to the numbers and relative abundance of species within a community.

A biological community has an attribute which is called species diversity (Krebs, 1989). The concept of species diversity in community ecology has been intensely debated by ecologists over the years. In fact, Hurlbert (1971) went so far as to suggest that diversity was probably best described as a “mono-concept” because of the many semantic, conceptual, and technical problems associated with its use.

In spite of debates and numerous cautionary remarks put forth by many regarding their use, biological species diversity has remained very popular with ecologists. The most obvious measure of species diversity is the number of species of some taxonomic groups in an area. Number of species is not the only way of looking at diversity, however, the abundance of each species can also be taken into consideration.

The importance of considering the relative abundance as well as number of species can be illustrated by two hypothetical “communities” both containing 10 same species and 100 individuals. Although both communities consist of 10 same species, the relative abundance may be different. Suppose, in community one, 90% of the individuals belong to a single species and the remaining 10% are distributed among the nine other species.

On the other hand, in community two, ten species each accounted for 10% of the total number of individuals. In the first case the evenness of species distribution is considered low, whereas it is maximum in the second case. It is hoped that a distribution will be found fitting the data from many different types of communities and allowing the comparison of different communities through the parameters determining the two distributions. Some of these distributions were proposed as purely empirical fits to the data.

Others, however, were derived from hypotheses about how the abundance of the species in the community should be related to each other. It had been hoped that, by specifying a set of conditions and deriving the distribution resulting from these hypotheses, specific conclusions about the interactions and relationships between the species and their environment could be tested.

This approach has not been very fruitful, because the same distribution can often be derived from contrasting sets of initial premises. In addition, two distributions derived from conflicting postulates can sometimes both adequately fit an observed set of data. The situation is analogous to the problem of fitting mathematical distributions to the observed spatial dispersion pattern of a species.

Therefore, even if the hypothesized distribution does fit the observed species abundance relationship data, the fit neither proves nor disproves the postulates of the model. However, in terms of purely subjective value, the -use of these models can help to summarize an observed species abundance relationship, and to create heuristic hypothesis about interactions among the species in a community.

Because of the impossibility of determining the numbers of all the species in an area, species diversity studies are invariably carried out on collection of particular taxonomic groups, such as the soil arthropods in 1 square meter of soil or the planktons in 1 cubic meter of water-body. A community is necessarily arbitrarily limited both to area and taxonomy. Commonness is also a relative factor.

A deer population of five individuals per acre is common, but a bacterial species with 100,000 individuals per acre is quite rare. The effect of relative commonness can seriously bias measures of species diversity if disparate taxonomic groups are both included in the group of species taken to be the community. For example, it is difficult to make meaningful comparisons between deer and bacteria purely on the basis of their abundances. In this article only species diversity measurement is discussed.

However, interested reader may consult the following references for species-area relationships and species-abundance relationships. Species-area relationships can be measured by following Preston method (Preston, 1962) and species- abundance relationship can be assessed through geometric distribution (niche preemption hypothesis), broken-stick distribution, and lognormal distribution (Whittaker, 1972 May, 1981 Giller, 1984 see also Poole, 1974 and Ludwig and Reynolds, 1984).

Species diversity may be thought of as being composed of two components. The first is the number of species in the community, which ecologists often refer to as species richness. The second component is species evenness or equitability. Evenness refers to how the species abundances are distributed among the species.

Over the years, a number of indices have been proposed for characterizing species richness and evenness. Such indices are termed as richness indices and evenness indices. Indices that attempt to combine both richness and evenness into a single value are referred to as diversity (heterogeneity) indices. In the following sections different types of indices and the procedures for computing of these are described.

Species Richness:

Some communities are simple enough to permit a complete count of the number of species present, and this is the oldest and the simplest concept of species diversity – the number of species in the community. Margalef (1958) coined the name species richness to describe this concept. Complete counts can often be done in some communities, such as in bird communities in small habitat. But it is often impossible to enumerate every species in most of the communities. The larger the sample, the greater the expected number of species. The following procedures have been used to measure different species richness indices.

