2.1 Individuals, populations, communities, and ecosystems

Organism → population → community → ecosystem → biosphere: each level adds relationships or physical context.

Use the smallest level that fits the question. A community contains interacting populations; an ecosystem adds abiotic environment; the biosphere contains all ecosystems.

One wolf is an organism; wolves of one species in one area are a population; wolves plus prey and soil form part of an ecosystem.

No. It is the total ecological system of life interacting with physical environments.

A biosphere is not just a list of organisms; it includes ecosystems and their physical interactions.

Under the biological species concept, members of one species can interbreed and produce fertile offspring.

Producing any offspring is not enough: fertility maintains gene flow between populations. The test is harder for fossils and asexual organisms, so other evidence may be needed.

Horses and donkeys can produce mules, but sterile mules do not maintain gene flow; horses and donkeys are classified as different species.

They fit the same biological species under this concept, assuming the populations can interbreed naturally.

Any offspring is not sufficient; fertility is the key condition.

Binomial nomenclature gives each species two words: capitalized genus, lower-case specific name, italicized in print.

The shared format avoids ambiguity from local common names and lets scientists compare organisms consistently.

Homo sapiens: Homo is the genus and sapiens the specific name.

Homo sapiens; only genus is capitalized.

Do not capitalize both words.

Identify an unknown organism by matching the tool to the evidence: dichotomous key for paired traits, field guide for recognizable local forms, and DNA comparison when appearance is insufficient.

Use diagnostic features and trusted references; an app is a starting aid, not automatic proof. Museums, herbaria and databases can verify uncertain results.

A leaf with clear paired features can be narrowed with a dichotomous key; a cryptic fungus may need DNA and reference specimens.

Check diagnostic traits or a trusted reference; do not accept the first output.

Tool output is not the same as reliable identification.

A population is made up of organisms from one species living in a defined area at a defined time. This definition makes a population count meaningful: we know exactly what has been counted.

A population has three boundaries:

• One species — the organisms can potentially interbreed.
• One area — the counting boundary is stated.
• One time — the survey refers to a defined moment or period.

If one boundary changes, you may be describing a different population.

A Tuesday survey finds 18 mallards and 6 Canada geese in one pond. The place and date are the same, but the species are different. Therefore, this describes two populations, not one population of 24 birds.

The same rule explains why “20 frogs were counted this month” is incomplete. The report must also state the frog species, exact area and survey window before the number can be compared with another count.

Key idea: same species + defined area + defined time.

Sharing a place does not make organisms one population. The species, area and time boundaries all matter.

Abiotic factors are non-living conditions such as temperature, light, pH and water; biotic factors arise from living organisms such as competitors, predators, disease or food.

Classify the factor itself, then explain its effect. A biological response to pH does not make pH biotic.

A plant may be absent because acidic soil limits physiology, or because another plant outcompetes it in suitable soil.

No. Predation is an interaction between living organisms, so it is biotic.

Classify the factor, not the consequence.

Choose an abiotic measurement because it could explain the distribution: light for shading, moisture for water supply or pH for nutrient availability.

Use consistent positions, times, units and calibrated tools; repeat readings along the distribution gradient. Relevance matters more than collecting many variables.

To test whether moss prefers shade, measure light intensity at sites with and without moss rather than only recording air temperature.

Same method, calibration, unit and sampling conditions.

Many irrelevant measurements do not strengthen an investigation.

A habitat is where an organism lives; its niche is the conditions, resources, timing and interactions that describe how it survives and functions.

Use niche when explaining requirements or ecological role, not just location. One habitat can contain many niches.

A warbler’s habitat may be young jack-pine forest; its niche includes sandy soil, nesting timing, food and migration.

Habitat. Add feeding and competition to describe niche.

Habitat answers where; niche includes role and requirements.

Mutualism is +/+, competition −/−, and predation, herbivory, parasitism and disease generally harm one partner; the mechanism predicts population and selection effects.

Translate label to fitness: food loss, injury, infection or benefit. Then ask which traits are favoured over time.

Predators lower prey survival; prey defenses become more valuable, while parasites may favour host resistance without rapidly killing the host.

No. The parasite benefits while the living host is harmed.

Parasitism is not the same as predation; a living host is normally maintained.

Apply each relationship with a named mechanism: grazing deer reduce plant biomass (herbivory, +/−); sea stars consume mussels and can limit mussel abundance (predation, +/−); ticks gain food while reducing host fitness (parasitism, +/−); flowering plants gain pollination while bees gain food (mutualism, +/+); a transmissible pathogen can lower host survival and spread faster at high density (disease, pathogen +/host −); and two plant species using the same limited light or nutrients can reduce each other's growth (competition, −/−). Then predict population feedback and selection, such as defence, resistance, resource partitioning or traits that improve capture or cooperation.

