1. Introduction: Key Attributes of Populations

In biology, a population is defined as a group of organisms of the same species inhabiting a particular location.

1a. Population size

The diagram to your left, which uses data from the United Nations, shows how the size of humanity’s population has changed over the past two hundred years. The Earth’s human population was about one billion in 1800. In 1961 (when Mr. W. was born), it was three billion. Now, it’s about 7.5 billion.

What about the future? It depends on a variety of factors, especially birth rate (the number of births/year). The low projection sees human population reaching a maximum size in about 2040, then starting to decline. Why? because in many parts of the world, birth rates are falling. The medium projection sees population growth continuing, but at a slower rate. And the high scenario sees the current rate continuing.

As human populations have grown, populations of wildlife have fallen. These numbers are less precise, but it’s thought that there were about 12 million African elephants in the early 1900s. Now there are about 350,000 (source: Africa Geographic).

Size is perhaps the most obvious way to characterize a population. Let’s continue by looking at some others.

1b. Density

A school of fish. What’s the size of this population? How would you determine its density?

Density is usually measured as the number of individuals/unit area. For example a 1990 study of lions in Serengeti National Park determined, in one area of the park, a density of one lion/4 km2. However, when an organism inhabits a three-dimensional space, like air or water, density is usually expressed as individuals/unit volume. For example, a liter of seawater may contain as many as 100 billion viruses.

1c.Range

Range. The range is where a population is found. That can be the specified area of a population that you’re studying, such as the wolves of Yellowstone National park. Or, as is shown on the left, it can be the total possible range of a species.

Note that the range is dynamic, shifting over time. In the summer, populations of Golden crowned sparrows will surge in Northwest Canada and Alaska. During the winter the sparrows retreat to the warmer temperatures and more abundant food that can be found in western British Columbia, Washington, Oregon, California, and Baja California.

Aggregating anemones: an example of clumped dispersion. Photo by Glenn Wolkenfeld

1d. Dispersion Pattern

Within a population’s range, a population will generally clump around a particular resource or habitat. For example, in an intertidal zone, you’ll find sea anemones clumped around tide pools. In an African savannah, there will be dense arrays of wildlife around watering holes.

Clumping (“B,” below) is the most common dispersion pattern. However, individuals within a population can also, in specific circumstances, space themselves out in a uniform pattern (see “A” below). This is seen in bird colonies, where territorial interactions force nesting birds to spread themselves out at a safe distance from their neighbors (who would be happy to eat an unguarded chick in a neighboring nest). Another dispersion pattern is seen in the way dandelions spread out over a lawn. Since  each plant’s location is a function of where a wind-blown seed randomly lands, the resulting plants will often be randomly distributed (“B” below).

Uniform (a), random (b), and clumped dispersion patterns (c). Each red circle represents an individual organism.

 

1e. Age structure

Populations can be characterized by the number of individuals of various age brackets. A population’s age-structure can be represented by a population pyramid (though these pyramids might not be very pyramidal in shape, as you’ll see below).

Take a moment and study the graph below to see what you can learn from it.

You can think of an age structure graph as being two bar graphs. In this particular age structure graph, the bars on the right are for this country’s females (note the label in the upper right corner).  The bars on on the left are for the males. The Y axis lists age brackets in increments of 5 years: the bottom bar is 0 to 4 years of age the next is 5-9 years, etc. The X axis is number of individuals of that sex in that bracket (as well as, in this case, the percentage of the population).

[qwiz qrecord_id=”sciencemusicvideosMeister1961-pop_ecol, age structure”]

[h] Age structure graphs

[i] A key thing to note about an age structure graph is the relative number of individuals of pre-reproductive age, reproductive age, and post reproductive age, as shown below

[q] In the diagram below, which number represents the age bracket of individuals who are of childbearing age?


[textentry single_char=”true”]

[c*] 2

[f] Yes! Number 2 represents individuals of childbearing age.

[c] Enter word

[c] *

[f] No. You’re looking for the individuals who are neither too young to bear children, nor too old. Which number would that most likely be?

[q] In the diagram below, which letter represents the population that will grow the most in the future?

[textentry single_char=”true”]

[c*] A

[f] Nice job! In the future, the large group of pre-reproductive individuals will enter their reproductive years. When they do, that large group of individuals will create many offspring, causing the population to grow.

[c] Enter word

[c] *

[f] No. You’re looking for the population which, in the future, will have the most individuals entering into their reproductive years. Which population is that?

[q] In the diagram below, which letter represents the population that will grow the least in the future?

[textentry single_char=”true”]

[c*] A

[f] Nice job! In population A, there’s a relatively small number of individuals in their pre-reproductive years. That means that in the future, this population will have relatively few reproducing individuals. They’ll produce relatively few offspring, causing that country’s population to decline.

[c] Enter word

[c] *

[f] No. You’re looking for the population which, in the future, will have the fewest individuals entering into their reproductive years. Which population is that?

[q] In the graph below, females between 65 and 69 make up about what percentage of the population?

[textentry single_char=”true”]

[c*] 3

[f] Excellent. The percentage of females between 65 and 69 is about 3%.

[c] Enter word

[c] *

[f] No. Find the pink bar that’s at the 65 and 69 age group. What percentage of the population does that bar make up (the answer is on the X axis, and be sure to look at %). 

[/qwiz]

 

1f. Life Tables and Survivorship Curves

Population pyramids like the ones we just examined are built from life tables. A life table represents the survivorship of individual organisms or people from a certain population. What is survivorship? It’s a number that represents, for each age bracket, the probability that an individual will die before his or her next birthday (adapted from wikipedia).

Here’s a life table for the U.S. population in 2003.

