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Chapter 5: Population Ecology Copyright © 2003 Pearson Education, Inc. publishing as Benjamin Cummings The Spread of Shakespeare's Starlings • In the 1800s and early 1900s, introducing foreign species of animals and plants to North America was a popular, unregulated activity • In 1890, a group of Shakespeare enthusiasts released about 120 starlings in New York's Central Park – It was part of a project to bring to America every bird species mentioned in Shakespeare’s works Copyright © 2003 Pearson Education, Inc. publishing as Benjamin Cummings • Today, the starling range extends from Mexico to Alaska • Their population is estimated at well over 100 million Current 1955 Current 1955 1945 1935 1925 1945 1905 1915 1935 1925 1925 1935 Copyright © 2003 Pearson Education, Inc. publishing as Benjamin Cummings • Over 5 million starlings have been counted in a single roost • Starlings are omnivorous, aggressive, and tenacious • They cause destruction and often replace native bird species • Attempts to eradicate starlings have been unsuccessful Copyright © 2003 Pearson Education, Inc. publishing as Benjamin Cummings • The starling population in North America has some features in common with the global human population – Both are expanding and are virtually uncontrolled – Both are harming other species • Population ecology is concerned with changes in population size and the factors that regulate populations over time Copyright © 2003 Pearson Education, Inc. publishing as Benjamin Cummings 35.1 Populations are defined in several ways • Ecologists define a population as a singlespecies group of individuals that use common resources and are regulated by the same environmental factors – Individuals in a population have a high likelihood of interacting and breeding with one another • Researchers must define a population by geographic boundaries appropriate to the questions being asked Copyright © 2003 Pearson Education, Inc. publishing as Benjamin Cummings POPULATION STRUCTURE AND DYNAMICS 35.2 Density and dispersion patterns are important population variables • Population density is the number of individuals in a given area or volume • It is sometimes possible to count all the individuals in a population – More often, density is estimated by sampling Copyright © 2003 Pearson Education, Inc. publishing as Benjamin Cummings • One useful sampling technique for estimating population density is the mark-recapture method Figure 35.2A Copyright © 2003 Pearson Education, Inc. publishing as Benjamin Cummings • The dispersion pattern of a population refers to the way individuals are spaced within their area – Clumped – Uniform – Random Copyright © 2003 Pearson Education, Inc. publishing as Benjamin Cummings • Clumped dispersion is a pattern in which individuals are aggregated in patches – This is the most common dispersion pattern in nature – It often results from an unequal distribution of resources in the environment Figure 35.2B Copyright © 2003 Pearson Education, Inc. publishing as Benjamin Cummings • A uniform pattern of dispersion often results from interactions among individuals of a population – Territorial behavior and competition for water are examples of such interactions Figure 35.2C Copyright © 2003 Pearson Education, Inc. publishing as Benjamin Cummings • Random dispersion is characterized by individuals in a population spaced in a patternless, unpredictable way – Example: clams living in a mudflat – Environmental conditions and social interactions make random dispersion rare Copyright © 2003 Pearson Education, Inc. publishing as Benjamin Cummings 35.3 Idealized models help us understand population growth • Idealized models describe two kinds of population growth – exponential growth – logistic growth Copyright © 2003 Pearson Education, Inc. publishing as Benjamin Cummings Example of Exponential Growth Describes the rate of expansion of a population under ideal, unregulated conditions. Figure 35.3A Copyright © 2003 Pearson Education, Inc. publishing as Benjamin Cummings • Logistic growth is slowed by populationlimiting factors – It tends to level off at carrying capacity – Carrying capacity is the maximum population size that an environment can support at a particular time with no degradation to the habitat Figure 35.3B Copyright © 2003 Pearson Education, Inc. publishing as Benjamin Cummings How carrying capacity plays a role in exponential growth and logistic growth: Copyright © 2003 Pearson Education, Inc. publishing as Benjamin Cummings • The logistic growth model predicts that – a population's growth rate will be low when the population size is either small or large – a population’s growth rate will be highest when the population is at an intermediate level relative to the carrying capacity Copyright © 2003 Pearson Education, Inc. publishing as Benjamin Cummings 35.4 Multiple factors may limit population growth • Density-dependent limiting factor- a factor that causes population growth to decreases a result of the population size – Competition – Predation – Parasitism – Disease Copyright © 2003 Pearson Education, Inc. publishing as Benjamin Cummings • Density-independent limiting factorsaffect all populations in similar ways, regardless of the population size – Weather – Natural Disasters – Seasonal Cycles – Human Activities • Damming rivers • Clear-cutting forests • DEVELOPMENT Copyright © 2003 Pearson Education, Inc. publishing as Benjamin Cummings • Abiotic factors may limit many natural populations – Aphids show exponential growth in the spring and then rapidly die off when the climate becomes hot and dry in the summer Figure 35.4B Copyright © 2003 Pearson Education, Inc. publishing as Benjamin Cummings • Most populations are probably regulated by a mixture of factors – Density-dependent birth and death rates – Abiotic factors such as climate and disturbances • Populations often fluctuate in number – A natural population of song sparrows often grows rapidly and is then drastically reduced by severe winter weather Copyright © 2003 Pearson Education, Inc. publishing as Benjamin Cummings Figure 35.4C 35.5 Some populations have "boom-and-bust" cycles • Some populations go through boom-and-bust cycles of growth and decline • Example: the population cycles of the lynx and the snowshoe hare – The lynx is one of the main predators of the snowshoe hare in the far northern forests of Canada and Alaska Copyright © 2003 Pearson Education, Inc. publishing as Benjamin Cummings – About every 10 years, both hare and lynx populations have a rapid increase (a "boom") followed by a sharp decline (a "bust") Figure 35.5 Copyright © 2003 Pearson Education, Inc. publishing as Benjamin Cummings • Recent studies suggest that the 10-year cycles of the snowshoe hare are largely driven by excessive predation – But they are also influenced by fluctuations in the hare's food supply • Population cycles may also result from a time lag in the response of predators to rising prey numbers Copyright © 2003 Pearson Education, Inc. publishing as Benjamin Cummings THE HUMAN POPULATION 35.8 Connection: The human population has been growing exponentially for centuries • The human population as a whole has doubled three times in the last three centuries • The human population now stands at about 6.7 billion and may reach 9.3 billion by the year 2050 Current Clock • Most of the increase is due to improved health and technology – These have affected death rates Copyright © 2003 Pearson Education, Inc. publishing as Benjamin Cummings How to Calculate Growth Rates: Four influences: Birth rate Death rate Immigration Emigration Birth + Immigration – Death – Emigration = Population Count Copyright © 2003 Pearson Education, Inc. publishing as Benjamin Cummings • The history of human population growth Figure 35.8A Copyright © 2003 Pearson Education, Inc. publishing as Benjamin Cummings • The ecological footprint represents the amount of productive land needed to support a nation’s resource needs • The ecological capacity of the world may already be smaller than its ecological footprint http://www.myfootprint.org/ Copyright © 2003 Pearson Education, Inc. publishing as Benjamin Cummings • Ecological footprint in relation to ecological capacity Figure 35.8B Copyright © 2003 Pearson Education, Inc. publishing as Benjamin Cummings • The exponential growth of the human population is probably the greatest crisis ever faced by life on Earth Figure 35.8C Copyright © 2003 Pearson Education, Inc. publishing as Benjamin Cummings 35.9 Birth and death rates and age structure affect population growth • Population stability is achieved when there is zero population growth – Zero population growth is when birth rates equal death rates Copyright © 2003 Pearson Education, Inc. publishing as Benjamin Cummings • The demographic transition is the shift from high birth and death rates to low birth and death rates – During this transition, populations may grow rapidly until birth rates decline Figure 35.9A Copyright © 2003 Pearson Education, Inc. publishing as Benjamin Cummings • The age structure of a population is the proportion of individuals in different agegroups – Age structure affects population growth Copyright © 2003 Pearson Education, Inc. publishing as Benjamin Cummings RAPID GROWTH SLOW GROWTH ZERO GROWTH/DECREASE Kenya United States Italy Male Female Male Female Ages 45+ Ages 45+ Ages 15–44 Ages 15–44 Under 15 Percent of population Male Female Under 15 Percent of population Percent of population Figure 35.9B Copyright © 2003 Pearson Education, Inc. publishing as Benjamin Cummings • Age-structure diagrams not only reveal a population's growth trends – They also indicate social conditions • Increasing the status and education of women may help to reduce family size Copyright © 2003 Pearson Education, Inc. publishing as Benjamin Cummings