ECOL 182R 1st Edition Lecture 15 Outline of Last Lecture I Cosmology II Origins of Life III Exobiology A Life nearby IV Mass Extinctions A The Big Five B Recent future extinctions Outline of Current Lecture I Population II Unlimited growth A Bacteria B Generation time III Limited growth A Carrying capacity B Logistic growth IV Estimating population size Current Lecture Population individuals of the same species occurring in the same place interbreeding They change over time and they change over space Dynamics differ enormously across populations species Why should we care It d be useful to be able to predict how population size changes over time Helps with disease viruses etc For medicine information Conservation Agriculture Social issues How large the human population will be Unlimited Growth Bacteria Binary fission 1 cell 2 cells Population can double every half hour 1 million after 10 hours Graphed out it would show an exponential growth the exponential curve Which captures the phenomenon constant doubling time accelerating growth rate The bigger the population the greater its capacity to grow more Rate of population increase pop size x avg birth rate per indiv avg death rate per indiv OR dN dt N b d Maximum b d for a population is its intrinsic rate of increase r So often b d is replaced with r Sometimes populations do grow really fast Most undergo some exponential growth before stabilizing or crashing Generation time the avg length between birth of parents and birth of offspring Ex Human generation time is 20 years Limited Growth Exponential growth assumes pop can grow forever when in reality resources start running out Carrying capacity the largest pop size that can persist on a given amount of resources Often called called K To determine K you d need to know What resources are needed in what amount Rates of resource renewal How much habitat are needed Result of limited resources logistic or sigmoid S shaped growth Pop growth rate exponential growth rate x a factor that slows growth as pop approaches carrying capacity dN dt b d N x K N K If 50 suddenly dies during exponential phase growth resumes At a rate that was identical to when the population was this size before to a carrying capacity that is the same carrying capacity is set by the environment What is carrying capacity is exceeded Populations can t respond instantly to resource depletion there s a time lag So populations must shrink many ways this might happen though Estimating Population Size How to figure out if it s growing or shrinking Follow it over time or use indirect evidence like age structure But how do you figure out how big a population is in the first place Saving endangered species Dealing with pests The best approach will depend upon population size and population mobility Option 1 Count everyone small population or if there s a very even distribution Sometimes seems feasible but isn t Ex US census what about homeless people Option 2 Count some then estimate large moving population Use quadrats area of known size enclosing countable individuals Count number in qudrat then calculate number of quadrats in entire area Mark recapture method mark many individuals let them go wait long enough for them to intermix capture a bunch estimate pop size based on percent of second sample that have marks of marker indivis in 2nd sample total 2nd sample marked originally population size population size being the unknown This method makes 3 assumptions 1 You re equally likely to catch marked and unmarked ones 2 Marking doesn t do any harm 3 There s no reproduction immigration or emigration between captures
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