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history of the species but not whether species were native or introduced, even in cases where the
species had been reported from the East or West coast of the United States.
5) Historical Introductions. There is a long history of ship traffic among Asian countries and
between Asia and the rest of the world. As a result of this historical trade, it is possible that the
origins of some of the Asian species may be lost in antiquity, and that some of the “native”
species in Asia were actually introduced a century or more ago. A similar case occurred on the
U.S. East coast with the snail Littorina littorea, which required extensive genetic analysis of both
the snail and its parasite to demonstrate that it had been introduced from Europe about 500 years
ago (Blakeslee et al., 2008). With heavy ship trading not being a major vector until about the
mid- to late-1800s for Hawaii and the Pacific coast of the United States, such under reporting of
historically introduced species is presumably relatively less important in the NEP and Hawaii.
6) Language barriers. Language barriers limited our access to the original Asian literature. In
many cases, we only had access to an English abstract while in other cases we were aware of a
paper but either could not obtain it or it did not have an English abstract. There is no doubt that
this language barrier limited our ability to fully parse the distributions and natural history
characteristics of Asian species. However, we do not believe that the language barrier resulted in
a substantial underestimate of the number of reported NIS in the NWP. First, we worked with
PICES members from each of the countries to identify the reported NIS in their respective
countries. Second, there have been a number of relatively recent English summaries for NIS in
each of these countries (e.g., Zvyagintsev et al., 2011). While the language barrier is not likely to
have been a major factor in enumerating the previously reported NIS, it is a greater limitation for
taxonomists evaluating foreign taxonomic literature. Without the ability of taxonomic experts to
scrutinize foreign literature, there is a danger that introduced species will be described as new
native species in a region (see Carlton, 2009).
7) Greater propagule pressure in Hawaii and the NEP. The NWP could be less invaded if it
experiences a substantially lower propagule pressure (rate and extent of new invaders being
introduced). However, given the extent of commercial shipping between Asia and the rest of the
world it seems highly unlikely that there is a lower propagule pressure from ballast water or hull
fouling in Asia. In addition, aquaculture escapees should be at least as important in Asia as in
North America and Hawaii. While certain specific vectors may be more important in the NEP
and/or Hawaii (e.g., airplanes for insects in Hawaii) it seems unlikely that a reduced propagule
pressure was a factor in the lower number of reported NIS in the NWP.
8) Greater invasibility in NEP and Hawaii compared to the NWP. The last possibility is that
NEP and Hawaii ecosystems have a greater invasibility; that is they are easier to invade. As used
here, invasibility refers to strength of the ecological interactions resisting the establishment of a
new invader. Invasibility is the result of species interactions, which is different than the NEP and
Hawaii having more benign environments mentioned above in the environmental matching
argument (#1). We have no evidence to support or refute the invasibility hypothesis, but note that
if it is a major factor it would have to operate across a number of different taxa and habitat types,
including fouling, soft-bottom, and plankton assemblages.
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