1. Landscape diversity - the number of different types of large scale communities that are located on the landscape.
    2. Habitat diversity - the number of different habitats within a single community (such as tree canopy, trunk, branches, soil,
                                        etc.)
    3. Species diversity - the number of species within an area
    4. Genetic diversity - the amount of genetic variation within a single species

    Biodiversity is all of these concepts together.  When we talk about the diversity of life, we should keep these multiple meanings in mind.

Who Coined the Term Biodiversity?
    E.O. Wilson, who is probably the most well known scientist alive today, next to perhaps, Stephen J. Gould (both from Harvard, by the way!), invented the term biodiversity to provide a theoretical framework for future research into this subject.

Where is all the biodiversity?
    Major Geographical Trends
    One of the great ecological patterns in the world is the latitudinal gradient in species diversity worldwide.  As one moves from the poles to the equator, species diversity goes up dramatically.  As an example, there are perhaps 5-6 species of trees/ha in the tundra, 15-25/ha in the Smokies, whereas you can find up to 125/ha in Costa Rica!!  The same gradient applies to other plants, fungi, animals, and so on.  Why this occurs has been the object of much study, and the issue is still not resolved.
        Some Hypotheses to Explain the Latitudinal Gradient in Species Diversity
        1. Age - some think tropical areas are older, and have had more time for species to evolve
        2. Stability - tropical areas don't suffer disturbances as much as temperate and polar areas, allowing more species to persist
        3. Energy - more energy in tropics, which allows more productivity, and hence more species
        4. Predation - more species-specific predation in tropics, which keeps other species from being competitively dominant,
                            hence more species can co-exist
        5. Disturbance - tropical areas may suffer moderate levels of disturbance that allow more species to co-exist.  Without
                            the disturbance, certain species dominant and exclude others.  With too much disturbance, only a few species
                            can survive.  This is known as the Intermediate Disturbance Hypothesis.
        6. Area - if you calculate the land area in the tropics, it is very large and exceeds that of any other areas around the world.
                            With more land area, there is room for more speciation, and there are plenty of refuges, so that extinction
                            rates are lower in tropical vs temperate areas
                            There is a strong correlation of diversity with land area - larger areas harbor more species

    The Role of History in Determining Species Diversity
    There are exceptional patterns of diversity around the world that make us consider other factors with regards to diversity.  For example, if we compare three areas of relatively the same size (Europe, Eastern North America, and eastern Asia - all of which have
about 1.2 million km2) we find that eastern Asia has 3X the number of tree species of North America, and 6X the number in Europe.  Why?
    In Europe, the major mountains run east-west.  This means that during the glaciations, they formed formidable barriers to migration, and after the glaciations were over, species had a tough time migrating northwards.  Consider that in England, there are only 12 native tree species, less than 1/2 what you could find on average in a single acre of woods in Boone!!  So, past history is important!!!!
    But what about the difference between North America and eastern Asia?  Most taxa in eastern North America originated in eastern Asia (eastern Asia has maples, tulip trees, devil's walking sticks, firs, spruces, and so on, all of which occur in North America).  So most likely, these species migrated from Asia to North America when the continents were closer together.  Not every species in Asia successfully made the trip to North America, thus we started off with a smaller species pool.  Once the migration routes were cut off speciation continued in Asia, and this contributes to the higher species diversity there.

    Island Biogeography
    Since Alexander von Humboldt's travels in the late 1700's, we have known that islands contain fewer species than nearby mainland areas.  Why is this so?
    The number of species on an island can be considered to be the result of two major processes:
                1. Immigration
                2. Extinction

    As long as immigration exceeds extinction, the number of species will increase in an area.  A third process, speciation, or evolution, can also contribute to the number of species, but only over a long period of time.

    In the 1960's, E.O. Wilson and Robert MacArthur devised the Theory of Island Biogeography.  It says the following:
            There are two things that are important in determining the number of species on an island.  These affect both immigration and extinction.  Affecting immigration, the factors are:
                1. Distance from the mainland - the farther away it is, the less likely a species will make it out the island
                2. Size of the island - the larger an island, the more likely a species that is dispersed will find it

            Affecting extinction, the main factor is:
                1. Size of the island - the smaller the island, the more competition there will be as more species arrive on the island,
                    and the more likely a species will go extinct.  Also, small islands have fewer habitats, and once all the habitats get
                    filled up, no more species that arrive later can survive

            At some point, there will be an equilibrium between immigration and extinction, and this will ultimately determine the number of species on an island.  Data clearly back these claims up.  If you compare similarly sized islands, those closer to the mainland usually have more species.  If you compare small to large islands that are the same distance from the mainland, the larger ones will have more species.

