Chapter 13 - Lecture

Chapter 13: Species Interactions and Competition

Concept Overview

• Concept 13.1: Laboratory and field studies reveal intraspecific competition.

• Concept 13.2: The competitive exclusion principle states that two species with identical niches cannot coexist indefinitely. This leads to the prediction that coexisting species will have different niches.

• Concept 13.3: Mathematical and laboratory models establish a theoretical foundation for studying interspecific competition in the natural world.

• Concept 13.4: Competition can significantly influence the ecology and evolutionary pressures on the niches of species.

Introduction to Species Interactions

• Species interact in various ways, resulting in diverse ecological outcomes.

• Possible impacts on interacting organisms:

  • Positive Impact: Benefits to one or both species.

  • Negative Impact: Energy expenditure or harm to one or both.

  • Neutral Impact: No significant effect on either species.

• Symbiosis occurs when one organism spends most or all of its life cycle on or in another organism.

Modes of Competition

• Types of Competition:

  • Interference Competition: Direct interactions between individuals, including:

    • Aggressive territorial defense

    • Production of chemical toxins

    • Resource limitation leading to reduced access.

  • Intraspecific Competition: Competition within a species.

  • Interspecific Competition: Competition between individuals of different species, which often reduces the fitness of both species.

Intraspecific Competition Among Plants

• Research indicates higher plant biomass in low-density populations due to reduced competition for resources.

• Intense competition for nitrogen occurs at higher densities, often resulting in self-thinning and increased mortality.

Evidence from Plant Studies

• Study on S. nutans showed growth variations at different densities:

  • High-density populations led to smaller plant sizes due to competition.

  • Low-density populations had larger sizes due to less competition.

Intraspecific Competition Among Planthoppers

• Studies by Denno and Roderick identified intraspecific competition in Homoptera:

  • Factors contributing to competition include aggregation habits, rapid population growth, and mobility of food sources.

  • Intraspecific competition was evident in Prokelisia marginata populations due to limited resources.

Competitive Exclusion Principle

• Gause’s competitive exclusion principle: two species with the same niche cannot coexist long-term.

  • One species is often a superior competitor, leading to the exclusion of the other.

• Niche differentiation helps in understanding ecological interactions.

Feeding Niches of Darwin’s Finches

• Selection for beak size among finches based on seed type:

  • Finch species demonstrate specialization in feeding niches correlating with beak size.

  • Drought events impact seed availability and influence finch mortality rates based on beak size.

Mathematical and Laboratory Models

• Mathematical models simplify natural systems to provide insight:

  • Particularly useful in understanding interspecific competition.

• The Lotka-Volterra model predicts coexistence when interspecific competition is less intense than intraspecific competition.

• Isoclines of Zero Population Growth: Predicts population growth halts at specific resource limits, indicating potential for coexistence.

Experiments with Paramecia and Flour Beetles

• Gause’s resource limitation experiments with P. caudatum and P. aurelia showed competitive dynamics under varying resource availability.

• Tribolium beetles in ecological studies indicated the restriction of realized niches through interspecific competition.

Competition and Niche Dynamics

• Strong competitive interactions may lead to evolutionary adaptations in species, changing their fundamental niche.

• Examples include studies on the bedstraw Galium spp. and niche overlap in barnacles, showing how competition shapes species distributions and interactions.

Character Displacement

• Character Displacement: A process where species evolve differences to reduce competition, often resulting in morphological adaptations.

• In Darwin’s Finches, differences in beak size are greater in areas where species overlap, leading to distinct feeding niches.

• Criteria for assessing character displacement include metrics of resource use, genetic basis of differences, and observed competition's correlation with similarity in traits.

Review Topics

• Intraspecific Competition: Studies on plants and planthoppers.

• Competitive Exclusion and Niches: Insights from Darwin’s finches and mathematical models.

• Competition Effects: Research involving Tribolium and the ecological implications for rodents and barnacles.

• Character Displacement: Evidence from Darwin’s finches and its significance in understanding evolutionary adaptations.