Unit 5.1 Population Ecology

Population Ecology

1. Processes influencing density, dispersion, and demographics
- Density & Dispersion
- Demographics
a. Life tables
b. Survivorship curves
c. Reproductive rates

2. Exponential model
- Per capita rate of change
- Exponential population growth

3. Logistical model
- Logistic growth model
- Logistic model & real populations

3. Life history traits
- Diversity of Life Histories
a. When Reproduction Begins
b. Frequency of Reproduction
c. Number of Offspring per Episode
- “Trade-offs” and Life Histories
a. Trade-offs
b. Selection of traits

4. Density-Dependent Factors
- Population Change & Density
- Mechanisms of Density-Dependent
Population Regulation
a. Competition for resources
b. Disease
c. Territoriality
d. Intrinsic factors
e. Toxic wastes
- Population Dynamics
a. Stability & Fluctuation
b. Population Cycles: Scientific Inquiry
c. Immigration, Emigration, and Metapopulations

5. Increase of human population
- The Global Human Population
a. Regional Patterns of Population Change
b. Age Structure
c. Infant Mortality & Life Expectancy
- Global Carrying Capacity
a. Estimates of Carrying Capacity
b. Limits on Human Population
Size

Processes influencing density, dispersion, and demographics

- Density & Dispersion
- Demographics

Abundance

- population size

Density

- number of individuals per unit area or
volume

counting, average, homogeneous, mark, initially, second, total

Density & Dispersion
How is density determined?
1) _______ all individuals within the
boundaries of the population

2) calculating the _______ density in the plots, and then extending the estimate to the population size in the entire area
○ accurate when the habitat is fairly ________

3) ____-recapture method for wildlife
populations
○ marking an initial random sample of individuals in the population then capturing a second set of individuals after a few days or
weeks
○ population size, N = sn/x
where
s = # of individuals _____ marked and released
x = # of marked individuals captured in ______ sampling
n = _____ # of captured individuals in second sampling

static

Density & Dispersion
- Density is not a _____ property but changes as individuals are added to or removed from a population.

add

Births and immigration ___ individuals to a population.

birth

all forms of reproduction

Immigration

influx of new individuals from other areas

remove

Mortality and emigration ______ individuals from a population.

mortality

death

emigration

movement of individuals out of a population and into other locations

Dispersion

- pattern of spacing among individuals within the boundaries of the population
- spatial relationship bet. members of a population within habitat

Habit

- growing patterns

Dispersion

1. Clumped
2. Uniform
3. Random

Clumped

individuals are aggregated in patches

e.g. Sea stars group together where food is abundant.

Uniform

evenly spaced

e.g. Nesting king penguins exhibit nearly uniform spacing, maintained by aggressive interactions between neighbors.

Random

unpredictable spacing

e.g. Dandelions grow from windblown seeds that land at random and later germinate.

Demography

study of vital statistics of populations and how they change over time

Demographics

1. Life tables
2. Survivorship curves
3. Reproductive rates

rates, age, cohort, females

Life tables
● summarizes the survival and reproductive ______ of individuals in specific ___-groups within a population
○ follows the fate of a ____ from birth until all of the individuals are dead

Demographers who study sexually reproducing species often ignore the males and concentrate on the ______ in a population because only females produce offspring.

Life tables

1. Age (years)
2. Number alive at start of year
3. Proportion alive at star of year (proportion of the original cohort of 653 individuals that are still alive at the start of a time interval)
4. Death rate (proportion of individuals alive at the start of a time interval that die during that time interval)
5. Average number of female offspring per female

