Intro to Stable Isotopes

Question 1

Historically, food web structures were developed with a focus on what 2 components?

Answer 1

taxonomy and organization

life history

Question 2

Which guy developed a highly influential paper that most current methods of food web ecology are still based on? What important number did he figure out?

Answer 2

Lindeman - looking at the ‘trophic-dynamic aspect of ecology’ in a pond ecosystem

he figured out that the loss value between trophic levels is ~10x

Question 3

What was unique and novel about Lindeman’s ‘trophic-dynamic perspective’ compared to the historical animal ecology food cycle?

Answer 3

Lindeman organized his food web:
- focused less on taxonomy and grouped organisms based on function
- focused on the math to organize the species

his had two distinct chains from distinct sources and showed that they interact - microbes and nutrients from abiotic contributions - included the recycling nutrients

the animal ecology one provides more taxonomic info, the Lindeman one is more informative and provides a functional understanding

Question 4

What does the study about fiddler crabs and the stable carbon isotope tracer tell us?

Answer 4

looking at the stable carbon isotope in various photosynthetic life which the fiddler crab consumes (C3 and C4 plants) in salt marshes tells us about the diet

how well does the crab’s signature match the photosynthetic plants?

Question 5

T or F: carbon isotopes vary among dietary sources

Answer 5

true

Question 6

What does the 1:1 linear line in a graph tell us?

Answer 6

the 1:1 line shows us what the relationship would look like if the carbon isotopes in the animal do not vary from the carbon isotope in the primary producer that is thought to be the dietary source

Question 7

what happens to the nitrogen stable isotope as you go up the trophic levels?

Answer 7

it increases (gets heavier)

Question 8

What does carbon tell us about the organism?

Answer 8

dietary source - ‘you are what you eat’

Question 9

What does nitrogen tell us about the organism?

Answer 9

trophic position

“you are what you eat + more”

Question 10

T or F: there is only variation in the stable nitrogen isotope between trophic levels

Answer 10

false, there can also be variation between individuals of the same species/same trophic level

Question 11

Explain the details of the carbon stable isotope graph we’re always shown (diet C v. animal C)

Answer 11

there is some variation within the dietary source species but not much between trophic levels (between the dietary source species)

most of them are close to the 1:1 line

‘you are what you eat’

Question 12

Explain the details of the nitrogen isotope graph we’re always shown (diet N v. animal N)

Answer 12

vary a bit within dietary sources (species), but more variation between dietary sources =

more variation from the 1:1 line (most of them are above the line)

‘you are what you eat + more’

Question 13

When you put the stable carbon and nitrogen isotopes together, what do you get?

Answer 13

a food web

with d13C on the x axis and d15N on the y axis

Question 14

What does the atomic mass of an element represent?

Answer 14

the number of protons + neutrons (each one has mass = 1)

Question 15

What does the atomic number of an element represent?

Answer 15

the number of protons (and same as number of electrons)

Question 16

So, for 12C where atomic mass = 12 and atomic number = 6; how many protons, neutrons, and electrons are there?

Answer 16

atomic mass = 12
atomic number = 6

there are 6 protons and 6 electrons (atomic number)

and 6 neutrons (atomic mass - atomic number)

Question 17

What is an isotope?

Answer 17

an element with the same number of protons and electrons but different number of NEUTRONS

Question 18

What number of neutrons do 13C and 14C have?

Answer 18

13C has: 13-6 protons = 7 Neutrons

14C has: 14-6 protons = 8 neutrons

Question 19

What does it mean for an isotope to be stable?

Answer 19

it means the element does not decay over time, it is stable

Question 20

What does it mean for an isotope to be radioactive?

Answer 20

it decays overtime as its nucleus changes, it is not stable

Question 21

How do isotopes behave?

Answer 21

heavier isotopes chemically behave like carbon, but physically behave different

Question 22

How are isotopes ‘hidden’ in the periodic table?

Answer 22

‘iso’ = same
‘topos’ = place
found in the same place

ie., isotopes are hidden in the periodic table because it is organized by atomic number (number of protons) and isotopes have the same atomic numbers/number of protons

Question 23

What are nuclides?

Answer 23

isotopes

Question 24

How is the chart of nuclides (isotopes) arranged?

Answer 24

rows are by element (same element across) = number of protons

columns are by the number of neutrons (increasing order)

each row represents a list of isotopes

each column represents a list of isotones (different protons but same neutron numbers)

on a diagonal to the left = isobars (different elements with same atomic mass)

on a diagonal to the right = isodiapher (difference between stable isotopes is the same)

Question 25

What predicts whether an isotope will be stable or radioactive?

Answer 25

how different the number of neutrons and number of protons are

the more different neutrons and protons are = the more likely to be radioactive

Question 26

What is the ratio of neutrons/protons for radioactive elements ?

Answer 26

neutrons/protons > or = 1.5

so for example, C and 13C and 14C

13C: 6 protons and 7 neutrons = difference of 1 < 1.5 = stable

14C: 6 protons and 8 neutrons = difference of 2 > 1.5 = radioactive

Question 27

What 5 major elements have stable isotopes used in ecology? what are they?

Answer 27

1. carbon - d13C
2. nitrogen - d15N
3. Sulfur - d34S
4. hydrogen (deuterium) - dD
5. oxygen - d18O

plus other trace elements like silicon and iron

Question 28

Which isotope of carbon is most naturally abundant?

Answer 28

12C

Question 29

Which isotope of nitrogen is most naturally abundant?

Answer 29

14N

Question 30

what do the most naturally abundant isotopes of carbon and nitrogen (and others) have in common?

Answer 30

these are the balanced elements - where the number of protons = number of neutrons (= number of electrons)

Question 31

What 3 ways can ecologists represent the relative distribution of heavy: light isotopes in a sample?

Answer 31

1. fraction (atomic percent) = % heavy isotope in a sample (F or AP)
2. ratio = heavy: light in a sample (R)
3. delta = most common ; a ratio of ratios

Question 32

What is the equation for delta?

Answer 32

[(Rsample / Rstandard) - 1] x 1000

where R = ratio of heavy to light isotopes (ex. 15N/14N)

Question 33

Calculate the AP(F), R and delta for the following sample:

sample 1 has 0.38 15N atoms for every 99.62 14N