9.4 Organisms and Their Environment
Key Takeaways
- Population growth may be exponential when resources are unlimited but becomes logistic as density-dependent limits approach carrying capacity (K).
- Communities are structured by competition, predation, mutualism, commensalism, and parasitism; ecological succession replaces species sets over time after disturbance.
- Ecosystem energy flows one way from producers through consumers to decomposers, with major losses as heat at each trophic transfer; matter cycles (C, N, H₂O, P).
- Biomes are large climate-defined communities (tropical rainforest, savanna, desert, temperate forest, tundra, etc.); the Philippines’ tropical setting favors rainforest and coral-reef productivity patterns.
- Human impacts—deforestation, overfishing, pollution, climate change, and invasive species—alter carrying capacity, succession trajectories, and biogeochemical balances.
9.4 Organisms and Their Environment
Quick Answer: Ecology links individuals to populations, communities, and ecosystems. Energy flows and dissipates across trophic levels; nutrients cycle. Population growth hits carrying capacity under density-dependent limits, while species interactions and succession shape communities—patterns increasingly modified by human activity.
CEM’s Biology content area Organisms and Their Environment rewards clear definitions, graph literacy (growth curves), and cause–effect reasoning about energy, nutrients, and human impact—not exhaustive field natural history.
Levels of Ecological Organization
| Level | Definition |
|---|---|
| Organism | Individual living being |
| Population | Same species in an area that can interbreed |
| Community | Interacting populations of different species |
| Ecosystem | Community + abiotic environment (energy and nutrient exchanges) |
| Biome | Large-scale community type shaped mainly by climate |
| Biosphere | All life and life-supporting environments on Earth |
Abiotic factors: temperature, light, water, nutrients, pH, salinity, oxygen. Biotic factors: other organisms as competitors, predators, prey, symbionts, pathogens.
Population Ecology
Density, distribution, and vital rates
Population density is individuals per unit area/volume. Dispersion may be clumped (most common), uniform, or random. Change in population size depends on births, deaths, immigration, and emigration.
Growth models
Exponential growth (J-shaped curve) assumes unlimited resources: dN/dt = rN, where r is the intrinsic rate of increase. Real populations may approximate this briefly (invasive species, rebounds after catastrophe, lab cultures early on).
Logistic growth (S-shaped curve) incorporates carrying capacity (K)—the maximum population size the environment can sustainably support:
dN/dt = rN (K − N)/K
Growth slows as N approaches K because of density-dependent factors: competition for food/space, disease transmission, predation pressure that intensifies with density, waste accumulation. Density-independent factors (severe storms, volcanic ash, many human catastrophes) hit populations regardless of density.
| Concept | NMAT cue |
|---|---|
| r-selected tendencies | Many offspring, little parental care, rapid colonization |
| K-selected tendencies | Fewer offspring, more parental investment, competitive environments near K |
| Survivorship curves | Type I (high early survival, e.g., humans), Type II (constant risk), Type III (high early mortality, many invertebrates/plants) |
Human demography adds age structure and fertility rates; expanding pyramids suggest rapid growth potential—useful Social Science crossover, but Biology items stay closer to logistic/exponential contrasts.
Communities and Species Interactions
| Interaction | Effect on species A | Effect on species B | Example |
|---|---|---|---|
| Competition | − | − | Two reef fish species limited by nesting sites |
| Predation | + (predator) | − (prey) | Grouper eating smaller fish |
| Herbivory | + | − | Insect feeding on rice leaves |
| Parasitism | + | − | Plasmodium in human (malaria) |
| Mutualism | + | + | Mycorrhizae–plant; cleaner fish–client fish |
| Commensalism | + | 0 | Epiphyte on tree (approx.; sometimes debated) |
Symbiosis broadly means intimate long-term association; exam stems may use it for mutualism only—read options carefully. Keystone species have outsized community impact relative to biomass (classic sea star or sea otter examples; conceptually, a top predator structuring a food web). Invasive species can restructure communities by lacking natural enemies.
Ecological succession
Succession is directional community change after disturbance or on new substrate:
- Primary succession — starts on bare rock/ash/sand without soil (lichens/mosses → herbs → shrubs → trees in many temperate sequences).
- Secondary succession — soil remains after fire, logging, abandoned fields; recovery is faster.
A climax community concept describes a relatively stable late-successional assemblage under a given climate, though modern ecology emphasizes patch dynamics and recurring disturbance—enough to know that early colonizers differ from late competitors.
Ecosystems: Energy Flow
Producers (autotrophs) capture energy (mostly photosynthesis; some chemosynthesis). Consumers eat living organisms; decomposers/detritivores recycle dead organic matter.
Energy transfer is inefficient: typically on the order of ~10% of energy passes to the next trophic level (rule of thumb; actual values vary). Consequences:
- Food chains are short.
