AEC 360  Ecology

Patterns of biodiversity

I.  Background
    A.  What is biodiversity?

    B.  How many species are there on Earth?

    C.  Biodiversity tends to be higher in the tropics

        1.  Though a closer look reveals more subtle patterns in mammals, trees, amphibians, and reptiles


        2.  Variation in biodiversity is also explained by geographical differences

II. Why are there more species in the tropics?

    A.  Time
        1.  Tropical climates have existed on earth for a longer period of time
        2.  Tropical habitats remained in equatorial regions during glaciation events
        3.  Rate of evolution may be faster in the tropics, due to greater energy input

    B.  More productive areas are more diverse

            1.  More energy can be used by more species
            2.  More energy may support larger populations,  lower extinction
            3.  More energy may lead to faster diversification
            4.  Energy and water inputs are important to biodiversity

    C.  Increased variability in resources

        1.  In the tropics, there may be a greater diversity of environments

        2.  Vegetation structure overrides productivity in determining diversity

    D.  Tropical species may have smaller environmental tolerances
        1.  Remember that a niche is the ranges of conditions that an organism can tolerate and the ways of life that an organism can pursue


        2.  Greater specialization in resource use?

       3.  Greater variety of roles


    E.  Area:  larger areas have a larger number of species


    F.  Other ecological factors
        1.  Greater predation (remember keystone species)
        2.   There may be an intermediate level of disturbance in the tropics relative to other biomes

    G.  Greater diversity may be a statistical artifact of how land is distributed over the earth
        1.  The Mid-domain hypothesis

 III.   I.  Why are there so many species on earth?
    A.  We can explain consumer diversity in terms of the number of plants
        1.  350,000 plant species on earth, each providing resources for 20-30 consumers
        2.  Each plant provides several different resources for herbivores (roots, stems, leaves, flowers, etc)
        3.  Each herbivore is a resource for different parasites and predators
        4.  Question:  How do we explain the diversity of plants on earth?

    B.  Five diverse continents, and parallel speciation and diversification has produced roughly equal # of species/continent

    C.  Within a continent ~200 different life zones can support species adapted to an area's abiotic conditions (climate, resources, etc.

    D.  How is it that a few hundred plant species can coexist in any area?

         1. In early ecological theory, the number of species was thought to be limited by the number of resources they consume
         2. A result of competition
         3. Plants only consume about a dozen resources (light, nitrogen, carbon dioxide, etc.), but there are more than a dozen plant species


IV.  Paradox of diversity
        1.  First addressed by Hutchinson (1959, 1961), and may be resolved with several answers
        2.  Trophic complexity

                -Predation is density-dependent
                -Predator consumes best competitor for resources (which should be most abundant)
                -Permits other competing species to exist

        3.  Spatial variation in resources
               - Each species may persist on a different combination of resources, for example, different levels of nitrogen and phosphorous.
                - Thus, different parts of the environment are suitable for different species

        4.  Variation in conditions over time

            -- Competition for resources not important
            -- Density-independent factors important
            -- A 'non-equilibrium' view of maintenance of diversity

        5.  Recruitment limitation

                -New sites become available due to disturbance (eg, trees knocked down by hurricane)
                -Plants must first colonize these new sites before they can establish and compete for resources
                 - There are tradeoffs, and the best competitors are the poorest dispersers

    C.  According to classic ecological theory, the number of species is limited by the number of resources. 

        By removing simple assumptions about communities, an infinite number of species can be supported