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I am a senior at Haverford College.  I am majoring in biology, with a minor in physics.

Biodiversity is a way to describe the make-up of an area with respect to groups of organisms: it describes the abundance of a group of organisms (grouped by some similar characteristic; here I will be using species as my scale for measuring biodiversity) and the relative distribution of the members of the group within an area.

As organisms interact with each other and their surroundings, population sizes and distributions will change.  These fluctuations mean that biodiversity is an ever changing phenomenon.  While I believe that these changes have always occurred, I suspect that the rate at which these changes take place has accelerated during the past century or two in such a way that biodiversity is decreasing in general on a global scale.

During the past century or so there have been many industrial, agricultural, and technological achievements whose effects in the natural world are not entirely known.  These human influences are changing the environment in rapid ways (as compared to geological and evolutionary time scales).  Thus, organisms must adapt or evolve to these changes, or become extinct.  Because the anthropogenic changes are taking place faster than most species can evolve or adapt, I think that rates of extinction are increasing.  Since here I am viewing biodiversity as a measurement of species abundance and distribution, biodiversity is decreasing as a result of these extinctions in the sense that the numbers of different species (the richness) in certain areas (such as in cities, for example), i.e., the way in which they are distributed (the evenness), is decreasing. 

From an environmental point of view, a decrease in biodiversity could lead to huge changes in the biomes of the earth.  A larger variety of organisms means more of a balance in an area because if a sudden change takes place there is more of a chance that some of the species in the area will be able to deal with the change.  This is important because almost all species of organisms on earth depend on their interactions with species.  From an anthropocentric point of view, humans have adapted to live in the world as it has been for thousands of years.  Significant changes in biodiversity can completely change that world in drastic ways.  It is possible that humans would not be able to adapt to such changes and our lifestyles would completely change.  A less drastic and more realistic way in which human lifestyles would be affected is through global warming which may lead to a decrease in biodiversity, but could also be aided by a decrease in biodiversity in so much as plant populations are concerned.  Also in answering this question, the intrinsic value of biodiversity on earth should be considered.

Research is needed to determine what can be done about it.  Since the problem stems from humans, policy may need to be implemented in order to curb human influences on the environment and biodiversity.  This means information should be gathered and presented to the public.