This estimate is based on the observed frequency of unique species in the community and is obtained as follows (Helteshe and Forrester, 1983). A unique species is defined as a species that occurs in one and only one quadrat. Unique species are spatially rare species and are not necessarily numerically rare, since they could be highly aggregated. From Heltshe and Forrester (1983),

The Jackknife estimate of the number of species is as under:

where (S) = Jackknife estimate of species richness

s = observed total number of species present in n quadrats

n = total number of quadrats sampled

k = number of unique species

The variance of this Jackknife estimate of species richness is given below:

Where var (S) = Variance of Jackknife estimate of species richness

Fj = number of quadrats containing j unique species (j = 1, 2, 3, …… ,s)

k = number of unique species

n = total number of quadrats sampled

This Variance can be Used to Obtain Confidence Limits for the Jackknife Estimator as follows:

This index tends to overestimate the number of species in a community (Heltshe and Forrester, 1983). This bias is usually less than the negative bias of the observed number of species (5), which, as a rule, is always less than the true value of species richness in the community. Jackknife estimator of species richness is twice the observed number of species. So, this approach cannot be used for communities with exceptionally large numbers of unique species or communities that have been sampled too little (so that S is low).

Bootstrap Procedure:

This is an alternative method of estimating species richness from quadrat samples (Smith and van Belle, 1984). This method is related to the Jackknife, but it requires simulation on a computer to obtain estimates.

The Essence of the Bootstrap Procedure is as follows:

1. Draw a random sample of size n from the q quadrats within the computer, using sampling with replacement this is the “bootstrap sample”.

2. Calculate the estimate of species richness from the equation (Smith and van Belle, 1984) below:

Where B(S) = Bootstrap estimate of species of richness

5 = observed number of species in original data

Pi = proportion of the n bootstrap quadrats that have species i present

3. Repeat steps 1 and 2 N times in the computer, where N is between 50 and 200.

The variance of this bootstrap estimate is given below:

where var [B(S)] Variance of the bootstrap estimate of species richness

qij = proportion of the n bootstrap quadrats that have both species i and species j absent

According to Smith and van Belle (1984), the Jackknife Index is appropriate when the number of quadrats is small and, in case of large number quadrats, bootstrap is more useful.

Species Heterogeneity or Diversity:

Diversity or heterogeneity indices incorporate both species richness and evenness into a single value.

Whittaker (1972) Defined three Distinct Levels of Diversity of Interest to Ecologists:

(1) Alpha (α) or within habitat diversity

(2) Beta (β) or between habitat diversity (i.e., changes along environment gradients) and

(3) Gamma (γ) or large scale landscape diversity (a composite of α and β diversity). First alpha diversity is dealt in detail and then the β and γ diversity.

The Shannon—Wiener Index (H’):

This index has probably been the most widely used index in community ecology. It is based on information theory (Shannon and Wiener, 1949) and is a measure of the average degree of “uncertainty” in predicting to what species an individual chosen at random from a collection of S species and N individuals will belong.

This average uncertainty increases as the number of species increases and as the distribution of individuals among the species becomes even. Thus, H’ has two properties that have made it a popular measure of species diversity: (1) H’ = 0 if and only if there is one species in the sample, and (2) H’ is maximum only when all S species are represented by the same number of individuals, that is, a perfectly even distribution of abundances.

The equation for Shannon—Wienner function, which uses natural logarithms (In), is as under:

where H’ is the average uncertainty per species in an infinite community made up of S species with known proportional abundances p1, p2, p3, …….. ps. S and pi‘s are population parameters and, in practice,

H’ is estimated from a sample as below:

where niis the number of individuals belonging to the ith of S species in the sample and n is the total number of individuals in the sample. Equation (7) is the most frequent from of this index used by ecologists. However, this estimation is biased because the total number of species in the community (S) will most likely be greater than the number of species observed in the samples (S). Fortunately, if n is large, this bias will be small.

