Ecology — Practice Quiz

A Biology cheat sheet for Ecology — every key formula with its symbols defined — plus a medium-level practice quiz to test recall.

Formulas & key concepts

Ecology: Study of interactions among organisms and between organisms and their environment. Biosphere: All life on Earth + all parts where life exists (land, water, atmosphere, 8km above to 11km below ocean surface).
Ecology & The Biosphere
Individual organism → Population (same species, same area) → Community (different populations in area) → Ecosystem (community + physical environment) → Biome (similar climates, typical organisms) → Biosphere.
Ecological Levels of Organization
Biotic: Living components (animals, plants, bacteria, fungi). Abiotic: Non-living (sunlight, temperature, water, precipitation, soil, wind, humidity). Interact to shape ecosystem.
Biotic vs. Abiotic Factors
Observation: Identify species, behaviors, patterns. Experimentation: Test hypotheses, manipulate variables (greenhouse studies, field experiments). Modeling: Predict long-term/large-scale events, based on data.
Ecological Methods
Habitat: Physical location where organism lives (address). Niche: Organism's role in ecosystem (profession) - what it eats, when active, where lives, how interacts. Fundamental niche (potential) vs. Realized niche (actual with competition).
Habitat vs. Niche
Autotrophs: Make own food. Photosynthesis: Use sunlight (plants, algae, cyanobacteria: 6CO₂ + 6H₂O + light → C₆H₁₂O₆ + 6O₂). Chemosynthesis: Use chemical energy (deep-sea vent bacteria, sulfur compounds). Foundation of food chains.
Producers (Autotrophs)
Herbivores: Eat plants (primary consumers). Carnivores: Eat animals (secondary/tertiary consumers). Omnivores: Eat both plants + animals. Scavengers: Eat dead animals. Detritivores: Eat detritus (earthworms, crabs). Decomposers: Break down organic matter (bacteria, fungi).
Consumers (Heterotrophs)

Linear sequence showing energy transfer. Example: Grass (producer) → Rabbit (primary consumer/herbivore) → Fox (secondary consumer/carnivore) → Decomposers. Each step = <b>trophic level</b>.

Food Chains

Complex network of interconnected food chains. Shows multiple feeding relationships. More realistic than simple chains. Organisms often eat multiple things and are eaten by multiple predators.

Food Webs
10% Rule: Only ~10% of energy transfers to next trophic level (90% lost as heat from metabolism). Biomass pyramid: Total mass of living tissue decreases at higher levels. Limits food chain length to ~4-5 levels.
Energy Pyramids

Efficiency of energy transfer between levels. Typically 5-20% (average 10%). Lost energy: respiration (heat), waste, incomplete digestion. Why top predators are rare.

Trophic Efficiency
Decomposers (bacteria, fungi): Break down dead organisms, return nutrients to soil. Essential for recycling. Detritus: Dead organic matter. Without decomposers, nutrients locked up in dead bodies.
Decomposers & Nutrient Recycling

Energy from chemical reactions (not sunlight). <b>Deep-sea vents</b>: Bacteria use H₂S + O₂ → organic compounds. Support entire ecosystems without photosynthesis. Also in hot springs, tidal marshes.

