Biodiversity: Meaning, Types, Evolution, Factors and Measures

Read this article to learn about the meaning, types, evolution, factors and measures of biodiversity.

Meaning of Biodiversity:

The biosphere (the web of life that lives within and depends upon the inorganic spheres) constitutes a vital life support system for man and its existence in a healthy and functional state is essential for the existence of human race.

It is the presence of innumerable organisms, the biological diversity, which makes our life pleasant and possible.

The term biodiversity was coined by Walter and Rosen (1985) and is the abbreviated word for Biological Diversity. Life originated on earth almost four billion years ago and nature took more than 1 billion year to develop this wide and complex spectrum of life on earth. Scientists believe that the total number of species on earth is in between 10-80 million (Wilson 1988) of which 1.4 million species have been enlisted so far.

However, we are losing this heritage of millions of years at a very fast rate. The reduction in diversity in life forms is bound to have grave consequences for the entire living world. It has become extremely important to study simultaneously the various life forms on earth and the causes of their destruction. Biodiversity is the total variety of life on our planet.

The total number of races, varieties or species i.e., the sum total of various types of microbes, plants and animals present in a system is referred to as biodiversity. The word biodiversity is now very widely used not only by the scientific community, but also by the common people, environmental groups, conservationists, industrialists and economists. So it is very important to have clear idea about the definition of biodiversity which is recognized as a separate scientific discipline with its own principles.

Some of the Important Definitions of Biodiversity are given here:

(i) Biodiversity is the variety of life in all its forms, levels and combinations. It includes species diversity, genetic diversity and ecosystem diversity (International Union for Conservation of Nature and Natural Resources—IUCN, United Nations Environment Programme—UNEP and World Wildlife Fund—WWF 1991).

(ii) United Nations Earth Summit in Rio de Janeiro defined biodiversity as – The variability among living organisms from all sources, including terrestrial, marine and other aquatic ecosystems and the ecological complexes of which they are part. This includes diversity within species, between species and of ecosystems.

(iii) According to U.S. Congressional Biodiversity Act – Biological Diversity is the variety and variability among living organisms and the ecological complexes in which they occur and encompases ecosystem diversity, species diversity and genetic diversity.

(iv) In the simplest terms, biological diversity is the variety of life and its processes and it includes the variety of living organisms, the genetic differences among them and the communities and ecosystems in which they occur. In this article we will present some fundamental aspects of biodiversity and its conservation.

Types of Biodiversity:

Biodiversity is usually studied at three different levels—Species diversity, Genetic diversity and Ecosystem diversity.

(i) Species Diversity:

Evolution of species diversity has probably been possible because of habitat diversity on earth. It refers to the variety of species within a region. This diversity could be measured on the basis of number of species in a region. The term biodiversity is commonly used as a synonym of species diversity.

It actually refers to species richness, in terms of number of species in a site or habitat. Global diversity is typically represented in terms of total number of species of different taxonomic groups. As mentioned before, an estimated 1.4 million species have been identified to date. Species diversity, again, is studied at three levels: alpha diversity (number of species coexisting at a site), beta diversity (difference in species complement between patches) and gamma diversity (number of species in a large area, e.g. a country).

This series can further be extended to delta diversity for biomes (biomes are climatically and geographically defined areas of ecologically similar climatic conditions such as communities of plants, animals and soil organisms and are often referred to as ecosystems) and omega diversity for the entire biosphere.

Some authors call it taxon diversity (variety of taxa within a community of an area). It is generally studied at the species level and hence called species diversity. When the taxonomic levels such as genus and family are considered, the term taxon diversity is more appropriate. This term is similar to taxic diversity.

(ii) Genetic Diversity:

Within a species there are a number of subspecies, varieties (subspecies and varieties are recognizable morphological variations within a species), forms (form is generally used to recognize and describe sporadic variations in a single morphological feature) or strains which slightly differ from each other.