Cohort

➔ a group of individuals of the same age

Survivorship curves

● shows the proportion or numbers in a cohort still alive at each age
1. Type 1
2. Type 2
3. Type 3

low, elephants

Type 1
○ ___ death rates during early and
middle life, increase in death rates
among older age-group
○ Ex. organisms that produce few
offsprings but provide good care such as humans and ______

constant, annual

Type 2
○ _______ death rate over the organism’s life span
○ Ex. rodents, invertebrates, lizards
and _____ plants

high

Type 3
○ very ____ death rates for the young,
decline in death rates for those that
survive the early die-off
○ Ex. organisms that produce very large numbers of offsprings but provide little or no care such as long-lived plants, fishes and many marine invertebrates

between, birds, stair, molts, species

* Many species fall somewhere ______ these basic types of survivorship or show more complex patterns.
* In _______, mortality is often high among the youngest individuals (as in a Type III curve) but fairly constant among adults (as in a Type II curve).
* Some invertebrates, such as crabs, may show a “___-stepped” curve, with brief periods of increased mortality during ____, followed by periods of lower mortality when their protective exoskeleton is hard.
* In addition to such variation among species, survivorship curves also can differ among the populations of a single _____.

breeding, genetic

Reproductive rates
● The simplest way to describe the reproductive pattern of a population is to identify how reproductive output varies with the number of ______ females and their ages.
● Approaches:
1. direct counts
2. mark-recapture method
3. molecular method
■ Ex. using _____ profiles from loggerhead turtle eggshells to identify which female laid the eggs

sexual, average, squirrels

Reproductive rates
● Reproductive output for ______ organisms such as birds and mammals is typically measured as the _____ number of female offspring produced by the females in a given age-group.
○ For some organisms, the number of offspring for each female can be counted directly; alternatively, molecular methods can be used
■ Researchers directly counted the offspring of the Belding’s ground _____, which begin to reproduce at age 1 year.
■ The squirrels’ reproductive output rises to a peak at 4–5 years of age and then gradually falls off in older females (see Table 53.1).

Age, growth

● ___-specific reproductive rates vary considerably by species.
○ Squirrels, for example, have a litter of two to six young once a year for less than a decade
○ Oak trees drop thousands of acorns each year for tens or hundreds of years.

● Mussels and other invertebrates may release millions of eggs and sperm in a spawning cycle.
● However, a high reproductive rate will not lead to rapid population ______ unless conditions are near ideal for the growth and survival of offspring

Exponential model

Populations have the potential to expand greatly when resources are abundant.

Exponential model

- Per capita rate of change
- Exponential population growth

Per capita rate of change

1. Change in population size (verbal form)
2. Change in population size (math form)
3. Change in population size per capita (per individual)

minus,

Change in population size (verbal form)
● If immigration and emigration are ignored, a population’s growth rate equals birth rate ____ death rate. For the next equations, both immigration and emigration are ignored

size, time, -

Change in population size (math form)
where
ΔN = change in population ___
Δt = change in ___
R = births _ deaths

average

Change in population size per capital (per individual): rΔt
● R can be substituted as rΔtN, where rΔt = contribution that an ______ member of the population makes to the number of individuals added to or subtracted from the population during the time interval Δt:

Change in population size

a. Change in population size: 16/1000 = 0.016
b. Population size: 500

ideal, constant, abundant

Exponential population growth
● A pattern of growth where a population experiences such ____ conditions increases in size by a _______ proportion at each instant in time.
● Assumes that resources remain ______.
● This pattern cannot be sustained for long in populations.
1. Equation
2. Model

rate, size, intrinsic

Equation for exponential growth
dN/dt = ___ at which population is increasing in size at each moment in time
N = population ____
r = _____ rate of increase (constant)

larger, J, catastrophic, rebounding, higher

Graph for exponential growth
● More individuals are added per unit time when the population is ______ than when it is smaller; resulting in a steeper, __-shaped curve.
● J-shaped curves are characteristic of some populations that are introduced into a new environment or whose numbers have been drastically reduced by a ________ event and are ______.
● Rate of growth is also dependent on the intrinsic rate of increase r. The ____ the r, the faster the growth.

