ENDY 6013 - ENVIRONMENTAL DYNAMICS

HOMEWORK ASSIGNMENT #7a - "IT'S A GOOD DAY TO DIE"

DUE FRIDAY, 29 OCTOBER 2004

During the last decade, there has been increasing concern over the state of Earth's biosphere. In particular, biologists have focused on biodiversity and the "quantity" of biodiversity within the biosphere. While there is no doubt that human activity has been responsible for the extinction of a number of species, is there justification for the claim that humans are effecting a "mass extinction" event? (Note: Paleontologists characterize mass extinctions as events where significant numbers of organism families and orders, not species or even genera, become extinct).

Lomborg (2001) suggests the background extinction rate might be on the order of 1 species per decade, and concedes that the present extinction rate might be as high as 1 species per year (i.e. an order of magnitude greater than his claimed background extinction rate). Boggs and Roughgarden (2000) suggest that locally, extinction rates may be 10,000 times the background rate, though most estimates place the number somewhere between 100 and 1,000 times the background rate. Let's do a thought experiment: Let's assume Lomborg's (2001) background extinction rate of 1 species per decade and that present extinction might be 100 to 1,000 times the background rate and consider these values as global rates.

To be complete, we should attempt to determine what the present background rate of speciation is. Oddly, this number seems far more difficult to obtain than those for extinction rates. However, Boggs and Roughgarden (2000) provide an example of a "hotbed" of speciation - East African Rift lakes. Here, they suggest that approximately 300 species of cichlid fish may have originated during the last 12,000 years (i.e. 1 new species every 40 years). However, some entomologists suggest that insect species might be emerging at rates between 1 and 100 species per year. Since we don't have any good estimates for speciation, let's assume a middle magnitude for speciation of 10 species per year as an estimate of the global background rate of speciation.

Finally, we need to estimate a quantity for biodiversity (i.e. number of species) for the biosphere on the whole. Estimates of this number range from 1 million to 100 million. Let's be conservative and pick a number of mid-magnitude, 10 million.

Calculate the following:

1. Using the above values for extinction and speciation, what is the global net annual change in biodiversity?
2. Using an extinction rate 100 times Lomborg's background estimate, how many years would it take to reduce biodiversity by 50% (which most paleontologists would agree amounts to a mass extinction event).
3. Using an extinction rate 1,000 times Lomborg's background estimate, how many years would it take to reduce biodiversity by 50% (which most paleontologists would agree amounts to a mass extinction event).
4. What is the probability that human activity might maintain these extinction rates for the interval necessary to declare a mass extinction has occurred?
5. Can these numbers be trusted to conclude that there is a mass extinction underway? Use your knowledge of the biosphere to evaluate the claim of an ongoing mass extinction event.