Population Ecology (Chapter 52)

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Population Ecology
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The study of populations in relation to the environment, including environmental influences on population density and distribution, age structure, and variations in population size.
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Population
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Group of individuals of a single species living in the same general area. Every population has specific boundaries and a specific size.
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Density and Dispersion
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Once boundaries are defined, population can be described in terms of density (the number of individuals per unit area or volume) and dispersion (the pattern of spacing among individuals within the boundaries of the population).
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Mark-recapture method
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It is impractical/impossible to count all individuals in a population. Instead ecologists use a variety of sampling techniques to estimate densities and total population size. In the mark-recapture method, researchers place traps within the boundaries of the population under study. Captured animals are marked and then released. After some time, traps are set again. The second capture yields both marked and unmarked individuals. From this, researchers can estimate the total number of individuals in the population. The limitation is that it assumes each marked individual has the same probability of being trapped as each unmarked individual. (Total Number Captured)(Total Number Marked)/(Total # Recaptured)
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Dynamic Interplay of Density
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Birth and immigration (influx of new individuals from other areas) add individuals to a population. Death and emigration (movement of individuals out of a population) removes individuals from a population. As population density increases, each individual has access to fewer resources.
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Patterns of Dispersion
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Local densities may vary substantially due to environmental differences or social interactions.
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Patterns of Dispersion: Clumped
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The most common pattern, with individuals aggregated in patches. Many animals spend their time in a particular environment that satisfies their requirements (Forest insects and salamanders under logs). Clumping may also be associated with mating behavior (mayflies swarming to increase mating chances). Group living may also increase the effectiveness of certain predators (wolf packs are more efficient at killing than a single wolf).
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Patterns of Dispersion: Uniform
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Dispersion is evenly spaced, possibility resulting from direct interactions between individuals in the population. Animals often exhibit uniform dispersion due to antagonistic social interactions, such as territoriality (the defense of a bounded physical space against encroachment by other individuals). Uniform patterns are not as common in populations as clumped patterns.
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Patterns of Dispersion: Random
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Unpredictable spacing occurring in the absence of strong attractions or repulsions among individuals of a population. Key physical or chemical factors relatively homogeneous across the study area is also another factor. The position of each individual is independent of other individuals. Random patterns are not common.
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Demography
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The study of the vital statistics of populations and how they change over time.
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Life tables
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Age-specific summaries of the survival pattern of a population. The number of individuals that die in each age group must be known to calculate the proportion of the cohort (group of individuals of the same age, from birth until all are dead) surviving from one age to the next.
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Survivorship Curves
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A plot of the proportion or numbers in a cohort (Group of individuals of the same age, from birth to death) still alive at each age. Type 1 curve is flat at the start, reflecting low death rates during early/middle life, then drops steeply as death rates increase among older age groups (Pattern of humans and mammals that produce few offspring but provide them with good care). Type III curve drops sharply at the start, reflecting very high death rates for the young, but then flattens out as death rates decline for those few individuals that have survived to a certain critical age (Pattern of organisms that produce very large numbers of offspring but provide little or no care) Type II curves are intermediate, with a constant death rate over the organism’s life span.(Pattern of rodents, various invertebrates, some lizards, and some annual plants). In populations without immigration or emigration, survivorship is one of two key factors determining changes in population size. (The other key factor is reproductive rate).
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Reproductive Table (Fertility Schedule)
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An age-specific summary of the reproductive rates in a population. The table measures the reproductive output of a cohort from birth until death, tallying the number of female offspring produced by each age group.
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Life History
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The traits that affect an organism’s schedule of reproduction and survival. Three basic variables include: age reproduction begins, how frequently organism reproduces, and how many offspring are produced during each reproductive episode. Life history traits are evolutionary outcomes reflected in the development, physiology, and behavior of an organism.
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Big-bang Reproduction (Semelparity)
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Producing thousands of small eggs in a single reproductive opportunity. (Examples: Salmon and Agave plant). Favored when survival rate of offspring is low, as in highly variable or unpredictable environments. Production of large numbers increases the probability that at least some will survive.
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Repeated Reproduction (Iteroparity)
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Repeating reproductive act annually for several years. (Example: Lizards) Favored in more dependable environments, where competition for resources may be intense. A few well-provisioned offspring will have a better change of surviving to reproductive age.
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Natural Selection (Trade-offs)
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Organisms have finite resources. There is a trade-off between reproduction and survival. Selective pressure also influence the trade-off between the number and size of offspring. Generally, extra investment on the part of parents greatly increases the offspring’s chances of survival.
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Population Growth
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Eventually, a population stops increasing due to limited resources and other factors.
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Per Capita Birth Rate
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The expected number of offspring produced per unit time in a population of any size. (The average number of births per individual during the specified time interval). If individuals cannot obtain sufficient resources to reproduce, the per capita birth rate will decline.
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Per Capita Death Rate
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The expected number of deaths per unit time in a population of any size. (The average number of deaths per individual during the specified time interval). If individuals cannot consume enough energy, the per capita death rate will increase.
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Zero Population Growth
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Occurs when the per capita birth and death rates are equal.
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Exponential (Geometric) Population Growth Model
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The pattern for population increase under ideal conditions. J-shaped growth curve is characteristic of some populations that are introduced into new environments or whose numbers have been drastically reduced by a catastrophic event and are rebounding. The model assumes resources are unlimited.
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Intrinsic Rate of Increase
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The maximum per capita rate of increase for a specie
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Carrying Capacity
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The maximum population size that a particular environment can support. Carrying capacity varies over space and time with the abundance of limiting resources (energy, shelters, refuges from predators, soil nutrients, water, and suitable nesting sites). The symbol for carrying capacity is K.
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Logistic Population Growth Model
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The per capita rate of increase declines as carrying capacity is reached. An S-shaped growth curve is formed. This model assumes that populations adjust instantaneously to growth and approach carrying capacity smoothly. It predicts different per capita growth rates for populations of low or high density relative to the carrying capacity.
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K-selection (Density-dependent Selection)
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Selection for life history traits that are sensitive to population density. This selection maximizes population size and operates in populations living at a density near the limit imposed by their resources (carrying capacity, K)
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r-selection (Density-independent Selection)
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Selection for life history traits that maximize reproductive success in uncrowded environments. This selection maximizes the rate of increase and occurs in environments in which population densities fluctuate well below carrying capacity or individuals are likely to face little competition.
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Density Independent
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A birth rate or death rate that does not change with population density.
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Density Dependent
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A death rate that rises as population density rises and/or birth rate declines as density rises.
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Density-Dependent Population Regulation
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Density-dependent birth and death rates control population growth. Negative feedback mechanisms are: competition for resources, territoriality, health, predation, toxic wastes, and intrinsic factors.
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Population Dynamics
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Focuses on the complex interactions between biotic and abiotic factors that cause variation in population size.
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Metapopulation
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Formed when a group of populations is linked. Higher level of immigration combined with a higher survival rate result in greater stability in the native population.
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Demographic Transition (Human Population)
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Associated with an increase in the quality of health care and sanitation as well as improved access to education, especially for women.
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Age Structure (Human Population)
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The relative number of individuals of each age, commonly represented in \”pyramids.\” The diagrams not only predict a population’s growth trends but also illuminate social conditions.
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Infant Morality
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The number of infant deaths per 1,000 live births
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Life expectancy at birth
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The predicted average length of life at birth.
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Ecological Footprint
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Summarizes the aggregate land and water area appropriated by each nation to produce all the resources it consumes and to absorb all the waste it generates. Types of ecologically productive areas are: arable land, pasture, forest, ocean, built-up land, and fossil energy land.

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