Instructor R. ZamoraInstructor R. Zamora

AP Environmental ScienceAP Environmental Science

Edinburg North High SchoolEdinburg North High School

Core Case StudyCore Case StudySouthern Sea Otters:

Are They Back From the Brink of Extinction?Historical abundance

and distribution: 1 million along NA Pacific Coast

Habitat: Kelp ForestsHabits: Use tools to

eat shellfish

By early 1900s, hunted to near extinctionBy early 1900s, hunted to near extinctionFurFurViewed as competitors for shellfishViewed as competitors for shellfish kelp forests disappearedkelp forests disappeared

Keystone speciesKeystone speciesDepredates herbivorous invertebrates (e.g., sea Depredates herbivorous invertebrates (e.g., sea

urchins)urchins) Maintains ecologically and economically important kelpsMaintains ecologically and economically important kelps

Recovery of otter populations Recovery of otter populations recovery of kelp forests and overall diversityrecovery of kelp forests and overall diversity upsets commercial and recreational shellfishersupsets commercial and recreational shellfishers

Focus of this chapter, Focus of this chapter, Population dynamicsPopulation dynamics – – study of how populations change in their study of how populations change in their distribution, numbers, age structure, and density.distribution, numbers, age structure, and density.

Focus QuestionsWhat are the major characteristics of

populations?How do populations respond to changes in

environmental conditions?How do species differ in their reproductive

patterns?

Population Dynamics and Carrying CapacityPopulation Distribution

Three patterns of distribution or dispersion: Clumping – most common Uniform – when there is intense competition for

resources Random – least common

Four reasons for clumping distributions Resources vary from place to place Living in groups provides protection against

predators Living in groups gives some predators a better

chance at getting food Mating or caring for young

Changes in Population Size: Entrances and ExitsPopulation size is influenced by:

Births Deaths Immigration Emigration

EmigrationDeathsnImmigratio Births Population in Change

Age structure: Young Populations Can Grow FastRate of population change depends on age structure

– proportion of individuals at various ages. Usually described by three main categories:

Pre-reproductive ages (juvenile or immature) Reproductive ages (adult) Post-reproductive (senescent)

Senescents

Adults

Immatures Growing

Pop.StablePop.

Decreasing Pop.

Limits on Population Growth: Biotic Potential vs. Environmental ResistanceNo pop. can grow indefinitely limits to growth

in nature (lesson from one of nature’s four sustainability principles)

Pops. vary in their biotic potential – capacity for growth. Intrinsic rate of increase (r) – rate at which a pop

would grow if it had unlimited resources. Populations with high r:

Reproduce early in life Have short generation times Can reproduce many times Have many offspring each time they reproduce

Example: House fly 5.6 x 106 descendants in 13-moExample: Bacteria w. generation time of 20-min 0.3-

m deep layer over the earth in 36-h

There is a size limit to growth imposed by limiting factors. Limiting factors: water, light, living space,

nutrients, competition, predation, and disease.Environmental resistance – all factors that limit

growth of a pop Negative, or corrective feedback

Biotic potential and environmental resistance lead to carry capacity (K) – the maximum population size that a particular habitat can sustain indefinitely w/o degrading the habitat.

Exponential and Logistic Population Growth: J-curves and S-curvesWith ample resources a pop can grow rapidly,

but as resources become limited, its growth rate slows and levels off.

With few, or no limitations populations grow exponentially (exponential growth) at a fixed rate (e.g., 2%). N-t plot produces a J-shaped curve

Logistic growth involves rapid growth followed by a steady decline w/ time until pop size levels off. Decrease occurs as pop experiences environmental

resistance N-t plot produces a S-shaped (or sigmoid) curve

Figure 8-3. No population can continue to increase in size indefinitely.

Figure 8-4. Logistic growth of sheep after being introduced to the island of Tasmania

Brown tree snake Multiplied

exponentially Up to 5000 km-2

Venomous Caused more than

2000 power outages Caused the extinction

of 8 out of 11 of Guam’s forests birds.

Figure 8-5. Brown tree snake was accidentally introduced to Guam during WWII.

http://www.npswapa.org/gallery/album59/Brown_tree_snake_Boiga_irregularis_USGS_Photograph

What influence would a decline in population size of a keystone species have on community composition? Decrease in populations of species dependent on

the keystone species. Increase in species that move in to occupy part or

all of vacant niches.

Exceeding Carrying Capacity: Move, Change Habits, or Decline in SizeThe transition from

exponential growth to logistic growth may not be smooth. Occurs because of a

reproductive lag time. Dieback, or crash

ensues (Fig. 8-6)Figure 8-6. Exponential growth, overshoot, and population crash after introduction to St. Paul Island in Bearing Sea in 1910.

