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Optimal Foraging Theory • Predators should optimize energetic gains by balancing the costs/benefits of capturing prey. • Costs – Search time – Handling time – Digestion • Benefits – Calories assimilated Predator strategies • Ambush predator – use stealth and crisps, wait for prey to come to you. • Active hunter – use speed and strength to capture prey. • Grazer – may or may not kill prey, different degrees of selectivity. • Parasite – do not directly kill prey, allow it to provide continuous supply of food. Optimal Foraging, Predator Choice and Search Image • Predators “know” which prey are most beneficial and will switch to more beneficial prey. Should switch as prey become scarce (optimizing cost/benefit) • Search image – mental picture of prey predators are search for. Giving up density • Giving Up Density (GUD) – density of prey at which a predator will abandon a prey type. – Prey are depleted – cost/benefit of pursuing prey no longer beneficial – Competing predator more efficient – cost/benefit of prey no longer beneficial – Predator response – switch or find new patch 1 Predator effects on communities • Eliminate weak and sick • “Switch” to most abundant species, preventing one from dominating – Biodiversity implications: • Prevents competitive exclusion, increasing diversity • More energy = more trophic levels = more predators = more diversity Prey Responses - Mimicry • Mimicry – look like something toxic, dangerous or inedible – Batesian – harmless species looks like dangerous one Prey Responses • Crypsis – increase predator search time • Escape • Avoidance – move to an area w/out predators • Both increase search times Prey Responses • Armor, physical defenses – increase handling time • Swamping – prey group together/produce more young than predators can consume. Safety in numbers. – Mullerian – two dangerous species look similar 2 Functional Response Predator-Prey oscillations • Predator and prey populations often tightly linked. – Prey (hare) and predator (lynx) population sizes are clearly linked…why? – What controls dynamic? – Two hypotheses Number Eaten • Predator/prey functional response – describes predation rate at different prey densities. – Note – assume # predators constant, number eaten ~ benefit. Prey Density Top-down control • Top-down control – consumption at higher tropic levels controls population sizes – lynx prey on hare, reduces hare population – Fewer hare → support fewer lynx, reduce lynx – Fewer lynx → hare recover and increase – More hare → support more lynx – cycle continues Fox Bottom-up control • Bottom-up control – productivity in lower trophic levels controls population sizes – Hare population increases → resources (primary producer biomass) limited → Hare population declines – Hare population declines → fewer lynx supported, primary producers recover → hare population recovers Fox rabbit rabbit Grass Grass 3 Which is it? Modern revision to “the world is green” • “The world is green” (Hairston et al. 1960) • Primary produces are not limited by resources • Herbivores limited by carnivores, not primary production • Primary produces are climate or nutrient controlled on the local scale. • Different climates and nutrient environments will favor selection for different traits. • Problems – Many parts of the world are not green – Much primary production does not represent a quality meal Trophic Cascades Food Web Interactions • Trophic Cascade - reduction or removal at one trophic level has the opposite affect at alternating lower levels. Hawk Snake Bird + Grasshopper Grasses - + Hawk - Snake + Bird - Grasshopper Grasses • Communities more complex than 1 predator, 1 prey or linear food chains. • Removal of one species can have ripple effect through community. + - 4 • Species who’s importance to the ecosystem is disproportionate to its abundance (biomass) • Traditionally keystone predators – species that controls abundant species, prevents competitive exclusion. • Modern view - does not have to be predator (eg. figs). Keystone Species Change in Ecosystem Function if Removed Keystone Species Keystone species – rare, but vital to ecosystem Abundant species And vital role in ecosystem Moderate abundance moderate role in ecosystem Rare species not a vital role in ecosystem Species Biomass Parasitism • Similar to predation, predator usually does not kill host (prey). Parasite dependent on host for nutrition. – Microparasites – bacteria, viruses etc. – Macroparasites – flea, tick, mosquito, tapeworm etc. – Biotropic – parasite can only live as long as host lives – Necrotrophic – parasite may kill host, feed on corpse – Parasitoids – parasite always kill host Other species interactions • Parasitism and Predation (+/-) • Mutualism (+/+) – both species benefit from relationship – Intestinal fauna aid in digestion – Leaf cutter ants • Plant (-) • Ant (+) • Fungi (+) • Comensalism (+/0) - one species benefits, the other not affected – Shark-ramora 5 Humans as Predators • Overexploitation – • Switching - Humans as Predators • Most desirable fish – large body – Other Competitor (K-selected) traits associated • Slow maturation • Long life span • Few, large young (low fecundity) Endangered Megafauna Of the worlds 20 largest freshwater fishes, 17 (18) are endangered due in part to overharvest. 6 Review of some basic definitions Community Properties • community - all populations of organisms that live together & potentially interact in particular area at particular time • ecosystem – collection of communities in an area & their biotic and abiotic interactions with the environment Example of Resistance and Resilience • Diversity - # of species, relative abundances (S and H`) • Productivity – Primary productivity and energy transfer through ecosystem • Trophic structure – number of trophic levels, food web structure • Species composition – What species occur in a region and why? • Patterns of colonization and local extinction – What species persist or disappear from a community? • Resistance and Resilience – Is the community resistant to change (do we observe the same species through time)? – Is the community resilient (if disturbed, does the community return to pre-disturbance conditions)? Example of Resistance and Resilience 16 14 Chickasawhay River 12 Diversity Leaf River 10 8 6 Pascagoula River Chickasawhay Leaf Pascagoula 4 2 Pre Storm Fall 2005 '2006 Time Period 7 Species Pools and Environmental Filters Ecological Succession • Communities tend to transition from one set of species to another • Generally, r selected species replaced by K selected species Disturbance • Natural process that removes biota, generally produces a patch of bare habitat. • Measure in frequency and intensity • Aquatic and terrestrial Ecological Succession • Primary succession – Start with empty habitat, Pioneer community colonizes • Secondary succession – Start with pioneer community, is replaced by later successional community 8 Ecological Succession • Succession involves changing plant, animal and microbial communities. Ecological Succession - Climax Community • Fredrick Clements – all communities naturally progress towards a stable climax • Made the analogy that succession was similar to the growth and development of an organism • Community composition is not random but predetermined Ecological Succession and Facilitation • Succession changes both the biotic and abiotic environment • Facilitation – presence of one species facilitates the presence of others (provide a resource other species require) • Bare land to forest change – Erosion – Nutrient export – Temperature – Shade –… Gaia Hypothesis • Gaia Hypothesis - Earth is a superorganism – Species are analogous to cell types – Communities (analogous to tissues) will change due to disturbance, but ultimately return to original state. – Popular with the environmental movement…no scientific basis. 9 Ecological Succession – Individualistic hypothesis • Henry Gleason – individualistic hypothesis. – There is no stable climax. Random events and properties of individual species determine final successional state. – Determined by • Local climate • Population interactions (competition, colonization ability) • Disturbance frequency Krakatoa – a natural experiment Post Disturbance Community Composition • Climax community – Only early succesional species suitable – Community progress predictably, reaches climax • Individualistic hypothesis – First species are the best dispersers, best adapted for postdisturbance communty – Later successional species are poorer dispersers, but better competitors Dominance vs. Founder Controlled Community • Founder controlled community - Early community determined by dispersal ability – R-selected • All life destroyed on island by 1880 volcanic activity. • Change in community structure observed over 100+ years since – K-selected 10 Dominance vs. Founder Controlled Community • Dominance Controlled Community – Over time, poorer dispersing species (better competitors) arrive and dominate. – R-selected – K-selected 11