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Topic 5.3 / Option G.1: Community Ecology 2

Topic 5.3 / Option G.1: Community Ecology 2. Populations and Sampling Methods Assessment Statements: 5.3.1-5.3.4, G.1.3-G.1.4. Populations. A group of organisms that occupy the same space (defined) at the same time.

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Topic 5.3 / Option G.1: Community Ecology 2

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  1. Topic 5.3 / Option G.1: Community Ecology 2 Populations and Sampling Methods Assessment Statements: 5.3.1-5.3.4, G.1.3-G.1.4

  2. 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.

  3. 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.

  4. 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.

  5. 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

  6. 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.

  7. 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.

  8. 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

  9. 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

  10. 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.

  11. 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.

  12. 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

  13. 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.

  14. 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 1m2 quadrats, therefore there are 450 trilliums in the 100 m2 study area.

  15. 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.

  16. 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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