Here’s how a life table works. Find the top row, which has data for 0-10 year olds. For every 100,000 individuals aged 0-10, 884 die, making the probability of their survival through this time period 0.991. Another way to think of this is that someone between ages 0 and 10 has a 99.1% chance of surviving to their next birthday. When these individuals are between age 50 and 60, their probability of their surviving to the next age group is 0.938. However, by the time an individual reaches the 80-90 bracket, their chances of surviving to the next age bracket are down to 0.405 (40.5%).

You can graph these data onto what’s called a survivorship curve. Here’s one built from similar data to what’s shown in the life table above.

Here’s how to read it. The Y axis represents the number of survivors, starting from an initial population of 1000. Note that the Y axis has a logarithmic scale: the intervals go from 1 to 10 to 100 to 1000. The X axis is linear, and represents the lifespan of this particular species.

The survivorship curve is the red line. Follow it from left to right. Between 0 and 50 (birth and halfway through the lifespan), survivorship slopes very gently downward. That corresponds to the life table above. For example, at ages 30 to 40, most of the individuals born to this population are still alive.

Look again at the survivorship curve. Even 75% of the way through the lifespan, the slope of the line isn’t very steep. But as you get to 80 or 90% of the way through life, the curve starts to slope downward more and more as the death rate climbs and the survivorship declines.

A population like this shows “late loss of life.” That’s true of many human populations today, especially those who live in advanced industrialized countries.

Now let’s look at some survivorship curves for other species.

Belding’s Ground Squirrel

One is Belding’s ground squirrel. This rodent lives in the mountains of the Western United states. Whereas humans have late loss of life, these squirrels experience a constant mortality across the range of their lifespan. As a result, the survivorship curve for these squirrels is constant. That’s what you see in the line labeled “Type II” survivorship curve below.

Now think of animals like an oysters, or frogs. These animals produce enormous numbers of young. Early in life, there’s high mortality. A relatively small percentage the young will survive to become adults. This is shown in the “Type III” survivorship curve.

Survivorship curves are associated with specific reproductive strategies. In species with a type 1 survivorship curve, the parents tend to have relatively few offspring, and invest a lot in their survival. In species with a type III survivorship curve, the strategy is to put energy into having enormous numbers of offspring, but to invest relatively little in their survival.

2. Key Attributes of Population: Checking Understanding

[qwiz random = “true” qrecord_id=”sciencemusicvideosMeister1961-pop_ecol, population attributes”]

[h]Key Attributes of Populations

[i]

[q] In the diagram below, which image represents the dispersion pattern that results from territorial interactions, where individuals want to maintain a certain amount of space around themselves?

[textentry single_char=”true”]
[c*] A

[f] Nice job. The uniform distribution pattern in “A’ results from territorial interactions.

[c] Enter word

[c] *

[f] No. Think of it this way. Which pattern would result if every individual forced every other individual to keep a certain distance away?

[q] Imagine a species of grazing animals. These animals congregate around waterholes, which are distributed around a large grassland. Which dispersion pattern would result?

[textentry single_char=”true”]
[c*] C

[f] Nice job. The clumped dispersion pattern in “C” results from clustering around a resource.

[c] Enter word

[c] *

[f] No. Here’s a hint. Which pattern would result if a population subdivided itself into groups that cluster around a specific resource?

[q] Of the three populations below, which one do you anticipate will grow the most in the near future?

[textentry single_char=”true”]
[c*] A

[f] Nice job. The large number of individuals who will soon be entering their childbearing years will cause population A to grow the fastest.

[c] Enter word

[c] *

[f] No. Here’s a hint. The population that will grow the fastest is the one that will have the most individuals entering into their childbearing years.

[q] Of the three populations below, which one do you anticipate will be the most stable in terms of population size?

[textentry single_char=”true”]
[c*] B

[f] Good work! In population B, the number of individuals of child bearing years will be replaced by an equal number of such individuals. As a result, the number of offspring born in the future should remain relatively constant over time.

[c] Enter word

[c] *

[f] No. Here’s a hint. Populations will grow when the number of individuals of child-bearing years increases. Populations decrease when the number of individuals of child-bearing years decreases. They’ll stay the same when the number of individuals of child bearing years stays the same. In which population will the number of individuals of child rearing years stay the same?

[q] Which survivorship curve is characterized by early loss of life?

[textentry single_char=”true”]
[c*] C

[f] Nice work! Survivorship curve C shows “early loss of life.”

[c] Enter word

[c] *

[f] No. Here’s a hint. Find the curve where most individuals die within the first 10 to 20 percent of their maximum lifespan, and relatively few live beyond the middle of the lifespan.

[q] Which survivorship curve is characterized by late loss of life.

[textentry single_char=”true”]
[c*] A

[f] Nice work! Survivorship curve A shows “late loss of life.”

[c] Enter word

[c] *

[f] No. Here’s a hint. Find the curve where very few individuals die within the first 3/4 of their maximum lifespan, and relatively few die when very young.

[q] Which survivorship curve is characterized by constant loss of life.

[textentry single_char=”true”]
[c*] B

[f] Awesome! Survivorship curve B shows “constant loss of life.”

[c] Enter word

[c] *

[f] No. Here’s a hint. The reason why this graph uses a logarithmic scale is that it shows constant loss of life as a constant slope. Which line fits that description?

[q]The graph below is called a [hangman] curve. Line A represents a population with [hangman] loss of life. The vast majority of the individuals in this population [hangman] past the middle of this population’s expected lifespan. By contrast, curve B represents [hangman] loss of life, and curve C represents [hangman] loss of life.

[c]survivorship

[c]late

[c]survive

[c]constant

[c]early

[/qwiz]

 

3. Understanding Population Growth

To learn how populations grow, complete the interactive reading below.

[qwiz qrecord_id=”sciencemusicvideosMeister1961-pop_ecol, Population Growth interactive reading”]

[h]Population growth: interactive reading

[i]

 

[q labels = “top”] Think of the city or town where you live. On the most fundamental level, only four events will determine whether your town or city’s population goes up or goes down. Think about what these are and complete what’s below.

