The findings support the hypothesis that the world is experiencing a mass extinction on par with the other five mass extinctions that have punctuated the history of life.

Not really, no matter how prestigious the journal. An awful lot of species are indeed under the hammer and declining from having to either compete with us or put up with the trashing we and our fellow world-travelling species indirectly inflict. But most species could lose 99+% of their existing numbers and still not be at any serious risk of extinction, so I don't believe stuff like this gives us any real handle on the extinction problem. One reason it doesn't is that when a known species in a place like England gets in serious trouble, human attention swings in to greatly increase its survival chances. (Whether this works in the longest term or whether a number of these species were dead men walking long before they came to our attention is another question.)

It might be taken to show we can't be complacent about species we don't know, but actually that holds irrespective of declines in known species. If something has a small area of critical habitat and you wipe that whole area out before you find it, then it's gone. Finding species and knowing something about them before we needlessly wipe them out through ignorance is one of the biggest problems. It doesn't help that by the time we get around to studying some areas they have already been heavily modified and species we never even knew about are lost.

Is it a crisis? Yes if you care about keeping other species alive, quite a few more simply aren't going to make it while others will need a lot of help. Are we headed for a "sixth mass extinction?" I don't believe so. Barring major catastrophes (runaway global warming, global nuclear war etc) I'd expect that the loss of species will be some high single-figure percentage, not really a "mass extinction" in the classical sense, more of a noticeable blip. If humans become extinct in the future, many of the species we are keeping on life support will probably snuff it not long after, but also many of the species we've carted around the world will speciate like crazy. At species level our presence on the planet could even end up proving positive. :eek