Key Ecological Concepts

Why This Matters

Ecology is the backbone of AP Environmental Science—nearly every unit connects back to how energy moves, how populations grow, and how species interact. You're being tested on your ability to explain why ecosystems function the way they do, not just what happens. The concepts here—trophic dynamics, carrying capacity, succession, and species interactions—show up repeatedly in multiple-choice questions and form the foundation for FRQ responses about environmental disruptions, conservation strategies, and human impacts.

Don't just memorize definitions. For each concept below, know what principle it demonstrates and how it connects to real-world environmental issues. When you understand that the 10% rule explains why ecosystems can't support many top predators, or that limiting factors determine carrying capacity, you'll be able to tackle any question the exam throws at you. Energy flow, population regulation, community dynamics, biogeochemical cycling—these are your conceptual anchors.

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Energy Flow and Trophic Dynamics

Energy moves through ecosystems in a predictable, one-way path. Unlike nutrients, which cycle, energy is lost as heat at each transfer—this fundamental thermodynamic constraint shapes everything from food web structure to ecosystem productivity.

Energy Flow in Ecosystems

• Sunlight drives primary production—producers (autotrophs) capture solar energy through photosynthesis, converting it to chemical energy stored in biomass
• Linear energy transfer means energy flows in one direction through trophic levels, unlike nutrients which cycle repeatedly through the system
• Gross primary productivity (GPP) represents total energy captured, while net primary productivity (NPP) is what remains after producers use energy for respiration

The 10% Rule and Trophic Transfer

• Only ~10% of energy transfers between trophic levels—the rest is lost as metabolic heat, following the second law of thermodynamics
• Energy pyramids always narrow at the top because insufficient energy remains to support large populations of apex predators
• Ecological efficiency explains why food chains rarely exceed 4-5 levels and why ecosystems have far more herbivores than carnivores

Trophic Levels and Food Webs

• Trophic levels categorize organisms by feeding position: producers → primary consumers (herbivores) → secondary consumers → tertiary consumers → decomposers
• Food webs show interconnected feeding relationships, revealing how energy and matter flow through multiple pathways simultaneously
• Trophic cascades occur when changes at one level ripple through the web—removing a top predator can trigger population explosions in prey species

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Population Regulation and Growth

Populations don't grow indefinitely—they're constrained by resources, space, and interactions with other species. Understanding what limits populations is essential for predicting environmental impacts and designing conservation strategies.

Carrying Capacity

• Carrying capacity (K) is the maximum population an environment can sustain long-term given available resources and conditions
• Logistic growth produces an S-shaped curve as populations approach K and growth rate slows due to density-dependent factors
• Overshoot and collapse occurs when populations temporarily exceed K, depleting resources and causing rapid decline

Limiting Factors

• Limiting factors restrict population growth and can be density-dependent (competition, predation, disease) or density-independent (weather, natural disasters)
• Liebig's law of the minimum states that growth is controlled by the scarcest essential resource, not the total amount of resources available
• Identifying limiting factors is crucial for wildlife management—addressing the wrong factor won't improve population outcomes

Population Dynamics

• Birth rates, death rates, immigration, and emigration determine whether populations grow, shrink, or remain stable
• Age structure influences future growth potential—populations with many young individuals will likely expand even if birth rates decline
• r-selected vs. K-selected species represent different life-history strategies: rapid reproduction with high mortality versus slow reproduction with parental investment

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Community Structure and Species Interactions

Species don't exist in isolation—their survival depends on complex webs of competition, predation, and cooperation. Community ecology explains how these interactions shape biodiversity and ecosystem stability.

Community Ecology

• Communities are assemblages of interacting species in a defined area, structured by both biotic interactions and abiotic conditions
• Species diversity includes both richness (number of species) and evenness (relative abundance), both affecting community stability
• Ecological resilience describes a community's ability to recover from disturbance while maintaining function and structure

Species Interactions

• Competition occurs when species share limited resources—interspecific competition between species can lead to competitive exclusion or niche partitioning
• Predation regulates prey populations and can drive evolutionary adaptations like camouflage, warning coloration, and mimicry
• Symbiosis includes mutualism (both benefit), commensalism (one benefits, other unaffected), and parasitism (one benefits, other harmed)

Keystone Species

• Keystone species have ecosystem impacts disproportionate to their abundance—their removal triggers dramatic community changes
• Sea otters exemplify keystone effects: they control sea urchin populations, preventing urchins from destroying kelp forest ecosystems
• Protecting keystone species is a conservation priority because their loss cascades through food webs, reducing overall biodiversity

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Habitat, Niche, and Biodiversity

Where organisms live and how they make a living determines community composition and ecosystem function. These concepts connect directly to conservation biology and human impacts on the environment.

