Biological Communities and Species Interactions

Biological Communities

Biological communities are composed of populations of different species living and interacting in a particular area. The structure and dynamics of these communities are shaped by the various interactions among species.

• Community: An assemblage of populations of different species living close enough for potential interaction.

• Community structure: Determined by species composition and the types of interactions among species.

Species Interactions

Types of Interactions

Species interactions are biological processes that determine community composition. These interactions can be positive, negative, or neutral, and influence the fitness of the parties involved.

	Species 2 (+)	Species 2 (0)	Species 2 (-)
Species 1 (+)	Mutualism	Commensalism	Predation/Parasitism
Species 1 (0)	Commensalism	Neutralism	Amensalism
Species 1 (-)	Predation/Parasitism	Amensalism	Competition

Additional info: Table summarizes the effects (+, -, 0) of interactions on each species.

Major Types of Species Interactions

• Mutualism (+/+): Both species benefit from the interaction. Example: Bees pollinating flowers.

• Commensalism (+/0): One species benefits, the other is unaffected. Example: Barnacles on whales.

• Amensalism (-/0): One species is harmed, the other is unaffected. Example: Algae shading out smaller plants.

• Predation/Parasitism (+/-): One species benefits at the expense of the other. Example: Wolves preying on deer, tapeworms in mammals.

• Competition (-/-): Both species are harmed by the interaction due to shared resource use.

Consumption Interactions

Types of Consumption

• Herbivory: Animals consume plant material.

• Predation: One organism kills and eats another.

• Parasitism: Parasite derives nourishment from host, usually without killing it immediately.

Plant Defenses Against Herbivory

• Chemical defenses: Production of toxic or deterrent compounds (e.g., nicotine, caffeine, THC, alkaloids).

• Mechanical defenses: Physical structures such as thorns and trichomes.

• Arms race: Ongoing evolutionary adaptations between plants and herbivores.

Constitutive vs Inducible Defenses

• Constitutive defenses: Always present (e.g., bark, thorns, toxins, spines).

• Inducible defenses: Produced only in response to herbivore attack (e.g., chemicals, sticky substances, increased mucilage).

Coevolution

• Reciprocal evolutionary adaptations between interacting species (e.g., passionflower and butterfly).

• Can lead to specialized relationships and increased biodiversity.

Adaptations to Predation

• Camouflage coloration: Allows organisms to blend into their environment.

• Secretion of toxins: Chemical defenses to deter predators.

• Aposematic coloration: Bright warning colors to signal toxicity or unpalatability.

Mimicry

• Batesian mimicry: A harmless species mimics a harmful one.

• Müllerian mimicry: Two or more harmful species resemble each other.

Parasitism

• Parasites consume small amounts of host resources, often without killing the host immediately.

• Endoparasites: Live inside the host (e.g., tapeworms).

• Ectoparasites: Live on the outside of the host (e.g., ticks, lice).

• Parasites can manipulate host behavior to enhance their own transmission.

Competition

• Occurs when individuals use the same resources, especially when resources are limited.

• Competitive Exclusion Principle: Two species competing for the same limiting resource cannot coexist indefinitely.

• Niche overlap: Leads to competition.

Fundamental vs Realized Niche

• Fundamental niche: The full range of resources a species could use in the absence of competition.

• Realized niche: The range actually occupied due to competition and other biotic factors.

Resource Partitioning

• Species evolve to use different resources or habitats to minimize competition.

• Example: Anole lizards occupy different parts of trees to reduce competition.

Character Displacement

• Natural selection favors divergence in traits where species overlap, reducing competition.

• Example: Galápagos finches have different beak sizes when living together versus apart.

Keystone Species

• Species with a disproportionately large effect on community structure relative to their abundance.

• Removal can cause dramatic shifts in community composition and trophic structure.

Food Chains and Trophic Webs

• Food chain: Linear sequence of who eats whom in a community.

• Trophic web (food web): Complex network of feeding relationships, showing that many organisms occupy multiple trophic levels.

Trophic Webs and Community Stability

• Communities with more trophic connections are more stable and resilient to disturbances.

• Loss of keystone species can destabilize communities.

Community Response to Disturbance

• Communities with high resistance do not change much after disturbance.

• Communities with high resilience recover quickly after disturbance.

• Low resistance and resilience lead to major changes and slow recovery.

Species Diversity

• Species richness: Number of species in a community.

• Species evenness: Relative abundance of each species.

• Species diversity: Combines richness and evenness; higher diversity often means greater stability and resilience.

Ecological Succession

• Orderly pattern of species replacements in a community over time.

• Primary succession: Occurs in newly exposed habitats (e.g., after a volcanic eruption).

• Secondary succession: Occurs after a disturbance in an existing community (e.g., after a fire).