Identify two different scales on which biodiversity can be measured.

Biodiversity refers to the enormous variety of life on Earth. It has three scales: habitat, species, and genetic diversity.

Habitat diversity considers the variety of ecosystems that arise in different geographical and physical conditions. If habitat diversity is high, species diversity will generally be high. Think of a country such as Ecuador, with its flat Pacific coast, the Andes mountains bisecting it from North to South, and the Amazon rainforest in the East. This geography creates many different habitats for a large number of species.

Species diversity refers to the number of species that inhabit a particular area. Higher species diversity means a more complex food web, which is generally more stable.

Genetic diversity considers the variation in genes within a single species or population. Higher genetic diversity is a protective factor against extinction because a population with a wide range of genes is more likely to survive environmental changes, as it can adapt.

Each measure of diversity can help us identify effective ways to conserve biodiversity. Since genetic diversity protects against extinction, we should maintain large enough populations of each species. Likewise, since higher species diversity leads to more stable ecosystems, we should strive to protect most, if not all, species in an area. And finally, since habitat diversity supports high species diversity, we should protect a variety of habitats to maximise conservation of species diversity.

In IB ESS, we learn how to quantify species diversity using Simpson’s reciprocal index, because the number of distinct species alone is insufficient to help us understand how complex or stable an ecosystem might be. This formula combines species richness, the number of different species, with species evenness, the relative abundance of each species.

$\hspace{10em} D = \dfrac{\textrm{N(N-1)}}{\sum n(n-1)}$

$\hspace{3.8em}$ where
$\hspace{7em}$ N = total number of individuals, and
$\hspace{7.2em}$ n = the number of individuals of one species

The lowest possible value is 1, with higher values indicating higher overall species diversity.

Think about it. An ecosystem with 100 species, with 100 individuals each, will be more stable than oan ecosystem with 100 species, where one species has 9000 individuals and the others each have around 10.