Lecture 21 Interspecific Competition and mutualism

Competition

Two species vying for the same limited resource (food, water, territory), resulting in negative effects on both.

Competitive Exclusion Principle

Two species cannot coexist indefinitely on the same limiting resource because one will outcompete the other.

Exploitation Competition

Two species negatively affect each other indirectly by both consuming a shared limiting resource.

Interference Competition

Species directly harm each other through behaviors like aggression or chemical inhibition, reducing access to resources.

Apparent Competition

Two species appear to compete but are actually both suppressed by a shared predator or parasite.

Intraguild Predation

One competitor directly preys upon another competitor within the same guild.

Why does resource overlap lead to competitive exclusion?

When two species have highly similar niches, one species is more efficient at exploiting the shared resource and drives the other to local extinction.

How does resource partitioning allow competing species to coexist?

Species divide up available resources by using different portions of the habitat, food sizes, or times of activity, reducing direct competition.

Character Displacement

When two competing species evolve morphological differences (e.g., beak size) in areas where they co-occur, reducing niche overlap.

What did MacArthur's warbler study demonstrate?

Five warbler species coexist in Maine by foraging in distinct zones of spruce trees, illustrating resource partitioning.

Keystone Species

A species with a disproportionately large effect on community structure relative to its abundance or biomass.

How did Pisaster (sea star) act as a keystone predator in Paine's experiment?

By preferentially eating mussels, Pisaster prevented mussels from monopolizing the rock surface, maintaining diversity of 28 intertidal species.

What happened when Pisaster was removed in Paine's experiment?

Mussels took over 100% of rock cover, reducing the community to just 1 intertidal species.

How did kangaroo rat removal change desert habitat?

Without kangaroo rats consuming large seeds, grasses outcompeted small-seeded plants, converting desert shrubland to grassland.

Mutualism

A species interaction where both partners receive a net fitness benefit (+/+).

Symbiosis

A close, long-term physical association between two species; not necessarily mutualistic.

How do fig wasps and figs form a mutualism?

Female wasps pollinate fig flowers while laying eggs inside the syconium; some ovules become seeds for the fig and others become wasp offspring.

How do ants and acacias form a defensive mutualism?

Ants live in acacia thorns and feed on Beltian bodies; in return they aggressively defend the acacia against herbivores and competing plants.

Why do leaf-cutter ants cultivate fungi rather than eat leaves directly?

Leaves are nitrogen-deficient; the cultivated fungi fix nitrogen via associated bacteria, converting leaves into a nitrogen-rich food source.

Partner-Fidelity Feedback

A mechanism maintaining mutualism where individual A directs benefits to individual B, and B later returns benefits to A, creating a positive feedback loop.

Vertical Transmission

Symbionts passed directly from parent to offspring, typically through eggs or seeds (usually mother to offspring).

Horizontal Transmission

Symbionts acquired from the external environment or unrelated individuals rather than from parents.

Why does vertical transmission favor mutualism over parasitism?

Vertically transmitted symbionts share the host's reproductive fate, so increasing host fitness also increases symbiont fitness, selecting for cooperation.

Cytoplasmic Incompatibility

A reproductive manipulation by some symbionts (e.g., Wolbachia) where infected males sterilize uninfected females, spreading the symbiont.

How is mutualism considered an evolutionary innovation?

Long-term mutualisms enabled key transitions such as the eukaryotic cell (mitochondria), photosynthesis in eukaryotes (chloroplasts), and plant colonization of land (mycorrhizae).

Parasitism-Mutualism Continuum

The relationship between two species can shift along a spectrum from harmful to neutral to beneficial depending on environmental conditions.

Lotka-Volterra Competition Coefficient (α)

A value describing how much one individual of species 2 suppresses species 1's population growth relative to an individual of species 1.

Niche Expansion

When a competitor is removed, a species broadens its resource use to occupy a wider portion of its fundamental niche.

Commensalism

A species interaction where one species benefits and the other is unaffected (+/0).

Amensalism

A species interaction where one species is harmed and the other is unaffected (−/0).