Various Terms in Ecology: Complete Notes

Ecological Hierarchy • Competitive Exclusion • Ecological Niche • Fundamental Niche • Realized Niche • Connell's Experiment • Resource Partitioning

CSIR-NET • GATE • DBT • ICMR • MSc Biotechnology

Study Tip: Ecology becomes easier when the major ideas are connected in a logical sequence: organism → population → community → ecosystem → niche → competition → coexistence → resource partitioning

Competitive exclusion and niche theory are closely related. When two species use exactly the same limiting resource in the same way, long-term coexistence becomes difficult. However, natural communities contain many environmental dimensions, and species may differ in where, when, or how they use resources. This difference can reduce competition and allow species to coexist.

📚 Table of Contents / Index

Introduction to Ecological Terms
Ecological Hierarchy
Levels of Ecological Organization
Population and Community
Competition in Ecology
Gause's Competitive Exclusion Principle
Ecological Niche Concept
Fundamental Niche
Realized Niche
Fundamental vs Realized Niche
Connell's Barnacle Experiment
Resource Partitioning
Species Coexistence
Examples from Natural Ecosystems
Important Comparisons
Important Exam Points
Quick Revision Notes
10 MCQs with Hidden Answers
Final Summary

1. Introduction to Various Terms in Ecology

Ecology is the branch of biology that deals with the relationships between organisms and their environment. The word ecology comes from the Greek words oikos, meaning house or place to live, and logos, meaning study. In simple terms, ecology tries to answer questions such as where organisms live, what they eat, how they interact with other organisms, how environmental conditions affect them, and how different species coexist in the same habitat.

To understand ecology properly, it is necessary to understand a number of terms that describe the organization and interaction of living systems. Ecological hierarchy tells us how biological units are arranged from individual organisms to the biosphere. The concept of competition explains what happens when organisms require the same limited resources. The ecological niche concept goes one step further and describes the complete set of environmental conditions and resources associated with the existence of a species.

One of the most important ideas in community ecology is that two species cannot indefinitely occupy exactly the same ecological niche if they depend on the same limiting resources in the same way. This idea is associated with the competitive exclusion principle developed from the experimental work of Georgy Gause.

However, natural ecosystems contain many examples of species living together despite apparently similar requirements. Resource partitioning provides one explanation. Species may use different food resources, different parts of a habitat, different depths of water, or different times of the day. These differences reduce direct competition.

Core idea:

The ecological niche is much broader than simply the physical place where an organism lives. A habitat describes the place, whereas a niche describes the species' ecological role, resource use, environmental requirements and interactions.

2. Ecological Hierarchy

Living organisms are organized into different levels. Ecology studies these levels from individual organisms to the entire biosphere. Each level has properties that cannot be understood simply by looking at the level below it.

Main levels of ecological organization

3. Levels of Ecological Organization

3.1 Organism

An organism is an individual living entity. It is the basic unit at which natural selection acts and is the starting point for many ecological studies.

It interacts directly with both biotic and abiotic components of its environment.
Examples include a single tree, bird, fish, bacterium or human.
Organismal ecology studies how individuals respond to environmental conditions.

3.2 Population

A population is a group of individuals of the same species living in a particular area at a particular time and having the potential to interact and reproduce with one another.

Population size refers to the number of individuals.
Population density refers to the number of individuals per unit area or volume.
Population distribution describes how individuals are spatially arranged.
Birth rate and death rate influence population size.
Immigration and emigration also influence population size.

3.3 Community

A community consists of populations of different species living and interacting in the same area.

For example, a pond community may contain algae, aquatic plants, bacteria, protozoans, insects, fish and other organisms. These populations interact through predation, competition, parasitism, mutualism and other ecological relationships.

3.4 Ecosystem

An ecosystem consists of a biological community together with the physical environment with which it interacts. Thus, both living and non-living components are included.

Biotic components: plants, animals, fungi, bacteria and other organisms.
Abiotic components: water, temperature, light, soil, minerals, gases and nutrients.
Energy flows through ecosystems, while nutrients are continuously cycled.

3.5 Biosphere

The biosphere represents the global zone of life. It includes all ecosystems and the regions of Earth where organisms exist.

4. Population and Community: Important Terms

Population

Individuals of the same species occupying a particular area and capable of interacting and reproducing.

Community

Populations of different species living and interacting in the same area.

Habitat

The physical place or environment where an organism normally lives.

Niche

The ecological role and requirements of a species, including its resource use and interactions.

Exam Alert:

Habitat and niche are not the same. A habitat is often described as an organism's address, while the niche is better thought of as its ecological role and way of life.

