Beetles in hollow oaks : the effects of traits on community structure

Abstract

Habitat fragmentation and habitat loss have long been acknowledged as two of the most important drivers of biodiversity loss all over the world. A way to preserve biodiversity is to protect the remaining hotspots. Hotspots can be large-scale areas representing the rich biodiversity of a biographic region. Yet there are also local hotspots which house high richness of threatened species that depend on the unique qualities of smaller habitat patches. Hollow oaks represent one important local hotspot habitat because they contain important dead wood that houses many threatened saproxylic species.

In Norway, the number of hollow oaks is in decline, and most of the remaining trees are scattered in small isolated patches across the southern part of the country. Within Norway, there are also regional differences in the species richness patterns of saproxylic beetles inhabiting hollow oaks. The western hollow oak region represents a different region than the east with tall mountains and fjords leading to a different landscape structure and a different climate pattern than the eastern hollow oak region. Species richness of hollow oak specialist species has been shown to be lower in the western region, but the understanding of the causes behind this east-west decline gradient remain limited.

How these oak-associated beetle communities respond to both isolation effects and geographical differences likely depends on their morphological traits, which influence their ecology. Species with good dispersal abilities have a greater chance to reach isolated habitats that are more scattered in the landscape. Further, the extinction risk of beetles has been shown to depend on body size. Smaller species are more resilient to isolation effects, because of their broader habitat breadth, higher reproduction rate and higher abundance. The focus in this study was therefore to investigate the effects of traits on community structure of oak associated beetles, including red-listed beetles. I have therefore chosen relative wing length as a proxy for dispersal ability and body size as a proxy for extinction risk of oak associated beetle communities. Traits were summarized as community weighed means, as I wanted to see a generalized pattern of how trait diversity of all oak- associated beetles respond to isolation effects and regional effects. I used a dataset in which beetles were sampled from eleven solitary hollow oaks and four groups of clustered hollow oaks to investigate isolation effects. These sites were distributed across two geographical zones to represent the eastern and western distribution of hollow oak habitats in Norway. This dataset included 116 saproxylic oak-associated beetle species (1706 individuals), of which 8 species (938 individuals) were red-listed. Mean relative wing length and mean body size did not differ significantly by habitat isolation status or between western and eastern regions. However, in the western region, species richness was 35% lower (p = 0.01) and red-listed species richness was 92% lower (p = <0.0001) relative to the eastern region. Richness did not differ significantly by isolation status.

A relatively small sample size likely made detecting any differences among highly variable beetle communities difficult. My results showed an east/west difference of species richness of all and red-listed beetle species associated with hollow oaks. I encourage researchers to also investigate species composition, as that was not included in this study design. The importance of region vs. isolation effects for richness patterns implies that region has a great impact on the variation in community structure of saproxylic beetles. On a small scale, conservation planning must focus on habitats with high density of large hollow oaks and promote occurrence of dead wood. Planning on large scale must focus on preventing further increasing the distance between habitat patches, as it is important to protect the large variation of all saproxylic oak-associated communities.