Food Chains and Webs


I. Food chains
    A. Background -- Ray Lindeman; Cedar Bog Lake
    B. Conceptual:
        light + nutrients
        primary producers (phytoplankton) first trophic level
        primary consumers (herbivorous zooplankton) second trophic level
        secondary consumers (carnivorous zooplankton or planktivorous fish) third trophic level
        tertiary consumers (piscivorous fish) fourth trophic level

        1. trophic levels – contain functionally similar organisms that utilize similar food resources

        2. trophic dynamics – transfer of energy from one part of the ecosystem to another

    C. Currency –
        1. calories – 1 mg C ~= 10 cal  (energy)
        2. C or N (conservation of mass within the system)
        3. dry mass (as a proxy for C and N; C ~ 50 % dry weight)
        4. ash-free dry weight -- parts of a sample that aren’t organic (minerals) are excluded; good for diatoms or for mussels
        5. limitations of currency measures – vitamins, nutrients, lignin
    D. Ecological efficiency – efficiency at each link

            change in energy content of trophic level N= energy income from N-1 minus losses (metabolism/respiratory)

            Increased efficiency of transfer as you move up the food chain

    E. Length of food chains – food webs usually are short – 4-5 levels – why?
        1. Energetic Hypothesis – length limited by inefficiency

        2. Dynamical stability – long food chains are less stable

        3. Ecosystem size - more habitat? More stable habitats...Do tend to find longer food chains in larger lakes

    F. Body size –
        1. Often organisms at each successive level in aquatic systems are larger than those at the previous level
        2. This is not often true in terrestrial systems
II. Food webs
    A. Food chain transformed into a web due to:
        1. omnivory – feeding on several trophic levels at once
            a. finding that this is more and more common
            b. mixotrophy – both a primary producer and a heterotroph
            c. Or a predator like cyclopoid copepods, that will also sometimes consume flagellated algae (acts as an herbivore).
            d. Omnivory is thought to be more common in aquatic systems 

        2. ontogeny – may change the food level an organism feeds on 

        3. temporal shifts in diet 

    B. Microbial loop

    C. Mathematical descriptions of food webs – can allow us to make comparisons between different food webs
        1. stability

        2. connectance – draw lines between species in food webs – connections between different trophic levels and species,
                and examine how many of the potential lines are filled in
                            actual interations/possible interactions

                As you decrease the number of species you increase connectance
                Aquatic predators generally are connected to 2.5-3 prey items
        3. diversity

    D. Stable Isotopes and Food Webs  (see attached sheet)
        Extra neutron doesn’t usually affect the chemical properties (doesn’t change outer electron shell)
        In some reactions it makes it harder to get activation energy because it is heavy – isotope discrimination
        If a reaction proceeds to completion then there is small fractionation (C4 plants have less fractionation than C3 plants)
        For carbon you are what you eat
        For nitrogen you are what you eat plus 3 0/00

        Can determine potential versus realized food webs
        Can separate terrestrial and aquatic sources

        Whole lake experiments feasible

        Fractional trophic levels

III. Major paradigms of what controls the organisms in an ecosystem
        A. Bottom-up control – nutrient regulate

        Nutrients   -> 1 producers -> 1 consumers ->  2 cons (sm. fish) ->3 cons. (lg. fish)
        increase             increase             increase              increase                 increase
 

    B. Top-down control
        1.  ‘odd-even link paradigm’/’saw tooth paradigm’, cascading trophic interactions

        Increase pred -> decrease sm. fish -> increase zooplankton -> decrease phytoplankton

        2. keystone species (Paine)
        3. biomanipulation (Shapiro) – controlling of algal blooms – get more big zooplankton by increasing
                piscivorous fish
               'biomanipulation' – main idea to decrease small fish so increase large zooplankton so decrease algae
        4. trophic cascade hypothesis (Carpenter and Kitchell)
    C. What is evidence for each?
        1. Bottom Up

        2. Top down
            Add big fish, almost always decrease small fish, usually get larger zooplankton (but not necessarily more zooplankton),
                but not always, rarely get algal decreases or nutrient effects.

    D. Where and why do these controls break down in food web?
        1. need to take into account ontogeny – often piscivorous fish are planktivorous when young and will
            themselves eat zooplankton
        2. predator-prey cycles – periodic releases from predation
        3. rearrangement of trophic structure during perturbation – may have species changes
            i. smaller zooplankton are less good grazers and don’t often reduce phytoplankton
            ii. may get shifts to inedible algae (e.g., blue-greens)
        4. variability of diet, plasticity in feeding

    E. Effectiveness of biocontrol often depends on trophic condition
        1. oligotrophic lakes – small responses of biotic release (top-down) –
                there are so few zooplankton that they can’t graze down the phytoplankton
        2. mesotrophic lakes – greatest response of biomanipulation and greatest overlap of top-down and bottom up controls
        3. eutrophic – fewer species at high and low end – less opportunity for these effects; more inedible algae -- but this is
                where they hoped it would work

    F. Synthesis –
        1. Both types of control operate most strongly closest to where they are initiated (piscivores to planktivores;
                nutrients to phytoplankton)
        2. Both operate at some time in almost all ecosystems
        3. There are also other physical and chemical controls imposed on food webs
                Mixing resets the system
                Oxygen limitation of fish
                Temperature

Stable Isotopes   Lecture note supplement

Not all atoms of the same element have identical masses.  For some elements, such as carbon and nitrogen a small percentage of atoms is enriched with extra neutrons, making them heavier.  Atoms with the same number of protons, but different numbers of neutrons are known as isotopes.  Some isotopes are unstable or radioactive, and decay over time (e.g., 14C).  Other isotopes are stable and the atoms do not decay.  Most of the atoms of carbon and nitrogen exist as 12C and 14N.  A small percentage of each element exists as a stable isotope, 13C and 15N, known as heavy isotopes due to the extra mass of their additional neutrons.  In nature, materials, including the tissues of organisms, contain some mixture of light and heavy isotopes.  Stable isotopes can be used to solve many problems in ecology, among them the understanding of feeding relationships.

 

The isotopic composition of materials can be measured very precisely with a mass spectrometer.  This isotopic composition is generally expressed as a ratio of heavy to light isotope in a sample relative to that in a standard; these relative ratios are δ values, given in units of parts per thousand (‰).  Increases in δ values indicate increases in the relative amount of heavy isotopes in a sample.  Decreases in the δ values indicate decreases in the heavy isotope content (and a corresponding increase in the light isotope content). 

 

The magnitude of these ratios in an organism reflects the isotopic ratios of the food or elements that were used to build the tissue.

 

  1. The rule for carbon and sulfur is that you are what you eat.  In other words, the isotopic ratio in your tissue is the same as the isotopic ratio of the food metabolized to make the tissue.  Different food sources (e.g., C3 versus C4 plants, terrestrial versus aquatic detritus, near shore versus offshore material) often have characteristic isotopic compositions.  The isotopic composition of the consumer will reflect the source of its food.
  2.  The rule for nitrogen is that you are what you eat +3 heavier.  In other words, your tissues will be enriched by 3 per mil over the ratio in your food.  This results from the fact that the lighter isotope (14N) takes part in more biochemical reactions during protein breakdown and is excreted at a higher rate than the heavier isotope (15N).  This leaves the remaining protein and tissue enriched in the heavier isotope.  Typically consumers are +2 ‰ to +5 ‰ (average ~ +3‰) heavier (or enriched in 15N) than their diet.  At each successive trophic link, the consumer’s diet contains prey that are more isotopically enriched in 15N, so that δ15N values of different organisms in a food chain reflect their relative trophic position.

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