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Lynx-Hare Cycle Assumptions • N1 and N2 dependent only on each other • predator can find and consume prey at any prey density • no Allee effect for predator or prey at low densities Variations in predator – prey interactions could be due to: --how well predators can find and consume prey --how well prey can avoid predation --whether or not a refugium exists where prey can be safe from predators Refugium may exist: place for prey to survive without presence of predator e.g., prickly pear cactus introduced to Australia In 1830s for hedge rows, gardens Also resisted drought and had no predators, so spread rapidly and became a nuisance species Cactus moth introduced in 1926, quickly spread and helped control cactus Cactus still present in refugia and expand from them until moth population resurges jaeger Snowy owl Short-eared owl Numerical Response Response of predatory birds to different densities of the brown lemming near Barrow, Alaska 1951 1952 1953 Brown lemming 1 to 5 Pomarine jaeger Uncommon, no breeding Breeding pairs 4 Mi2 Breeding pairs 18 Mi2 Snowy owl Scarce, no breeding Breeding pairs 0.2 to 0.5 Mi2 many nonbreeders Breeding pairs 0.2 to 0.5 Mi2 few nonbreeders Short-eared owl Absent Source: Pitelka et al. 1955. 15 to 20 One record 70 to 80 (ind per acre) Breeding pairs 3-4 Mi2 Charles Krebs: detailed analysis of Lynx-Hare Cycle • Disease and parasites? studied parasite loads in hares for years none caused direct mortality • Quality of Food? measured winter food abundance hares only eat 20-40% of what’s available food addition experiments showed initial growth response, then decline • Predation? marked hares 95% mortality due to predation by lynx, owls, coyotes exclusion showed high survival rate Why are cycles synchronous across large areas and only in high latitudes? • predators tend to disperse over large areas, e.g., one marked lynx moved 1100 km, owls forage over large areas • more seasonal variations at high latitudes may drive higher oscillations in cycles there • get some cycles at lower latitudes, but not as obvious Optimal Foraging Theory • developed by MacArthur and Pianka (1966) • predicts minimal foraging effort for maximum energy gain • minimal effort means lowest search and handling time • if food patchy, predators should select best patches, lowest S&H time Glimcher (2002) Krebs Experiment: Bird Foraging over Conveyor Belt In Krebs' experiment, hungry birds stand over a conveyor belt. An experimenter places mealworm segments of two sizes on the belt in a pseudorandom sequence. The bird faces a serial decision problem; it must decide which segments to eat and which to ignore. The decisions the bird makes are influenced by the mean rates at which both prey types are encountered, the difficulty of capturing and eating the segments, and the relative values of the two different size pieces. Optimal Foraging in the Wild whelks Northwestern Crow http://www.asnailsodyssey.com Why the world is green: 1. Insects and other herbivores controlled by their predators and parasites Hairston, Slobodkin, and Smith (HSS Model) 2. Not all plants are edible Nitrogen content of typical plant: Stem Leaf Seed 0.002 % 1-5 % (folivores) 10+ % (granivores) Also, as plant grows from sprout to mature, water and protein content declines Cellulose and lignin increase, need bacteria and protozoa in gut to digest Plant defenses from herbivory: 1. Morphological --thick leaves, thorns, needles 2. Chemical --secondary compounds Secondary Compounds Chemicals produced by plants solely for defense Take considerable energy to produce 1. Nitrogen compounds --derived from amino acids --alkaloids include nicotine, morphine --mostly toxic, bitter tasting 2. Terpenoids --oils and resins --mostly bitter tasting --includes terpetines, solvents 3. Phenolics --tannins that hinder digestion --used in dyes, tanning, inks Allelopathy: plants use of secondary compounds for defense against competition from other plants --can affect growth and development of other plants around them Corn plants: use terpenoids to attract a parasitic wasp, lays eggs in caterpillar feeding on plant Ethnobiology: study of anthropology and biology, how humans, past and present, used or use plant and animal resources in their culture Herbivore response to plants: 1. Morphological --teeth, gut 2. Behavioral --detoxify secondary compounds Hindgut fermentation enlarged cecum Foregut fermentation enlarged stomach Behavioral responses: 1. Eat clay to detoxify compounds --primates, parrots 2. Coprophagy -- rabbits A specialized diet on plants is not without costs: 1. Red tree vole, eats only conifer needles --high in tannins, thick cellulose --slow metabolism, growth 2. Lynx-hare cycle --browsing by hares stimulates plants to produce secondary compounds --food becomes less digestible --hare numbers decline --populations cycle without predator Avoiding Predation • cryptic and warning coloration • Batesian mimicry • Müllerian mimicry katydid Cryptic coloration walking stick Biston betularia Warning coloration Batesian Mimicry Monarch Viceroy coral snake king snake Red on yellow, harm a fellow Red on black, friend of jack Müllerian Batesian Mimicry in Plants Heliconius butterflies and eggs on Passion flower leaves Predators can be specialists or generalists Specialist versus generalist relates to optimal foraging theory as well -- generalist flowers with small nectar rewards usually are common species -- specialist flowers with large nectar rewards usually are rare species -- a specialist predator seeks out rarer plants with large rewards rather than waste time getting small rewards from abundant plants -- ensures pollination of rare plant