5.2 Population & Community Ecology

From Genes to Numbers

Where section 5.1 tracked alleles, population ecology tracks how many individuals there are and why the number rises or falls. The diploma exam tests growth curves, the meaning of carrying capacity, the factors that limit growth, and the ways species interact within a community (all the populations sharing a habitat). Expect both multiple-choice interpretation of graphs and numerical-response growth-rate questions.

Exponential vs Logistic Growth

Exponential growth occurs when resources are unlimited: the population grows by a constant proportion each interval, producing a steep J-shaped curve. It is unsustainable and seen in invasions, lab cultures, or recovering populations.

Logistic growth is realistic: growth is rapid at first but slows as resources become scarce, producing an S-shaped (sigmoid) curve. The population levels off at the carrying capacity (K) — the maximum number the environment can support indefinitely. At K, birth rate equals death rate and the growth rate approaches zero. Populations may briefly overshoot K, then fall back toward it.

Carrying Capacity (K)

Carrying capacity (K) is set by limiting resources — food, water, space, nesting sites. Key facts the exam expects:

• Below K, per-capita growth is positive and the population rises.
• As the population approaches K, growth slows because resources are shared among more individuals.
• At K, the population stabilizes (growth rate ≈ 0); the S-curve flattens.
• K is not fixed — it shifts if resources, predation, or disease change.

Reading a graph: the horizontal asymptote of a logistic curve marks K.

Calculating Growth Rate

Numerical-response questions often ask for a change in population or a growth rate. The population change over a period is:

Change = (births + immigration) − (deaths + emigration)

The per-capita growth rate is usually written as a percent of the starting population. For example, a herd of 200 caribou has 40 births and 16 deaths in a year (ignore migration):

• Net change = 40 − 16 = 24
• Growth rate = 24 / 200 = 0.12 = 12% per year

A positive rate means the population is growing, zero means it is stable (as at K), and a negative rate means it is declining. Read whether the question wants a count or a percent.

r-selected vs K-selected Species

Reproductive strategies fall on a continuum:

r-selected species exploit unstable environments with rapid reproduction; K-selected species succeed in stable environments by investing in fewer, competitive offspring.

Density-Dependent vs Density-Independent Factors

Limiting factors determine how a population approaches K:

• Density-dependent factors intensify as the population becomes more crowded: competition for food, predation, disease/parasites, and accumulation of waste. These factors push the population toward K and create the logistic plateau.
• Density-independent factors affect the population regardless of its size: weather extremes, drought, fire, floods, and volcanic eruptions.

Exam tip: if the effect gets stronger when the population is large, it is density-dependent; if a flood kills the same proportion no matter the density, it is density-independent.

Survivorship Curves

Survivorship curves plot the proportion of a cohort still alive against age:

• Type I — low youth mortality, most deaths late in life (humans, large mammals; K-selected).
• Type II — constant death rate at all ages (many birds, rodents).
• Type III — very high early mortality, few survive to adulthood (oysters, many fish/insects; r-selected).

These link directly to reproductive strategy: high parental care (Type I) goes with K-selection; huge numbers of unprotected offspring (Type III) go with r-selection.

Community Interactions and Symbiosis

Within a community, populations interact:

• Competition — two species use the same limited resource; the harm is mutual (−/−).
• Predation — one organism (predator) kills and eats another (prey); predator–prey cycles are density-dependent.
• Symbiosis — a close, long-term relationship between species:
  • Mutualism (+/+): both benefit, e.g. pollinators and flowers.
  • Commensalism (+/0): one benefits, the other is unaffected, e.g. barnacles on a whale.
  • Parasitism (+/−): the parasite benefits while harming the host, e.g. tapeworms.

Note the sign pattern — the diploma often tests whether you can classify a described relationship.

Ecological Succession

Ecological succession is the gradual, predictable change in a community over time toward a relatively stable climax community.

• Primary succession begins on bare, lifeless substrate with no soil — bare rock after a glacier, cooled lava. Pioneer species such as lichens and mosses break down rock to build the first soil; it is slow.
• Secondary succession occurs where a disturbance (fire, flood, abandoned farmland) cleared the community but soil remains. Because soil and seeds survive, recovery is much faster.

The presence or absence of existing soil is the single feature that distinguishes the two types.

Predator-Prey Cycles

Predation is a key density-dependent interaction, and predator and prey populations often oscillate in linked cycles (the classic lynx and snowshoe hare data):

1. Abundant prey → predators have plenty of food → predator numbers rise.
2. More predators → heavier predation → prey numbers fall.
3. Scarce prey → predators starve → predator numbers fall.
4. Fewer predators → prey recover → prey numbers rise, restarting the cycle.

The predator peak lags behind the prey peak because predators respond after prey become abundant. These cycles help keep both populations near their carrying capacities and illustrate how community interactions, not just resources, regulate population size.