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Chapter 54

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Biological Sciences
Mary Olaveson

Biology – Chapter 54 Population: the individuals of a species within a given area Population Structure: The age distribution of individuals in a population and the ways those individuals are spread over the environment Population Density: the number of individuals of a population per unit area (volume) Demographic Events: Events such as births, deaths, immigrations, and emigrations that influence the change of the structure of a population The study of the births, deaths, and movement rates that create a population dynamics are known as demography To study these populations, it requires ecologists to count the number of individuals in the area as well as the number of deaths, births, and immigrations/emigrations. This is often done by mapping/tagging the animals Electronic tagging also tell a lot about the environment and the animal Population densities In many cases, population densities are far too vast to count; therefore densities must be estimated by counting the number of individuals in a sample of representative habitats and extrapolating them to cover the entire area A problem may be encountered when counting the amount of mobile individuals within a species as they can immigrate/emigrate out of the sample area This is solved by marking all the individuals within the area, allowing them to roam free and interact with other individuals outside the same area, and then another sample is taken. The proportion of individuals in the new sample that are marked can be used to estimate the size of the population using the formula: m 2 = n 1 n1= # of marked individuals in first sample(all of them) n2 N n2 = # of individuals in the second sample m 2 # of marked individuals in the second sample N = The estimated size of the population This method has its faults, however. This method can only be accurate if it assumes that the populations of tagged and untagged individuals randomly mix. Also it would be inaccurate if the individuals learn to avoid traps or become trap-happy (because of the free meals) Birth/Death Rates The rate (number per unit time) of births, deaths, and movement of a population can be calculated using population density Ecologists use the equation to determine the rate: N = N + B - D + I - E N = # of individuals at time 1 1 0 1 N0= # of individuals at time 0 B = # of individuals born between time 0 and 1 D = # of individuals that died between time 0 and 1 I = # of individuals that immigrated between time 0/1 E = # of individuals that emigrated between time 0/1 Performing this several times can determine how population density changes over time Another way of tracking such changes includes using a life table. This is done by tracking a group of individuals born at the same time (called a cohort) and seeing how many of them are still alive after later dates (survivorship) and sometimes the children of the survivors (foundity) All populations have the potential to increase exponentially. As the population increases, the number of individuals in a population increases and the number of new members added increases, even if the rate of growth (per capita growth rate) remains constant R = = (b-d) N r = the net reproductive rate = Change in number of individuals = change in time B = average population per capita birth rate D = the average population per capita death rate The highest possible value for reproductive rate (r) is called the intrinsic rate of increase and be expressed = max N. This rate can be achieved unde
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