Community Structure and Dynamics

Introduction

Communities are complex assemblages of different species living together in a particular area. The structure and dynamics of these communities are shaped by the interactions among their constituent organisms and their environment. Understanding these relationships is fundamental to ecology and helps explain patterns of biodiversity and ecosystem function.

Community Structure

Definition of a Community

• Community: All the organisms inhabiting a particular area and interacting with each other.

• Communities are organized by the types and strengths of interactions among species.

• Biological communities are the next level of organization above populations, consisting of multiple populations of different species living in close proximity.

Interspecific Interactions

Interspecific interactions are relationships between individuals of different species within a community. These interactions are fundamental in shaping community structure and population dynamics.

• Types of Interspecific Interactions:

  • Competition: Both species are harmed (-/-).

  • Mutualism: Both species benefit (+/+).

  • Commensalism: One species benefits, the other is unaffected (+/0).

  • Predation: One species benefits, the other is harmed (+/-).

  • Herbivory: An animal eats plants or algae (+/-).

  • Parasitism and Pathogens: Parasite/pathogen benefits, host is harmed (+/-).

Symbiotic Relationships

Definition and Types

A symbiotic relationship is an interaction between two or more species that live together in direct contact. There are three main types of symbiotic relationships within communities:

• Parasitism

• Commensalism

• Mutualism

Parasitism

• One organism (the parasite) lives on or in the host and benefits by feeding on it, while the host is harmed.

• Examples: Tapeworms, fleas, disease-causing protozoa, bacteria, and viruses.

Commensalism

• One species benefits, and the other is neither helped nor harmed.

• Example: Barnacles hitching a ride on the skin of a whale.

• Other examples: Epiphytes (plants growing on trees), cattle egrets and grazing animals.

Mutualism

• Both species benefit from the interaction.

• Example: Lichens, which are associations between fungi and algae. The algae provide food by photosynthesis, and the fungi provide protection and structure.

• Other examples: Nitrogen-fixing bacteria and legumes; Acacia trees and ants of the genus Pseudomyrmex.

Unusual Nutritional Adaptations

• Epiphytes: Plants that grow on other plants for support but do not harm them.

• Parasitic plants: Non-photosynthetic plants that absorb sugar and minerals from living hosts.

• Carnivorous plants: Plants that obtain nutrients by trapping and digesting insects or other small animals.

Summary Table: Symbiotic Relationships

Key Terms

• Community: An assemblage of populations of different species living close enough for potential interaction.

• Symbiosis: A close and long-term biological interaction between two different biological organisms.

• Epiphyte: A plant that grows on another plant but is not parasitic.

• Parasitic plant: A plant that derives some or all of its nutritional requirements from another living plant.

• Mutualism: A symbiotic relationship in which both participants benefit.

• Commensalism: A symbiotic relationship in which one organism benefits and the other is neither helped nor harmed.