Energy Flow: Pyramids, Food Chains, and Food Webs

    1) Nutrients are cycled up food chain, down to the soil, and back through the plants.  Each animal and plant of the community has its place in the food chain and nutrient cycles called its niche.

    2) A food chain is a chain of eating and being eaten that connects larger and carnivorous animals to their ultimate plant food.  Ex. Pine tree>aphids >spiders>chickadee>hawk

    3) The arrows in the food chain point in the direction of the energy flow.  It shows what is doing the eating.

    4) A food web is necessary because many kinds of plants live side by side, and because most animals eat more than one kind of food.  The more complex the web, the more stable the ecosystem.  The more choices that are available.

    5) While the nutrients are cycled through the ecosystem, energy (calories) movement is unidirectional.  Energy is reduced in the process according the second law of thermodynamics (Energy is “lost” as heat to he environment at each transfer).

    6) First Law of Thermodynamics = energy can not be created or destroyed.

    7) Animals high on food chains are larger and rarer than animals lower down the food chain.  This is called the pyramid of numbers.

    8) Each layer on the pyramid represents kinds of animals living at parallel levels on food chains.  All herbivores are on one level.  All primary consumers on the next. All secondary consumers on the next.  These levels are called tropic levels.

    9) Producers – organisms that make their food from non-food molecules. Ex.  Plants use H2O and CO2 to make sugar.

    10) Consumers -  organisms that cannot make their own food
        a) Herbivores – plant eaters; primary consumers
        b) Carnivores – meat eater; secondary consumer
        c) Omnivores – eats plants and meat
        d) Decomposers – decay dead organic material
        e) Predator – feeds on animals that if must hunt and kill (prey)
        f) Scavenger – carnivore that feeds on organisms have died naturally or that another animals has killed
        g) Parasite – feed on still living organisms.

    11) The Number of organisms at the highest level is directly related to the # of levels in the chain.  The shorter the   chain the more people or top consumers that can be supported by the bases.
        a) Ex.  27,000 lbs. alfalfa > 3300 lbs. Beef > 150 lbs. Human tissue
        b) The main ingredient in Chinese food is rice.  This reduces the number of links in the food chain so that more people can be sustained.

    12) Principle of Food size – animals tend to be the size that let them thrust their prey into their mouths whole.  There is an optimum size for any animal.  They must be large enough to easily catch its prey but small enough so that it doesn’t have to catch too much prey.

    13) Why are big animals particularly big carnivores, rare?  Small animals low on the pyramid are the food of larger animals up the food chain so that the energy enters in the bottom of the pyramid.  Energy received by each trophic level is than the previous level.  (lose 90%)

    14) Living biomass (organic matter at any level in the food chain) is a store of energy and its maintenance cost are a function of its mass.  Less biomass can be supported by the food energy received in each higher tropic level.  Those animals higher on the food chain are bigger so they can consume more food to make-up for the decrease in energy flow.  Since more food is needed per individual, the fewer organisms that will be at the top of the food chain.

    15) Other factors that may affect population numbers
    a) Reproductive rates
    b) Mortality rates

    16) As animals become larger and rarer, they also have larger home ranges.  This is a necessity when energy is in short supply high on the food chain.

    17) Attributes that should change along a gradient from herbivory to carnivory up the food chain-
        a) Fewer species
        b) Lower reproductive levels
        c) Lower population levels
        d) Increased home range
        e) Higher maintenance cost
        f) Food of higher caloric value
        g) Reduced feeding specialization
        h) More complex behavior
        i) Longer life expectancy

    18) Several meanings of niche
        a) Class I niche:  the animals place in  the biotic environment, its relation to food and enemies ( community function)
        b) Class II:  specific set of capabilities for remaining resources from the environment (species definition)
        c) Class III: a set of conditions that allows a species to exploit an energy source effectively ( quality of environment)
    19) Exotic predators can collapse and simply the structure of food webs
        a) The introduction of exotic species can reduce biodiversity through competitive replacement
        b) They also reduce biodiversity through their effects on food webs: net effect is reduction in size, complexity of food webs.
            i) Ex. Nile perch in Lake Victoria
        (1) Lake Victoria has over 400 species of fish (many only found in this location)
        (2) Introduction of Nile perch and Nile tilapia around 1954; population really exploded beginning in 1980’s
        (3) Fish catch now dominated by only 3 species (perch, tilapia, and a native species
        (4) May be the largest mass extinction of vertebrates in modern times.