Ecology- Predation and Herbivory

Predators and herbivores can limit the abundance of populations

• Can be direct or indirect, artificial or natural

• Ex. Lizard Predation on Spiders

  • Islands with predatory lizards had lower spider density

  • Islands without predatory lizard had much higher spider densities

  • Tested experimentally to verify this natural occurrence is caused by predation by lizards

• Ex. Snowshoe Hares and Lynx

  • Major case study

  • Approximately 10 years cycles

  • Fluctuate closely- indication of predators impacting prey cycles very strongly

Biocontrol

Use biological agent to control another biological phenomenon

• Commonly used for invasive species

• Can backfire so must be done extremely carefully

Ex. Prickly Pear Cactus and Moths

• Example of successful biocontrol

• Brought in a moth that specialized in eating invasive prickly pear cactus

Ex. Brown Tree Snake and Poisoned Mice

• Another example of successful biocontrol

• Bring in prey species to control predator species

• Created poison parachutes with dead baby mouse stuffed with acetaminophen (tylenol) to control invasive brown tree snake

Ex. Red Scale Insects and Parsitoid Wasps

• Another successful biocontrol

• Red scale insects are a nuisance that attach to citrus fruit trees

• Parasitoid wasp introduced to control scale insects by laying eggs right underneath them and larva hatching and eating scale insects

Huffaker’s Predator-Prey Lab Experiment

Initial set up:

• Established populations of predator and prey mites on large trays that contained oranges and rubber balls

• Varied the distribution on each tray

With this set up:

• Without predators, prey population rapidly increased and leveled off

• With predators, predator population rapidly increased and wiped out prey population, then predators went extinct without prey

Modified set up:

• Vaseline- barrier to slow dispersal of walking predators

• Vertical pegs- jumping- off points for prey to parachute off of prey and allowed prey to escape predators and find new orange to colonize 

With modified set up:

• Produced a series of 3 population cycles

• Distribution of predators and prey throughout tray continually shifted over time

• Created a metapopulation

Coevolution

2 or more species evolve in response to each other; can result in:

• Behavioral Defenses

• Structural Defenses

• Crypsis/Camouflage

• Chemical Defenses

• Warning Coloration

• Müllerian Mimicry

• Batesian Mimicry

Behavioral Defenses

Ex. Dragonflies (predator) and Tadpoles (prey)

• Control- tadpoles without dragonfly present

• Dragonfly caged- unable to eat tadpoles, tadpoles knew dragonfly was present, tadpoles decreased activity

Structural Defenses

Ex. Carp 

• Grow larger with larger muscles to swim faster in the presence of predators

• Gives them a higher chance to escape predation

Crypsis/Camouflage

Both predators and prey do this

Ex. Owl

Chemical Defenses

Ex. Bombardier Beetle

• Sets off an explosion that releases a boiling hot stinging acid

• Can aim the spray

Warning Coloration

Strategy in which distastefulness evolves in association with very conspicuous colors and patterns

• Also known as aposematism

Müllerian Mimicry

When several unpalatable species evolve a similar pattern of warning coloration

• All actually toxic

Batesian Mimicry

When palatable species evolve warning coloration that resembles unpalatable species

• Not really toxic

• Ex. Wasp (toxic), Fly (not toxic), Moth (not toxic)

Lotka-Volterra Model

Model of predator-prey interactions that incorporates oscillations in the abundances of predator and prey populations and shows predator numbers lagging behind those of their prey

Isocline Trends

Want to reach equilibrium/isocline

• Above isocline = pop decreasing

• Below isoline = pop increasing

Have to pay attention to both species

• Both increasing?

• Both decreasing?

• One increasing and other decreasing?

Correspond with population trajectories

• Dotted line = isocline

• Highlighted portions correspond with isocline trends

Functional Response

Relationship between the density of the prey population and an individual predator’s rate of food consumption

Type I Functional Response

Predator’s rate of prey consumption increases in a linear fashion with an increase in prey density until satiation occurs

Type II Functional Response

Predator’s rate of prey consumption begins to slow down as prey density increases and then plateaus when satiation occurs

Type III Functional Response

Predator exhibits low prey consumption under low prey densities, rapid prey consumption under moderate prey densities, and slowing prey consumption under high prey densities

Numerical Response

Change in the number of predators through population growth or population movement due to immigration or emigration