Carrying capacity is the maximum population or load an environment can support sustainably under current limiting conditions.

Food, water, space and nesting sites set the limit; change the conditions and K changes. A temporary overshoot is not sustainable capacity.

Deer rise above winter food supply, then mortality increases and the population falls below the temporary peak.

Yes, if it increases a limiting resource; K is conditional, not permanent.

K is not a fixed number independent of conditions.

A density-dependent factor strengthens as population density rises, reducing growth and creating negative feedback around carrying capacity.

Crowding increases competition or disease transmission; births fall or deaths rise. When density falls, pressure weakens and growth can recover.

More deer sharing winter food increases competition, lowers survival and pulls the population back toward K.

Usually no; its effect is not produced by crowding, so it is density-independent.

A disturbance that reduces numbers is not automatically density-dependent.

J-shaped growth shows accelerating exponential increase; S-shaped growth slows toward K; boom–bust growth overshoots resources and crashes.

Link curve shape to limiting factors. A curve near K still has births and deaths; the net change is small because gains and losses balance.

Reindeer rise while lichen is abundant, deplete it, then crash after a severe winter: a boom–bust pattern.

No. It means net growth is near zero, with births/immigration balanced by deaths/emigration.

Leveling off is not biological stillness.

Agriculture, sanitation, medicine, transport and trade can weaken local limiting factors, but they do not remove global resource and waste limits.

Trace the change: resource import or health improvement raises local support, then check extraction, consumption and ecosystem degradation elsewhere.

Imported food lets a city support more people than local farms could, while the water and land impacts occur in distant regions.

No. It relocates the resource demand and may postpone the local limit.

Lowering a local limit is not the same as removing a planetary limit.

Human carrying capacity is conditional on diet, consumption, technology, trade, efficiency, spatial scale and environmental degradation.

State the lifestyle and time horizon before estimating. Efficiency may expand access, but rebound in total use and depleted natural capital can reduce the gain.

Two cities of equal population need different land and energy capacity if one imports food and consumes more per person.

No. It depends on stated assumptions about lifestyle, technology, scale and time.

Technology cannot expand carrying capacity indefinitely without natural-capital limits.

Use random sampling for equal selection chance, systematic sampling for even coverage, and transects when abundance changes along a gradient.

Choose from the ecological question, organism mobility and spatial pattern. Fixed intervals can bias results if they align with a repeating habitat pattern.

Use a belt transect to measure plants from shore to dune; use random quadrats to estimate a uniform meadow.

A line or belt transect, because the gradient is the question.

Systematic is not automatically unbiased if the interval matches a repeating pattern.

For non-mobile organisms, random quadrats estimate abundance from mean count × total area ÷ quadrat area.

Use counts for population size; percentage cover and frequency describe occupancy patterns, not direct individual totals.

Mean 4 plants per 1 m² quadrat across a 100 m² meadow estimates 400 plants.

Not directly; frequency tells how many quadrats contain the species, not the number per quadrat.

Do not multiply percentage cover by area and call it population size.

Lincoln estimate N = M × C ÷ R, where M is marked first capture, C total second capture and R marked recaptures.

Assume a closed population, no mark loss or effect, mixing and equal capture chance. Low R makes the estimate unstable.

Mark 40, catch 50 later and recapture 10 marked: N = 40×50÷10 = 200.

The estimate rises to 400; a low recapture count makes it sensitive to small sampling changes.

C is the total second sample; R is only marked recaptures.

A community is all interacting populations living in an area at the same time; abiotic conditions belong to the ecosystem around it.

Include multiple species populations and their feeding, competition or decomposition links. Add water chemistry and sunlight only when defining the larger ecosystem.

Algae, insects, fish and microbes in a pond form a community through feeding and decomposition.

No; water is abiotic, so it belongs to the ecosystem, not the community.

A community contains populations, not the abiotic environment.

A habitat is the location in which an organism, population, species or community lives.

Use habitat for place and niche for how the organism survives and interacts. One habitat can contain many niches.

A rocky intertidal shore is a barnacle habitat; tolerance of exposure and competition describe its niche.

Niche; it describes behaviour and role, not location.

Habitat answers where; it does not describe ecological role.

An ecosystem links a community with its abiotic environment, and energy and matter cross its boundary.

Sunlight enters, heat leaves, and water, gases, nutrients and organisms can cross. Drawing a boundary helps analysis but does not isolate the ecosystem from neighbours.

