4.2 Ecology: Interrelationships & Problems
Key Takeaways
- Ecology studies interactions among organisms and between organisms and their environment across population, community, and ecosystem levels
- Energy flows one way through food chains and webs and is lost as heat; biomass and available energy typically decrease at higher trophic levels
- A niche is an organism’s role and resource use; carrying capacity is the maximum population size an environment can sustain long-term
- Symbiosis includes mutualism, commensalism, and parasitism; competition and predation also structure communities
- Human impacts—pollution, climate change, and habitat loss—disrupt ecosystems; carbon, nitrogen, and water cycles move matter through biotic and abiotic parts of the environment
Ecology is the study of how organisms interact with each other and with the nonliving environment. On the NLN NEX Science exam, ecology questions sit in the Biology domain and often test levels of organization, food chains and energy pyramids, niches, population growth limits, symbiotic relationships, human impacts, and the big matter cycles (carbon, nitrogen, water). Think in systems: energy flows, matter cycles, and populations respond to resources and interactions.
Levels of Ecological Organization
Biologists zoom in and out along a scale. For NEX, own these nested levels:
| Level | Definition | Example |
|---|---|---|
| Organism | A single living individual | One white-tailed deer |
| Population | Individuals of the same species in the same area that can interbreed | All deer in a forest tract |
| Community | All populations of different species living and interacting in an area | Deer, trees, ticks, wolves, fungi in that forest |
| Ecosystem | Community plus abiotic factors (energy, water, soil, climate, nutrients) | The forest community with sunlight, rainfall, and soil chemistry |
| Biosphere | All regions of Earth inhabited by life | Global sum of ecosystems |
Abiotic factors are nonliving (temperature, light, pH, minerals). Biotic factors are living or once-living influences (predators, pathogens, competitors, decomposing organic matter). Ecosystem questions almost always require both.
Energy Flow: Food Chains, Webs, and Pyramids
Energy for nearly all ecosystems originates as sunlight captured by producers (autotrophs)—mainly plants, algae, and photosynthetic bacteria. Consumers (heterotrophs) eat other organisms. Decomposers (many fungi and bacteria) break down dead matter and wastes, returning nutrients to the environment.
A food chain is a linear sequence of who eats whom:
Grass → grasshopper → frog → snake → hawk
A food web is a network of interconnected chains reflecting real diets. Webs are more realistic; chains are simplified teaching tools.
Trophic levels classify position in the energy path:
| Trophic level | Role | Examples |
|---|---|---|
| Producers (1st) | Fix energy into organic molecules | Plants, phytoplankton |
| Primary consumers | Herbivores / eat producers | Caterpillars, zooplankton |
| Secondary consumers | Eat primary consumers | Small fish, insectivorous birds |
| Tertiary (and higher) | Eat other consumers | Large predators |
| Decomposers | Recycle nutrients from all levels | Bacteria, fungi |
Energy Pyramid
Energy transfer between trophic levels is inefficient. Roughly on the order of ~10% of energy at one level is incorporated into the next (the rest is lost as heat through metabolism, incomplete consumption, and waste—exact percentages vary, but "most energy is lost" is the exam point). Consequently:
- Biomass and available energy usually decrease toward the top of the pyramid
- Food chains are short; there is not enough energy to support many successive predator levels
- Toxins that accumulate in tissues (biomagnification) can become more concentrated at higher trophic levels—important for environmental health awareness
Energy flows through ecosystems and is continually lost as heat; it is not recycled like matter. Matter (carbon, nitrogen, water) cycles.
Niches and Habitats
- Habitat: the place where an organism lives ("address")
- Niche: the organism’s role—resources used, interactions, timing of activity, and conditions tolerated ("profession")
Two species cannot permanently occupy identical niches in the same place without competitive exclusion (one outcompetes the other) or niche differentiation. Generalists use wide resource ranges; specialists depend on narrow ones and may be more vulnerable to change.
Population Growth and Carrying Capacity
Populations grow when birth + immigration exceed death + emigration. Idealized exponential growth produces a J-shaped curve when resources are unlimited. Real populations often approach a limit and form an S-shaped (logistic) curve as growth slows.
Carrying capacity (K) is the maximum population size an environment can sustain indefinitely given resources and conditions. Density-dependent limits include competition, predation, disease, and waste buildup. Density-independent factors (severe weather, natural disasters) can reduce numbers regardless of density. Overshooting K can damage resources and cause a crash.
For human health context: crowded conditions can facilitate disease transmission—linking ecology to epidemiology without leaving the science framing.
