To test this, they had first to establish a reliable list of invaders. That is not as easy as it sounds. As they observe, “although there are many lists of invasive species published by governmental agencies, inclusion of a given species in the lists （3）►may not be entirely free of political motivation◄”. Instead, they polled established researchers in the field of alien species, asking each to list ten invasive species and, for c_________⑤, ten aliens that just ★rubbed along quietly with[5] their neighbours. The result was a list of 21 species widely agreed to be invasive and, for comparison, 18 non-invasive aliens.

为了证实这一猜测，他们必须首先列出一些已经确认的入侵物种。这可不是件轻松的事。正如他们所说，“虽然政府部门公布了许多入侵物种名单，但是把指定物种归入此名单也许多少有些出于政治上的考虑。”为此，他们调查了外来物种领域一些有名望的研究人员，请每人举出10种入侵物种，以及10种与本地物种能和睦共生的外来物种用于对照。最后得到的名单中包括21种被广泛认可的入侵物种和18种用于对照的非入侵性外来物种。