Biotic Interactions Part I

Biotic interactions can be examined in

• Ecosystem Ecology (trophic relationships)

• Community Ecology (Determine/shape communities (species richness, diversity, etc)).

• Population Ecology (Effects on population growth)

Community

Different species living together in a specific habitat and interacting directly or indirectly.

• All organisms and their interactions (competition, predation, parasitism)

Theoretical Ideas behind the concept of community

• Frederick Clements - Organismal community concept

• Henry Gleason - Individualistic community concept.

Biotic interactions can shape communities (0,+,-) 

(++) Mutualism

(-+) Predation/parasitism 

 (--) Competition 

(0+) Commensalism 

(0-) Amensalism 

(0+) Neutralism

Competition 

Organisms from the same species or different species compete for resources when their niches overlap

Niche does what

Individuals from the same species or from different species compete b/c their niches overlap, affecting the composition, structure and functioning of populations/communities. 

Niche (Charles Elton 1990-1991)(OR Joseph Hrinnel) 

Role an organism plays in community 

• Due to this definition, niche and habitat are interchangeable

Niche (The Hutchinsonian Niche)

Each “n-dimension’ represents each one of the abiotic and biotic resources + conditions/tolerances that an individual or species requires to survive and reproduce.

overlap is there competition is formed

Hutchinsonian Niche (called fundamental niche)

Of an individual or species. Multidimensional space symbolizing the complete range of environmental conditions and the individual species can tolerate. 

Realized Niche

Interaction (eg; competition) restricts space in which individual species may live. More restricted “n-dimension” space. 

Competition (important)

Effect on population.

• Density dependent (may be related) 

Competition among Individuals of

Same species: Intraspecific Competition

Different Species: Interspecific Competition

Two of same means 

2 Primary means of competition

1) Scramble - (Indirect) species exploit a common resource: each species diminishes the resource available for others

2) Interference Competition - (direct) Depletion of one another’s resources via aggressive behaviour, fighting. 

Effects of competition on population growth: 

• The Competitive Exclusion Principle (“The Principle”) 

• The Lotka-Volterra model of competition (“The Model”) 

Georgii Gause

Conducted some experiments with Paramecium spp. and identified conditions that led to competitive exclusion principle of one species by one another.

Principle of Competitive Exclusion (Gause’s Law)

Two species compete for the same requirements cannot have stable co-existence. 

*One species will be slightly more efficient than the others and will reproduce, reach higher levels of abundance, causing the extinction of other species.

Requires certain conditions (Competition in Lab)

Low abundance levels for both species, low K for both species. 

Alfred Lotka and Vitto Volterra (1920s)

mathematicians interested in biological problems who did their studies independently, but arrived at these conclusions 

Equation (affected by presence of another species)

dN/dt = rN[(K-N)/K]

α and β

α and β are competition coefficients or “per capita effect of a competing species on the growth of other species

• Values tell us which interspecific and inter-specific competition has the most significant effect on species 1 and 2. 

Values of α and β

α>1: one individual from another species have more

β<1: Inhibitory effect on one individual from its own species. Interspecific competition has a greater effect on population growth. 

α<1: One individual from another species has less

β>1: Inhibitory effect than one individual from its own species. Interspecific competition has greater effect on population growth. 

Lotka-Volterra Model Predictions 

• Species 1 excludes species 2 (outcompetes species 2). Competitive exclusion: K1 is much higher than K2

• Species 2 excludes species 1 (outcompetes species 1). Competitive exclusion: K2 is much higher than K1. 

• Unstable coexistence between the two species: Both species could reach high K. Dependence on the initial abundance of each species, ability to quickly adapt to condition changes. 

Stable coexistence between two species

• Both have low K

• No species really monopolizes resources: they share or partition resources equitably.