Study Notes on Predation and Herbivory

Chapter 13: Predation and Herbivory

Introduction to Predation and Herbivory

• Predators and herbivores can limit the abundance of populations.

• Example: 5 islands in the Bahamas demonstrated the effects of predators on anoles.

• Each island was introduced to 20 orb-weaving spiders, and spider populations were censused annually to monitor changes in population dynamics.

Effects of Parasitoids

• Parasitoids affect populations by targeting host insects, such as red scale insects.

Introduced Species

• Introduced Species: A species introduced to a region where it historically did not exist. Also referred to as exotic species or non-native species.

• Invasive Species: An introduced species that spreads rapidly and negatively impacts other species, human recreation, or human economies.

• Example: The brown tree snake introduced to Guam in the 1940s led to 9 birds, 3 bats, and several lizard species either declining or becoming extinct within 20 years.

Herbivores

• Case Study: The prickly pear cactus and cactus moth (Cactoblastis cactorum ) in Australia, illustrating herbivore impacts over three years.

• Example: Beetle herbivory demonstrated on Klamath weed.

Management of Herbivores

• Fencing out deer in British Columbia as a management strategy to protect vegetation.

Population Cycles

• Cyclic Fluctuations: The populations of snowshoe hares and lynx fluctuate regularly, as detailed in studies conducted by the Hudson Bay Company.

Huffaker’s Predator–Prey Lab Experiment

• Key Findings:

  • Predators and prey cannot coexist without prey refuges.

  • Spatial arrangements in the lab provided a dispersal advantage to prey species.

Laboratory Mite Dynamics

• In predator-prey dynamics, extinction can be avoided through prey using dispersal advantages to find predator-free areas.

• Predation cycles are influenced by the slower dispersal rates of predators and lagged reproductive responses of predators to prey abundance.

Lotka–Volterra Predator-Prey Model

• Lotka-Volterra Model: A mathematical model representing predator-prey interactions, exhibiting oscillations in population abundances where predator numbers lag behind prey numbers.

  • Equations:

  • Prey: $\frac{dN}{dt} = rN - cNP$

  • Predator: $\frac{dP}{dt} = acNP - mP$

  • Definitions:

  • $N$ = number of prey

  • $P$ = number of predators

  • $r$ = intrinsic growth rate of prey

  • $c$ = capture efficiency (probability of captured prey)

  • $a$ = conversion efficiency (predator reproduction from food)

  • $m$ = predator mortality rate.

Equilibrium Isoclines in Population Dynamics

• Equilibrium Isocline: The population size of one species that maintains the stability of another species' population.

  • Factors affecting prey growth and predator mortality lead to different trajectory outcomes.

• Joint Equilibrium Point: The intersection of isoclines for predator and prey populations.

Simplifying Assumptions of the Lotka-Volterra Model

• Assumptions made for the model include:

  • No individual variation (homogeneity).

  • Closed system (no migration).

  • Immediate responses with no time lags.

  • No refuges for prey (homogeneous environment).

  • No carrying capacity effects on prey and predators.

  • No satiation of predators deserting variable response regarding prey availability.

Functional Response of Predators

• Functional Response: Relationship between prey density and an individual predator's food consumption rate.

  • Forms of functional responses:

  • Type I: Linear increase in consumption until saturation, typical for filter feeders like zooplankton.

  • Type II: Consumption levels off after initial linear increase due to handling time after capture.

  • Type III: Low consumption at low densities, high consumption at moderate densities, and slowing at high densities, related to search image development.

Mechanisms behind Type III Functional Response

• Limited Number of Prey Refuges: Provide varying availability to predators.

• Search Image Development: Predators learn to recognize specific prey amidst variations in density.

• Prey Switching: Predators change their prey preference based on availability relative to alternatives.

Numerical Response in Predation

• Numerical Response: Adjustments in predator populations through reproduction or movement driven by prey population changes.

Defenses Against Predators

• Behavioral Defenses: Strategies employed by prey to evade capture.

• Crypsis: Camouflage techniques allowing prey to blend into their environment for concealment.

• Structural Defenses: Physical adaptations such as hard shells or spines.

• Chemical Defenses: E.g. Bombardier beetles use volatile compounds to deter predators, generating high-temperature defensive sprays.

  • Mechanism breakdown results in defense system via explosive chemical reactions.

Warning Coloration and Aposematism

• Warning Coloration: Distastefulness evolved alongside conspicuous patterns to prevent predation (also termed aposematism).

• Mimicry Types:

  • Batesian Mimicry: Palatable species evolve to mimic unpalatable species’ colorations.

  • Müllerian Mimicry: Unpalatable species converge on similar warning patterns.

Costs of Defenses

• Defenses against herbivores entail trade-offs, reflected in the costs of sustaining various defensive strategies in species such as tobacco plants.

• Experimental analysis indicated that induced defenses by herbivore damage caused differential resource allocation impacting growth and reproduction.

Key Concepts Recap

• Predators and herbivores exert a top-down control influencing population dynamics, which can be complemented by bottom-up controls driven by resource availability.

• Population dynamics of consumers and consumed exhibit regular oscillation cycles, calculable using mathematical models like the Lotka-Volterra.

• The coevolution of defenses and counter-defenses between predators and herbivores is a fundamental aspect of ecological interactions.

