Ecology and Population Dynamics

Levels of Ecological Organization

Ecology is organized in a hierarchical framework from small to large. Organismal ecology studies how individual organisms interact with their environment—physiological, morphological, and behavioral adaptations. Population ecology examines groups of individuals of the same species in a defined area, focusing on population size, density, distribution, age structure, and growth dynamics. Community ecology studies interactions among populations of different species—predation, competition, mutualism, and their effects on community structure and diversity. Ecosystem ecology adds the abiotic component: energy flow and nutrient cycling through both living (biotic) and nonliving (abiotic) compartments. Landscape ecology considers spatial patterns and the exchange of organisms, materials, and energy across multiple ecosystems. Global ecology (biosphere ecology) studies the planetary-scale circulation of energy and matter. The MCAT focuses primarily on population, community, and ecosystem levels, with emphasis on being able to identify which level a given scenario or data set describes.

Energy Flow and Trophic Levels

Energy flows through ecosystems in a unidirectional path: sunlight (or chemical energy) is captured by primary producers (autotrophs: plants, algae, cyanobacteria) via photosynthesis or chemosynthesis and converted to chemical bond energy. This energy is passed to consumers: primary consumers (herbivores), secondary consumers (carnivores that eat herbivores), tertiary consumers (carnivores that eat other carnivores), and ultimately to decomposers (bacteria, fungi) that break down dead organic matter. Each step is a trophic level. The 10% rule is a rough estimate: only about 10% of the energy at one trophic level is transferred to the next; the remaining ~90% is lost as metabolic heat, used for respiration, or excreted as waste. This energy inefficiency explains why food chains rarely exceed 4–5 levels and why biomass pyramids (total dry mass at each level) are typically upright—producer biomass must vastly exceed consumer biomass. Energy pyramids are ALWAYS upright (energy is lost, never gained). Inverted biomass pyramids can occur in aquatic ecosystems where phytoplankton reproduce rapidly despite low standing biomass. Food webs, more realistic than simple chains, show interconnected trophic relationships. Bioaccumulation and biomagnification: fat-soluble toxins (e.g., DDT, mercury) increase in concentration at successively higher trophic levels because they are not readily excreted and are concentrated by each consumer.

Carbon, Nitrogen, and Water Cycles

Biogeochemical cycles describe the movement of elements and compounds between biotic and abiotic reservoirs. The carbon cycle: atmospheric CO₂ is fixed by photosynthesis into organic carbon; respiration, decomposition, and combustion return CO₂ to the atmosphere. Major reservoirs: atmosphere (CO₂), oceans (dissolved CO₂, bicarbonate, carbonate), fossil fuels, biomass, and sedimentary rocks. The nitrogen cycle begins with nitrogen fixation: N₂ gas is converted to ammonia (NH₃) by nitrogen-fixing bacteria (free-living soil bacteria like Azotobacter and symbiotic Rhizobium in legume root nodules) or by lightning. Nitrification: ammonia is oxidized to nitrite (NO₂⁻) then nitrate (NO₃⁻) by nitrifying bacteria. Assimilation: plants take up nitrate and ammonium, incorporating nitrogen into amino acids and nucleic acids. Ammonification: decomposers convert organic nitrogen in dead matter back to ammonium. Denitrification: denitrifying bacteria convert nitrate back to N₂ gas, closing the cycle. The water (hydrologic) cycle: evaporation from oceans/lakes, transpiration from plants, condensation into clouds, precipitation back to surface, runoff and groundwater percolation. The MCAT may test which process or organism mediates a specific transformation in these cycles.

Population Growth Models

Population growth follows two fundamental models. Exponential growth (J-curve): dN/dt = rN, where N is population size and r is the intrinsic rate of increase (birth rate minus death rate). This occurs when resources are unlimited—bacteria in fresh culture medium, invasive species in a new habitat, human population historically. Exponential growth is unsustainable in any finite environment. Logistic growth (S-curve): dN/dt = rN(K−N)/K, where K is the carrying capacity. Growth slows as N approaches K due to density-dependent limiting factors (competition, predation, disease, waste accumulation). At N = K/2, growth rate is maximal—this is the maximum sustainable yield point. K-selected species (e.g., elephants, humans, whales) live near K: few offspring, extensive parental care, long lifespan, stable environments. r-selected species (e.g., insects, weeds, bacteria) emphasize rapid reproduction: many offspring, minimal care, short lifespan, unstable or unpredictable environments. Survivorship curves: Type I (high juvenile survival, most mortality in old age—humans, large mammals), Type II (constant mortality rate throughout life—birds, some reptiles), Type III (very high juvenile mortality, high survival once established—oysters, fish, plants producing many seeds). Age-structure pyramids show the proportion of individuals in each age class; expanding populations have broad bases (many young), stable populations have relatively even distribution, declining populations have narrow bases.

