AP Biology: Community Ecology – Species Interactions (Competition, Predation, Symbiosis), Niche

Community Ecology – Species Interactions (Competition, Predation, Symbiosis), Niche

Concept Summary

• Community Ecology: Study of interactions among species in a shared environment, shaping biodiversity and ecosystem function.
• Niche: Full range of biotic/abiotic conditions a species uses; fundamental niche (theoretical) vs. realized niche (actual due to competition).
• Competition (-/-): Interaction where species vie for limited resources, leading to competitive exclusion or resource partitioning.
• Predation (+/-): One species (predator) consumes another (prey), driving adaptations like cryptic coloration or aposematic warning signals.
• Symbiosis: Close, long-term interactions; mutualism (+/+), commensalism (+/0), parasitism (+/-).

Core Questions

WHAT (definitional)

Q: What is a niche? A: The role and position of a species in its environment, including all interactions and resource requirements. Trap/Clarification: A niche is not just habitat—it includes behavior, diet, and tolerances (e.g., temperature).

Q: What is competitive exclusion? A: When two species with identical niches cannot coexist, leading to local extinction of one. Trap/Clarification: Exclusion is not instantaneous; it may take generations and depends on resource overlap.

WHY (causal/explanatory)

Q: Why is resource partitioning important? A: It reduces competition by allowing species to use different subsets of a resource (e.g., feeding at different times). Trap/Clarification: Partitioning doesn’t eliminate competition—it just minimizes it.

Q: Why do predators and prey coevolve? A: Reciprocal selective pressures (e.g., faster prey-more agile predators) drive adaptations like mimicry or toxins. Trap/Clarification: Coevolution isn’t always balanced; human-induced changes (e.g., overhunting) can disrupt it.

HOW (process/application)

Q: How do you determine a species’ realized niche? A: Compare its fundamental niche (lab conditions) to its actual use of resources in the wild (field observations). Trap/Clarification: Realized niches can expand if competitors are removed (e.g., invasive species).

Q: How is the strength of competition measured? A: Via Lotka-Volterra equations: dN?/dt = r?N?(K?-N?-?N?)/K?, where ? = competition coefficient. Trap/Clarification: ? isn’t constant—it varies with environmental conditions (e.g., drought).

CAN (conditions/possibilities)

Q: Can two species with identical niches coexist? A: No, unless resource partitioning or environmental variation (e.g., spatial/temporal) reduces overlap. Trap/Clarification: "Identical" is rare—even slight differences (e.g., microhabitat) can allow coexistence.

Q: Can mutualism be obligate? A: Yes (e.g., fig wasps and fig trees), where one or both species cannot survive without the other. Trap/Clarification: Obligate mutualism isn’t always symmetrical—one partner may depend more than the other.

Quick Facts & Traps

• Fact: Keystone species (e.g., sea otters) disproportionately affect community structure despite low abundance.
• Trap: "All predators are carnivores"-Reality: Some (e.g., Venus flytraps) are plants; others (e.g., herbivores) can be "predators" of plants.
• Fact: Batesian mimicry = harmless species mimics a toxic one; Müllerian mimicry = multiple toxic species share warning signals.
• Trap: "Symbiosis = mutualism"-Reality: Symbiosis includes all close interactions (e.g., parasitism, commensalism).
• Fact: Character displacement = divergence in traits (e.g., beak size) when species coexist vs. alone.
• Trap: "Competition only occurs between different species"-Reality: Intraspecific competition (same species) is often more intense.

Rapid-Fire True/False

• Statement: Two species can occupy the same realized niche indefinitely. Answer: FALSE Why the common mistake happens: Confusing realized (actual) with fundamental (theoretical) niche—competition forces differentiation.

• Statement: Parasitism always kills the host. Answer: FALSE Why the common mistake happens: Overgeneralizing from extreme cases (e.g., parasitoids); most parasites reduce host fitness without killing.

• Statement: Aposematic coloration is a form of mimicry. Answer: FALSE Why the common mistake happens: Aposematism is warning coloration (e.g., poison dart frogs); mimicry copies it (e.g., Batesian mimics).