Logistical model

- Logistic growth model
- Logistic model & real populations

slowly, fewer

Logistical model
● Describes how a population grows more _____ as it nears its carrying capacity
● As the size of a population increases, each individual has access to _____ resources

Carrying capacity (K)

➔ Maximum population size that a
particular environment can sustain
➔ Varies over space and time with the
abundance of limiting resources

Limiting, E, Shel, Refuge, Nutrient, nesting, bats

Carrying capacity (K)
➔ Limiting factors:
1. _nergy
2. _____ter
3. _____ from predators
4. ______ availability
5. Water
6. Suitable _____ sites
➔ Carrying capacity for ___ may be high in a habitat with abundant flying insects and roosting sites but lower where there is abundant food but fewer suitable shelters

Crowding, resource, decline, increase, drop

● ______ and _______ limitations can have a profound effect on population rate
○ If individuals cannot obtain sufficient resources, per capita birth rate will _____
○ If starvation or disease increases with density, per capita death rate may ______
● Falling per capita birth rates or rising per capita death rates will cause per capita rate of population growth to ____ (opposite of constant per capita growth rate r, as seen in exponential growth)

Logistic growth model

● We can modify our model so that per capita population growth rate decreases as N increases
1. Equation
2. Graph

zero

Equation for logistic growth
➔ Per capita rate of population growth
approaches ____ as the population size nears carrying capacity
1. When (K-N)/K is close to 1
2. When (K-N)/K is close to 0
3. When (K-N)/K is 0

small

When (K-N)/K is close to 1, N is ____ compared to K
○ Per capita rate of population growth will be close to r
○ Intrinsic rate of increase seen in exponential growth

large

When (K-N)/K is close to 0, N is ____ compared to K
○ Per capita rate of population growth is small

equal

When (K-N)/K is 0, N is ___ to K
○ Population stops growing

sigmoid, intermediate, decreases

Graph for logistic growth
● Produces a _____ (S-shaped) growth curve when N is plotted over time (red line)
● New individuals are added most rapidly at _______ population sizes
● Number of individuals added ______ dramatically as N approaches K
○ Population growth rate (dN/dt) also decreases as N approaches K

Logistic model & real populations

1. Approximates logistic growth
2. Does not correspond to logistic growth

Paramecium, constant

Approximates logistic growth
● Growth of laboratory animals (beetles, crustaceans) and microorganisms (bacteria, ______, yeasts) fit an S-shaped curve under conditions of limited resources
○ Are grown in a ______ environment w/o predators and competing species

match

● Populations in nature rarely ____ the predictions of the logistic model
○ Logistical model does not apply to all populations
○ Assumes populations adjust instantaneously to growth and approach carrying capacity smoothly

delay, overshoot

Does not correspond to logistic growth
● In reality, there is often a ____ before the negative effects of an increasing population are realized
○ If food is a limiting factor, reproduction will decline eventually and females may use their energy reserves to continue reproducing for a short time
○ Causes an _____ in carrying capacity (see water fleas)

Hardy-Weinberg, conservation

Logistic model & real populations
● Provides a useful starting point for thinking about how populations grow and for constructing more complex models
○ Similar to the role played by the ________ equation for thinking about the evolution of populations
● Also important in ______ biology
○ Conservation biologists can also use
the model to estimate the critical size below which populations of certain organisms may become extinct (white rhinoceros- Ceratotherium simum)

Life history traits

- Diversity of Life Histories
a. When Reproduction Begins
b. Frequency of Reproduction
c. Number of Offspring per Episode
- “Trade-offs” and Life Histories
a. Trade-offs
b. Selection of traits

Trade, frequency, number, parental

Life history traits
● Natural selections favors traits that improve an organism’s chances of survival and reproductive success
● ____-offs between survival and reproductive traits:
○ _______ of reproduction
○ _______ of offspring (seeds,
litter/clutch size)
○ Investment in _______ care

