Maarg Manthan · Topic 1.7

Structure and Characteristics of an Ecological Community

Environment & Ecology › Basic Concepts of Ecology · Topic 1.7

An ecological community is an assemblage of populations of different species living together in an area and interacting with one another. Its structure refers to its species composition and the way they are arranged, and its main characteristics are species richness, species diversity, dominance, relative abundance, growth form and structure, vertical stratification, trophic structure, and ecological succession. Species richness is the number of species, evenness is how equally individuals are spread among them, and diversity combines both and is measured by indices such as the Shannon index. R H Whittaker described alpha diversity within a habitat, beta diversity between habitats and gamma diversity across a region. Frederic Clements saw the community as a superorganism, while H A Gleason saw it as a chance association of species.

5 min read · UPSC Prelims · Environment & Ecology · Basic Concepts of Ecology

At a Glance

Richness Number of species
Evenness Equal spread of individuals
Alpha diversity Within a habitat
Beta diversity Between habitats
Gamma diversity Across a region
Exam link UPSC Prelims, Environment & Ecology: Basic Concepts of Ecology

Where Does It Fit?

This topic follows the account of ecological principles. It describes the features of a community, which prepares for the next topics on stratification, ecotone and succession. Previous topic: Ecological Principles. Next topic: Stratification, Ecotone and Ecological Dominance.

What Are the Structural Features of a Community?

Feature	Meaning
Species composition	The list of species present in the community and their numbers
Species richness	The number of species. Tropical rainforests have the highest richness
Species evenness	How equally individuals are distributed among species. A community with equal numbers is more even
Species diversity	A measure that combines richness and evenness, calculated by indices such as the Shannon-Wiener index and Simpson’s index
Relative abundance	The share of each species in the total number of individuals
Growth forms	Trees, shrubs, herbs, climbers and epiphytes, as described in Raunkiaer’s life forms
Trophic structure	Producers, consumers and decomposers arranged in food chains and food webs

What Are Dominance, Diversity Types and Community Concepts?

Dominance: a few species exert strong control over the community because of numbers, size or activity, for example sal in a sal forest
Stratification: vertical layers of vegetation such as tree, shrub, herb and ground layers
Alpha diversity: species diversity within a particular habitat or community
Beta diversity: change in species composition from one habitat to another along a gradient
Gamma diversity: total diversity over a large region or landscape
Clements’ view: the community is like a superorganism that develops toward a climax
Gleason’s view: the individualistic concept; species respond separately to the environment, so communities are loose assemblages
Periodicity: changes in community with time of day and season

What Else Should You Remember?

Life forms: Raunkiaer classified plants by the position of their buds during unfavourable season: phanerophytes, chamaephytes, hemicryptophytes, cryptophytes and therophytes
Frequency classes: Raunkiaer’s law of frequency describes how species are spread among classes
Keystone species: species whose effect is large compared with their numbers, as in Endemic Species, Keystone Species, Indicator Species, Invasive Species
Foundation species: species that create habitat, such as corals and kelp
Importance value: the sum of relative density, frequency and dominance of a species
Edge effect: more species and density at the junction of communities
Community boundaries: often gradual, forming an ecotone
Community types: forest, grassland, pond and desert communities
Major and minor communities: major communities are self-sufficient, minor depend on others
Latitudinal gradient: species richness is highest near the equator
Species-area relationship: larger areas hold more species
Island biogeography: MacArthur and Wilson explained species numbers on islands
Disturbance: moderate disturbance can increase diversity (intermediate disturbance hypothesis)
Redundancy: several species that play similar roles give resilience
Sampling: quadrats and transects are used to study communities
Biodiversity link: diversity types are further covered in Definition and Types of Biodiversity (Genetic, Species, Ecosystem)
Succession link: change in community over time is in Climax community Exam…">Ecological Succession: Types and Process
Raunkiaer life form example: therophytes are annual plants that survive the dry season as seeds, common in deserts
Equitability: another word for evenness
Simpson index: gives the chance that two random individuals belong to different species
Rank abundance: a curve showing how common each species is in order
Community productivity: more diverse communities often capture more energy
Functional groups: sets of species that play a similar role, such as nitrogen fixers
Niche structure: each member occupies a niche, as in Habitat and Ecological Niche
Indian example: the Western Ghats rainforest has very high richness and many endemic species
Mangrove community: low richness but high dominance of a few salt tolerant tree species
Human effect on community: introduced species and fragmentation reduce evenness and richness
Stratification link: see Stratification, Ecotone and Ecological Dominance