Chemosynthesis
Evaporation (liquid → vapor), Transpiration (plants release water), Condensation (vapor → clouds), Precipitation (rain/snow), Runoff (flows to rivers/oceans), Infiltration (groundwater). Driven by solar energy.
Water Cycle
Photosynthesis: CO₂ → organic compounds. Respiration: Organic → CO₂ (returns to atmosphere). Decomposition: Dead organisms → CO₂. Combustion: Burning fossil fuels/wood → CO₂. Oceans: Dissolve/release CO₂. Sediments: Long-term storage (fossil fuels).
Carbon Cycle
Nitrogen fixation: N₂ → NH₃/NH₄⁺ (bacteria: Rhizobium in root nodules, cyanobacteria, lightning). Nitrification: NH₄⁺ → NO₂⁻ → NO₃⁻ (bacteria). Assimilation: Plants absorb NO₃⁻, make proteins. Ammonification: Dead organisms → NH₄⁺. Denitrification: NO₃⁻ → N₂ (back to atmosphere).
Nitrogen Cycle
Weathering: Rocks release phosphate (PO₄³⁻). Absorption: Plants take up from soil. Food chain: Passes through consumers. Decomposition: Returns to soil. Sedimentation: Washes to ocean, forms rocks. No atmospheric component (unlike C, N). Slow cycle.
Phosphorus Cycle
Law of the Minimum: Nutrient in shortest supply limits growth. Terrestrial: Often nitrogen. Aquatic: Often phosphorus or nitrogen. Adding limiting nutrient increases productivity (can cause eutrophication).
Limiting Nutrients
Size: Number of individuals. Density: Number per unit area. Distribution: Clumped (groups, most common), Uniform (evenly spaced, territorial), Random (unpredictable, rare). Age structure: Proportion in each age group.
Population Characteristics

Population change = (Birth rate + Immigration) - (Death rate + Emigration). <b>Immigration</b>: Individuals moving in. <b>Emigration</b>: Individuals moving out. <b>Demography</b>: Study of population statistics.

Population Growth Factors
J-curve: Accelerating growth when resources unlimited. Equation: dN/dt = rN (r = intrinsic rate of increase). Population doubles at constant intervals. Unsustainable long-term. Occurs after colonization or low density.
Exponential Growth
S-curve: Growth slows as approaches carrying capacity. Carrying capacity (K): Maximum population environment can sustain. Growth rate highest at K/2. More realistic than exponential. Equation: dN/dt = rN(K-N)/K.
Logistic Growth

Maximum population size environment can support indefinitely. Determined by limiting resources (food, water, space, shelter). Varies with environmental conditions. Population fluctuates around K.

Carrying Capacity

Effects depend on population density. <b>Competition</b> (for resources), <b>Predation</b> (more prey = more predators), <b>Disease/Parasitism</b> (spreads faster when crowded), <b>Stress</b>, <b>Waste accumulation</b>. Regulate populations near carrying capacity.

Density-Dependent Factors

Effects regardless of density. <b>Weather</b> (drought, freeze, flood), <b>Natural disasters</b> (fires, hurricanes), <b>Seasonal cycles</b>, <b>Climate</b>. Can cause drastic population changes.

Density-Independent Factors
r-selected: Unstable environments. Many offspring, small size, little parental care, mature quickly, short lifespan (insects, mice, weeds). K-selected: Stable environments. Few offspring, large size, much parental care, mature slowly, long lifespan (elephants, whales, humans).
r-selection vs. K-selection
Type I: Low infant mortality, most die old (humans, large mammals, high parental care). Type II: Constant death rate throughout life (birds, rodents). Type III: High infant mortality, few reach adulthood (fish, insects, plants, many offspring).
Survivorship Curves
Negative feedback: Density-dependent factors slow growth as population increases. Oscillations: Predator-prey cycles (e.g., lynx-hare). Zero population growth: Birth rate = death rate. Populations stabilize around carrying capacity.
Population Regulation
Competition (-/-), Predation (+/-), Herbivory (+/-), Mutualism (+/+), Commensalism (+/0), Parasitism (+/-). Shape community structure and evolution.
Species Interactions Overview
Interspecific: Between different species for same resources. Intraspecific: Within same species (usually more intense). Competitive exclusion principle: Two species cannot coexist if they occupy identical niche. Resource partitioning: Species divide resources (different times, places, foods).
Competition
Predation: One organism kills/eats another. Drives evolution. Prey defenses: Camouflage (blend in), Warning coloration (bright = dangerous), Mimicry (Batesian: harmless looks dangerous; Müllerian: both dangerous look similar), Physical (spines, shells).
Predation & Defenses

Close, long-term interactions. <b>Mutualism (+/+)</b>: Both benefit (bees + flowers, lichen, mycorrhizae). <b>Commensalism (+/0)</b>: One benefits, other unaffected (barnacles on whales). <b>Parasitism (+/-)</b>: Parasite benefits, host harmed (tapeworms, ticks, mistletoe).

