Chapter 6

Population Ecology

• They were over-hunted to the brink of extinction by the early 1900’s and are now making a comeback.

Core Case Study: Southern Sea Otters: Are They Back from the

Brink of Extinction?

Figure 8-1Figure 8-1

Population Trends from 1990 - 2012

17,000 in 1800’s Less than 100 in 1914

Case Study: CA Condor

• Large pre-historic population; ice age

• In 1987, only 22 condors were left.

• Captured all wild birds; captive breeding program

• Decline due to:– Pesticides (DDT)– Poisoning (lead)– Starvation– Accidents

Levels of Complexity

Population• Population – composed of all individuals that

belong to the same species and live in a particular area at a given time.– Unit of evolution– Cause of decrease and increase in population– Boundaries can be arbitrary– Dynamic (always changing)

Changes in Population Size: Inputs & Outputs

• Populations increase through births and immigration

• Populations decrease through deaths and emigration

Communities & Ecosystems

• Community - incorporates all of the populations of organisms within a given area.– Boundaries can also be arbitrary– Scientists interested in interaction between species– Ex: Maple-Beech-hemlock community– Communities that experience similar temperature & rainfall are

grouped into biomes• However, species of trees varies from community to community. Ex:

Eucalyptus vs. oak (Aust. vs N. America/Europe)

• Ecosystems – consists of all abiotic & biotic components in a particular location.– Focused on the flow of energy & matter; cycling of nutrients

through a system

Population Ecology

births + immigration > deaths + emigration = population growth

births + immigration < deaths + emigration = population decrease

Population Ecology is the study of the factors that cause populations to change.

– Knowing factors that affect population are useful for controlling it– CA Condor (extinction) or Argentine ants (invasive)

Population Characteristics

• Population Size (N) – total number of individuals within a defined area @ a given time.

• Population Density – is the number of individuals per unit area (or volume in case of aquatic organisms).– Help estimate if species is rare, abundant, or

overpopulated– Help aid in wildlife management

Population Characteristic

• Population Distribution – description of how individuals are distributed with respect to one another.– How a population occupies space– Three types:

• Random – Trees in a forest• Uniform – territorial nesting birds, tree farm• Clumped – schooling fish, flocking birds, & herding

animals

Population Distribution

• Most populations live in clumps although other patterns occur based on resource distribution.

Figure 8-2Figure 8-2

Population Characteristics

• Population Sex Ratio – the ratio of males to females– Usually close to 50-50; fig wasps (20:1 females/males)– Knowing ratio helps scientists estimate number of

offspring in next generation

• Population Age Structure – description of how many individuals fit into a particular age category.– Helps predict how rapidly a population can grow– Large proportion too young or too old to reproduce will

help understand the growth or decline of a population

Age Structure: Young Populations

Can Grow Fast• How fast a population grows or declines

depends on its age structure.– Prereproductive age: not mature enough to

reproduce.– Reproductive age: those capable of

reproduction.– Postreproductive age: those too old to

reproduce.

Age Structure Diagram

Factors That Influence Population Size

• Density Dependent Factors: influence an individual’s probability of survival & reproduction that depends upon the size of population.– Amount of available food (greater affect on large

populations); predation, disease, competition, parasitism

– Gause Experiments – shows how food supply controls population growth

• Rapid growth early, then population plateaued later • Double the amount of food, populations also doubled• Confirmed limiting resource = food• Limit to how many individuals the food supply can sustain is

Carrying Capacity (K) without destroying the habitat.– Predict how much individuals an environment can sustain

• Density-Independent Factors: have same effect on an individual’s survival & reproduction at ANY population size.– Examples: tornado, hurricanes, floods, fires, volcanic

eruptions, and other climatic events– Mortality (death) increases in such an event

regardless of population size– Bird populations are regulated by these factors

Factors That Influence Population Size

Gause’s Experiments

Population Growth Models

• Important tools in explaining population fluctuations.

• Used to predict population size at any moment in time.

• Growth Rate – number of offspring an individual can produce in a given time period, minus the deaths.

• Maximum potential for growth is called the intrinsic growth rate (r). Under ideal conditions.

Population Growth Models

• Exponential Growth Model:– N0 = starting population

– r = intrinsic growth rate– N = future size of population– t = time elapsed @ which population grows– r = 0 if population is constant; r > 0 if

population is increasing; r < 0 if population is decreasing

Exponential Growth Model

• Populations not limited by resources, growth is very rapid.

• Rate is fixed• More births occur with

each step in time.• This model produces a J-

Shaped curve .• Exponential growth is

density independent.• Gause’s experiment

showed that no population can grow indefinitely

The Logistic Growth Model

• Modified model to incorporate environmental limits on exponential growth.

• Logistic growth model – a population whose growth is initially exponential but slows as population reaches carrying capacity (K).

• Produces an S-shaped curve

Variations on the Logistic Growth Model

• Overshoot – producing more offspring than environment can hold, leading to a die-off– Seasonal: mate in winter but

population is larger than spring carrying capacity

– Reindeer are a good example (see graph)

– Overshoot & die-off can be a recurring cycle as populations will oscillate around the carrying capacity.

Types of Population Change Curves in Nature

• Population sizes may stay the same, increase, decrease, vary in regular cycles, or change erratically.– Stable: fluctuates slightly above and below carrying

capacity.– Irruptive: populations explode and then crash to a

more stable level.– Cyclic: populations fluctuate and regular cyclic or

boom-and-bust cycles.– Irregular: erratic changes possibly due to chaos or

drastic change.

Natural Population Curves

Fig. 9-7 p. 194

Population grew exponentially then crashed to only 8 individuals . . . . Most likely due to lack of food

The Role of Predation in Controlling Population Size

The Role of Predation in Controlling Population Size

Top-down control: predator controls the cyclic fluctuations of the prey

Bottom-up control: herbivore-plant interaction influences the cyclic fluctuations of the predator

Reproductive Strategies & Survivorship Curves:

r-Selected• High intrinsic growth rate• Reproduce often w/ lots of

offspring• Large fluctuation around

carrying capacity• Ex: insects, rodents, small fishK-Selected• Low intrinsic growth rate• Abundance of species is

determined by carrying capacity.• Small fluctuations• Ex: Large mammals, most birds

Survivorship Curves: Short to Long Lives

The way to represent the age structure of a population is with a survivorship curve.

Late loss population (Type I) high survival rate; live to an old age.

• Elephants, rhinoceros, humans, whales, large reptiles

Constant loss population (Type II) constant; die at all ages.

• Corals, Jellyfish, songbirds, rodents

Early loss population (Type III) low survival rate; die at young ages.

• Annual plants• Insects