Population Ecology

Summary

In this environmental science video, Mr. Andersen explores population ecology through the conservation story of the whooping crane. He discusses factors affecting population growth, including births, deaths, immigration, and emigration, and introduces the intrinsic growth rate. The video explains density-dependent and independent factors, carrying capacity, and limiting resources. It covers exponential and logistic growth models, and the strategies of K-selected and r-selected species. The importance of population size, density, distribution, sex ratio, and age structure are highlighted, along with survivorship curves, providing a comprehensive look at population dynamics.

Takeaways

• 🕊️ The whooping crane is a conservation success story, with numbers rebounding from just 15 individuals in 1938 to a higher population.
• 📈 Population health is determined by its size, influenced by births, immigration, deaths, and emigration, which contribute to the intrinsic growth rate.
• 🌿 Density and distribution, sex ratio, and age structure are additional factors that affect population dynamics beyond the intrinsic growth rate.
• 🚫 Density-dependent factors are limitations such as food, water, or shelter that increase as population density grows, leading to a carrying capacity (K).
• 🌪️ Density-independent factors are random events like floods or fires that can limit population size, independent of density.
• 📊 Exponential growth models illustrate how populations can increase rapidly over time, assuming no constraints on growth.
• 📚 Logistic growth models show population growth that eventually levels off at the carrying capacity, considering environmental limits.
• 🐰 K-selected species, like whooping cranes, invest more in parental care and have fewer offspring, stabilizing around a carrying capacity.
• 🐇 r-selected species, such as arctic hares, produce many offspring with less parental care, leading to volatile population cycles.
• 🔍 Survivorship curves help identify species strategies, with type 1 curves indicating K-selected species that provide extensive parental care and type 3 curves suggesting r-selected species with high offspring mortality.

Q & A

What was the historical population of whooping cranes in the U.S., and what was their lowest recorded number?

Whooping cranes used to number around 10,000 in the U.S., but by 1938, their numbers had dropped to only 15 individuals.

What factors are responsible for the increase or decrease in population size?

Population size can be increased through births and immigration, while it can be decreased through deaths and emigration.

What is the intrinsic growth rate and how is it calculated?

The intrinsic growth rate is calculated by the formula: (births - deaths + immigration - emigration) / initial population size.

What are the two types of factors that can affect population growth outside of the intrinsic growth rate?

The two types of factors are density dependent and density independent factors. Density dependent factors limit growth based on the population's density, while density independent factors are related to chance events like floods or fires.

What is the carrying capacity (K) and how does it relate to population growth?

The carrying capacity (K) is the maximum number of individuals an area can support, beyond which the population growth levels off due to limiting resources such as food, water, or shelter.

What are the two strategies that species use for population growth, and how do they differ?

Species use either K-selected or r-selected strategies. K-selected species have a stable population that increases until it hits the carrying capacity, while r-selected species have a boom-and-bust cycle with rapid increases followed by crashes.

How does the sex ratio and age structure of a population contribute to its health?

The sex ratio and age structure are important because they influence the potential for reproduction and the overall stability of the population. A balanced sex ratio and a diverse age structure are typically healthier for a population.

What is the exponential growth model and how does it represent population growth?

The exponential growth model represents population growth that increases rapidly over time without an upper limit. It uses the formula Nt = No * e^(rt), where Nt is the population at time t, No is the initial population, r is the intrinsic growth rate, and t is time.

What is the logistic growth model and how does it differ from the exponential growth model?

The logistic growth model also shows exponential growth initially but eventually reaches a carrying capacity (K), leading to an S-shaped curve. It accounts for limiting resources and population stability, unlike the exponential growth model which does not have an upper limit.

How do survivorship curves help in understanding species strategies?

Survivorship curves show the probability of an individual surviving to a certain age. Type 1 curves are indicative of K-selected species with high parental care, Type 2 curves are for species with constant mortality rates, and Type 3 curves are for r-selected species with high early mortality and few survivors.

What is the significance of the whooping crane conservation story in the context of environmental science?

