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A Population of Mexican Poppies Ecology • Ecology is the study of how living organisms and the physical environment interact in a complicated web of relationships • Biologists call interactions among organisms biotic factors and interaction between organisms and their nonliving, physical environment abiotic factors • Abiotic factors include precipitation, temperature, pH, wind, and chemical nutrients Levels of Biological Organization • Above the level of the individual organism, life is organized into populations, communities, ecosystems, landscapes, and the biosphere • Each level has its own characteristic composition, structure, and functioning • An individual belongs to a population, members of the same species that live together in a specified area at the same time Features of Populations • Features that characterize populations include population density, population dispersion, birth and death rates, growth rates, survivorship, and age structure • Because populations share a common gene pool, natural selection acts directly on allele frequencies to produce adaptive changes in populations Features of Populations (cont.) • Population ecology considers the number of individuals of a particular species in an area and the dynamics of the population • Population dynamics is the study of changes in populations – how and why their numbers increase or decrease over time • Biologists also study populations’ reproductive success or failure, evolution, genetics, and the way they affect the normal functioning of communities and ecosystems Population Density • Researchers study a large population by sampling part of it and expressing the population in terms of density • Population density is the number of individuals of a species per unit of area or volume at a given time • Different environments vary in the population density of any species they can support • Population density may be determined in large part by biotic or abiotic factors in the environment Dispersion • Individuals in a population may exhibit characteristic patterns of spacing (dispersion) relative to one another • Random dispersion occurs when individuals are spaced in a manner that is unrelated to the presence of others (rare) • Clumped dispersion occurs when individuals are concentrated in specific parts of the habitat (most common) • Uniform dispersion occurs when individuals are more evenly spaced than a random pattern Dispersion of Individuals Within a Population (a) Random dispersion, illustrated for comparison, rarely, if ever, occurs in nature. Fig. 53-1a, p. 1155 (b) Clumped dispersion is evident in the schooling behavior of certain fish species. Shown are bluestripe snappers (Lutjanus kasmira), photographed in Hawaii. This introduced fish, which grows to 30 cm (12 in), may be displacing native fish species in Hawaiian waters. Fig. 53-1b, p. 1155 (c) Uniform dispersion is characteristic of these nesting Cape gannets (Morus capensis) on the coast of South Africa. The birds space their nests more or less evenly. Fig. 53-1c, p. 1155 Clumped Dispersion • Clumped dispersion (aggregated distribution or patchiness) often results from patchy distribution of resources in the environment • It also occurs among animals because of the presence of family groups and pairs, and among plants because of limited seed dispersal or asexual reproduction • Clumped dispersion is advantageous when social animals benefit from their association (e.g. schooling fish) Uniform Dispersion • Uniform dispersion occurs: • when competition among individuals is severe • when plant roots or leaves that have been shed produce toxic substances that inhibit growth of nearby plants • when animals establish feeding or mating territories • Example: A colony of seabirds nesting in a homogeneous environment place their nests at approximately equal distance from one another Changes in Population Size • Change in the number of individuals in a population (ΔN), over a given period of time (Δt ) is ultimately caused by two factors, expressed on a per capita (per individual) basis: • Natality (b), the average per capita birth rate • Mortality (d), the average per capita death rate ΔN /Δt = N (b − d) Changes in Population Size (cont.) • Growth rate (r), or rate of change of a population on a per capita basis, is the birth rate minus the death rate: r=b−d • The rate at which the population is growing at a particular instant in time (instantaneous growth rate, dN/dt) can be expressed as: dN/dt = rN Dispersal • Movement of individuals among populations (dispersal) must be considered when examining changes in populations on a local scale • There are two types of dispersal: • Immigration occurs when individuals enter a population and increase its size • Emigration occurs when individuals leave a population and decrease its size Dispersal (cont.) • The growth rate of a local population must take into account birth rate (b), death rate (d), immigration rate (i),and emigration rate (e) on a per capita basis • The per capita growth rate equals the birth rate minus the death rate, plus the immigration rate minus emigration rate: r = (b − d) + (i − e) Exponential