Symbiosis Types

Species with disproportionately large impact on ecosystem relative to abundance. Removal causes major changes. Examples: Sea otters (control urchins), wolves (control herbivores), sea stars (control mussels). Maintain biodiversity.

Keystone Species

Form major structural component of habitat. Create environment for other species. Examples: Coral (reefs), kelp (forests), trees (forest canopy). Usually abundant, high biomass.

Foundation Species
Primary succession: Starts on bare rock/new land (glacial retreat, volcanic island). Pioneer species: Lichens, mosses. Secondary succession: After disturbance (fire, farming, logging), soil present. Faster than primary. Climax community: Stable end point, characteristic of climate.
Ecological Succession
Species richness: Number of different species. Relative abundance: Evenness of distribution. High biodiversity increases: ecosystem stability, productivity, resilience to disturbance. Highest in tropical rainforests, coral reefs.
Biodiversity
Biotic: Producers, consumers (primary, secondary, tertiary), decomposers. Abiotic: Sunlight, temperature, water, soil, minerals, climate. Energy flows through (not recycled), matter cycles.
Ecosystem Components
Gross Primary Productivity (GPP): Total energy captured by producers. Net Primary Productivity (NPP): GPP - respiration (energy available to consumers). Standing crop: Total biomass at given time. Highest in tropical rainforests, estuaries, coral reefs.
Ecosystem Productivity

Determined by temperature + precipitation. <b>Tropical rainforest</b> (high temp, high rain, high diversity). <b>Temperate forest</b> (moderate temp, seasonal, deciduous trees). <b>Grassland</b> (moderate rain, periodic fire). <b>Desert</b> (low rain, extreme temps). <b>Taiga</b> (cold, conifers). <b>Tundra</b> (very cold, permafrost, low diversity).

Terrestrial Biomes
Freshwater: Lakes (littoral, limnetic, profundal zones), rivers/streams, wetlands (filter water, biodiversity). Marine: Photic zone (sunlit, 0-200m), Aphotic zone (dark). Intertidal (tides), Coral reefs (high diversity), Estuaries (salt + fresh mix, high productivity), Open ocean.
Aquatic Biomes

Climate (long-term temperature + precipitation patterns) determines biome. <b>Latitude</b>: Tropical (equator) → temperate → polar. <b>Altitude</b>: Similar to latitude (higher = colder). <b>Rain shadow</b>: Mountains block moisture, desert on leeward side.

Climate & Biomes
Aquatic: Littoral (shallow, near shore), Limnetic (open water, sunlit), Profundal (deep, no light), Benthic (bottom). Terrestrial: Canopy (treetops), Understory (shrubs/small trees), Forest floor (decomposition).
Ecological Zones
Deforestation: Clearing forests (agriculture, logging, development). Urbanization: Cities replace natural habitats. Fragmentation: Habitats broken into small patches (edge effects, reduced population sizes). Leading cause of species extinction.
Habitat Destruction
Air: Smog, particulates, greenhouse gases (CO₂, CH₄), acid rain precursors (SO₂, NOₓ). Water: Sewage, chemicals, oil spills, thermal pollution, eutrophication. Soil: Pesticides, heavy metals, industrial waste. Noise/Light: Affects animal behavior.
Pollution Types
Greenhouse effect: CO₂, CH₄, N₂O trap heat. Global warming: Average temp rising (~1°C since 1880). Impacts: Ice melting, sea level rise, extreme weather, range shifts, coral bleaching, extinction risk. Causes: Fossil fuel burning, deforestation.
Climate Change

Non-native species introduced to new area. Often lack natural predators/parasites. <b>Impacts</b>: Outcompete natives, alter habitats, disrupt food webs, cause extinctions. Examples: Zebra mussels, kudzu, cane toads, Asian carp. Prevention: biosecurity, quarantine.