The whooping crane conservation story is significant as it demonstrates the impact of human intervention in reversing the decline of a species and highlights the importance of understanding and protecting habitats to ensure species survival.

Outlines

00:00

🐦 Population Ecology and Conservation

The paragraph discusses the importance of understanding population ecology in conservation efforts, using the whooping crane as an example. Once numbering 10,000, their population plummeted to just 15 by 1938, prompting scientists to investigate their breeding locations and protection strategies. The intrinsic growth rate, determined by births, deaths, immigration, and emigration, is crucial for population health. Factors affecting population growth include density-dependent (limited by resources like food, water, and shelter) and density-independent factors (chance events like floods or fires). Models such as the exponential growth model and logistic model help describe population dynamics, with the latter incorporating the carrying capacity (K), the maximum sustainable population size.

05:03

📈 Exponential and Logistic Growth Models

This section delves into the exponential growth model, which predicts rapid population increase over time, forming a J-shaped curve. The logistic growth model, however, accounts for the carrying capacity, leading to an S-shaped curve where population growth eventually levels off. The paragraph explains the exponential growth equation, involving the mathematical constant e, and demonstrates how to calculate future populations based on the initial population, growth rate, and time. The logistic model is also mentioned, which includes the carrying capacity, but the detailed mathematical explanation is reserved for another video.

10:07

🦊 K and r Selection, Survivorship Curves, and Population Strategies

The final paragraph explores the concepts of K-selected and r-selected species, which represent different strategies for survival and reproduction. K-selected species, like humans and whooping cranes, invest heavily in parental care and have fewer offspring, stabilizing around a carrying capacity. In contrast, r-selected species, such as arctic hares, produce many offspring with little parental care, leading to volatile population cycles. The predator-prey relationship between arctic hares and Canada lynx exemplifies how population fluctuations can affect entire ecosystems. Survivorship curves, which plot the probability of survival over time, help classify species into type 1 (K-selected), type 2, and type 3 (r-selected), although many species exhibit strategies that fall between these categories.

Keywords

💡Population Ecology

Population ecology is the study of how populations interact with their environment and how they change over time. It is central to the video's theme as it sets the stage for understanding the dynamics of species populations, such as the whooping crane, and how they can be affected by various factors. The video discusses how population size is influenced by births, deaths, immigration, and emigration, which are all key components of population ecology.

💡Intrinsic Growth Rate

The intrinsic growth rate refers to the natural increase or decrease in a population size over time, assuming no external factors. It is a key concept in the video as it helps to understand how populations can grow exponentially if resources are abundant. The video uses the intrinsic growth rate to explain how populations can increase until they reach their carrying capacity.

💡Carrying Capacity (K)

Carrying capacity is the maximum population size that an environment can sustain indefinitely, given the food, habitat, water, and other resources available. The video explains that as populations grow, they eventually reach a point where resources become limiting, and this is represented by the carrying capacity. The whooping crane example illustrates how conservation efforts aim to protect and increase the population up to its carrying capacity.

💡Density Dependent Factors

Density dependent factors are those that affect a population's growth rate based on the population's density. In the video, it is explained that as the population density increases, factors like limited food, water, or shelter can slow down population growth, eventually leading to a stable population size at the carrying capacity.

💡Density Independent Factors

Density independent factors are external events that can affect a population's size regardless of its density. The video gives examples such as floods or fires, which can reduce a population size by chance and are not related to the population's density. These factors introduce variability and unpredictability into population dynamics.

💡Exponential Growth Model

The exponential growth model is a mathematical model that describes the rapid increase in population size when resources are abundant and there are no limiting factors. The video uses this model to illustrate how populations can grow very quickly if there are no constraints, resulting in a J-shaped growth curve.

💡Logistic Growth Model

The logistic growth model is another mathematical model that accounts for the carrying capacity of an environment, leading to an S-shaped growth curve. The video explains that unlike the exponential model, the logistic model shows population growth that eventually levels off as the population approaches the carrying capacity, which is more reflective of real-world scenarios.

