Isotopes to Quantify Food Webs Flashcards
What kind of information did early stable isotope studies provide?
descriptive and qualitative
ex. from the graph we always see of uplands v. plankton v. salt marsh plants, we can see that C4 plants and plankton support more production in the salt marsh than the uplands C3 plants, but we can’t say by how much
ex. the martens shifting their diet to salmon - we can make broad claims about seasonal diet variations but no quantitative measurements
Why do the organisms that make up the marten diet vary in their isotopes? Ex. why is the salmon so different from the deer mice, voles, squirrels and berries?
trophic position? dietary source of salmon and p.p. in marine ecosystems different than terrestrial?
Interpret the graph with the marten and its diet
the first graph depicts the martens dietary sources in the fall and summer
summer - when the marten diet consists mostly of terrestrial sources (deer mice, vole, squirrels, berries) - this is shown because the marten’s isotopic signatures are closer to the terrestrial diet sources’
fall - when salmon are spawning, there is more variation in signatures between dietary sources and the marten’s more closely matches salmon’s (marine) - so the martens are probably ingesting more marine dietary sources than terrestrial
what are 3 useful food web metrics?
- sources of production
- trophic positions (dNanimal v. dNdiet)
- changes in entire food web structure (before and after a disturbance such as salmon spawning)
How is looking at the sources of production useful for quantifying food webs?
because stable isotopes vary among primary producers, there is variation between dietary sources (ex. marine v. terrestrial C) and measuring the difference between them can tell us how much the organism derived from dietary sources
How can we quantify the sources of production for a food web? why?
fractionation between primary producers and secondary consumers is responsible for creating the variability between sources of production and different trophic levels
ex. it explains why C3 and C4 plants are different, or why N-fixers v. nitrate users are different, or phytoplankton in ocean gyres v. coastal zones are different
it creates the diversity in signatures
How do we quantify the sources of production in a food web?
by calculating the % carbon (or nitrogen or energy) that an organism derived from each production source
T or F: the isotopic signatures of consumers reflect a MIXTURE of the isotopic signatures of their dietary sources
true
What are mixing models?
models used to determine proportions of carbon (or N) an organism derived from different sources of production
ie., how we quantify sources of production
What are the 4 families of mixing models?
geometric
linear
IsoSource
Bayesian
Describe the geometric mixing model
uses Euclidean distance measurements on isotopic bi-plots (ex. martens and their diet sources) to measure the distance between consumers and their dietary sources
what are the pros and cons of the geometric mixing model?
PROS:
easy
measures euclidean distances (basically puts numbers on an isotopic biplot) = intuitive results
could be good for comparing seasonal diets
CONS:
susceptible to bias - you can put whatever you want on your plot, even if the animal isn’t eating it, you can calculate an ED which can lead to:
- over-estimating the contribution of abundant prey
- underestimating the proportion of rare prey
limited application
- all you can really say is that there is a difference between the sources
only provide point estimate
What is the Euclidean distance formula?
ED2 = (Xa - Xb)2 + (Ya - Yb)2
where X and Y are coordinates on the isotopic bi-plot
describe linear mixing models
uses simple equations (algebra) to calculate the proportion of the isotope the animal is getting from source A and B including fractionation of the sources in its diet
2 sources + 1 isotope
dXanimal = Pa * dXa + Pb * dXb
where Pa + Pb = 1 (because it’s a proportion/% = 100%)
and P = fraction of the source
what are the pros and cons of the linear mixing model?
PROS:
more robust than geometric - gives more detailed information
CONS:
most food webs have more than 2 dietary sources and we don’t have enough isotopes to work with to be able to find a unique solution
only provide point estimate
In a salt marsh food web,
d13C for a C4 plant = -10 ppt
d13C for a C3 plant = -30 ppt
d13C for the crab = -15 ppt
calculate the contributions of each source to the crab (PC3 and PC4), assuming big delta (trophic) = 0 ppt cause there is no fractionation between the animals and their diets
use the linear mixing model formula
dXcrab = PC4 * dCC4 + PC3 * dCC3
and Pa + Pb = 1
If the previous question about calculating the contributions for each dietary source for the crab in the salt marshes was looking at N isotope instead of C, what would we have to consider in the equation?
looking at N, the trophic fractionation (fractionation between the crab and its diet) would not equal 0, like it does for C, instead it would be 3.5 ppt
If you had 3 dietary sources in the linear mixing models, how many isotopes would you need?