  • Things that increase population: _________ and _____________ (movement into a population).
  • Things that decrease population: _________ and _____________ (movement out of a population.

[l]birth

[fx] No. Please try again.

[f*] Excellent!

[l]death

[fx] No. Please try again.

[f*] Excellent!

[l]immigration

[fx] No, that’s not correct. Please try again.

[f*] Great!

[l]emigration

[fx] No. Please try again.

[f*] Correct!

 

[q labels = “top”]To simplify what’s going to come, we’re going to assume that immigration and emigration are equal. With that assumed, then the growth rate (the change in population over time) can be expressed as follows

________ rate = birth rate – _________ rate

or, with symbols

r = __ – d

[l]death

[fx] No. Please try again.

[f*] Great!

[l]growth

[fx] No. Please try again.

[f*] Correct!

[l]b

[fx] No, that’s not correct. Please try again.

[f*] Good!

[q]Just to review:

r = b – d

means

growth rate = [hangman] rate – [hangman] rate.

That means, of course, that population only grows if [hangman] rate is greater than the [hangman] rate. If the reverse is true, population will [hangman].

[c]birth

[c]death

[c]birth

[c]death

[c]decrease

[q]Another way to express this is to think about the change in a population’s size over time. This is typically expressed by the following formula.

dN/dt=rN

where

  • dN means “the change in population”
  • dt means “the change in time”
  • r is the “growth rate”
  • N is the population

In this formula, if r > 1, then the population will _________. If r < 1, then the population will __________.

[l]decrease

[fx] No. Please try again.

[f*] Great!

[l]grow

[fx] No. Please try again.

[f*] Great!

 

[q]To connect this to something that might be more familiar, just think about money. If you put $100 into a bank account that paid 2% yearly interest, how much money would you have at the end of the year? Note that I’m using “$” to indicate a quantity of money.

Start by converting 2% to 1.02

Then use the formula d$/dt=r$

Your change in value would be calculated as

d$/dt=1.02(100)

d$/dt=$102

[q]Populations will grow in the same way as money in the bank will. The key point is that each year’s ending point becomes the next year’s starting point. So if your interest rate were 3%, and your initial investment was  $1000, your money would grow as follows.

Growth of a $1000 investment at 3%

Note that each year, your money grows a bit more. By year 14 you’re adding $44/year, compared to year 1, when you added only $30. The result is what’s called exponential growth.

[/qwiz]

 

Exponential Growth

If you graph a system (like a population) that’s growing exponentially, you get something that looks like what’s shown on the left.

Note that it doesn’t matter if your rate of growth is 0.03% or 5%: any growth rate will lead to exponential growth, as long as the growth rate is constant over time. The only difference is how long it’ll take before the line starts to steeply shoot upward. For example, the gray line on the right shows growth at 40%/year. The blue line shows 30%/year. Notice how in both cases, there’s a long takeoff period. Many generations have to go by before the lines start to rise upward to any appreciable degree. Think of that takeoff as being similar to the time an airplane spends moving down the runway until it’s moving quickly enough to take off. The higher the growth rate, the less time required on the runway.

Here’s another example of exponential growth. This one relates to the very start of the COVID-19 pandemic. The graph below shows cases of COVID-19 over a two week period in China from January 10 to January 24, 2020. Take a look at the legend, and note how closely the number of reported cases matches the number of cases predicted by exponential growth model.

Source: www.i24news.tv

In countries where effective public health measure were not implemented, COVID-19 has continued to spread at an exponential rate. Here’s a graph of how the pandemic unfolded in Brazil between February and June of 2020. Click on the image to enlarge it. Note how the time scale and number of cases differs from the graph of the China’s experience, but that the shape of the line is the same.

Biotic potential is “the ability of a population …. to increase under ideal environmental conditions — sufficient food supply, no predators, and a lack of disease. An organism’s rate of reproduction and the size of each litter are the primary determining factors for biotic potential.” (source: populationeducation.org).

Biotic potential is represented as rmax. When nothing impedes rmax, populations grow exponentially.

4. Carrying Capacity, Limiting Factors, and Logistic Growth

On a finite planet, exponential growth can’t go on forever. Eventually, a growing population reaches the environment’s carrying capacity. Complete the interactive reading below to see how carrying capacity works.

[qwiz style=”width: 730px !important;” qrecord_id=”sciencemusicvideosMeister1961-pop_ecol,Carrying Capacity, Logistic Growth”]

[h] Carrying Capacity and Logistic Growth: Interactive Reading

[q labels = “top”] Carrying capacity is the maximum number of individuals of a particular species that a specific environment can support. It’s represented by the letter K. As a population’s size (N) approaches K, then population growth _______________. Why? Either b ___________ or d ___________ (or both).

[l] decreases

[f*] Good!

[fx] No. Please try again.

[l] increases

[f*] Correct!

[fx] No, that’s not correct. Please try again.

[l]slows down

[f*] Excellent!

[fx] No, that’s not correct. Please try again.

[q labels = “top”] Here’s the formula for logistic growth. Translate it into words by dragging in the correct terms

 

biotic potential

[f*] Good!

[fx] No, that’s not correct. Please try again.

[l]carrying capacity

[f*] Excellent!

[fx] No. Please try again.

[l]population

[f*] Correct!

[fx] No. Please try again.

[l]time

[f*] Good!

[fx] No. Please try again.

[q] In the diagram at left, the dotted line at “E” represents the carrying capacity. Carrying capacity is a function of limiting factors. Imagine a small population of deer that swim to a large island that lacks both deer and predators, but which has plenty of grass and shrubs. Without any limits, the deer grow at their rmax. The growth is slow at first. This initial phase (shown at A) is called a lag phase.

