Competition (11)

Competition

Definition of Competition

  • Competition: An interaction with negative effects between two individuals depending on the same limiting resource.

    • Key aspects:

    • Competing individuals experience a detriment due to sharing resources.

Types of Competition

  • Intraspecific Competition: Competition that occurs within species; this is related to density-dependent population growth.

  • Interspecific Competition: Competition that occurs among individuals of different species; this will be the focal point of today’s discussion.

Importance of Studying Competition

  • Understanding competition is crucial as it:

    • Organizes ecological processes that control the composition of natural communities.

    • Has broad applications in the management of agriculture, forests, wildlife, and natural resources.

Overview of Topics

  • Measuring competition, including niches and resources.

  • Modeling competition and its outcomes.

  • Types of competition.

Historical Context of Competition Studies

Tansley's 1917 Experiment

  • Tansley (1917) conducted the first test of competition in plants by measuring competition among closely related plants.

  • Observations and Hypotheses:

    • Observation: Closely related species tend to grow in different habitats.

    • Hypothesis: Each species is adapted to particular soil conditions and thus will be competitively superior in its native habitat.

    • Predictions:

    1. When grown alone, each species will exhibit optimal growth in its native soil type.

    2. When grown together, the outcome of competition will depend on the soil type.

Experiment Details

  • Plant Species Studied:

    • Galium saxatile (Heath Bedstraw): Acidic soil specialist, grows best in acidic conditions.

    • G. sylvestre (White Bedstraw): Alkaline specialist, grows best in alkaline conditions.

  • Results:

    • In acidic soil, G. saxatile outcompetes G. sylvestre.

    • In alkaline soil, G. sylvestre outcompetes G. saxatile.

Laboratory Competition Experiments

Chihuahuan Desert, Arizona

  • Brown and Munger (1985): Conducted exclosure experiments to measure animal competition focusing on granivores (seed eaters).

    • Focused on:

    • Insectivores: Kangaroo rat (large) and pocket mouse (small).

    • Grasshopper mouse (small).

  • Method: Compared plots from which large granivores were excluded (termed "exclosures") to control plots.

Evidence of Competition in Animals Using Exclosures

  • Results from Exclosure Plots:

    • Large granivores were successfully excluded.

    • Populations of small granivores (pocket mice) increased.

    • Non-competing insectivores (grasshopper mice) remained unaffected by the exclusion of large granivores.

Conceptual Understanding of Competition

Competition & Niche Relationships

  • The degree of niche overlap affects the degree of competition encountered.

  • Complete Competitors: Two species that share the same niche and are limited by the same one resource.

    • Competitive Exclusion Principle: Complete competitors cannot coexist indefinitely.

Limiting Resources

  • Not all resources impose limits on consumer populations.

  • Liebig’s Law of the Minimum: Population increases until the supply of the most limiting resource prevents further growth.

    • Illustrations for various limitations could include:

    • Nesting space

    • Prey availability

    • Hiding spots

Examples of Experimental Evidence

Gause’s Experiment (1934)

  • Studied two paramecium species grown with bacteria as a food source.

  • Findings showed that when grown together, only one species survived, illustrating the competitive exclusion principle.

    • The persisting species was the one that could drive resource abundance the lowest without going extinct, indicating survival with fewer resources.

Joseph Connell’s Barnacle Experiment

  1. Species Studied: Chthamalus in upper intertidal areas; Balanus in wetter areas.

  2. Competition Analysis:

    • Investigating the resources they compete for and identifying the better competitor.

    • Measuring competition through space occupation in their respective environments.

Coexistence of Competing Species

  • Competing species can coexist if:

    • They are not complete competitors.

    • They are limited by different (>1) resources.

  • Learnings:

    • Two species may coexist, or one may outcompete and exclude the other.

    • The greater the ecological difference between two species, the less competition they experience and thus the higher the likelihood of coexistence.

Outcomes of Competition Modeling

Overview of Modeling Approaches

  • Goals: Predicting the outcomes of competition using models such as the Lotka-Volterra model.