These differences are due to slight variations in their genetic organization. This diversity in the genetic make-up of a species is referred to as genetic diversity. A species with a large number of varieties or strains is considered to be rich and diverse in its genetic organization. Genetic variations arise in individuals of a species by genie or chromosomal mutations. Genetic variation within populations is considered a “prerequisite for adaptation and evolutionary change”, and as such an important aspect of biodiversity.

Genetic variation is often expressed in terms of alleles (genes occupying the same locus in a chromosome) and is mainly studied at the population level. Genetic variations can be measured by different recent techniques such as allozyme analysis, DNA fingerprinting, polymerase chain reaction, restriction site mapping and DNA sequencing.

Diversities go on increasing at the micro level. Differences in the level of varieties are followed by differences among the subspecies, varieties and species. Accumulation of these differences at infra-specific level will automatically lead to distinctive character at the species level.

(iii) Ecosystem Diversity:

In ecosystem, there may exist different land-forms, each of which supports different and specific vegetation. Ecosystem diversity in contrast to genetic and species diversity is difficult to measure since the boundaries of the communities which constitute the various sub-ecosystems are not distinct. Ecosystem diversity could best be understood if one studies the communities in various ecological niches within the given ecosystem, each community is associated with definite complexes.

These complexes are related to composition and structure of biodiversity. Loss of ecosystem diversity may be considered as ultimate cause of loss of species and genetic diversity. Community diversity is a synonym of ecosystem diversity and is defined as the diversity of community types within larger areas (ecological, units). It should not be confused with habitat variety which is an expression mainly used for different species of animals which have different habitats.

It has been observed and reported that there is no direct effect of the number of species on ecosystem processes. The effect on the ecosystem arises from functional differences between species. It is suggested that ecosystem with lots of functional traits will operate more efficiency in terms of productivity, resilience and resistance to invaders.

Consequently, it was suggested (Hooper, 1998) that functional diversity should be measured in a species pool that summarizes the extent of functional differences. In simple form, functional diversity is the range of functions that are performed by organisms in a system.

The species within a habitat or community can be divided into different functional types such as feeding guilds or plant growth forms or into functionally similar taxa such as suspension feeders or deposit feeders. Functionally similar species may be from quite different taxonomic entities. A common measure of functional diversity is the number of functional groups represented by species in a community.

To cluster species into functional groups, first a set of characters significant for ecosystem functioning is measured for each species obtaining a trait matrix. The trait matrix is then converted into a distance matrix. Finally, the distance matrix is clustered with standard multivariate methods to divide species among functional groups.

Evolution of Biodiversity:

Biodiversity is created by evolutionary and ecological processes. The ecology describes patterns of and provides explanations for the biodiversity of extant ecosystems. Ecological processes also have evolutionary consequences. They interact with genetic diversity via adaptation, microevolution and speciation.

The environment provides continual pressures to diversify via adaptation, innovation and exploitation of new ways of life. The species diversity is the most conspicuous result of ecological and evolutionary processes driving the multiplication of species. Ecological factors like age of the ecosystem, environmental gradient, isolation, nature of physical environment, architecture of the habitat, interaction between species, natural disturbance, migration and dispersal of a species.

Darwin (1859) proposed that species compete and only the fittest survive in nature. It is inferred that under a strong pressure of natural selection the less fit species are eliminated. From this concept has arisen the competitive exclusion principle (Hardin 1960), which is based on the idea that no two species can be exactly equally fit. It states that if two or more species exist in the same habitat, ultimately all but one of them will be excluded. This is the paradox of biodiversity: we expect few species but we find many in nature.

Mechanisms that may be Responsible for Preventing Loss of Species by Competitive Exclusion, and Allow Species Diversity to be Maintained are as follows (Newman 2000):

(1) Each species has an exclusive ecological niche and subjected to conditions where it is fitter than its competitors.

(2) A perfect balance is maintained between species loss and gain. The slightly less fit species are eliminated by competitive exclusion, but this process is so slow that there will be time for other species to arise by evolution or to invade from other region.

