Population Ecology

Here is a brief introduction to some of the important parameters that we will need to understand to be able to study population ecology. For each of the parameters it is important that you know (1) the name of the parameter, (2) the algebraic symbol used to represent the parameter, (3) the units of measurement for the parameter, (4) how to calculate the parameter, and (5) how to describe (in words) what a particular value of that parameter means.

It is probably easiest for me to introduce these concepts using an example.
Imagine that in a population of 100 elephants that in one year 10 elephants are born and 5 elephants die.

1) Population Size (N) units- individuals. Measures the number of individuals in a population.

N = 100 individuals

In this population, there are 100 elephants.

2) Population Birth Rate (B) units- number of births per time. Measures the number of births per time that occur in a population.

B = 10 births/year

In this population, each year there are 10 births.

3) Population Death Rate (D) units- number of deaths per time. Measures the number of deaths per time that occur in a population.

D = 5 deaths/year

In this population, each year there are 5 deaths.

4) Population Growth Rate (dN/dt) units- number of idividuals per time. Measures the rate of change of the population size.

dN/dt = B - D

dN/dt = 10 births/year - 5 deaths/year = 5 individuals/year

In this population, the population size increases by 5 individuals each year.

5) Per Capita Birth Rate (b) units- births per time per individual. Measures the number of births per time averaged across all members of the population.

b = B/N

b = (10 births/year)/100 individuals = 0.10 births/year/individual

In this population, each year 0.10 babies are born for each individual in the population.

6) Per Capita Death Rate (d) units - deaths per time per individual. Measures the number of deaths per time averaged across all members of the population.

d = D/N

d = (5 deaths/year)/100 individuals = 0.05 deaths/year/individual

In this population, each year 0.005 individuals die for each individual in the population.

7) Per Capita Growth Rate (r) units = individuals/time/individual. Measure the rate of change in population size averaged across all individuals. The per capita growth rate can be calcuated two ways.

a) r = b - d

r = 0.10 births/year/individual - 0.05 deaths/year/individual = 0.05 ind/year/ind

b) r = (dN/dt)/N

r = (5 individuals/year)/100 individuals = 0.05 individuals/year/individual

In this population, each year 0.05 individuals are added for each individual in the population.

Practice Problem

In a population of 50 tigers, in one year 10 tigers are born and 20 tigers die. What is B, D, dN/dt, b, d, r?

Readings


Population- http://www.eoearth.org/article/Population

Population ecology- http://www.eoearth.org/article/Population_ecology

Population growth rate- http://www.eoearth.org/article/Population_growth_rate

Exponential growth- http://www.eoearth.org/article/Exponential_growth

Logistic growth- http://www.eoearth.org/article/Logistic_growth

Carrying capacity- http://www.eoearth.org/article/Carrying_capacity


Expected Learning Outomes

By the end of this course a fully engaged student should be able to

- define and calculate the value of basic population ecology parameters

- draw and interpret the following graphs

a) how population size changes over time in exponential growth

b) how population size changes over time in logistic growth

- explain why exponential growth is an unrealistic pattern of growth for most species

- define and explain the carrying capacity

- discuss the factors that regulate population size, be able to distinguish between density dependent and density independent factors that regulate population growth and give examples

Cultural Selection

In humans there are examples of alturistic behaviors that appear to be difficult to explain by kin selection of reciprocal altruism (e.g. soldiers sacrificing their lives in battle, police or firefighters risking their lives, catholic priests remaining celibate).

Genes are self replicating molecules. Genes produce our bodies which in turn produce more copies of their genes. Richard Dawkins has suggested that we think about genes as being "replicators" and our bodies as being "vehicles" whose job it is to make more copies of the replicators. If we can not explain altruistic behaviors as strategy for increasing the transmission of genes into the next generation them maybe we need to search for another kind of "replicator". Dawkins has suggested that "ideas" (he calls them "memes") are also capable of self replication. Because ideas differ in how long they survive and how well they are passed on it should be possible to have selection for ideas (cultural selection).