- Biomass pyramids usually narrow upward.
- Top consumers need large base productivity.
Food chains are linear paths; food webs are interconnected networks more realistic for NMAT diagrams. Trophic categories: primary producers → primary consumers (herbivores) → secondary/tertiary consumers (carnivores/omnivores) → decomposers at all levels of dead matter.
Gross primary productivity (GPP) is total photosynthesis; net primary productivity (NPP) = GPP − producer respiration—the energy available to consumers. Warm, wet, nutrient-rich systems (tropical rainforests, estuaries, coral reefs with symbiotic algae) often show high NPP; open ocean can be low per area despite huge global contribution by volume.
Biogeochemical Cycles (Overview)
Matter cycles; energy does not.
| Cycle | Key reservoirs / processes |
|---|---|
| Water | Evaporation, condensation, precipitation, runoff, transpiration, groundwater |
| Carbon | CO₂ in atmosphere/ocean, photosynthesis/respiration, fossil fuels, sedimentation; human combustion raises atmospheric CO₂ |
| Nitrogen | N₂ fixation (bacteria, industrial), nitrification, assimilation, ammonification, denitrification; fertilizers can cause eutrophication |
| Phosphorus | Rocks/minerals, slow weathering, no major atmospheric gas phase; often limiting in freshwater; runoff → algal blooms |
Eutrophication: excess N/P → algal blooms → death/decomposition → hypoxia/dead zones. Coastal and lake examples matter for environmental health questions.
Biomes (High-Level)
Climate (temperature + precipitation patterns) sets biome type:
| Biome | Climate sketch | Biota sketch |
|---|---|---|
| Tropical rainforest | Warm, high rainfall year-round | Highest terrestrial biodiversity; layered canopy |
| Tropical dry forest / savanna | Warm; seasonal rain | Grasses, scattered trees; large herbivores in classic savannas |
| Desert | Low precipitation | Sparse vegetation; water-conserving adaptations |
| Temperate grassland | Seasonal temperature; moderate rain | Fertile soils; grasses |
| Temperate deciduous forest | Seasonal; adequate rain | Broadleaf trees that shed leaves |
| Boreal (taiga) | Cold; moderate precip | Conifers |
| Tundra | Very cold; low precip; permafrost | Low shrubs, lichens, migratory fauna |
| Aquatic systems | Marine vs freshwater; photic zones, currents, nutrients | Plankton base; coral reefs, mangroves, open ocean, lakes, rivers |
Philippine-relevant framing (appropriate, not touristy)
The Philippines is a tropical archipelago with rainforest remnants, extensive coastal mangroves, seagrass beds, and coral reefs that support fisheries and coastal protection. Typhoons, volcanic soils, and high endemism shape ecology. Pressures include deforestation, destructive fishing, plastic and nutrient pollution, and climate-driven coral bleaching risk. Mangroves and reefs also buffer storm surge—linking ecosystem services to human well-being, a concept NMAT may test indirectly via “role of producers” or “human impact” stems.
Human Impact Themes
| Pressure | Ecological effect |
|---|---|
| Habitat destruction/fragmentation | Lower K, edge effects, biodiversity loss |
| Overexploitation | Population crashes, trophic cascades |
| Pollution | Toxicity, eutrophication, bioaccumulation/biomagnification up food chains |
| Invasive species | Competition, predation on natives |
| Climate change | Range shifts, phenology mismatches, coral bleaching, extreme weather |
Bioaccumulation is buildup in an individual over time; biomagnification is increasing concentration at higher trophic levels (classic lipophilic toxins). Conservation strategies—protected areas, sustainable harvest rules, restoration (including mangrove reforestation)—aim to keep populations below damaging exploitation rates and preserve community structure.
NMAT Strategy for Ecology Items
- Identify the level (population vs community vs ecosystem).
- For graphs: J-curve ≈ exponential; S-curve ≈ logistic near K.
- For energy: arrows show direction of energy flow; decomposers recycle matter but energy still exits as heat.
- For interactions: assign +/+/− signs before picking the named relationship.
- Prefer mechanistic answers (e.g., “nutrient runoff causes algal bloom and hypoxia”) over vague “pollution is bad” options.
Ecology closes the organismal arc of NMAT Biology: cells and genes build organisms; development shapes them; homeostasis maintains them; populations and ecosystems place them in energy and nutrient networks that medicine and public health continually intersect.
In the logistic growth model, carrying capacity (K) is best defined as:
A + / + interaction in which both species benefit, such as mycorrhizal fungi and plant roots exchanging nutrients, is called:
Why are food chains typically limited to only a few trophic levels?
Secondary succession differs from primary succession primarily because secondary succession: