Cavity size

This study has not been able to show any significant effect by cavity size on differences between beetle composition in different oaks. No clear correlation was found between species diversity and cavity size, but another study showed that ɑ-diversity increased with increasing size of hollow entrance. Species diversity decreased with increasing decomposition of wood mould and at medium stage of decomposition they found the highest number of saproxylic beetles. Specialized species may thrive in heavily decomposed wood mould hence, ꞵ-diversity might increase. In a study from 2016 where they investigated the effect of wooden boxes, mimicking decaying wood, species richness decreased during a 10-year study which they explained might be due to a decreasing volume of wood mould available. Even so, there was an increase in the total abundance of species and over time they showed that specialized hollow tree species becomes more common. This was explained by the fact that artificial wood mould becomes more realistic as times passes. One study showed that cavity size and trunk circumference correlated with the amount of available wood mould but no wood mould was investigated and hollow sizes inside oaks were not measured which might cause some loss of significant results regarding the cavities.

Based on the results from this study, I think that the size of cavity entrance is not the cavity parameter that is of importance. Instead, significant results, regarding cavity characteristics, might come from parameters like size of hollows inside trees or the amount and quality of wood mould. A factor that might blur results in this study regarding cavity size and its effect on beetle fauna (at least for guilds like hollow and nest) is the distance between my studied oaks. Some trees were standing very close together while others were standing far from other oaks and cavity density has been shown to be important for biodiversity of saproxylic beetles. In this case though, cavity density could influence what species that were found in each tree, e.g. oaks with small cavities had species that live in larger cavities nearby. In the results, only the non-significant effect by general flying activity on species number and individuals is shown, not possible interactions from nearby trees regarding what specific species that were found.

Forest regrowth

What this study could confirm is the importance of forest regrowth and trunk circumference that were shown to have the largest impact on differences in beetle compositions. The importance of large oaks, trunk circumference, and forest regrowth for species richness has also been shown in a previous study. They also showed that high levels of surrounding regrowth had a negative effect on species which strengthens my findings. Diversity on landscape level may increase when both pasture oaks and forest oaks are available. Another study found that species composition differed between the two types. As management they suggest periodic removal of trees standing too close to forest oaks in a way to enhance the value of these oaks. Also, oaks in stand with lower density (<100 trees/ha) have been shown to have lower mortality (1.24 %) than oaks in dense stands (>100 trees/ha, 3.21 %). Other studies describe the importance of sun exposed, larger trunks for rare saproxylic beetles. The relevance of large downed trunks for beetle diversity has also been confirmed by other studies.

Combining these data with my results regarding forest regrowth it is possible to understand the importance of clearcutting in oak stands, at least smaller bushes and young pioneer species like hazel and aspen. These oaks might, like mentioned above, be important habitats for saproxylic beetles for a long time, even as dead logs. Oaks in areas with many other oaks might experience canopy cover by its neighbors which could be more difficult to do anything about, especially when this study shows how little of the total variance that is explained by several parameters. Without knowing more about what is affecting beetle composition, removing some oaks might be bad for the beetle community. Many species which seems to correlate with the presence of O. eremita and in areas where hermit beetles were found, species richness of other saproxylic beetles often was higher. This and that it is easy to identify, makes O. eremita useful as both an indicator species and umbrella species. Even if results from this study showed no difference in species number depending on presence of O. eremita (p>0.05), this study did not focus on this beetle. Hence, O. eremita is probably still worth focusing on in conservation work.

Even though explanation level of this study on species composition is relatively low, high temperatures during the summer of 2018 (a mean temperature of 17.8 °C May-August compared with 2017 with a mean of 14.55 °C) should enable more flying activity in general amongst species. Hence, the weather is probably not the problem when trying to analyze why this level of explanation is low. Instead, there are probably many explanatory variables that needs to be included in studies similar to this one.

In the model using the amount of species found from each guild, fungi and branch species were positively affected by forest regrowth and canopy cover respectively, which indicates that they are more shade favored guilds. However, no environmental parameters were significant which means there are no certain conclusions that can be drawn.

Individual species

I also checked for correlations between individual species and my explanatory variables using general linear models. Ampedus cardinalis and Ampedus hjorti are two click beetles living as predators in brown rot in oaks. Both were positively correlated with trunk circumference (p=0.015 and p=0.002 respectively). My results also indicate that A. hjorti is favored by increasing canopy cover i.e. shady environments (p=0.014). One species (Orchesia micans) was shown to be favored by smaller trunks (p=0.019).

Euglenes oculatus, belonging to the Aderidae family and develops in oak cavities, was significantly affected by cavity size (p=0.049) and the estimate shows that E. oculatus is favored by larger cavities. Another species, significantly correlated with cavity size, was Quedius dilatus (p=0.014) but in this case, smaller cavities was preferred. Q. dilatus is a predator in the Staphylinidae family that prey on oak trunks and in cavities.

Both A. cardinalis and A. hjorti was sun-loving in a study from 1959 but indifferent according to a more recent study. Strangely, my results indicates that A. hjorti is more shade-loving (p=0.014 with an estimate of 0.069) but A. cardinalis was still indifferent, i.e. no significant results regarding canopy cover. In the study from 1959, O. micans was indifferent regarding sun and shade, while in the study from 2000 it was found to be sun-loving. In my results, no correlation between O. micans and canopy cover was found. These findings seem to only enhance uncertainty when it comes to conservation work due to various results regarding sun preferences.

Ampedus cardinalis has been shown to be more common in cavities that are situated higher up on trunks, but in my results no such correlation was found.

When having no specific model that can explain species composition and no clear picture of where to find a lot of the species in this study, it is important to remember that this makes conservation work both harder but also easier. Like mentioned before, conservation work has limited resources and with these results in mind, decision making has not been easier. There are no specific trees that needs saving and to enhance the possibility of surviving for the majority of saproxylic beetles, large areas with heterogeneity amongst oaks needs to be preserved. Studies investigating dead wood also show that diversity amongst logs and snags is important for beetles favored by these habitats. If diversity is lost, we might lose species that, without our knowledge, were in fact depending on a certain variable which in more oriented conservation could have been the main focus. On the other hand, without any certain model to follow during conservation work, all hollow oaks are important. Regardless if all oaks have tiny cavities or large ones close to the ground, all might serve as important habitats. More important however, is the fact that trunk circumference and forest regrowth were significant. The majority of the beetle species in this seem to prefer open landscapes with large oaks but there still are exceptions. Describes how all oaks (>400 years old) have some sort of cavity while only 1 % of younger oaks have cavities. To summarize, old large oaks in an open area are what need protection and keep younger ones that, with time, can replace older oaks when they die.