Patterns of biodiversity: latitudinal gradients and species-area relationships
Species diversity decreases from the equator towards the poles. Within a region, species richness rises with area, following log S = log C + Z log A.
Latitudinal gradients
In general, species diversity decreases as we move away from the equator towards the poles. The tropics (latitudes 23.5° N to 23.5° S) have more species than temperate or polar regions. Colombia, near the equator, has nearly 1,400 species of birds; New York at 41° N has 105; Greenland at 71° N has only 56. India, with much of its land in tropical latitudes, has more than 1,200 bird species. A tropical forest in Ecuador has up to 10 times as many species of vascular plants as an equal area of temperate forest in the Midwest of the USA. The largely tropical Amazon rain forest has the greatest biodiversity on Earth. It holds more than 40,000 species of plants, 3,000 of fishes, 1,300 of birds, 427 of mammals, 427 of amphibians, 378 of reptiles and more than 1,25,000 invertebrates.
Why are the tropics richer in species?
- More evolutionary time: speciation is a function of time. Temperate regions were repeatedly covered by glaciers in the past, but the tropics stayed relatively undisturbed for millions of years, giving species a long time to diversify.
- A stable climate: tropical environments are less seasonal, more constant and predictable. This favours niche specialisation, which leads to more species.
- More solar energy: the tropics receive more sunlight, which gives higher productivity, and this may indirectly support greater diversity.
Species-area relationship
The German naturalist Alexander von Humboldt, exploring the wilds of South America, noticed that within a region, species richness increased with the explored area, but only up to a limit. For many groups, such as flowering plants, birds, bats and freshwater fishes, the graph of species richness against area is a rectangular hyperbola. On a logarithmic scale it becomes a straight line: log S = log C + Z log A. Here S is species richness, A is area, Z is the slope of the line (the regression coefficient) and C is the Y-intercept.
What the Z value tells you
| Situation | Z value | Meaning |
|---|---|---|
| Areas within a region, whatever the group or region | 0.1 to 0.2 | The slopes are remarkably similar for very different groups and regions |
| Very large areas, such as whole continents | 0.6 to 1.2 | The line is much steeper: species richness rises fast with area |
| Frugivorous (fruit-eating) birds and mammals in tropical forests of different continents | 1.15 | An example of a steep slope across continents |
How to draw the species-area graphs
- Draw two graphs side by side.
- Graph 1: x-axis area (A), y-axis species richness (S). Draw a curve that rises steeply and then flattens (a rectangular hyperbola). Write S = C Aᶻ beside it.
- Graph 2: x-axis log A, y-axis log S. Draw a straight, rising line. Label its slope Z and the point where it meets the y-axis log C. Write log S = log C + Z log A.
- Write the title: Species-area relationship. Note that the second graph uses log scales.
Worked example: using the species-area equation
In a region, C = 10 and Z = 0.25 for flowering plants. How many species are expected in an area of 10,000 km²?
- Use log S = log C + Z log A.
- log C = log 10 = 1, and log A = log 10,000 = 4.
- log S = 1 + 0.25 × 4 = 1 + 1 = 2.
- S = 10² = 100.
About 100 species. Check: S = C Aᶻ = 10 × 10,000^0.25 = 10 × 10 = 100.
Kerala's rain forests in the Western Ghats, closer to the equator, hold far more species than the cold desert of Ladakh. Conservation planners use the species-area rule to judge how many species a shrinking forest may lose.
1 marks: “What is the usual range of Z in species-area relationships within a region?” 0.1 to 0.2.
2 marks: “Write the equation for the species-area relationship on a log scale. What do S, A, C and Z represent?” log S = log C + Z log A. S = species richness, A = area, Z = slope (regression coefficient), C = Y-intercept.
3 marks: “Give three reasons why the tropics have greater biodiversity than temperate regions.” Long, undisturbed evolutionary time (no repeated glaciation); a less seasonal, more constant and predictable climate that allows niche specialisation; more solar energy, giving higher productivity.
Tropics win with TSS: more Time, Stable seasons, more Sun.