BIO326 : Predation/Competition : Predation : Lesson

Glossary terms that are important in this lesson: Antibodies, antigens, cross-resistance, food chain, functional response, maximum sustainable yield, net primary production, numerical response, parasite, parasitoid, population trajectory, rate, schistosomiasis, search image, stable cycles, switching.

I. Introduction

1. Questions
   • What factors influence the size and stability of populations?
   • Do predators reduce the size of prey populations below carrying capacity?
   • Do dynamics of predator-prey interactions cause oscillations?
   • Relevance
     • Management of crop pests, game populations, endangered species
     • Structure of biological communities
     • Stability in natural ecosystems

2. Distinctions among consumers
   • Predators: consumes an individual and removes it from the population
   • Parasite: host remains alive; increases host mortality or reduces its fecundity
   • Parasitoid: eventually kills its host
   • Herbivores: eat plants, whole or partially
     • Grazers eat grasses and herbaceous vegetation
     • Browsers eat woody vegetation
   • Detritivores: consume detritus

II. Limitation of resource populations

1. Cyclamen mite pest of California strawberries preyed on by Typhlodromus mite
   • Pest population remained low in presence of the predator
   • Pest population reached damaging levels where insecticide was applied; it depressed the predator more than the pest
   • Effectiveness of predator
     • Voracious appetite
     • High fecundity, equivalent to that of prey
     • Strong dispersal powers
     • Switch to alternative food resources when primary prey unavailable

2. Opuntia in Australia: introduced pest
   • Cactus moth (Cactoblastis cactorum) brought in from S. America
   • Caterpillar feeds on pods and inoculates cactus with pathogens and rot-causing organisms
   • Cactus and moth are in a low-level equilibrium in a continually shifting mosaic of isolated patches; cactus colonizes new areas before it is discovered by the moth

3. Sea urchin predators
   • Predator removal was followed by an increase in large brown algae
   • Other algae were competitively eliminated when predation removed

4. Parasite-host systems
   • Parasites adversely affect host populations
   • Balance between parasite and host populations is mediated by immune response
     • Host produces antibodies in response to parasite antigens
     • Immune response takes time to develop
     • Schistosomiasis is a major parasite of many human populations; sometimes produces cross-resistance

5. Herbivore and plant populations
   • Klamath weed pest in California controlled by Chrysolina beetle from Australia
   • Proportion of net primary production consumed by herbivores depends on digestibility of vegetation
     • Consume about 10% of forest vegetation, most enters detritus pathway
     • Consume 30-60% above-ground vegetation in grasslands
     • Consume most of algal production in littoral zone
     • Occasional outbreaks of forest insects can defoliate vegetation
   • Some plants produce chemical compounds in response to herbivory

III. Predator-prey cycles

1. Animals of the Canadian boreal forests and tundra
   • Lynx and snowshoe hare: synchronized cycles of 10-11 years
   • Small herbivores and their predators have 4-year cycles
   • Large herbivores and their predators have 9-10 year cycles
   • Tundra species have shorter cycles, forest species have longer cycles
   • Period of cycle is 4-5 times the response lag

2. Laboratory experiments
   • Azuki bean weavils (Callosobruchus chinensis) and parasitoid braconid wasps (Heterospilus) produced recurring population cycles
   • Paramecium and Didinium went extinct unless there was a refuge for Paramecium or recolonization by Didinium

3. Huffaker's mites
   • Six-spotted mite (Eotetranychus sexmaculatus) ate oranges and Typhlodromus occidentalis ate the herbivore
   • Stable predator-prey cycles if the habitat was complex enough to provide the prey a temporal refuge from the predator

4. Three-level consumer system
   • Willow-hare-lynx
     • Both food and predation influenced peak numbers of hares
     • Food and predation had a greater influence together than the sum of the two acting alone
   • Oak-insects-birds
     • Extreme herbivory when birds were excluded depressed leaf production and sapling growth
   • Trees-leaf chewing herbivores-ants
     • Ant removal resulted in severe effects on trees
   • Influence of consumer extends down the food chain

IV. Models of predator-prey interactions

1. Lotka-Volterra predator-prey model
   • Growth rate of prey population depends on its intrinsic rate of increase and the loss to predation: dR/dt = rR - cRP
   • Growth rate of predator population depends on prey captured and death of predators
   • Equilibrium lines when population change equals zero
     • Stable equilibrium where both equilibrium lines cross
     • Stable cycles everywhere else
   • Change in predator and prey populations follows a population trajectory
   • Density-dependence leads to joint equilibrium
   • Fit to data on voles and weasels

2. Functional response
   • Individual predator's rate of food consumption relative to prey density
     • Rate is a ratio, e.g., predation rate is the number of prey items consumed by each predator, or the number of prey consumed divided by the total number of prey present
   • Type I: rate of predation is independent of prey density
   • Type II: predation rate declines as prey density increases
   • Type III: initial increase in predation rate at low prey density, then declines as prey density continues to increase
   • Search image is acquired by experience
   • Predators often switch to alternative prey at low prey density

3. Numerical response
   • Increase in numbers of predators as prey density increases
   • Many predators have lower reproductive potential and longer life spans than their prey
   • Response may be a combination of population growth and immigration
   • Example: bay-breasted warbler and spruce budworm; raptors and lemmings

V. Stability in predator-prey systems

1. Conclusions from models: long-term persistence of predator-prey interactions are promoted by:
   • Predator inefficiency
   • Density-dependent limitation by external factors
   • Alternative food for the predator
   • Refuges for prey at low prey density
   • Reduced time lags

2. Multiple stable states
   • Equilibrium occurs at every intersection of the recruitment curve and the predation curve
     • Stable equilibrium: any change from the equilibrium density results in a reaction which returns the population to the equilibrium density
     • Unstable equilibrium: any change from the equilibrium density results in a reaction which increases the distance of the population from its equilibrium density
     • Stable cycle: continuous oscillation of constant amplitude
   • Curves represent rates (proportion of prey) at each prey density

3. Maximum sustainable yield
   • Highest rate of recruitment of a prey population
   • Occurs at a density of prey intermediate between zero and the carrying capacity
   • Predators often achieve an equilibrium with their prey populations
   • Different predator-prey systems seem to achieve equilibrium at different ratios
   • Concept used to manage world's fisheries, e.g., Atlantic swordfish, menhaden

When you have completed this lesson, go on to Review Questions