Maarg Manthan · Topic 6.2

Patterns of Biodiversity

Environment & Ecology › Biodiversity Basics and Classification · Topic 6.2

Patterns of biodiversity are the regular ways in which the number of species changes across space. The best known is the latitudinal gradient: species richness is highest in the tropics and falls towards the poles, with Colombia having over 1,400 bird species and New York around 105. Reasons offered include more solar energy and productivity in the tropics, stable climate over long periods, larger area of tropical regions and long evolutionary time, since temperate areas were hit by glaciation. Altitudinal gradient is similar, with diversity falling as height increases. The species-area relationship, found by Alexander von Humboldt, shows that species number rises with area in a rectangular hyperbola on a linear scale and as a straight line on a log scale, written log S = log C + Z log A, with Z usually between 0.1 and 0.2 for small regions and 0.6 to 1.2 for very large areas.

5 min read · UPSC Prelims · Environment & Ecology · Biodiversity Basics and Classification

At a Glance

Richest region Tropics
Trend Falls towards poles
Species-area Humboldt
Z value 0.1 to 0.2 typical
Island theory MacArthur and Wilson
Exam link UPSC Prelims, Environment & Ecology: Biodiversity Basics and Classification

Where Does It Fit?

This topic follows the definition and types of biodiversity. It explains how species numbers vary across the globe, before the classifications and hotspots that use these patterns. Previous topic: Definition and Types of Biodiversity (Genetic, Species, Ecosystem). Next topic: Bio-Geographical Classification of the World.

What Are the Main Patterns and Their Causes?

Pattern	Description and reasons
Latitudinal gradient	Diversity falls from the equator to the poles. Tropical rainforests such as the Amazon hold the most species. India with tropical areas has over 1,200 bird species, against about 105 for New York
Reason: time	Tropics were relatively undisturbed by glaciation, giving more time for species to evolve
Reason: climate stability	Seasons are less variable, favouring specialisation and narrow niches
Reason: energy and productivity	More solar energy supports higher productivity and more species
Reason: area	The tropical belt is the largest biome by area
Altitudinal gradient	Diversity usually falls as elevation rises because of cooler temperature, thinner air and less area
Species-area relationship	Larger areas hold more species. On a log scale it is a straight line: log S = log C + Z log A

What Do the Species-Area Curve and Island Theory Tell Us?

S, A, C, Z: S is species richness, A is area, C is the Y-intercept, Z is the slope of the line (regression coefficient)
Z values: 0.1 to 0.2 for areas of different sizes within a region, irrespective of taxon or region
Large areas: For very large areas such as whole continents, the slope is steeper, with Z from 0.6 to 1.2
Alexander von Humboldt: observed in South American forests that species richness increases with area up to a limit
Island biogeography: MacArthur and Wilson (1967) proposed that the number of species on an island is a balance between immigration and extinction rates
Island size and distance: large islands near the mainland hold more species than small, far islands
Implication: habitat loss lowers species, with the area reduced by 90 percent causing loss of about half the species
Conservation use: reserve design needs large, connected areas

What Else Should You Remember?

Rapoport’s rule: species ranges are narrower in the tropics than at high latitudes
Species-energy hypothesis: diversity follows available energy
Mid-domain effect: species ranges overlap most in the middle of a bounded area
Hotspots: regions of high endemism and threat; see Biodiversity Hotspots
Longitudinal pattern: less marked than latitudinal pattern
Depth gradient: marine diversity varies with depth, often highest at mid depths
Coral triangle: highest marine diversity
Amazon: about 40,000 plant species, 1,300 birds, 427 mammals, 378 reptiles and over 3,000 fish
Western Ghats: a hotspot with high endemism in amphibians and plants
Himalaya: shows an altitudinal gradient with mid elevation peaks
Edge effect: more species at habitat boundaries
Ecotone: transition zone between ecosystems
Habitat fragmentation: breaks areas into smaller pieces, cutting species
Extinction debt: species lost over time after habitat loss
Wallace line: a faunal boundary between Asia and Australia
Alfred Russel Wallace: a founder of biogeography
Link to previous: see Definition and Types of Biodiversity (Genetic, Species, Ecosystem)
Link to next: see Bio-Geographical Classification of the World
Link to threats: see Threats to Biodiversity and Conservation Strategies macro
Caution: the latitudinal gradient has exceptions, such as high diversity of some groups in temperate zones
Marine pattern: shallow tropical seas such as coral reefs also show high diversity, while polar seas are less rich
Island examples: the Andaman and Nicobar Islands and Lakshadweep show the theory of island biogeography
Practical value: the species-area rule helps estimate species lost when a forest is cut
Number tip: remember Z of about 0.1 to 0.2, and that it is the same for different taxa
Exam hint: state the five reasons for the tropical richness: time, stability, energy, area and less glaciation
Tropical regions: cover the largest area of any climate belt and hold most of the world’s species, which is why conservation attention concentrates there