(3) Competition is reduced or prevented, because the main controls on abundance are physical disturbance, stresses (e.g. low temperature, toxic substances), predation and disease, hence competitive exclusion does not occur.

Factors that Promotes High Diversity:

Biodiversity varies greatly from site to site, over both large and small distances (Huston 1994). This variation is due to certain factors that we discuss here.

(i) Favourable Environmental Conditions:

It is quite natural to think that species diversity would be greater where the conditions for growth are very favourable for plants and animals. But this is not universally true. Curriae (1991) studied the numbers of species of trees, mammals, birds, reptiles and amphibians across the USA and Canada in relation to environmental conditions. Species richness of each group showed positive correlation with temperature and solar influx.

In relation to these two factors biodiversity increases with increasing favourableness. The results from the long-term Park Grass Experiment at Rothamsted, England, showed that the more favourable the nutrient regime for plant growth — the lower was the biodiversity. Grime (1973) proposed the highest diversity at intermediate stress or favourableness. Therefore, it can be inferred that there is no universal relationship between species-diversity and the prevalence of favourable condition.

(ii) Reducing Soil Fertility:

There is a negative relationship between diversity and soil fertility (Newman, 2000). It is necessary to reduce soil fertility to achieve high species diversity in grasslands. The low diversity in high productivity grasslands is because a few species grow tall, there is intense competition for light and low growing species are eliminated.

(iii) Disturbance:

Minor disturbances help to maintain local species diversity. Disturbance of forests by felling of trees, fires affects the subsequent species composition. Grazing animals in grasslands can be considered disturbance and they can increase diversity. In the past tropical rainforests had been used for shifting cultivation.

The diversity was initiated as a response to such disturbance. The present day forest represent a mosaic of small patches at different stages of succession following disturbance that provides niches and contributes to diversity (Connell 1979). The conclusion is that disturbance can augment biodiversity and diversity managers need to consider carefully what disturbance to allow or introduce.

(iv) Heterogeneity of the Environment:

Environmental heterogeneity increases β-diversity but has no such effect on α-diversity (Newman 2000). On a landscape scale patches and mosaics of varying vegetation can be related to differences in exposure, steepness, soil depth, wetness, rock type affecting soil properties and other factors of microclimate and soil.

Each species responds differently to the environmental factors and so the proportions of species change. Whittaker (1956) showed that each woody species in the Great Smoky Mountains had a different distribution to altitude and exposure. So if we want to promote p-diversity we should pay attention to heterogeneity in the physical environment.

(v) Plant Species Diversity may Promote Insect Diversity:

High plant diversity may promote insect diversity. This is primarily because of coevolution between plants and insects involving secondary chemicals (Harborne 1993). The most of the secondary chemicals in plants such as alkaloids, terpenoids and flavonoids, are poisonous to most animals.

However, there are examples where one insect species being tolerant to one secondary chemical. This gives the insect the ability to eat something that most other insects cannot eat and it may then specialize in eating one plant species. Thus many insects eat only one or a few plant species. Other herbivorous animals tend to show less specificity in their diet. They show preferences between plant species but rarely confine their feeding to one plant species. Thus plant species diversity is likely to promote diversity of insects, but not necessarily of other animals.

Measures of Biodiversity:

Biodiversity can be measured in different ways. Two main factors taken into account when measuring diversity are richness and evenness. Species richness is the number of different species present in an area. However, diversity depends not only on richness, but also on evenness. Evenness compares the similarity of the population size of each of the species present. Evenness is a measure of the relative abundance of the different species making up the richness of an area.

Let us consider, two communities, A and B of species 1 and 2, both with 100 individuals:

The species richness of community B would equal that of community A. However, community B has more evenness than A. Community B must be considered more diverse: one is more likely to get both species there than m community A. A community dominated by one or two species is considered to be less diverse than one in which several different species have a similar abundance. As species richness and evenness increase, the diversity increases.