Carrying capacity if an area or volume is not fixed. Habitat may be degraded by the population that

exceeded K. Also, K varies temporally increasing or decreasing

seasonally or year to year. Weather Climate Other factors

K for a population man increase by developing adaptive traits through natural selection.

Population may migrate when K has been exceeded.

Humans are not exempt from population overshoot and dieback. Ireland, 1845, 1 million died, 3 million migrated Polynesians on Eater Island, pop crashed after using up

most of island trees Earth’s carrying capacity for humans has been extended

by technological, social, and cultural changes.

Population Density and Population Change: Effects of CrowdingPopulation density – the number of individuals in a

population found in a particular area or volume. Pop density can affect how rapidly it can grow or

decline. Some control factors are not affected by population

density.Density-dependent factors can control population

size increase as the density increases. Competition, predation, parasitism, and diseases (e.g.,

bubonic plague in the 14th century) Tend to regulate a pop at a fairly constant size, often

near KDensity independent factors control independently

of pop density. Mostly abiotic

Examples: freezes, floods, hurricanes, fire, pollution, and habitat destruction

Types of Population Change Curves in NatureFour general patterns:

Stable – size fluctuates slightly above and below K Characteristic of species in stable environments

Irruptive – explosive growth to a high peak and then crash. Characteristic of short-lived, rapidly reproducing species Linked to seasonal changes in weather and nutrient availability

Cyclic – regular cycles of increase and decrease Rise and fall of lemmings every 3-4 years Lynx and snowshoe hare, 10-yr

(Fig 8-7) Top-down pop regulation Bottom-up regulation

Irregular – no pattern in change of population size

Figure 8-7

Cases Study: Exploding White-tailed Deer Cases Study: Exploding White-tailed Deer Population in the USPopulation in the USSince the 1930s the white-tailed deer Since the 1930s the white-tailed deer

population in the US has exploded.population in the US has exploded. By 1900, reduced to 500 000By 1900, reduced to 500 000 1920s and 30s laws passed to protect deer, and 1920s and 30s laws passed to protect deer, and

wolves and mountain lions nearly eliminatedwolves and mountain lions nearly eliminated Today there are 25-30 millionToday there are 25-30 million

Problem with the reboundProblem with the rebound EncroachmentEncroachment SuburbanizationSuburbanization Vector for Lyme diseaseVector for Lyme disease

SolutionsSolutions Change hunting regulationsChange hunting regulations Trap and relocateTrap and relocate Birth controlBirth control

Reproductive PatternsWays to reproduce: Sexual Partners Not

Always NeededAsexual Reproduction

Produces clones Common in taxa such as bacteria, plants and some

animals such as corals.Sexual Reproduction

Mixes genetic material of two parents producing offspring w/ genetic traits of each parent.

Disadvantages of sexual reproduction First, males don’t give

birth; female has to produce twice as many offspring to break even.

Second, increased change of genetic errors separation and recombination of chromosomes.

Third, courtship and mating is expensive (time and energy budgets), can cause disease, and injury may be inflicted in males that combat for mates.

Advantages Provides genetic diversity

in offspring Males of some species can

help raise young

Figure 8-8. Courtship display

Reproductive Patterns: Opportunists and CompetitorsSpecies differ in reproductive strategies to help ensure

survival.

(instead opportunists) (good competitors)

Most species have reproductive patterns between extreme r- and K- selected species.

Reproductive patterns may give a species a temporary advantage, but the ultimate population regulator is available habitat.

Figure 8-9. Positions of r-selected and K-selected species on the sigmoid population growth curve.

Survivorship CurvesA representation of age

structure that shows the percentage of members surviving at different ages (Fig. 8-11) There are three

generalized curves: late loss, early loss, and constant loss.

A life table shows projected life expectancy and probability of death for individuals at each age in a survivorship curve.

Figure 8-11. Survivorship curves for populations of different species.

The next chapter applies the principles of The next chapter applies the principles of population dynamics discussed in this chapter population dynamics discussed in this chapter to the growth of human population and its to the growth of human population and its environmental impact.environmental impact.

The principle of population dynamics are also The principle of population dynamics are also used to help us harvest fish and wildlife used to help us harvest fish and wildlife resources more sustainably.resources more sustainably.

The problems to be faced are vast and complex, but come down to The problems to be faced are vast and complex, but come down to this: 6.7 billion people are breeding exponentially. The process of this: 6.7 billion people are breeding exponentially. The process of fulfilling their wants and needs is stripping earth of its biotic fulfilling their wants and needs is stripping earth of its biotic capacity to support life; a climactic burst of consumption by a single capacity to support life; a climactic burst of consumption by a single species is overwhelming the skies, earth, waters, and fauna.species is overwhelming the skies, earth, waters, and fauna.

-Paul Hawken-Paul Hawken

Figure 9-1. Crowded Figure 9-1. Crowded street in China.street in China.