Why Does It Matter for the Exam?

Gradients: latitude, altitude
Law: species-area
Z value: 0.1 to 0.2
Current affairs link: habitat fragmentation and linear projects
Common errors: saying diversity rises towards the poles or confusing slope Z with intercept C

As an analytical point, these patterns explain why tropical forests and large intact areas are conservation priorities. The species-area rule also warns that shrinking habitat eventually costs species in a predictable way.

Why Is This Topic Important?

Gradient Tropics richest
Area Bigger holds more
Islands Balance of arrival and loss

What Came Next?

The next topic covers the bio-geographical classification of the world. Next topic: Bio-Geographical Classification of the World.

Key People and Terms

Latitudinal gradient Diversity change with latitude
Species-area curve Species rise with area
Z Slope of the line
Island biogeography Theory on island species
Ecotone Transition zone
Extinction debt Delayed species loss

Exam Corner

Points to Remember

Richest: tropics.
Species-area by Humboldt.
Z is 0.1 to 0.2.
Island theory by MacArthur and Wilson.
Diversity falls with altitude.

Do Not Confuse With

C and Z Intercept versus slope
Ecotone and edge effect Transition zone versus its richer biota
Latitudinal and altitudinal gradient Poleward versus upward decline

Memory Hook

Warm, old, stable and big: that is why the tropics are rich.

Mains Angle

Questions ask for causes and the curve. These points help.

Patterns Gradients
Causes Reasons
Use Reserve design

A Question You May Face

An original practice question, not a past paper question.

“Explain the latitudinal gradient in species diversity and the species-area relationship, and discuss their relevance for conservation.”

How to Answer

Gradient Latitude.
Causes Reasons.
Curve Species-area.
Use Reserves.

GS Relevance

Prelims: latitudinal gradient, species-area curve, island biogeography, causes. GS Paper 3: biodiversity and conservation.

Frequently Asked Questions

What is the latitudinal gradient of biodiversity?

The latitudinal gradient is the pattern in which the number of species is highest near the equator and decreases towards the poles. For example, the tropical forests of the Amazon hold far more species of trees, birds and insects than the forests of high-latitude regions. This is one of the oldest patterns recognised in ecology.

Why are the tropics richer in species?

The tropics have a stable climate, high solar energy that supports productivity, a large area, and a long unbroken evolutionary history because they escaped the glaciations that wiped out temperate life. These factors allowed more species to evolve, survive and specialise in narrow niches.

What is the species-area relationship?

The species-area relationship states that the number of species increases with the size of the area explored, in the form of a rectangular hyperbola. On a logarithmic scale it becomes a straight line described by log S equals log C plus Z log A, where Z is the slope, usually 0.1 to 0.2.

What is the theory of island biogeography?

The theory of island biogeography, proposed by MacArthur and Wilson in 1967, says that the number of species on an island is a dynamic balance between the rate at which new species arrive and the rate at which resident species go extinct. Large islands near the mainland have more species than small, distant ones.

How does altitude affect species diversity?

Species diversity generally declines with increasing altitude because temperature falls, the air becomes thinner, the growing season shortens and the area available becomes smaller. In mountains such as the Himalaya, many groups show highest diversity at low or mid elevations and fewest species near the snowline.

PYQ Practice — Statement Analysis

1 Species diversity generally decreases from the equator to the poles.

True

The latitudinal gradient.

2 The species-area relationship was described by Charles Darwin.

False

By Alexander von Humboldt.

3 The slope Z of the species-area line is typically 0.1 to 0.2.