In the microcosm (astronomically speaking) of our own single planet we can see the folly of trying to force one environmental template onto other environments; I live in the southeastern USA... we have plenty of snakes, alligators, frogs, etc...is it reasonable to look for rattlesnakes on the North Slope of Alaska? How about alligators in central Saudi Arabia? Mangrove trees in Siberia? Sperm whales in Chad? About as likely as your chance of meeting a native Scotlander by the name of Yitzhak Abdulla MacGreggor (cheekiness intended).
These examples sound extreme, but they are no more erroneous than looking for 'Earth' life on other worlds. One almost has the impression that NASA expects to bump into Vulcans or Klingons(that is, basically earth-pattern life), and have a nice chat with them.
To date, we have seen no indications of large scale flora or fauna on the Martian surface; therefore, it DOES make sense to look for micro-scale indications...but it does NOT make sense to look for earth-cycle life (must be water...). IMHO, we have seen ENOUGH of shattered, boulder-strewn landscape (Opportunity is a welcome departure, to an extent). If we want to look for life on mars, it makes the most sense to direct our search to areas where there appears to have been liquid flow (water or not), and the areas cyclically covered by icecaps; I think our best chance is to look at areas that appear to have had two different environment types coming into contact, giving us a small geographic area in which we can search both enviro-types.