The composition of the arthropod fauna of the canopies of some species of oak (Quercus)

1. A study was made by knockdown sampling and branch clipping of the arthropod fauna of the canopy of two native oak species (Quercus robur and Q. petraea) and of two introduced species (Q. cerris and Q. ilex) in woods near Oxford, UK, and of two native species (Q. pubescens and Q. ilex) in southern France. Sampling was undertaken for five years in the UK and for four years in France. All the specimens from the UK, except Acarina, Collembola and Nematocera were identified to species or, occasionally, to morphotype. 2. In terms of overall numbers, species richness and biomass, the results for Q. robur and Q. petraea in the UK were very similar. Their samples contained over three times the biomass and nearly double the number of species compared with those from the introduced oak species (Q. cerris and Q. ilex). 3. The ordinal composition was very similar on Q. robur and Q. petraea, and that on Q. cerris showed closer concordance to the composition of these trees than to that of Q. ilex in the UK. The ordinal composition of the faunas of the two native oak species in France showed closer concordance to each other than to any of the oak species in the UK. The proportion of Psocoptera was much higher in the UK, and that of Thysanoptera was higher in France. 4. The guild composition in terms of numbers, species richness and biomass were closely correlated on Q. robur and Q. petraea, and on Q. robur in two different woods, which argues for an underlying determinism in the development of community structure. The guild composition on the introduced species (Q. cerris and Q. ilex) was most closely correlated when expressed as species richness. 5. The similarity of the guild composition on the oak species in the UK in terms of the actual species found was tested by a modified Sorensen's Index. This showed that the guilds could be ranked in the following order of decreasing similarity: epiphyte fauna, tourists, scavengers, phytophages, parasitoids-predators. 6. There were distinct seasonal patterns. In terms of species richness the values showed a general trend peaking in summer and early autumn, but biomass peaked in May on the native oak species, mainly due to Lepidopterous larvae. The ratio of prey to predator and parasitoid biomass had a distinct seasonal pattern, there being an excess of prey in May, but almost an equivalence throughout the rest of the season. This may reflect the high population of spiders in the late summer and autumn, which are likely to feed on tourists. Most classical ecosystem models do not allow for this latter important source of food that is not itself dependent on the communities' habitat. 7. The epiphyte dwelling guild (principally grazing Psocoptera) was a major component of the fauna in terms of numbers on the evergreen Q. ilex in UK. This has been reported for other studies of evergreen species in temperate climates.