Ecological Relationships

Why This Matters

Ecological relationships are the backbone of Unit 8 in AP Biology, and you're tested on your ability to explain how and why organisms interact, not just that they do. The exam connects 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.

Every relationship has consequences for energy transfer, population regulation, and community structure. Don't just memorize that wolves eat elk; know that this illustrates top-down control and trophic cascades. 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

Predators exert top-down control on prey populations, and those effects cascade through the food web to lower trophic levels. When wolves hunt elk, for instance, reduced elk browsing allows vegetation to recover, which in turn supports more songbirds and insects.

• Lotka-Volterra dynamics describe the cyclical pattern: prey populations rise first, predator populations follow with a time lag, then prey crash as predation intensifies, and predators decline after that. These oscillations repeat.
• Coevolution drives adaptations in both species. Prey develop defenses like camouflage, toxins, or speed, while predators evolve countermeasures such as sharper senses or resistance to toxins.

Competition (Interspecific and Intraspecific)

• Competitive exclusion principle: two species competing for identical resources in the same niche cannot coexist indefinitely. One will always outcompete the other.
• Interspecific competition occurs between different species, while intraspecific competition happens within the same species. Intraspecific competition is often more intense because individuals have identical resource needs.
• Character displacement can result when competition drives species to evolve different traits over time, reducing niche overlap. Darwin's finches are the classic example: species living on the same island evolved different beak sizes to exploit different food sources, while populations on separate islands showed no such divergence.

Symbiosis: Mutualism, Commensalism, and Parasitism

• Mutualism (+/+) benefits both partners. Mycorrhizal fungi extend a plant's root network in exchange for sugars; pollinators get nectar while transferring pollen between flowers.
• Commensalism (+/0) benefits one species with no measurable effect on the other. Epiphytic orchids grow on tree branches to access sunlight without harming the tree. Cattle egrets follow grazing mammals and eat insects stirred up by their movement.
• Parasitism (+/−) benefits the parasite at the host's expense. Parasites typically don't kill their hosts outright, since a living host provides a longer-term resource. This means parasites often regulate host populations through gradual weakening rather than immediate death.

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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 and resources where a species could survive if no other species were present. Realized niche is the narrower range it actually occupies after accounting for competition, predation, and other biotic interactions.
• Niche partitioning allows similar species to coexist by dividing up resources. MacArthur's warblers are a great example: five warbler species coexist in the same spruce trees by feeding at different heights and on different parts of branches.
• Competitive exclusion occurs when niches overlap completely and neither species can partition resources. The inferior competitor gets driven to local extinction.

Habitat and Ecosystem

• Habitat refers to the physical location where an organism lives, defined by 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. A beaver dam creates ponds and wetlands that support fish, amphibians, waterfowl, and aquatic plants that wouldn't otherwise be present.

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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. The total energy they fix is called gross primary productivity (GPP).
• Net primary productivity (NPP) equals GPP minus energy the producer uses for its own respiration: $NPP = GPP - R$. NPP is what's actually available to consumers.
• Ecological efficiency (the 10% rule) explains why ecosystems support far fewer top predators than herbivores. If producers fix 10,000 kcal, only about 1,000 kcal reaches primary consumers, 100 kcal reaches secondary consumers, and just 10 kcal reaches tertiary consumers.

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 as metabolic heat. This is why energy pyramids can never be inverted.
• Biomass pyramids can occasionally invert. In open ocean ecosystems, phytoplankton reproduce so rapidly that their standing biomass at any given moment is less than the zooplankton feeding on them, even though phytoplankton productivity is higher over time.

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 populations, which overgraze riparian vegetation, which destabilizes stream banks and reduces habitat for songbirds and beavers.

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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

The carbon cycle moves carbon through several processes:

• Photosynthesis pulls $CO_2$ from the atmosphere into organic molecules.
• Cellular respiration and decomposition release $CO_2$ back into the atmosphere.
• Combustion of fossil fuels returns geologically stored carbon to the atmosphere on a much faster timescale than it was sequestered.

The nitrogen cycle requires bacterial transformations at each step:

1. Nitrogen fixation: bacteria (often in root nodules of legumes) convert atmospheric $N_2$ into ammonia ($NH_3$).
2. Nitrification: other bacteria convert $NH_3$ into nitrates ($NO_3^-$), the form most plants can absorb.
3. Denitrification: anaerobic bacteria convert $NO_3^-$ back into $N_2$, returning it to the atmosphere.

Decomposers (bacteria and fungi) break down dead organic matter, returning nutrients to the soil where producers can absorb them. Without decomposers, nutrients would remain locked in dead tissue and ecosystems would grind to a halt.

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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 and crowd together. Density-independent factors like severe weather, natural disasters, and seasonal changes affect populations regardless of size.
• Carrying capacity (K) represents the maximum population an environment can sustain given available resources. Populations tend to oscillate around K rather than settling exactly at it.

Community Interactions

• Community structure emerges from the sum of all species interactions. Predation, competition, and symbiosis collectively determine which species thrive and in what abundances.
• Species diversity has two components: richness (the number of different species present) and evenness (how equally individuals are distributed among those species). A forest with 10 tree species in roughly equal numbers is more diverse than one with 10 species where 95% of trees belong to a single species.
• Top-down control occurs when predators regulate community structure from above. Bottom-up control occurs when resource or producer availability limits what consumer populations can be supported.

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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 generally increases biodiversity.

Succession (Primary and Secondary)

• Primary succession begins on bare substrate with no soil, such as cooled lava flows or land exposed by retreating glaciers. Pioneer species like lichens and mosses colonize first, gradually breaking down rock into soil over centuries.
• Secondary succession occurs after a disturbance destroys a community but leaves soil intact, such as after a forest fire or on abandoned farmland. Because soil and seed banks are already present, recovery is much faster (decades rather than centuries).
• Climax community is the traditional term for the stable endpoint of succession. Modern ecology recognizes that many ecosystems are maintained in earlier successional stages by periodic disturbances like fire, flooding, or storms.

Keystone Species

A keystone species has an impact on community structure that is disproportionately large relative to its abundance. Removing a keystone species triggers cascading effects across the ecosystem.

• Keystone predators like sea stars (Pisaster) or gray wolves control herbivore populations, preventing any single herbivore from dominating and thus maintaining diversity.
• Ecosystem engineers like beavers physically create new habitats (ponds, wetlands) that support entire communities of other organisms.
• Keystone mutualists like fig trees fruit during periods when little other food is available, sustaining frugivore communities through resource scarcity.

Biodiversity and Species Richness

• Biodiversity encompasses three levels: species diversity (variety of species), genetic diversity (variation within populations), and ecosystem diversity (range of habitat types across a landscape).
• High biodiversity increases ecosystem resilience because functional redundancy means if one species declines, others performing similar roles can compensate.
• Simpson's Diversity Index ($D = 1 - \Sigma(n/N)^2$) quantifies diversity by accounting for both richness and evenness. Values closer to 1 indicate higher diversity. Here, $n$ = number of individuals of each species, and $N$ = total number of individuals.

Invasive Species and Their Impacts

• Invasive species lack natural predators or competitors in their new environment, allowing their populations to grow unchecked and outcompete native species.
• Through competitive exclusion and predation, invasives can drive native species to extinction, simplifying food webs and reducing biodiversity. Brown tree snakes introduced to Guam, for example, eliminated most native forest birds.
• Facilitation can worsen invasions when one invasive species creates conditions favoring others, compounding the damage to native communities.

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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.