💡K-Selected Species

K-selected species are those that live in stable environments and invest heavily in the survival of a few offspring, reaching a stable population size close to the carrying capacity. The video uses the whooping crane as an example of a K-selected species, emphasizing the importance of conservation efforts to protect such species and their habitats.

💡r-Selected Species

r-selected species are those that produce many offspring with little parental care, in the hope that a few will survive in unpredictable environments. The video contrasts this with K-selected species, using the arctic hare as an example, which experiences boom and bust cycles in population size.

💡Survivorship Curves

Survivorship curves are graphical representations that show the proportion of a cohort surviving over time. The video discusses three types of survivorship curves: Type 1 (most survive to old age, like humans), Type 2 (constant mortality throughout life, like songbirds), and Type 3 (high early mortality, like acorns). These curves help to classify species as either K-selected or r-selected based on their life history strategies.

Highlights

The whooping crane's population recovery story is one of the greatest in conservation biology.

In 1938, whooping crane numbers dropped to a critically low 15 individuals.

Understanding the breeding and protection areas of species is crucial for conservation.

Population health is tied to its size, influenced by births, immigration, deaths, and emigration.

Intrinsic growth rate is a key metric in determining whether a population is increasing or decreasing.

Density and distribution, sex ratio, and age structure are essential characteristics of a population.

Density-dependent factors limit growth based on population density, such as food, water, or shelter.

Carrying capacity (K) is the maximum number of individuals an area can support.

Density-independent factors, like natural disasters, can affect population size irrespective of density.

Exponential growth model illustrates how population size increases over time without limits.

Logistic growth model shows population growth reaching a carrying capacity, forming an S-shaped curve.

K-selected species gradually reach a carrying capacity with characteristics like parental care and few offspring.

r-selected species have a boom-and-bust cycle, producing many offspring with little parental care.

Survivorship curves help determine species strategies, with humans exemplifying K-selected species.

Arctic hare and Canada Lynx demonstrate predator-prey relationships affecting population dynamics.

Species strategies are not strictly r or K selected; many exist in a spectrum between the two.

Population size is determined by a balance of births, deaths, immigration, and emigration.

The intrinsic growth rate formula provides a simple method to calculate population changes.

Transcripts

Hi. It’s Mr. Andersen and this is environmental science video 12. It is on population ecology.

One of the greatest conservation stories in biology is the story of the whooping crane.

They used to number 10,000 in the U.S. but by 1938 their numbers had dropped to only

15 individuals. So scientists had to figure out where are they, where are they breeding,

how do we protect those areas and you can see the population is starting to rebound.

But the health of the population is dependent upon the size of the population. How do we

increase the size of a population? Through births and immigration. New individuals coming

into the population. Likewise, how do we decrease it? Through deaths and emigration. These things

contribute to what is called the intrinsic growth rate. Is it increasing? Or is it decreasing?

It is not the only characteristic. We also have the density and distribution. We have

the sex ratio and the age structure as well. But what other factors, outside of this intrinsic

growth rate can affect their growth? Well we break that into two groups. Density dependent

and independent. Density dependent factors are factors that limit growth based on the

density of the population. So if you think about it as the population’s density increases,

if there is not enough food or water or shelter, we call those limiting resources. And what

happens to the population? It will eventually level off. It hits something called the carrying

capacity or K. It is the maximum number of individuals an area can support. We also have

density independent. And those are going to be things just related to chance. So a flood

or a fire could be examples that limit the size of a population. So in population ecology

we are studying these factors. And scientists come up with models that help to describe

what is going on in a population. So a famous model is the exponential growth model. What

we are looking at is this growth rate and how it is increasing the population over time.

And then we have a logistic model. It is also showing exponential growth but eventually

it is reaching what is called a carrying capacity or this limit of population growth. Scientists

also study strategies that species have. Some are what are called K selected. That means

their population size will increase until it gradually hits a carrying capacity. And

those who live more of a boom or bust cycle, that are r selected. And we can look at how

long individuals survive and that tells us a little bit about which strategy they are

using. And so the population size is incredibly important. So if we have these rabbits, so

we have 9 rabbits and their N value at this point would be 9. If we lose 2 of them our

N value is 7. If we gain 3 now our N value is going to be 10. It is the set number we

have. But also density is important. That is the number of individuals we have in a

given area. And so we could call this one density but we would call this greater density.

