Botanists digging through bog sediments on the Galapagos island Santa Cruz have discovered that six plant species thought to be invasive were actually already there when humans first arrived [$-a]. The key data are fossilized pollen grains buried in the sediments - pollen from all six plants are found in sediments formed up to 7,000 years before humans settled the Galapagos.
J.F.N. van Leeuwen, C.A. Froyd, W.O. van der Knaap, E.E. Coffey, A. Tye, K.J. Willis (2008). Fossil Pollen as a Guide to Conservation in the Galapagos Science, 322 (5905) DOI: 10.1126/science.1163454