Chapter 52 Population Ecology. Characteristic of Populations Population: group of individuals of a single species that simultaneously occupy the same.

Chapter 52

Population Ecology

Characteristic of Populations

• Population: group of individuals of a single species that simultaneously occupy the same general area– rely on the same resources, influenced by similar

environmental factors, and have a high likelihood of breeding with and interacting with one another

• Shaped by interactions between individuals and their environments, and natural selection can modify these characteristics

Density and Spacing

• At any point in time, every population has geographic boundaries and a population size

• Begin by studying a population by defining boundaries appropriate to the organisms– May be nature ones, such as a specific island, or

arbitrarily defined, such as oak trees in Minnesota• Population density is the number of individuals

per unit of area or volume• Dispersion is the pattern of spacing among

individuals within the geographic boundaries

Measuring Density

• Rarely, you can count all individuals inside the boundaries

• Use a variety of sampling techniques to estimate densities and total population sizes– More sample plots = more reliable data

• Mark-recapture method: traps are placed within the boundaries of the study area, and captured animals are marked with tags, collars, bands, or dye spots and then immediately released. After a certain time (enough for the marked animals to mix with unmarked animals) traps are set again. The proportion of marked animals is assumed equivalent to the proportion of marked animals in the total population.

Measuring Density

Number of recaptures in second catch Total number in second catch

= Number marked in first catch Total population N

Measuring Density

• Thus, if there have been no births, deaths, immigrations, or emigration, the following is an estimate of N

N = Number marked in first catch X Total number in second catch

/ number of recaptures in second catch

Measuring Density

• Example:– 50 snowshoe hares are captured in traps, marked

with ear tags, and released– Two weeks later, 100 hares are captured, and 10

have already been marked (recaptures = 10%)– Since 50 hares were originally marked, the entire

population would be about 500 hares

Patterns of Dispersion

• Clumped: most common type, with individuals aggregated in patches– Plants may be clumped in areas where soil

conditions and other environmental factors favor germination and growth

– Herd animals– Used during mating seasons– “Safety in Numbers”


• Uniform: evenly spaced distribution– May result from direct interactions between

individuals of a population– Tendency toward uniform spacing in plants may be

due to shading and competition for water and minerals• Some plants also secrete chemicals that inhibit growth

and germination of nearby plants• Animals that are territorial and aggressive


• Random: “unpredictable dispersion” occurs in the absence of strong attractions or repulsions among individuals of the population– Position is independent of each other– Trees in a forest are sometimes randomly

distributed– Not very common at all

Demography

• Changes in population size reflect rates of processes that add individuals to the population and eliminate some from it

• Additions are birth and immigration• Eliminations are deaths and emigration• Demography: study of the vital statistics that

affect population size

Life Tables and Survivorship Curves

• Began when the first life insurance policies were started

• Life table: an age-specific summary of the survivorship of a population– Construct by following the fate of a cohort, a

group of individuals the same age, from birth until all are dead


• Survivorship Curves: plot of the proportion or numbers in a cohort still alive at each age– Can be classified into 3 different types:

• Type 1: relatively flat at the start, reflecting low death rates during early and middle life, then drops steeply as death rates increase among older age-groups– Humans (parental care and few offspring)

• Case 3: curve drops sharply at the left of the graph, reflecting high mortality in youth, but then flattens out, and declines as it reaches older age-groups– Very large numbers of offspring, provide little or no care– Ex: Oyster- releases millions of eggs but only few survive, those that

do survive tend to live for a long time

• Case 2: intermediate, constant death span over time– Annual plants, some lizard and rodent species


• Some species are a combination of curves:– Birds: mortality is highest among the young, but

fairly constant in adults– Invertebrates, such as crabs, have a stair-stepped

curve with brief periods of increased mortality during molts• Greater vulnerability to predators, and physiological

problems

Reproductive Rates

• Ignore males and concentrate on females• View populations in terms of females giving

birth to new females• Reproductive Table: fertility schedule, is an

age-specific summary of the reproductive rates in a population– Measure the reproductive rates in a cohort from

birth to death

Life Histories

• Life history: traits that affect an organisms’ schedule of reproduction and survival

• Big-bang reproduction: adults have a single reproductive opportunity to produce large numbers of offspring– Ex: salmon, agave, desert wildflowers– Also called semelparity• Semel = once• Parito = to beget

Life Histories

• Repeated reproduction: adults produce large numbers of offspring for many years– Lizards that produce two eggs a year for 20 years– Oaks that produce vast amounts of seeds for a

century or more– Also called iteroparity• Itero = repeat

Life Histories

• What factors influence which life history a population will have?– If the offspring’s chance of survival is poor or

inconsistent, repeated production will be favored– If you need high reproductive rates, semelparitous

organisms typically produce two to five times as many offspring as closely related species that reproduce often

Trade-Offs

• Studies show that animals that reproduce and have many offspring end up surviving less than those who don’t reproduce as many