A forest receives rain and sunlight, exports heat and dissolved nutrients, and exchanges animals with surrounding habitat.

No. Matter can cycle internally and also enter or leave the ecosystem.

‘Cycles’ does not mean the ecosystem is closed.

Ecosystem sustainability can appear as dynamic steady state: inputs and outputs balance over time while stores, populations and flows keep changing.

Look for a stable range, regeneration and feedback after disturbance, not frozen numbers. Individuals are born and die within the balance.

A forest’s biomass stays near an average while births, deaths and nutrient flows continue.

No; sustainable function can persist while individual populations fluctuate within limits.

Steady state is dynamic balance, not a frozen ecosystem.

A tipping shift occurs when disturbance plus reinforcing feedback pushes an ecosystem beyond recovery to its previous stable state.

Show threshold → feedback → persistent new conditions. In deforestation, less transpiration lowers rainfall, raising fire and mortality risk and causing more tree loss.

A forest becomes drier after tree loss; fires then remove more trees, shifting toward a different equilibrium.

No. Show self-reinforcing change and a persistent shift toward another state.

Large disturbance alone does not prove a tipping point.

A keystone species has an effect on community structure much larger than its abundance would suggest.

Ask what changes when the species is removed: trophic control or habitat engineering can reorganize many other populations.

Sea stars limit mussel dominance and retain intertidal diversity; elephants remove woody vegetation and maintain grassland patches.

No. Test the disproportionate consequence of removal.

Keystone status is about effect size, not body size or abundance.

Biosphere integrity concerns diversity and ecosystem function; extinction rates and population declines indicate pressure beyond a safe planetary boundary.

Use both evolutionary diversity and functional capacity. Boundary crossing signals rising systemic risk, not synchronized collapse of every ecosystem.

Rapid population loss across pollinators can threaten ecosystem function even before every species is extinct.

No; it indicates elevated risk and need for pressure reduction.

A planetary boundary is not a simultaneous-collapse forecast.

Biosphere-integrity action works when it protects niche conditions—habitat, food, water, timing and interactions—not only the species name.

Trace pressure to reproduction and survival. Protecting a forest bird may require managing forest age and fire, not only preventing direct killing.

A bird returns to its nesting forest, but food insects emerge earlier; habitat protection alone misses the timing mismatch.

Monitor and protect food conditions and seasonal interactions, not only nesting space.

Species protection is not only preventing direct killing.

A clade contains an ancestor and all descendants; on a cladogram, the more recent shared node indicates closer inferred relatedness.

Compare branching nodes, not horizontal tip position or branch length unless the diagram explicitly scales them. New DNA evidence can change the hypothesis.

Taxa A and B sharing a more recent node are inferred closer relatives than A and C sharing an older node.

Not necessarily; compare their most recent common node.

Tip position and branch length do not automatically show relatedness.

Traditional ranks organize names, but rank labels do not measure equal evolutionary distance because lineages diverge unevenly.

Use domain-to-species ranks for communication, then use clades and evidence for evolutionary relationships.

Two families can have the same rank name while representing very different amounts of evolutionary divergence.

No; compare the actual cladogram or sequence evidence.

A taxonomic rank is useful for naming, not a ruler of evolutionary time.

The fundamental niche is the potential range without biotic restriction; the realized niche is the range actually occupied after competition, predation and other interactions.

Compare conditions with and without the competitor or predator. Biotic constraints can narrow the observed range.

Chthamalus barnacles survive lower on shore when a competitor is removed, so competition narrows their realized niche.

Fundamental niche; realized niche excludes the occupied range after competition.

Realized niche is not always the physiologically ideal range.

r-selected traits favour early, rapid reproduction with many small offspring; K-selected traits favour later reproduction, fewer offspring and greater investment near carrying capacity.

Use the continuum to predict boom–bust versus slower recovery. Real species combine traits; r and K are not value judgements.

A disturbed-site weed can produce many seeds quickly; a large mammal produces fewer young and recovers slowly after adult loss.

K-leaning species, because later maturity and fewer offspring slow replacement.

Species are not locked into two boxes; r–K is a comparative continuum.

Predict disturbance response by combining which population is affected, what conditions it needs and how quickly its life history can replace losses.

Trace disturbance → niche mismatch or loss → reproduction/survival response → recovery speed. Protecting one habitat feature may miss timing or food interactions.

Warming advances caterpillar emergence before a migratory bird breeds; a late-maturing bird then loses food synchrony and recovers slowly.

No; monitor timing and food interactions as well as space.

A habitat can remain while niche conditions or interactions become unsuitable.