Chapter 27 Population Growth Lectures by Gregory Ahearn

27. 1 How Are Populations Distributed In Space And Time? § Organisms arrange themselves in space in many different ways. § Ecologists recognize three major types of spatial distribution: • _________ • _____ Copyright © 2009 Pearson Education Inc.

27. 1 How Are Populations Distributed In Space And Time? § Individuals in many populations clump together in groups, and include social groups such as elephant herds, wolf packs, lion prides, flocks of birds, and schools of fish. § What are the advantages of clumping? Copyright © 2009 Pearson Education Inc. • Many eyes can search for localized food. • Part of a large group may reduce the odds that one individual will be killed by a predator. • A group may increase the individual’s chance of finding a Fig. 27 -1 a mate.

27. 1 How Are Populations Distributed In Space And Time? § Some individuals disperse themselves evenly. • Populations with uniform distributions maintain relatively even spacing between individuals. • This type of distribution occurs among animals that defend territories. • Male Galapagos iguanas establish regularly spaced breeding territories. § Creosote bushes release chemicals into the soil around them that inhibit germination of seeds from other plants. Copyright © 2009 Pearson Education Inc.

27. 1 How Are Populations Distributed In Space And Time? § In a few populations, individuals are distributed at random. • In a population with random distribution, the distance between individuals varies unpredictably. • Individuals in such populations do not form social groups. • The resources they need are not in short supply and are available throughout the year. § Trees and other plants in rain forests may be randomly distributed. Copyright © 2009 Pearson Education Inc.

27. 2 How Do Populations Grow? § Births, deaths, and migration determine population growth. • A population’s size remains stable if, on average, as many individuals join as leave. • Individuals enter a population by ____ or _______, and leave it by _____ or ________. • A population grows when the number of births plus immigrants exceeds the number of deaths plus emigrants. • Populations shrink when the opposite occurs. § A simple equation for the change in population size within a given period is as follows: • (births – deaths) + (immigrants – emigrants) = change in population size Copyright © 2009 Pearson Education Inc.

27. 2 How Do Populations Grow? § Population growth can be expressed as a rate. • The per capita growth rate (r) of a population is measure of how fast a population grows, expressed as a change in population size per individual per unit of time (that is, as a percentage). • This value is determined by subtracting the per capita death rate (d) from the per capita birth rate (b). • b (births) – d (deaths) = r (growth rate) • For example, to calculate the annual growth rate of a human population of 10, 000 in which there are 1, 500 births and 500 deaths each year, we first calculate the annual per capita birth and death rates. • b = 1, 500 births/10, 000 people = 0. 15 births person per year • d = 500 deaths/10, 000 people = 0. 05 deaths person per year Copyright © 2009 Pearson Education Inc.

27. 2 How Do Populations Grow? • Finally, we calculate the per capita growth rate by subtracting the death rate from the birth rate. • r = 0. 15 births person per year – 0. 05 deaths person per year = 0. 10 (that is, 10% increase per year) • If the death rate exceeds the birth rate, the growth rate will be negative and the population will shrink. Copyright © 2009 Pearson Education Inc.

27. 2 How Do Populations Grow? § A constant growth rate increases population size rapidly. • To calculate the number of individuals added to a population in a year, multiply the annual per capita growth rate (r) by the original population size (N). • Population growth = r. N • In our example, population growth in the first year (r. N) is 0. 10 x 10, 000 = 1, 000 people; at the end of year 1, the population has 11, 000 people. • If the per capita growth rate r remains constant, the number of people added to the population increases each year. • This pattern of continuously accelerating increase in population size is exponential growth. Copyright © 2009 Pearson Education Inc.

