Larry,

I know none other than John Rummel made this point in rebuttal to Zubrin, but it really misses the point. The point wasn't that humans are not in danger so who cares about elm trees. The point is that the more divergent the species the less likely pathogens are to cross the species barrier. Given that Martian life is obviously distantly if at all related to Earth life and most likely single celled if it exists at all, there is next to nil chance it will be pathogenic. I'd like the alarmists to point out a case were a single cell pathogen has infected higher life forms plant or animal. Zubrin was not trying to claim that human's are some superior species, but rather that most life on Earth has little to worry. The older report at http://www.nationalacademies.org/ssb/bcmarsmenu.htm reaches essentially the same conclusion using other forms of reasoning. I find those more convincing that Zubrin's, but I think his point is rational unlike many of DiGregorio's.

Regards,
Mark