Key Takeaways
- Ecology is the study of interactions between organisms and their environment; an ecosystem includes both biotic (living) and abiotic (nonliving) components
- Energy flows through ecosystems in food chains: producers → primary consumers → secondary consumers → tertiary consumers → decomposers
- Only about 10% of energy is transferred between each trophic level (10% rule)
- Biogeochemical cycles (water, carbon, nitrogen) recycle matter through the ecosystem
- Natural selection is the mechanism of evolution: organisms with advantageous traits survive and reproduce more successfully
- Taxonomy classifies organisms from broadest to most specific: Domain, Kingdom, Phylum, Class, Order, Family, Genus, Species
- The three domains of life are Bacteria, Archaea, and Eukarya (which includes Protista, Fungi, Plantae, Animalia)
- Scientific naming uses binomial nomenclature: Genus species (e.g., Homo sapiens, Staphylococcus aureus)
Ecology, Evolution & Classification
The HESI A2 Biology section includes questions on ecology, evolution, and biological classification. These topics provide context for understanding microbiology, infectious disease, and the biological basis of nursing science.
Ecology
Ecology is the study of interactions between organisms and their environment.
Levels of Ecological Organization
| Level | Description | Example |
|---|---|---|
| Organism | A single living individual | One deer |
| Population | All organisms of one species in an area | All deer in a forest |
| Community | All populations (different species) in an area | Deer, wolves, trees, fungi in a forest |
| Ecosystem | Community + nonliving environment | Forest ecosystem (organisms + soil, water, climate) |
| Biome | Large region with similar climate and organisms | Temperate deciduous forest |
| Biosphere | All ecosystems on Earth | The entire planet |
Ecosystem Components
| Component | Description | Examples |
|---|---|---|
| Biotic factors | Living or once-living organisms | Plants, animals, bacteria, fungi, decomposing matter |
| Abiotic factors | Nonliving physical and chemical factors | Temperature, water, sunlight, soil, pH, oxygen |
Energy Flow in Ecosystems
Energy flows through ecosystems in food chains and food webs:
| Trophic Level | Role | Examples |
|---|---|---|
| Producers (autotrophs) | Make their own food via photosynthesis | Plants, algae, cyanobacteria |
| Primary consumers (herbivores) | Eat producers | Rabbits, deer, caterpillars |
| Secondary consumers (carnivores) | Eat primary consumers | Frogs, small birds, foxes |
| Tertiary consumers (top predators) | Eat secondary consumers | Hawks, wolves, sharks |
| Decomposers | Break down dead organisms, recycle nutrients | Bacteria, fungi, earthworms |
The 10% Rule: Only about 10% of the energy at one trophic level is transferred to the next. The remaining 90% is lost as heat through metabolic processes. This is why food chains rarely have more than 4-5 trophic levels.
Biogeochemical Cycles
Matter is recycled through ecosystems via biogeochemical cycles:
Water Cycle
- Evaporation — liquid water → water vapor
- Condensation — water vapor → clouds (liquid droplets)
- Precipitation — rain, snow, sleet, hail
- Runoff and infiltration — water returns to oceans, lakes, and groundwater
- Transpiration — water released from plant leaves
Carbon Cycle
- Producers absorb CO2 during photosynthesis
- Consumers release CO2 during cellular respiration
- Decomposers release CO2 by breaking down dead organisms
- Fossil fuels (coal, oil, gas) store carbon for millions of years
- Burning fossil fuels releases CO2 → contributes to the greenhouse effect
Nitrogen Cycle
- Atmospheric nitrogen (N2) is converted to usable forms by nitrogen-fixing bacteria
- Plants absorb nitrogen from soil as nitrates
- Animals obtain nitrogen by eating plants
- Decomposers return nitrogen to the soil
- Denitrifying bacteria convert nitrates back to N2 gas
Ecological Relationships
| Relationship | Description | Example |
|---|---|---|
| Mutualism | Both species benefit (+/+) | Bacteria in human gut (bacteria get food; host gets vitamins) |
| Commensalism | One benefits, other unaffected (+/0) | Birds nesting in trees |
| Parasitism | One benefits, other is harmed (+/-) | Tapeworm in human intestine |
| Predation | Predator kills and eats prey (+/-) | Wolf eating a rabbit |
| Competition | Both species compete for same resources (-/-) | Two plant species competing for sunlight |
Evolution and Natural Selection
Evolution is the change in the inherited characteristics of a population over successive generations.
Charles Darwin's Theory of Natural Selection:
- Variation — Individuals in a population have different traits (due to mutations and genetic recombination)
- Overproduction — More offspring are produced than can survive
- Competition — Organisms compete for limited resources
- Survival of the fittest — Individuals with advantageous traits are more likely to survive and reproduce
- Inheritance — Favorable traits are passed to offspring, becoming more common in the population over time
Evidence for Evolution:
- Fossil record — shows changes in organisms over time
- Comparative anatomy — homologous structures (same origin, different function: human arm, whale fin, bat wing)
- Embryology — similar embryonic development across species
- DNA/molecular evidence — similar DNA sequences indicate common ancestry
- Observed evolution — antibiotic-resistant bacteria (MRSA)
Nursing Relevance: Understanding evolution explains why antibiotic resistance develops — bacteria with resistance genes survive antibiotic treatment and reproduce, creating resistant strains. This is why proper antibiotic stewardship is critical.
Biological Classification (Taxonomy)
Taxonomy organizes living things into hierarchical categories:
| Level | Human Example | E. coli Example |
|---|---|---|
| Domain | Eukarya | Bacteria |
| Kingdom | Animalia | Bacteria |
| Phylum | Chordata | Proteobacteria |
| Class | Mammalia | Gammaproteobacteria |
| Order | Primates | Enterobacterales |
| Family | Hominidae | Enterobacteriaceae |
| Genus | Homo | Escherichia |
| Species | sapiens | coli |
Mnemonic: Dear King Philip Came Over For Good Spaghetti
Binomial Nomenclature
- Developed by Carl Linnaeus
- Every organism has a two-part scientific name: Genus species
- The genus is always capitalized; the species is always lowercase
- Both are italicized (or underlined when handwritten)
- Examples: Homo sapiens (humans), Staphylococcus aureus (MRSA), Escherichia coli (E. coli)
The Three Domains of Life
| Domain | Cell Type | Examples |
|---|---|---|
| Bacteria | Prokaryotic | E. coli, Streptococcus, Staphylococcus |
| Archaea | Prokaryotic | Extremophiles (thermophiles, halophiles) |
| Eukarya | Eukaryotic | Animals, plants, fungi, protists |
The four kingdoms within Eukarya:
- Protista — single-celled eukaryotes (amoeba, algae, malaria parasite)
- Fungi — multicellular decomposers (mushrooms, yeast, mold)
- Plantae — multicellular photosynthesizers (trees, flowers, ferns)
- Animalia — multicellular heterotrophs (humans, insects, fish)
According to the 10% rule, if producers capture 10,000 kcal of energy, how much energy is available to secondary consumers?
Natural selection requires all of the following EXCEPT:
The correct order of taxonomic classification from broadest to most specific is:
In the scientific name Staphylococcus aureus, "Staphylococcus" is the _____ and "aureus" is the species.
Type your answer below
Match each ecological relationship to its description.
Match each item on the left with the correct item on the right
Antibiotic resistance in bacteria is an example of:
Which of the following are abiotic factors in an ecosystem? (Select all that apply)
Select all that apply
Arrange the levels of ecological organization from simplest to most complex.
Arrange the items in the correct order