Discussion and Conclusion
Successional process in the boxes favours tree hollow specialists
We hypothesized that since the artificial wood mould in the boxes should decrease over time, due to larval consumption and fungal activity, richness and abundance would also decrease. The wood mould decreased with almost 10 % every year, with 15-30 % the first three years and finally after ten years the wood mould had decreased with 73-83 % (95% CI). The hypothesis was only partly confirmed as the boxes experienced a decrease in species richness between year four and the final year but on the other hand had more specimens per species on average in the final year. In addition, all categories experienced a decrease in species richness from the fourth to the final year but surprisingly the specimens in the categories tree-hollow species, wood rot species and nest species increased with almost 40 %. Almost 9 out of 10 specimens in the final year were from these three categories which are strongly associated with wood mould hollows. One explanation for their dominance in the later stages of the succession could be that the artificial wood mould becomes more similar to real wood mould over the years. But as the volume wood mould decreases over the years so does the species richness. The remaining hollow tree specialists, however, takes advantage of the niche to the full extent, which increases their abundance. Consequently, the boxes develops from a varied to a more specific wood mould habitat.
In conclusion, the fact that 114 saproxylic beetle species and five pseudoscorpion species were detected to use the boxes of this ten year study, clearly shows that the boxes are sufficient as artificial habitats.
Five boxes were destroyed during the ten year field period. The metallic band where only attached to the back side of the box and thus the whole side could come loose. Therefore, should they be used in conservation management, the suspension of the boxes must be improved so that all sides are connected with the metallic band.
The placement of the boxes should also be considered. The boxes in this study were placed on the shadiest side of the trunks, to ensure a similar as well as stable condition for all boxes. This could be a disadvantage for colonization since sun-exposed trees have a greater species richness and species frequency. Furthermore, large stands of hollow oaks comprise a greater species richness. Consequently, the conservation value of a single box may be best in a sun-exposed placement and when arranged in a cluster.
There was a weak tendency for the dead hen to attract more species than the other substrates. Amongst the four different substrates that were added, a dead hen probably resembles the real conditions in a hollow better than the other substrates which increases the richness. The hollows are often colonized by birds and some of their offspring are often left for dead which a dead hen can mimic.
With placement alternations, box design changes and refilling of artificial wood mould the boxes could well be used as temporal and/or spatial stepping stones in a fragmented, habitat poor landscape prior to the development of real tree hollows.
Responsible for this page: Agneta Johansson
Last updated: 05/11/15