After the lag phase, growth accelerates ( shown at B). During this period the growth is similar to what you’d see in a population that’s growing exponentially. Accordingly, “B” is called the exponential growth phase.

At a certain point, the deer population grows to a point where there begins to be a significant amount of competition for food. Food scarcity starts to limit population growth. At “C” you can see population growth starting to level off. This is often referred to a a “deceleration phase.” At “D,” the population has reached its carrying capacity.

[q] In the diagram below, which number represents the carrying capacity?

[textentry single_char=”true”]
[c*] E

[f] Excellent. “E” represents carrying capacity

[c] Enter word

[c] *

[f] No. You’re looking for a line that represents the maximum population that this area or environment could support.

[q] In the diagram below, which letter represents the point at which the population is growing the fastest?

[textentry single_char=”true”]

[c*] B

[f] Nice job. “B” is the point at which this population is increasing the fastest.

[c] Enter word

[c] *

[f] No. Look for the point in the black line where the slope is the steepest.

[q] In the diagram below, which letter represents the point at which the population has reached the environment’s carrying capacity?

[textentry single_char=”true”]

[c*] D

[f] Awesome. “D” is the point at which this population has reached its carrying capacity.

[c] Enter word

[c] *

[f] No. Look for the point in the black line where the slope is the steepest.

[q] Pretend that the population represented below had feelings. Which letter represents the point at which the population would start to feel resistance to its exponential growth?

[textentry single_char=”true”]

[c*] C

[f] Good job. “C” is the point at which this population would start to feel resistance from the environment. That’s the point where it’s approaching its carrying capacity.

[c] Enter word

[c] *

[f] No. Look for the point in the black line where the line is starting to slope less steeply upward. That decreased slope is from resistance from the environment, causing either a decrease in the birth rate, or an increase in the death rate. Where on the black line does the slope start to decrease?

[q] In the diagram below, which letter represents the lag phase?

[textentry single_char=”true”]

[c*] A

[f] Excellent. “A” is the lag phase, a period of very slow growth the occurs when a population is just getting started.

[c] Enter word

[c] *

[f] No. Imagine that the population was an airplane. Before it takes off, it has to move down the runway and pick up speed. Which letter would best represent the runway before takeoff?

[/qwiz]

 

5. Density Dependent and Density Independent Limiting Factors

The two models of population growth described above have a variety of terms connected to them. Take a minute to study the table below.

Graph
Graph description J Curve S Curve, Sigmoid Curve
Growth Model Exponential Growth Logistic Growth

Carrying capacity, a key part of the logistic growth model, can be thought of as environmental resistance. As a population grows towards the environment’s carrying capacity, various factors in the environment start to limit a population’s capacity for continued growth. These are called limiting factors, and they fall into two categories.

Density-dependent limiting factors become more intense as the density of a population increases. For example, as a population’s density increases,

  • There’s more competition for food, shelter, mates, etc.
  • It becomes easier for parasites to spread from one individual to the next.
  • If the members of this population are potential prey for another species, then predation might increase.
  • Wastes can accumulate, fouling the environment (which can then become more conducive to parasites).

All of the factors listed above are classified as extrinsic factors: that’s because they come from outside the population. Other density-dependent limiting factors are intrinsic. For example, increased crowding might cause stress, which leads to a decrease the birth rate.

Density-independent limiting factors aren’t a function of increased population density. Consider a population of anole lizards on an island in the Caribbean. A hurricane strikes the island, completely swamping it with a tidal surge. Whether the lizard’s population is large or small, this storm will kill many, if not all of the lizards. The same could be said of almost any natural disaster.

6. What happens as populations approach K?

The logistic growth model is a mathematical model. In nature, or in experimental studies with actual organisms, a variety of things can happen as a population approaches the environment’s carrying capacity.

In the 1930s, Russian biologist Georgy Gause studied growth rates of Paramecia, a single celled, ciliated eukaryote. In two separate species (only one of which is shown at left), the pattern of growth roughly followed the logistic growth model.

Here’s a different representation of what happens as a population reaches its carrying capacity.

Modified from an image from bioninja.org.

In this scenario, the population’s growth decelerates (C) as it approaches carrying capacity. The peak population that’s reached (E)  is actually above carrying capacity (D). Consumption of resources beyond carrying capacity degrades the environment and depletes available resources. This lowers carrying capacity. As a result, population falls (as shown at F). With the population at this level, the environment recovers, allowing the population to grow. What we have, in other words, is a stable oscillation around carrying capacity.

Here’s another scenario:

 

Think back to the small group of deer who swim to an island with abundant vegetation and no predators. The blue line shows the subsequent growth of the deer population. In this case, the population grows significantly beyond the island’s initial carrying capacity (at “1”). This is called an overshoot, and it’s indicated by the double headed arrow at “2.”

The overshoot causes significant depletion of the island’s resource base. As the population of shrubs diminishes, so does the population of the deer. By the end of the process, the vegetation is gone, and the island’s deer have become locally extinct.

Interactions between predators and their prey can result in population oscillations, as shown in the graph below.

Source: modified from an image on Wikipedia.

The Canada Lynx (Lynx canadensis) is a medium-sized cat that’s about twice the size of a housecat. It’s  adapted for life in Canada and Alaska. The lynx’s primary prey is the snowshoe hare (Lepus americanus).

The data in this graph is based on pelts collected by trappers and sold to the Hudson Bay Company for about a 100-year period.

While this looks like density dependent regulation of the hare’s population by the lynx, the interaction is more complex. Read the lightly edited passage below from Wikipedia:

A specialist predator, the Canada lynx depends heavily on snowshoe hares for food. Snowshoe hare populations in Alaska and central Canada undergo cyclic rises and falls—at times the population densities can fall from as high as 2,300/km2 (6,000/sq. mi) to as low as 12/km2 (31/sq. mi). Consequently, a period of hare scarcity occurs every 8 to 11 years. An example of a prey-predator cycle, the cyclic variations in snowshoe hare populations significantly affect the numbers of their predators—lynxes and coyotes—in the region. When the hare populations plummet, lynxes tend to move to areas with more hares, and tend to not produce litters.

