13 – Exploitative Interactions

Exploitative interaction definition

An interaction that increases the fitness of one organism (consumer) while reducing the fitness of the other (resource).

Five main types of exploitation

1) Parasite/host

2) Pathogen/host

3) Parasitoid/host

4) Predator/prey

5) Herbivore/plant.

How parasites can manipulate host behavior

Infected amphipods move to the surface (positive phototaxis) to increase the chance of being eaten and transmitting the parasite.

Pathogen transmission mechanisms

Host responses like sneezing or diarrhea are often manipulations by the pathogen to enhance transmission.

Parasitoid definition

An organism whose larvae develop in or on a host, eventually killing it.

Krebs et al. (1995) Hare-Lynx study result

The largest population increase occurred when food was added AND predators were removed simultaneously.

Conclusion of the Hare-Lynx field study

Both food availability and predation pressure interact to strongly affect population dynamics.

Bean weevil and parasitoid lab study result

Demonstrated that exploitative interactions lead to oscillating population dynamics over generations.

Four stages of a predator-prey oscillation cycle

1) Prey increase

2) Predator increase

3) Prey decline (predation)

4) Predator decline (starvation).

Lotka–Volterra Predator-Prey model;

A mathematical model used to describe how predator and prey populations influence each other's growth and decline.

L-V Parameter

Nh;The population size of the prey (host).

L-V Parameter: Np

The population size of the predator.

L-V Parameter: p

The predation rate or capture efficiency of the predator.

L-V Parameter: dp

The death rate of the predator population.

L-V Parameter: c

The rate at which prey are converted into predator offspring. Prey growth formula interpretation

Prey growth = intrinsic growth rate minus the rate of predation.

Predator growth formula interpretation

Predator growth = consumption of prey (converted to offspring) minus the predator death rate.

Purpose of ecological refuges

To provide hiding places or escape routes that prevent prey populations from falling to zero.

Types of physical refuges

Burrows, trees, or habitats predators cannot reach (e.g., air vs. water).

Protection in numbers as a refuge

Behaviors like schooling or herding that make it difficult for a predator to pick off an individual.

Huffaker’s (1958) Mite study conclusion

Spatial heterogeneity (a patchy habitat) acts as a refuge that stabilizes dynamics and allows for delayed cycles.

Predator satiation definition

A defense tactic where prey occur at such high densities that predators cannot possibly eat them all.

Example of predator satiation

Periodical cicadas emerging every 13 or 17 years in massive numbers to overwhelm predators.

Impact of refuges on population stability

Refuges allow prey populations to recover, preventing extinction and stabilizing the predator-prey cycle.