Expected Learning Outcomes

By the end of this course a fully engaged student should be able to

- identify examples of altruistic behavior that might be explained by cultural selection
- be able to compare and contrast "natural selection" with "cultural selection"

Further Readings

Cultural evolution http://plato.stanford.edu/entries/evolution-cultural/

One of my favorite books of all time is "The Selfish Gene" by Richard Dawkings. He discusses some of his ideas about cultural selection in the final chapter of this book. Here is a link to that chapter in case you are interested
http://www.rubinghscience.org/memetics/dawkinsmemes.html

I think this chapter is a must read for any educated person, I hope you enjoy.

Reciprocal Altruism and Evolutionary Psychology

Drawing of a hungry vampire bat (right) solicits food from a potential donor, first by grooming around the stomach area (c) and then licking the donor's face (d). The donor bat then responds by regurgitating blood (e) if receptive. (Wilkinson 1990)

Altruistic acts among non relatives can be understood by reciprocal altruism. As we discussed in class we would expect reciprocal altruism to be limited to species that show long term associations and are "smart" emough to be able to recognize individuals and remember who owes them and who does not.

Further Readings

Reciprocal Altruism http://www.bbc.co.uk/nature/animals/mammals/explore/altruism.shtml

Reciprocal Altruism in Vampire Bats http://www.bio.davidson.edu/people/vecase/behavior/Spring2002/Perry/altruism.html

The Evolution of Reciprocal Altruism. Robert Trivers. 1971
http://www.nbb.cornell.edu/wkoenig/wicker/NB4340/Trivers%201971.pdf

If you are interested in learning more about Evolutionary Psychology here is a link to a bunch of Frequently asked questions. Some of this goes into way more detail than we need to be worried about for this class.
FAQ Evolutionary Psychology http://www.anth.ucsb.edu/projects/human/evpsychfaq.html

Video

Stone Age Minds: A conversation with evolutionary psychologists Leda Cosmides and John Tooby
http://www.youtube.com/watch?v=nNW_B8EwgH4

David Buss and Richard Dawkins on Evolutionary Psychology
http://www.youtube.com/watch?v=QZw3lxyuhEU

Expected Learning Outcomes

By the end of this course a fully engaged student should be able to

- define reciprocal altruism
- discuss the conditions under which altruistic acts can be explained by reciprocal altruism
- examine an example of an altruistic behavior and determine whether reciprocal altruism is the best explanation
- explain how participants benefit by being involved in reciprodal altruism in real world examples (e.g, vampire bats)
- define a cheater in a reciprocal altruism system and discuss (a) why cheating is a problem in the system and (b) what organisms can do to reduce cheating
- discuss Trivers' ideas about how human psychology has been influenced by reciprocal altruism (be able to provide your opinion about Trivers' ideas and be able to back up your opinions)

Altruism Part 1

Photo: Belding's Ground Squirrels, the subject of research on altruism


From our discussion about natural selection you should have learned that organisms have the traits they do because traits that produce phenotypes that are more successful at transmitting genes to the next generation (surviving and reproducing) become more common in a population over time. Thus, we expect organisms to have traits that maximize their individual survival and reproduction (we call these selfish traits).

Examples of Altruistic Behaviors

1. Broken wing display by kildeer
kildeer live around here so you should be able to see this behavior later on this spring (that will require going outside!)

http://www.youtube.com/watch?v=TNG7y0caqj0

2. Group defense in musk oxen
you can't see this around here even if you go outside.

http://www.youtube.com/watch?v=pb6Rke7jiTc

3. Food sharing in African Wild Dogs

http://www.sandiegozoo.org/animalbytes/t-wild_dog.html

4. Prairie dogs giving warning calls

http://www.youtube.com/watch?v=rXCPaNWcTFo

We should originally be a little bit confused when we learn about altruistic traits. How can genes that produce traits that decrease an organisms abilty to survive or reproduce become more common in a population?!? Luckily, we have learned that understanding what happens in natural selection requires us to focus on the transmission of genes. Apparently, organisms that behave altruistically are actually passing on more genes by behaving altruistically than they would by behaving selfishly. How can this be? (this problem perplexed Darwin).

