Interspecific competition

Species interactions

Early ecological models (e.g. exponential and logistic growth) treat populations in isolation. However, in real ecosystems, species interact constantly, and these interactions directly affect population growth rates, survival, and evolution.

Key interaction types include:

• Interspecific competition

• Predation

• Mutualism

• Commensalism

• Amensalism

• Neutralism

Mutualism (+/+)

Both species benefit.

• Classic case: clownfish and sea anemones

  • Clownfish gain protection from predators

  • Anemones benefit from cleaning and nutrient input

✔ Case study significance: Demonstrates tight co-evolution and niche interdependence.

case study: (Loccoz, 2014)

Pollination Mutualism (Plants & Pollinators)

• System: Flowering plants and pollinating insects (e.g. bees)

• Mechanism:

  • Pollinators gain nectar (energy)

  • Plants gain pollination → reproduction

• Demonstrates co-evolution and ecosystem dependence

• Can link to biodiversity and ecosystem resilience

Commensalism (+/0)

One benefits, the other unaffected.

• Example: polar bears scavenging whale carcasses without affecting other scavengers significantly

✔ Often subtle and difficult to prove experimentally

Case study: (Hellmann, 2013)

Cattle Egrets and Grazing Mammals

• Cattle egret feeds on insects disturbed by cattle

• Cattle unaffected

Remora Fish and Sharks: (Mougi, 2016)

• Remoras attach to sharks for transport and food scraps

• Shark largely unaffected

• Shows phoresy (transport-based commensalism)

Neutralism (0/0)

No measurable effect between species.

• Example: camels and tarantulas in desert ecosystems

✔ Rare in practice because most species interactions are indirect

Camels and Tarantulas

• No measurable interaction or overlap

• True neutralism is rare and difficult to prove experimentally

• Many ecologists argue neutralism is theoretical because indirect effects are common

Competition (-/-)

Both species are negatively affected.

Gause’s Paramecium Experiment (1934)

• Gause (1934)

• Paramecium aurelia vs Paramecium caudatum

• P. aurelia outcompetes P. caudatum

• Demonstrates Competitive exclusion principle

Red vs Grey Squirrels (UK)

• Grey squirrel outcompetes Red squirrel

• Via disease (parapox virus)

• Tompkins et al. (2003)

• Apparent competition

Amensalism (0/-)

One harmed, the other unaffected.

• Example: Penicillium fungi producing antibiotics that inhibit bacteria

• Example: Black walnut trees releasing juglone toxin suppressing nearby plant growth

✔ Important in plant ecology and microbial competition

Case study: (Mougi, 2016)

Penicillium and Bacteria

• Penicillium produces penicillin

• Kills surrounding bacteria

Case study:

Black Walnut Tree (Allelopathy)

• Black walnut releases juglone toxin

• Inhibits growth of nearby plants

• Chemical competition vs true competition distinction

Case study: (Mougi, 2016)

Large Animals Crushing Vegetation

• Large herbivores unintentionally destroy smaller organisms

Contramensalism (+/-)

One benefits while harming another indirectly.

• Example: otters displacing other species through habitat modification

Otters Altering Habitat

• Otters modify aquatic environments

• Benefit themselves but negatively affect other species

• Shows indirect negative effects without direct competition

Interspecific Competition Definition

Competition between species for a shared limiting resource, reducing growth rates of both

Interference Competition (Direct)

• Involves aggression, territoriality, or intimidation

Case Study 1: Arctic Fox vs Red Fox

• Red fox expanding north due to climate change

• Outcompetes Arctic fox through size and dominance

• (Hersteinsson & Macdonald, 1992)

✔ Key point: Climate change shifts competitive balance.

Case Study 2: Cheetahs vs Lions & Hyenas

• 73% of cheetah cubs killed by lions/hyenas (Laurenson, 1994)

• Not for food → reduces future competition

✔ Interpretation: Competition can involve pre-emptive suppression of competitors

Exploitation Competition (Indirect)

• Species compete by consuming shared resources

Case Study 1: Red Deer vs Takahe (New Zealand)

• Introduced deer degrade grass habitats

• Endangered Takahe declines

• (Lee & Jamieson, 2001)

Case Study 2: Goats vs Galápagos

• Introduced goats destroy vegetation

• Impacts Galápagos tortoise

• (Project Isabela, 2004)

✔ Conservation link: Removing invasive species restores ecosystem balance

Apparent Competition (Indirect via Third Party)

Two species share a common predator or pathogen, leading to indirect competition.

Case Study: Red vs Grey Squirrels (UK)

• Grey squirrel carries parapox virus

• Red squirrel is highly susceptible

• Greys outcompete reds due to shared pathogen

• Explains speed of red squirrel’s demise

• (Tompkins et al., 2003)

✔ Explains rapid decline of red squirrels
✔ Not resource competition → disease-mediated exclusion

The Niche Concept

Hutchinson (1958)

Defined niche as an n-dimensional hypervolume of environmental conditions

Connell (1961) Barnacle Experiment

Species:

• Semibalanus balanoides

• Chthamalus stellatus

Findings:

• Semibalanus excludes Chthamalus from lower shore

• Chthamalus survives only in upper zone

✔ Conclusion: Competition restricts realised niche

Fundamental vs Realised Niche

Fundamental niche

Full potential range without competitors

Realised niche

Restricted range due to competition

Competitive Exclusion Principle

Gause (1934)

Two species with identical niches cannot coexist indefinitely.

Case Study: Paramecium Experiments

Species:

• Paramecium aurelia

• Paramecium caudatum

Results:

• Grown separately → both thrive

• Grown together → P. aurelia outcompetes P. caudatum

✔ Demonstrates competitive exclusion in controlled conditions

Real-World Case: Mallards vs Black Ducks

• Mallard expanding range

• Native American black duck declines

✔ Shows exclusion in natural ecosystems.

Environmental Influence on Competition

Case Study: Tribolium Beetles (Park, 1954)

Species:

• Tribolium castaneum

• Tribolium confusum

Results:

• 34°C / 70% RH → T. castaneum wins

• 24°C / 30% RH → T. confusum wins

✔ Key insight: Competitive outcomes depend on environmental conditions

Coexistence & Niche Differentiation

Species can coexist if they:

• Partition resources

• Shift niches

Case Study: Darwin’s Finches (Character Displacement)

• Galápagos finches

• Different beak sizes reduce competition

• Greater divergence when species coexist (Santa Cruz)

✔ Conclusion: Competition drives evolutionary change

Mathematical Models of Competition

Logistic Growth Model:

Population growth limited by carrying capacity:

dN/dt = rN((K−N)/K)

Lotka–Volterra Competition Model:

For two species:

dN₁/dt = r₁N₁((K₁ − N₁ − α_1,2N₂) / K₁)

dN₂/dt = r₂N₂((K₂ − N₂ − α_2,1N₁) / K₂)

Where:

• α = competition coefficient

• Measures impact of one species on another

Outcomes of the Model

Condition	Outcome
Intraspecific > interspecific competition	Stable coexistence
Interspecific > intraspecific competition	Competitive exclusion
Mixed conditions	Unstable equilibrium