Predation & Herbivory
Chapter 14
Predation
 Review types of predation
– Carnivory
– Parasitism
– Parasitoidism
– Cannabalism
Lotka-Volterra
 Predators control prey populations and prey control predator populations
 Prey populations described by:
– dN/dt = rN - mortality due to predators term
Predation rate
 Per capita rate at which predators consume prey assumed to increase linearly
– cNprey
– c is predator efficiency
Total rate of predation
 The product of per capita rate of consumption (cNprey) and the numbers of
predators (Npred)
– cNpreyNpred
 Therefore, the population growth of the prey population is:
– dNprey/dt = rNprey – cNpreyNpred
– With no predators, the population will grow exponentially
– When rNprey = cNpreyNpred, population growth is zero
 Npred = r/c
Predator population
 Predator populations described by:
– dN/dt = amount of prey consumed and transferred into biomass - constant
mortality term
Predator production
 Amount of predators produced rises linearly with number of prey consumed
 b is efficiency with which food is converted to reproduction
Predator growth model
 Birth rate is a product of the rate of predation (cNpreyNpred) and efficiency of
conversion (b)
– dNpred/dt = b(cNpreyNpred) – dNpred
 d = probability of mortality
– When b(cNpreyNpred) = dNpred, population growth is zero
 Nprey = d/bc
Zero growth isoclines
Combined zero growth isocline
Equations graphed
Predator population growth
 Dependent on rate at which prey are captured, which is related to density of
prey
– Functional response
 Dependent on increase in predator reproduction
– Numerical response
Per capita rate of predation
 Can be expressed as amount of prey eaten by a predator per unit time
– Ne = (cNprey)Ts
– Te is period of search time
Type I Functional Response
 Assumed for Lotka-Volterra model
Time
 Total time spent foraging includes searching and handling
– T = Ts + (NeTh)
Type II Functional Response
 As number of prey consumed per unit time increases, handling time also
increases
 Declining mortality rate
 Most common pattern
Type III Functional Response
 As number of prey consumed per unit time increases, handling time also
increases
 Refugia
 Search image
 Switching
Functional response curves
Prey switching
Numerical response
Numerical response
Optimal foraging theory
 Recognizes that foragers must balance foraging activities with activities related
to survival, growth and reproduction
– Natural selection will favor most efficient foragers
 Choices include what to eat, how to search, where to search, how long
to search
 Costs and benefits explored
Food choice
 Organism will choose prey item where net energy gained per unit time is
maximized
– E1/Th1
Food choice
 Will predator always consume most profitable prey?
– Depends on search time
 When P2 is encountered, energy gain is lower E 2/Th2
 If E2/Th2 > E1/(Th1 + Ts1), then predator will eat P2 rather than searching
for P1
Foraging behavior of gulls
 What is the best food choice?
 Should they eat an urchin or go on and find a chiton?
 Should they eat a mussel or search for a chiton?
 If E2/Th2 > E1/(Th1 + Ts1), then predator will eat P2 rather than searching for P1
Marginal Value Theory
 Predicts the length of time a forager should stay in a patch before leaving to
find another
– Dependent on distance between patches, quality of patch and time
required to extract resource
Marginal Value Theory
 When predator encounters a patch, cumulative energy gain rises quickly, then
flattens out as prey are depleted
Marginal Value Theory
 Cumulative energy gain per unit time (G/T) must be maximized
Marginal Value Theory
 Maximum rate of return is the tangential line of cumulative energy gain and
time
Marginal Value Theory
 Distance to patch will affect leaving time
Marginal Value Theory
 Quality of patch will affect leaving time
Predation
 Risk of predation also affects foraging behavior
– Sohenen found that birds restricted their foraging during years of low vole
abundance (food of owls)
Predator-Prey Coevolution
Predator saturation
Predator-Prey
 Induced responses
 Phenotypic plasticity
Herbivory
 Herbivory is a form of predation
 Defoliation leads to flush of new growth, which compromises growth,
reproduction and survival
Herbivores take many forms
 For example:
– Browsers
– Burrowers
– Sap-suckers
Many grasses are tolerant of grazing
Plants defend against predators
 Structural defenses
 Low quality food
 Secondary chemicals
– Tannins
– VOCs
Plants, herbivores and carnivore interact
Defense against predators comes at a cost
Predators have non-lethal effects