In northern Canada, the abundance of lynx can be estimated from the records kept of the number caught each year for their fur; records have been kept by the Hudson’s Bay Company and Canadian government since the 1730s. These cycles…are caused by the interplay of three major factors—food, predation and social interaction. A study involving statistical modelling of the interspecific relations of the snowshoe hare, the plant species it feeds on and its predators (including the Canada lynx) suggested that while the [changes in population size] of the lynx depend primarily on the hare, the hare’s [population size] depends both on the plant species in its diet and predators, of which the Canada lynx is just one.

7. Carrying Capacity and Limiting Factors: Checking Understanding

[qwiz qrecord_id=”sciencemusicvideosMeister1961-pop_ecol, Carrying Capacity, Limiting Factors, checking understanding”]

[h] Carrying Capacity and Limiting Factors

[i]

[q] The growth curve below can be described by a single letter: _____.

[textentry single_char=”true”]
[c*] J

[f] Excellent. Exponential growth is typically describe as a “J” curve.

[c] Enter word

[c] *

[f] No. Just run the letters of the alphabet through your head. Which letter looks most similar to the blue line in the graph above?

[q] What letter is used to describe the growth curve below?

[textentry single_char=”true”]

[c*] S

[f] Nice job. Logistic growth is frequently described as an “S” curve.

[c] Enter word

[c] *

[f] No. Just run the letters of the alphabet through your head. Which letter looks most similar to the blue line in the graph above?

[q]The kind of population growth shown below is [hangman] growth. It only occurs when there are no [hangman] factors at work.

[c]exponential

[c]limiting

[q]The kind of population growth shown below is called [hangman] growth. In this growth model, a population’s growth slows at it reaches the environments’ [hangman] [hangman].

[c]logistic

[c]carrying

[c]capacity

[q]As a population grows within a confined geographical area, there come to be more individuals/unit of area or volume. The limiting factors that come into play in this situation are called [hangman] [hangman] limiting factors.

[c]density

[c]dependent

[q]Environmental factors such as fires, floods, landslides, and volcanic eruptions would all be classified as [hangman] [hangman] limiting factors

[c]density

[c]independent

[q]Limiting factors such as competition, parasitism, predation, and waste accumulation are all classified as [hangman] factors, because they come from outside of the population itself.

[c]extrinsic

[q]Sometimes, a population’s growth can be impeded by stress that’s induced by overcrowding, which lowers the population’s birth rate. These types of limiting factors are called [hangman] factors.

[c]intrinsic

[q]In the diagram below, letter “D” represents the [hangman] [hangman].

[c]carrying

[c]capacity

[q]In the diagram below, letters “E” through “F” show the population [hangman] around its carrying capacity.

[c]oscillating

[q] In the diagram below, which number or letter refers to the “overshoot.”

[textentry single_char=”true”]

[c*] 2

[f] Nice job. The overshoot is represented by “2.”

[c] Enter word

[c] *

[f] No. The overshoot is the point where the population grows beyond the environment’s carrying capacity. Where would that be?

[q] In the diagram below, which number or letter represents carrying capacity?

[textentry single_char=”true”]

[c*] 1

[f] Awesome. Carrying capacity is represented by “1.”

[c] Enter word

[c] *

[f] No. Carrying capacity is the maximum population that an area can support. In this graph, take a look at where the resource base (starts) to drop steeply downward. That’s a good indication that you’ve exceed the carrying capacity.

[q]The graph below shows the results of a computer model of interaction between a population of predators and a population of prey. Which number represents the predator?

[textentry single_char=”true”]

[c*] 2

[f] Awesome. The predator’s population is represented by “2.”

[c] Enter word

[c] *

[f] No. Just think about what you know about the relationship between predators and prey in nature. Think through the blanks in this hint, and you’ll have the answer: there are always fewer _________ than prey.

[/qwiz]

8. r vs. K selection

Atlantic Right Whale Mother and Calf

Let’s compare two mammals.

We’ll start with whales, like the Atlantic Right Whale mother and calf shown at left. Whales almost always give birth to one calf. This birth follows a long gestation period (pregnancy), which ranges from 10 months to up to a year and a half (depending on the type of whale). After her calf is born, a whale mother will nurse her calf for up to two years (again, the time period depends on the species). Once that calf grows to be an adult, it can live for up to 70 years.

Mice

Now let’s think about mice. Mice can live for 12 to 18 months. Their gestation period ranges from about 3 weeks to a month. A litter typically consists of 5 – 6 pups. Nursing lasts for about three weeks. By six weeks of age, a female mouse is sexually mature.

In a previous tutorial about adaptation and natural selection, we explored how a population is selected by the environment for traits that maximize fitness. Fitness involves maximizing the number of one’s descendants. But maximizing one’s descendants doesn’t necessarily mean having a lot of offspring. In some circumstances, fecundity — the ability to produce abundant offspring — is indeed the best evolutionary strategy. In other environments, having very few offspring maximizes fitness.

Among mammals, mice and whales represent the two extremes of selection for traits related to how many offspring are typically born to mothers, and how much energy parents invest in their offspring. Mice are “r-selected.” The “r” refers to selection for reproductive rate. Species that are r-selected live in environments with less predictable resources. They produce more offspring, and invest only a little in each one’s survival.

Whales are “K-selected.” The “K” refers to carrying capacity. Species that are K-selected live in more stable, predictable environments. They produce relatively few young, but invest a lot in each one, trying to ensure each offspring’s successful survival.

Based on what you’ve read above, complete the questions below.