Competition and Symbiosis

Species interactions can be categorized by the effect on each participant (+, −, 0). Competition (−/−): both species are harmed. Interspecific competition occurs between different species for a shared limiting resource. The competitive exclusion principle (Gause) states that two species cannot occupy exactly the same niche—one will outcompete and exclude the other, or resource partitioning will occur (character displacement, niche differentiation). Predation (+/−): one species (predator) kills and consumes another (prey). Predator-prey cycles (e.g., lynx and snowshoe hare) show coupled oscillations: prey increase → predator increase with lag → prey decrease → predator decrease → prey increase. Herbivory (+/−): consumption of plants by animals. Symbiosis encompasses close, prolonged interspecific associations. Mutualism (+/+): both benefit (e.g., nitrogen-fixing bacteria in legume roots, mycorrhizal fungi and plant roots, coral polyps and zooxanthellae, gut microbiome and human host). Commensalism (+/0): one benefits, the other is unaffected (e.g., barnacles on whales, remoras on sharks, birds nesting in trees). Parasitism (+/−): one (parasite) benefits at the expense of the other (host), but typically does not kill the host immediately (unlike predation). Parasitoids (e.g., certain wasps) lay eggs in a host that is eventually consumed —this blurs the line between parasitism and predation.

Carrying Capacity

Carrying capacity (K) is the maximum population size an environment can sustain indefinitely given the available resources (food, water, habitat, space). K is not fixed—it can change with environmental conditions (seasonal variation, climate change, habitat degradation, resource depletion). Density-dependent factors limit population growth more strongly as population density increases: competition for resources, spread of disease, predation pressure, accumulation of wastes, territorial behavior, and stress-induced physiological changes. Density-independent factors affect populations regardless of density: natural disasters (fires, floods, volcanic eruptions), weather extremes, and human activities (habitat destruction, pollution). In logistic growth, populations can overshoot K temporarily—consuming resources faster than they renew, followed by a population crash (die-off) as resources are depleted. This overshoot-crash pattern is common in boom-bust species (e.g., reindeer on St. Matthew Island). Human carrying capacity is debated because technology can increase K (agricultural advances, fossil fuel use) but also depletes resources unsustainably (soil degradation, aquifer depletion, biodiversity loss). The ecological footprint concept quantifies human demand on Earth's ecosystems relative to biocapacity.

Ecological Succession

Ecological succession is the predictable, directional change in community composition over time following a disturbance or the creation of new substrate. Primary succession occurs on newly exposed surfaces with no soil: bare rock (from volcanic lava, glacial retreat), sand dunes, artificial substrates. Pioneer species—lichens, mosses, hardy grasses—colonize first, weathering rock and building soil through their decomposition. Over time, soil depth increases and supports successively larger plants: grasses → shrubs → pioneer trees → climax community. Secondary succession occurs in areas where soil remains after a disturbance (fire, logging, abandoned farmland, hurricane). It is faster than primary succession because soil and seed banks already exist. Early-successional (pioneer) species are typically r-selected: fast-growing, short-lived, shade-intolerant, good dispersers. Late-successional (climax) species are K-selected: slow-growing, long-lived, shade-tolerant. The climax community concept is now understood as dynamic—disturbances create a shifting mosaic of successional stages across a landscape (patch dynamics). A keystone species has a disproportionately large effect on community structure relative to its abundance (e.g., sea otters controlling sea urchin populations, which in turn determines kelp forest extent; gray wolves in Yellowstone). Removing a keystone species triggers a trophic cascade—ripple effects through the food web.

How it works

Ecology can be reduced to two master equations. Energy flow: sunlight → photosynthesis → trophic transfers (∼10% efficiency per level) → heat dissipation. Matter cycles: carbon fixed by producers, consumed up the food chain, respired back as CO₂, or decomposed to re-enter the soil; nitrogen fixed from atmosphere, cycled through nitrification, assimilation, ammonification, denitrification; water evaporated, condensed, precipitated, transpired, and runoff. Population regulation: exponential growth when unconstrained, logistic growth limited by K, with density-dependent feedbacks maintaining populations near equilibrium. Species interactions are classified by their +/−/0 effects, and the net outcome determines community composition. Disturbance resets the successional clock, and the sequence of species replacement is predictable because each stage modifies the environment in ways that facilitate (or inhibit) the next. The MCAT expects you to read graphs of these dynamics—population curves, age pyramids, energy and biomass pyramids, and survivorship curves—and deduce which model or principle applies.

