The diversity of saproxylic beetles’ from the Natural Reserve Voievodeasa forest, North-Eastern Romania

Abstract

Nature reserves harbor considerable richness and diversity of saproxylic organisms since dead wood is preserved in situ , this being also the case of Voivodeasa beech-spruce-fir forest in North-Eastern Romania, the area investigated under the present research. The employed sampling design consisted in 20 flight interception traps placed in a square grid (100 x 100 m). The beetle content of the traps was collected every two weeks, from May to September. The number of intercepted beetles reached 13,554 individuals, of which 7,174 individuals (assigned to 336 species placed in 217 genera and 58 families) were identified at species level and 6,390 individuals, at genus or family level. The majority of the identified species were obligate saproxylic species (217 species). However, the unexpected high species richness corresponded to an area with modest representation of deadwood due to previous status of commercial forest. The identified beetles were members of different habitat-guilds depending on what type of substrate they colonize: recently dead wood (23%), decomposed dead wood (41%), wood inhabiting fungi (34%) and tree-hollow detritus (2%). According to their trophic position, the identified saproxylic beetles pertained to the following guilds: xylophagous (40%), mycetophagous (39%), predatory (14%), and species relying on other food resources (5%). Non-metric Multidimensional Scaling ordination using Bray-Curtis distance, performed to compare the saproxylic beetles’community at different sampling dates across the vegetation season suggested that species turn-over took place in the time window of one month. The analysis of abundance/dominance structure of the saproxylic beetle community employing classical community indices (abundance, dominance, constancy and Dzuba index) showed that 7% of the species were abundant, and 68% were rare, Fisher’s log series fitting the distribution of abundances.  In terms of constancy, 11% were eu-constant and 62% - accidental species. Considering the dominance, only 0.4% represented eu-dominant species while 89% were sub-recedent species. According to Dzuba ecological significance index, four species were characteristic for the saproxylic beetle community: Ptilinus pecticornis, Enicmus rugosus, Cis rugulosus and Taphorychus bicolor , most of the identified species being accessory or accidental (33% and 65%). PCA ordination performed on abundance constancy, dominance and ecological significance scores, showed that all indices were highly correlated with PC1 and further testing using multivariate regression with dominance held as independent variable, showed high correlation among indices. Species ranked according to these indices as rare, accidental or accesory clustered separatelly in the ordination space while dominant and eu-dominant species established another distinct cluster. Species richness estimation based on Chao 1 nonparametric index exceeded the observed richness of saproxylic beetles (estimated number of species = 266.67, observed number of species = 217). Inspection of the individual-based rarefaction curve showed that the number of species did not stabilize after pooling all captured individuals in all samples across the sampling period. The observed richness corresponds to the case of hyperdiverse communities where sampling never leads to the stabilization of species richness under a realistic sampling scheme. Species diversity: The rank-abundance curves constructed on aggregated catches and catches of saproxylic beetles at different sampling dates showed variation in terms of species richness and eveness, with species re-ordering across the sampling period. The diversity profiles constructed on Shannon, Gini-Simpson, Berger-Parker and eveness indices for the pooled sample and for separate samples across the vegetation season indicated  the aggregated saproxylic community as highly diverse and also highly uneven, with rich representation of rare species, dominated by few abundant species contrasting the community at the end of the vegetation season characterized by high eveness and few species. Network structure: We assembled four bipartite, unweighted, and undirected networks to approach the temporal changes across the sampling period extended over one vegetation season using the specific metrics derived from the community network analysis: connectance, nestedness, average degree, modularity. The networks corresponded to aggregated community of saproxylic beetles and to xylophagous, mycetophagous and predatory guilds. The topology of beetles’ community and of the three main trophc guilds networks linked to time sequences are characterized by high connectance, high nestedness and modularity, with the exception of the mycetophgous sub-network not displaying significant modularity. The topologies reflect the temporal partition of the ecological niche, with nestedness and modularity relating to species’phenologies during the adult stage. Among the identified species, 13% indicate high degree of naturalness of the Voievodeasa forest: of these, 9 species are old-growth forests relicts. Other 62 species are included in the Red List of European Saproxylic Beetles of which five are near threatened ( Protaetia fieberi, Cucujus cinnaberinus, Crepidophorus mutilatus, Ceruchus chrysomelinus, Prostomis mandibularis ), Ischnodes sanguinolentus is vulnerable and Rhysodes sulcatus is an endangered species. During the study, two Coleoptera species, new for Romanian insect fauna were identified: Denticollis interpositus Roubal, 1941 and Hylis procerulus (Mannerheim 1823).