Competition and Interspecific Interactions

Introduction to Competition

Competition is a fundamental ecological interaction that occurs when organisms vie for the same limited resources. It can take place within a population (intraspecific) or between populations of different species (interspecific). Understanding competition is essential for studying population dynamics and community structure in ecology.

• Competition: An interaction where both participants are negatively affected due to the shared use of a limiting resource.

• Occurs at multiple ecological scales: within populations and among species in communities.

• Examples: Competition for space in intertidal zones, light in rainforests, or food among nestlings.

Types of Ecological Interactions

Ecological interactions are classified based on the effects on the organisms involved:

• Competition: Both organisms are negatively affected (-/-).

• Exploitation: One benefits, one is harmed (+/-); includes predation and parasitism.

• Mutualism: Both benefit (+/+).

• Amensalism: One is harmed, the other is unaffected (-/0).

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

• No interaction: Neither is affected (0/0).

Direct and Indirect Interactions

Interactions among organisms can be direct or indirect:

• Direct interactions: Occur when two species interact without intermediaries (e.g., physical competition for food).

• Indirect interactions: Mediated by a third species (e.g., one species affects another through its impact on a shared predator or resource).

Mechanisms of Competition

Types of Competition

• Intraspecific competition: Occurs among individuals of the same species (e.g., nestlings competing for food).

• Interspecific competition: Occurs between individuals of different species (e.g., trees of different species competing for light).

Three Main Mechanisms of Competition

• Interference competition: Direct interactions, often involving aggression or territoriality (e.g., animals fighting over mates or food).

• Exploitation (scramble) competition: Indirect competition where individuals consume resources, reducing their availability for others (e.g., plants absorbing soil nutrients).

• Apparent competition: Indirect interaction where one species increases the abundance of a shared predator, negatively affecting another species (e.g., increased rabbits lead to more foxes, which also prey on pheasants).

Asymmetrical Competition

Competition is often asymmetrical, meaning one species or individual is a superior competitor and suffers less from competition, while the other is more negatively affected.

Testing and Evidence for Competition

Experimental Approaches

• Removal experiments: Remove one species and observe changes in the abundance of the other (e.g., removing ants increases rodent abundance and vice versa).

Classic Studies

• Gause's Paramecium Experiments: When cultured separately, Paramecium aurelia and P. caudatum each reached their carrying capacity. When cultured together, P. aurelia outcompeted P. caudatum, demonstrating the competitive exclusion principle.

Competitive Exclusion Principle

The competitive exclusion principle states that two species with identical niches cannot coexist indefinitely; the superior competitor will exclude the other.

• Example: Warblers with similar niches in trees; only one persists in a given niche.

Resource Partitioning

Resource partitioning allows similar species to coexist by utilizing different resources or the same resource in different ways.

• Example: Two species shift their prey size preferences over time to reduce overlap.

Niches and Population Distributions

Fundamental vs. Realized Niche

• Fundamental niche: The full range of environmental conditions a species can potentially occupy.

• Realized niche: The actual conditions occupied, limited by competition and other interactions.

• Example: Chthamalus barnacles have a larger fundamental niche but are restricted to upper intertidal zones due to competition with Semibalanus.

Mathematical Models of Competition

Logistic Growth Equation

The logistic growth equation models population growth with intraspecific competition:

• Equation:

• r: Intrinsic rate of increase

• N: Population size

• K: Carrying capacity

Lotka-Volterra Competition Equations

The Lotka-Volterra equations extend the logistic model to include interspecific competition using competition coefficients:

• α (alpha): Effect of Species 2 on Species 1

• β (beta): Effect of Species 1 on Species 2

For two competing species:

• Species 1:

• Species 2:

Zero-Growth Isoclines

Zero-growth isoclines are lines on a graph where the population growth rate of a species is zero. They help predict the outcome of competition.

• For Species 1:

• For Species 2:

• Isoclines are plotted with N1 (Species 1) on the x-axis and N2 (Species 2) on the y-axis.

Interpreting Isoclines

• Below the isocline: Population increases.

• Above the isocline: Population decreases.

• The intersection of isoclines determines whether one species excludes the other or if coexistence is possible.

Possible Outcomes of Competition

• Species 1 excludes Species 2 (Species 1's isocline is above Species 2's).

• Species 2 excludes Species 1 (Species 2's isocline is above Species 1's).

• Unstable equilibrium: Outcome depends on initial population sizes.

• Stable coexistence: Isoclines cross and both species persist below their carrying capacities.

Factors Facilitating Coexistence

• Abundant, non-limiting resources

• Environmental variability

• Interactions with other species

• Immigration from other areas

• Character displacement: Evolutionary divergence in traits to reduce competition (e.g., Darwin's finches)

• Recruitment: Arrival of new individuals can maintain populations even if one is a superior competitor

Summary Table: Types of Ecological Interactions

Key Equations

• Logistic Growth:

• Lotka-Volterra for Species 1:

• Lotka-Volterra for Species 2:

• Zero-growth isocline for Species 1:

• Zero-growth isocline for Species 2:

Example Applications

• Removal experiments in Arizona: Removing ants increases rodent abundance and vice versa, demonstrating competition for food.

• Gause's Paramecium: Demonstrates competitive exclusion when two species with similar niches are grown together.

• Barnacle distribution: Chthamalus and Semibalanus occupy different zones due to competition and environmental tolerance.

Additional info: Character displacement is an evolutionary process where differences among similar species are accentuated in regions where the species co-occur, but are minimized or lost where the species' distributions do not overlap. This reduces competition and facilitates coexistence.