Brillouin Index:

Many community samples would be treated as collections rather than as a random sample from a large biological community (Pielou 1966). In any case in which the data can be assumed to be a finite collection and sampling is done without replacement,

The appropriate information theoretic measure of diversity is Brillouin’s formula:

where H = Brillouin’s index

N = total number of individuals in entire collection

n1 = number of individuals belonging to species 1

n2 = number of individuals belonging to species 2 (etc.)

Margalef (1958) was the first to propose using Brillouin’s index as a measure of divesity.

There is much argument in the literature about whether the Brillouin index or the Shannon-Wiener index is a better measure of species diversity (Peet, 1974 Washington, 1984). In practice, this argument is irrelevant to field ecologists. Because H 1 and H 1 are nearly identical for most ecological samples (when N is large). Legendre and Legendre (1983) also point out that Brillouin’s index cannot be used when biomass, cover, or productivity is used as a measure of species importance in a community. Only the number of individuals can be used in equation (8).

Beta and Gamma Diversities and their Relationships with Alpha Diversity:

Alpha diversity is also known as local diversity it is the number of species in a small area of more or less uniform habitat. Gamma diversity or regional diversity is the total number of species observed in all habitats within a region. By region, ecologists generally mean a geographic area that includes no significant barriers to dispersal to organisms.

Thus, the boundaries of a region depend on which organisms are considered. The important point is that, within a region, distributions of species should reflect their selection of suitable habitats rather than their inability to disperse to a particular locality (Ricklefs and Miller, 1999).

When each species is found in all habitats within a region, local (α) and regional (γ) diversities are equal. Ecologists refer to the difference in species from one habitat to the next as beta diversity. The greater the difference, or turnover, of species between habitats, the greater will be the beta diversity.

An example can be cited to discuss the relationship between alpha, beta and gamma diversity. Suppose there are four regions and each region containing four habitats. In first case (a) the diversity in each habitat (alpha diversity) is the same of all four habitats (let each habitat contains species A and B, species richness of 2 (average = 8/4 = 2).

The regional diversity (gamma) is also 2 (in the region there is only 2 species). The beta (turnover) diversity is gamma/alpha = 2/2 = 1. In second case (b) out of four habitats, one contains species A and B, next C, next D and last one E. Alpha diversity is 2 for one habitat (species A and B) and 1 for the other three (species C, D, and E occur alone in each habitat, yielding an average alpha diversity (<2 + 1 + 1 + 1>/4) of 1.25. Gamma diversity is 5 (altogether 5 species in all four habitats), so beta diversity is gamma/alpha (5/1.25) = 4.

In third case (c) two habitats contain A species and another two with B species. The alpha diversity is (1 + 1 + 1 + 1)/4 = 1, and in the region there are two species and hence, the gamma diversity is 2, beta diversity is 2/1 = 2. In last case (d), each habitat is occupied with the same three species A, B and C.

Therefore, alpha diversity is 12/4 = 3, in the region there are three species and gamma diversity is 3 and beta is 3/3 = 1. Regions (a) and (d) have same beta diversity but alpha and gamma diversities are different, indicating little species turnover in those areas.

Evenness Measures:

In a sample when all species are equally abundant, it seems intuitive that an evenness index should be maximized and decreased toward zero as the relative abundance of the species diverges away from evenness.

Two Formulations are Possible:

where D = observed index of species diversity

Dmax = maximum possible index of diversity, given S species and N individuals

Dmin = minimum possible index of diversity, given S and N.

These two measures (labeled V’ and V by Hurlbert, 1971) are convergent for large samples, and evenness measures of the first type (V’) are most commonly used in the literature. All evenness measures range from 0 to 1.

