ECOL 182R 1st Edition Lecture 21 Outline of Last Lecture I The niche II Competition III Competition the niche IV How do so many species coexist Outline of Current Lecture I Coexistence II Predator prey dynamics A Limit population sizes B Competing prey coexist Current Lecture Coexistence continued If different resource use is genetically fixed how do you know if natural selection for resource partitioning caused it Darwin s finches This idea has been examined in 2 species of Darwin s finches Beak depths food sizes differ on the island where both are present Specifically on the island where they occur together But where the same birds occur alone on their separate islands their bills are different Beak depths are similar between different species where birds have never occurred together An example of character displacement divergence in morphology due to evolution of niche differences No longer competitors because they re eating different seeds There must be mechanisms that allow similar species to coexist Disturbance Population sizes are kept small resources never start running out No disturbance leads to competitive exclusion Extensive disturbance all species die regardless of competitive ability Intermediate disturbance resources don t run out before pop sizes cut back Poorer competitors can hang on indefinitely Predation Herbivory Predator Prey Dynamics What s so interesting about it Predators can limit prey population sizes Which is sometimes really important to us as humans mosquitoes that carry diseases Otherwise we d poison them with pesticides but we now know that there are major drawbacks such as the evolution of resistance Also pesticides kill harm nontarget species too High concentrations of DDT massive losses in birds Another way we could reduce the species we don t want would be using the pest s antagonists called biological control pest control using predators parasites or diseases Ex Prickly pear Australia Their economy ran on agriculture with these infesting all the land cows couldn t live there etc Brought in the herbivore Cactoblastis moth to eat it which worked was destroyed Cactoblastis is a specialist what if it had a more generalized diet It d be terrible because it would eat many other things too The Gypsy Moth would do this Predation can permit competing prey to coexist Hypothesis predators reduce prey number so that allows more prey to coexist on a limiting resource Test remove predators and see what happens Starfish eat Limpets Mussels So Limpets and Mussels compete for space on rocks So the starfish was removed turns out that coexistence wasn t possible Mussels outcompeted the Limpets The starfish is the keystone predator Predator prey population growth are not independent Predator number always peaks after prey number Prey is always more abundant than predators But this isn t proof
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