2 because it only works if you have 1 less isotope than the number of dietary sources
it is impossible to determine the contribution of 3 things to an animal if you only have 1 thing to consider - you can’t substitute to solve (infinite solutions) iPa + Pb + Pc = 1
Describe IsoSource mixing models
can handle multiple dietary sources in a food web without needing to consider more than 2 isotopes but it creates a range for which each dietary source may be contributing to the animal
it uses principles of mass balance to consider all possible combinations that created the animal’s isotopes
Pros and cons of IsoSource
PROS:
free and available online
doesn’t have restrictions for number of isotopes v. number of dietary sources
CONS:
only computes a range for the % contribution of each dietary source
Describe Bayesian mixing models and how they overcome the challenges of the other models
uses prior information to narrow down possible solutions and accounts for sources of error, then considers all possible combinations that led to the animal’s isotopic value, and provides a probability of distributions of solutions (not just a range, but how likely and how confident) that you can calculate confidence intervals for
the other models provide only point estimates or ranges, which you cannot do statistical analyses on cause you can’t calculate means, SE or SD (variability)
What are examples of prior information which can inform a Bayesian model?
gut content analysis
natural history
isotopic turnover
fractionation
food quality
Pros and cons of Bayesian models
PROS:
- free packages on R
- provides probability distributions for your range of contribution %
- can do statistical analyses on these data
- uses prior information about the organism to determine which solutions are most likely and calculate CIs to determine how confident
CONS:
- not always necessary / too fancy for some questions
What are some other potential biases in quantifying sources of production for a food web?
accurate measures of trophic fractionation (especially when looking at N where it contributes ~3.5 ppt every trophic level)
isotopic overlap between different food sources
turnover differences between consumer and diet (especially in herbivores) - ex. fish eat zooplankton, zooplankton have way shorter lifespans and are responding to changes in the environment much faster than fish so a samples of a fish will not represent the zooplankton it ate that day = the more different the organisms are, the more variation there is
accounting for variation in time and space
Define trophic position
the distance between an animal and the base of the food web - ie., how many trophic steps are between the animal and the base? (how many 3.5 ppt fractionation steps)
What 3 things do you need to know to calculate trophic position? Which isotope is always considered?
only considering Nitrogen isotope
- the isotopic signature of the BASE of the food web (trophic position of the base = 0) - ie., what are the primary producers doing?
- the isotopic signature of the animal (usually it’s 3.5 ppt, unless lit suggests otherwise)
- trophic fractionation (big delta)
What is the trophic position equation for an animal feeding on one diet source?
TP = TPbase + [(d15Nanimal - d15Nbase) / big delta trophic]
in words: the trophic position of an animal is = to the trophic position of the base + the isotopic difference between the animal and the base, divided by the trophic fractionation
TP = the number of steps that have occurred
almost always, the TPbase = 0 because working from primary producers
If the base of the food web is a single source, and you’re calculating the lemur TP where
dNlemur = 7 ppt
dNlegume = 0 ppt
TPbase = TPlegume = 0ppt
big delta trophic = 3.5 ppt
What is the TP of the lemur? What does the TP suggest?
TP = TPbase + [(d15Nanimal - d15Nbase) / big delta trophic]
TP = 0 + [(7 - 0) / 3.5]
TP = 7/3.5 = 2
if the lemur was only a herbivore, then the trophic level should be closer to 1, but obviously it is consuming something else - which is true, lemurs are omnivores
How can we adapt the TP equation to examine an organism that is consuming more than 1 diet source? for example, a salt marsh crab consumes both a C3 and C4 plant?
TP = TPbase + [(d15Nanimal - d15Nbase) / Dtrophic]
we need to calculate what the d15N for the base of the food web is when there are 2 diet sources
d15Nbase = Psource1 * d15Nsource1 + Psource2 * d15Nsource2
where Psource1 + Psource2 = 1
you can also use Psources from the d13C calculations if you’ve already done them from the linear model
A salt marsh crab consumes both a C3 and C4 plant. Use the following to calculate the TPcrab:
PC4 = 0.75
d15NC4 = 5 ppt
PC3 = 0.25
d15NC3 = 0 ppt
d15Ncrab = 8 ppt
assume Dtrophic = 3.5 ppt and TPbase = 0
And what information can we glean from these data?
from this data, we can guess that the C3 plant is an N-fixer (d15NC3 = 0 ppt) and C4 plant is a nitate user (d15NC4 = 5 ppt)
and the crab gets 75% of its N from the C4 plant and 25% of its N from the C3 plant and fractionates at 3.5 ppt
TPcrab = 0 + [(8 - d15base) / 3.5)] –> need to solve for d15Nbase first
d15Nbase = PC4 * d15NC4 + PC3 * d15NC3, where PC4 + PC3 = 1
d15Nbase = 0.755 + 0.250.25 = 3.75 –> sub this back into the TP equation
TPcrab = 0 + [(8 - 3.75) / 3.5] = 1.21 –> the crab is not quite at TP = 1, we have assumed with the Psource1 + Psource 2 = 1 equation that we have accounted for all the dietary sources, so:
- it’s possible that the 3.5 ppt fractionation is off, but more likely that:
– the crab is a herbivore so it’s starved for protein which inflates the d15N
Why might a herbivore like the salt marsh crab have an inflated TP?
a herbivore’s diet is not high in protein, so a lack of protein can cause the TP to be inflated
