Chapter 13: Interspecific Competition

What species was introduced into Gatun Lake (Panama) in 1967?

Peacock bass (Cichla ocellaris)

Where is the peacock bass native to?

The Amazon–Orinoco watershed.

What happened after its introduction in Panama?

It devastated native fish populations through predation.

How many native fish populations were severely reduced or eliminated?

Six

What does the peacock bass case study highlight?

The unintended consequences of species introductions.

What are the major types of interspecific interactions?

Competition, predator–prey, host–parasite, mutualism.

Why do interspecific interactions matter?

They influence birth and death rates → affect population dynamics.

How can interspecific interactions act as agents of natural selection?

They influence which individuals survive and reproduce.

What did Darwin call interspecific competition?

The “struggle for existence.”

What is interspecific competition?

When individuals of different species compete for the same limited resource.

What is intraspecific competition?

Competition among individuals of the same species.

What is exploitation competition?

Indirect competition by using up shared resources first.

What is interference competition?

Direct interaction preventing another species from accessing resources.

What is consumption competition?

One species inhibits another by consuming a shared resource.

What is preemption competition?

First organism prevents others from using the space (sessile organisms).

What is overgrowth competition?

One organism grows over another and blocks resources like light.

What is chemical competition?

Use of toxins/inhibitors (allelopathy in plants).

What is territorial competition?

Behavioral exclusion from a defended area.

What is encounter competition?

Interference from direct encounters (scavengers fighting over food).

The Lotka–Volterra model builds on what basic equation?

Logistic growth equation.

What does “a” represent in the model?

Competition coefficient: effect of species 2 on species 1.

What does “b” represent?

Competition coefficient: effect of species 1 on species 2.

What happens if a = 0 or b = 0?

No interspecific competition — each species grows to its own K.

What are the possible outcomes predicted by Lotka-Volterra?

1. Species 1 wins.

2. Species 2 wins.

3. Unstable coexistence → one eventually wins.

4. Stable coexistence.

What condition allows stable coexistence?

Each species inhibits itself more than it inhibits the other.

What did Gause’s Paramecium experiment show?

P. aurelia outcompeted P. caudatum → competitive exclusion.

What happened when P. caudatum was grown with P. bursaria?

Coexistence due to different feeding zones.

What did Tilman’s diatom study find?

Synedra used silica more efficiently → drove Asterionella extinct when together.

What is the competitive exclusion principle?

Species with identical niches cannot coexist.

What conditions must be met for competitive exclusion to apply?

1. Same resource requirements.

2. Constant environment.

Why is competitive exclusion rare in nature?

Environmental conditions vary.

Are temperature, pH, humidity, and salinity resources?

No — they are environmental conditions, not consumable.

What did Bazzaz’s study show?

Species differ in germination responses to temperature → affects competition.

How does environmental variability influence competition?

Allows coexistence because competitive advantage shifts over time.

What did P. Dye find in African grasses?

Dominant species changed over 10 years due to rainfall variation.

How do density-independent factors affect competition?

Extreme events can reduce populations → reduce competition.

What happened when peacock bass were introduced to Gatun Lake?

They devastated native fish populations through predation, causing elimination or severe reduction of six native species.

What does this peacock bass case study illustrate?

The unintended consequences of species introductions.

What types of interactions occur among species in the same area?

Competition, predator–prey, host–parasite, mutualism.

How do interspecific interactions influence population dynamics?

They affect birth and death rates and serve as agents of natural selection.

What is interspecific competition?

When individuals of different species compete for a shared, limited resource, harming both populations.

How does interspecific differ from intraspecific competition?

Intraspecific occurs within one species; interspecific occurs between different species

Two forms of interspecific competition?

Exploitation (indirect) and interference (direct).

What is consumption competition?

One species inhibits another by consuming a shared resource.

What is preemption competition?

Occupying space that prevents others from establishing (common in sessile species).

What is overgrowth competition?

One organism grows over another, blocking access to resources like light.

What is chemical interaction (allelopathy)?

Releasing toxins or inhibitors to reduce competitor growth.

What is territorial competition?

Behavioral exclusion from a defended area.

What is encounter competition?

Harmful interactions during random encounters (e.g., scavengers fighting over carcasses).

What do Lotka–Volterra equations describe?

How two species competing for the same resource affect each other’s population growth.

What does competition coefficient “a” represent?

Per capita effect of species 2 on species 1.