  • Two Species Model:

    1. Represented by two equations.

    2. Account for the abundance of the second species: either $N1$ or $N2$.

    3. Incorporate the impact that each individual of the second species has on the first species, via competition coefficients: $eta$ and $eta$.

Lotka-Volterra Competition Model - Coefficients

  • Competition Coefficients:

    • $eta$ = competitive effect of species 2 on species 1 (e.g., “the effect of deer on elk”).

    • $eta$ = competitive effect of species 1 on species 2 (e.g., “the effect of elk on deer”).

  • Example:

    • Food for 100 elk (species 1) or 200 deer (species 2):

    • $100$ elk = $200$ deer implies $eta = 0.5$ (converts deer into “elk equivalents”).

    • Conversely, $200$ deer = $100$ elk implies $eta = 2$ (converts elk into “deer equivalents”).

Predictions Using the Lotka-Volterra Equations

Equilibrium Conditions

  • One Species: Observed growth of a population = 0.

  • Two Species: Both must have observed growth = 0 for equilibrium.

  • Aim for this two-species equilibrium in population dynamics.

  • The equations allow for connection of points on a graph for combinations of $N1$ and $N2$ when species 1 is at equilibrium.

Isoclines Representation

Species 1 Equilibrium

  • 0-Growth Isocline: Represents all combinations of $N1$ and $N2$ resulting in species 1 stability.

  • Direction of growth:

    • As $N_1$ approaches this line along the x-axis, growth vectors indicate how populations will either increase or decrease.

Species 2 Equilibrium

  • Trajectory for Growth Vectors:

    • $N_2$ moves towards this line along the y-axis as growth vectors dictate increase or decrease in population.

Phase-Plane Diagram Analysis

Overview of Predictions

  • Combine species 1 and species 2 isoclines into one graph.

  • Identify initial population sizes as starting points (x,y coordinates).

  • Determine four possible outcomes based on isocline positioning:

    • Outcome 1: Species 1 isocline is farther out than species 2, leading to species 2 extinction.

    • Outcome 2: Species 2 isocline is farther out, causing extinction of species 1.

    • Outcome 3: Unstable equilibrium where one species goes extinct while the other reaches its carrying capacity K.

    • Outcome 4: Stable equilibrium with coexistence; arises when interspecific competition is weaker than intraspecific competition ($eta < 1$, $eta < 1$).

Summary of Competitive Outcomes and Equilibrium Points

  • Outline of expected results for different competitive scenarios:

    • Species 2 goes extinct at $N1 = K1$.

    • Species 1 goes extinct at $N2 = K2$.

  • Identify stable points of coexistence with final population sizes marked on the diagram.

Competition Mechanisms

Types of Competition Mechanisms

  • Exploitation Competition: Where individuals most efficiently consume the resource, driving down resource abundance to levels that prevent persistence of others.

    • Example: Paramecia species.

  • Interference Competition: When one species blocks another via behavioral or chemical means from utilizing the resource.

    • Example: Barnacle competition where one can physically dominate a space.

Example of Interference Competition

  • Allelopathic Zones: Surrounding sage shrubs in coastal California exhibit interference competition through allelo-chemicals that kill neighboring plants.

Apparent Competition

  • Defines scenarios where effects mimic competition but do not occur because of shared resources. Instead, effects are mediated through a shared predator or parasite.

    • For instance, one prey species negatively impacts another due to increased predator presence, leading to an indirect negative impact.

Practice Problem

  • Take-home Practice Problem: Based on the Lotka-Volterra competition model:

    • Draw a phase-plane diagram to predict competition outcomes.

    • Indicate vectors, plot initial population sizes, track the two populations' movements, identify equilibrium points, and interpret these outcomes for both populations.

    • Example parameters for the practice problem:

    • Species 1: $K = 90$, $eta = 1.5$, $N = 100$.

    • Species 2: $K = 40$, $eta = 0.8$, $N = 50$.

Notes for Next Class

  • Focus on Population Dynamics.