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Population Ecology Population: Group of interbreeding members of the same species that exist in a specified area Ecology: Interactions among organisms and between organisms and the environment How Do We Describe Populations? Demographics Statistics that describe a population Size, density and distribution of a population Population Demographics Population size: total number of individuals in a population Population density: number of individuals per unit area or volume Population distribution: spacing of members within a population in a specific area Boundary of a population: natural or arbitrary Population Size Plot Sampling Uniform conditions Mostly non mobile species Used to estimate population size in a defined area based on direct counts in small portions of the area Mark - Release - Recapture Involves marking a number of individuals in a natural population, returning them to that population This technique is commonly used by fish and wildlife managers to estimate population sizes before fishing or hunting seasons Patterns of Distribution Clumped: Attraction Members of a population group in patches/ most common Unequal distribution of resources Social Reduce risk of predation Uniform: Active repulsion Even distribution Competition for limited resources Plants competing for resources/ Allelopathy Territorial behavior in animals Random: minimal influence Uniform distribution of resources No pattern Rare because of social interactions and varying habitats Why does the size of a population change? Interactions between biotic and abiotic factors cause variations in population size Birth and immigration Death and emigration How is population growth measured? Zero population growth Time interval in which the number of births is equal to the number of deaths Per capita growth rate (r)= per capita birth rate (b) – per capita death rate (d) Exponential growth Population grows by a fixed proportion of its total in successive time intervals (assuming r remains constant) Population growth rate (G) = per capita growth rate (r) x number of individuals (N) Death will slow the rate of increase but does not stop the exponential growth Factors Affecting Population Growth Density-dependent factors: Intraspecific competition (food, territory and other resources), predation, diseases Density-independent factors: Weather changes, fires, habitat destruction,…. How Has the Size of the Human Population Changed Over Time? For most of history, the human population grew very slowly Growth rate began to increase about 10,000 years ago, then soared during the past two centuries Expansions and Innovations Invention of agriculture Use of energy in fossil fuels Advances in the medical field and sanitation Migration into new habitats and expansion into new climate zones Development of new technologies that increased the carrying capacity of existing habitats US Census Bureau (2013) Rank 1 Country or Area China Population 1,349,585,838 2 3 4 5 6 7 8 9 10 India United States Indonesia Brazil Pakistan Nigeria Bangladesh Russia Japan 1,220,800,359 316,668,567 251,160,124 201,009,622 193,238,868 174,507,539 163,654,860 142,500,482 127,253,075 Fertility and future growth Fertility rate Age structure: the age distribution of individuals Reproductive base of a population: Members of the prereproductive and reproductive age categories Future Growth World population is expected to reach 8.9 billion by 2050, and possibly to decline as the century ends China and India each hold more than one billion people (together, 38% percent of the world population); the United States is third, with 317 million