Explain competition (interspecific, intraspecific)

What IS Competition? (Definition from First Principles)

The mechanism: When individual A uses a unit of resource R, there's less R available for individual B. This creates a negative feedback loop that slows population growth as density increases.

The Two Types: By WHO Competes

==Intraspecific competition== = competition within a species (same species members compete)
==Interspecific competition== = competition between different species

Intraspecific Competition: The Family Fight

Derivation: How Intraspecific Competition Limits Population Growth

Start with the exponential growth model (unlimited resources):

$\frac{dN}{dt} = rN$

where $r$ = per capita growth rate (constant), $N$ = population size.

Problem: This predicts infinite growth. Reality? As $N$ increases, resources per individual decrease, so $r$ must decline.

Step 1: Make $r$ density-dependent. The simplest linear form:

$r_{actual} = r_0 \left(1 - \frac{N}{K}\right)$

where:

$r_0$ = intrinsic growth rate (maximum, when $N \approx 0$ )
$K$ = carrying capacity (maximum $N$ the environment can sustain)
$\frac{N}{K}$ = proportion of carrying capacity used

Why this form? When $N = 0$ , no competition, so $r = r_0$ . When $N = K$ , $r = 0$ (growth stops). When $N > K$ , $r < 0$ (population declines).

Step 2: Substitute into growth equation:

$\frac{dN}{dt} = r_0 N \left(1 - \frac{N}{K}\right)$

This is the ==logistic growth equation==—the mathematical signature of intraspecific competition.

Worked example Worked Example 1: Yeast in a Flask

A lab culture of yeast starts with $N_0 = 10$ cells. The carrying capacity is $K = 1000$ cells, and $r_0 = 0.5 \text{ hr}^{-1}$ .

Calculate: Growth rate when $N = 100$ , = 500 $, and$ N = 900$.

Solution: At $N = 100$ : $\frac{dN}{dt} = 0.5 \times 100 \times \left(1 - \frac{100}{1000}\right) = 50 \times 0.9 = 45 \text{ cells/hr}$

Why this step? When population is small (10% of $K$ ), intraspecific competition is weak—90% of resources remain available.

At $N = 500$ : $\frac{dN}{dt} = 0.5 \times 500 \times \left(1 - \frac{500}{1000}\right) = 250 \times 0.5 = 125 \text{ cells/hr}$ Why faster than at $N=100$ ? Even though competition is stronger (only 50% of resources left), the larger population base ( $N = 500$ instead of 100) more than compensates. This is the peak growth rate.

At $N = 900$ : $\frac{dN}{dt} = 0.5 \times 900 \times \left(1 - \frac{900}{1000}\right) = 450 \times 0.1 = 45 \text{ cells/hr}$

Why this step? Competition is intense (90% of $K$ used). Growth slows dramatically despite large $N$ .

Mechanisms of Intraspecific Competition

==Exploitation competition== (scramble): All individuals have equal access to resources; they just consume them. Everyone gets less. Example: grass plants competing for soil nitrogen—all roots absorb it, none get enough.
==Interference competition== (contest): Direct antagonistic interactions. Dominants monopolize resources; subordinates get little or nothing. Example: territorial birds—winners breed, losers don't. Creates hierarchies.

Worked example Worked Example 2: Tadpole Density Experiment

An ecologist places tadpoles in tanks at densities of 5, 20, 50, 100 per liter. She measures average mass at metamorphosis:

Density (tadpoles/L)	Avg mass (g)	Survival (%)
5	2.4	95
20	1.8	88
50	1.1	70
100	0.6	45

Interpretation:

Mass declines = exploitation competition for algae (food). Each tadpole gets less as density rises.
Survival declines = some tadpoles starve when competition is extreme.
Why this matters: Lower mass → smaller frogs → lower reproductive success. Intraspecific competition regulates the population through reduced fecundity and increased mortality.