5. Competition in Ecology

Competition occurs when organisms use the same resource and that resource is insufficient to satisfy the requirements of all individuals. Competition can occur between members of the same species or between different species.

Types of competition

Intraspecific competition

Competition between individuals of the same species is called intraspecific competition.

Two plants of the same species competing for sunlight.
Two birds competing for nesting sites.
Individuals competing for food or mates.

Interspecific competition

Competition between individuals belonging to different species is called interspecific competition.

Two plant species competing for soil nutrients.
Different herbivore species competing for the same food.
Different microorganisms competing for a limiting substrate.

Why competition occurs

Food may be limited.
Water may be limited.
Space may be limited.
Light may be limited.
Nutrients may be limited.
Nesting or breeding sites may be limited.
Suitable environmental conditions may be restricted.

Important:

Competition is strongest when two species have highly overlapping resource requirements and when one or more of those resources are limiting.

6. Gause's Competitive Exclusion Principle

The competitive exclusion principle is associated with the Russian ecologist Georgy F. Gause. The principle states, in simplified form, that two species cannot indefinitely coexist when they occupy exactly the same ecological niche and compete for the same limiting resources. One species may eventually outcompete the other under the conditions being considered.

Gause supported this idea through laboratory experiments using Paramecium species. When certain species were grown separately, they were able to survive and increase in population. When species with overlapping resource requirements were grown together, competition changed their population growth.

Basic idea

Gause's Paramecium experiments

Gause studied competition among closely related species of Paramecium. The experiments demonstrated that species with similar resource requirements could show strong competition when grown together.

Paramecium aurelia and Paramecium caudatum were able to grow when cultured separately.
When grown together under certain laboratory conditions, P. aurelia outcompeted P. caudatum.
The result illustrated the importance of limiting resources and competition.
The outcome is not a universal prediction that one species always eliminates another in every natural environment. Environmental conditions and differences in resource use matter.

Competitive Exclusion Principle:

If two species have identical ecological requirements and depend on the same limiting resource in the same way, stable coexistence is unlikely.

7. Ecological Niche Concept

The term ecological niche refers to the conditions, resources and interactions that determine how a species lives in an ecosystem. It includes environmental requirements as well as the way an organism obtains resources and interacts with other organisms.

The niche concept is one of the central ideas in ecology because it helps explain why species occur in particular environments and why closely related species can sometimes coexist.

What can be included in a niche?

Temperature range tolerated by a species.
Moisture requirements.
Food resources used.
Type of habitat occupied.
Breeding requirements.
Time of activity.
Interactions with predators.
Interactions with competitors.
Interactions with mutualistic organisms.
Use of space.
Use of nutrients and other limiting resources.

Habitat versus niche

Feature	Habitat	Niche
Meaning	Physical place where an organism lives.	Ecological role and resource requirements.
Simple analogy	Address	Profession or way of life
Includes	Physical environment	Conditions, resources, interactions and ecological role
Example	Pond	What a particular pond organism eats, where it feeds and how it interacts

8. Fundamental Niche

The fundamental niche represents the full range of environmental conditions and resources under which a species could potentially survive and reproduce in the absence of limiting biotic interactions such as competition, predation or parasitism.

It represents the potential ecological space available to the species. In nature, however, organisms rarely experience a situation where all competitors and predators are absent. Therefore, the actual niche occupied by a species is often smaller.

Characteristics

Represents potential ecological distribution.
Includes a broad range of tolerated environmental conditions.
Considers conditions under which the species can survive and reproduce.
Does not primarily represent the restrictions imposed by competitors and predators.
Can be larger than the niche actually occupied in nature.

Easy way to remember:

Fundamental niche = What the species could potentially use.

9. Realized Niche

The realized niche is the portion of the fundamental niche that a species actually occupies under natural conditions. It is shaped by interactions with other organisms and by environmental constraints.

Competition can force a species to occupy only part of the conditions or resources that it could otherwise tolerate. Predation, parasitism, disease and mutualistic interactions can also influence where and how successfully a species occurs.

Factors that can reduce or modify the realized niche

Interspecific competition.
Predation.
Parasitism.
Disease.
Availability of food.
Availability of nesting or breeding sites.
Dispersal limitations.
Other environmental constraints.

Easy way to remember:

Realized niche = What the species actually uses in nature.

10. Fundamental Niche vs Realized Niche

Feature	Fundamental Niche	Realized Niche
Meaning	Potential range of conditions and resources.	Actual range used under natural conditions.
Competition	Usually considered without major biotic restrictions.	Competition can restrict the niche.
Predation	Not the primary restriction in the idealized concept.	Predation may restrict actual distribution.
Size	Often broader.	Often narrower.
Natural occurrence	Represents potential ecological space.	Represents actual ecological space.