[qwiz qrecord_id=”sciencemusicvideosMeister1961-pop_ecol,r and K selection”]

[h]r vs K selection

[i]

 

[q labels = “top”]Start by completing this table.

r-selection K-selection
body size _____________ _____________
Life span _____________ _____________
Energy investment by parent(s) _____________ _____________
number of offspring produced _____________ _____________
survivorship curve _____________ _____________
Population regulation _____________
_____________
_____________
_____________
Timing of sexual maturation _____________ _____________

 

[l]density dependent

[fx] No. Please try again.

[f*] Good!

[l]density independent

[fx] No. Please try again.

[f*] Excellent!

[l]early

[fx] No. Please try again.

[f*] Great!

[l]few

[fx] No, that’s not correct. Please try again.

[f*] Excellent!

[l]high

[fx] No, that’s not correct. Please try again.

[f*] Excellent!

[l]large

[fx] No, that’s not correct. Please try again.

[f*] Correct!

[l]late

[fx] No. Please try again.

[f*] Excellent!

[l]long

[fx] No. Please try again.

[f*] Great!

[l]low

[fx] No. Please try again.

[f*] Excellent!

[l]many

[fx] No. Please try again.

[f*] Excellent!

[l]small

[fx] No, that’s not correct. Please try again.

[f*] Good!

[l]short

[fx] No. Please try again.

[f*] Good!

[l]Type 1

[fx] No. Please try again.

[f*] Great!

[l]Type 3

[fx] No, that’s not correct. Please try again.

[f*] Good!

[q labels = “top”]Let’s think a few of these concepts through in a bit more detail. In an ___________ and/or unpredictable environment, the chance of an organism’s young surviving is very ______. With odds like that, it makes sense to have a lot of ___________, but to invest relatively little in each one, hoping that at least ______ survives.

That’s the strategy taken by _____________ species. In the plant world, it’s exemplified by the dandelion shown at left. With the seeds randomly distributed by the _________, it makes evolutionary sense to ________ very little in each one. This is typical of species with a _________ survivorship curve.

[l]invest

[fx] No. Please try again.

[f*] Good!

[l]low

[fx] No, that’s not correct. Please try again.

[f*] Excellent!

[l]offspring

[fx] No. Please try again.

[f*] Correct!

[l]one

[fx] No. Please try again.

[f*] Great!

[l]r-selected

[fx] No. Please try again.

[f*] Correct!

[l]Type 1

[fx] No, that’s not correct. Please try again.

[f*] Great!

[l]unstable

[fx] No. Please try again.

[f*] Correct!

[l]wind

[fx] No. Please try again.

[f*] Good!

[q labels = “top”]By contrast, imagine an environment that’s predictable and stable. In that type of environment, it’s a _________ bet that there will be food, shelter, and other ___________ required for survival. In that case, it makes evolutionary success to __________ a lot in each young’s survival. In addition, species in this kind of environment will be selected to have a _______ survivorship curve, with most organisms living into ____________. And a big factor behind of survivorship will be the ___________ investment that each individual receives.

[l]adulthood

[fx] No, that’s not correct. Please try again.

[f*] Good!

[l]good

[fx] No. Please try again.

[f*] Correct!

[l]invest

[fx] No. Please try again.

[f*] Great!

[l]resources

[fx] No, that’s not correct. Please try again.

[f*] Excellent!

[l]parental

[fx] No, that’s not correct. Please try again.

[f*] Great!

[l]type I

[fx] No. Please try again.

[f*] Good!

[/qwiz]

9. Human Population Growth

Humans, like all primates, are K-selected. Partly, that has to do with our tree-dwelling origins. Trees are dangerous places to raise a family. Survival is greatly enhanced if mothers can take care of their offspring one at a time (as opposed to having multiple births). Navigating life in the trees requires a lot of learning. With long childhoods and long weaning periods, births are widely spaced, and many years pass between birth and sexual maturity.

If that’s the case, then why does human population growth look like what’s shown in the red line, above and to the left?

9a. A very brief and simplified history of human population growth

For most of our existence as a species, humans were hunter-gatherers. We lived in temporary settlements, and lived off the land or sea. If we overharvested the local resources, we moved on to the next area. The birth rate and the death rate were approximately equal, and human population stayed relatively small and constant for tens of thousands of years. Ten thousand years ago, there might have been as few as six million human beings on all of planet Earth.

About 10,000 years ago, humanity shifted from hunting and gathering to agriculture. How this happened goes beyond the scope of this course (you can read about it here), but in terms of human population, the consequences were vast. Over the next few thousand years, with more food available, population began to grow. Food availability decreased the death rate, and sedentary lifestyles increased the number of children a woman could have over the course of her lifetime. By the time of the Roman Empire, world population is estimated to have been somewhere between 170 million and 400 million.

The next major change in population involved two more cultural shifts: the scientific revolution and the industrial revolution. Both led to increases in agricultural productivity, which had the effect of decreasing infant mortality. By about 1850, world population had reached one billion.

A widely unknown boost to human population growth came in 1918. In that year,  the Haber-Bosch process for creating nitrogen fertilizer from atmospheric nitrogen was developed. This removed a major limit on agricultural productivity. Environmental scientists estimate that without this one innovation, the maximum human population that the Earth could support would be about 4 billion. As of the time of this writing (summer, 2020), world population is at 7.5 billion.

Another factor decreasing the death rate has come about through modern medicine. The first widely used antibiotic, penicillin, was  first discovered in the 1930s. Along with other antibiotics, this has led to a significant decrease in deaths from bacterial infections. Similarly, widespread immunization campaigns throughout the 20th century have lead to significant decreases in death from viral (and bacterial) infections.