Population Growth • A plot of population size versus time (under optimal conditions) has a J shape characteristic of exponential population growth • Accelerating growth rate occurs when optimal conditions allow constant per capita growth rate – the larger the population gets, the faster it grows • Populations will increase exponentially as long as their per capita growth rates remain constant Exponential Population Growth Environmental Limits • Organisms cannot reproduce indefinitely at their intrinsic rate of increase because of environmental limits • Environmental limits include limited availability of food, water, shelter, and other essential resources (resulting in increased competition), and limits imposed by disease and predation • At or near the limits of the environment to support the population, population growth rate may decrease to nearly zero Carrying Capacity • Carrying capacity (K) is the largest population that can be maintained for an indefinite period by a particular environment, assuming no changes in the environment • In nature, carrying capacity is dynamic and changes in response to environmental changes Logistic Population Growth • When a population regulated by environmental limits is graphed over longer periods, the curve has a characteristic S shape (logistic population growth) • Initial exponential increase is followed by a leveling out as carrying capacity of the environment is approached The Logistic Model • The logistic model describes a population increasing from a small number of individuals to a larger number of individuals that are ultimately limited by the environment • The logistic equation takes into account the carrying capacity of the environment: dN/dt = rN [(K − N)/K] • The element [(K − N)/K] reflects decline in growth as population size approaches its carrying capacity Carrying Capacity and Logistic Population Growth Population Crash • A population rarely stabilizes at K (carrying capacity) but may temporarily rise higher, then drop back to, or below K • Sometimes a population that overshoots K will experience a population crash, an abrupt decline from high to low population density • Population crashes are observed in bacterial cultures, zooplankton, and other populations whose resources have been exhausted KEY CONCEPTS 53.2 • Changes in population size are caused by natality, mortality, immigration, and emigration Density-Dependent Factors • Factors that affect population size fall into two categories: density-dependent factors and density-independent factors • If a change in population density alters how an environmental factor affects that population, then the environmental factor is density-dependent • The effects of density-dependent factors on population growth increase as the population density increases Density-Dependent Factors (cont.) • Density-dependent factors act as negative feedback systems • As population density increases, density-dependent factors tend to slow population growth by causing an increase in death rate and/or a decrease in birth rate • Population density declines Density-Dependent Factors and Negative Feedback Density-Dependent Factors (cont.) • Predation, disease, and competition are examples of densitydependent factors • As the density of a population increases: • Predators are more likely to find prey • The chance of transmitting parasites and infectious disease organisms increases • Competition for resources such as living space, food, cover, water, minerals, and sunlight increases Cyclic Changes in Density • Lemming populations in the arctic tundra have a three- to four-year cyclical oscillation (“boom or bust”) • Possible explanations: • Dense lemming populations overgraze the food supply; reduced food reduces the population • Population density of predators that eat lemmings increases in response to increasing density of prey – reducing lemming populations • Lemming populations decrease in years when climate restricts growth of food Lemming Fig. 53-5, p. 1159 Competition • Competition is an interaction among two or more individuals that attempt to use the same essential resource (food, water, sunlight, or living space) that is in limited supply • Use of the resource by one individual reduces the availability of that resource for other individuals • Competition occurs within a population (intraspecific competition) and among populations of different species (interspecific competition) Intraspecific Competition • Interference competition (contest competition) • Dominant individuals obtain an adequate supply of the limited resource at the expense of other individuals • Example: red grouse • Exploitation competition (scramble competition) • All individuals in a population share the limited resource equally – at high population densities none of them obtains an adequate amount • Example: moose population on Isle Royale Effects of Density-Dependent Factors • In natural communities, it is difficult to evaluate the relative effects of different density-dependent factors • Example: Effects of lizards on spider populations • Spider population densities were higher in lizard-free enclosures than in enclosures with lizards • Enclosures without lizards