Conclusion

• The complexity of interactions in predator-prey relationships, the mathematical modeling of these dynamics, and the implications of evolutionary adaptations build a robust understanding of ecological principles.

Introduction to Predation and Herbivory

• Predators and herbivores can limit the abundance of populations.

• Example: 5 islands in the Bahamas demonstrated the effects of predators on anoles.

• Each island was introduced to 20 orb-weaving spiders, and spider populations were censused annually to monitor changes in population dynamics.

Effects of Parasitoids

• Parasitoids affect populations by targeting host insects, such as red scale insects.

Introduced Species

• Introduced Species: A species introduced to a region where it historically did not exist. Also referred to as exotic species or non-native species.

• Invasive Species: An introduced species that spreads rapidly and negatively impacts other species, human recreation, or human economies.

• Example: The brown tree snake introduced to Guam in the 1940s led to 9 birds, 3 bats, and several lizard species either declining or becoming extinct within 20 years.

Herbivores

• Case Study: The prickly pear cactus and cactus moth (Cactoblastis cactorum) in Australia, illustrating herbivore impacts over three years.

• Example: Beetle herbivory demonstrated on Klamath weed.

Management of Herbivores

• Fencing out deer in British Columbia as a management strategy to protect vegetation.

Population Cycles

• Cyclic Fluctuations: The populations of snowshoe hares and lynx fluctuate regularly, as detailed in studies conducted by the Hudson Bay Company.

Huffaker

Laboratory Mite Dynamics

• In predator-prey dynamics, extinction can be avoided through prey using dispersal advantages to find predator-free areas.

• Predation cycles are influenced by the slower dispersal rates of predators and lagged reproductive responses of predators to prey abundance.

Lotka

• Lotka-Volterra Model: A mathematical model representing predator-prey interactions, exhibiting oscillations in population abundances where predator numbers lag behind prey numbers.

• Equations:

  • Prey: $\frac{dN}{dt} = rN - cNP$

  • Predator: $\frac{dP}{dt} = acNP - mP$

• Definitions:

  • $N$ = number of prey

  • $P$ = number of predators

  • $r$ = intrinsic growth rate of prey

  • $c$ = capture efficiency (probability of captured prey)

  • $a$ = conversion efficiency (predator reproduction from food)

  • $m$ = predator mortality rate.

Equilibrium Isoclines in Population Dynamics

• Equilibrium Isocline: The population size of one species that maintains the stability of another species' population.

• Factors affecting prey growth and predator mortality lead to different trajectory outcomes.

• Joint Equilibrium Point: The intersection of isoclines for predator and prey populations.

Simplifying Assumptions of the Lotka-Volterra Model

• Assumptions made for the model include:

  • No individual variation (homogeneity).

  • Closed system (no migration).

  • Immediate responses with no time lags.

  • No refuges for prey (homogeneous environment).

  • No carrying capacity effects on prey and predators.

  • No satiation of predators deserting variable response regarding prey availability.

Functional Response of Predators

• Functional Response: Relationship between prey density and an individual predator's food consumption rate.

• Forms of functional responses:

  • Type I: Linear increase in consumption until saturation, typical for filter feeders like zooplankton.

  • Type II: Consumption levels off after initial linear increase due to handling time after capture.

  • Type III: Low consumption at low densities, high consumption at moderate densities, and slowing at high densities, related to search image development.

Mechanisms behind Type III Functional Response

• Limited Number of Prey Refuges: Provide varying availability to predators.

• Search Image Development: Predators learn to recognize specific prey amidst variations in density.

• Prey Switching: Predators change their prey preference based on availability relative to alternatives.

Numerical Response in Predation

• Numerical Response: Adjustments in predator populations through reproduction or movement driven by prey population changes.

Defenses Against Predators

• Behavioral Defenses: Strategies employed by prey to evade capture.

  • Crypsis: Camouflage techniques allowing prey to blend into their environment for concealment.

• Structural Defenses: Physical adaptations such as hard shells or spines.

• Chemical Defenses: E.g. Bombardier beetles use volatile compounds to deter predators, generating high-temperature defensive sprays.

  • Mechanism breakdown results in defense system via explosive chemical reactions.

Warning Coloration and Aposematism

• Warning Coloration: Distastefulness evolved alongside conspicuous patterns to prevent predation (also termed aposematism).

• Mimicry Types:

  • Batesian Mimicry: Palatable species evolve to mimic unpalatable species

  • Müllerian Mimicry: Unpalatable species converge on similar warning patterns.

Costs of Defenses

• Defenses against herbivores entail trade-offs, reflected in the costs of sustaining various defensive strategies in species such as tobacco plants.

• Experimental analysis indicated that induced defenses by herbivore damage caused differential resource allocation impacting growth and reproduction.

Key Concepts Recap

• Predators and herbivores exert a top-down control influencing population dynamics, which can be complemented by bottom-up controls driven by resource availability.

• Population dynamics of consumers and consumed exhibit regular oscillation cycles, calculable using mathematical models like the Lotka-Volterra.

• The coevolution of defenses and counter-defenses between predators and herbivores is a fundamental aspect of ecological interactions.

Conclusion

• The complexity of interactions in predator-prey relationships, the mathematical modeling of these dynamics, and the implications of evolutionary adaptations build a robust understanding of ecological principles.