Biodiversity: Concept and Levels

Biodiversity, short for biological diversity, refers to the enormous variety of life found on Earth -- the full range of living organisms, the genetic variation carried within them, and the ecological communities and ecosystems they form. The term captures not just how many different kinds of organism exist, but the whole living fabric of variation at every scale, from the molecular differences between individuals of a single species right up to the differences between whole ecosystems spread across a continent. Understanding biodiversity, and why it is worth protecting, begins with recognising that it is not a single, uniform property but is studied at three distinct, nested levels.

Genetic diversity refers to the variation in genes that exists within a single species. Even organisms belonging to exactly the same species are rarely genetically identical to one another, and this internal variation can be considerable. India's own rice crop, Oryza sativa, is a frequently cited example: thousands of genetically distinct varieties of rice are grown across the country, differing from each other in traits such as grain size, colour, aroma, yield and resistance to drought, pests or salinity, even though every one of these varieties belongs to the same single species. Similarly, the medicinal plant Digitalis purpurea (foxglove), used as a source of cardiac drugs, is known to show wide variation in the potency of its active chemical compounds from one population to another. Genetic diversity of this kind matters enormously in practice: it is the raw material plant breeders draw on to develop new crop varieties with improved yield or disease resistance, and it is what allows a species as a whole to adapt, over generations, to changing environmental conditions -- a genetically uniform population has far less capacity to survive a new disease or a shifting climate than a genetically diverse one.

Species diversity refers to the variety of species found within a given region, and is the level of biodiversity most people picture first when they hear the word. It is usually assessed in terms of species richness, the sheer number of different species present in an area, though ecologists also consider species evenness, how evenly individuals are distributed across those species rather than being dominated by just one or two. Species diversity varies enormously across the planet and across different taxonomic groups -- some regions and some groups of organisms (insects and flowering plants, for instance) show vastly higher species richness than others (mammals or birds, by comparison), and this unevenness itself is an important pattern in how biodiversity is distributed globally, examined further in the next section of this chapter.

Ecological diversity, sometimes called ecosystem diversity, refers to the variety of ecosystems, habitats and communities present within a defined geographical region, rather than the variety of genes or species within any one of them. A single Indian state, for instance, might contain wetlands, grasslands, dry deciduous forest and montane forest all within its boundaries -- each a distinct ecosystem type with its own characteristic set of species, physical conditions and ecological processes. A country as large and physically varied as India, spanning deserts, wetlands, coastlines, plains and the Himalayan mountain range, shows exceptionally high ecological diversity by world standards, which is itself one reason India is recognised as one of the world's mega-diverse countries.

These three levels are not independent of one another but are nested: genetic diversity exists within each species, many species together make up the species diversity of a region, and many different ecosystems together make up the ecological diversity of a still larger landscape. Losing biodiversity at any one of these levels -- even the loss of a single crop variety's unique gene pool, without the outright extinction of the crop species itself -- represents a real and often irreversible loss, which is why conservation efforts, examined later in this chapter, work to protect biodiversity at all three levels together, not species alone.