Breaking down complex Saproxylic communities: understanding sub-networks structure and implications to network robustness

Sex specificity of dispersal behaviour and flight morphology varies among tree hollow beetle species

BACKGROUND

Flight performance and dispersal behaviour can differ between sexes, resulting in sex-biased dispersal. The primary sex ratio of populations may also explain dispersal bias between sexes, as this bias may evolve with the primary sex ratio to reduce intrasexual competition. Although dispersal bias between sexes is relevant to population dynamics, there are few studies on sex-biased dispersal in insects. We studied the flight performance and dispersal behaviour of seven saproxylic beetle species associated with tree hollows from a sex perspective. We also analysed the possible coevolution of flight performance with the primary sex ratio.

METHODS

Wing loading and wing aspect ratio were used as measures of the flight performance of species and sexes. Dispersal behaviour was explored by analysing the frequency of each sex in interception traps versus the primary sex ratio obtained by tree hollow emergence traps using contingency tables and posthoc standardized residuals. A more active flight behaviour was expected for the sex with higher capture frequency in the interception traps. To explore the causes of flight performance bias between sexes, we searched for possible correlations between wing loading or wing aspect ratio and primary sex ratio using Pearson's correlation coefficient.

RESULTS

Wing loading and wing aspect ratio differed between species and sexes, with flight performance being higher in males than in females for four of the seven species analysed. Dispersal behaviour and flight performance matched in the case of Elater ferrugineus; males showed higher flight performance and were the most collected sex in the interception traps (more active flyers). In contrast, the higher flight activity of Cetonia carthami aurataeformis females was not correlated with a higher flight performance than that of males. Moreover, we found that a bias in the primary sex ratio towards females is often correlated with a decrease in female flight performance.

CONCLUSIONS

We stress that flight performance and dispersal behaviour of sexes do not always go hand in hand. Moreover, the relationship between the sex ratio and flight performance bias between sexes is not driven by competition within the most abundant sex. The inclusion of a sex perspective in insect dispersal studies would be useful to detect dispersal bias between sexes and its causes and would allow for further analysis of its effects on population dynamics.

Climate-induced tree senescence leads to a transient increase in reproductive success of a large woodpecker species

Climate change-induced mortality of trees is a concerning phenomenon for global forest ecosystems. The rapid decay and death of long-lived trees can significantly impact forest dynamics, with effects that transmit through ecological networks, becoming more evident in organisms occupying high trophic levels, such as large and specialized woodpecker species. However, understanding how populations of high trophic level species respond to climate change is still a challenge. In this study it was analyzed 32-year data of social groups of the Magellanic Woodpecker (Campephilus magellanicus) in North Patagonia, a region facing increasingly frequent droughts and increased temperatures. A positive trend in the size of woodpecker social groups as a response to climate-induced tree senescence was tested. A causal structural equation model examining climate- tree senescence- woodpecker relationships was used. Increasing nonlinear trends and positive interannual growth rates (>10%) for tree senescence and group size were found. Lowland forest sites had higher levels of tree senescence and more numerous social groups. The causal model supported the positive effect of mean temperature on tree senescence and the positive association of woodpeckers with tree senescence. These results provide evidence of a climate-induced increase in tree senescence that causes an increase in the size of woodpecker social groups. It is suggested that accelerated decay and mortality of trees in the northern Patagonian forests will decrease the stocks of deadwood in the long term, threatening the persistence of this large woodpecker species.

Saproxylic Cetoniidae (Coleoptera: Scarabaeoidea): A 'Females' World' or a Question of Dependence on Deadwood?

