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”
Patterns of Dispersion
• 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
Patterns of Dispersion
• 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
Life Tables and Survivorship Curves
• 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
Life Tables and Survivorship Curves
• 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?
Population Limiting Factors
• 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
Population Limiting Factors
• 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…
Population Limiting Factors
• Some populations have regular boom-andbust 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
Population Limiting Factors
– 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
Human Population Growth
• 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
Human Population Growth
• 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 “babyboomers” are
– Can be used to predict future social concerns