This is the Australian landhopper or woodhopper Arcitalitrus dorrieni (an amphipod crustacean). It is not native to the UK, but has become increasingly established over the last century or so. It lives in deciduous or mixed leaf litter, and can be confused with native springtails. There are no native terrestrial amphipods in Britain; only aquatic species such as Gammarus pulex.
Thanks to a tip-off from a former colleague (Chris Dresh at ARC Trust), I know that A. dorrieni is present in the Bournemouth area. I took this picture of a landhopper I caught at Hurn in Dorset in August 2012.
Today, however, I spent the afternoon systematically searching for Australian landhoppers in numerous places in the Bournemouth and Poole area. I selected parks and gardens with deciduous or mixed coniferous/deciduous tree cover and thick leaf litter.
Out of the 33 locations I surveyed, I got positive records from 22 places. At each location, I used a small trowel to scrape back leaf litter, and look for landhoppers. I spent up to five minutes at each location (though five seconds was often enough for a positive record).
As far as I know, these are all new records for A. dorrieni. In most cases, I found landhoppers in my first trowel scrape. They jump about immediately, sometimes in their hundreds. They are usually no more than about 8mm long, and it's difficult to get a good look at them without catching one. They have a distinctive movement compared to springtails though (which are also generally smaller).
Evidently A. dorrieni is widespread and well-established in southeast Dorset. They are known to be widely established in Britain, particularly in the Southwest, but also the east of England and as far north as Wester Ross. They have probably gone unnoticed in many more places than they are known.
I haven't heard of any known negative impacts from these non-natives. We don't have any native terrestrial amphipods, but it is conceivable that they could compete with other detritivores such as springtails.