Why Does It Matter for the Exam?

Terms: richness, evenness, diversity
Whittaker: alpha, beta, gamma
Concepts: Clements versus Gleason
Current affairs link: biodiversity assessments
Common errors: mixing richness with evenness or alpha with gamma diversity

As an analytical point, communities with higher diversity and several species performing similar roles are usually more stable and productive. This is a key reason for protecting biodiversity and not just single species.

Why Is This Topic Important?

Features Richness, evenness
Scales Alpha, beta, gamma
Concepts Two views

What Came Next?

The next topic covers stratification, ecotone and ecological dominance. Next topic: Stratification, Ecotone and Ecological Dominance.

Key People and Terms

Species richness Number of species
Evenness Equal abundance
Alpha diversity Local diversity
Beta diversity Turnover between habitats
Gamma diversity Regional diversity
Dominance Control by few species

Exam Corner

Points to Remember

Whittaker: alpha, beta, gamma.
Richness = number; evenness = equality.
Clements: superorganism; Gleason: individualistic.
Raunkiaer: life forms.
Highest richness near the equator.

Do Not Confuse With

Richness and diversity Count versus count plus evenness
Alpha and beta diversity Within versus between habitats
Clements and Gleason Organism-like versus individualistic

Memory Hook

Count the species, spread the numbers; alpha at home, beta between, gamma overall.

Mains Angle

Questions ask for diversity types and community concepts. These points help.

Structure Features
Diversity Three scales
Concepts Debate

A Question You May Face

An original practice question, not a past paper question.

“Describe the structural characteristics of an ecological community and explain alpha, beta and gamma diversity.”

How to Answer

Define Community.
Features Richness, strata.
Diversity Three types.
Concepts Clements, Gleason.

GS Relevance

Prelims: community features, diversity types, concepts. GS Paper 3: ecology.

Frequently Asked Questions

What is an ecological community?

An ecological community is a group of populations of different species that live together in the same area and interact with one another through feeding, competition and other relationships. Examples include the community of a pond, a forest or a coral reef. It has features such as diversity, dominance and layering.

What is the difference between species richness and species evenness?

Species richness is the number of different species in a community, while species evenness describes how equally the individuals are divided among the species. A community with ten species of equal numbers is more even than one with ten species where a single species has most of the individuals.

What are alpha, beta and gamma diversity?

Alpha diversity is the species diversity within a single habitat or community. Beta diversity is the change or difference in species composition between habitats. Gamma diversity is the total diversity of a larger region or landscape. R H Whittaker introduced these terms in 1960.

What is dominance in a community?

Dominance means that one or a few species have a major influence on the community because of their numbers, size or activity, and determine the conditions for others. For example, sal trees dominate a sal forest and kelp dominates a kelp forest. These species are the dominants.

What is the difference between Clements' and Gleason's views of community?

Frederic Clements saw a community as a kind of superorganism whose species are tightly linked and which develops in a predictable way to a climax. H A Gleason argued that a community is just a chance grouping of species that happen to share the same environmental needs, and each species responds individually.

PYQ Practice — Statement Analysis

1 Alpha diversity refers to diversity within a habitat.

True

Introduced by Whittaker.

2 Gamma diversity is the diversity within one habitat.

False

It is regional diversity.

3 Clements regarded a community as a superorganism.