Species diversity

1. Meaning and components of species diversity

Biodiversity is commonly studied at genetic, species and ecosystem levels. Species diversity concerns the variety of species within a defined area and their relative abundances. The area may be a pond, forest plot, landscape or biogeographic region. A species is often understood as a group of naturally interbreeding organisms reproductively isolated from other such groups. However, this biological species concept is difficult to apply to fossils, asexual organisms and some hybridising groups, so scientists also use morphological and phylogenetic evidence.

Species richness is simply the number of species recorded. Species evenness describes how uniformly individuals are distributed among those species. Consider two plots, each containing four tree species and 100 trees. A plot with 25 individuals of each species has greater evenness than one with 97 individuals of one species and one individual of each remaining species. Their richness is identical, but diversity indices incorporating abundance generally assign greater diversity to the first plot.

Species diversity must be distinguished from related concepts. Endemism means restriction to a particular geographical area; an endemic species is not necessarily threatened. Rarity may reflect small populations, narrow ranges or specialised habitat requirements. Functional diversity concerns ecological traits and roles, while phylogenetic diversity concerns evolutionary relationships. Therefore, counting species alone cannot fully represent a community’s ecological character or conservation significance.

• High abundance does not imply high diversity: a dense monoculture may contain very few species.
• A local increase caused by introduced species does not necessarily represent improved conservation status.

2. Measurement, sampling and spatial scales

Alpha diversity describes diversity within a particular habitat or sampling site. Beta diversity captures variation in species composition among sites, including turnover and differences arising when one assemblage is a subset of another. Gamma diversity is the total diversity across a larger region containing several communities. A landscape can therefore have substantial regional diversity even when each individual site contains relatively few species, provided the sites support different assemblages.

Beta diversity has several mathematical definitions. In a multiplicative partition based on richness, beta is regional richness divided by mean local richness. Other approaches use dissimilarity measures such as Jaccard or Sørensen indices, with results depending on the chosen convention. For examination purposes, the essential idea is compositional difference between communities, not one universally applicable formula.

The Shannon index is H′ = −Σpᵢ ln pᵢ, where pᵢ is the proportion of individuals belonging to species i. It incorporates both richness and evenness. Simpson’s concentration, D = Σpᵢ², increases with dominance; commonly used diversity forms, 1 − D and 1/D, increase with diversity. Consequently, a question mentioning the Simpson index must be read carefully to identify the form being used.

Reliable comparisons require comparable sampling effort, area, season and identification methods. Quadrats suit many plants and sessile organisms; transects, point counts and camera traps serve different animal groups. Species-accumulation curves show how recorded richness changes as sampling increases. Rarefaction permits comparisons at standardised sampling levels. Environmental DNA can detect otherwise overlooked species, but detection alone generally does not establish population size.

3. Geographical patterns and the species–area relationship

Many groups show a latitudinal diversity gradient: tropical regions generally contain more species than temperate and polar regions. Proposed explanations include greater solar energy and productivity, long evolutionary histories, climatic conditions favouring specialisation, and differences in speciation and extinction. No single explanation accounts for every group. Diversity also varies with elevation, depth, rainfall, habitat complexity and disturbance; elevational patterns may decline steadily or peak at intermediate elevations.

Alexander von Humboldt described an association between the area explored and the number of species encountered. The species–area relationship is expressed as S = CAᶻ, where S is species richness, A is area, C is a constant and Z is the slope on a logarithmic plot. NCERT gives typical Z values of 0.1–0.2 for many smaller-scale relationships and higher values, about 0.6–1.2, for comparisons involving very large areas such as continents. These are empirical ranges, not universal constants.

Island biogeography explains island richness through a dynamic balance between immigration and extinction. Nearer islands generally receive more immigrants, while larger islands often support larger populations and more habitats, reducing extinction risk. Habitat fragments can function like ecological islands, although the intervening landscape may be partly usable rather than completely hostile. Thus, reserve area, connectivity and the nature of the surrounding landscape all influence conservation outcomes.

• Species–area curves do not predict extinction with certainty; habitat quality, spatial arrangement and delayed ecological responses also matter.
• The Western Ghats illustrate how rainfall, elevation and habitat variation can support strong species turnover and local endemism.

4. Ecological importance and threats

Species contribute to primary production, decomposition, pollination, seed dispersal, nutrient cycling and food-web regulation. Diversity can improve resource use when species occupy complementary niches. It can also stabilise ecosystem functioning when species respond differently to drought, disease or other disturbances. Such insurance effects are important, but the relationship between diversity and stability depends on the ecosystem, the functions measured and species identities.

Not all species make interchangeable contributions. Keystone species have effects disproportionately large relative to their abundance; ecosystem engineers modify habitats physically. Losing a key predator, pollinator or habitat-forming organism can reorganise a community even if the decline in total species richness appears small. Functional redundancy offers some buffering, but apparently similar species may respond differently to future environmental stress.

Major pressures include habitat loss and fragmentation, overexploitation, invasive alien species, pollution and climate change. NCERT groups habitat loss and fragmentation, overexploitation, alien species invasions and co-extinctions as the ‘evil quartet’. Co-extinction occurs when the loss of one species causes the loss of an obligately dependent species. Biotic homogenisation is another concern: communities become increasingly similar as widespread generalists expand and distinctive local species disappear.

5. Conservation applications and Indian institutions

Conservation planning should combine richness with endemism, threat status, habitat representation and connectivity. Complementarity means selecting sites that collectively protect different species rather than repeatedly protecting similar assemblages. Naturally species-poor ecosystems, including some deserts and alpine habitats, should not be neglected. Likewise, natural grasslands should not be treated as degraded forests merely because they have few trees.

In situ conservation maintains species in their ecological settings through national parks, wildlife sanctuaries, conservation reserves, community reserves and other managed landscapes. Ex situ measures, including seed banks, botanical gardens and conservation breeding, can support threatened species but cannot reproduce all ecological interactions. Corridors may facilitate movement and gene flow; their effectiveness depends on habitat suitability, human pressures and species-specific requirements.

In India, the Wild Life (Protection) Act, 1972 provides major species and habitat protection mechanisms. The Biological Diversity Act, 2002, amended in 2023, addresses conservation, sustainable use and equitable benefit-sharing. The National Biodiversity Authority, State Biodiversity Boards and local Biodiversity Management Committees operate at different levels. People’s Biodiversity Registers document local biological resources and associated knowledge, providing useful baselines when supported by scientific verification and periodic updating.

• Monitor native species composition, abundance and trends, not merely the total number of species.
• Combine ecological information with local participation, livelihood considerations and long-term habitat management.

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