We explored the dependence of some Cetoniidae species on saproxylic environments and microhabitats in a Mediterranean oak forest by analyzing species collected using different kinds of traps-log emergence, hollow emergence, and interception traps-and the sex ratio of the species in each trap. Comparing the sex ratio of the species collected via emergence versus interception was useful to unravel the degree of dependence on saproxylic microhabitats. Among the species studied, Cetonia aurataeformis Curti, 1913 (Coleoptera: Cetoniidae) was the only obligate tree hollow inhabitant. Special attention should thus be paid to the maintenance of tree hollows for the species' conservation in Mediterranean forests. A gradient of dependence on tree hollows was established from the more dependent Protaetia (Potosia) cuprea (Fabricius, 1775) (Coleoptera: Cetoniidae) and Protaetia (Potosia) opaca (Fabricius, 1787) (Coleoptera: Cetoniidae) to the less dependent Protaetia (Netocia) morio (Fabricius, 1781) (Coleoptera: Cetoniidae). All the latter species can be considered facultatively dependent, to varying degrees, on tree hollows. By contrast, the saproxylic affinity of Protaetia (Netocia) oblonga (Gory and Percheron, 1833) (Coleoptera: Cetoniidae), Tropinota squalida (Scopoli, 1783) (Coleoptera: Cetoniidae) and Oxythyrea funesta (Poda, 1761) (Coleoptera: Cetoniidae) was doubtful. Generally, the sex ratio of the studied species was female-biased. A possible explanation may be local male competition for females, suggesting the Cetoniinae is a female world. However, the range of difference in the female-biased sex ratio among species suggests it is important to explore other possible causes, such as differences in dispersal abilities.

Spatio-temporal variation of Cerambycidae-host tree interaction networks

Despite its high ecological importance, the commensal interactions at community level are poorly studied. In tropical dry forests (TDF) there is a great diversity of species adapted to the high seasonality that characterizes them; however, little is known regarding how the spatial and temporal availability of resources generates changes in the pattern of commensal interactions. We experimentally studied changes in the diversity, composition, and pattern of interactions in spatio-temporal associations between the saproxylophagous beetles and their host trees in a TDF in Morelos, Mexico. A total of 65 host tree species were selected, from which 16 wood sections were obtained per species. These sections were exposed in the field to allow oviposition by the cerambycids under four different (spatio-temporal) treatments. We analyzed the network structure and generated indices at species level (i.e., specialization, species strength, and effective partners) and those related to physical characteristics of the wood (hardness and degradation rate) and the cerambycids (body size). In total, 1,323 individuals of 57 species of cerambycids emerged. Our results showed that, independently of the space and time, the network presented a nested and modular structure, with a high specialization degree and a high turnover of cerambycid species and their interactions. In general, we found that the cerambycids are mostly associated with softwood species with a lower decomposition rate of wood, as well as with the most abundant host species. The commensalistic interactions between the cerambycids and their host trees are highly specialized but are not spatio-temporally static. The high turnover in the interactions is caused by the emergence patterns of cerambycids, which seem to restrict their use to certain species. The knowledge of the spatio-temporal variation in Cerambycidae-host tree interactions allows us to predict how environmental and structural changes in the habitat can modify the species ensemble, and therefore its interactions.

Contrasting functional structure of saproxylic beetle assemblages associated to different microhabitats

Saproxylic beetles depend on diverse microhabitats that are exploited by different species assemblages. We focused on anlyse the functional trait patterns and functional diversity components of two main assemblages that were collected with window traps (WTs) and hollow emergence traps (HETs) respectively, between three protected areas of the Iberian Peninsula. For that we measured phenological, physiological, morphological, and ecological traits. Results showed that the main microhabitats exploited by each assemblage (WT and HET) constrain most of the morphological traits and trophic guilds. In addition, relative elytra length and predator guild, together with adult activity period, responded to differences at the habitat level (among study areas). HET assemblages were less taxonomically diverse but more functionally diverse than those of WTs, enhancing the functional relevance of tree hollows. Additionally, niche filtering dominated WT assemblages, which were characterised by a narrower functional space and a higher redundancy. In contrast, in the HET assemblages the coexistence of functionally dissimilar species is driven by the niche heterogeneity. HET and WT assemblages differed in the functional space occupied by each within areas, but both assemblages reflected coincident patterns among areas that pointed to a reduction of functional space with management.

