Cyclic Population Fluctuations in Ecology

Natural Fluctuations

• Cyclic population fluctuations occur in laboratory populations, and can be affected by factors such as delays in development stages and changes in adult food abundance.
• Example: Experiment on the Sheep Blowfly (Lucilia C.) - STEP 1, where the larvae are fed a fixed amount of food (K is defined) and the adults are fed an unlimited amount, resulting in low irregular cycles in population fluctuations.
• In another experiment, when the adults are fed a limited amount of food, the population crashes and the larvae population increases.

Consumer-Resource population cycles

• Consumer-resource population cycles occur due to time delays between the consumer and resource populations.
• Case study: "Lynx" vs "Snowshoe Hare" - trapping records from Hudson Bay company show a natural population fluctuation with a cycle period of 9-10 years, with the lynx cycles lagging 2 years behind the hare cycles.
• Other Canadian large herbivores and their predators follow similar patterns with connected delays, with medium-sized animals fluctuating with a 9-10 year cycle period and small-sized animals fluctuating with a 4-year cycle period.
• Understanding consumer-resource cycles requires examining them in the context of population models, which can help identify the mechanisms underlying the cycles.

Creating artificial predator-prey cycles

• Experiment on species of mites of citrus: Typhlodromus (predator) and Six-spotted mite (prey) - prey grows well on trays with few rubber balls, but when predators are added, the prey population declines, and eventually, both species go extinct.
• On trays with more rubber balls, the prey population declines more slowly, but both species still go extinct eventually.
• If predator dispersal is reduced, both species can coexist better over time.

Mobility of the mites

• Predatory mites disperse by walking, while prey mites use a silk line to float on wind currents.
• Slowing down the predators by creating a maze-like pattern of Vaseline barriers among oranges, or enhancing prey escape by installing vertical wooden pegs, can lead to a predator-prey cycle.

Mathematical models for predator-prey cycles

• Lotka-Voltera model: incorporates oscillations in predator and prey populations, with the predator numbers always lagging behind the prey's.
• The model consists of two equations: one for the prey population change rate and one for the predator population change rate.
• The model can be used to predict the response of one population to changes in the other.

Cycling in the Lotka-Voltera model

• Equilibrium conditions are defined by a zero change in the populations, and are represented by isoclines.
• The joint population trajectory is a graph that combines the prey and predator isoclines, and shows the cycling behavior of the populations.
• The cycling starts when the populations deviate from the center, and the direction is counter-clockwise.

Functional and Numerical responses

• Functional response: the relationship between the density of the prey and the rate of consumption by the predator.
• There are three types of functional responses: Type I, Type II, and Type III, each with different characteristics.
• Type III response is characterized by low prey consumption when prey is low, rapid consumption when prey density is moderate, and slow consumption when prey density is high.

Delayed density dependence

• Delayed density dependence occurs due to time delays for development stages, and is related to the shift in life history stages (e.g., larvae to adult).
• Example: Experiment on the Water Flea (Bosmina L.) - when the population approaches K, the numbers decrease rapidly, and there is no large oscillation observed.

This quiz covers cyclic population fluctuations, including laboratory populations and natural fluctuations. It explores the relationship between larval food, eggs, and adult populations.