Interspecific Competition: The Neighborhood War

Derivation: The Lotka-Volterra Competition Model

Now we have two species competing. Let:

$N_1$ , $N_2$ = population sizes of species 1 and 2
$K_1$ , $K_2$ = carrying capacities (if alone)
$r_1$ , $r_2$ = intrinsic growth rates

Step 1: Start with logistic growth for species 1 (alone):

$\frac{dN_1}{dt} = r_1 N_1 \left(1 - \frac{N_1}{K_1}\right)$

Step 2: Add the effect of species 2. Species 2 uses some of the resources that species 1 needs. We model this as: one individual of species 2 is equivalent to $\alpha$ individuals of species 1 in consuming resources.

$\alpha$ = ==competition coefficient== = per capita effect of species 2 on species 1.

Why this parameterization? It converts species 2's population into "species 1 equivalents" so we can add them to the intraspecific competition term.

Step 3: The "effective population" competing with species 1 is:

$N_1 + \alpha N_2$

Substitute into logistic:

$\frac{dN_1}{dt} = r_1 N_1 \left(1 - \frac{N_1 + \alpha N_2}{K_1}\right)$

By symmetry, for species 2 (where $\beta$ = effect of species 1 on species 2):

$\frac{dN_2}{dt} = r_2 N_2 \left(1 - \frac{N_2 + \beta N_1}{K_2}\right)$

The Four Outcomes: Predicting Who Wins

To find equilibrium, set $\frac{dN_1}{dt} = 0$ and $\frac{dN_2}{dt} = 0$ :

Species 1 isocline (where species 1 stops growing): $N_1 + \alpha N_2 = K_1 \quad \Rightarrow \quad N_1 = K_1 - \alpha N_2$

Species 2 isocline: $N_2 + \beta N_1 = K_2 \quad \Rightarrow \quad N_2 = K_2 - \beta N_1$

The outcome depends on four inequalities (comparing interspecific vs. intraspecific competition):

Species 1 always wins: $K_1 > K_2$ and $\frac{K_1}{\beta} > K_2$ (species 1 can outcompete species 2 at all densities)
Species 2 always wins: $K_2 > K_1$ and $\frac{K_2}{\alpha} > K_1$
Unstable equilibrium: $\frac{K_1}{\alpha} < K_2$ and $\frac{K_2}{\beta} < K_1$ (interspecific competition > intraspecific; first to arrive wins)
Stable coexistence: $\frac{K_1}{\alpha} > K_2$ and $\frac{K_2}{\beta} > K_1$ (intraspecific competition > interspecific; both species self-limit before excluding the other)

Worked example Worked Example 3: Paramecium Competition (Gause's Experiments)

Gause cultured two ciliate species:

P. aurelia: $K_1 = 100$ , $r_1 = 0.8$
P. caudatum: $K_2 = 80$ , $r_2 = 0.7$

Measured competition coefficients: $\alpha = 1.3$ , $\beta = 1.1$

Predict the outcome:

Check condition 1 (species 1 wins):

$K_1 = 100 > 80 = K_2$ ✓
$\frac{K_1}{\beta} = \frac{100}{1.1} = 91 > 80 = K_2$ ✓

Conclusion: P. aurelia should exclude P. caudatum.

Why this step? Species 1 has both higher carrying capacity AND its intraspecific competition ( $\beta = 1.1$ ) is weak enough that species 2 cannot invade when species 1 is at equilibrium.

Experimental result: After 16 days in mixed culture, P. caudatum went extinct. P. aurelia reached $N \approx 100$ . Theory confirmed.

The Competitive Exclusion Principle

Why it works: If niches are identical, $\alpha = \beta = 1$ (or $> 1$ ), so interspecific competition is as strong as (or stronger than) intraspecific. The species with higher $\frac{r}{K}$ ratio (or higher $K$ alone if $r$ is similar) wins.

