Biodiversity science: mass extinctions
Just because a question has been asked on mass extinction. Worth looking at Barnosky, A. D., et al. (2011). "Has the Earth/'s sixth mass extinction already arrived?" Nature 471(7336): 51-57. And Barnosky, A. D., et al. (2012). "Approaching a state shift in Earth/'s biosphere." Nature 486(7401): 52-58.