MEV 011: Unit 07 - Community Organisation and Interaction among Organisms

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 UNIT 7 – COMMUNITY ORGANISATION AND INTERACTION AMONG ORGANISMS

7.1 Introduction

An ecological community is composed of different species living in the same area and interacting with one another in various ways—competition, predation, symbiosis, etc. The study of communities focuses on how species coexist, how energy and nutrients flow through the system, and how changes in the environment affect the community's structure and function. Understanding these complex interactions is essential for ecology, conservation biology, and environmental management.

7.2 Objectives

After studying this unit, you should be able to:

·         Define an ecological community and explain its characteristics.

·         Understand how species are organized within a community.

·         Describe key concepts like habitat, niche, keystone species, and community stability.

·         Explain species interactions such as competition, predation, and mutualism.

·         Understand how competition shapes species behavior and evolution.

7.3 What is Community?

A community is defined as a group of populations of different species living in the same geographic area and interacting with each other. These interactions may be direct or indirect and are essential for the functioning of ecosystems.

Characteristics of a community include:

·         Species richness (number of species)

·         Species diversity (abundance and variety)

·         Trophic structure (feeding relationships)

·         Dominance (presence of dominant species)

·         Succession (temporal change)

7.4 Community Gradients and Boundaries

Communities are not isolated units with clear-cut borders. Instead, they blend into one another in a gradient.

·         Ecotone: A transitional zone between two ecosystems (e.g., forest and grassland), often rich in biodiversity.

·         Edge effect: Greater diversity of life in the boundary zone between two ecosystems.

Communities may be:

·         Closed (with sharp boundaries)

·         Open (with gradual transitions and overlapping species)

7.5 Community Organisation

Community organization refers to how species interact and their roles in maintaining the community structure.

7.5.1 Habitat and Niche

·         Habitat: The physical environment where an organism lives.

·         Niche: The functional role a species plays in its ecosystem (what it eats, where it nests, how it behaves).

·         Two species cannot occupy the same niche indefinitely (competitive exclusion principle).

7.5.2 Functional Roles and Guilds

·         Functional role: Refers to the role a species plays (e.g., decomposer, herbivore, predator).

·         Guild: A group of species that exploit the same resources in similar ways (e.g., nectar-feeding insects).

7.5.3 Keystone Species

A keystone species has a disproportionately large effect on its community relative to its abundance. Its removal leads to significant changes in ecosystem structure.

Examples:

·         Sea otters (control sea urchin population)

·         Wolves in Yellowstone (regulate herbivores)

7.5.4 Dominant Species

Dominant species are the most abundant or most influential species in a community. They shape the environment by their biomass or energy flow.

Examples:

·         Pine trees in coniferous forests

·         Grasses in grasslands

7.5.5 Stability

Community stability refers to its ability to resist or recover from disturbances (resilience). Diverse communities tend to be more stable and productive.

7.6 Species Interaction

Species interact in many different ways:

·         Mutualism (+/+): Both species benefit (e.g., bees and flowers)

·         Commensalism (+/0): One benefits, other unaffected (e.g., barnacles on whales)

·         Parasitism (+/−): One benefits at the expense of the other (e.g., tapeworm in humans)

·         Predation (+/−): Predator kills and eats prey

·         Amensalism (−/0): One is harmed, the other unaffected (e.g., algal bloom harming fish)

These interactions define the ecological network of the community.

7.7 Competition

Competition occurs when two or more species vie for the same resource (food, space, mates).

7.7.1 Competition in Laboratory Populations

·         Studied by G.F. Gause, who demonstrated competitive exclusion using Paramecium species.

·         When grown together, one species outcompeted the other.

7.7.2 Competition in Natural Populations

In nature, competition is influenced by:

·         Resource availability

·         Environmental fluctuations

·         Behavioral adaptations

Species may reduce competition through niche differentiation.

7.7.3 Results of Competition

Competition can lead to:

·         Competitive exclusion: One species eliminates the other.

·         Coexistence through resource partitioning.

·         Character displacement: Evolutionary changes in species traits to minimize overlap (e.g., beak size in Darwin’s finches).

7.7.4 Evolution of Competitive Ability

Over generations, species evolve traits that enhance their competitive edge:

·         Faster reproduction

·         Efficient resource use

·         Better defense mechanisms

This evolutionary arms race shapes the structure and diversity of the community.

7.8 Let Us Sum Up

In this unit, you learned:

·         A community is a group of different populations living and interacting in a specific area.

·         Communities can be structured and categorized by gradients, ecotones, and species composition.

·         Habitat and niche are key concepts in understanding species’ roles.

·         Keystone and dominant species play crucial roles in community stability.

·         Species interactions—competition, mutualism, parasitism, etc.—drive community dynamics.

·         Competition influences not only population sizes but also evolutionary traits and community diversity.

·         Both experimental and field studies confirm the effects of interspecies competition on ecological balance.

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