Week_7

Community Ecology

Spring 2025 Overview

  • Focus: Understanding community interactions and their impact on ecology.

  • Date: 2/26/25

Objectives

  • Define and classify interspecific interactions.

  • Explain competition and the competitive exclusion principle.

  • Define ecological niche; explain resource partitioning, fundamental niches, and realized niches.

  • Describe predation, herbivory, symbiosis, and facilitation with examples.

  • Investigate the importance of interspecific interactions in shaping community structure.

Introduction to Biological Communities

  • Definition: A biological community consists of populations of different species living in proximity to interact.

  • Community Boundaries: Defined by ecologists to address specific research questions, e.g., decomposers on a rotting log.

  • Types of Interspecific Interactions:

    • Competition

    • Predation

    • Herbivory

    • Symbiosis (e.g., parasitism, mutualism, commensalism)

    • Facilitation

Classifying Interspecific Interactions

  • Symbolic Representation:

      • = benefit

      • = harm

    • 0 = no effect

  • Interactions are classified based on their effects on species involved.

  • Ex. Predation (+/-), Mutualism (+/+)

  • Historical Focus: Negative interactions, now recognizing the significance of positive interactions.

Competition

Definition and Examples

  • Interspecific Competition: ( -/- ) interaction for limited resources.

    • Example: Weeds vs. garden plants competing for nutrients and water.

    • Rarely limiting resources (like oxygen) typically don't lead to competition.

Competitive Exclusion Principle

  • Studies by G. F. Gause (1934) with Paramecium species:

    • Finding: When grown together, one species (P. caudatum) became extinct due to competition for resources.

    • Principle: Two species competing for the same limiting resources cannot coexist indefinitely; the superior competitor will eliminate the other.

  • Coexistence Conditions: Understanding conditions that allow coexistence despite resource competition.

Ecological Niches and Resource Partitioning

  • Niche Definition: A species' biotic and abiotic resource usage in its environment.

    • Habitat vs. Niche: Habitat = address; niche = profession.

    • Example: Tropical tree lizard’s niche includes temperature preference, branch size, time of day, and diet.

  • Resource Partitioning: Niche differentiation allowing similar species to coexist.

    • Concept of “Ghost of Competition Past.”

Fundamental and Realized Niche

  • Fundamental Niche: The potential niche a species could occupy.

  • Realized Niche: The actual niche occupied in a given environment.

    • Example: Joseph Connell’s barnacle study—removal of Balanus allowed Chthamalus to expand its niche, indicating competition limits the realized niche.

Character Displacement

  • Definition: Traits of species diverge more in overlapping (sympatric) than non-overlapping (allopatric) populations.

    • Example: Galapagos finch beak sizes correlate with food resource differentiation.

  • Temporal Partitioning: Alternative method to reduce competition, illustrated by spiny mice with diurnal and nocturnal behaviors.

Predation

Definition and Adaptations

  • Definition: (+/-) interaction where the predator kills and eats the prey, including herbivory as predation.

  • Predator Adaptations:

    • Acute senses, claws, speed, camouflage, e.g., rattlesnakes using heat sensors.

  • Prey Defenses:

    • Hiding, fleeing, herding, alarm calls, cryptic coloration, poison (e.g., aposematic coloration) and mimicry.

Types of Mimicry

  • Batesian Mimicry: Harmless species mimics harmful species.

    • Example: Hawkmoth larva resembles a snake.

  • Müllerian Mimicry: Two unpalatable species resemble each other.

    • Example: Cuckoo bee and yellow jacket.

Herbivory

  • Definition: (+/-) interaction where an organism consumes plant parts or algae.

    • Examples include various mammals and marine animals.

  • Plant Defenses: Toxins and physical defenses (e.g., spines); examples include strychnine, nicotine, and tannins.

Symbiosis

Types of Symbiotic Relationships

  • Definition: Direct and intimate contact between species; includes:

    • Parasitism (+/-): Parasite benefits at the host's expense.

    • Mutualism (+/+): Both species benefit; types include:

      • Obligate: One species cannot survive without the other.

      • Facultative: Both can survive independently.

    • Commensalism (+/0): One benefits, the other unaffected; examples include algae on turtles and cattle egrets with herbivores.

Facilitation

  • Definition: (+/0) or (+/+); species positively affect others' survival/reproduction without intimate contact.

    • Example: Black rush (Juncus gerardi) in salt marshes preventing salt buildup and oxygen depletion.

Summary

  • Interspecific interactions include competition, predation, herbivory, and various symbiotic relationships.

  • Competition can lead to competitive exclusion, resource partitioning, division of niches, and character displacement.

  • Understanding these interactions is vital for grasping community structure.

  • Final Thought: Consider how climate or land use changes may impact these interspecific interactions.