Considerable attention has been focused on the concept of Propagule Pressure (number of individuals introduced and introduction events) as a predictor of invasion success (975 papers). Much less well studied is the role of Colonization Pressure (number of species introduced; 24 studies), the complement of propagule pressure. Here we review the invasion history of the Laurentian Great Lakes to predict the risk of a future invasive (i.e. producing adverse ecological effects on other species) non-indigenous species based upon the number of species introduced (colonization pressure), using the recorded history of invasions in this system as our starting point. Historically, 52% of the fishes that were introduced and became established in the Great Lakes were subsequently identified in the literature as invasive, whereas the value for invertebrates (16%) was much lower. Assuming future invaders have similar invasion attributes as those already present, the risk of getting at least one high impact species is positively and asymptotically related to the number of species introduced, though the rate is substantially higher for fishes than for invertebrates. Our study provides support for the contention that managers ought to focus initially on vectors transmitting multiple species when attempting to prevent invasion of their system by species likely to become problematic.

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