• Three basic decisions must be made:– When to begin reproducing– How often to breed– How many offspring to produce during each

reproductive episode

Population Growth

• Change in population size = Births – Deaths• N = population size• t = time (appropriate to life span or generation

time of species)– ∆N / ∆t = B – D

• b = per capita birth rate – If a population of 1,000 has 34 births/ year, b =

0.034– B = bN

Population Growth

• ∆N /∆t = bN – dN• r = difference in per capita birth and death rates– r = b – d– Will tell us if the population is growing or declining

• Zero population growth (ZPG): when r = 0– Deaths and births equal one another

• ∆N / ∆t = rN• dN / dt = r N (for instantaneous growth rate)– d doesn’t mean death rate here, it is a small change

Population Growth

• Exponential Growth: geometric population growth– Would assume the maximum growth rate,

intrinsic rate of increase, denoted as rmax

– dN / dt = rmax N

Population Growth

• As a population increases, its density may influence the ability of individuals to harvest sufficient resources for maintenance, growth, and reproduction

• Carrying Capacity: the maximum population size that a particular environment can support at a particular time with no degradation to the habitat– K– Energy limitation is one of the most significant

determinants of carrying capacity• Others are shelter, refuges from predators, soil nutrients,

water, suitable nesting and roosting sites

Population Growth

• Logistic population growth: incorporates the effect of population density on the per capita of increase, allowing this rate to vary from maximum at low population size to zero as carrying capacity is reached– When they are well below K, population increases

drastically and slows down when it reaches K

Logistic Growth Equations

• K – N = how many additional individuals the environment can support

• (K-N) / K tells us what fraction of K is still available for growth

• dN/dt = rmax N (K-N /K)• Produces a sigmoid shaped curve

How is it Really?

• Can only be “logistical” if under laboratory settings

• Model incorporates ideas that even at low population levels, each individual added to the population has the same negative effect on population growth rate– Some have the Allee effect, where individuals may

have a more difficult time surviving or reproducing if the population is too small

How is it Really?

• Model also assumes that populations adjust instantaneously and approach carrying capacity smoothly– There is a lag time before the negative effects of an

increasing population are realized• Can cause a population to overshoot their K, and then drop

below it

• Logistic model is a useful starting point for thinking about how populations grow– Still useful in conservation biology and pest control

because it gives an idea about growth rates

Population Growth and Life Histories

• At a high population density, selection favors adaptations that enable organisms to survive and reproduce with few resources– Competitive ability and maximum efficiency of

resource utilization is favored in populations that are at or near their K

• At a low density, increased fecundity, and earlier maturity would be selected for

Population Growth and Life Histories

• K-selection: selection for life history traits that are sensitive to population density– Density-dependent selection– Maximize population size and operates in populations

living at a density near K• r-selection: selection for life history traits that

maximize reproductive success in uncrowded environments– Density-independent selection– Maximize r, the rate of increase, and occurs in variable

environments where population densities fluctuate below K or in open habitats where they face little competition

Population Limiting Factors

• Why do all populations eventually stop increasing?

• What environmental factors stop a population from growing?

• Why is the population density of a particular species greater in some habitats than others?


• Must find out how the rates of birth, death, immigration, and emigration change as density rises

• Density-dependent: death rate rises as a population density rises, and birth rate falls with rising density– Negative feedback

• Density-independent: birth and death rate that does not change when population density increases


• Negative Feedback:– Resource limitation in crowded populations

• Reduce seed production in plants• As bird density increases, females lay fewer eggs

– Predation• Target some because their densities are large

– Accumulation of toxic wastes• Why wine is only 13% alcohol, because yeast die off in a higher

concentration

– Disease– Intrinsic Factors

• Ex: Woodchucks- stress leads to atrophy of sexual organs, and increased aggression…


• Some populations have regular boom-and-bust cycles– Ex: Snowshoe hare and lynx• Lynx numbers reflect hares, but hares fluctuate greatly• Cycles may be caused by food shortage during winter• Cycles may be due to predator-prey interactions• Cycles may be due to a combination of the two


– Why the cycles?• Stress resulting from high population density may alter

hormonal balance, which could reduce fertility and increase aggressiveness. – Stress could also be from unsuccessful predator attacks,

having to search for food, or parasites that are found in high densities

• Result from a time lag in the response of predators to rising prey • Availability of prey/food

Human Population Growth

• Explosive growth of human population, and massive consumption of the planet’s resources by developing nations, is the primary cause of severe environmental problems and loss of species

• Human growth has been exponential for three centuries


• Population increases by about 80 million each year– 214,000 a day• It says in the book “Equivalent to adding a city the size

of Amarillo, Texas every day”

• Demographic transition: a shift from zero population growth in which birth rates and death rates are high to zero population growth characterized by low birth and death rates


• In developed countries, populations are near equilibrium, with reproductive rates near the replacement level (2.1 children per female)

• Age structure: relative number of individuals of each age group– Stable ones indicate stable population size– Kenya has a “bottom- heavy” structure that favors

young, and they will grow up and reproduce and add to the population immensely

– US is steady except for a bulge where the “baby-boomers” are

– Can be used to predict future social concerns

UNITED STATES

Chapter 52 Population Ecology. Characteristic of Populations Population: group of individuals of a single species that simultaneously occupy the same.

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