12: Competition

Competition:

What are the two types of competition?

Intraspecific = within the same species

Interspecific = between different species, both reduce fitness

What is intraspecific competition?

Individuals of the same species compete for the same limited resources (e.g., food, space), leading to reduced growth, survival, or reproduction

What is interspecific competition?

Different species compete for shared resources, negatively affecting both populations

Example of interspecific competition (lions & hyenas)?

Both compete for the same prey → shared resource use limits growth of both populations

Mechanisms:

What is interference competition?

Direct competition through physical interactions (territorial defense, aggression, toxins) that prevent access to resources

What is exploitative competition?

Indirect competition where individuals consume shared resources, reducing availability for others

Example of exploitative competition?

Trees block sunlight, limiting growth of understory plants

Can interference and exploitative competition occur in both types?

Yes, both intra- and interspecific competition can involve either mechanism

Models:

What do Lotka–Volterra competition models do?

They mathematically describe how two species interact and predict outcomes like coexistence or exclusion

What do N₁ and N₂ represent?

Population sizes of species 1 and species 2

What do r₁ and r₂ represent?

Intrinsic growth rates of each species (how fast they grow without limits)

What do s₁₁ and s₂₂ represent?

Intraspecific competition (self-limitation within each species)

What do s₁₂ and s₂₁ represent?

Interspecific effects (impact of one species on the growth of the other)

What do negative s values indicate?

They reduce population growth; more negative = stronger competitive effect

Interspecific Competition:

What are the four possible outcomes?

Species 1 wins, species 2 wins, coexistence, or competitive exclusion (unstable, depends on starting sizes)

What is competitive exclusion?

One species outcompetes the other, leading to its local extinction

When is exclusion unstable?

When the outcome depends on initial population sizes rather than fixed conditions

Intraspecific:

What happens as population density increases?

Competition intensifies, reducing survival, growth rate, and body size

Grasshopper example (Denno & Roderick)?

High density leads to lower survival, slower development, and smaller adult size

Plant example (Tilman)?

Higher density → reduced biomass and smaller plants due to competition for resources

How does this relate to logistic growth?

As populations approach carrying capacity, self-limitation (intraspecific competition) increases

What is self-thinning?

Over time, crowded populations lose individuals as competition eliminates weaker ones

What is the self-thinning rule?

High initial density results in fewer but larger individuals later

Environment & Parameters:

How does nitrogen availability affect competition?

More nutrients increase growth rate (r), allowing faster population growth

How does density affect s₁₁?

Higher density increases self-limitation (more negative s₁₁)

What causes negative s values?

Limited resources create competition that reduces growth

How does environment affect competition models?

Changes both growth rates (r) and competition strength (s values)

Lab Studies:

What did Gause (1934) demonstrate?

Competitive exclusion using Paramecium species in controlled lab conditions

What happened when Paramecium species were grown separately?

Only intraspecific competition occurred, and populations grew normally

What happened when grown together?

One species (P. aurelia) outcompeted the other → competitive exclusion

When is interspecific competition stronger than intraspecific?

When s₁₂ and s₂₁ are larger than s₁₁ and s₂₂ → exclusion occurs

When can species coexist in experiments?

When they use different resources or occupy different niches

Why are lab studies important?

They allow control of variables and clear testing of competition outcomes

Environment:

How can environment change competition outcomes?

By altering survival, reproduction, and competitive ability of species

Example (Tribolium beetles)?

Warm/humid conditions favor one species

Cold/dry conditions favor another

What biological factors change with environment?

Reproduction rate, survival, development speed, and behaviors like cannibalism

How does this relate to L-V models?

Environmental changes modify r and s values, shifting competition outcomes

Field Studies:

What did Connell (1961) show with barnacles?

Competition plays a key role in limiting species distributions in nature

What happened when Balanus was removed?

Chthalamus expanded into the mid intertidal zone

Why is Chthalamus normally absent there?

It is excluded by interspecific competition from Balanus

What determines realized niches in nature?

A combination of competition, predation, and environmental stress

What limits the fundamental niche?

Abiotic stress (e.g., desiccation, temperature)

Why are field studies important?

They show how multiple factors interact in real ecosystems

Character Displacement:

What is it?

Evolutionary change in traits due to competition, making species more different

Character Displacement: When is it most evident?

In sympatric populations (species living together)

Character Displacement: Why does it occur?

To reduce niche overlap and decrease competition

Finch example?

G. fortis evolves larger beaks; G. fuliginosa smaller beaks when together

What happens in sympatry vs allopatry?

Sympatry → greater trait differences; allopatry → more similar traits

What happens during drought in finches?

Large, hard seeds dominate → birds with larger beaks survive better

How does competition drive evolution?

Strong competition favors traits that reduce overlap in resource use

How does this relate to L-V models?

Strong competition (large negative s₁₂, s₂₁) selects for reduced overlap, lowering competition over time