Alternatives to competition. Lecture 13. Facilitation. Functional types of consumers. Stress Gradient Hypothesis

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1 Lecture 13 Finishing Competition and Facilitation Consumer-Resource interactions Predator-prey population dynamics Do predators regulate prey? Lotka-Volterra predator-prey model Predator behavior matters: functional response Case history: rodent cycles Evolutionary consequences of predation Alternatives to competition Facilitation Positive interactions among species 1 spp may have positive net effect on another Examples - Succession Nurse plants Invasional meltdown Abiotic stress Facilitation Stress Gradient Hypothesis Functional types of consumers 1. Those that remove prey from prey pops : Predators kill and eat prey Parasitoids capture prey, lay eggs on/in it, and kids feed on prey - e.g. wasps and flies will use caterpillars and spiders 2. Those that harm prey by consuming some of their resources (i.e. don t kill) Herbivores eat plants or seeds Parasites consume part of an organism or insides Social parasites parasitize parental care Callaway et al. 1

2 Do predators regulate prey? Evidence: predator removal studies lead to increases in prey density (with exceptions) What is regulation? Density-dependence in birth and/or death rates (r) Do predators regulate prey? Pattern: Many predator and prey populations show very regular fluctuations in density A: amplitude change in density is from lowest to highest levels P: period how much time it takes for density cycle to repeat itself How do predators regulate prey? Increase in predation with increase in prey density (d is density-dependent) N t Do predators regulate prey? Do predators regulate prey? Pattern: Many predator and prey populations show very regular fluctuations in density SO : Does predation cause these cycles? Do prey cycle for other reasons and predator cycles are a result of prey cycles? or 2

3 Building a LV predator-prey model Putting prey and predator isoclines together: Influence of Predator behavior 2 predator responses that add stability: N P 1. Numerical response number of predators increases with prey density # prey consumed per predator constant but total # predators 2. Functional response predation rates increase w/ prey density (# prey consumed/predator ) t V Predator behavior influences predator-prey population dynamics European kestrel & voles Types of functional responses Type I predator consumes more as prey density What L-V models assume But unrealistic b/c of satiation and handling time # prey eaten per predator Prey mortality rate 3

4 types of functional responses: Type II predator consumes more as prey density but at a decreasing rate (diminishing returns) Incorporates satiation and handling time types of functional responses: Type III predation rate is accelerated at low prey density but decreases at high prey density Also incorporates satiation and handling time, plus # prey eaten per predator Prey mortality rate # prey eaten per predator Prey mortality rate What causes Type III functional response? At low prey densities: Search image Prey refugia Prey switching At high prey densities: Handling time and satiation (converges on Type II) Return to Competition Apparent Competition can happen when: predators consume prey at a rate that increases with prey abundance Both prey spp lower densities when they are together vs. separate Appears like competition 4

5 Evolutionary responses to predation Coevolution is the evolutionary outcome of species interactions (reciprocal evolutionary response) Aposematism warning coloration Mimicry having a phenotype similar to a different sp Mullerian mimicry: several different species that are each toxic/nasty converge on same signal Batesian mimicry: non-toxic species mimicking a toxic species Evolutionary responses to predation Why mimic? - Benefit from lesson learned by predators How does this work when nasty species are lethal? - Dead predators can t learn! Natural selection favors those individuals with innate ability to recognize toxic species Evolutionary responses to predation Case history #1: Experiment to test for innate recognition of toxic species by predators (Smith) Predator: mot-mots (eats snakes) Question: do mot-mots have innate recognition of coral snake pattern? Mot-mots raised in captivity (no snake experience) exposed to 3 treatments: 5

6 Evolutionary responses to predation, 2 Emory Oak (Quercus emoryi) with catkins (flower spike) Evolutionary responses to predation Case history #2: Developmental mimicry polymorphism What is basis of these polymorphisms in caterpillars? - Diet, temperature, or photoperiod? Experiment: split broods from a single mother; expose each group to different factors Results: Caterpillars fed pollen catkin mimics Caterpillars fed leaves twig mimics Diet determines difference, not photoperiod or temperature! photo Erick Greene Sex-limited Batesian mimicry in Papilio dardanus Evolutionary consequences of predation What are the various adaptations that prey evolve in order to reduce their predation risk? in this species, only females are mimetic species is polymorphic different models are mimicked in different areas Mullerian mimicry Heliconius spp. models mimics (nasty) (yummy) 6