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populations • Population – Group of organisms of the same species living in a specific area that can mate and produce fertile offspring Why measure populations • They share a common gene pool – Reflects interaction between a population and its environment (natural selection) • Determine their health or stability – Is there growth or decline – How is a new species – What factors affect growth, stability, decline • 1st step measuring size Problems measuring size • Population density – more meaningful than total number unless close to extinction – Number of individuals per unit volume or area – Comparisons over space and time Problems measuring size • Distribution – The characteristic pattern of spacing of individuals within a population Types of distribution – Random/Patchy – scattered suitable areas w/ population boundries – Clumped/grouped - most common – herds, flocks- soil conditions or gathering for reprod. – Uniform/Even – fairly consistent distance Age sex distribution • Age sex differences between populations are significant indicators of the health of a population • Studied by ecologists concerned with a population’s future Age structure diagram • X axis = percentage of population • Y axis = age groups • Males left - females right Age Structure • • • • • Population growth depends, in part, on how many people of different ages make up a given population Demographers can predict future growth using models called age-structure diagrams, or population profiles Age-structure diagrams show the population of a country broken down by gender and age group Each bar in the age-structure diagram represents individuals within a 5-year group Percentages of males are to the left of the center line and females to the right in each group Age Structure • Growing population – usually has more young individuals than adults at beyond reproductive age. • Stable population – equal numbers of young members and adults • Declining population – more adults and fewer young Patterns in Populations Characteristics of populations interact to form several strategies for survival • Birth rate - number of births per individual within the population per unit time • Death rate - number of deaths per individual within the population per unit time • Life expectancy -average survival time for an individual Plotting patterns • Survivorship curves – graph the number of all individuals still living for each age • 3 types – Type I – Type II – Type III • Type I – highest survival rates w/ individuals living to old age – Parental care of offspring – Low infant death rates – Produce few offspring • Type II – Uniform death rates throughout life span – Constant loss pattern – Birds and asexual species • Type III – High death rates at young age; few live to old age – Parents produce high # of young to offset high death rates – no parental care – Plants, invertebrates, fish Exponential Growth • • • • • • • The figure at right shows a graph with the size of the bacterial population plotted against time As you can see, the pattern of growth is a J-shaped curve The J-shaped curve indicates that the population is undergoing exponential growth Exponential growth occurs when the individuals in a population reproduce at a constant rate At first, the number of individuals in an exponentially growing population increases slowly Over time, however, the population becomes larger and larger until it approaches an infinitely large size Under ideal conditions with unlimited resources, a population will grow exponentially Logistic Growth • Obviously, neither bacteria nor elephants cover the planet • This means that exponential growth does not continue in natural populations for very long • What might cause population growth to stop or to slow down? Population Growth on a J-shaped curve • Shows that population grows slowly during the early lag phase and then very rapidly during the exponential phase Logistic Growth Logistic Growth • Logistic Growth This graph shows the Sshaped curve of logistic growth • As resources become less available, the population growth rate slows or stops • The growth of this population has leveled off at its carrying capacity Carrying Capacity • • • • If you look again at the graph to the right, you will see a horizontal line through the region of the graph where the growth of the yeast population has leveled off The point at which that line intersects the y-axis tells you the size of the population when the average growth rate reaches zero That number, in turn, represents the largest number of individuals—in this case, yeast cells—that a given environment can support Ecologists call this number the carrying capacity of the environment for a particular species Population Growth on an S-shaped curve • Levels off after the exponential phase • The top of this curve indicates the carrying capacity of an ecosystem Limiting Factors • • • • • Recall from Chapter 3 that the primary productivity of an ecosystem can be reduced when there is an insufficient supply of a particular nutrient Ecologists call such substances limiting nutrients A limiting nutrient is an example of a more general ecological concept: a limiting factor In the context of populations, a limiting