ECOL 182R 1st Edition Lecture 18 Outline of Last Lecture I Black carbon tropospheric ozone II Politically controversial III IPCC reports A Positive negative IV Coral reefs V Predicted impacts VI Prediction rates compared to the past A Major extinctions Outline of Current Lecture I Density dependent independent factors II Predictability III Life history traits A Natality B Mortality C Dispersal patterns Current Lecture Why does a population grow slower as it gets bigger Because of density dependent factors Things limiting growth more and more as the population grows Often biotic food limits disease attacks May or may not lead to stabilization around the carrying capacity But populations may alternatively be limited by things unrelated to population size These density independent factors Act regardless of population size Often abiotic weather volcanoes tsunami Especially important for small organisms Carrying capacity isn t helpful in understanding this Species differ predictably in 1 Whether populations are primarily regulated by density dependent or independent factors Large vertebrates are usually density dependent limitation Ex Salmon are huge and are maintained near their carrying capacity Ex Whiteflies are driven by weather 2 The timing and numbers of births deaths dispersal Life history the set of predictable significant events in the life cycle of organisms of a given species It s variable across species and they vary in all characteristics Life history traits Pattern in birth natality Patterns in death mortality Dispersal patterns Natality Age at first reproduction All else being equal population grows faster when age of first reproduction is lower because there s more generations per unit time This holds within species too Number of reproductive episodes in life Iteroparous can more than once Semelparous only once and usually dies after Also called Big Bang Reproduction All else being equal population grows faster when there are more reproductive episodes per lifetime Average number of births per reproductive episode Mortality Timing of death is semi predictable for a given species based on Major mortality risks Vulnerability at different ages Age at 1st reproduction Survivorship curves illustrate mortality patterns Type 1 Many individuals reach oldest possible age Type 2 Mortality uniform not age related It s a straight line Type 3 Most die early the few who survive have long lives Study these Dispersal Patterns Species differ in frequency timing of movements in space Immigration movement into population Emigration movement out of population Dispersal rates may change as population grows Emigration is often a response to overcrowding Lemmings throw themselves off a cliff when population gets too large but not for the good of the population why Organisms can t predict the future and then adjust behavior to hold down population size Such suicidal behavior couldn t spread the non suicidals would leave more offspring How can there be so much variation among species Organisms can t maximize every life history trait because resources for reproduction are not infinite If zillions of offspring are produced each must be tiny and tiny means vulnerable If they re reproducing early and often can t live for long Life fast die young These are life history tradeoffs Size number trade off Survival reproduction trade off So all else is not equal Life history strategy a species predictable combo of life history traits Neither wins both work By having an early onset of reproduction pygmies trade off time spent growing allowing them to avoid extinction in the face of high mortality
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