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, r max
, 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
Type III Produce vast numbers of offspring, few survive to
Type I
Individuals are likely to die at any age
Example: hydra
reproduce
Once established mortality is low
Example: oysters
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