Ecological Relationships

Why This Matters

Ecological relationships are the backbone of Unit 8 in AP Biology, and you're being tested on your ability to explain how and why organisms interact—not just that they do. The exam loves to connect these relationships to bigger concepts: energy flow through trophic levels, the cycling of matter, population dynamics, and ecosystem stability. When you see an FRQ about declining biodiversity or a food web disruption, you need to trace the cause back to specific ecological interactions.

Think of this topic as the "so what?" of ecology. Every relationship—whether it's a predator hunting prey, two species competing for the same resource, or a mutualistic partnership—has consequences for energy transfer, population regulation, and community structure. Don't just memorize that wolves eat elk; know that this is an example of top-down control and trophic cascade. The College Board wants you to connect mechanisms to outcomes, so as you study each relationship type, ask yourself: what principle does this illustrate?

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Species Interactions: The Forces That Shape Communities

Species don't exist in isolation—their populations rise and fall based on who they eat, who eats them, and who they compete with. These interactions can be characterized by positive (+), negative (−), or neutral (0) effects on each species involved.

Predator-Prey Relationships

• Top-down control of populations—predators regulate prey numbers, which cascades through the food web to affect lower trophic levels
• Lotka-Volterra dynamics describe the cyclical oscillations where prey populations rise, predators follow, then prey crash as predation intensifies
• Coevolution drives adaptations in both species—prey develop defenses (camouflage, toxins, speed) while predators evolve countermeasures

Competition (Interspecific and Intraspecific)

• Competitive exclusion principle—two species competing for identical resources cannot coexist indefinitely; one will outcompete the other
• Interspecific competition occurs between different species, while intraspecific competition happens within the same species and is often more intense
• Character displacement can result when competition drives species to evolve different traits, reducing niche overlap (Darwin's finches with different beak sizes)

Symbiosis: Mutualism, Commensalism, and Parasitism

• Mutualism (+/+) benefits both partners—classic examples include mycorrhizal fungi and plant roots or pollinators and flowering plants
• Commensalism (+/0) benefits one species with no effect on the other—think epiphytic orchids growing on trees or cattle egrets following grazing mammals
• Parasitism (+/−) benefits the parasite at the host's expense, often regulating host populations without killing them outright

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Niche Theory: How Species Coexist

The concept of the niche explains why dozens of species can share the same habitat without driving each other to extinction. Resource partitioning and niche differentiation reduce competition and allow coexistence.

Niche Concept

• Fundamental niche represents all conditions where a species could survive; realized niche is the actual range it occupies after accounting for competition and predation
• Niche partitioning allows similar species to coexist by using different resources, foraging at different times, or occupying different microhabitats
• Competitive exclusion occurs when niches overlap completely—the superior competitor drives the other to local extinction

Habitat and Ecosystem

• Habitat refers to the physical location where an organism lives, providing abiotic factors like temperature, moisture, and soil type
• Ecosystem encompasses both the biotic community and abiotic environment, functioning as an integrated system of energy flow and nutrient cycling
• Ecosystem engineers like beavers physically modify habitats, creating conditions that benefit other species and increase biodiversity

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Energy Flow: The One-Way Street

Unlike matter, energy doesn't cycle—it flows through ecosystems in one direction, from producers to consumers, with massive losses at each step. Only about 10% of energy transfers between trophic levels; the rest is lost as heat through cellular respiration.

Energy Flow in Ecosystems

• Primary producers (autotrophs) capture energy through photosynthesis, converting light energy into chemical energy stored in gross primary productivity (GPP)
• Net primary productivity (NPP) equals GPP minus energy used for the producer's own respiration—this is what's actually available to consumers
• Ecological efficiency (the 10% rule) explains why ecosystems support fewer top predators than herbivores—energy pyramids are always bottom-heavy

Trophic Levels

• Trophic levels organize organisms by feeding position: producers → primary consumers (herbivores) → secondary consumers → tertiary consumers
• Energy pyramids always narrow at the top because approximately 90% of energy is lost at each transfer through metabolic heat
• Biomass pyramids can occasionally invert (ocean ecosystems with fast-reproducing phytoplankton), but energy pyramids never do

Food Chains and Food Webs

• Food chains show linear energy transfer but oversimplify real ecosystems where organisms feed at multiple trophic levels
• Food webs capture the complexity of interconnected feeding relationships and help predict how disturbances propagate through communities
• Trophic cascades occur when changes at one level ripple through the web—removing wolves increases elk, which overgrazes vegetation, which affects songbirds

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Nutrient Cycling: Matter Recycles, Energy Doesn't

While energy flows one way and exits as heat, matter cycles continuously between organisms and the environment. Biogeochemical cycles demonstrate conservation of matter and are essential for sustaining life.