9b. The demographic transition

While the growth curve above seems to show an inexorable rise in world population, there are trends that are causing human population growth to slow. Population scientists have identified a process called the demographic transition in which a country’s population growth pattern shifts over time in a predictable way. The pattern definitely applies to the developed, industrialized nations of Europe, North America, and Asia. It seems to be unfolding similarly in South America and some parts of Africa.

Study the graphic and the text below to see how it works.

 

In stage 1, both birth and death rates are high, and roughly in balance. This was humanity’s situation during the hunter-gatherer phase, and even during the first few thousand years of the agricultural revolution.

Stage 2 is what happens as countries begin to develop. Improved agricultural technology increases food supplies, leading to increased life expectancies. Improved sanitation reduces the death rate. Together, these combine to put population growth on a sharply upward trajectory.

In stage 3, birth rates start to fall. Much of this fall is associated with urbanization (development of cities). With less people working on farms, large families become less desirable. As woman receive more educational and economic opportunities, the age at which women bear their first child is delayed, which reduces the number of children a woman will have over the course of her life. Availability of contraception accelerates this trend.

In stage four, death rates are low, and so are birth rates. As a result, population stabilizes and even begins to shrink.

As countries progress through the demographic transition, the world’s population growth rate has been steadily dropping, even as the absolute number of people has continued to increase. When Mr. W was born, in 1961, the growth rate peaked at its highest level ever, about 2%. World population in that year had just surpassed 3 billion.

A 2% growth rate might not seem very high, but a population growing at 2% will double in size in about 35 years. Since Mr. W’s birth in 1961, the world’s population has, in fact, more than doubled. As stated above, world population recently surpassed 7.5 billion: an increase of 4.5 billion people in about 60 years.

For those concerned about the environmental and sociological impacts of population growth, the good news is that  the planetary growth rate continues to drop. In 2018, the growth rate had dropped to to 1.1%. Because of ongoing trends, among the most important of which are urbanization, and the increase in women’s economic status and education, the growth rate is anticipated to continue to drop.

We’ll turn again to population growth and human impacts upon the environment at the very end of this course. For now, let’s consolidate everything that you’ve learned about population ecology with the following quiz.

10. Population Ecology: Summative Quiz

This quiz will test you on everything above.

[qwiz random = “true” qrecord_id=”sciencemusicvideosMeister1961-pop_ecol, summative quiz (w human pop growth)”]

[h]Population Ecology: Summative Quiz

[i]

[q] In the diagram below, which letter represents the population that will grow the most in the future?

[textentry single_char=”true”]

[c*] A

[f] Nice job! In the future, the large group of pre-reproductive individuals will enter their reproductive years. When they do, that large group of individuals will create many offspring, causing the population to grow.

[c] Enter word

[f] Sorry, that’s not correct.

[c] *

[f] No. You’re looking for the population which, in the future, will have the most individuals entering into their reproductive years. Which population is that?

[q] In the diagram below, which letter represents the population that will grow the most in the future?

[textentry single_char=”true”]

[c*] A

[f] Nice job! In the future, the large group of pre-reproductive individuals will enter their reproductive years. When they do, that large group of individuals will create many offspring, causing the population to grow.

[c] Enter word

[f] No.

[c] *

[f] No. You’re looking for the population which, in the future, will have the most individuals entering into their reproductive years. Which population is that?

[q] In the diagram below, which letter represents the population that will grow the least in the future?

[textentry single_char=”true”]

[c*] A

[f] Nice job! In population A, there’s a relatively small number of individuals in their pre-reproductive years. That means that in the future, this population will have relatively few reproducing individuals. They’ll produce relatively few offspring, causing that country’s population to decline.

[c] Enter word

[f] No, that’s not correct.

[c] *

[f] No. You’re looking for the population which, in the future, will have the fewest individuals entering into their reproductive years. Which population is that?

[q] In the diagram below, which image represents the dispersion pattern that results from territorial interactions, where individuals want to maintain a certain amount of space around themselves?

[textentry single_char=”true”]

[c*] A

[f] Nice job. The uniform distribution pattern in “A’ results from territorial interactions.

[c] Enter word

[f] No.

[c] *

[f] No. Think of it this way. Which pattern would result if every individual forced every other individual to keep a certain distance away?

[q] Which survivorship curve is characterized by early loss of life?

[textentry single_char=”true”]

[c*] C

[f] Nice work! Survivorship curve C shows “early loss of life.”

[c] Enter word

[f] No.

[c] *

[f] No.

[q] Which survivorship curve would most likely be associated with a K-selected species?

[textentry single_char=”true”]

[c*] A

[f] Nice work! Survivorship curve A shows “late loss of life,” and is most associated with species that are K selected?

[c] Enter word

[f] Sorry, that’s not correct.

[c] *

[f] No. Here’s a hint. Find the curve where most individuals survive past middle age. That type of survival usually comes about from significant parental investment in their offspring’s survival, which is what K selection is all about.

[q] The graph below is called a [hangman] curve. Line A represents a population with [hangman] loss of life. The vast majority of the individuals in this population [hangman] past the middle of this population’s expected lifespan. By contrast, curve B represents [hangman] loss of life, and curve C represents [hangman] loss of life.

[c] survivorship

[f] Excellent!

[c] late

[f] Good!

[c] survive

[f] Great!

[c] constant

[f] Great!

[c] early

[f] Good!

[q] In the diagram below, which letter represents the carrying capacity?

[textentry single_char=”true”]

[c*] E

[f] Excellent. “E” represents carrying capacity

[c] Enter word

[f] No, that’s not correct.

[c] *

[f] No. You’re looking for a line that represents the maximum population that this area or environment could support.

[q] In the diagram below, which letter represents the point at which the population has reached the environment’s carrying capacity?

[textentry single_char=”true”]

[c*] D

[f] Awesome. “D” is the point at which this population has reached its carrying capacity.

[c] Enter word

[f] Sorry, that’s not correct.