Fortunately, a lot of really smart scientists have thought about altruism and have recognized that their are a variety of different ways that organisms behaving altruistically could pass on more genes than organisms acting selfishly. There are at least 4 different hypotheses that can explain the evolution of altruistic behaviors (one of these will probably only help to explain altruism in humans).

The first explanation for why organisms were altruistic was the idea of group selection. Group selection is the idea that organisms have traits becasue those traits "assure the survival of the species". At first glance this seems like a pretty useful idea, but it actually does not work and it has been a very difficult idea to remove from the minds of the general public even though scientists have know that it is wrong and unecessary (there are much better theories about the causes of altruism) for over forty years. It would take a while for me to explain why group selection doesn't work so I won't spend any more time talking about it either in class or here on the blog. However, if you are interested in learning more about this I would be happy to chat with you.

It is important for this class that you are able to understand under which conditions the other hypotheses could explain the presence of altruistic behaviors.

Hamilton's Rule

Hamilton's Rule is a mathematical equation that helps scientists understand under which conditions organisms should behave altruistically and when they should behave selfishly. It is important that you understand (1) how sceintists use mathematical models to help us understand the world and (2) what Hamilton's Rule tells us about when organisms should behave altruistically.

Suggested Readings

Biological Altruism- http://plato.stanford.edu/entries/altruism-biological/


Expected Learming Outcomes

By the end of the course a fully engaged students should be able to

1) define altruistic traits and provide several examples

2) compare and contrast selfish traits and altruistic traits

3) explain why altruistic traits at first glance appear to be difficult to understand based on what we know about the process of natural selection

4) discuss at least four possible hypotheses that explain the presence of altruistic traits and explain under which circumstances these theories are expected to apply

5) use “Hamilton’s Rule” as an example to illustrate how biologists use mathematical models to help them understand biology

6) discuss how Sherman’s work with Belding’s Ground Squirrels provided support for Hamilton’s Rule

7) be able to determine which hypothesis best helps you understand any examples of altruistic traits that I give you and be able to justify that answer

The Influence of the Abiotic Environment on the Dominant Plant Types

Here is a link to a slideshow I prepared exploring the physical environment (primarily precipitation and temperature) affect the dominant plant types. This seems like a great place for us to start out exploration.

Deserts, prairies, and forests http://www.slideshare.net/secret/aVrFdv9S7038HJ

The Physical Environment- Global Patterns

Introduction

The physical environment can have a profound influence on ecology at a variety of levels. For example, the physical environment can act as a strong selective presssure to produce adaptations or can influence the rates of nutrient cycling through an ecosystem. For our simple purposes here, the two most important components of the physical environment are temperature and precipitation. I suggest that we can predict a lot about what is going on ecologically in an environment if we know something about temperature and precipitation patterns.

From watching the nightly news we all know how difficult it is for the local weatherperson to accurately predict what the weather is going to be like tomorrow. Fortunately, it is much easier to understand broad patterns of variation in temperature and precipitation.

The following information should be a review of what you have already learned from Dr. Lee.

Temperature



The dominant global temperature pattern is that it tends to get cooler as you move away from the poles. The cause of this is relatively simple. Because the earth is so far from the sun, the light rays hitting the earth are basically paralell to each other. Because of the curvature of the earth, sunlight hitting the earth near the equator falls over a smaller area than sunlight hitting near the poles. Because the same amount of light energy is hitting a smaller area near the equator, the concentration of energy/area is greater near the equator than the pole thus resulting in higher temperatures.

Elevation is another factor that influences global temperatures. Because there is less insulating atmosphere above areas of high elevation temperatures tend to decrease as you go up in elevation.

Large bodies of water can mediate temperature variations. For example, seasonal and daily variation in temperatures are much lower in areas near the ocean (maritime climates) than they are in areas far from the ocean (continental climates).

Global temperature patterns can also be affected by patterns of ocean circulation. For example, the west coast of continents are often cooled by cool water flowing from the poles to the tropics while the east coasts of continents can be warmed by warmer water from the tropics to the poles (e.g., the Gulf Stream). If you have ever been to the beach in southern California you surely noticed how cold the water was; east coast beaches at similar latitudes have much warmer water.