For the Shannon-Wiener Function, Maximum Possible Diversity Occurs According to the following formula:

Probably the Most Common Evenness Index used by Ecologists is Based on the Shannon-Weiner Function:

where J’ = evenness measure (range 0 to 1)

H’ = shannon-Weiner function (equation 6)

H’max = maximum value of H (equation 11)

There is general problem with all measures of evenness all assume that the total number of species in the community is known (Pielou, 1969). But this number is almost always impossible to determine for species rich communities. Since observed species numbers must always be less than true species numbers in the community, the evenness ratios are always overestimated (Sheldon, 1969). Peet (1974, 1975) and Routledge (1983) recommended that evenness measures should not be used in ecological work unless the number of species in the whole community is known. This may be possible in some temperate zone communities and in well-studied tropical communities (Krebs, 1989).


Species diversity is a dual concept that includes the number of species in the community and the evenness with which the individuals are divided among the species. There are many ways of measuring species diversity and much controversy about which indices of diversity are best.

All Diversity measures have some limitations and this includes species richness indices, diversity indices, and evenness indices. These measures are easy to compute, but are usually difficult to interpret. In some communities with few species it is easy to determine the species richness. But in all other cases, the species list and the sample size are directly proportional.

The rarefaction technique allows one to adjust a series of samples to a common sample size (number of individuals) so that species richness can be compared among samples (Krebs, 1989). Jackknife estimate of species richness can be made for quadrat sampling. Species richness and evenness are confounded by heterogeneity measures in a single index of diversity. Heterogeneity measures place most weight on the rare species in the sample, and the Shannon-Wiener function is an example of these measures.

With the help of the ratio of observed heterogeneity to maximum possible heterogeneity, when all species have the same number of individuals, evenness can be estimated. Evenness measures have not been used to much advantage in community analysis because all measures are biased upward unless the total number of species in the community is known.

Alpha, beta, or gamma: where does all the diversity go?

Global taxonomic richness is affected by variation in three components: within-community, or alpha, diversity between-community, or beta, diversity and between-region, or gamma, diversity. A data set consisting of 505 faunal lists distributed among 40 stratigraphic intervals and six environmental zones was used to investigate how variation in alpha and beta diversity influenced global diversity through the Paleozoic, and especially during the Ordovician radiations. As first shown by Bambach (1977), alpha diversity increased by 50 to 70 percent in offshore marine environments during the Ordovician and then remained essentially constant for the remainder of the Paleozoic. The increase is insufficient, however, to account for the 300 percent rise observed in global generic diversity. It is shown that beta diversity among level, soft-bottom communities also increased significantly during the early Paleozoic. This change is related to enhanced habitat selection, and presumably increased overall specialization, among diversifying taxa during the Ordovician radiations. Combined with alpha diversity, the measured change in beta diversity still accounts for only about half of the increase in global diversity. Other sources of increase are probably not related to variation in gamma diversity but rather to appearance and/or expansion of organic reefs, hardground communities, bryozoan thickets, and crinoid gardens during the Ordovician.

T Cells

Alpha/beta (&alpha&beta) T cells

  • alpha chain gene segments on chromosome 14 and
  • beta chain gene segments on chromosome 7

And like B cells, the greatest diversity in the receptors of &alpha&beta T cells occurs in the third complementarity determining region (CDR3) of the alpha and beta chains because of

  • the junctional diversity between the V, J, and D segments and
  • the addition of N region nucleotides.

However, T cells do not seem to use somatic mutation to increase receptor diversity. Actual measurements of the repertoire in humans reveals a figure of about 2.5 x 10 7 .

What about reinfections after vaccination?

To answer this question, experts cite data from a population surveillance study in the UK, showing that genomically documented COVID-19 reinfections are "rare", and even more rare are ones that are serious. One study shows ≤ 0.4% reinfection rate out of 4 million cases was dubbed as "good news". "It tells us prior covid immunity is solid (but even stronger w/ 1 dose vaccine). This will help #BlockDelta," said Dr. Eric Topol, professor of molecular medicine and founder-director of the Scripps Research Translational Institute, Medicine, and Executive Vice-President of Scripps Research.

The current data shows that there is a low risk of reinfection with SARS-CoV-2. There were 15,893 possible reinfections with SARS-CoV-2 identified up to 30 May 2021 in England throughout the pandemic, out of nearly 4 million people with confirmed infections.

