In total, around 94,0000 insect and spider individuals were caught in the 60 studied trees over the 14 catch periods:

That high number of individuals (representing only a fraction of all arthropods in the case trees) prove the great importance of urban trees as habitats for insects and spider animals.

Detailed evaluations of the window traps and manual samplings until family and species niveau showed, that on native urban tree species a higher amount of insects were found as on non-resident urban tree species. However this is only true for certain animal groups and also not for all tree species (figure 1).

With e.g. 41 beetle and 42 hymenopteran families (bees, bumblebee, wasps, ants; fig. 2), including 57(!) wild bee species, the biodiversity was high.

In comparison with species diversity of wild bees on native and south-east European tree species no significant differences were found in the biodiversity (fig. 3), similar to other animal groups.

Because 91% of the caught bee species create ground nests for their breed, which they supply with pollen of various pollen ressources, they rely on a constant grass verge under the trees as a partial habitat - like many other insect species. When all insects of all until species level investigated animal groups are sorted by emergence to only native, only south-east European or both urban tree species, it was found out that the vast majority belogns to the crown fauna of both urban tree species groups (43 %), a third only occured at native trees and a quarter only occured at south-east European climate tree species (figure 4).


Conclusion and Outlook

Already young urban street trees displayed a high abundance of individuals and an amazing insect diversity in their crowns. South-east European urban tree species can therefore contribute to species biodiversity in tree crowns similar as the close related native species. In contrast to mono-species alleys, alleys should be comprised by mixed urban tree species to foster arthropod diversity and to counteract the expansion of more and more frequent, newly introduced plant diseases and pests. Linking grass verges (instead of single tree pits) are an important habitat of many crown living insects. To provide recommendations regarding biodiversity potentials of different urban tree species for municipalities, departements and associations for ecological conservations, further studies on the ecological value of non-resident urban tree species should be conducted.