41_Communities Ecology

Species Interactions

• Concept 41.1: Interactions within a community may help, harm, or have no effect on the species involved.

  • Relationships called interspecific interactions can be positive (+), negative (−), or neutral (0).

  • Examples: competition, predation, herbivory, parasitism, mutualism, commensalism.

Competition

• Interspecific competition (−/−): occurs when species compete for limited resources.

  • Can lead to competitive exclusion: local elimination of a competing species.

  • Competitive exclusion principle: two species competing for the same resources cannot coexist permanently.

Ecological Niches and Natural Selection

• Ecological niche: set of biotic and abiotic resources used by an organism; an organism's ecological role.

  • Similar species may coexist if significant niche differences exist.

Character Displacement

• A tendency for characteristics to diverge more in sympatric (geographically overlapping) populations than in allopatric populations.

  • Example: beak size variation in Galápagos finches.

Exploitation

• Exploitation: A +/− interaction where one species benefits by feeding on the other.

  • Includes predation, herbivory, and parasitism.

Predation

• Predation (+/−): one species (predator) kills and eats another (prey).

  • Predators adapt acute senses; preys develop defensive adaptations:

    • Behavioral defenses: hiding, fleeing, forming groups.

    • Morphological/physiological defenses: cryptic coloration (camouflage), aposematic coloration (warning colors).

    • Batesian mimicry: harmless species mimics harmful models to evade predators.

Herbivory

• Herbivory (+/−): interaction where an herbivore eats parts of a plant or alga.

  • Adaptations in herbivores: chemical sensors, specialized teeth.

  • Plant defenses: chemical toxins, protective structures (spines, thorns).

Parasitism

• Parasitism (+/−): a parasite derives nourishment from a host, harming it.

  • Endoparasites: live within the host; ectoparasites: live on the host’s surface.

  • Can alter host behavior to increase parasite fitness.

Positive Interactions

• Positive interactions: +/+ or +/0 interactions where at least one species benefits without harming the other.

  • Mutualism (+/+): both species benefit; can be obligatory or facultative.

  • Commensalism (+/0): one benefits, the other is unaffected.

Diversity and Trophic Structure

• Concept 41.2: Structure of communities characterized by species diversity and trophic relations.

  • Species diversity: variety of organisms, consists of species richness (number of species) and relative abundance (proportion of each species).

Trophic Structure

• Trophic structure: feeding relationships among organisms, links producers to top carnivores; food webs illustrate complex interactions.

Ecosystem Engineers and Controls

• Keystone species: have a large impact on community structure; not necessarily abundant.

  • Example: sea stars as keystone species.

• Bottom-up control model: nutrient availability affects community structure (N → V → H → P).

• Top-down control: predation influences community organization (N ← V ← H ← P).

Disturbances and Succession

• Concept 41.3: Disturbance affects species diversity and community composition.

  • Disturbance: event that alters community structure; can be high or low intensity/frequency.

  • Ecological succession: sequence of changes in community composition after disturbance.

    • Primary succession: begins in lifeless areas; secondary succession: occurs where soil remains after disturbance.