Patterns of Biodiversity

The latitudinal gradient is the pattern in which species richness is highest in the tropics and decreases towards the poles.

Regions near the equator contain more species than temperate or polar regions. The tropical belt lies roughly between 23.5°N and 23.5°S.

• Tropical regions show greater species richness than higher latitudes.
• As one moves from the equator towards the poles, biodiversity generally decreases.
• Tropical rainforests are among the most species-rich ecosystems on Earth.
• The Amazon rainforest is characterised by very high species richness, including about 40,000 plant species, 1,300 bird species, 427 mammal species, and a very large number of insect species.

Why Are Tropics Richer in Species?

• Tropical regions have remained relatively undisturbed for long evolutionary periods, allowing more time for speciation.
• Tropical climates are less seasonal and more stable, which supports niche specialisation.
• Greater solar energy input contributes to higher productivity, which can support more organisms.

The altitudinal gradient is the pattern in which species diversity is greater at lower altitudes and declines with increasing altitude.

Mountain slopes often show more diversity at lower elevations than at higher elevations.

• Species richness generally decreases with increasing altitude.
• Higher altitudes experience harsher environmental conditions.
• Temperature decreases, and climatic conditions become more variable with increasing altitude.
• Seasonal variation and reduced productivity make survival difficult for many species at high elevations.

As altitude increases, the environment becomes less favourable for many organisms. Therefore, fewer species can survive and maintain stable populations.

The species-area relationship states that within a region, species richness increases with increasing explored area, though only up to a limit.

This relationship was described by Alexander von Humboldt through observations on species richness across larger areas.

Pattern:

• On a normal graph, the relationship appears as a rectangular hyperbola.
• On a logarithmic scale, the relationship becomes a straight line.

Formula:

Where:

• S = species richness
• A = area
• C = constant
• Z = slope of the line or regression coefficient

Species area relationship; note that on log scale the relationship becomes linear

Remember, for smaller areas, the value of Z is usually in the range of 0.1 to 0.2.
For very large areas such as entire continents, the slope becomes much steeper and Z may range from 0.6 to 1.2.

Why This Matters:

Larger areas usually contain a greater variety of habitats, more resources, and more ecological niches. As a result, they can support more species.

Latitudinal Gradient

• Species richness is high near the equator (tropics: 23.5°N to 23.5°S) and decreases towards the poles. Example: The Amazon rainforest has 40,000 plant species, 1,300 bird species, and 427 mammal species.
• The tropics have high biodiversity due to a stable climate, reduced glaciation, abundant sunlight, higher rainfall, and greater niche specialisation.

Altitudinal Gradient

• Species diversity decreases at higher altitudes due to drastic climatic changes and seasonal variations.

Species-Area Relationship

• Observed by Alexander Von Humboldt, species richness increases with area but only up to a limit. For many species, this forms a rectangular hyperbola.
• Expressed as: log⁡ S = log⁡ C + Z log⁡ A, where S = species richness, A = area, Z = slope, and C = Y-intercept. On a logarithmic scale, it gives a straight line.
• Z-value for smaller areas = 0.1-0.2. For larger areas (continents) = 0.6 to 1.2 (steeper slope - species increase faster than area explored).