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

Species Studied: Chthamalus in upper intertidal areas; Balanus in wetter areas.
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: $\beta$ and $\beta$ .

Lotka-Volterra Competition Model - Coefficients

Competition Coefficients:
- $\beta$ = competitive effect of species 2 on species 1 (e.g., “the effect of deer on elk”).
- $\beta$ = 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 $\beta = 0.5$ (converts deer into “elk equivalents”).
- Conversely, $200$ deer = $100$ elk implies $\beta = 2$ (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 ( $\beta < 1$ , $\beta < 1$ ).

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: $K = 90$ , $\beta = 1.5$ , $N = 100$ .
- Species 2: $K = 40$ , $\beta = 0.8$ , $N = 50$ .

Notes for Next Class

Focus on Population Dynamics.