What is the difference between a food chain and a food web?	A food chain is a single linear energy pathway; a food web is a network of interconnected food chains showing all feeding relationships in an ecosystem.
What is the 10% rule?	Only ~10% of energy at one trophic level transfers to the next; the rest is lost mainly as heat from respiration.
How does primary succession differ from secondary succession?	Primary succession starts on bare substrate with no soil (e.g., after volcanic eruption); secondary succession starts where soil remains after a disturbance, so it proceeds faster.
What are pioneer species?	First colonizers in primary succession (e.g., lichens, mosses) that break down rock into soil and enable later species to establish.
How does exponential growth differ from logistic growth?	Exponential growth (J-curve) is unlimited (dN/dt = rN). Logistic growth (S-curve) slows as the population approaches carrying capacity K (dN/dt = rN[(K-N)/K]).
What is carrying capacity (K)?	The maximum population size an environment can sustain indefinitely given available resources.
What are density-dependent limiting factors?	Factors whose impact increases with population density (e.g., competition, predation, disease).
What are density-independent limiting factors?	Factors that affect populations regardless of density (e.g., natural disasters, extreme weather, pollution).
What is the difference between biotic and abiotic factors?	Biotic factors are living components (organisms, interactions). Abiotic factors are non-living components (temperature, water, sunlight, soil pH).
What are the three types of symbiosis?	Mutualism (both benefit), commensalism (one benefits, other unaffected), and parasitism (one benefits, other harmed).
What is a keystone species?	A species with a disproportionately large ecological impact relative to its abundance (e.g., sea otters preventing urchin overgrazing of kelp).
What roles do photosynthesis and respiration play in the carbon cycle?	Photosynthesis removes CO2 from the atmosphere and fixes it into organic molecules; cellular respiration releases CO2 back.
Why can't most organisms use atmospheric nitrogen directly?	N2 has a strong triple bond. Nitrogen-fixing bacteria (e.g., Rhizobium) must convert it to ammonia or nitrate before plants can use it.
What is the difference between GPP and NPP?	GPP is total energy fixed by producers; NPP = GPP minus producer respiration, representing energy available to consumers.
What is an ecological niche?	The full range of conditions and resources a species uses to survive and reproduce, including habitat, diet, and interactions.
What is the competitive exclusion principle?	Two species competing for the same limited resources cannot coexist indefinitely; one will outcompete the other or they must partition resources.
What is biomagnification?	Progressive increase in concentration of persistent toxins (e.g., DDT, mercury) at each higher trophic level in a food chain.
What characterizes a tropical rainforest biome?	High year-round temperatures, heavy rainfall (200-400+ cm), extremely high biodiversity, stratified vegetation layers, and nutrient-poor soil.
What characterizes a tundra biome?	Extremely cold temperatures, low precipitation, permafrost, short growing season, low biodiversity, and no trees.
What distinguishes r-selected from K-selected species?	r-selected: many small offspring, little parental care, thrive in unstable environments. K-selected: few large offspring, extensive care, thrive near carrying capacity.
What are the key processes of the water cycle?	Evaporation, transpiration, condensation, precipitation, infiltration, and runoff, driven by solar energy.
How does the phosphorus cycle differ from other biogeochemical cycles?	Phosphorus has no significant atmospheric phase. It cycles through rock weathering, soil uptake by plants as phosphate, food chains, and decomposition.
What is a trophic cascade?	A change at one trophic level that ripples through others (e.g., wolves reducing elk, allowing vegetation recovery in Yellowstone).
What is the difference between a population, community, and ecosystem?	Population: same species in one area. Community: all species interacting in an area. Ecosystem: community plus abiotic factors, linked by energy flow and nutrient cycling.
What is eutrophication?	Excess nutrient runoff (N, P) causes algal blooms; decomposition of dead algae depletes oxygen, creating dead zones that kill aquatic life.