How it works

Ecology can be reduced to two master equations. Energy flow: sunlight → photosynthesis → trophic transfers (∼10% efficiency per level) → heat dissipation. Matter cycles: carbon fixed by producers, consumed up the food chain, respired back as CO₂, or decomposed to re-enter the soil; nitrogen fixed from atmosphere, cycled through nitrification, assimilation, ammonification, denitrification; water evaporated, condensed, precipitated, transpired, and runoff. Population regulation: exponential growth when unconstrained, logistic growth limited by K, with density-dependent feedbacks maintaining populations near equilibrium. Species interactions are classified by their +/−/0 effects, and the net outcome determines community composition. Disturbance resets the successional clock, and the sequence of species replacement is predictable because each stage modifies the environment in ways that facilitate (or inhibit) the next. The MCAT expects you to read graphs of these dynamics—population curves, age pyramids, energy and biomass pyramids, and survivorship curves—and deduce which model or principle applies.

Comparisons

• B/B (Population graphs): Interpret exponential vs. logistic growth curves, identify K and K/2, distinguish r-selected from K-selected species from life-history data.
• B/B (Biogeochemical cycles): Nitrogen cycle is the most tested—know which organisms perform each transformation (fixation, nitrification, denitrification).
• B/B (Trophic levels): Energy pyramids are ALWAYS upright; biomass pyramids can be inverted in aquatic systems. The 10% rule limits food chain length.
• C/P (Thermodynamics): Energy flow through ecosystems obeys the first and second laws—energy is conserved but degraded to heat, explaining trophic inefficiency.
• P/S (Human ecology): Population growth models (Malthusian theory, demographic transition) appear in sociology passages, connecting to exponential/logistic growth.
• B/B (Symbiosis and microbiome): Mutualism with gut microbiota directly links ecology to human physiology—increasingly tested in passage-based questions.

Common confusions

• Confusing the direction of energy vs. matter flow. Energy flows one-way (sunlight → heat); matter cycles (C, N, H₂O are reused). The MCAT loves to ask ‘which is recycled?’
• Thinking energy pyramids can be inverted. Energy pyramids are ALWAYS upright because energy is lost at each trophic level. Biomass pyramids can be inverted (aquatic systems), but energy pyramids cannot.
• Forgetting that the 10% rule is approximate, not a law. The actual ecological efficiency varies (5–20%) depending on organism type (endotherms are less efficient than ectotherms).
• Mixing up nitrification and denitrification. Nitrification = NH₃ → NO₂⁻ → NO₃⁻ (by bacteria, adds oxygen). Denitrification = NO₃⁻ → N₂ (by bacteria, removes nitrogen, anaerobic conditions).
• Assuming carrying capacity is a fixed constant. K changes with environmental conditions—habitat degradation lowers K; technological advances can temporarily raise it.
• Confusing primary and secondary succession based on the presence of soil. Primary = no soil (bare rock, lava, glacier retreat). Secondary = soil present (abandoned farmland, after fire, after logging).
• Misidentifying mutualism vs. commensalism. If both benefit demonstrably, it is mutualism (bacteria + legume = N for plant, C for bacteria). If one benefits and the other is truly unaffected, it is commensalism (rare; often an unmeasured mutualism).
• Treating r/K selection as a strict binary. Most species fall on a continuum; it is a comparative framework, not an absolute classification.

Quick review

• Levels of organization: Organism → Population → Community → Ecosystem → Landscape → Biosphere.
• Energy flow is unidirectional (sunlight to heat); matter cycles (C, N, H₂O) between biotic and abiotic reservoirs.
• The 10% rule: ~10% of energy transfers between trophic levels. Energy pyramids are ALWAYS upright; biomass pyramids can be inverted.
• Exponential growth: dN/dt = rN (J-curve, unlimited resources). Logistic growth: dN/dt = rN(K−N)/K (S-curve, limited by K).
• Maximum growth rate occurs at K/2 (maximum sustainable yield).
• r-selected: many offspring, little care, short life, unstable environments. K-selected: few offspring, high care, long life, stable environments.
• Survivorship curves: Type I (high juvenile survival), Type II (constant mortality), Type III (high juvenile mortality).
• Nitrogen cycle: Fixation (N₂→NH₃, bacteria/lightning), Nitrification (NH₃→NO₃⁻, bacteria), Assimilation (plants take up N), Ammonification (decomposers→NH₃), Denitrification (NO₃⁻→N₂, bacteria).
• Species interactions: Competition (−/−), Predation (+/−), Mutualism (+/+), Commensalism (+/0), Parasitism (+/−).
• Primary succession: no soil (bare rock, lava). Secondary succession: soil present (after fire, abandoned farm). Pioneer species → climax community.
• Competitive exclusion principle: two species cannot occupy the exact same niche indefinitely.
• Keystone species: disproportionately large effect on community structure relative to abundance. Removal causes trophic cascade.