Humans, songbirds, and oysters die in completely different rhythms. Most humans survive childhood and old age claims us all at once, late. Songbirds face a roughly constant chance of death every year. An oyster releases millions of larvae, and almost all die in the first days — but the rare survivor may live for decades. Plot survivors against age and you get three signature shapes that ecologists use as a fingerprint of a species' life strategy. This lesson reads those survivorship curves, connects them to the r/K strategies from Lesson 7, and adds the vocabulary of biotic potential, dispersion, and the factors that decide how fast a population can actually grow.
A survivorship curve plots the number (or proportion) of individuals from a cohort still alive at each age, usually on a logarithmic y-axis. Three types:
[DIAGRAM: Survivorship curves on a log y-axis (number surviving) vs. age (x). Type I = high flat line that drops steeply at old age (concave down). Type II = straight diagonal line (constant rate). Type III = steep early drop that flattens (concave up). Label I = elephant/human (K), III = oyster/insect (r).]
Connecting to r/K: Type I ↔ K-selected (few offspring, high care, long life); Type III ↔ r-selected (many offspring, no care, high early death). Type II is intermediate.
Biotic potential is the maximum reproductive rate of a population under ideal conditions (unlimited resources). It's influenced by: - Number of offspring per reproductive event - Age at first reproduction (earlier = faster growth) - Frequency of reproduction (how often) - Reproductive lifespan and survival of offspring to reproductive age
Species with high biotic potential (r-selected, Type III) can rebound quickly from crashes and colonize fast. Species with low biotic potential (K-selected, Type I) recover slowly and are vulnerable to overharvesting because they can't replace losses fast enough.
Environmental resistance (all the limiting factors — competition, predation, disease, resource limits) opposes biotic potential. The realized growth is the balance: high biotic potential − environmental resistance → actual population change. Carrying capacity is essentially where environmental resistance fully offsets biotic potential.
How individuals are spread across space: - Clumped (most common) — individuals cluster where resources are patchy or for social/protective reasons (herds, schools, plants near water). - Uniform — evenly spaced, usually from competition or territoriality (nesting birds, allelopathic plants). - Random — no pattern; rare, seen when resources are uniform and there's no strong interaction (some wind-dispersed plants).
As in Lesson 7: - Density-dependent factors intensify with crowding: competition, predation, disease, parasitism, waste/stress. - Density-independent factors act regardless of density: weather, natural disasters, temperature, human habitat destruction.
Predator–prey cycles are a classic density-dependent dynamic: prey rise → predators rise (more food) → prey fall (heavy predation) → predators fall (less food) — producing linked oscillations (e.g., lynx and snowshoe hare).
Survivorship-curve identification is a near-guaranteed data-interpretation item, and it ties directly to r/K strategy and to conservation (why slow-reproducing K-species like whales and sharks are so vulnerable to overharvest). Age structure previews Lesson 9's human demographics.
A species shows very high mortality among juveniles but survivors live long. Which survivorship curve, and is it r- or K-selected?
Solution: Type III (early loss); it's r-selected — many offspring, little parental care, high early death.
Interpretation: Early cliff on the curve = Type III = r-strategist.
Explain why a whale population recovers from overhunting far more slowly than a herring population.
Solution: Whales are K-selected with low biotic potential — few offspring, late maturity, long gaps between births — so they replace losses slowly. Herring are r-selected with high biotic potential — many eggs, early maturity — so they rebound quickly. Low biotic potential makes whales far more vulnerable to overharvesting.
Interpretation: Reproductive rate governs recovery speed; slow reproducers stay endangered longer.
Lynx and snowshoe hare populations oscillate, with lynx peaks lagging hare peaks. Explain the mechanism and classify the limiting factor.
Solution: Abundant hares provide food, so lynx numbers rise; heavy lynx predation then drives hares down; with less food, lynx numbers fall; reduced predation lets hares recover — and the cycle repeats. Predation is a density-dependent limiting factor, so lynx peaks lag hare peaks.