Source: Public Health England (PHE), published June 17, 2021


The Convention on Biological Diversity (CBD) requires that its signatory nations undertake to make inventories of their biodiversity, monitor changes in biodiversity and make plans as to how biodiversity can be conserved. This is highly laudable, but makes the assumption that we know how to measure and monitor biodiversity. In the marine domain, this assumption is far from correct.

Biodiversity encompasses a range of different levels of organisation from the genetic variation between individuals and populations, to species diversity, assemblages, habitats, landscapes and biogeographical provinces. I have earlier given a general review of marine biodiversity (Gray, 1997a) but here will consider the methodological problems in measurement of the number of species and individuals in a given area.

Most studies of biodiversity are confined to the number of species in a given area, the species richness. But the number of species alone does not describe the structure of the assemblage of species in a given area because the number of individuals per species varies. A variety of indices have been derived taking into account the proportional abundance of species (see Magurran, 1984 for a full account). These indices consider both species richness and how evenly the individuals are distributed among species (evenness or equitability). Following Peet (1974), I call this heterogeneity diversity.

Whittaker (1960) suggested that there was a range in scales of species richness. He called the number of species found in a sample alpha (or within habitat) diversity, and felt that this was the basic unit of diversity. Whittaker (1960) also suggested there was beta (or between habitat) diversity, and species diversity measured at the largest scale he called gamma diversity. In contrast, Rosenzweig (1995) felt that the term gamma diversity was redundant since it as simply the total number of species in a biological province. Whilst Whittaker’s scheme appears to have a logic in practical terms, there are difficulties in defining and interpreting these scales. Thus, there is a need to consider these terms in detail to arrive at acceptable definitions and agreed ways to measure the different scales of species richness and heterogeneity diversity.

There are also problems when trying to compare diversity from different areas. For example when trying to publish recent papers comparing coastal with deep-sea diversities (Gray, 1994 Gray et al., 1997), the referees stated that it was not valid simply to compare coastal and deep-sea species richness. The reason given was that the deep-sea samples were always from a single uniform habitat (within habitat diversity) whereas the coastal samples were from many habitats, which the reviewers called between habitat diversity. Such an argument is wrong on two counts. First, it is quite justifiable to ask the question: are there more species in a comparable area of coastal compared with deep-sea sediment, irrespective of possible variations in the habitat? After all, it is valid to compare species richness of a tropical rain forest with that in a boreal oak-forest, even though these are different habitats. But, perhaps more importantly, the referees had not understood what between habitat (beta) diversity really is. Whittaker (1960) defined beta diversity as the extent of species replacement or biotic change along environmental gradients. Beta diversity is not a measure of the number of species in different habitats in an area.

Most studies of diversity and the terminology accompanying them relate to terrestrial systems. Yet marine systems differ from terrestrial in a number of ways. Subtidally, one is usually sampling blind because the extent of a habitat or assemblage often cannot be determined and boundaries are less distinct than on land. Marine systems are open with many species dispersing over large areas by means of larvae. Steele (1985) suggested that in the sea scales of differences in time and space are different from those on land. Studies of marine diversity have developed their own methods and terminology that have been ignored by terrestrial ecologists. For example, Sanders (1968) rarefaction technique for comparing samples of differing size was reinvented by Coleman (1981) and gives almost identical results (Brewer and Williamson, 1994). This review analyses the above issues, proposes a uniform notation for species richness and uses some Norwegian data to illustrate approaches that can be taken.

Assertion Reason Questions for Biology Chapter 37 Biodiversity and its Conservation

Directions: In the following questions, a statement of assertion is followed by a statement of reason.
Mark the correct choice as:
(a) If both Assertion and Reason are true and Reason is the correct explanation of Assertion.
(b) If both Assertion and Reason are true but Reason is not the correct explanation of Assertion.
(c) If Assertion is true but Reason is false.
(d) If both Assertion and Reason are false.

