Opilio canestrinii Another Invasive Alien!

Opilio canestrinii

The rise of Opilio canestrinii in the UK has been monumental making other fairly recent colonists like Dicranopalpus look rather slow. It almost feels like invertebrate colonisation happens much faster than it used to with species colonising the south, then rapidly expanding northwards. O. canestrinii however unlike many of the other southern expanders, we await so patiently up north, spells potential bad news as a species considered which is considered invasive by most. 

The curious thing about this harvestman is nobody knows where it came from, whilst we know much of its recent European history, it has long been not considered to be a species which originated elsewhere. European records began in Italy pre-1962, from where it has rapidly spread through Switzerland, Austria, Germany, the Netherlands and Denmark before being recorded from the Lea Valley in 1999. Since 1999 it has reached as far north as Elgin in northern Scotland. I probably don't look at Harvestmen as closely as I should, or as I do with other groups but its enough of an interest which would have picked up this species sooner, but interestingly this was my first sighting of the species (of two) at Wykeham Lakes recently.  

So another invasive, what is the fuss, well this species is considered aggressive and competitive and studies from Europe suggest that it displaces the other two native Opilio species from these areas O. parietinus and O. saxatilis. Studies in the Netherlands also suggest the rapid decline or complete extinction of O. parietinus populations there. This could potentially lead to biodiversity loss in the UK with two native species being replaced by one non-native one. 

O. canestrinii is typically found around buildings walls (unlike the ones which I found in woodland). House walls are a habitat frequented by O. parietinus and presumably this competition for a similar niche occupancy is what has driven the latter almost to extinction across large areas (time to get out there looking for them!). There is less direct competition with O. saxatilis which is generally a ground dweller of open dry habitats like drained grasslands. O. canestrinii can easily be split from O. parietinus in the field by the orange coxa and trochanter (the two leg joints closest to the body). As ever nothing is completely conclusive with some authors recording both species together with O. parietinus showing no evidence of a decline and co-existing perfectly. Certainly something I will be looking out for from now on.