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Predation Lecture 15 • • • • • Overview Chapter in Text: 15, 17 Predation and Herbivory Responses of individuals to predation Responses of populations to predation – refuges Importance of Predators – Maintenance of ecosystem diversity – as a Keystone species What is a predator? • Narrow sense • Broad sense • Ecological definition of predator: – Herbivores: • Grazers and browsers – consume part of plant/plant not killed • Seed eaters + planktinovores: consume entire plant – Parasite: generally do not kill host – Parasitoid: lay eggs in host – feeding by larvae eventually kill host – Under simple laboratory conditions, the predator often exterminates its prey • It then becomes extinct itself having run out of food! Lotka-Volterra Models and Predator-Prey cycling: • Developed during 1920s • Assume mutual interaction of predator and prey numbers • Predict persistence of both predator and prey populations • The Lotka–Volterra equations for predator and prey populations link the two populations – Each population functions as a density-dependent regulator on the other • Predator as a source of density-dependent regulation on the mortality of the prey population • Prey as a source of density-dependent regulation on the birthrate of the predator population • The paired equations, when solved, show that the two populations rise and fall in oscillations • The cycle can continue indefinitely — the prey is never quite destroyed; the predator never completely dies out “Now, here you see, it takes all the running you can do, to keep in the same place” – the Red Queen • Coevolution: as prey species evolve ways to avoid being caught, predators evolve more effective means to capture them • Natural selection (think in terms of fitness) – “smarter,” more evasive prey – “smarter,” more skilled predators • Prey defenses to avoid being detected, selected, and captured by predators: – Chemical defense • Alarm pheromones • Repellants • Toxins – Cryptic coloration – Warning coloration – Protective armor – Behavioral defense Chemical Defenses • Some animals receive an added benefit from eating plants rich in secondary chemical compounds – Caterpillars of monarch butterflies concentrate and store these compounds • They then pass them to the adult and even to eggs of next generation • Birds that eat the butterflies regurgitate them Blue jay I’m not eating this again! Plant Responses to Herbivores • Physical – Thorns – Height – Heavy seed coat • Chemical – Toxins – Digestion inhibitors • Nutritional – Low levels of N in older foliage – Tough, difficult to masticate foliage Chemical Responses of Plants to Herbivory • Mustard oils protected plants from herbivores at first – At some point, however, certain insects evolved the ability to break down mustard oil • These insects were able to use a new resource without competing with other herbivores for it –Cabbage butterfly caterpillars Adult Green caterpillar Animal Defenses against Predation • Physical • Behavioral • Chemical – Toxins • Coloration – Cryptic – Warning coloration – aposmatic • Batesian mimicry – harmless mimics Monarch butterfly Viceroy butterfly – After Henry Bates, a 19th century British naturalist • Müllerian mimicry – common coloration of toxin bearing spp – After Fritz Müller, a 19th century German biologist Self Mimicry • Involves adaptations where one animal body part comes to resemble another – This type of mimicry is used by both predator and prey – Example • “Eye-spots” found in many butterflies, moths and fish Müllerian Mimicry • Two or more unrelated but protected (toxic) species come to resemble one another Yellow jacket – Thus a group defense is achieved Masarid wasp Sand wasp Anthidiine bee Yellow jacket Predation and Behavior Modification - Refuges • Schooling of prey fish – response to predator attack – some survive • Alarm calls – Prairie dogs, ground squirrels • Song birds mob and harass predator bird species • Avoidance – temporal, spatial • Refuges Refuges • A mechanism that allows exploited population to escape predation/parasitism – many forms: – Place/form of cover, schooling, synchronized reproduction (large numbers at one time), size – May not provide absolute sanctuary, enough for species to survive – Important for survival of predator too! Protection in Numbers • Living in a large group provides a “refuge.” • Predator’s response to increased prey density: Prey consumed x Predators = Prey Consumed Predator Area Area • Wide variety of organisms employ predator satiation defense. – Prey can reduce individual probability of being eaten by living in dense populations. Examples of Predator Satiation • Synchronous widespread seed and fruit production by plants - masting. • Synchronized emergence of Cicadas – 16-17 year cycle – Williams estimated 1,063,000 cicadas emerged from 16 ha study site. • 50% emerged during four consecutive nights. • Losses to birds was only 15% of production Size As A Refuge • If large individuals are ignored by predators, then large size may offer a form of refuge. – Peckarsky observed mayflies (Family Ephenerellidae) making themselves look larger in the face of foraging stoneflies. • In terms of optimal foraging theory, large size equates to lower profitability. • Is regulation top-down or bottom-up? • ie. primary productivity versus limits imposed by predator populations Ch 18 p 344 • Diffuse predator–prey interactions – The lynx, coyote, and horned owl are responsible for the periodic cycles in the snowshoe hare population • Diffuse mutualism – A single plant species may depend on a variety of animal species for successful reproduction Hare popul crashes as: 1. Reduced forage weakened hares, high lynx prdation 2. Forage produced after heavy browsing accumulates plant defense chemicals less palatable Lynx predates weakened hares – eventually crashes • Is regulation topdown or bottomup? • ie. primary productivity vs. limits imposed by predator populations Predators and Diversity – see pages 340-344 • Alter competitive balance amongst prey spp. – Robert Paine studies: sea star exclusion in intertidal plots decreased prey diversity (15 8 spp) • Selective alteration of competitive relationships – Peter Morin studies – altered competitive relationships amongst immature frog spp by predatory newt Keystone Predator (or Keystone Consumer –) • Species essential to maintenance of ecosystem structure/diversity • Example (there are many): CA sea otter – kelp forest community THINGS TO WORRY ABOUT • Your Pores — Portals for Invasion? • Musty Dankness • Fleas & Ticks — Tiny Terrorists What's Embedded in Your Bed? • What Your Mother Never Told You About Those Hidden Corners and Cracks • Pink Mold — Slime or Scourge? Mildew — Mold's Evil Twin