Intraspecific competition

definition

Intraspecific competition refers to interactions between individuals of the same species competing for a shared, limiting resource, resulting in reduced survivorship, growth, and/or reproduction of at least some individuals.

• It is a key driver of density dependence, meaning population processes change as population density increases.

• It regulates populations around a carrying capacity (K).

• Central to population ecology, influencing:

  • Population size

  • Stability

  • Evolutionary strategies

A foundational principle comes from Thomas Malthus, whose work on population limits underpins modern density-dependent theory.

Scramble Competition (Exploitative)

Individuals compete indirectly by consuming a shared resource; all individuals are affected roughly equally.

Key Features:

• Equal resource division (in theory)

• No direct interaction

• Can lead to population overshoot and crash

• Associated with high b values (b > 1)

Scramble competition Case Study 1: Reindeer on St. Paul Island

• Introduced population: 4 males, 22 females

• By 1938: population exploded to ~2000

• Overshot carrying capacity (K)

• Result: catastrophic crash due to food depletion

Interpretation:

• Classic scramble competition collapse

• All individuals over-exploited lichen resources

• Demonstrates density-dependent starvation

Scramble competition Case Study 2: Soay Sheep on St Kilda

• Sheep feed on shared grass resources

• When population exceeds K:

  • Winter mortality increases sharply

• Mortality pattern:

  • Highest in lambs

  • Then adult males

  • Then adult females

Key Insight:

• Mortality increases exponentially with density

• Environmental stress (winter) amplifies competition

Reference:
Studies by Tim Coulson on Soay sheep dynamics

Contest Competition (Interference)

Individuals compete directly, often aggressively, for resources.

Key Features:

• Unequal resource distribution

• “Winners” secure enough resources

• “Losers” may get none (binary outcome)

• Associated with b ≈ 1

Contest Competition Case Study: Red Grouse in Scotland

• Territorial birds compete for space

• Most aggressive individuals:

  • Secure territories

  • Achieve higher reproductive success

• Non-territorial individuals:

  • Fail to reproduce

Experimental evidence:

• Removal of territory holders → immediate replacement

Key Insight:

• Population regulated by territory availability, not total individuals

Modelling Intraspecific Competition

Basic Population Growth Model

N_t+1 = N_t×R

• R: finite rate of increase

• Assumes no competition

Density-Dependent Model

N_t+1 = N_tR/1+aN_t

• a = R−1/K

• Incorporates density dependence

Interpretation:

• As NtNt​ increases → denominator increases → growth slows

Modelling Intraspecific Competition Case Study: Serengeti National Park Wildebeest

• Late 1800s: population < 50,000 due to rinderpest

• 1950s: vaccination of cattle reduces disease

• 1980s: population stabilises ~1.35 million

Growth rate:

• R ≈ 1.5

Interpretation:

• Early phase: near-exponential growth

• Later: density dependence stabilises population

The Competition Parameter (b)

Introduced by John Maynard Smith and George Slatkin (1973):

N_t+1 = N_tR / 1+(aN_t)^b

Biological Meaning of b

• Measures strength of density dependence

• Controls how sharply mortality increases with density

Values of b and Population Dynamics

b Value	Type	Dynamics	Example
b = 0	No competition	Exponential growth	Yeast
b = 1	Contest	Stable equilibrium	Wildebeest
b > 1	Scramble-like	Overshoot & crash	Reindeer
b >> 1	Strong scramble	Oscillations	Sheep
b >>> 1	Extreme	Limit cycles	Voles
b >>>> 1	Very extreme	Chaos	Theoretical

The Competition Parameter (b) Key Discovery: Robert May (1970s)

• Increasing b leads to instability

• Simple deterministic models can produce:

  • Oscillations

  • Chaos

Implication:

• Complex population dynamics can arise from simple rules

The Competition Parameter (b) Case Study: Voles in Scandinavia

• Exhibit 3–4 year population cycles

• Driven by:

  • Density dependence

  • Resource depletion

Interpretation:

• Example of stable limit cycles

• High b → oscillatory dynamics

The Competition Parameter (b) Case Study: Sheep in Tasmania

• Overshoot carrying capacity

• Followed by damped oscillations

Use in essay:
Illustrates overcompensation dynamics (b > 1).

Role of Parameter a

• a = (R−1)/K

• Represents susceptibility to density

Key relationships:

• Higher a → lower K

• Higher a → earlier slowdown in growth

Environmental Change and CompetitionCase Study: Coral Reefs (Rachel Horwitz et al. 2017)

Location: Red Sea

Experiment:

• Coral assemblages under:

  • Current pH

  • Future ocean acidification (OA)

Findings:

• Reduced growth under OA

• Stronger effects under intraspecific competition

• Shifts in competitive hierarchy between species

Example:

• Galaxea corals showed:

  • Control > OA (significant difference, p < 0.001)

Interpretation:

• Environmental stress intensifies competition

• Alters ecosystem structure