Species Interactions and Symbiosis
| Interaction | Effect on species A | Effect on species B | Notes |
|---|---|---|---|
| Competition | − | − | Both harmed when sharing limited resources |
| Predation | + (predator) | − (prey) | Includes herbivory on plants in broad treatments |
| Mutualism | + | + | Both benefit (e.g., gut microbiota aiding digestion) |
| Commensalism | + | 0 | One benefits; other roughly unaffected |
| Parasitism | + (parasite) | − (host) | Parasite lives on/in host; often does not kill immediately |
Symbiosis means living in close association; the three classic subtypes for exams are mutualism, commensalism, and parasitism. Predation and competition are critical community interactions even when not labeled "symbiotic" in every textbook.
Human Impacts and Environmental Problems
Human activity is a dominant ecological force. NEX-relevant problem areas:
Pollution
Introduction of harmful substances or energy into air, water, or soil. Examples: industrial chemicals, excess nutrients causing aquatic eutrophication and oxygen-poor "dead zones," plastics, heavy metals, and air pollutants that harm respiratory health. Point sources are localized; nonpoint sources (runoff) are diffuse.
Climate Change
Increased greenhouse gases (notably CO₂ from fossil fuel combustion and deforestation, plus methane and others) trap heat, altering global climate patterns. Consequences include shifting habitats, extreme weather, sea-level rise, and stress on food and water systems—public health relevance without needing physics deep-dives.
Habitat Loss and Fragmentation
Conversion of forests, wetlands, and grasslands for agriculture, cities, and infrastructure is a leading cause of biodiversity loss. Fragmentation isolates populations, reducing gene flow and increasing extinction risk. Invasive species introduced by humans can outcompete natives and restructure communities.
| Human impact | Ecological effect |
|---|---|
| Pollution | Toxicity, nutrient overload, reduced water/air quality |
| Climate change | Range shifts, phenology changes, ecosystem stress |
| Habitat loss | Smaller populations, extinctions, less ecosystem service |
| Overharvesting | Collapse of populations (fisheries, etc.) |
| Invasives | Competition, predation, novel diseases |
Ecosystem services—clean water, pollination, soil fertility, climate regulation—decline when ecosystems are degraded, which is why ecology appears in health and community wellness discussions.
Biogeochemical Cycles (Brief)
Matter cycles between biotic and abiotic reservoirs.
Water Cycle
Evaporation and plant transpiration move water to the atmosphere; condensation forms clouds; precipitation returns water to land and oceans; runoff and groundwater flow complete paths. Human use, damming, and pollution alter local water availability and quality.
Carbon Cycle
Photosynthesis pulls CO₂ into organic molecules; respiration and decomposition return CO₂. Fossil fuel burning and deforestation increase atmospheric CO₂. Oceans absorb large amounts of carbon. Carbon moves among atmosphere, organisms, soils, rocks, and waters on different timescales.
Nitrogen Cycle
Most atmospheric N₂ is unusable by animals and plants until nitrogen-fixing bacteria convert it to ammonia/ammonium. Other microbes perform nitrification and denitrification. Plants take up nitrate or ammonium; consumers obtain nitrogen in organic form. Excess fertilizer runoff drives eutrophication—linking the nitrogen cycle to pollution problems.
| Cycle | Key processes to remember |
|---|---|
| Water | Evaporation, transpiration, condensation, precipitation, runoff/groundwater |
| Carbon | Photosynthesis, respiration, combustion, ocean exchange |
| Nitrogen | Fixation, nitrification, uptake, denitrification, decomposition |
Nursing and Exam Tie-Ins
- Water quality and vector ecology (mosquitoes, ticks) connect environment to infection risk
- Nutrition and food systems depend on productive ecosystems and stable climate
- Antimicrobial stewardship is selection ecology applied to pathogens (preview of micro)
- Community health nursing often addresses environmental exposures—pollution, heat, housing—as determinants of health
NEX Application Tips
- Label population vs community vs ecosystem from short descriptions
- Trace arrows in food chains from producer toward top consumer; energy originates with producers (usually sunlight-driven)
- Remember energy is lost as heat; matter is recycled
- Match symbiosis type by who benefits/harmed
- Link human activities to specific disruptions (habitat loss → extinction risk; fertilizer → nitrogen-driven algal blooms)
Ecology questions reward precise vocabulary and simple energy/matter logic. Build mental pictures of a pond or forest, then walk through trophic levels, K, interactions, and cycles until the terms feel automatic.
Which ecological level includes all living populations in an area plus abiotic factors such as sunlight and soil nutrients?
Why do energy pyramids typically narrow at higher trophic levels?
Nitrogen-fixing bacteria are important in ecosystems because they