Life History

➔ Traits that affect an organism’s schedule of reproduction and survival
➔ Life history traits are evolutionary outcomes reflected in its development, physiology, and behavior

Evolution

Diversity of Life Histories
● _____ accounts for diversity of life histories
● Three key components of an organism’s life history:
1. When Reproduction Begins
2. Frequency of Reproduction
3. Number of Offspring per Episode

first, turtle, Coho

When Reproduction Begins
● Age at ___ reproduction / age at maturity
● Varies considerably across species
○ 30 years old for loggerhead _____
○ 3-4 years old for ___ salmon

Frequency of Reproduction

1. Semelparity
2. Iteroparity

Semelparity

➔ From the Latin semel (once) and parere (to beget)
➔ Organisms that undergo a “one-shot” pattern of big-bang reproduction
1. Coho salmon
2. Agave or “century plant”

Pacific

Semelparity (Coho salmon)
◆ Hatches in the head water of a freshwater stream and then migrates to the _____ Ocean (requires a few years to mature)
◆ Eventually returns to the same stream to spawn, producing thousands of eggs before it dies

arid, wet

Semelparity (Agave or “century plant”)
◆ Grow in ___ climates with unpredictable rainfall and poor soils
◆ Grow for years until there is
an unusually ___ year in which sends up a large
◆ An adaptation to harsh desert
environment

Iteroparity

➔ From the Latin iterare (to repeat)
➔ Repeated reproduction
1. Loggerhead turtle
2. Horses and other large mammals, fish, sea urchins, and long-lived trees (maples and oaks)

clutches

Iteroparity (Loggerhead turtle)
◆ Produces 4 _____ (300 eggs) in a year
◆ Then waits 2-3 years before laying more eggs as it lacks sufficient resources to produce that many eggs
every year
◆ Mature turtle may lay eggs for 30 years after first clutch

mammals, fish, trees

Iteroparity
- Horses and other large _____, ____, sea urchins, and long-lived ____ (maples and oaks)

White, plants

Number of Offspring per Episode
● Vary in how many offspring they reproduce
○ _____ rhinoceros
■ Produces a single calf when they reproduce
○ Insects and many ____
■ Produce large numbers of offspring

“Trade-offs” and Life Histories

1. Trade-offs
2. Selection of traits

Trade-offs

1. between the number of offspring and the amount of resources a parent can devote to each offspring
2. between number and size of offspring
3. Extra investment on the part of the parents greatly increases offspring’s chances of survival

number, resources, reduce, kestrels, deer

● Trade-off between the _____ of offspring and the amount of ______ a parent can devote to each offspring
● Occur because organisms do not have access to unlimited resources
○ Use of resources for one function (e.g., reproduction) can ____ resources available for another function (e.g., survival)

● Eurasians ______
○ Caring for a larger number of young
lowered survival rates of the parents

● Red ____
○ Females that reproduced in the summer were more likely to die the next winter than were females that did not reproduce

number, size, small, spread, predation

● Selective pressures also influence trade-offs between _____ and ___ of offspring
● Plants and animals whose young have low chance of survival often reproduce ____ offspring

○ Plants
■ Plants that colonize disturbed
environments usually produce small seeds
■ Small size may also increase
_____ by enabling the seeds
to be carried longer distances

○ Animals
■ Animals that suffer high ______ rates (e.g., quail, sardines, and mice) tend to produce many offspring

increases, large, learning

● Extra investment on the part of the parents greatly ______ offspring’s chances of survival

● Brazil nut and walnut trees
○ Produce ____ seeds packed with
nutrients that help the seedlings
become established

● Primates
○ Generally bear only 1-2 offspring at a time
○ Parental care and an extended period of ______ in the first several years are important

