Biologists have proven that opportunities for outside species grow more numerous as native species grow sparser. Conversely, the more abundant is native life, the smaller is the foothold available for outsiders.
生物界也證實，當本地生物愈少時，外來生物能夠入侵的機會愈大；反之，本土生物愈多，留給外來種的空間愈小。