Diversity and deadwood-based interaction networks of saproxylic beetles in remnants of riparian cloud forest

We studied the saproxylic beetle community inhabiting deadwood in remnants of riparian cloud forests in “La Antigua” basin, in central Veracruz (Mexico). We assessed the influence of deadwood features (tree species, trunk position, trunk diameter, trunk volume and decomposition stages) on saproxylic beetle diversity. In order to assess the stability of beetle species-deadwood interactions, we also analyzed the ecological networks structure. A total of 63 deadwood trunks, belonging to four tree species, were sampled by standardized hand-collection throughout well-preserved remnants of riparian cloud forest. We found that tree species and deadwood decay stage are the main drivers that determine the diversity and stability of saproxylic beetle species interactions. Our results indicate that Quercus corrugata is the main tree species in terms of maintaining the significantly highest saproxylic beetle diversity, but with no stable interactions (saproxylic beetle-deadwood). A nested network structure was detected for Clethra mexicana and Liquidambar styraciflua, with a pool of core (generalist) saproxylic beetle species. We observed that beetle diversity from the early and late deadwood stages comprises distinct assemblages and the four stages of decomposition showed a nested network structure. During deadwood succession, community composition and guilds changed among networks; the early successional stage had more specialized xylophagous beetles, while other guilds (mycophagous, saprophagous and zoophagous) arrive later and become the core species in the advanced stages of decomposition networks. Heliscus tropicus (Passalidae) is a key species constituting the core of all of the networks and could be considered an ecosystem engineer in cloud forests. By exploring links between saproxylic beetles and deadwood characteristics, we can further our understanding of species interaction in order to develop management strategies oriented towards the protection of species and their habitats in this threatened ecosystem.

Temporal Shifts and Cactus-Beetle Networks in an Intertropical Semiarid Zone in Mexico

Cactus-dominated semiarid scrubland covers 40% of Mexican territory. This ecosystem is highly dynamic and undergoes drastic changes caused by seasonality. These temporal changes influence saprophagous insect communities associated with Cactaceae species. In this study, we analyzed the beetle community associated with decayed cactus species at the Barranca de Metztitlán Biosphere Reserve in central Mexico. We compared the diversity and composition of beetle communities in rainy and dry seasons; moreover, the network architecture of cactus-beetle interactions was examined. High dominance and abundance were detected in rainy assemblages, whereas the dry season had less abundance but more ecological diversity. A nested structure was found between individual cactus species and beetle species, as well as in an intrapopulation network between fragments of the columnar cacti Isolatocereus dumortieri (Scheidw.) Backeb. (Cactaceae), and beetle species for both seasons (rainy and dry). This finding shows more generalist than specialist beetle species inhabiting cactus species. Further research is still needed to understand whether the presence of these beetle species is determined by microhabitat conditions or the abundance of prey associated with decayed cacti. This is the first step in untangling the complex interactions among cactus-beetle species involved in the decomposition process of cacti in semiarid environments. This study provides evidence of temporal shifts in abundance and diversity patterns of these beetles associated with decayed cacti; furthermore, we did not detect an influence of seasonality on the structure of cactus-beetle interactions.

Unraveling Saproxylic Insect Interactions in Tree Hollows from Iberian Mediterranean Forest

Tree hollows are complex microhabitats in which a variety of abiotic and biotic factors shape the community assembly of saproxylic insects. Detecting non-random species co-occurrence patterns is a fundamental goal in ecology in order to understand the assembly mechanisms of communities. We study association patterns of species of Coleoptera and Diptera (Syrphidae), belonging to different trophic guilds, on 72 tree hollows at a local and regional scale in three protected areas in Mediterranean forests using a fixed-fixed null model. Our matrix-level analysis shows a tendency for segregation in species association (species exclusion) at the regional and site levels. However, the high complexity of tree-hollow habitats, offering different resources for a more or less specialized fauna, makes it difficult to prove competition interactions. Indeed, pairwise analysis shows a dominance of non-random aggregation patterns (species coexistence) at the local and regional levels. Both aggregation and segregation of non-random patterns were more common among species from different trophic guilds than within the same guilds, with predators being a common denominator for a high percentage of the inter-guild pairs. Our results suggest that predation and facilitation interactions, together with habitat segregation, are the main factors shaping tree-hollow assemblages, while competition seems to be less important. We conclude that species interactions take an important part of the process of assemblage structuration and special attention should be paid to 'ecosystem engineers' and threatened species in the conservation of tree hollow assemblages.