Q.1. Assertion : Alpha diversity is said· to be higher if the dissimilarity between communities is higher.
Reason :Alpha diversity is a measure of diversity between the communities.

Answer Answer: (d) Alpha diversity (within-community diversity) refers to the diversity of organisms sharing the same community/ habitat. A combination’ of species richness and equitability/evenness is used to represent diversity within a community or habitat. Generally, greater the species richness, greater is the species diversity. Species frequently change when habitat or community changes. The rate of replacement of species along a gradient of habitats or communities is called beta diversity between-community diversity. Higher the heterogeneity in the habitats in a region or greater the dissimilarity between communities, higher is the beta diversity. Diversity of the habitats over the total landscape or geographical area is called gamma diversity.

Q.2. Assertion: The species diversity present in a given community or habitat is referred to as alpha diversity.
Reason: Alpha diversity is usually expressed by species richness and species evenness in that community habitat.

Answer Answer: (a) Alpha diversity within community diversity is species diversity in a given community or habitat. It is dependent upon species richness and species evenness/ equitability. There is a lot of competition, adjustments and interrelationships amongst members of the same community. The number of species per unit area is called species richness. Number of individuals of different species represent species evenness or species equitability.

Q.3. Assertion : Diversity observed in the entire geographical area, is called gamma diversity.
Reason : Bio-diversity decreases from high altitude to low altitude.

Answer Answer: (c) Biodiversity is not uniform on the earth. It varies with change in latitude or altitude. Biodiversity increase, when we move from high to low latitude (i.e. from the poles to the equator).

Q.4. Assertion : A biosphere reserve is a specified area.
Reason : No restriction on human activities has been imposed in biosphere reserve.

Q.5. Assertion : In tropical rain forests. O-horizon and A-Horizon of soil profile are shallow and nutrient-poor.
Reason : Excessive growth of micro-organisms in the soil depletes its organic content. [AIIMS 2006]

Answer Answer: (c) O-horizon occupies the topmost soil and is rich in mineral and decomposed organic matter (humus). A-horizon is dark coloured and has abundant minerals mixed with humus.

Q.6. Assertion: Communities that comprise of more species tend to be more stable.
Reason: A higher number of species results in less animal variation in total biomass. [AIIMS 2017].

Answer Answer: (a) Communities with higher number of species are more stable as it can resist occasional disturbances. A stable community should show less variation in productivity from year to year and resistance towards alien species.

Q.7. Assertion: Community with more species tends to be more stable than those with less species.
Reason: More will be the species, less will be year to year variation in total biomass.

Answer Answer: (a) Communities with more species tend to be more stable than those with less species. It is able to resist occasional disturbance . A stable community should not show too much variation in productivity from year to year it must be resistant to invasions by alien species. David Tilman’s long term experiments showed the plots with more species, experience less year to year variation in total biomass.

Q.8. Assertion: A stable community should not show too much variation in productivity from year to year.
Reason: A stable community must be resistant to invasions by the alien species.

Answer Answer: (b) A stable community should not show too much variation in productivity from year to year it must be either resistant or resilient to occasional disturbances (natural or man-made), and it must also be resistant to invasions by alien species.

Q.9. Assertion: Decrease in species diversity occurs as we ascend a high mountain.
Reason: Decrease in species diversity occurs with increase in altitude due to rise in temperature.

Answer Answer: (c) Barring arid/semiarid and aquatic habitats, biodiversity shows a latitudinal and altitudinal gradient. A decrease in species is observed as we ascend a high mountain due to drop in temperature (lapse temperature being 6.5°C for 1 km or 1000 m) and greater seasonal variability.

Q.10. Assertion : Most common forest type in India is tropical dry deciduous forests.
Reason : They are common in West Bengal.

Answer Answer: (c) The tropical monsoon deciduous forests are found in areas receiving an annual rainfall of 100 to 200cms in India, with a distinct dry and rainy season and minimum temperature. The south western ghats moist deciduous forests are a tropical moist broad leaf forest ecoregion of southern India. It covers the southern portion of the Western Ghats range and the Nilgiri Hills between 250 and 1000 meters elevation in Kerala, Karnataka and Tamil Nadu states.

