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Topic 5.3 / Option G.1: Community Ecology 2 Populations and Sampling Methods Assessment Statements: , G.1.3-G.1.4

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Populations A group of organisms that occupy the same space (defined) at the same time. Population size is affected by additions to the population and subtractions from the population. Additions occur due to natality (birth) and immigration Subtractions occur due to mortality (death) and emigration.

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Demography Is the study of population statistics and how they change over time. Survivorship curves represent the proportion of a particular cohort that is alive at various points in time. There are 3 basic types. These show where the most births and deaths are occuring in the population. Understanding the type of survivorship curve a species has can help ecologists manage threatened and endangered populations.

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Reproductive rates By looking at the reproductive rates of species as well as the ages of the individuals in the population, population ecologists can predict the amount of growth that would occur at a given time. For example, a population with the majority of females in reproductive age would increase faster than a population with the majority of females past reproductive age.

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The exponential model of population growth Assuming that all females in a population can reproduce at their maximum capacity, that there is enough resources, and that there is no immigration or emmigration, then we can estimate population growth rate as: dN/dt = b(N) – d(N) dN = the change in the number of females, dt = the change in time, b= theoretical birth rate and d = theoretical death rate Assume r = b-d, then we have the growth rate as dN/dt=rN

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Carrying Capacity Environments cannot support unlimited population growth as represented by the exponential growth curve. We introduce a new factor, K which is the environmental carrying capacity, or the maximum number of individuals an environment can support. dN/dt = rN(K-N/K) Then, as N reaches K, the growth rate gets smaller. When N passes K, the growth rate is negative (the population declines) The curve that is created is called the logistic growth curve.

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Parts of the logistic growth curve Exponential phase: This is when N is much much less than K, so rN is multiplied by a factor close to 1. Then birth rate is much greater than death rate. Transition phase: This is when N is getting close to K, so rN is multiplied by a factor close to 0. Then birth rate is not much greater than death rate. Plateau phase: This is when N is equal to K, so rN is multiplied by 0. The birth rate = the death rate and the population is stable.

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What really happens: Populations rarely approach their carrying capacities in so neat a way. Usually they overshoot. When N is greater than K, rN gets multiplied by a negative number and becomes negative. In this case the birth rate is lower than the death rate, so the population declines. Populations usually go up and down around K before stabilizing

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What limits an environment’s carrying capacity? Population regulation can be density dependent or density independent. Density independent: a population is controlled by factors that kill similar proportions of populations regardless of density. –Examples include seasonal changes, natural disasters, climate change

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Density dependent factors set carrying capacity These factors are “made worse” the larger a population is. They act as a negative feedback mechanism to reduce population size. –Competition: resources are limited, and the more that have to share, the less everyone gets. –Territoriality: this is competition for space. If there are no nesting sites, you can’t breed.

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–Health: The more crowded it is, the easier it is for disease to spread throughout the population –Predation: predators may concentrate on a species that is easy to catch because there are so many. –Waste buildup: accumulation of waste is toxic to organisms, the more there is, the less likely it is to be decomposed or dispersed. –Intrinsic factors: depend on the physiology of the animal. For example, mice that are crowded are more violent toward each other.

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Why sample populations? To know the size of the population in a specified area To know the density of the population To know the proportion of the population with a specific characteristic To know how a population varies in relationship to particular environmental factor

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Random Samples In order to get an accurate measure of a population without checking each individual, it is necessary to use random samples, and use those results to represent the entire population. A random sample means that each member of the population has an equal chance of being checked. If we did not take a random sample, for example, only checked in areas that were easy to get to, we could not be sure of an accurate reflection of the whole population.

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The Quadrat/Sample plot method Involves counting the number of individuals within random sample plots or quadrats. You must know the size of the quadrat compared to the size of the whole area being considered. Count the number of individuals in all the quadrats, then compare that to the total area of the study site. Example: there are 45 trilliums in 10 1m 2 quadrats, therefore there are 450 trilliums in the 100 m 2 study area.

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Assumptions of the quadrat method The number of individuals in the quadrat must be known exactly. The size of the quadrats must be known. The quadrats must be representative of the whole study area. The quadrats must be placed randomly, not uniformly.

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Transects The transect uses the quadrat method, only instead of placing quadrats randomly throughout an area, they are placed along a specified line that denotes a change in an abiotic variable. This allows one to study the relationship between population(s) and abiotic factors. Examples include: –Intertidal zone, which includes variations in salinity, light, water coverage, temperature etc. –Forest to clearing, which includes variations in light –Stream/lake/ocean depth, which includes variations in depth, light and temperature

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