27. 2 How Do Populations Grow? § A population grows during periods when births exceed deaths. § Growth will persist if, on average, each individual produces more than one surviving offspring during its lifetime. § Each individual, of course, has the potential to replace itself many times during its lifetime. § Every species has a built-in capacity for population growth, but the speed of this potential growth varies among species, dependent upon the following factors. • The age of first reproduction • The frequency of reproduction • The average number of offspring produced each time • The length of an organism’s reproductive life span Copyright © 2009 Pearson Education Inc. • The organism death rate

27. 3 How Is Population Growth Regulated? § A population’s growth is influenced by its _____ potential: the maximum rate at which a population can increase, assuming ideal conditions that allow the highest possible birth rate and the lowest possible death rate. • The ultimate size of a population is also affected by limits that oppose this potential for growth. • These limits, which are set by the living and nonliving environments, are collectively known as _______ resistance. Copyright © 2009 Pearson Education Inc.

27. 3 How Is Population Growth Regulated? § Environmental resistance is imposed by several things. • • • Availability of food and space Interactions with competitors Predators Disease-causing organisms Natural catastrophes (e. g. , storms, fires, freezing weather, floods, and droughts) § Environmental resistance limits population size by the following factors: • Increasing death rates • Decreasing birth rates • Both factors together Copyright © 2009 Pearson Education Inc.

27. 3 How Is Population Growth Regulated? § Some populations fluctuate cyclically. • Short-term explosive growth occurs in populations that undergo regular cycles of rapid population growth followed by a sudden, massive die-off—a boom-and-bust cycle. population density Nutrients are depleted Favorable growth conditions occur 0 Jan Copyright © 2009 Pearson Education Inc. Mar May “boom” “bust” Jul Nov Sep month

27. 3 How Is Population Growth Regulated? § Ecosystem changes may allow temporary rapid growth. • A population may grow rapidly if population- controlling factors, such as _____, are eliminated or if the ____ supply is increased. • Growth can also be explosive when individuals invade a new habitat that has favorable conditions and few competitors. Copyright © 2009 Pearson Education Inc.

27. 3 How Is Population Growth Regulated? § Environmental resistance limits population growth. • Populations that grow rapidly must eventually stabilize or crash. • They tend to stabilize at or below their ecosystem’s carrying capacity, the maximum population of a particular species that an ecosystem can support indefinitely. • As a growing population approaches the carrying capacity of the environment, its growth rate gradually declines and finally stops when the population reaches a state of ______. • In this equilibrium, the birth rate is balanced by the death rate, and population size is stable. Copyright © 2009 Pearson Education Inc.

27. 3 How Is Population Growth Regulated? • The factors that usually maintain populations at or below the carrying capacity of their environment can be classified into two broad categories. • ______-_______factors limit population size regardless of the population density (number of individuals per given area). • ______-_______factors increase in effectiveness as the population density increases. Copyright © 2009 Pearson Education Inc.

27. 3 How Is Population Growth Regulated? § Density-independent factors limit population size. • The most important natural density-independent factor is ______. • hurricanes, droughts, floods, and fire • Human activities, such as the use of _____ and ______, can also limit population growth in ways independent of population density. Copyright © 2009 Pearson Education Inc.

27. 3 How Is Population Growth Regulated? § Density-dependent factors have a greater effect as population density increases. • Density-dependent factors become increasingly effective as population density increases, thus exerting negative feedback that limits the size of populations. • Conversely, density-dependent factors become less effective as population density decreases, allowing population size to stabilize or grow. Copyright © 2009 Pearson Education Inc.

27. 3 How Is Population Growth Regulated? • The most important density-dependent factors are predation and competition. • In predation, one organism feeds on another, harming it in the process. • Often, one organism (the _____) kills another (its ______) in order to eat it, but the prey is not always killed. Copyright © 2009 Pearson Education Inc.

27. 3 How Is Population Growth Regulated? • Predation plays an increasingly important role in population control as prey populations increase. • The increased frequency of encounters with predators increases the prey populations’ death rate, because predators tend to eat larger numbers of whichever prey species is most abundant and easiest to find. • Increasing numbers of prey organisms also increase the numbers of predators, and this tends to control prey numbers. Copyright © 2009 Pearson Education Inc.

