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Lecture 9

Lecture 9 - Population Growth and Regulation

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Western University
Biology 2483A
Hugh Henry

LECTURE 9: POPULATION GROWTH AND REGULATION Human Population Growth: A Case Study  Humans have a large impact on the global environment: our population has grown explosively, along with our use of energy and resources  Human population reached 6.8 billion in 2010, more than double the number of people in 1960  Our use of energy and resources has grown even more rapidly. From 1860 to 1991, human population quadrupled in size, and energy consumption increased 93-fold  For thousands of years, our population grew relatively slow, reaching 1 billion for the first time in 1825. Now we are adding 1 billion people every 13 years.  Growth rate has slowed recently, to about 1.18% per year, and continues to slow  By 2080, it is predicted there will be roughly 9-10 billion people on Earth  Is 10 billion above the carrying capacity of the human population?  Many people have tried to estimate human carrying capacity.  Researchers must make assumptions about how people would live and how technology would influence our future.  Estimates range from fewer than 1 billion to more than 1,000 billion.  Ecological footprint – total area of productive ecosystems required to support a population  Uses data on agricultural productivity, production of goods, resource use, population size, and pollution  The area required to support these activities is then estimated.  In 2006: o 11.9 billion hectares of productive land available globally o Average ecological footprint: 2.6 hectares o Suggests a carrying capacity of 4.6 billion o Population: 6.6 billion, a 40% overshoot of carrying capacity  If everyone in the world used resources at the same rate as: o U.S. citizens in 2006, carrying capacity would be 1.3 billion people o Indian citizens in 2006, carrying capacity would be 14 billion people  Ecological footprint is much lower in India, compared to that of the U.S. Introduction  One of the ecological maxims is: o “No population can increase in size forever”  The limits imposed by a finite planet restrict a feature of all species: a capacity for rapid population growth  Ecologists try to understand the factors that limit or promote population growth  Life table – summary of how survival and reproductive rates vary with age  Information about births and deaths is essential to predict future population size  Life data table for the grass Poa annua were collected by marking 843 naturally germinating seedlings and then following their fates over time  S = survival rate – chance that an individual of age x x will survive to age x + 1 o Survival rate is highest when the species is young in this case, but it decreases as they get older o Could increase in other cases  x = survivorship – proportion of individuals that survive from birth to age x  Fx= fecundity – average number of offspring a female will have at age x  Birth and death rates can vary greatly between individuals of different ages  Gambians’ survivorship depends on the season of birth  Gambians born during the “hungry season” (when food stored from the previous year is depleted) had lower survivorship than those born at other times of the year  In some species, age is not important o e.g. in many plants, reproduction is more dependent on size (related to growth conditions) than age  Life tables can also be based on size or life cycle stage  Survivorship curve – plot of the number of individuals from a hypothetical cohort that will survive to reach different ages o Cohort – group of organisms born at the same time  Survivorship curves can be classified into three general types: o Type I: most individuals survive to old age (Dall sheep, humans)  Not a linear scale, number of survivors = log scale o Type II: the chance of surviving remains constant throughout the lifetime (some birds) o Type III: high death rates for young, those that reach adulthood survive well (species that produce a lot of offspring)  Most commonly seen in nature  A population can be characterized by its age structure – the proportion of the population in each age class  Age structure influences how fast a population will grow  If there are many people of reproductive age (15 to 30), it will grow rapidly  A population with many people older than 55 will grow more slowly  If the population starts with 100 individuals: o Age class 0 (n ) = 20 individuals 0 o Age class 1 1n ) = 30 individuals o Age class 2 2n ) = 50 individuals  To predict population size for the following year, calculate: o The number of individuals that will survive to the next time period o The number of offspring those survivors will produce in the next time period  Growth rate (λ) – ratio of population size in year t +t+1(N ) to population size
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