11. Connell's Barnacle Experiment

Joseph Connell conducted an influential field study involving two species of intertidal barnacles, Balanus and Chthamalus. The study is widely used to explain the difference between fundamental and realized niches and the role of competition in determining species distributions.

The basic observation

In the intertidal zone, the two barnacle species were distributed at different vertical levels on the rocky shore. Chthamalus was found higher on the shore, while Balanus occurred lower down.

The important question was whether Chthamalus was unable to survive in the lower region because of physical environmental conditions, or whether competition with Balanus restricted its distribution.

Experimental approach

Connell removed Balanus from some experimental areas.
He observed what happened to the distribution of Chthamalus.
When the competitor was removed, Chthamalus was able to occupy areas lower on the shore that it normally did not occupy.
This suggested that competition with Balanus restricted the distribution of Chthamalus.

Conclusion

The study demonstrated that the realized niche of Chthamalus was smaller than its fundamental niche because competition with Balanus restricted its distribution.

Exam Point:

Connell's barnacle study is an important example of how interspecific competition can reduce a species' realized niche.

12. Resource Partitioning

Resource partitioning occurs when competing species use different portions of an available resource. Instead of using exactly the same resources in exactly the same way, species differ in the location, timing or type of resource they use.

Resource partitioning can reduce competition and allow species with overlapping requirements to coexist.

Major forms of resource partitioning

1. Spatial partitioning

Species use different physical locations within the same general habitat.

Different bird species feeding at different heights in a tree.
Different fish species occupying different depths of a lake.
Different plant species using different zones of soil.

2. Temporal partitioning

Species use the same type of resource at different times.

One species may feed during the day.
Another species may feed at night.
Different organisms may reproduce during different seasons.

3. Trophic partitioning

Species use different food types or occupy different positions in resource-use patterns.

Two herbivores may feed on different plant parts.
Two birds may consume different types of insects.
Different fish may specialize on different food sources.

Resource partitioning and coexistence

If species reduce their overlap in resource use, competition may become weaker. This can increase the possibility of stable coexistence.

Key idea:

Resource partitioning does not necessarily mean that species never compete. It means that differences in resource use reduce the intensity of competition enough to permit coexistence under suitable conditions.

13. Species Coexistence

Species coexistence refers to the ability of different species to persist in the same community over time. Coexistence becomes easier when species differ in their ecological requirements or use resources in different ways.

Mechanisms that can promote coexistence

Resource partitioning: Species use different portions of a resource.
Spatial separation: Species use different locations.
Temporal separation: Species use resources at different times.
Dietary differences: Species consume different food resources.
Microhabitat specialization: Species occupy different small-scale habitats.
Differences in environmental tolerance: Species perform better under different environmental conditions.
Predator-mediated interactions: Predators can sometimes influence competitive relationships among prey species.

Niche overlap

Niche overlap describes the degree to which two species use similar resources or environmental conditions. High niche overlap can increase competition when the overlapping resource is limiting.

However, niche overlap alone does not automatically mean that one species will be eliminated. The strength of competition depends on which resources are limiting, how efficiently species use them, environmental variability and other ecological interactions.

14. Examples of Niche Differentiation in Natural Ecosystems

Resource partitioning is common in natural ecosystems. A community may contain many species that appear to use similar resources, but closer examination often reveals differences in feeding location, feeding time, food type or environmental tolerance.

Bird communities

Several insect-eating bird species may live in the same forest. Instead of feeding everywhere in the same way, different species may specialize on different parts of trees. One may forage near the canopy, another on branches, and another closer to the trunk.

Such spatial differences reduce direct competition for the same insects.

Intertidal organisms

Rocky shores provide a useful example because environmental conditions change strongly with vertical position. Organisms living higher on the shore experience longer exposure to air, while organisms lower on the shore experience greater immersion in seawater but may face stronger biological competition.

Forest plants

Plants may partition light and soil resources by occupying different heights and rooting zones. Canopy trees capture light high above the ground, while understory plants tolerate lower light levels.

Aquatic ecosystems

Fish and aquatic organisms may partition resources according to water depth, feeding time, food type and location within the water column.

15. Important Comparisons for Competitive Exams

Concept	Meaning	Key Point
Organism	Individual living entity.	Basic ecological unit.
Population	Individuals of the same species in an area.	Same species.
Community	Populations of different species interacting.	Multiple species.
Ecosystem	Community plus physical environment.	Biotic + abiotic components.
Habitat	Physical place where an organism lives.	Address.
Niche	Ecological role and resource requirements.	Way of life.
Fundamental niche	Potential ecological range.	Broader potential space.
Realized niche	Actual ecological range.	Often restricted by biotic interactions.
Intraspecific competition	Competition within one species.	Same species.
Interspecific competition	Competition between species.	Different species.
Competitive exclusion	Strong competition can prevent stable coexistence when niches are effectively identical.	Associated with Gause.
Resource partitioning	Species use different portions of resources.	Can reduce competition.