            Islands can be found in habitats other than the ocean - for example, we can consider mountain tops to be small islands amidst a sea of lower elevation habitats.  Or we could consider lakes to be islands of water in a sea of land (sounds weird I know!).  In all cases, the Theory of Island Biogeography serves well to explain patterns of diversity in these habitats as well.

What is the Diversity of the World?
Believe it or not, we still do not have a good grasp off the total species diversity of the world.  Only a small fraction of the species that exist have been officially catalogued and described.  We have only best estimates of the diversity.  What do you all think it might be?
        Plants - about 275,000 species described.  But we think there might be 2-3 million out there.  About 2,000 new species are
                    described per year.
        Amphibians and Reptiles - we know of about 5,200 species.  Probably at least 6,000 or perhaps twice that out there.
        Mammals - we know of about 5,000 species.  Some new ones still being found (monkey recently, deer also).
        Birds - we know of about 10,000 species.  Could still be many new ones out there.
        Insects - 8 out of every 10 animals is a beetle!!!!  Insects are the most prevalent animals on earth - may be millions of species
        Fungi and Microbes - the great unknown!

        As an example: Terry Erwin fumigated a single tree in Peru in the tropics looking for insects.  One tree had 3,099 individuals on it, and these comprised over 1,000 different species!!  From one tree!!  Many of these were new to science.

        From work like this, Erwin estimated that there could be up to 100 million species of organisms on Earth.  Others say it is lower and closer to 3 million.  Today, we think it must reside somewhere between these two extremes.  Over 50% of all these species live only in the tropics.  Norman Myers and others have mapped the distribution of species and found certain "hot spots" where diversity is unusually high.  These areas constitute only 0.5% of the land area, but contain up to 20% of the world's species!!  Thus losing even a small percentage of the tropics could have devastating consequences for species survival!!

Past and Present Extinctions
    Species naturally go extinct - nothing lives forever.  Some evolve away, others simply can't cope with changing environmental conditions, and are lost.  There are many factors that contribute to extinction, and they include:
            1. changing climate
            2. new species which outcompete resident ones
            3. violent disturbances, such as volcanoes, or asteroids/comets

    Mass Extinctions
       Mass extinctions refer to periods in the Earth's history when unusually large numbers of species went extinct - levels much higher than normal background levels of extinction.  There have been at least FIVE major extinctions in the past.  One at the end of the Permian Period, about 250 million years ago, was the worst - nearly 90% OF ALL LIFE PERISHED in this event!!  The most famous is the one that happened at the end of the Cretaceous - at the K-T Boundary, about 65 million years ago.  This was the final straw for most dinosaurs (they were declining anyway at this time) and no dinosauers survived this event (birds may be dinosaur descendants though!!).  Today, we are experiencing extinction rates that rival or exceed those of the these major mass extinctions.  But instead of being due to natural causes, or asteroids, they are being caused by humans.  Why and how?  This is sometimes referred to as the Sixth Extinction.

        Reasons for the Sixth Extinction
        1. Loss of habitat - humans now have altered much of the landscape.  We have cut down forests, converted prairies to farms, dammed up rivers, and destroyed wetlands and coastal habitats.   All contribute to loss of species.  Overpopulation is a major
contributor to loss of habitat and overconsumption of resources.
        2. Consumption of Resources - humans now appropriate 45% of all the earth's productivity, leaving little for other species.
        3. Invasive Species - the movement of species to new habitats in which they did not evolve is causing massive species
                extinctions in certain areas.  Some call our age the Homogocene!

Can Biodiversity Loss Be Prevented?
        Here is what E.O. Wilson proposes in his new book, The Future of Life:
            1. salvage the world "hot spots" - places at greatest risk, but which harbor abundant diversity
                Some 88% of these areas are already wiped out or imperiled.  The time to act is now.
            2. keep frontier forests intact - don't subdivide forests, or cut down old growth.
            3. stop logging old-growth forests everywhere.
            4. concentrate on lakes and streams, which are among the most threatened of habitats.
            5. find and preserve marine hotspots.
            6. extend and improve our ability to catalogue and describe species.
            7. use GIS and other global sensing techniques to help find and preserve important conservation areas.
            8. make conservation profitable so people will want to do it.  Find ways to raise the income of those who live near
                    conservation areas - engage them in the process of preservation.  Promote ecotourism.  Swap debt for land.
            9. show how biodiversity is economically important to world agriculture and society as a whole.
            10. begin restoring degraded habitats.
            11. improve zoos so that endangered species have a respite period if they can't be put back into the wild right away.
            12. control population growth - all of the above are probably fruitless if we can't get population growth controlled.