We could also look at their distribution. I would say that these rabbits are now randomly

distributed. But they could be distributed uniformly. Or they could be just clumped in

their distribution. And we could also look at their sex ratio. So how many are males

and how many of them are going to be females. And we could expand that to look at what is

called their age structure. Not only what is their gender but also how old are they.

So we could organize them like this where this is going to be our first year female

rabbits, second year and third year. And we can do the same thing with males. But when

it comes to the health, the population size is incredibly important. It is dictated by

births, deaths, immigration and emigration. And so we have a formula that allows us to

look at that. And the calculations are very simple. You can do them just in your head.

And so let’s say we have a population of 10. So our N naught is going to be 10. That

is our initial population. Here is our equation. So it is really simple. The change in N is

going to be the births minus the deaths plus the immigration minus the emigration. So let’s

look at this population over here and see what happens. So this rabbit gave birth to

3 other rabbits. And so if we write this out what is our births going to be? It is going

to be 3. Now let’s watch the population again. So you can see 1 of the rabbits died.

And so we are going to be put a 1 here in the deaths. We could look at immigration,

how many come in. It looks like just 1. So we would put a 1 right here. And then how

many emigrate? It looks like 2 left. And so we would put a 2 right here. And so the delta

N or the change in N is simply going 3 minus 1 plus 1 minus 2, or 1. That is the change.

Or we have seen an increase in 1. Now what is the growth rate? The growth rate is going

to be the change divided the initial population. So 1 divided by 10 gives us a 10 percent growth

rate of 0.1 is our growth rate. We call that the intrinsic growth rate. And as long as

we have no other factors outside that population, that will remain constant over time. And you

could solve a really hard problem. We could have a million people in an area. 100,000

are born. 10,000 die. If you are given the immigration and emigration you should be able

to calculate r for that population. So if we study a group of rabbits over time their

population will increase. But it will eventually level out at some point. Now that leveling

out point is called the carrying capacity or the K. Now why is a population going to

level out? It is because they are running out of something. They are running out of

food or water or shelter. And so we call all of those things limiting resources. Disease

could be another limiting resource. The more rabbits we have the more disease. And so it

is eventually going to level it off. Now it will not look perfect like that. The normal

population is going to have over shoots and it is going to have a lot of die off. But

we are going to have the average that we eventually hit. These are density dependent factors because

they are based on the density of the population. We can also have density independent. So imagine

that these rabbits over on this side are killed in a forest fire. That is just chance. It

is just chance taking over and so it is not based on the density of rabbits that we had.

So if we start to use models to explain how this works, a really important model is the

exponential growth model. And so the equation looks like this. It is a little scary but

it is really not that bad. N sub t is going to be the population at any time into the

future. N sub O is going to be the initial population. So let’s say we start with a

population of 10. r is going to be the growth rate. That is that intrinsic growth rate.

And t is going to be time. So the only thing that you really do not know in this equation

is e. e is going to be the mathematical constant. So it is a number. It is just like pi. It

is going to be 2.718. It just keeps going like that. So for our purposes we just think

of it as 2.71. And so let’s say we want to figure out what is going to happen to the

population in year 1. So if we want to figure out, we started at 10, what is going to be

the population probably at year 1? We just use this equation. So e is going to be the

same. So what is going to be our r value? Our r value will always be 0.5. That is that

intrinsic growth rate. What is our t value? Our t value is going to be time. What is our

initial population? It is going to be 10. So if I expand that a little bit or simply

multiply 1 times 0.5, 1 year times that growth rate. And so that is going to be 10 times

2.71, again that is e, raised to the 0.5 power. So that is really like taking the square root

of 2.71. And so that is 1.64. So if we work that out that is going to be around 16 rabbits

after 1 year. So let me graph that. And let’s go to year 2. So same thing. We are going

to plug in r value of 0.5 but now our t value is going to be 2. Still have that same initial

population. And so now it is going to be 2.71 raised to the 1 power. So what is that? That

is simply 2.71. So if we work this out now we are going to have 27 rabbits in that next

year. You can see the population is increasing. We are starting to see that exponential growth.