What does “b” represent?

Per capita effect of species 1 on species 2.

What is one predicted outcome?

One species always wins (competitive exclusion).

What is another predicted outcome?

Stable coexistence when each species limits itself more than its competitor.

What did Gause find in his Paramecium experiment?

P. aurelia outcompeted P. caudatum; P. caudatum went extinct.

When did two Paramecium species coexist?

When they used different parts of the test tube (resource partitioning).

What did Tilman discover about diatoms and silica?

Synedra used silica so efficiently it drove Asterionella to extinction when grown together.

What does the competitive exclusion principle state?

Complete competitors cannot coexist.

Conditions required for the principle?

Identical resource requirements and constant environmental conditions.

How do non-resource factors affect competition?

Factors like temperature, humidity, and pH influence growth but are not consumed.

What did Bazzaz find about temperature and plant seedlings?

Species differ in which temperatures allow maximal germination, influencing competition.

How does temporal variation affect competition?

Changing conditions allow coexistence where one species might otherwise be excluded.

Example of temporal variation affecting dominance?

African grass species dominance shifted over 10 years due to rainfall variation.

What did Groves & Williams find about clover and skeletonweed?

Clover outcompeted skeletonweed both above and below ground.

Why was clover the superior competitor?

Faster growth rate → taller → more light → higher water/nutrient demand.

What did Pickett & Bazzaz find along moisture gradients?

Species' competitive abilities changed across the gradient.

What did Austin find with thistle species and nutrients?

Superior competitor shifts with nutrient availability.

What sets upper and lower distribution limits in salt marsh plants?

Upper: competition for nutrients.
Lower: tolerance to physical stress.

What determines chipmunk species distributions in the Sierra Nevada?

Aggressive exclusion and tolerance to environmental stress.

What is a fundamental niche?

 Full range of conditions/resources a species can use.

What is a realized niche?

Portion of the fundamental niche actually occupied due to competition.

Which cattail species can grow deeper than 80 cm?

Typha angustifolia.

What happens when cattails grow together?

T. angustifolia shifts to deeper water due to competition.

What does niche overlap indicate?

Potential competition; high overlap suggests abundant resources or low competition.

What is competitive release?

Expansion of a species’ niche after competitor removal or colonizing new habitats. 

Example of competitive release?

Increase in krill predators after baleen whale decline.

Why is Stipa restricted to dry ridge crests?

It is competitively excluded from better habitats.

What is resource partitioning?

Coexistence through using different resources or using them differently.

Three annual prairie plants showed partitioning of which resource?

Water, through different root systems.

What did Dayan find in Middle Eastern wild cats?

Canine size differences correspond to different prey preferences.

How did Hutchinson describe a niche?

A multidimensional hypervolume of environmental conditions and resources.

Why can similar species coexist?

Niche differentiation.

Why is competition sometimes weak despite overlap?

Full niche involves many dimensions, reducing total overlap.

What causes resource partitioning?

Physiological, morphological, or behavioral adaptations.

What is the evolutionary origin of these adaptations?

They are considered outcomes of past interspecific competition.

What did P. and R. Grant study?

Medium and small ground finches feeding on overlapping seed sizes.

What happens to beak size distributions where finch species coexist?

They do not overlap; each species has distinct beak sizes.

What happens to finch beak size on islands where species live separately (allopatric)?

They show intermediate and overlapping beak-size distributions.

What is character displacement?

A shift in feeding niche that changes morphology, behavior, or physiology.

How was character displacement proven?

Through the Grants’ long-term finch research — the first direct observation.

Which two species demonstrated character displacement?

Large ground finch (Geospiza magnirostris) and medium ground finch (G. fortis).

What does G. magnirostris usually feed on?

Large, hard seeds (also eaten by G. fortis).

Q: What changed between 1982–2003 for G. magnirostris?

Its population grew and became a strong competitor to G. fortis.

What happened during the low-rain year 2003–2004?

G. magnirostris depleted large seeds → G. fortis forced to rely on smaller seeds.

What selection occurred in G. fortis during 2004–2005?

Strong directional selection against individuals with large beaks.

Why is competition difficult to study in nature?

• Environment can’t be controlled

• Unsure if populations are at/below carrying capacity

• Limited knowledge of life histories

• Subtle species differences are hard to detect

Why are removal experiments difficult to interpret?

Removing one species can cause hidden treatment effects — indirect changes to the environment.