16. Important Exam-Oriented Concepts

1. Habitat is not the same as niche

Habitat describes where an organism lives. Niche describes how the organism lives and how it interacts with its environment.

2. Fundamental niche is generally broader

The fundamental niche describes the potential ecological conditions available to a species. The realized niche is the portion actually occupied under real ecological interactions.

3. Competition can reduce realized niche

When another species competes strongly for a limiting resource, a species may be prevented from using parts of its potential niche.

4. Gause is associated with competitive exclusion

Gause's experiments with Paramecium helped establish the experimental foundation for the competitive exclusion concept.

5. Connell is associated with barnacle competition

Connell's work on Balanus and Chthamalus demonstrated how competition can influence the distribution of species and help distinguish fundamental from realized niches.

6. Resource partitioning promotes coexistence

Species can coexist when they reduce competition through differences in resource use, location, time or feeding strategy.

High-Yield Memory Trick:

Gause → Paramecium → Competitive Exclusion

Connell → Barnacles → Competition → Realized Niche

Resource Partitioning → Reduced Competition → Coexistence

17. Quick Revision Notes

⭐ Must-Remember Points

Ecology studies relationships between organisms and their environment.
The major ecological hierarchy can be represented as organism → population → community → ecosystem → biosphere.
A population contains individuals of the same species.
A community contains populations of different species.
An ecosystem contains biotic and abiotic components.
Habitat refers mainly to the physical place where an organism lives.
Niche describes the ecological role, resource use and environmental requirements of a species.
Competition occurs when organisms require the same limiting resource.
Intraspecific competition occurs within the same species.
Interspecific competition occurs between different species.
Gause is strongly associated with the competitive exclusion principle.
Competitive exclusion predicts that species with effectively identical ecological requirements cannot indefinitely coexist when they depend on the same limiting resources.
Fundamental niche represents the potential range of conditions and resources that can support a species.
Realized niche represents the range actually occupied under natural ecological interactions.
Competition can make the realized niche smaller than the fundamental niche.
Connell's barnacle experiment is an important example of competition affecting species distribution.
Balanus and Chthamalus were used in Connell's classic intertidal study.
Resource partitioning occurs when competing species use different portions of available resources.
Resource partitioning can occur spatially, temporally or trophically.
Resource partitioning can reduce competition and promote coexistence.
High niche overlap can increase competition when overlapping resources are limiting.
Competitive exclusion and resource partitioning are important concepts for understanding community structure.

19. Final Exam-Oriented Summary

The various terms used in ecology are closely connected. Ecological hierarchy describes the organization of life from an individual organism to the biosphere. At the community level, species interact through competition, predation, parasitism, mutualism and other relationships.

The ecological niche describes how a species fits into its environment. The fundamental niche represents the potential range of conditions and resources that could support the species, whereas the realized niche represents the portion actually occupied after ecological interactions and environmental restrictions are taken into account.

Gause's work with Paramecium provided important experimental support for the competitive exclusion principle. When species have essentially identical requirements for limiting resources, stable coexistence becomes difficult.

Connell's barnacle experiment provided a classic field example of competition affecting species distribution. Removing a competing species allowed Chthamalus to occupy areas from which it was normally excluded, demonstrating that competition can restrict the realized niche.

Natural ecosystems nevertheless contain many species that coexist. Resource partitioning helps explain this pattern. Species may divide resources according to space, time or type of food. By reducing direct competition, such differences can allow several species to persist in the same community.

Ecological hierarchy: Organism → Population → Community → Ecosystem → Biosphere.
Habitat: Physical place where an organism lives.
Niche: Ecological role and resource requirements.
Gause: Competitive exclusion and Paramecium.
Fundamental niche: Potential ecological range.
Realized niche: Actual ecological range under natural interactions.
Connell: Barnacle competition and niche restriction.
Resource partitioning: Division of resources among competing species.
Result: Reduced competition can promote species coexistence.

For competitive examinations such as CSIR-NET, GATE Biotechnology, DBT-BET and ICMR-JRF, questions are often framed around the distinction between habitat and niche, fundamental and realized niche, Gause's experiments, Connell's barnacle study and mechanisms of coexistence. Understanding the ecological logic behind these concepts is more useful than memorizing isolated definitions.

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Friday, 28 August 2026