Thomas, J.A., Telfer, M.G., Roy, D.B., Preston, C.D., Greenwood, J.J.D., Asher, J., Roy, R., Clarke, R.T. & Lawton, J.H. 2004. Comparative losses of British butterflies, birds, and plants and the Global Extinction Crisis. Science 303: 1881. doi:10.1126/science.1095046