How to construct food webs

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How to construct food webs?

• What are the ways you would construct a food web
• How do you determine who eats who
• What are the ways to do this?
• How do you determine what proportion of food /
  energy they get from one source versus another?

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How Are Food Webs Constructed?

• Gut content analysis
• Detailed quantification of the stomach content of
  consumers
• Very labor intensive in the laboratory
• Measures what was eaten, not what is actually
  converted to biomass
• Can create a complex picture of interactions

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Food Web Produced Using Gut Content Data
(Caddisflies Only)
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What are stable isotopes?

• Non-radioactive atoms of a given element that
  contain the same number of protons in their
  nuclei but differ in the number of neutrons
• Stable isotopes of an element differ in mass, but
  have essentially identical chemical reactivity

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Commonly used stable isotopes in ecologyand
their average relative abundances
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Stable Isotope Analyses of Food Webs

• Stable Isotopes
• 12C, 13C
• Indicative of source of carbon (e.g. macrophyte
  carbon vs algal carbon
• 14N, 15N
• Indicative of trophic position
• (3 / trophic level)
• 16O, 18O
• Hydrological applications
• Fractionization
• Heavy isotope often discriminated against in
  metabolic processes, and left behind in the
  organism, causing enrichment (i.e. isotopically
  enriched)

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Stable isotopic fractionation

• Definition alteration of the distribution of
  stable isotopes as a result of a chemical or
  physical process
• Examples

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Expression of stable isotope ratios

• dxE (o/oo) (Rsample - Rstandard)/Rstandard x
  1000
• where x is the atomic mass of the heavier isotope
  of element E (e.g., 13C)
• R - ratio of heavy to light isotopes of that
  element
• (e.g., 13C/12C, 2H/1H or 18O/16O)
• Internationally recognized standard materials for
  each element have d values of 0
• Samples with more positive d values contain
  higher percentage of heavier isotope
• more negative values indicate higher percentage
  of lighter isotope in a sample
• For example If a fish tissue sample had an
  isotopic ratio of 0.003696, and the Rstandard for
  N is air with a ratio of 15N/14N 0.003676
• Relative to the isotopic ratio of air, the fish
  tissue is enriched with 15N

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Measuring stable isotope ratios

• Isotope ratio mass spectrometry
• Sample is completely converted to a gas via
  chemical reaction or high temperature, rapid
  combustion
• Gases are separated into different types (CO2,
  N2, H2, etc.)

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Some ways in which stable isotope ratios vary in
ecosystems and examples of their utility in
ecology

• Stable isotope ratios (primarily C, N, S) can
  vary among organisms at the base of food webs
  (primary producers) what is the relative
  importance of different types of primary
  producers to production of organisms at higher
  trophic levels?
• Variation among trophic levels (primarily N)
  what is a particular consumers position in a
  food web?
• Variation among environments or geographic
  locations (H, C, N, O, S, Sr)
• what is the environmental history of an animal?
  (identify source, feeding locations, reconstruct
  migration)

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The primary basis for stable isotopes asa tool
in ecology

• You are what you eat and drink (or take in if
  youre a plant), and you reflect the environment
  in which you live

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Stable isotope mixing models

• To calculate contributions of multiple sources of
  a chemical element to something of interest

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Isotopic mixing models
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Isotopic mixing models
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Some commonly observed differences instable
isotope ratios among primary producers

• C3 vs. C4 plants (different photosynthetic
  pathways corn vs. soybeans example)
• Stream algae/phytoplankton vs. terrestrial
  vegetation
• Benthic algae vs. phytoplankton in lakes

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Effect of trophic transfers on stable
isotoperatios of biologically important elements

• d13C 1 or less
• d34S lt 0.5
• d2H, d18O less well understood,
• But fractionation appears to be minimal, at least
  in terrestrial food webs
• d15N about 3-4 per trophic level

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Stable isotope study of crayfish diet
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Some commonly observed differences in stable
isotope ratios among environments geographic
locations

• Effects of pollution
• Geologically driven differences
• Marine vs. terrestrial/freshwater ecosystems
• Differences among locations resulting from
  different types of primary producers found in
  each place
• Hydrogen/oxygen gradients in precipitation

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Naturally-occurring stable isotope markers
asindicators of environmental history

• Different geographic locations may carry distinct
  chemical fingerprints or signatures
• Chemical composition of an animals tissues
  reflects that of the environment in which it
  lives (for fishes, water and food)
• Animals that move among chemically distinct
  environments retain the chemical signature of
  previously occupied location (s) for some time,
  but also begin to acquire the signature of their
  new environment

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Persistence of signatures from previous diet or
previously-occupied areas depends on tissue type

• Metabolically active tissues (e.g., muscle)
  retain chemical signatures from previous diets or
  previously occupied environments until replaced
  by the chemical signature of the animals new
  diet/environment by new growth and elemental
  turnover (retention time of signal from old
  diet or environment varies among tissues)
• Other tissues or structures (e.g., bird flight
  feathers, fish earstones otoliths) are
  metabolically inert following formation and thus
  maintain a permanent record of the chemical
  signature from the environment in which they were
  synthesized

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Stable hydrogen oxygen isotopic composition of
precipitation varies geographically
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Using stable isotopes to determineorigins of
migratory birds

• Birds collected on wintering grounds in Central
  America

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Advantages of stable isotopes as a tool inecology

• Sample collection, preparation, and analysis
  relatively easy analyses still not cheap but
  becoming less expensive
• Many labs now have isotope ratio mass
  spectrometers
• Measures material assimilated, not just ingested
• Can provide insight into the relative importance
  of various sources of elements (C, H, O, N, S)
  for plants and animals that are more difficult to
  determine or less reliably estimated by other
  methods
• Can provide both long-term and short-term
  information on an animals dietary or
  environmental history
• Samples can be quite small and often obtained
  nonlethally

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and some disadvantages

• Quantifying relative contributions of elemental
  (e.g. C, N, S) sources to a plant or animal
  becomes increasingly difficult as the number of
  isotopically distinct sources increases
• Elemental sources or environments/geographic
  locations of interest may not be isotopically
  distinct
• Sometimes difficult to obtain clean samples of
  some materials