factor is a factor that causes population growth to decrease Some of the limiting factors that can affect a population are shown in the figure at right Limiting Factors POPULATION GROWTH • Limits to Growth – Density-Dependent Factors: factors that affect indifferent ways depending on population density • Food • Space • Light • Parasitic infections • Disease • Number of predators / prey • Oxygen – Density-Independent Factors: factors that affect populations regardless of population density • Usually abiotic – Changes in weather – Changes in temperature – Changes in humidity – Variations in the amount of sunlight – Amount of available energy Limiting Factors • A resource base that is limited can also affect the long-term survival of a species • For example, pandas depend for food on bamboo that grows in certain kinds of temperate forests in China • Since the time that these forests have been cleared for timber and farmland, panda populations have fallen dramatically and have become isolated in small pockets of remaining forest Density-Dependent Factors • A limiting factor that depends on population size is called a density-dependent limiting factor • Density-dependent factors become limiting only when the population density—the number of organisms per unit area—reaches a certain level • These factors operate most strongly when a population is large and dense • They do not affect small, scattered populations as greatly • Density-dependent limiting factors include competition, predation, parasitism, and disease Competition • When populations become crowded, organisms compete with one another for food, water, space, sunlight, and other essentials – Example: puffins must compete for limited nesting sites • Competition among members of the same species is a density-dependent limiting factor • The more individuals living in an area, the sooner they use up the available resources • Likewise, the fewer the number of individuals, the more resources are available to them and the less they must compete with one another Predation Parasitism and Disease • Parasites can also limit the growth of a population • Parasitic organisms range in size from microscopic, disease-causing bacteria to tapeworms 30 centimeters or more in length • These organisms are similar to predators in many ways • Like predators, parasites take nourishment at the expense of their hosts, often weakening them and causing disease or death Density-Independent Limiting Factors • Affect all populations in similar ways, regardless of the population size • Unusual weather, natural disasters, seasonal cycles, and certain human activities—such as damming rivers and clear-cutting forests—are all examples of density-independent limiting factors • In response to such factors, many species show a characteristic crash in population size • After the crash, the population may soon build up again, or it may stay low for some time Density-Independent Limiting Factors • For some species, storms or hurricanes can nearly extinguish a population – For example, thrips, aphids, and other insects that feed on plant buds and leaves might be washed out by a heavy rainstorm • Extremes of cold or hot weather also can take their toll on a population, regardless of the population's density – A severe winter frost, for example, can kill giant saguaro cactuses in the Arizona desert • In some areas, periodic droughts can affect entire populations of vegetation • Such events can, in turn, affect the populations of consumers within the food web Density-Independent Limiting Factors • Environments are always changing, and most populations can adapt to a certain amount of change • Populations often grow and shrink in response to such changes • Major upsets in an ecosystem, however, can lead to long-term declines in certain populations • Human activities have caused some of these major upsets • Human population continues to grow but many scientists believe that we overconsume resources and exceed the environment’s capacity to cycle nutrients and process waste. • Possible population crash is possible Factors that can lead to crisis • Agricultural system –”green revolution” depends on nonrenewable fossil fuels for fertilizers, pesticides, and irrigation • Most countries depend on almost entirely on petroleum to fuel industrialization and transportation Factors that can lead to crisis • Fresh water supplies are declining due to pollution and overuse • Habitat destruction due to agriculture, development, and mining • Burning of fossil fuels has brought about atmospheric change Sustainability • We must consider the impact of our decisions on the next seven generations • A sustainable activity or state can be maintained indefinitely, w/out compromising resources for the future. Sustainability • Sustainability of products and services considers complete life cycles – raw materials, manufacturing, transportation/distribution, use and re-use, maintenance, recycling and disposal. Sustainability • Way to estimate sustainability is your ecological footprint – The amount of land area you would need to sustain your current lifestyle – Considers resources you consume and pollution you generate then calculates the amount of land needed to produce equivalent RENEWABLE resources