Let’s go for year 3. So if we figure out year 3, again our intrinsic growth rate is

still 0.5. 3 is going to be the year we are at. Still have that same initial. And so this

is going to be 2.71 raised to the 1.5 power. You probably need a calculator to do this.

We now get 44.6 or, let’s say 45 rabbits. So if we graph it, you can see that the population

is increasing like that. We have what is called a j-shaped curve. And it is going to increase

rapidly over time. We are going to, the whole world would be filled with rabbits if we keep

following this model. And so we know that is not what occurs. And so not only intrinsic

growth rate is important but K, that carrying capacity. So if you are given a problem like

this could you graph what is going to happen over time if K is 70? Well you are going to

get something that looks like this. It is going to be j for awhile but is eventually

going to curve off and we are going have a s shaped curve. This is a logistic growth

model. There is also a mathematical model we will not work through. I will put a link

to another video where I do that down below. And so scientists, now that they have models,

they can start to apply that to nature. So what we have found is that species kind of

fall into one of two camps. We have what are called K selected species. Those are going

to be species that their population increases and then it will eventually hit a carrying

capacity and it stays there. What are some characteristics of species like that? They

are going to give a lot of parental care to their offspring. They are just going to have

a few offspring. And so the whooping crane would be an example of that. Humans are an

example of that. We do not just go up and down in our population. r selected are going

to do that. So an arctic hare is an example of that. A famous study was looking at the

pelts that were collected by the Hudson Bay Company. And they found from 1850 to 1930

that the population of arctic hare just went up and down and up and down. And so hares

are going to be groups of individuals that have lots of offspring. They do not get tons

of parental care and their population is going to increase and then it will crash. So we

have this boom and bust cycle. Now what is interesting is that there is another species.

And so the arctic hare are fed on by the Canada Lynx. And if we look at their population,

their population goes through a boom and bust as well. We have what is called a predator

prey relations where as the arctic hare population increases then we can have more lynx feeding

on it. But as they crash then the lynx are going to crash as well. Now a way to look

at which strategy species are using is figuring out their survivorship. And so we have time

on the bottom and then we have the survivors on the side. So if we look at humans as a

type 1 survivorship curve, what that means is when we are born almost all of the humans

survive. And then throughout their lifetime they all die right at the end. And so we give

a lot of parental care to our offspring. Almost all of them survive and then when we get into

our 80s, 90s, then we all die off. We could also have a type 2 survivorship curve. Songbirds

are an example of that. From the moment they are born they are dying off at a constant

rate. Or we could look at type 3. Those are individuals like the acorns from a tree. Almost

all of them die but a few of those survive and those make up the plants that we have.

And so could you link that to K or r selected species? Well type 1 individuals are generally

going to be those K selected species. And the type 3 are generally going to be those

r selected species. But there are so many examples that are in the middle. So if you

think about a sea turtle for example, they have lots of offspring. They do not give them

much parental care, but they live a long time. And so it is not as simply as are you r or

are you K? It is somewhere in the middle. But they are applying these different strategies

in life. And so did you learn the following? Could you pause the video at this point and

fill in the blanks? If not, population size is determined by immigration and birth. That

increases it. Decreased by emigration and deaths. We have other characteristics, density,

distribution, sex ratio and age structure. There are density independent and dependent

factors. Density independent remember are related to chance. Density dependent lead

to what is called a carrying capacity or K. We use models to study it. Exponential models

are built on the growth rate. Logistic models, also built on the growth rate but include

carrying capacity. And then we have different strategies in species. K selected, r selected.

Remember we are K selected. And then we have survivorship curves that we can study to get

that. That is a lot. I hope it made sense. And I hope that was helpful.