Q.11. Assertion : Tropical latitudes have greater biological diversity then temperate latitudes.
Reason : Tropical regions remain relatively undisturbed for millions of years.

Answer Answer: (a) Tropical latitudes have greater biological diversity. It is quite true. Ecologists and evolutionary biologists have proposed various hypothesis in support of this. Speciation is generally a function of time and unlike temperate regions, subjected to frequent glaciations. In the past, tropical latitudes remained undisturbed for millions of years, where species continued to flourish.

Q.12. Assertion: If the species-area relationships are analyzed among very large areas like the entire continents, the value of Z i.e., slope of line lies in the range of 0.1 to 0.2.
Reason: The value of Z i.e., slope of line of species area relationships lies in the range of 0.6 to 1.2 when analysis is done among small areas.

Q.13. Assertion: Speciation is a function of time and tropical regions had got a long evolutionary time for species diversification as compared to temperate regions.
Reason: Temperate regions have undergone frequent glaciations in the past whereas tropical regions have remained relatively undisturbed for millions of years.

Answer Answer: (a) Speciation is a function of time. Temperate regions have undergone frequent glaciations in the past, due to which many species had been killed. However, tropical latitudes have remained relatively undisturbed for millions of years and thus, had a long evolutionary time for species diversification.

Q.14. Assertion: Taiga is also called North coniferous forest.
Reason: The ground flora is absent in Taiga.

Q.15. Assertion: Temperate deciduous forest is two – storeyed forest.
Reason: Two stories are formed of soft wood and hard wood trees.

Measuring Biodiversity

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Biodiversity. The word evokes the splendor of a great forest, or the teeming richness of the ocean, and is simply defined as the variety of organisms in an ecosystem of interest. To protect biodiversity, scientists must be able to measure it. This means figuring out how many different species are living together in a particular space. What is a convenient way to count species?

Trying to count everything in an entire ecosystem would be impossible, so scientists use a tool called the quadrat, which is a frame of fixed size placed randomly in the environment in which to do the counting. After cataloging the species and individuals found in this small section, the process is repeated, placing more quadrats at random, or alternatively, at set positions along a line through the environment, referred to as a transect.

In order to then estimate the total number of species in an area, species accumulation curves are used. If the cumulative number of species found in a quadrat are plotted against the number of quadrats sampled, a curve will emerge. For example, in this data set, when four quadrats were investigated, it was found that there were 10 unique species. Six contained 17 and so on. The asymptote of this type of curve represents an estimate of the number of species supported by an environment. In this case, it's about 30. But while measuring diversity at a single site is incredibly useful, comparing sites over a greater area can give us an even larger scale indication of diversity.

In 1972, the ecologist Robert Whittaker described three major kinds of biodiversity, alpha, beta, and gamma. Alpha diversity refers simply to the number of species in an area and is often referred to as species richness. For example, at this site there are seven different species, so the alpha score is seven. A second site, site B, has five species, and a third, site C, has seven. But by comparing between sites, we can determine what is called the beta diversity, the sum of species unique to each area. So if we compare site A with site B, we see three species in common between the two. Counting the remaining species, we find that there are six. This means that there is a beta diversity between site A and site B of six. Sites A and C also have three species in common, leaving eight unique ones. This is a beta diversity of eight. Sites B and C have two common species between them, or a beta diversity value of eight. Finally, gamma diversity is the number of different species in all sites combined. In this example, there is a gamma diversity of 12. So to summarize the three kinds of biodiversity, we can look at them this way, alpha, beta, and gamma. As well as recording diversity, scientists often refer to species evenness, meaning how many individuals of each type are present. For example, these two sites have the same richness, or alpha diversity, as they both have seven species. But site A is relatively overrun by rabbits with low numbers of the other species, whereas site B has a pretty even distribution of species, so it is considered to have greater evenness compared to site A. Scientists generally considered ecosystems with higher richness and evenness, i.e. many evenly distributed species, to be the healthiest. Disturbed habitats, often due to the actions of humans, like farming or pollution, often have poor richness and evenness. Being able to compare sites is critical because it allows researchers to determine the relative health of ecosystems.
In this laboratory, you will carry out quadrat and transect sampling at three different environmental sites, as well as carrying out a laboratory simulation, and then analyze the data collected to describe the observed biodiversity.