● Especially important in habits with high population densities

logistical, range, carrying

Selection of traits
● One way to categorize variation in life history traits is related to _______ growth model
● These names follow from the variables of the logistic equation:
1. K-selection
2. r-selection
● Two concepts represent 2 extremes in a ____ of actual life histories
● Are both grounded in the idea of ______ capacity

high, K, Mature

K-selection
➔ Selection of traits that are
advantageous at ____ densities
➔ Operates in populations living at a
density near the limit imposed by their resources (carrying capacity, __), where competition is stronger
◆ ____ trees in an old-growth forest

low, maximize, Weeds

r-selection
➔ Selection of traits that maximize
reproductive success in uncrowded
environments (___ densities)
➔ Said to _______ r (intrinsic rate of
increase)
➔ Occurs in environments in which
population densities are well below
carrying capacity (low competition)
➔ Disturbed habitats that are being
recolonized
◆ ____ growing in an abandoned agricultural field

Density-Dependent Factors

1. Population Change & Density
2. Mechanisms of Density-Dependent
Population Regulation
3. Population Dynamics
a. Stability & Fluctuation
b. Population Cycles: Scientific Inquiry
c. Immigration, Emigration, and Metapopulations

reduce, increase, abundance

Density-Dependent Factors
● In cases wherein we seek to _____ the size of an unwanted population or ______ the size of one that is endangered, it is helpful to understand factors that affect population ______.

exceeds

Population Change & Density
● Ecologists study how the rates of birth, death, immigration, and emigration change as population density rises
● If immigration and emigration offset each other, then a population grows when the birth rate ______ the death rate and declines when the death rate exceeds birth rate
1. Density Independent
2. Density Dependent

not, Mortality, drought, physical

Density Independent
➔ Birth rate or death rate does ___
change with population density
➔ Example. _____ of dune fescue grass (Vulpia fasciculata) is mainly due to _____ stress that arises when the roots are uncovered by shifting sands
◆ ______ factor = density independent

changes, Reproduction, scarce

Density Dependent
➔ Birth rate or death rate ______ with population density
➔ Example. _______ of dune fescue grass (Vulpia fasciculata) declines as population density increases partly because water or nutrients become more _____

dependent, independent, stop

● In the dune fescue grass population, key factors affecting its birth rate are density _________ while its death rate is largely determined by density-_______ factors
● When immigration and emigration offset each other, the combination of density-dependent reproduction and density-independent mortality can ____ population growth (see Figure 53.15)

temperature, precipitation, dependent, regulated

● Variations in density-independent factors such as _______ and ________ can dramatically change population size
○ Example. A drought or a heat wave can sharply increase mortality rates, causing the population to plummet

● Density-independent factors cannot consistently cause a population to decrease in size when it is large or increase in size when it is small
○ Only density-_______ factors can
consistently cause changes

● A population is said to be ______ when one or more density-dependent factors cause its size to decrease when large or increase when small

dependent, Kelp, fluctuate

Mechanisms of Density-Dependent
Population Regulation
● Ultimately, at large population sizes, negative feedback is provided by density-______ regulation which halts population growth by reducing birth rates or increasing death rates
○ Without negative feedback, a population will never stop growing
○ No population can increase in size
indefinitely
○ Example. ___ perch’s (Brachyistius
frenatus) death rate rose proportionally as its density increased because the fish ran out of spaces in the kelp where they could hide from predators

● Increased densities cause population growth rates to decline by affecting reproduction, growth, and survival

● Negative feedback addresses why populations stop growing, but does not explain why some ______ dramatically while others remain relatively stable

Mechanisms of Density-Dependent
Population Regulation

1. Competition for resources
2. Disease
3. Territoriality
4. Intrinsic factors
5. Toxic wastes

fertilizers

Competition for resources
● Increasing population density leads to competition among members of a population for nutrients and other resources
● Leads to reducing reproductive rates
● Farmers counteract the competition on the growth of wheat (Triticum aestivum) and other crops by applying _______ to reduce nutrient limitations on crop yield

transmission, influenza, tuberculosis

Disease
● Density-dependent diseases have an increased rate of _______ as a population becomes more crowded
● In humans, the respiratory diseases ______ (flu) and ________ are spread through the air when someone sneezes or coughs
○ Both diseases strike a greater percentage of people in densely populated cities than in rural areas