Exceptions:

Environmental variation: Conditions fluctuate, so the "winner" changes over time—neither excludes the other.
Disturbances: Reset populations before exclusion completes.
Niche partitioning: Species evolve to use different resources (see below).

Ecological Consequences: How Competition Shapes Communities

1. Resource Partitioning (Niche Differentiation)

When competition is strong, natural selection favors individuals that use slightly different resources or the same resource in a different way.

2. Character Displacement

When two species overlap in range, natural selection strengthens differences in traits related to resource use.

Example: Darwin's finches on Galápagos islands. Geospiza fortis and G. fuliginosa have:

Sympatric (both present): Beak sizes diverge (one larger, one smaller) → different seed sizes
Allopatric (only one present): Beak sizes converge (intermediate size optimal)

Mechanism: Hybrids (intermediate beak) compete with both parents → lower fitness → selection favors divergence.

3. Apparent Competition (Indirect)

Two prey species share a predator. As prey1 increases, it supports more predators, which then eat more prey 2. Prey 2 declines not because it competes with prey 1 for resources, but because prey 1 increases predation pressure.

Key difference: No shared resource between prey1 and prey 2, but they still negatively affect each other (mediated by a third species).

Comparing Intraspecific vs. Interspecific Competition

Feature	Intraspecific	Interspecific
Who competes	Same species	Different species
Strength	Usually stronger (identical needs)	Usually weaker (different needs)
Effect on population	Density-dependent regulation	Can cause local extinction (exclusion)
Evolutionary outcome	Stabilizing selection (optimizes traits)	Divergent selection (niche differentiation)
Mathematical signature	Logistic growth curve	Lotka-Volterra system (isoclines)
Coexistence?	Always (can't exclude yourself)	Only if intraspecific > interspecific

Recall Explain to a 12-Year-Old

Imagine a pizza party with 10 slices for 10 kids. Everyone gets one slice—no problem. But what if 20 kids show up? Now there's only half a slice each. That's intraspecific competition: everyone wants the same thing (pizza), and the more kids there are, the less each kid gets.

Now imagine the party has 10 kids who love peperoni pizza and 10 kids who love cheese pizza. If there are 5 pepperoni slices and 5 cheese slices, both groups are fine. But if there are only 5 pepperoni slices and 10 kids want peperoni, those 10 kids fight over them (intraspecific). Meanwhile, the cheese-loving kids have no problem—they're not competing with the pepperoni kids. That's interspecific competition: different groups (species) want overlapping resources.

The twist: If both groups start liking the same pizza, the group that eats faster or grabs slices first will get more. Eventually, one group might give up and switch to eating cookies instead (that's niche partitioning). Or the slower group might starve out (that's competitive exclusion).

Connections

Logistic Growth Model – mathematical foundation of intraspecific competition
Carrying Capacity (K) – ceiling imposed by resource limits and competition
Density-Dependent Factors – competition is the main density-dependent regulator
Ecological Niche – competition drives niche differentiation
Competitive Exclusion Principle – theoretical limit of species coexistence
Character Displacement – evolutionary response to interspecific competition
Predation – alternative biotic interaction (one benefits, one harmed, vs. both harmed in competition)
Mutualism – opposite of competition (+/+ instead of −/−)
Community Structure – competition determines species composition and diversity
Apparent Competition – indirect competition mediated by shared predators

#flashcards/biology

What is competition in ecology? :: An interaction where two or more organisms require the same limited resource, resulting in reduced fitness (survival, growth, or reproduction) for both. It's a −/− interaction.

What is intraspecific competition?

Competition within a single species—members of the same species compete for the same resources.

What is interspecific competition?

Competition between different species for overlapping resources.

Which type of competition is usually stronger, and why?

Intraspecific competition is usually stronger because individuals of the same species have identical niche requirements (same food, habitat, mates).

Write the logistic growth equation and identify the competition term

\frac{dN}{dt} = r_0 N \left(1 - \frac{N}{K}\right)

. The term

\frac{N}{K}

represents intraspecific competition (proportion of carrying capacity used).

