BIO 360  Ecology

Lecture 29: Food webs and communities

I.  Communities
    A.  A group of populations of plants and animals that live together in an area
                and/or, a group of associated populations

    B.  The nature of communities

        1.  Interdependent communities (Clements) – species occur only in their community

            -- Example:  Ecotones (habitat edges) help to define community boundaries 

        2.  Intradependent communities (Gleason) – Communities are random assemblages of species

            -- Example:  Community zonation:  along gradients of physical conditions, species gradually replace one another 

    C.  What do ecologists measure in communities?

        1.  The abundance of organisms in a community
        2.  The diversity (or number of species) in a community
        3.  Interactions between species

II.  Food webs:  the species in the community and their interactions

    A.  Information in food webs

        1.  Connections between species in a community 

            -    Food webs with similar numbers of species can differ in their complexity  

        2.  Energy flow between species
        3.  Role of specific species in controlling populations
        4.  Keystone species are species that have have a particularly important impact on abundances of other species in the food web

    B.  The role of keystone species

        1.  Case 1:  Keystone predators:  Starfish in rocky intertidal zones

        2.  Case 2: Red-naped sapsuckers in aspen forests

    C.    A related concept is that of an ecosystem engineer 

            --   A species that modifies its environment in a way that substantially affects the community

    D.  Which are more important in controlling the structure of communities?

        1.  Bottom-up factors –higher levels of food webs regulated by factors lower down (production, prey)

        2.  Top-down factors –lower levels of food webs regulated by factors higher up (predation)

        3.  Both are important!

        4.  Top-down control advocated in a paper by Hairston, Slobodkin, and Smith

            1.  'The world is green', so herbivores are not food limited
            2.  Instead, herbivores are limited in abundance by predators
            3.  Plants compete for scarce resources

        5.  Response:  The world is green, but herbivores are food limited because plants defend themselves

    E.  Importance of bottom-up v. top down regulation determined by experiments
        1.  Bottom-up control examined through fertilization experiments

        2.  and top-down control is usually examined through predator removal or addition 

            --  Trophic cascade refers to the effects of predators down the foodchain  

    E.  Indirect interactions can be as important as direct interactions
        1.  Example: impacts of killer whale on food web