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Population Ecology Chapter 4 Population • An interacting group of individuals of a species that use common resources and are regulated by the same natural phenomena, such as temperature, water and food supply, and predation • Population density-the number of individuals of a species per unit area or volume Key Concepts Factors affecting population size Species reproductive patterns Species survivorship patterns Conservation biology and human impacts on ecosystems Spatial Distribution Dispersion – the pattern of spacing of a population within an area Population ranges- not all species occupy all habitats in the biosphere: limited and vast ranges Dispersion Patterns Uniform is even pattern of Dispersion that results in Interactions among Individuals of a population Creosote bush and black Bears are examples Clumped is when individuals are aggregated into patches, is the most common in nature. Elephants and the American Bison are clumped in groups or herds. Random occurs when Individuals are spaced, in a patternless, unpredictable way. Dandelions And white-tailed deer are examples. Density Independent Factors Limit Population Growth • Density Independent – density of the population doesn’t matter, the population will be affected • Natural disasters (climate, weather); environmental factors (fire, floods, storms); and habitat disruption by human activity Density Dependent Factors Limit Population Growth • Density Dependent – when population-limiting factors depend on the density of the population • Competition for resources • Predation • Parasitism • Disease What Limits a Population? • Birth • Death • Immigration • Emigration Population = (Birth + Immigration) - (Deaths + Emigration) Growth Models • Exponential Growth Model -gives an idealized picture of the unregulated growth of a population • Equation is G= rN describes the J-shaped curve, which is typical of exponential growth; G=growth, r=intrinsic rate of increase, N=population size • The bigger the value of N, the faster the population increases • The lower part of the J results from the relatively slow growth when the population is small • The steep, upper part of the J results from N being large Growth Models • Logistic Growth Model-a description of idealized population growth that is slowed by limiting factors; results in a sigmoidal or “S” shaped curve • Describes the effect of population-limiting factors as well as population growth • Equation is more complicated than exponential; (KN)/K; K is for “carrying capacity” • Carrying Capacity is the number of individuals in a population that the environment can maintain “carry” without a net increase or decrease. Population Growth Curves Exponential - J curve, starts slow then fast Logistic - s curve, exponential growth, environmental effects, then leveling Population size (N) Population size (N) K Time (t) Time (t) Exponential Growth Logistic Growth What affects K? (Carrying Capacity • competition • immigration & emigration • natural & human caused catastrophes • seasonal fluctuations affect food, water, shelter Reproductive Patterns • Depends on several factors • Species vary in the number of births per reproduction cycle, in the age reproduction begins, and in the life span of the organism • R-strategy is an adaptation for living in an environment where fluctuation of biotic and abiotic factors occur (availability of food or changes in temperature) • R-strategist is usually a small organism such as a fruit fly, mouse or locust. Generally have short life spans and produce many offspring. – Expend little or no energy in raising their young to adulthood – Controlled by density-independent factors – Do not maintain a population near its carrying capacity Reproductive Patterns • K-strategy or carrying capacity is an adaptation for living in predictable environments • K-strategist is generally a larger organism with a long life span – Produces few offspring whose population reaches carrying capacity – Have a better chance of survival because energy, resources and time is invested in caring for the young – Controlled by density-dependent factors Survivorship Curves • Type 1 survivorship curve: ex. Whales, elephants, humans High survival rates until old age. Example: humans % of survivors % of maximum life span Survivorship Curves • Type 2 survivorship curve: mortality constant over the life span; ex. Hydras, squirrels Intermediate between the extremes. Example: % of survivors squirrel % of maximum life span Survivorship Curves • Type 3 survivorship curve: High mortality (death) rates as larvae/very young but decreased mortality at later ages. Example: % of survivors oysters frogs insects % of maximum life span Survivorship Curves Percentage surviving (log scale) 100 10 1 0 Fig. 9.11, p. 206 Age Trends in Human Population Growth Demographic transition is a change in a population from high birth and death rates to low birth and death rates If (Birth + Immigration) = (Deaths + Emigration) then it’s called ZERO POPULATION GROWTH Age Structure Diagrams • Gives the proportion of individuals in different age groups Demography • The study of the human population size, density, distribution, movement and birth, and death rates