27. 3 How Is Population Growth Regulated? • Prey also often survive the special form of predation known as ______. • The predator (the parasite) lives on or inside another organism (its _______) and feeds on the its body without killing it—at least not immediately. Copyright © 2009 Pearson Education Inc.

27. 3 How Is Population Growth Regulated? § Parasites spread faster when the host population density is high. • Like other forms of predation, parasitism is densitydependent. • Most parasites have limited ability to move, so they spread more readily from host to host at high host-population densities. • Parasites can affect the death rate of a host population because the damage inflicted by the parasite on its host’s body may kill the host. Copyright © 2009 Pearson Education Inc.

27. 3 How Is Population Growth Regulated? § Populations can soar or crash when predator-prey relationships are disrupted. • The population balance in ecosystems can be disrupted when predators are introduced to regions in which they were not previously present and where prey species have had no opportunity to evolve defenses against them. • Rats, snakes, and mongooses were introduced in Hawaii and many other Pacific islands, and have drastically reduced or exterminated many native bird populations. Copyright © 2009 Pearson Education Inc.

27. 3 How Is Population Growth Regulated? • Prey populations can grow out of control when introduced to areas where they have no predators. • The prickly pear cactus was introduced into Australia from Latin America, and, lacking predators, it spread uncontrollably. • In the 1920 s, a cactus moth was imported from Argentina to feed on the cacti. • Within a few years, the cacti were almost eliminated, and today, the moth keeps the population density of its prey very low. Copyright © 2009 Pearson Education Inc.

27. 3 How Is Population Growth Regulated? § Competition for resources helps curb population size. • The resources that determine the carrying capacity are limited. • Therefore, organisms must compete for access to resources, and the competition among individuals limits population size in a density-dependent manner. • There are two forms of competition: ______ competition among individuals of different species, and ______ competition among individuals of the same species. Copyright © 2009 Pearson Education Inc.

27. 3 How Is Population Growth Regulated? § Competition may be indirect or direct. • Some organisms, including most plants and many insects, engage in scramble competition, a kind of free-for-all in which individuals independently seek resources without interacting directly. • Many animals have evolved contest competition, in which individuals interact to contest access to important resources. • Only the best competitors are able to defend adequate territories; poor competitors fail to secure territories and are unlikely to reproduce or survive. Copyright © 2009 Pearson Education Inc.

27. 3 How Is Population Growth Regulated? § Different age groups may experience different death rates. • The effects of population-limiting factors do not necessarily fall equally on all members of a population. • For example, the death rate in a population differs among individuals of different ages; survivorship varies with age. • A survivorship curve shows the likelihood that an individual will survive to a given age. number of survivors 1, 000 100 late loss (human) constant loss (American robin) 10 0 Copyright © 2009 Pearson Education Inc. early loss (dandelion) age (in percentage of maximum life span)

27. 3 How Is Population Growth Regulated? § Survivorship in populations follows three basic patterns. • Convex late-loss survivorship curves reflect populations in which the death rate is low for juveniles and in which most individuals survive to old age. • These curves are characteristic of humans and other large, long-lived animals, such as elephants and mountain sheep. • These species produce relatively few offspring that receive a great deal of parental care. Copyright © 2009 Pearson Education Inc.

27. 3 How Is Population Growth Regulated? • Early-loss survivorship produces a concave curve and is characteristic of organisms that produce large numbers of offspring that receive little or no parental care. • The death rate of juveniles is very high, but individuals than manage to reach adulthood have a good chance of surviving to old age. • Early-loss survivorship curves are common among invertebrates, plants, and fish. • Populations with straight-line, constant-loss survivorship curves have a fairly constant death rate that does not vary much with age. • This pattern has been found in some bird and lizard species, and in populations of some asexually-reproducing invertebrate species. Copyright © 2009 Pearson Education Inc.