Nutrient Cycling

• Carbon cycle moves carbon through photosynthesis (atmosphere → organisms), respiration (organisms → atmosphere), and decomposition
• Nitrogen cycle requires bacterial transformations: nitrogen fixation ($N_2 \rightarrow NH_3$), nitrification ($NH_3 \rightarrow NO_3^-$), and denitrification ($NO_3^- \rightarrow N_2$)
• Decomposers (bacteria and fungi) break down dead organic matter, returning nutrients to soil where producers can absorb them

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Population and Community Dynamics

Populations don't stay constant—they grow, shrink, and fluctuate based on births, deaths, and interactions with other species. Understanding these dynamics helps predict ecosystem responses to disturbance.

Population Dynamics

• Birth rates, death rates, immigration, and emigration determine whether populations grow, decline, or remain stable
• Density-dependent factors like competition, predation, and disease intensify as populations grow; density-independent factors like weather affect all population sizes equally
• Carrying capacity (K) represents the maximum population an environment can sustain—populations oscillate around this value

Community Interactions

• Community structure emerges from the sum of all species interactions—predation, competition, and symbiosis collectively determine which species thrive
• Species diversity includes both richness (number of species) and evenness (relative abundance of each species)
• Top-down control occurs when predators regulate community structure; bottom-up control occurs when producer availability limits consumer populations

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Ecosystem Change Over Time

Ecosystems aren't static—they change through succession, respond to disturbances, and can be destabilized by species introductions or removals. Succession leads to predictable changes in community composition and increased biodiversity.

Succession (Primary and Secondary)

• Primary succession begins on bare substrate with no soil (lava flows, retreating glaciers)—pioneer species like lichens slowly build soil over centuries
• Secondary succession occurs after disturbance destroys a community but leaves soil intact (forest fires, abandoned farmland)—recovery is much faster
• Climax community represents the stable endpoint of succession, though modern ecology recognizes that disturbances maintain many ecosystems in earlier stages

Keystone Species

• Keystone species have outsized impacts relative to their abundance—their removal triggers extinction cascades affecting many other species
• Keystone predators like gray wolves control herbivore populations, preventing overgrazing and maintaining plant diversity
• Ecosystem engineers like beavers create habitats for other species; keystone mutualists like fig trees support frugivore communities during resource scarcity

Biodiversity and Species Richness

• Biodiversity encompasses species diversity, genetic diversity within populations, and ecosystem diversity across landscapes
• High biodiversity increases ecosystem resilience—functional redundancy means multiple species can perform similar ecological roles
• Simpson's Diversity Index ($1 - \Sigma(n/N)^2$) quantifies diversity by accounting for both richness and evenness

Invasive Species and Their Impacts

• Invasive species lack natural predators or competitors in new environments, allowing populations to explode and outcompete natives
• Competitive exclusion and predation by invasives can drive native species to extinction, simplifying food webs
• Facilitation can worsen invasions when one invasive species creates conditions favoring others, leading to ecological collapse

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Quick Reference Table

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Self-Check Questions

1. Both competition and predation can regulate population sizes. How do their effects on prey/competitor populations differ in terms of mechanism and outcome?

2. A keystone predator is removed from an ecosystem. Using the concept of trophic cascade, predict three specific changes that might occur at different trophic levels.

3. Two bird species feed on the same insect prey in the same forest. According to the competitive exclusion principle, what are the only two possible long-term outcomes, and what ecological process might prevent extinction?

4. Compare and contrast how energy and matter move through ecosystems. Why can energy pyramids never be inverted while biomass pyramids occasionally can?

5. An FRQ describes a disturbed forest ecosystem recovering after a fire. Explain why this is secondary succession rather than primary succession, and predict how species diversity will change over the next 50 years.