Week 3: Interspecific Competition

Exploitation

Scramble competition (symmetric)

Interference

Contest competition (asymmetric)

• eg black walnut (Juglans nigra)→ secretes toxic chemicals into the soil so sensitive trees growing near roots often killed→ creates a kind of exclusion zone

Interspecific (heterospecific) competition

Competition with an individual from another species

Intraspecific (conspecific) competition

with members of the same species

Gause’s Principle

• Competitive exclusion principle

• Two species cannot coexist on the same limiting resource

R* (R-star) rule→ Tilman

• Update on Gause’s principle

• Tilman

• species that outcompetes the other is the one that drives the resource level to the lowest in isolation

  • Population that is largest in isolation (monoculture) doesn’t necessarily ‘win’

Coexistence

The ability of all species to recover from low density/abundance in the presence of the other (undisturbed) species

For a species to invade a landscape where another species is there (at equilibrium)

The competitive effect of species 1 on species 2 needs to be less than the competitive effect of species 1 on itself

Requirement for Coexistence

INTRASPECIFIC (CONSPECIFIC) competition needs to be more intense than INTERSPECIFIC (HETEROSPECIFIC) competition

→ Niche Differentiation

Fundamental Niche

The full range of environmental conditions (biological and physical) under which an organism can exist describes its fundamental niche

Realised niche

restriction by competitors→ range which that species will be in the presence of another

If conspecific competition is greater than heterospecific competition

• Implies species must somehow cut up resources, so that each is the best at exploiting some resource

• Niche differentiation

Example of adaptive radiation and niche differentiation within a group

• Darwin’s finches

• beak shape and size → different specialisation

Consequences for spatial distribution

1. that the presence or absence of a species could be determined by competition with other species

2. that conditions of the environment (in this case, soil type) affected the outcome of competition

3. that the present ecological segregation of species might have resulted from competition in the past

Mechanisms to generate coexistence

• spatial niche partitioning

• temporal niche partitioning

What is a Darwinian demon?

• Hypothetical organism that can maximize all aspects of fitness simultaneously and would exist if the evolution of species was entirely unconstrained → no such thing

• Such organisms would reproduce directly after being born, produce the maximum number of offspring, and live indefinitely.

• Trade-offs: can’t be good at everything.

• Each species needs to be slightly better at one thing than any other species

• Means for n-niche axes it is very hard to get more than n species

Trade Offs

• Growth-survival trade-off

  • fast growing species have low survival rates in poor environmental conditions

  • e.g fast growing trees don’t grow tall → short but shade tolerant

• Reproductive trade offs

  • big seeds= more nutrients = higher survival

  • more seeds = better colonization, more attempts by parent

  • Large animals = more parental care

Intermediate Disturbance Hypothesis

• Species diversity maximised when ecological disturbance is neither too rare nor too frequent

• Important for maintaining species richness over time

Disturbance in Forests/Shrublands/Grasslands

• isturbance important

• Tree-fall = light gaps

• Hurricanes/storms etc = light gaps

• Pioneer species (fast growing) exploit these gaps

• Succeeded by late successional shade-tolerant individuals

• Next disturbance event…….

• Too frequent/infrequent

  • Loss of diversity

No disturbance → loss of pioneers

too much → climax community never reached

What is the Janzen-Connell hypothesis?

• (Specialist) Natural enemies important for diversity

• Advantage of rarity

• Disadvantage of being common→ more likely to be hit by your specialist natural enemy e.g disease/ fungi

• Requires overcompensating density dependent effect→ population crashes after high population densities

Apparent/ Indirect Competition

• Most competition assumed for a shared resource

• Indirect negative effects

Example: Bats and Humans

• no shared resources/ direct competition

• Bats→ zoonotic diseases

• Many bats→ bad for human population