Space, cheetahs, surplus

Territoriality
● _____ becomes the resource for which individuals compete
● _______ (Acinonyx jubatus) use a chemical marker in urine to warn other cheetahs of their territorial boundaries
● Presence of ______ (nonbreeding) individuals is a good indicator of territoriality as a population growth restriction

aggressive, hormonal

Intrinsic factors
● Intrinsic physiological factors (operating within an individual organism) sometimes regulate population size
● Reproductive rates of white-footed mice (Peromyscus leucopus) can drop even when food and shelter are abundant because of _______ interactions and ______ changes within individual mice
○ Delays sexual maturation and depresses the immune system

Yeast, Ethanol, 13

Toxic wastes
● ____ (Saccharomyces cerevisiae) converts carbohydrates to ethanol in winemaking
● ____ that accumulates in the wine is toxic to yeasts and contributes to density-dependent regulation of yeast population size
○ Alcohol content of wine is usually less than ___%
■ This is the maximum concentration of ethanol that most wine-producing yeast cells can tolerate

Population Dynamics

➔ Population fluctuations from year to year or place to place
➔ Influenced by many factors and in turn affect other species
➔ Complex interactions between biotic and abiotic factors cause variations in population sizes

Population dynamics

1. Stability & Fluctuation
2. Population Cycles
3. Immigration, Emigration, and Metapopulations

Stability & Fluctuation

● Populations of large mammals were once thought to remain relatively stable, but long-term studies have challenged this idea

Moose, weather, predation, parasites, wolves, winter

Stability & Fluctuation
● Example. _____ population on Isle Royale in Lake Superior has fluctuated substantially since the 1900s
○ Harsh _____ (cold winters with heavy snowfall) weakens moose and reduces food availability ➔ decreasing population size
○ High moose numbers bring about _____ and an increase in the density of ticks and other ______ ➔ decreasing population size
○ Low moose numbers, mild weather,
and good food availability ➔ increasing population size

○ The moose population experienced 2 major collapses during the last 50 years
■ First major collapse - coincided with a peak in the number of _____ (predators) from 1975-1980
■ Second major collapse - Harsh _____ weather in 1995 increased the energy needs of moose and made it harder for them to find food under deep snow

boom, bust

Population Cycles

* Many populations fluctuate at unpredictable intervals, while others undergo regular___-and-___ cycles
* Small herbivorous mammals (voles and lemmings) tend to have 3-4 year cycles
* Birds (ruffed grouse and ptarmigans) have 9-11 year cycles

hares, lynx

Population Cycles
● Example. 10-year cycling of snowshoe ____ (Lepus americanus) and ___ (Lynx canadensis) in the far northern forests of Canada and Alaska
1. 1st Hypothesis
2. 2nd Hypothesis

rejected

1st Hypothesis
- Cycles may be caused by food shortage during winter
■ Hares eat the terminal twigs of small shrubs
■ Cycles should stop if extra food is provided to a field population
● Studies show that hare populations in areas with extra food increase in density but continue to cycle similar to unfed control populations
■ First hypothesis is _____

supported

2nd Hypothesis
- Cycles may be due to predator-prey interactions
■ Many predators other than lynx eat hares, and they may overexploit their prey
■ Predators (lynx, coyotes, hawks, and owls) killed 95% of hares in conducted studies
● No hares died due to starvation
■ Predator overexploitation thus seems to be an essential part of snowshoe hare cycles
● With no predators, it is unlikely that hare populations will cycle in Northern Canada
● When predators were excluded from the area, the collapse in the cycle was nearly eliminated
■ Second hypothesis is ______