Precipitation

In order to understand global precipitation patterns you need to understand global patterns of atmospheric circulation. Hopefully, after studying the article on atmospheric circulation you will be able to explain-

1. why there tends to be high precipitation in tropical regions and

2. why precipitation tends to be low at 30 degrees North and South of the equator.

Patterns of precipitation can also be influenced by the presence of mountains. As air masses containing moisture hit a mountain they are forced upward. Because rising air cools and cool air







holds less moisture, precipitation occurs on the windward side of mountains. Once the air mass has passed over the mountain in falls to lower elevations and gets warmer. Because most of the moisture has been lost as precipitation on the windward side of the mountain and the warmer air holds more moisture there is very little precipitation on the leward side of the mountain resulting in a "rainshadow desert".

Let's think about Lubbock!

Let's see if we can use our newfound understanding of some of the factors influencing temperature and precipitation to make predictions about what the climate should be like in Lubbock. What information do we need about the geographic location of Lubbock to help us understand the climate? First, we need to know the latitude; Lubbock is located approximately 33 degrees north. Second we need to know something about the proximity to the ocean. As an old beach boy, I can guarantee you that we are a long, long way from the ocean in Lubbock. Third, where is Lubbock in relation to mountains? Lubbock is located to the east of the southern extension of the Rockies.

Why is all of this important?

1. What can we learn from the latitude of 33 degrees North? This latitude is still close enough to the equator to be warm so we expect relatively high temperatures. Because Lubbock lies near the 30 degree zone of low precipitation we would predict relatively low precipitation. At 30 degrees North we would predict that Lubbock would receive predominately winds from the west.

2. From the continental location of Lubbock we would predict fairly extreme daily and seasonal fluctuations of temperatures.

3. Because Lubbock lies in the Westerlies most of the precipitation that is arriving in Lubbock comes from the Pacific Ocean. Because these winds have passed over the Rockies we would predict that Lubbock would lie in a rainshadow, again causing low precipitation.

How did we do. If anyone has ever been in Lubbock (especially in the spring time) you would know that the wind almost always blows in from the west. Temperatures are relatively warm but there is fairly large seasonal and daily variations in temperature. Lubbock has a semi-arid climate and receives on average about 18 inches of precipitation per year. Thus, with just a little bit of knowlege about the factors that influence global patterns of temperature and precipitation we were able to fairly accurately the climate in Lubbock. Thus, I would expect that organisms native to Lubbock should be well adapted to the low precipitation, continental climate of the region (the short grass prairie was the dominant vegetation type presettlement).

See use these patterns to understand climate in your town (note climate patterns in Texas are complicated in central and eastern Texas becasue of the influence of air masses coming up from the Gulf). Compare the temperature and precipitation of your town with that if very divergent locations around the globe.


Further Reading

If you would like some more detailed information about factors affecting climate and the atmosphere you can check out the Atmosphere Chapter in Michael Pidwirny's online Physical Geography textbook http://www.physicalgeography.net/fundamentals/contents.html.

Powerpoint Presentation

Click here to see the powerpoint presentation "Factors Influencing the Physical Environment".
http://www.slideshare.net/secret/EaVq4nm5KuSsBI

Expected Learning Outcomes

At the end of this course a fully engaged student should be able to

- describe global patterns of variation in temperature and precipitation and be able to explain the causes of these patterns

- for any location in the world, use your knowledge of the factors that affect global patterns to preict the local climate

Natural Selection

An understanding of the process of natural selection helps us to understand the amazing diversity of life on the earth.

Expected Learning Outcomes

By the end of the course a fully engaged students should be able to

1) define the process of natural selection

2) describe how the process of natural selection has produced a trait that is an adaptation to a particular environmental condition.

3) explain why organisms are not expected to be perfectly adapted to their environments


Readings

Natural selection http://www.eoearth.org/article/Natural_selection

Here is a link to a website from UC Berkeley that might be useful to take a look at-

http://evolution.berkeley.edu/evolibrary/article/evo_25