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 cn prey c is predator efficiency Total rate of predation Predator population The product of per capita rate of consumption (cn prey ) and the numbers of predators (N pred ) cn prey N pred Therefore, the population growth of the prey population is: dn prey /dt = rn prey cn prey N pred With no predators, the population will grow exponentially When rn prey = cn prey N pred, population growth is zero N pred = r/c Predator populations described by: dn/dt = amount of prey consumed and transferred into biomass - constant mortality term 1
Predator production Predator growth model Amount of predators produced rises linearly with number of prey consumed b is efficiency with which food is converted to reproduction Birth rate is a product of the rate of predation (cn prey N pred ) and efficiency of conversion (b) dn pred /dt = b(cn prey N pred ) dn pred d = probability of mortality When b(cn prey N pred ) = dn pred, population growth is zero N prey = 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 2
Per capita rate of predation Can be expressed as amount of prey eaten by a predator per unit time N e = (cn prey )T s 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 = T s + (N e T h ) 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 3
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 E 1 /T h1 Food choice Will predator always consume most profitable prey? Depends on search time When P 2 is encountered, energy gain is lower E 2 /T h2 If E 2 /T h2 > E 1 /(T h1 + T s1 ), then predator will eat P 2 rather than searching for P 1 4
Foraging behavior of gulls 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 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 E 2 /T h2 > E 1 /(T h1 + T s1 ), then predator will eat P 2 rather than searching for P 1 When predator encounters a patch, cumulative energy gain rises quickly, then flattens out as prey are depleted Cumulative energy gain per unit time (G/T) must be maximized Maximum rate of return is the tangential line of cumulative energy gain and time Distance to patch will affect leaving time 5
Predation Quality of patch will affect leaving time 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 6
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 7
Predators have non-lethal effects 8