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Lecture 7

Lecture 7-8 Notes.pdf

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Biological Sciences
Marc Cadotte

Lecture 7-8: Population Growth and Regulation Factors Affecting Human Population ▯ - you have to understand how population change over time ▯ - Very recently, human population has exploded in size ▯ - The reason for this has to do with technological innovations, industrial revolution, modern medicine, etc ▯ - However, the Bubonic plague caused a greater decrease in population ▯ - In 1975, the population was growing at an annual rate of nearly 2%. At this rate, a population will double in ▯ size every 35 years. If this growth rate were sustained, we would reach 32 billion by 2080 ▯ - But growth rate has slowed recently, to about 1.21% per year. If this rate is maintained, there would be ▯ roughly 16 billion people on Earth in 2080. Could Earth support 16 billion people? Introduction ▯ - One of the ecological maxims is: “No population can increase in size forever” ▯ - If no one can increase in size forever, what does that mean for the human population? ▯ ▯ - human population has declined ▯ ▯ - Through maybe, women waiting longer to reproduce, women getting more educated ▯ ▯ - we must understand why population grows Life Tables ▯ - Life tables show how survival and reproductive rates vary with age, size, or life cycle ages ▯ - Information about births and deaths is essential to predict trends or future population size ▯ - Data for a life table for the grass Poa annua were collected by marking 843 naturally germinating seedlings ▯ and then following their fates over time ▯ - Sx= age-specific survival rate: chance that an individual of age x will survive to age x +1 ▯ - x = survivorship: proportion of individuals that survive from birth (age 0) to age x ▯ - Fx= fecundity: average number of offspring produced by a female while she is of age x ▯ - Hereʼs the table ▯ - A cohort life table follows the fate of a group of individuals all born at the same time (a cohort). This is usually ▯ hard to do since you gave to follow individuals throughout time ▯ - In some cases, a static life table can be used- survival and reproduction of individuals of different ages during ▯ a single time period are recorded ▯ - A survivorship curve is a plot of the number of individuals from a hypothetical cohort that will survive to reach ▯ different ages ▯ ▯ - Survivor ship curves can be classified into 3 general types ▯ ▯ - Type I: high survival rate and then decline with old age ▯ ▯ - Type II: linear survival rate ▯ ▯ - Type III: rapid decline. Once you have made it through ▯ ▯ childhood, you have a higher chance of surviving Age Structure ▯ - Life table data can be used to project the future age structure, size, and growth rate of a population ▯ - A population can be characterized by its age structure- the proportion of the population in each age class ▯ - Age structure influences whether a population will increase or decrease in size ▯ - Three types: ▯ ▯ - Rapid growth (eg. Guatemala, Nigeria, Saudi Arabia) ▯ ▯ most young people are heading to the reproductive age ▯ ▯ - Zero growth (eg. Spain, Austria, Greece) ▯ ▯ have relatively the same people dying as are ▯ ▯ being born ▯ ▯ - Negative growth (eg. Germany, Bulgaria, Italy) ▯ ▯ more old people than younger people ▯ - Life table data can be used to predict age structure and population size ▯ - ▯ - Assume the population starts with 100 individuals ▯ ▯ Age class 0 = 20 individuals ▯ ▯ Age class 1 = 30 ▯ ▯ Age class 2 = 50 ▯ - Two things must be calculated: ▯ ▯ 1. Number of individuals that will survive to the next time period ▯ ▯ 2. Number of newborns those survivors will produce in the next time period ▯ - Results from this shows: ▯ ▯ - there is a lot of fluctuation since there is not a good age representation ▯ ▯ - the population is increasing/growing ▯ ▯ - Refer to graph above ▯ - The growth rate (⋋) can be calculated as the ration of the population size in year t + 1 (N ) to the t+1 ▯ population size ▯in year t (N) t ▯ - Equation: ⋋ = N /N t+1 t ▯ ▯ - If your population is growing then the top number will always be greater then the denominator ▯ ▯ - lambda (⋋) will be greater than 1 ▯ ▯ - When age-specific survival and fecundity rates are constant over time, the population ultimately grows at a ▯ fixed rate ▯ - The age structure does not change from one year to the next- it has a stable age distribution ▯ - In the example, the stable age distribution is 0.73 in age class 0, 0.17 in age class 1, and 0.10 in age class 2 ▯ - Any factor that alters survival or fecundity of individuals can change the population growth rate ▯ - Two things greatly affect population growth: survival and production ▯ - This can be used to develop management practices that decrease pest populations or increase an ▯ endangered population ▯ - Example: Loggerhead sea turtles are threatened by development on nesting sites and commercial fishing s t e ▯ n ▯ - Reasons for the decline on the turtle population: ▯ ▯ - fisheries, development of resorts that impose on their laying space, being eaten by predators ▯ ▯ - scientist are trying to find ways to increase survival rates. Some people have been focusing on ▯ ▯ eggs, but others are focusing on trying to get the nets to have holes so turtles can be released ▯ - Even if hatching survival were increased to 100%, loggerhead populations would continue to decline ▯ - Instead, population growth rate was most responsive to decreasing mortality of older juveniles and adults ▯ - Shrimping resulted in 5,000 to 50,000 loggerhead deaths per year! Exponential Growth ▯ - Populations can grow exponentially when conditions are favourable, but exponential growth cannot continue ▯ indefinitely ▯ ▯ - In general, populations can grow rapidly whenever individuals leave an average of more than one offspring ▯ over substantial periods of time ▯ - If a population reproduces in synchrony at regular time intervals (discrete time periods), and growth rate ▯ remains the same, geometric growth occurs ▯ - The population increases by a constant proportion, so the number of individuals added to the population ▯ becomes larger with each time period ▯ - When we use a life table, we are assuming the population is increasing in intervals (Geometric growth), the ▯ population is growing by a constant amount ▯ - Geometric growth: ▯ ▯ N t+1 = ⋋N t ▯ ▯ ⋋: geometric growth rate; also known as the (per capita) finite rate of increase f ▯ ▯ I ⋋ = 1 the population is replacing itself and not growing ▯ - Geometric growth rate predicts exponential increase in population size t ▯ - Geometric growth can also be represented by N = ⋋N t 0 ▯ ▯ - N 0ives you the starting population size today ▯ - This predicts the size of the population after any number of discrete time periods ▯ - In any species, indivi
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