Invasive Species

Harvesting organisms faster than they can reproduce. <b>Overfishing</b>: Cod, tuna population collapse. <b>Overhunting</b>: Passenger pigeon (extinct), whales (endangered). <b>Poaching</b>: Elephants (ivory), rhinos (horns). Unsustainable resource use.

Overexploitation
Cultural eutrophication: Excess nutrients (N, P from fertilizer runoff, sewage) enter water. Process: Algal bloom → blocks sunlight → algae die → decomposition uses O₂ → hypoxia/dead zones → fish/organisms die. Major problem in lakes, coastal areas.
Eutrophication
Bioaccumulation: Toxins build up in organism over time. Biomagnification: Toxin concentration increases up food chain. Top predators most affected. Examples: DDT (egg shell thinning in eagles), Mercury (fish consumption warnings), PCBs. Fat-soluble toxins persist.
Biological Magnification
Stratospheric ozone (O₃): Blocks UV radiation. CFCs (chlorofluorocarbons from refrigerants, aerosols) break down ozone. Ozone hole: Over Antarctica. Effects: Increased UV → skin cancer, cataracts, crop damage. Montreal Protocol: Banned CFCs, ozone recovering.
Ozone Depletion
Causes: SO₂ + NOₓ from fossil fuel combustion → H₂SO₄, HNO₃ in atmosphere. pH < 5.6. Effects: Damages forests (leaches nutrients from soil), acidifies lakes (kills fish/amphibians), corrodes buildings. Solutions: Scrubbers on smokestacks, catalytic converters, renewable energy.
Acid Rain
Soil erosion: Tree roots hold soil, removal causes erosion. Species loss: Habitat destruction, extinction. Climate: Less CO₂ absorption, more CO₂ release (burning/decomposition). Water cycle disruption: Less transpiration, altered rainfall. Indigenous displacement.
Deforestation Impacts

Science of protecting biodiversity. <b>Goals</b>: Prevent extinctions, preserve habitats, restore ecosystems. <b>Strategies</b>: Protected areas, captive breeding, reintroduction programs, habitat corridors, legislation (Endangered Species Act, CITES).

Conservation Biology
Renewable: Replace at/faster than use rate (solar, wind, forests, fisheries if managed). Non-renewable: Finite supplies (fossil fuels, minerals). Ecological footprint: Land/water needed to support lifestyle. Sustainability: Meeting needs without compromising future generations.
Sustainable Resource Use

Active restoration of degraded ecosystems. <b>Examples</b>: Wetland restoration, reforestation, stream restoration, removing invasive species. <b>Reintroduction</b>: Return species to former range (wolves to Yellowstone, condors). <b>Bioremediation</b>: Use organisms to clean pollution.

Restoration Ecology

Areas with high species richness + endemism (species found nowhere else) + threat level. <b>Criteria</b>: >1,500 endemic plant species, >70% habitat loss. <b>Examples</b>: Madagascar, tropical Andes, Caribbean, Philippines. Conservation priorities. ~36 hotspots globally.

Biodiversity Hotspots
National parks, Wildlife reserves, Marine protected areas (MPAs), Wilderness areas. Preserve habitats, prevent development. Challenges: Insufficient size, isolated (need corridors), enforcement, human-wildlife conflict. ~15% of land, ~7% of ocean protected.
Protected Areas

Human population ~8 billion, growing. <b>Demographic transition</b>: High birth/death → high birth/low death (rapid growth) → low birth/death (stable). <b>Carrying capacity</b>: Uncertain for humans (technology extends but finite). <b>Resource consumption</b>: Developed nations use disproportionate share. Need sustainable practices.

Human Population & Sustainability

Practice quiz

  1. Which of the following correctly represents the hierarchical order of ecological organization from smallest to largest?

