Published on December 30, 2007
PREDATION: PREDATION ENV 330 Lecture 9 Lynx and Snowshoe Hare:Revisiting the classic story: Lynx and Snowshoe Hare: Revisiting the classic story From the Hudson Bay Company records: Lynx eat hares, regulating their numbers which would otherwise grow exponentially Lynx “overharvest” hares, causing a decline in the lynx population, which causes a rebounding of hare population “Top-down” interpretation of data suggests that prey numbers are regulated by predators Unfortunately, careful analysis doesn’t support this interpretation Lynx and Snowshoe Hare (cont): Lynx and Snowshoe Hare (cont) Alternative explanation of data Changes in productivity of plant community grazed by hares. In good years, hares increase in abundance, and lynx populations track that increase. In bad years, hares decline, and after some lag time, the lynx population declines, too. So…what does control predator/ prey cycles, top-down, or bottom-up regulation? Predator strategies: Predator strategies Predator Strategies, and examples: Ambush Stalking Pursuit Predators form mental images – search images- that help them to find food quickly, concentrating on a particular prey “Switching” – when predators turn to an alternate, more profitable prey type. How do prey avoid predation?: How do prey avoid predation? Protective coloration Cryptic coloration Flashing coloration Warning coloration Mimicry Armor Behavior Group living – safety in numbers Synchronous timing of reproduction – first starve, then satiate predators How do prey avoid predation? (cont.): How do prey avoid predation? (cont.) Some bizarre examples: Mayflies can detect stoneflies chemically Predatory catfish in Lake Malawi Theory: Theory The Lotka-Volterra equations for predation For the prey: Assume prey grows exponentially in absence of predation What affects growth rate of prey? How often they get eaten # of predators How often they catch prey What affects how often predators catch prey? # of prey How good predators are at catching prey. Theory (cont.): Theory (cont.) In words: The change in the number of prey equals the exponential rate of increase for the prey, minus the amount of predation, which depends on the number of predators, the number of prey, and the skill of the predator in catching the prey when they find it. We know N1, N2, r1 We can say P = coefficient of predation –how good the predator is at catching prey. The equation: dN1/dt = r1N1 – N2N1P Theory (cont.): Theory (cont.) For the Predator Assume the growth rate of the predator depends on the number of prey consumed. What affects growth rate for predators? effectiveness of the predator death rate of predator In words: The change in the number of predators equals the initial number of predators times the number of prey consumed, minus the death rate of the predator, which is a product of the number of individuals and the per capita death rate. Theory (cont.): Theory (cont.) Say the death rate = d2 The equation: dN2/dt = N2(PN1) – d2N2 Thus, we have just modified our equations for population growth to incorporate the effects of predator/prey interactions on both the prey and the predator populations. Plant Herbivory: Plant Herbivory Costs to plant: Decrease in biomass Reduced vigor Increased susceptibility to further herbivory/disease Lower reproductive effort In the juvenile stage, plant is most vulnerable and least competitive Young leaves are net importers and consumers of nutrients, herbivores focusing on young leaves remove large quantities of nutrients. Plant Herbivory (cont.): Plant Herbivory (cont.) Responses of plants Reduce palatability of tissue using spines, thick cuticle, biochemicals Terpenes, tannins, alkaloids, cyanogenes, glycosides Production and storage of chemical defenses is expensive Responses of herbivores Breach chemical defenses Breakdown and/or absorb toxins Example: Monarch butterflies Can sequester them and use for their own defense! Others use in producing pheromones. Coevolution : Coevolution Definition: The reciprocal evolution of two or more interacting populations. (Ehrlich and Raven 1964) Most butterflies feed only on one or a few plants of a single plant family. Why? Biochemicals. Specific species or groups of species breached the chemical defenses, or else evolved ways of coping with them as they were evolving. The effect of a predator on the evolution of a prey species and vice versa, offers another example of coevolution The Red Queen Hypothesis : The Red Queen Hypothesis (Van Valen 1973) A species must continually evolve to survive in a world full of other evolving species Janzen: Ants and Acacias: Janzen: Ants and Acacias Who can summarize what Janzen discovered? How did he verify his ideas about coevolution?