[c] *

[f] No. Look for the point in the black line where the slope is the steepest.

[q] The kind of population growth shown below is [hangman] growth. It only occurs when there are no [hangman] factors at work.

[c] exponential

[f] Great!

[c] limiting

[f] Excellent!

[q] The kind of population growth shown below is called [hangman] growth. In this growth model, a population’s growth slows at it reaches the environments’ [hangman] [hangman].

[c] logistic

[f] Good!

[c] carrying

[f] Correct!

[c] capacity

[f] Correct!

[q] Environmental factors such as fires, floods, landslides, and volcanic eruptions would all be classified as [hangman] [hangman] limiting factors

[c] density

[f] Good!

[c] independent

[f] Good!

[q] Limiting factors such as competition, parasitism, predation, and waste accumulation are all classified as [hangman] factors, because they come from outside of the population itself.

[c] extrinsic

[f] Correct!

[q] In the diagram below, which number or letter refers to the “overshoot.”

[textentry single_char=”true”]

[c*] 2

[f] Nice job. The overshoot is represented by “2.”

[c] Enter word

[f] No.

[c] *

[f] No. The overshoot is the point where the population grows beyond the environment’s carrying capacity. Where would that be?

[q] The graph below shows the results of a computer model of interaction between a population of predators and a population of prey. Which number represents the predator?

[textentry single_char=”true”]

[c*] 2

[f] Awesome. The predator’s population is represented by “1.”

[c] Enter word

[f] No, that’s not correct.

[c] *

[f] No. Just think about what you know about the relationship between predators and prey in nature. Think through the blanks in this hint, and you’ll have the answer: there are always fewer _________ than prey.

[q] In the diagram below, which stage shows a dense human population living in a highly developed economy where both the birth and death rate are low?

[textentry single_char=”true”]

[c*] 4

[f] Excellent. Stage 4 of the demographic transition is found in highly developed, populous countries with a high standard of living, low birth rates and low death rates.

[c] Enter word

[f] No.

[c] *

[f] No. Look for a country where the population is high, but the rate of population growth has slowed (which you can tell by a very gentle slope to the line).

[q] In the diagram below, which stage shows a population of hunter-gatherers, or early agriculturalists.

[textentry single_char=”true”]

[c*] 1

[f] Excellent. Stage 2 of the demographic transition is found in hunter-gatherer societies or early agricultural societies, where both the birth and the death rate is high.

[c] Enter word

[f] No, that’s not correct.

[c] *

[f] No. Look for a stage where the population is low, and both the death rate and the birth rate are high.

[q] The diagram below represents the stages of the [hangman] [hangman]

[c] demographic

[f] Correct!

[c] transition

[f] Excellent!

[q] In the diagram below, which stage is associated with a country that is beginning to develop, resulting in changes that bring about a high rate of population growth.

[textentry single_char=”true”]

[c*] 2

[f] Nice job. Stage 2 of the demographic transition is found in developing countries, with rapidly growing populations.

[c] Enter word

[f] No, that’s not correct.

[c] *

[f] No. Look for a stage where the population starting to grow at a high rate, which you can tell by the steep slope of the line representing population growth.

 

[q multiple_choice=”true”] Of the descriptions below, which fits best with the demographic transition?

[c] A population starts with a high birth rate and a high death rate, and transitions into one with a high birthrate and a low death rate

[f] No. Think about the birth and death rates in a hunter gatherer society, and those in a highly developed nation (like those in Europe or Japan).

[c] A population moves from a high birth rate and a low death rate, and transitions to a low birthrate and low death rate.

[f] No. You’re right about the end, but populations at the beginning of the transition do not have a low death rate.

[c] A population moves from a low birth rate and a low death rate, and transitions to a low birthrate and high  death rate.

[f] No. Think about the birth and death rates in a hunter gatherer society, and those in a highly developed nation (like those in Europe or Japan).

[c*] A population moves from a high birth rate and a high death rate, and transitions to a low birthrate and a low death rate.

[f] Nice job. That’s exactly what happens during the demographic transition.

[q multiple_choice=”true”] Human population began to grow exponentially after

[c] The start of the industrial revolution.

[f] No. While the Industrial revolution did increase the rate of population growth, human population had already be growing exponentially for thousands of years before the onset of industrialization.

[c*] the discovery/invention of agriculture.

[f] Way to go. It was the agricultural revolution that started humanity’s population on its exponential pathway.

[c] the Black Death (bubonic plague)

[f] No. The black death (bubonic plague) drastically reduced many populations around the world.

[c] The development of the Internet.

[f] No. The development of the Internet occured as many (but not all) nations around the globe were moving out of exponential growth, and into a more stable phase of population growth. Note that this almost certainly a correlation, and not a cause.

[q multiple_choice=”true”] A population with a pyramid-shaped age structure is likely to

[c] grow slowly in the future.

[f] No. This type of population will have many people enter their reproductive years, priming it for rapid population growth in the future.

[c] decline in size over the coming years.

[f] No. This type of population will have many people enter their reproductive years, priming it for rapid population growth in the future.

[c*] grow rapidly in the near future.

[f] Excellent. This type of population will have many people enter their reproductive years, priming it for rapid population growth in the future.

[q multiple_choice=”true”] A species that is a K-strategist is most likely to have a

[c] short life span.

[f] No. K-strategists tend to be long-lived.

[c] small in size.

[f] No. Within their clade, K-strategists tend to be large.

[c] large number of offspring.

[f] No. K-strategists tend to have few offspring.

[c*] long period of parental care.

[f] Way to go. K-strategists tend to have few offspring, and to provide them with lots of care.

[q]Limiting factors such as disease from parasites, increased predation, and food scarcity are all density [hangman]. As this population continues to grow within the same area, these factors will become more [hangman].

[c]dependent

[c]intense

[/qwiz]