    • Individual organism \rightarrow Population \rightarrow Community \rightarrow Ecosystem \rightarrow Biome \rightarrow Biosphere
    • Population \rightarrow Individual organism \rightarrow Community \rightarrow Ecosystem \rightarrow Biome \rightarrow Biosphere
    • Individual organism \rightarrow Community \rightarrow Population \rightarrow Ecosystem \rightarrow Biome \rightarrow Biosphere
    • Ecosystem \rightarrow Community \rightarrow Population \rightarrow Individual organism \rightarrow Biome \rightarrow Biosphere

    Answer: Individual organism \rightarrow Population \rightarrow Community \rightarrow Ecosystem \rightarrow Biome \rightarrow Biosphere

  2. The process by which autotrophs convert light energy into chemical energy is called photosynthesis. Which of the following correctly represents the overall chemical equation for photosynthesis?

    • $6\text{CO}_2 + 6\text{H}_2\text{O} + \text{light} \rightarrow \text{C}_6\text{H}_{12}\text{O}_6 + 6\text{O}_2$
    • $\text{C}_6\text{H}_{12}\text{O}_6 + 6\text{O}_2 \rightarrow 6\text{CO}_2 + 6\text{H}_2\text{O} + \text{energy}$
    • $6\text{CO}_2 + 6\text{O}_2 + \text{light} \rightarrow \text{C}_6\text{H}_{12}\text{O}_6 + 6\text{H}_2\text{O}$
    • $\text{C}_6\text{H}_{12}\text{O}_6 + 6\text{H}_2\text{O} \rightarrow 6\text{CO}_2 + 6\text{O}_2 + \text{energy}$

    Answer: $6\text{CO}_2 + 6\text{H}_2\text{O} + \text{light} \rightarrow \text{C}_6\text{H}_{12}\text{O}_6 + 6\text{O}_2$

  3. According to the $10\%$ rule of energy transfer in an ecosystem, if producers have $10,000 \text{ kcal}$ of energy, how much energy would typically be available to secondary consumers?

    • $10,000 \text{ kcal}$
    • $1,000 \text{ kcal}$
    • $100 \text{ kcal}$
    • $10 \text{ kcal}$

    Answer: $100 \text{ kcal}$

  4. Which of the following processes removes carbon dioxide from the atmosphere and incorporates it into organic compounds?

    • Respiration
    • Decomposition
    • Photosynthesis
    • Combustion

    Answer: Photosynthesis

  5. The conversion of atmospheric nitrogen gas ($N_2$) into ammonia ($NH_3$) or ammonium ($NH_4^+$) by bacteria is known as:

    • Nitrification
    • Denitrification
    • Nitrogen fixation
    • Ammonification

    Answer: Nitrogen fixation

  6. A population exhibiting logistic growth will typically show an S-shaped curve. What does the 'K' in the logistic growth equation $dN/dt = rN(K-N)/K$ represent?

    • The intrinsic rate of increase
    • The maximum population size the environment can sustain
    • The current population density
    • The death rate of the population

    Answer: The maximum population size the environment can sustain

  7. A severe drought causes a significant decrease in the population of deer in a region, regardless of how many deer were initially present. This is an example of a:

    • Density-dependent factor
    • Density-independent factor
    • K-selected factor
    • r-selected factor

    Answer: Density-independent factor

  8. Lichens, which are a symbiotic association between a fungus and an alga, where both organisms benefit, represent which type of interaction?

    • Commensalism
    • Parasitism
    • Predation
    • Mutualism

    Answer: Mutualism

  9. After a volcanic eruption creates new land from hardened lava, the first organisms to colonize this barren environment are typically lichens and mosses. This process is an example of:

    • Secondary succession
    • Climax community formation
    • Primary succession
    • Resource partitioning

    Answer: Primary succession

  10. Why are top predators, such as eagles or large fish, often more severely affected by persistent environmental toxins like DDT or mercury than organisms at lower trophic levels?

    • They have a higher metabolic rate, which increases toxin absorption.
    • They are more susceptible to disease, which is exacerbated by toxins.
    • Toxins become more concentrated at successive trophic levels through biological magnification.
    • They consume a wider variety of food sources, increasing their exposure to different toxins.

    Answer: Toxins become more concentrated at successive trophic levels through biological magnification.

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