One aspect that is often overlooked when thinking about biological invasions is that in addition to the invading species, be it a bird, beetle or banana, there are usually associated parasite species that come along for the ride. More intriguingly, sometimes one of the reasons why invasive species are so successful is because the parasite species that come with them cause greater problems for native species allowing the newcomer to out-compete the locals. At other times invasive species succeed because they left a nasty parasite at home and escaped their influence. Studying the absence of parasite species is a challenging task! 
There are several reasons why a parasite species might be missing from the host species new home. And here things get a little nautical…
In the late 90s Adrian Paterson, studying feather lice on birds in New Zealand, speculated that many parasite species of introduce birds species would not be present because they had ‘missed the boat’. When birds, like sparrows and blackbirds, had been collected in Great Britain to be brought out with colonists to release in New Zealand, there was always a chance that a particular louse species might not be present on the individual birds collected and therefore never even had a chance to establish. Paterson had some limited data to support this idea.
A new study by Paterson and Richard Duncan (both Lincoln University) and Catriona MacLeod and Dan Tompkins (Landcare Research) has examined this idea in a more sophisticated manner and is just published in Ecology Letters. The team realised that New Zealand is in a unique situation to test this idea. Bird species introduced into New Zealand were reasonably well recorded. We know roughly how many individuals were brought to New Zealand, how often attempts were made to establish populations and how successful they were. We also know a great deal about the louse species present in the UK and NZ. Using newly collected data where birds were collected in the UK and their lice recorded, as well as similar records from early in the 20th century, MacLeod and colleagues were able to work out the louse population distribution over a number of bird species. They then asked the question ‘if, say, 33, individual greenfinches were used to establish a successful colony in NZ, how likely is it that these 33 individuals would have all or some of the louse species present in the UK for this particular sample?’. Complicated statistical methods ensued
The results were interesting. About 38% of UK louse species actually have colonised New Zealand successfully. Of those that failed, only 5% are predicted to have missed the boat. That is, it is likely, given the numbers of individual birds brought to NZ, that almost all louse species should have at least got onto the ships. Another third failed to establish because their host species also did not establish (which we term ‘sinking with the boat’). Finally, 27% are ‘lost overboard’ where their hosts are successful but for some reason the louse species did not survive the colonisation event (perhaps because of low numbers). This is a rather surprising result as it tells us that a successful host colonisation is simply not enough to ensure a successive parasite invasion. This is promising in our efforts to stop the spread of pest species.
There are lots of ways in which this research may progress. For example, we currently have a postgraduate looking at louse distributions in populations in more detail. In particular, he is focusing on sampling error, that is that there may be stowaway species that are in low numbers and have avoided detection thus far. And there are many more nautical terms left to explore…