Recent and dramatic increases to the size of the human population have had large effects on current issues today Population ecology is the study of how and why the number of individuals in a population changes over time and space Chapter 54 Population Ecology 54 1 Distribution and Abundance Range or geographic distribution is determined by two factors Abiotic factors such as temperature rainfall and the presence of geographical structures like mountains oceans and large scale ongoing historical processes like continental drift Biotic factors such as the past and current presence of other species that provide food habitat or competition Ranges are dynamic and in constant flux as abiotic and biotic factors change over time Population density is the number of individuals per unit area In general individual organisms can be arranged in different patterns within populations Random if position is independent of others Clumped if the quality of habitat is patchy or the organism associates in social groups Uniform if negative interaction occurs among individuals If a species range is small it may consist of a single population of interbreeding individuals If it is large it may consist of many populations If individuals from one species occupy many small patches of habitat so they form many independent populations they are said to represent a A metapopulation is a population of populations that are connected by migration Because humans are reducing large contiguous areas of forest and grasslands to isolated patches or reserves more and more species are being forced into To determine populations in sedentary species researchers count the individuals that occur along the lines of known position and length transects or inside metapopulations metapopulations rectangular plots quadrats 54 2 Demography The number of individuals present in a population depends on four processes Birth death immigration and emigration Populations grow as a result of births and immigration Populations decline due to deaths and emigration structure of populations through time Age structure is how many individuals of each age are alive Analyzing birth rates death rates immigration rates and emigration rates is fundamental to demography the study of factors that determine the size and Researchers use age structure along with how likely individuals of different ages are to survive the following year how many offspring are produced by females of different ages and how many individuals of different ages immigrate and emigrate each generation to predict the future of a population A generation is the average time between a mother s first offspring and her daughter s first offspring A life table summarizes the probability than an individual will survive and reproduce in any given time interval over the course of its lifetime in most cases biologists focus on females when calculating life table data because the number of males present rarely effects population growth Females are the limiting resource for reproduction in a population Age Class and Number Alive An age class is a group of individuals of a scecific age ex all female lizards between 4 and 5 years old A cohort is a group of the same age that can be followed through time Survivorship is the proportion of offspring produced on average to a particular age There are three basic types of survivorship curves Type I survivorship individuals survive for a relatively long time and most individuals survive to the maximum lifespan Type II survivorship shows individuals having the same chance of dying each year they are alive Type III curves have extremely high death rates for younger individuals but higher survivor rates later in life Fecundity is th enumber of female offspring produced by each female in a population Age specific fecundity is the average number of female offspring per female in each age class The sum of age specifc fecundity is the net reproductive rate which indicates whether the population is increasing or decreasing as long as immigration and Fitness trade offs occur because every individual has a restricted amount of time and energy as resources An individual that puts a lot of resources into reproduction has less resources to contribute to its immune system growth nutrient stores or other structures an organisms life history described how an individual allocates resources to growth reproduction and activities or structures related to survival Life history is shaped by natural selection in a way that maximizes an individuals fitness in its environment Life History Patterns Across Species Individuals from species with high fecundity tend to grow quickly reach sexual maturity at a young age and producce many small eggs or seeds Life Tables Survivorship Fecundity Reproductive Rate emigration are insignificant What are Fitness Trade offs related to survival cid 127 cid 127 cid 127 cid 127 cid 127 cid 127 cid 127 cid 127 cid 127 cid 127 cid 127 cid 127 cid 127 cid 127 cid 127 cid 127 cid 127 cid 127 cid 127 cid 127 cid 127 cid 127 cid 127 cid 127 cid 127 cid 127 cid 127 cid 127 cid 127 cid 127 cid 127 cid 127 cid 127 cid 127 cid 127 cid 127 cid 127 cid 127 cid 127 cid 127 cid 127 Individuals from species with high survivorship tend to grow slowly invest resources in traits that reduce damage from enemies and increase their own ability to compete for water light or food 54 3 Population Growth Quantifying the Growth Rate death rate per individual d cid 127 When r is positive the population is growing cid 127 When r is negative the population is declining N D rmax Exponential Growth A populations growth rate is the change in number of individuals in the population per unit time If no immigration occurs the population s growth rate equals the number of individuals N times the difference between the birth rate per individual b and The difference between the birth rate and death rate per individual is called the per capita rate of increase and is symbolize r cid 127 Optimal conditions for a species are when the birth rates b are high and the death rates d are low r reaches its maximum value called the intrinsic rate of increase rmax Exponential growth occurs when r doesn t change over time The growth rate does not depend on the number of individuals in the population This is called density independent growth In nature exponential growth is common in two circumstances Few individuals found a new population in a new habitat A population has been devastated by a
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