Diverse ecosystems are important for the health of the planet and our survival as humans it is therefore incredibly important for us to understand and measure biodiversity, which is defined as the variability among living organisms in an ecosystem. Biodiversity can be measured at many different levels including genetic, species, community, and ecosystem. One way to measure biodiversity is to assess species richness of an ecosystem, which is the total number of distinct species within a local community. While having many species generally coincides with having a diverse and healthy ecosystem, the evenness also needs to be considered. Evenness refers to the equality of the proportion of each species within an area or community. For instance, when one species dominates the area while the others are very rare, the biodiversity in this area is lower than in an area with species of equal abundance. Therefore, areas with many species that are relatively equal in abundance have the highest values of biodiversity.

Estimating Biodiversity

The differences in richness and evenness between two communities can be visualized by rank-abundance curves. If the number of species is equal, the shape of the line can tell us which community is more diverse. If the line is flat, there is high evenness among species. However, if the line quickly dips, the evenness is low. If richness and evenness are both different between two communities, biologists must use equations to calculate diversity. These equations weight the importance of each component differently, and a consensus on which equation is the best at calculating diversity is still debated.

Sometimes there are too many species in an area that it is unrealistic to count every single species. For example, a single tree in the Amazon Rainforest may contain hundreds of species of beetles. To circumvent this problem, ecologists use sampling tools called quadrats. A quadrat is simply a frame with a known internal area. For example, to measure the species richness of a one-acre field of grass, ecologists randomly place the quadrat in the field and count the species within the quadrat, instead of counting all of the species within the acre. They may also systematically sample by using transect tapes. Transects are stretched across the field, and quadrats are then placed along the transect at regular intervals. This method is semi-random and ensures ample coverage of sampling across the entire field to estimate its biodiversity.

While quadrats and transects may pick up most of the species, some rare species may go unnoticed. In this case, ecologists may use a species accumulation curve, which represents the cumulative number of species seen in a series of quadrats. 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. Alpha-diversity (α) refers to the number of species in an area. Beta-diversity (β) compares two different areas and is the sum of species unique to each area. Gamma-diversity (γ) is the number of species in many areas combined into a region. By using these measures, biologists can get an idea of diversity over space, including both small and large scales.

Threats to Biodiversity and their Implications

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 3-4 . 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 . Without protection of the sea otters, herbivores like sea urchins, which are usually eaten by the otters, are capable of completely devouring kelp forests leaving barren rocks where very few species could survive.


  1. Knowlton, Nancy. The future of coral reefs. PNAS. 2001, Vol. 98 , (10) 5419-5425.
  2. Andrea S. Downing, Egbert H. van Nes, Wolf M. Mooij, Marten Scheffer. The Resilience and Resistance of an Ecosystem to a Collapse of Diversity. PLoS One. . 2012 , Vol. 7(9): e46135.
  3. Wall, Monroe E. Camptothecin and taxol: Discovery to clinic. Med Res Rev. 1998, Vol. 18, 5 (299-314).
  4. Yuh Fen Pung, Peter T. H. Wong, Prakash P. Kumar, Wayne C. Hodgson, R. Manjunatha Kini. Ohanin, a Novel Protein from King Cobra Venom, Induces Hypolocomotion and Hyperalgesia in Mice. J Biol Chem. 2005, 280, 13137-13147.
  5. Estes, J.A., et al. Complex Trophic Interactions in Kelp Forest Ecosystems. Bulletin of Marine Science, Volume . 2004, Vol. 7, 3: 621-638.

Watch the video: Alpha diversity metrics (August 2022).