What does the competition coefficient

\alpha

represent in the Lotka-Volterra model?

The per capita effect of species 2 on species 1—how many "species 1 equivalents" one individual of species 2 represents consuming resources.

Write the Lotka-Volterra competition equations

\frac{dN_1}{dt} = r_1 N_1 \left(1 - \frac{N_1 + \alpha N_2}{K_1}\right)

and

\frac{dN_2}{dt} = r_2 N_2 \left(1 - \frac{N_2 + \beta N_1}{K_2}\right)

State the competitive exclusion principle

Two species with identical ecological niches cannot coexist indefinitely in the same habitat—the superior competitor will exclude the other.

What is exploitation competition?

(Scramble) competition where all individuals have equal access to resources; resources are consumed passively, and everyone gets less. Example: grass roots competing for nitrogen.

What is interference competition? :: (Contest) competition involving direct antagonistic interactions where dominants monopolize resources and subordinates are excluded. Example: territorial defense.

What condition allows stable coexistence in Lotka-Volterra competition?

When intraspecific competition > interspecific competition for both species:

\frac{K_1}{\alpha} > K_2

and

\frac{K_2}{\beta} > K_1

. Each species limits itself more than it limits the other.

What is resource partitioning?

Evolutionary divergence where competing species use different resources or the same resource in different ways (different times, places, or methods), reducing competition and enabling coexistence.

What is character displacement?

Evolution of greater differences in resource-use traits (like beak size) between two species where they coexist compared to where they live separately (allopatry). It's driven by interspecific competition.

What is apparent competition?

An indirect negative interaction between two prey species that share a predator. Increase in one prey supports more predators, which then increases predation on the other prey—not competition for a shared resource.

At what population size is growth rate maximum in logistic growth?

N = \frac{K}{2}

(half the carrying capacity), growth rate reaches maximum

\frac{dN}{dt} = \frac{r_0 K}{4}

Concept Map

Hinglish (regional understanding)

Intuition Hinglish mein samjho

Competition ka matlab hai do yazyada organisms ko same limited resource chahiye, aur dono ko loss hota hai—ya toh kam food milta hai, ya kam space, ya reproduction slow ho jata hai. Yeh ek negative-negative interaction hai, dono suffer karte hain. Do types hain: intraspecific (apni hi species ke members ke bech—jaise ek jungle mein deer apas mein grass ke liye compete karte hain) aur interspecific (alag-alag species ke bech—jaise deer aur rabbits dono grass khate hain, toh compete karte hain).

Intraspecific competition zyada strong hota hai kyunki same species ke sab members ko exactly same chezein chahiye—same food, same nesting sites, same conditions. Isi wajah se jab population badhti hai, growth slow ho jati hai (logistic growth curve dikhta hai). Ecology mein isko carrying capacity (K) se model karte hain: jab population K ke pas pahunchti hai, toh competition itna strong ho jata hai ki growth almost ruk jati hai.

Interspecific competition decide karta hai ki do species ek sath reh sakti hain ya nahi. Agar dono bilkul same resources chahiye (same niche), toh competitive exclusion hota hai—ek species jetti hai, dosri extinct ho jati hai locally. Lekin agar species evolve karke different resources use karne lagti hain (isko resource partitioning ya character displacement kehte hain), toh coexistence possible hai. Matlab, competition evolution ko drive karta hai—species alag-alag directions mein specialize hoti hain taki competition kam ho.

Ecology aur conservation mein yeh concept bahut important hai kyunki competition population size control karta hai, biodiversity shape karta hai, aur invasive species ka impact predict karne mein help karta hai. Agar koi naya species introduce hoti hai jo native species se resources ke liye compete karti hai aur better competitor hai, toh native species khatam ho sakti hai. Isliye competition samajhna zaroori hai ecosystem health ke liye.

Test yourself — Population & Community Ecology

Connections