Trophic level, successional age and trait matching determine specialization of deadwood-based interaction networks of saproxylic beetles

The specialization of ecological networks provides important insights into possible consequences of biodiversity loss for ecosystem functioning. However, mostly mutualistic and antagonistic interactions of living organisms have been studied, whereas detritivore networks and their successional changes are largely unexplored. We studied the interactions of saproxylic (deadwood-dependent) beetles with their dead host trees. In a large-scale experiment, 764 logs of 13 tree species were exposed to analyse network structure of three trophic groups of saproxylic beetles over 3 successional years. We found remarkably high specialization of deadwood-feeding xylophages and lower specialization of fungivorous and predatory species. During deadwood succession, community composition, network specialization and network robustness changed differently for the functional groups. To reveal potential drivers of network specialization, we linked species' functional traits to their network roles, and tested for trait matching between plant (i.e. chemical compounds) and beetle (i.e. body size) traits. We found that both plant and animal traits are major drivers of species specialization, and that trait matching can be more important in explaining interactions than neutral processes reflecting species abundance distributions. High network specialization in the early successional stage and decreasing network robustness during succession indicate vulnerability of detritivore networks to reduced tree species diversity and beetle extinctions, with unknown consequences for wood decomposition and nutrient cycling.

Influence of Traditional Management and Environmental Variables on Mediterranean Saproxylic Beetle Assemblages

Mediterranean oak forests of the Iberian Peninsula host a great diversity of saproxylic beetles. For centuries, humans have carried out traditional management practices in this area, at both habitat and tree level, causing changes in forest structure. The aim of this study was to evaluate the anthropic effect of these traditional practices on saproxylic beetle diversity by measuring a set of environmental variables related to forest structure at both plot and tree level. Fauna was collected using window traps over a period of 12 mo. Multiple regression procedures showed which variables significantly affected the diversity of the studied assemblage. Our results demonstrated that the different metrics used to assess the diversity of assemblages responded variably depending on the management strategies applied and the level at which they were carried out. Certain management practices that disrupted the landscape from its natural state, such as the introduction of livestock or the local removal of particular trees, maximized species richness but, nevertheless, had a negative effect on the rest of diversity metrics analyzed. However, other practices such as pollarding, which involves the suppression of the main branch of the tree, had a positive effect on all diversity metrics evaluated as it promoted the formation of potential microhabitats for saproxylic fauna. We concluded that not all types and degrees of traditional forest management favor saproxylic beetle diversity and that different diversity metrics should be taken into consideration in future strategies for the protection and conservation of this fauna.

Bacteria and Hoverflies (Diptera: Syrphidae) in Tree Hollows From the Iberian Mediterranean Forest

Saproxylic insect communities inhabiting tree hollows in Mediterranean forests depend on a combination of physical characteristics and interactions occurring between community member species. Despite the need to preserve these organisms, little is known about their interrelationships, in particular those relationships between saproxylic insects and microbiota occurring in these microhabitats. In tree hollows of Quercus rotundifolia Lamark that hold water and contain dead leaves, abundant microbial populations can be found. Developing on them are the larvae of Mallota dusmeti Andréu, 1926 (Diptera: Syrphidae), a vulnerable species (IUCN category: Marcos-García and Quinto 2011). This study provides the first data on the microbiota living inside the gut of the larvae of M. dusmeti, as well as the microbiota in the hollow where these larvae develop. Bacteria were identified by amplification and partial sequencing of the V1-V3 regions and the complete nucleotide sequence of 16S rRNA genes. We found eight species of bacteria living in tree hollows and three species in the gut of M. dusmeti larvae: Bacillus cereus, Bacillus toyonensis, and Lysinibacillus sphaericus. The filter-feeding mechanism characteristic of M. dusmeti larvae is selective in enabling ingestion of bacteria only above 2.1 µm in diameter.

