Why the Tropics Hold Far More Species Than the Poles

From the bacteria in soil to tigers in forests, life on Earth shows astonishing variety. Biologists measure that variety at several levels, look for patterns in where it is richest, and try to understand why it is being lost so fast.

This part covers the levels of biodiversity, how many species exist, patterns of diversity including the species-area relationship, and why diversity matters and how it is lost.

Biodiversity is the variety of life at every level: genetic diversity is variation within a species, species diversity is variety among species, and ecological diversity is variety among ecosystems.

Genetic diversity:

- The medicinal plant Rauwolfia vomitoria growing in different Himalayan ranges differs in the potency and concentration of reserpine, its active chemical
- India has more than 50 000 genetically different strains of rice and 1000 varieties of mango

Species diversity:

- The Western Ghats have greater amphibian species diversity than the Eastern Ghats

Ecological diversity:

- India, with deserts, rainforests, mangroves, coral reefs, wetlands, estuaries and alpine meadows, has greater ecosystem diversity than a Scandinavian country such as Norway

An everyday example. The many mango varieties in Indian markets, from Alphonso to Dasheri, show genetic diversity within a single species.

The substance. Genetic diversity lets a species adapt — without variation, one disease or a change in climate could wipe it out.

About 1.5 million species have been described so far, and a well-supported estimate puts the global total near 7 million; India, with only 2.4 per cent of the world's land area, holds about 8.1 per cent of global species diversity, and among all recorded species, animals — above all insects — form the largest share.

Global diversity:

- About 1.5 million species have been recorded
- A conservative estimate places the true total at about 7 million

India's diversity:

- 2.4 per cent of the world's land area, but about 8.1 per cent of its species diversity
- One of the world's 12 megadiversity countries

Proportions of major groups:

- Animals make up more than 70 per cent of recorded species
- Plants, including algae, fungi, bryophytes, gymnosperms and angiosperms, make up no more than 22 per cent
- Insects are the most species-rich animal group — about 7 in every 10 animal species

An everyday example. The beetles, ants, bugs and caterpillars on a single garden plant in the monsoon hint at why insects dominate species counts.

The substance. Described species are only a fraction of what exists — many microbes cannot even be grown in the laboratory to be identified.

Species diversity generally falls from the equator towards the poles, because the tropics have had a long, relatively undisturbed evolutionary history, a less seasonal and more predictable climate, and more solar energy that supports higher productivity; within a region, species richness rises with area according to log S = log C + Z log A.

Latitudinal gradient:

- Colombia, near the equator, has nearly 1400 bird species, New York at 41° N has 105, and Greenland at 71° N only 56

Species-area relationship:

$$\log S=\log C+Z\log A$$

where S is species richness, A is area, Z is the slope of the line, and C is the Y-intercept.

- For most regions, Z lies between 0.1 and 0.2
- For very large areas such as whole continents, Z is much steeper, 0.6 to 1.2

Worked example. With $Z=0.2$, a tenfold larger area multiplies species richness by

$$10^{0.2}\approx 1.58$$

so an area with $100$ species would hold about $158$.

An everyday example. A large national park holds far more bird species than a small city garden in the same region.

The substance. The relationship forms a straight line only on logarithmic scales — plotted on ordinary axes it is a rectangular hyperbola.

More diverse communities tend to be more stable and productive, and biodiversity is being lost mainly through the Evil Quartet — habitat loss and fragmentation, over-exploitation, alien species invasions and co-extinctions.

Why diversity matters:

- Tilman's long-term outdoor plots showed that plots with more species had less variation in total biomass over time and higher productivity
- Rivet popper hypothesis — an ecosystem is like an aircraft whose rivets are species; losing a few may not matter at first, but losing key species, like rivets on the wings, or many species can bring collapse

The Evil Quartet:

- Habitat loss and fragmentation — the most important cause; tropical rainforests are cleared for farming, and fragmentation harms animals needing large territories
- Over-exploitation — excessive hunting or harvesting drove species such as Steller's sea cow and the passenger pigeon to extinction
- Alien species invasions — the Nile perch introduced into Lake Victoria eliminated more than 200 species of cichlid fish; in India, Parthenium, Lantana and water hyacinth harm native species
- Co-extinctions — when a species dies out, species obligatorily linked to it also vanish, such as the unique parasites of a host fish, or a plant and its only pollinator

An everyday example. Water hyacinth choking lakes and ponds across India blocks light and oxygen from native plants and fish.

The substance. Habitat loss is usually the biggest threat — even species that are neither hunted nor invaded cannot survive without a home.