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Lecture 19: Intro to Predation But first… Back to Mount St. Helen’s Two studies: a) Morris & Wood 1989 b) Bishop 2002 Facilitation vs. Inhibition • Lupines: nitrogen-fixing pioneer species • Anaphilis: later invader • Epilobium (fireweed): later invader Pumice Plains • • • • True Primary Succession Covered in debris Then lava flows Low in nutrients (especially nitrates) & pyroclastic flows hold water poorly • Summer: high surface temperatures & low water availability 1 Introduction of Invaders Species • Seeds survival: Barren Areas > Dead Lupines > Live Lupines • Seedling growth & flowering: Best in areas with Lupines What Explains Seed Survival? 1) More pathogens or herbivores 2) Competition with Lupines 3) Allelopathic substances produced by Lupines Likely a combination… What Explains Seedling Growth? 1. Increased nitrate availability 2. Increase in water availability (mulching) 3. Amelioration of microclimate 2 What happened after first 10 years? • Originally very high population growth of lupines • But, expansion rates decreased 10 fold in 1991-1995 time period • Not a reflection of habitat availability • Early explanation: poor dispersal of heavy seeds (but, they managed to get there) Impact of Insect Herbivores? • Lots of Lepidopteran larvae & Anthomyiid flies • Feed on stems, leaves & seeds • Other populations: may or may not have effect on populations, succession or colonization Core Patches • • • • • More effect at the edge Effect not significant to population growth Competition appears to be more significant Fewer insects in core? Likely a result of presence of insectivores, parasitoids, vertebrate predators 3 Edge Patches • Strong effect of insects • Especially in relation to seed loss, but also serious effect on adult survival Conclusions Small Scale: • Aids succession by opening up spaces & decreasing inhibitory effects Large Scale: • Decreases colonization by lupines, therefore decreases site amelioration, eventually slows succession Now, back to predation What is a predator? • An organism that consumes all or part of another living organism. • Interaction is positive for predator & negative for individual prey 4 Predator-prey interactions Many trophic levels: • Herbivores on plants • Carnivores on herbivores • Carnivores on carnivores • Parasites/Parasitoids on host • Disease on victim Effects of Predation • Obvious direct effects • Less obvious indirect effects • Influence on energy flow & nutrient cycling (affects movement through trophic levels) • Effects are obvious through antipredator adaptations. Competition vs. Predation • In terms of adaptations: • Tend to be more complicated/sophisticated for predation (mimicry, crypsis, chemical defenses, behaviours, mechanical defenses) • Competition (size, morphology, behaviour) 5 Biological Control • Good way to find effect of predators on prey populations • Prickly pear cactus • St. John’s wort • Salvinia molesta • Rabbits Prickly Pear Cactus • • • • • Australia Rangeland problem: sheep can’t graze Control: Cactoblastis cactorum Larval moths specific to this cactus Low density of predator & prey St. John’s Wort • • • • California Rangeland: phototoxic to cattle, can’t sell Chrysolina beetle Only survives in shade (beetle survives poorly) 6 Salvinia molesta • Aquatic fern • Introduced from Brazil, weed in tropics (boats, fishing, ecosystem function) • Cyrtobagus singularis (weevil) • Originally wrong species so worked poorly • Very effective now Rabbits • Australia • Myxoma virus (vertebrate predators would & have seriously affected native species) • Did a decent job but evolved decreased virulence & rabbits evolved resistance Biological Control Examples Show: • Predators can directly limit populations well below that attained without native predators • Suggests that they are important in structuring communities. 7 Effect of Predation • Varies with type of community (e.g. freshwater vs. terrestrial) • Varies with strength of competition • Varies with specificity of predator • Varies with density & distribution of prey • +++ 8