Population cycles & predator–prey dynamics

1) Why some populations “cycle” instead of just growing logistically

Most real populations don’t grow smoothly. They overshoot their resources, then crash, then recover—creating a repeating cycle. The engine of the cycle is:

  • Delayed density dependence (feedback comes later, not instantly), plus

  • Density-independent shocks (e.g., unusual winters) that can amplify or reset the cycle.

Because the feedback is delayed, the population can keep rising past carrying capacity (K) before the “brakes” kick in, so it falls too far, then rises again—like a pendulum.

2) Tent caterpillars: why outbreaks, then sudden collapses?

Facts from the unit: very high fecundity (≈300 eggs per clutch), outbreaks about every ~10 years, peaks last ~2–4 years, followed by long lows.

Mechanisms behind the rise → peak → crash → low:

  • Bottom-up (food supply): Caterpillars defoliate huge areas. After peak feeding, trees (e.g., aspen in your text’s example) have less leaf biomass and often poorer quality leaves (trees can also ramp up defensive chemicals and need time to regrow). Result: starvation, slower growth, lower survival → crash.

  • Natural enemies (density-dependent): When caterpillars are abundant, their parasitoids/pathogens (wasps, flies, viruses) also build up (a “numerical response”). After a short lag, these enemies can wipe out large fractions of the population.

  • Weather (density-independent): A badly timed frost, cold spring, or heat wave can hammer eggs/larvae regardless of how many there are, accelerating the crash or prolonging the low.

  • The delay matters: Leaves don’t instantly regrow, enemies don’t instantly disappear—so recovery takes years, setting a ~10-year rhythm.

Story in four phases (good to remember for tests):
Increase → Peak (defoliation) → Crash (food poor + many enemies) → Low (trees recover; enemies drop) → repeat.

3) Lynx–hare cycles: why both species rise and fall together (with a lag)

Observed pattern (Hudson’s Bay pelt records): ~10-year cycles; hares peak first, lynx peak ~1–2 years later; both then crash.

Two primary drivers (as your unit states): Food supply and predation—plus an important time lag.

A) Food supply (bottom-up control)

  • Hares eat vegetation (your unit highlights white spruce needles; the key point is preferred foods are limited and slow to regrow in cold climates).

  • When hares are many, they overbrowse: less food per hare, and lower quality food (plants under stress often have tougher tissues/defenses).

  • Consequences for hares: lighter body mass, smaller litters, higher juvenile mortality → the hare population starts to decline.

B) Predation (top-down control)

  • When hares boom, lynx show two responses:

    • Functional response: each lynx can catch more hares per day when hares are plentiful.

    • Numerical response: with lots to eat, lynx reproduce more and young survive better; plus lynx move into high-hare areas.

  • Time lag: Lynx numbers depend on last year’s hare abundance (it takes time to breed and for kittens to grow). That’s why the lynx peak lags the hare peak by ~1–2 years.

C) The crash and the lag together

  • Once hares decline (food depletion + stress + predation), lynx are still numerous for a short while, so predation pressure stays high and hares fall even further.

  • With few hares left, lynx then crash (not enough food).

  • During the low phase, vegetation recovers, hare reproduction improves, and the cycle restarts.

D) Extra piece students often miss: stress effects

Even before hares actually starve, chronic predation risk (being chased a lot) can elevate stress hormones in hares, which reduces reproduction (smaller litters) for multiple years. This prolongs the crash and helps synchronize cycles over large regions—alongside weather (density-independent) that affects many populations at once.

4) Density-dependent vs. density-independent in the cycles

  • Density-dependent (gets worse with crowding): competition for food/nest sites, disease/parasites, predation pressure (predators focus on abundant prey), aggression/stress.

  • Density-independent (hits regardless of how many there are): cold snaps, heat waves, drought, fires, unusual winters.

The cycles emerge from their interaction: delayed, density-dependent feedbacks create the oscillation; density-independent events modulate the height and timing of peaks and troughs.

5) How to answer exam questions cleanly

Template (use this):

  1. State the pattern: “Populations of X and Y show ~10-year cycles; Y (predator) lags X (prey) by ~1–2 years.”

  2. Name the drivers: “Driven by a combination of bottom-up (food supply) and top-down (predation) forces, with delayed density dependence.”

  3. Explain the lag: “Predators respond numerically after prey peaks (reproduction/migration), so predator peaks come later.”

  4. Mention density-independent modifiers: “Weather can amplify or dampen peaks regardless of density.”

  5. (Optional depth) Stress/disease: “Risk-induced stress and pathogens further depress reproduction/survival during and after peaks.”

One-paragraph model:

The hare–lynx cycle arises from delayed density-dependent feedbacks in both food supply and predation. As hares increase, they overuse limited, slow-regrowing food, reducing food quantity/quality and lowering hare reproduction and survival (bottom-up control). Abundant hares simultaneously trigger stronger predation: lynx eat more per capita and, with a lag of ~1–2 years, increase in number via reproduction and immigration (top-down control). When hares start to decline, lynx remain temporarily abundant, prolonging high predation and deepening the hare crash; lynx then decline due to food shortage. Weather (density-independent) and stress/disease further modulate the amplitude and duration of peaks and troughs, producing the ~10-year cycles.

6) Quick self-check (2 items)

  • Why do lynx peaks lag hare peaks by ~1–2 years?
    Because lynx numbers respond to past hare abundance (time needed for breeding and juvenile survival), not today’s, creating a delayed numerical response.

  • Give one bottom-up and one top-down factor that help cause the hare crash.
    Bottom-up: depleted/low-quality food after overbrowsing.
    Top-down: high predation pressure because lynx are still abundant.