Interpretation: Coupled oscillations with a lag = classic density-dependent predator–prey dynamic.
Desert shrubs are often evenly spaced. What dispersion pattern is this, and what causes it?
Solution: Uniform dispersion, caused by competition for scarce water (and sometimes chemical inhibition/allelopathy) that spaces plants out to reduce overlap of root zones.
Interpretation: Uniform spacing usually signals competition or territoriality.
(C) Drought is density-independent. Others are density-dependent.
Type I: few offspring, high parental care, mortality concentrated in old age (e.g., humans/elephants). Type III: many offspring, little/no parental care, very high early (juvenile) mortality (e.g., oysters/insects). They are near-opposite life strategies (K vs. r).
(a) Type III (huge early loss, then slow decline). (b) r-selected. (c) With high biotic potential, the species can rebound quickly from a crash by producing many offspring.
FRQ rubric (10 pts): - (a) 1 pt mark/track a cohort born the same time; 1 pt record the number surviving at successive ages/intervals; 1 pt plot proportion surviving vs. age (log scale) to form the curve. (3) - (b) 1 pt IV = presence/level of nest predation (e.g., protected vs. unprotected nests); 1 pt DV = juvenile/egg survival rate. (2) - (c) 1 pt Type III; 1 pt because many eggs + no parental care → very high early mortality. (2) - (d) 1 pt density-dependent (predation intensifies as nest density rises; acceptable if justified). (1) - (e) 1 pt names action (nest cages/protection, predator management, head-starting hatchlings, habitat protection); 1 pt justification tied to raising juvenile survival. (2)
Researchers want to determine the survivorship pattern of a local turtle species and whether nest predation is a key limiting factor.
(a) Describe how a cohort life-table study could be designed to build a survivorship curve. (3 pts) (b) Identify the independent and dependent variables for testing the effect of nest predation. (2 pts) (c) Predict the survivorship curve type for a turtle that lays many eggs with no parental care, and justify. (2 pts) (d) Classify nest predation as density-dependent or density-independent and explain. (1 pt) (e) Propose one management action to raise juvenile survival and justify it. (2 pts)
MC: 1. (C) Type III = very high juvenile mortality. 2. (C) Elephant fits Type I. 3. (B) Straight diagonal (log scale) = Type II (constant rate). 4. (B) Biotic potential = max reproductive rate under ideal conditions. 5. (A) K-selected → Type I. 6. (B) Low biotic potential → slow recovery. 7. (B) Uniform dispersion from territoriality. 8. (B) Predator peaks lag prey peaks. 9. (C) Clumped is most common. 10. (C) Drought is density-independent. Others are density-dependent.
Type I: few offspring, high parental care, mortality concentrated in old age (e.g., humans/elephants). Type III: many offspring, little/no parental care, very high early (juvenile) mortality (e.g., oysters/insects). They are near-opposite life strategies (K vs. r).
(a) Type III (huge early loss, then slow decline). (b) r-selected. (c) With high biotic potential, the species can rebound quickly from a crash by producing many offspring.
FRQ rubric (10 pts): - (a) 1 pt mark/track a cohort born the same time; 1 pt record the number surviving at successive ages/intervals; 1 pt plot proportion surviving vs. age (log scale) to form the curve. (3) - (b) 1 pt IV = presence/level of nest predation (e.g., protected vs. unprotected nests); 1 pt DV = juvenile/egg survival rate. (2) - (c) 1 pt Type III; 1 pt because many eggs + no parental care → very high early mortality. (2) - (d) 1 pt density-dependent (predation intensifies as nest density rises; acceptable if justified). (1) - (e) 1 pt names action (nest cages/protection, predator management, head-starting hatchlings, habitat protection); 1 pt justification tied to raising juvenile survival. (2)
⭐ Exam strategy: When a survivorship curve appears, check the axis (usually log) and where the big drop is — start (Type III, r-selected) or end (Type I, K-selected). Then connect it to recovery ability: r-species bounce back fast; K-species stay vulnerable.
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