Lab 10 - Survivorship Fecundity PDF

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Name Oyinda Adeyemo

LAB EXERCISE 11:

Survivorship and Fecundity Introduction At its simplest, understanding population dynamics involves understand the balance between birth and death: mortality and fecundity. Mortality is studied using the concept of survivorship, which tells you how likely individuals of a certain age are to survive to another age. For example, if we have a group of 100 sea turtles that hatch in a single day, we could ask how many of them will still be alive 5 days later, a month later, or even a year later. Fecundity is generally described as the potential capacity of an individual to have offspring. In more technical terms, it is defined as the number of female offspring that a female can have over her lifetime. How are survivorship and fecundity related? It turns out that species that tend to have a high fecundity (lots of possible offspring), also tend to have a low survivorship of those offspring. Keep in mind that survivorship does not tell you how many offspring survive to a given age, just the probability of surviving to that age.

With this in mind, answer the following: 1. A species with a potential to have many offspring would generally have a: a. High fecundity and a high survivorship of offspring b. High fecundity and a low survivorship of offspring c. Low fecundity and a high survivorship of offspring d. Low fecundity and a low survivorship of offspring 2. A species that is capable of having relatively few offspring would be expected to have a: a. High fecundity and a high survivorship of offspring b. High fecundity and a low survivorship of offspring c. Low fecundity and a high survivorship of offspring d. Low fecundity and a low survivorship of offspring

To understand this interaction, we will first look at how a life table is created. A life table follows a group of individuals and tracks how many are alive at a given time, how many have died since the last observation, and what the per-capita mortality rate of that time period was. Life tables and survivorship curves can’t keep track of every individual in a population, but instead follow a cohort: a group of individuals in a population born at nearly the same time and followed over their lifetime. 3. A cohort would therefore be defined as: a. All of the offspring from a single mating pair b. The survivors of a decrease in the size of a population c. A group of individuals that died before they could mate d. A sample of individuals of the same age in a population Consider the following life table for a cohort of sea turtles: Age (years)

0-1 1-2 2-3 3-4 4-5 5-6 6-7 7-8 8-9 9-10

Number alive at Start of Year 852 415 276 153 98 61 34 27 12 3

Proportion Alive at Start of Year (Survivorship)

Number Dying during the Year

That Year

1.000 0.48 0.32 0.18 0.12 0.07 0.04 0.03 0.01 0.004

437 139 123 55 37 27 7 15 9 3

0.513 0.335 0.446 0.359 0.378 0.443 0.206 0.556 0.750 1.000

Complete the table by calculating how many individuals in the cohort died at each time interval, and then calculate the survivorship by dividing the number still alive by the total number in the cohort at the start. Example: At the beginning of the first year, all of the turtles are still alive, and the survivorship is 100% (1.000). At the beginning of the second year, only 415 turtles remain, meaning 437 turtles died, meaning the survivorship is 415/852 = 0.487. 4. At what age range was the survivorship the highest? 0-1

5. At what age was the survivorship the lowest? 9-10

Next, consider another life table for a cohort of elephants: (Note: elephants live sometimes up to 60-70 years, but to more easily compare elephants with sea turtles their lifespan has been altered for the sake of this activity. Use your imagination.)

Age (years 6. )

Number alive at Start of Year

0-1 1-2 2-3 3-4 4-5 5-6 6-7 7-8 8-9 9-10 what age range

57 56 53 49 41 36 30 21 12 1 was

Proportion Number Alive at Start Dying of Year during the (Survivorship) Year 1.00 1 0.98 3 0.93 4 0.86 8 0.72 5 0.63 6 0.53 9 0.37 9 0.21 11 0.02 1 the survivorship the highest?

Death Rate that Year 0.018 0.054 0.075 0.163 0.122 0.167 0.300 0.429 0.917 1.000

0-1

7. At what age was the survivorship the lowest? 9-10 The data from the two life tables can be graphed to create a survivorship curve, which will track the proportion of individuals that are still alive (on the Y-axis) over the designated age intervals (X-axis). Generally, three idealized survivorship curves are compared: Type I curve: Most young survive well beyond adulthood and die late in life. Type II curve: Individuals die at the same rate, regardless of age. Type III curve: Most individuals die young. Label these three curves on the graph below.

A t

100 Type III curve

Type II curve 50%

Type I curve 0

To edit this chart, right-click on it and select Edit Data. You will open up an embedded Excel workbook. There you can add all of your Survivorship data from your Elephants and Sea Turtles respectively. Your graph will be formed automatically.

Survivorship Curves 1 0.9 0.8

Survivorship

0.7 0.6 0.5 0.4 0.3 0.2 0.1 0 0

1

2

3

4

5

6

7

Age Survivorship Elephant

Survivorship Sea Turtle

8

9

10

8. Which survival curve did the sea turtle cohort most resemble?

Type II curve

9. Which survival curve did the elephant cohort most resemble?

Type I curve As we noted above, species that tend to have higher survivorship of offspring, tend to have fewer offspring. The elephant fits this trend perfectly. A mother elephant will have very few offspring over her lifetime because she can only carry one at a time, will carry it for more thana year, and will support that one offspring for years. As expected, survivorship of that offspring will be high. Species that follow this strategy (low fecundity, few offspring, lots of care and high survivorship of offspring) are considered to be K-selected. 10. K-selected species therefore tend to fit which of the following survivorship

curves? a. Type I b. Type II c. Type III Alternatively, species that tend to have low survivorship of offspring, generally have many more offspring (and therefore have a much higher fecundity). The sea turtle fits this pattern pretty well. A female sea turtle will lay up to 60 eggs at a time, and will potentially lay hundreds over her lifetime, but she will give those offspring very little support. As expected, survivorship of those offspring will be very low. Species that follow this strategy (high fecundity, many offspring, little to no care and low survivorship of offspring) are considered to be r-selected. 11. r-selected species therefore tend to fit which of the following survivorship

curves? a. Type I b. Type II c. Type III Final thought question: Why can’t a species have both? Why can’t a sea turtle lay 60 eggs and guarantee that all 60 will survive? Why can’t an elephant have 40 offspring a year and nurture all of them to ensure their survival? Survival of the animals is not solely dependent on just being to provide the needs of the offspring, there are other factors such as the environment, other animals, and diseases. If it were only one sea turtle or elephant in the world then maybe they could have both, but there are so many of the species competing for food, resources, and other things. Reproduction requires lots of energy and the offspring’s need to be fed properly and if all the offspring’s survive, there wouldn’t be enough...