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Chapter 52 Population Ecology Key Vocabulary Terms: population, density, dispersion, census, sample census, plot census, mark-returnrecapture census, clumped, uniform, random, demography, fecundity, generation time, cohort, survivorship curve, exponential growth, carrying capacity (K), zero population growth, rmax, N, Kstrategist, r-strategist, density-dependent limiting factor, density-independent limiting factor, demographic pyramid Chapter Outline POPULATIONS Have Characteristic Features Examples: size, density, dispersion and demography Occupies particular place and plays particular role defined as its niche POPULATION SIZE AND DISPERSION Population Size Is an Important Feature Counting organisms using actual census, sample census (plot or MRR) Indirectly relates to the ability of a given population to survive Very small populations are more likely to become extinct Inbreeding can be a negative factor Lowers vigor by direct genetic effects Produces reduced levels of variability Extinction is more likely to occur in areas that change radically Population Density Is Very Important With wide spacing, individuals may only rarely interact Related measure is dispersion: way in which individuals are arranged Randomly spaced Evenly spaced Clumped Clumped distributions are frequent in nature Individuals tend to group within particular microhabitats Microhabitats are not generally uniformly distributed MORTALITY AND SURVIVORSHIP Intrinsic Rate of Increase Depends on Age and Reproductive Performance Constant environment stabilizes a population`s age distribution Distribution varies by species and regions Sex distribution can also affect population growth statistics Generation time also affects rate of growth Survivorship Curves Express Characteristics of Populations Survivorship: percentage of original population that survives to a given age Mortality: rate of death Types of survivorship curves Type II Straight line Individuals are likely to die at any age Example: hydra Type III Produce vast numbers of offspring, few survive to reproduce Once established mortality is low Example: oysters Type I Relatively low mortality when young High mortality in postreproductive years Example: humans Many animal and protist populations are between type II and III Many plant populations are closer to type III POPULATION GROWTH Key Characteristic of a Population Is Its Capacity to Grow Population numbers remain constant regardless of offspring produced Unchecked, most populations would increase dramatically Under some situations populations can increase rapidly Must consider circumstances and factors that limit population growth Biotic Potential (exponential growth of idealized population) Intrinsic rate of natural increase dN/dt = rN fig. 52.11 N = number of individuals within a population DN = Births minus deaths (B – D) dN/dt = rate of change of population number over time r = intrinsic rate of growth for that population Difficult value to calculate Actual rate of population growth is more readily calculated figure Difference between birth rate and death rate per given number of individuals Actual rate of growth also affected by emigration and immigration Innate capacity for growth is exponential, represented by growth curve Rate of growth remains constant Actual increase in numbers accelerates as population increases Analogous to compounding interest on an investment Such patterns of growth occur for only short periods Carrying Capacity Populations always reach a limit imposed by environmental shortages Size for such stabilization is the carrying capacity A dynamic rather than static value Number of individuals fluctuates around a mean value dN/dt = rN(K-N/K) fig. 52.14 dN/dt = growth rate of the population r = rate of increase N = number of individuals present at any one time K = carrying capacity As a population grows in size, the rate of increase declines until N=K Competition among individuals for resources increases Build up of wastes Increased ratio of predation Relationship is an S-shaped sigmoid growth curve As the population stabilizes its rate of growth slows down r-strategists and K-Strategists Many species have fast rates of population growth Not a sigmoid curve Growth not effectively controlled by reductions in population size Small populations quickly enter an exponential pattern of growth Population reduction in slow-breeding organisms may cause extinction Populations with sigmoid growth curves limited by carrying capacity (K) Include relatively slow-breeding organisms Tend to live in stable, predictable habitats Called K-strategists Other species characterized by exponential growth and sudden crashes Have high intrinsic rate of growth (r) Called r-strategists Many organisms are not clearly delineated as pure r or k strategist Have reproductive strategies between the two extremes Change from one extreme to other with environmental conditions Reproduction in r-strategists Reproduce early, have many offspring Offspring are small, mature rapidly, receive little parental care Generations are relatively short, large brood size Examples: dandelions, aphids, mice, cockroaches Reproduction in K-strategists Reproduce late, have small broods Offspring are large, mature slowly, receive intensive parental care Generations are relatively long Examples: coconut palms, whooping cranes, whales Many organisms in danger of extinction are K strategists Density-Dependent and Density-Independent Effects Density-dependent effects Depend on size of population, regulate its growth Accompanied by hormonal changes that alter animal behavior In general have an increasing effect as population increases Density-independent effects Operate regardless of the population size Include factors such as weather and physical disruption of habitat Agriculture depends on characteristics of a sigmoid growth curve After an area has been cleared, populations grow rapidly Very high net productivity Commercial fisheries exploit populations in rapid growth phase Harvest at the steep, rapidly growing part of the curve Produces optimal yield, maximum sustainable catch from population Over harvesting smaller population can destroy its productiveness Human Populations Like all other organisms, size is controlled by the environment Humans have expanded populations by technical innovations Early in history controlled by density-dependent and density-independent factors Migration influences adjustment of human populations to particular areas Changes in technology have fostered explosive population growth DEMOGRAPHY Statistical Study of Populations Measurement of people, therefore the characteristics of populations Helps predict ways in which sizes of populations will alter the future Accounts for age distribution and changing population size over time Stable Population Population with constant size through time Birth + immigration = death + emigration Age structure also remains constant Population Pyramid Graphical illustration of a population`s characteristics Male and female counts on opposite sides of the vertical age axis Shows population composition by age and sex as percent of population Can view historical trends of demographic events Examples of human populations Number of females disproportionately larger than males Females generally have longer life expectancy