had more species of spiders • In this experiment, effects of two density-dependent factors (predation and interspecific competition) cannot be evaluated separately Density-Independent Factors • Any environmental factor that affects the size of a population but is not influenced by changes in population density is a density-independent factor • Density-independent factors are typically abiotic, such as random weather events (e.g. blizzards, hurricanes) • Example: Mosquito populations in arctic environments • No adult mosquito survives winter; the entire population grows in summer from eggs and hibernating larvae r Selection • r selected populations have traits that contribute to a high population growth rate (r) • r-selected species have small body size, early maturity, short lifespan, large broods, and little or no parental care • r-strategists are opportunists in variable, temporary, or unpredictable environments where long-term survival is low • Examples: Insects such as mosquitoes; plants such as desert annuals that rapidly grow, flower, and die K Selection • K selected populations have traits that maximize survival in an environment near the carrying capacity (K) • K-selected species produce few offspring, have long lifespans, late reproduction, and large body size • K strategists tend to be found in relatively constant or stable environments, where they have a high competitive ability • Examples: Redwood trees; tawny owls, which pair-bond for life and invest in parental care of their young Survivorship Curves • Survivorship is the probability that a given individual in a population or cohort will survive to a particular age • Plotting the logarithm (base 10) of the number of surviving individuals against age, from birth to the maximum age reached by any individual, produces a survivorship curve • Ecologists recognize three main types of survivorship curves: Type I, Type II, and Type III Survivorship Curves (cont.) • In Type I survivorship, probability of survival decreases more rapidly with increasing age; mortality is greatest later in life • Example: humans • In Type II survivorship, probability of survival does not change with age; death is equally likely across all age groups • Example: some lizards • In Type III survivorship, probability of survival increases with increasing age; young are most likely to die • Example: oysters Survivorship Curves Human Population Growth • It took thousands of years for the human population to reach one billion, around 1800 • It took 130 years to reach two billion (1930), 30 years to reach three billion (1960), 15 years to reach four billion (1975), 12 years to reach five billion (1987), and 12 years to reach six billion (1999) • The United Nations projects that the human population will reach about seven billion by 2012 Human Population Growth Human Population Growth (cont.) • Recent increase in human population is due to a decrease in death rate (d), not an increase in birth rate (b) • Greater food production, better medical care, and improved sanitation practices have increased life expectancies • Example: From 1920 to 2000, the death rate in Mexico fell from 40 per 1000 individuals to 4 per 1000 – the birth rate dropped from 40 per 1000 individuals to 24 per 1000 Birth and Death Rates: Mexico Human Demographics • The science of human demographics deals with human population statistics such as size, density, and distribution, and provides information on populations of various countries • Countries can be classified into two groups based on their rates of population growth, degrees of industrialization, and relative prosperity: highly developed and developing • Developing countries have two subcategories: moderately developed and less developed Highly Developed Countries • Highly developed countries (such as the US) • Highly industrialized • Low rates of population growth • Low birth rates • Low infant mortality rates • Long life expectancies • High average GNI PPP per capita Developing Countries • Moderately developed countries (such as Mexico) • Birth rates and infant mortality rates generally higher than in highly developed countries, but declining • Medium level of industrialization • Lower average GNI PPP per capita • Less developed countries (such as Bangladesh) • Highest birth rates and infant mortality rates • Lowest life expectancies • Lowest average GNI PPP per capita Fertility Changes Table 53-4, p. 1168 Age Structure • Age structure is the number and proportion of people at each age in a population • An age structure diagram represents the number of males and females at each age, from birth to death • The overall shape of an age structure diagram indicates whether the population is increasing, stationary, or shrinking Age Structure (cont.) • For highly developed countries, age structure diagrams have more tapered bases – a smaller proportion of the population is prereproductive • The age structure diagram of a stable population shows approximately the same number of people at prereproductive and reproductive ages • In a population that is shrinking, the prereproductive age group is smaller than either the reproductive or postreproductive group Age Structure Diagrams