Habitat and Niche

• Habitat is the physical location where a species lives, defined by abiotic factors like temperature, moisture, and substrate
• Ecological niche describes a species' functional role—what it eats, when it's active, where it reproduces, and how it interacts with other species
• Niche differentiation allows similar species to coexist by partitioning resources—different feeding times, locations, or food sources reduce competition

Biodiversity and Conservation

• Biodiversity operates at three levels: genetic diversity (variation within species), species diversity (variety of species), and ecosystem diversity (variety of habitats)
• High biodiversity increases resilience—diverse ecosystems recover faster from disturbances and provide more ecosystem services
• Conservation priorities focus on protecting habitat, preventing invasive species introductions, and addressing climate change impacts on species ranges

Biomes and Aquatic Ecosystems

• Biomes are large-scale ecosystems defined by climate and dominant vegetation—temperature and precipitation determine biome distribution
• Aquatic ecosystems are classified by salinity (freshwater vs. marine) and physical characteristics (flowing vs. standing water, depth, light penetration)
• Ecotones are transition zones between biomes or ecosystems, often supporting high biodiversity due to species overlap

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Ecosystem Change and Succession

Ecosystems are dynamic—they change over time through predictable patterns of species replacement. Understanding succession helps explain ecosystem recovery after natural and human-caused disturbances.

Succession

• Ecological succession is the gradual replacement of species over time, moving toward a stable climax community
• Primary succession occurs on bare substrates (volcanic rock, glacial till) where no soil exists—pioneer species like lichens begin soil formation
• Secondary succession follows disturbance to existing ecosystems (fire, logging, abandonment)—proceeds faster because soil and seed banks remain

Natural Disruptions to Ecosystems

• Disturbance regimes describe the frequency and intensity of natural disruptions—some ecosystems depend on periodic disturbance (fire-adapted forests)
• Volcanic eruptions, hurricanes, and wildfires reset succession, creating habitat heterogeneity that supports diverse species assemblages
• Ecological resilience determines recovery speed—ecosystems with high biodiversity and intact soil typically recover faster

Climate and Adaptations

• Climate determines species distribution by setting physiological limits on survival, growth, and reproduction
• Organisms adapt through behavioral changes (migration, hibernation), physiological adjustments (acclimatization), and evolutionary responses over generations
• Climate change is shifting species ranges poleward and upward in elevation, disrupting established community interactions

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Biogeochemical Cycles

Matter cycles continuously between living organisms and the physical environment. Human disruption of these cycles—adding carbon to the atmosphere, nitrogen to waterways—drives many environmental problems you'll encounter on the exam.

Biogeochemical Cycles

• Essential elements cycle through reservoirs (atmosphere, hydrosphere, lithosphere, biosphere) via biological, geological, and chemical processes
• Carbon, nitrogen, phosphorus, and water each have distinct pathways—know the major reservoirs and transfer processes for each
• Human activities disrupt cycles through fossil fuel combustion (carbon), fertilizer application (nitrogen, phosphorus), and water diversion

Ecosystem Structure and Function

• Ecosystems integrate biotic and abiotic components through energy flow and nutrient cycling—neither occurs without the other
• Producers, consumers, and decomposers form the biological framework, while climate, soil, and water provide the physical template
• Ecosystem services (water purification, carbon storage, pollination) depend on intact structure and functioning cycles

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Quick Reference Table

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Self-Check Questions

1. Both carrying capacity and limiting factors regulate population size—how do they relate to each other, and which concept is broader?

2. Compare energy flow and nutrient cycling in ecosystems. Why does energy require constant input while nutrients can be recycled?

3. A forest recovers after a wildfire versus lichens colonizing bare volcanic rock—which succession type is each, and why does one proceed faster?

4. How would removing a keystone predator affect both its prey population and the plant community? Trace the trophic cascade.

5. An FRQ describes a lake experiencing algal blooms after agricultural runoff increases. Which biogeochemical cycle is disrupted, what's the limiting factor being added, and what ecological consequences would you predict?