Saproxylic Beetle Assemblage Selection as Determining Factor of Species Distributional Patterns: Implications for Conservation

The knowledge of the distributional patterns of saproxylic beetles is essential for conservation biology due to the relevance of this fauna in the maintenance of ecological processes and the endangerment of species. The complex community of saproxylic beetles is shaped by different assemblages that are composed of species linked by the microhabitats they use. We evaluate how different the species distribution patterns that are obtained can be, depending on the analyzed assemblage and to what extent these can affect conservation decisions. Beetles were sampled using hollow emergence and window traps in three protected areas of the Iberian Peninsula. Species richness, composition, and diversity turnover were analyzed for each sampling method and showed high variation depending on the analyzed assemblage. Beta diversity was clearly higher among forests for the assemblage captured using window traps. This method collects flying insects from different tree microhabitats and its captures are influenced by the forest structuring. Within forests, the assemblages captured by hollow emergence traps, which collect the fauna linked to tree hollows, showed the largest turnover of species, as they are influenced by the characteristics of each cavity. Moreover, the selection of the forest showing the highest species richness strongly depended on the studied assemblage. This study demonstrates that differences in the studied assemblages (group of species co-occurring in the same habitat) can also lead to significant differences in the identified patterns of species distribution and diversity turnover. This fact will be necessary to take into consideration when making decisions about conservation and management.

Mother knows the best mould: an essential role for non-wood dietary components in the life cycle of a saproxylic scarab beetle

Beetles living in tree hollows can feed on a wealth of substrates-e.g. the rotten wood surrounding the cavity, leaf humus falling into the hole, and larval frass accumulating in the cavity. In this paper, we examine the role of these main substrates in Finnish tree hollows in the female preference and larval growth of the hermit beetle Osmoderma barnabita. We rear larvae on diets consisting of wood material (as affected by brown-rot), leaf humus, and larval frass, in varying proportions. To pinpoint the effects of microbes, we contrast larval growth on sterilized versus unsterilized larval frass, and on pure mycelia of the cavity-creating fungus Laetiporus sulphureus. Finally, to relate larval performance to female preference, we examine female choice among the three main substrates used in the larval rearings. We found that the presence of one substrate modifies the influence of another, with larval growth and survival being highest on pure leaf humus. Microbes came with both positive and negative impacts on larval performance, as larvae grew quicker on unsterilized than on sterilized larval frass, but were also struck by higher mortality. On pure fungal mycelia, larvae neither grew nor survived. Female preference reflected larval performance, with leaf humus being preferred over other resources. Overall, our study suggests that organisms inhabiting tree holes may be dependent on subsidies entering the cavity from outside, and that ovipositing females may specifically respond to the presence of such subsidies. Thus, the quality of a microhabitat may depend on what enters it from outside.

Association patterns in saproxylic insect networks in three Iberian Mediterranean woodlands and their resistance to microhabitat loss

The assessment of the relationship between species diversity, species interactions and environmental characteristics is indispensable for understanding network architecture and ecological distribution in complex networks. Saproxylic insect communities inhabiting tree hollow microhabitats within Mediterranean woodlands are highly dependent on woodland configuration and on microhabitat supply they harbor, so can be studied under the network analysis perspective. We assessed the differences in interacting patterns according to woodland site, and analysed the importance of functional species in modelling network architecture. We then evaluated their implications for saproxylic assemblages' persistence, through simulations of three possible scenarios of loss of tree hollow microhabitat. Tree hollow-saproxylic insect networks per woodland site presented a significant nested pattern. Those woodlands with higher complexity of tree individuals and tree hollow microhabitats also housed higher species/interactions diversity and complexity of saproxylic networks, and exhibited a higher degree of nestedness, suggesting that a higher woodland complexity positively influences saproxylic diversity and interaction complexity, thus determining higher degree of nestedness. Moreover, the number of insects acting as key interconnectors (nodes falling into the core region, using core/periphery tests) was similar among woodland sites, but the species identity varied on each. Such differences in insect core composition among woodland sites suggest the functional role they depict at woodland scale. Tree hollows acting as core corresponded with large tree hollows near the ground and simultaneously housing various breeding microsites, whereas core insects were species mediating relevant ecological interactions within saproxylic communities, e.g. predation, competitive or facilitation interactions. Differences in network patterns and tree hollow characteristics among woodland sites clearly defined different sensitivity to microhabitat loss, and higher saproxylic diversity and woodland complexity showed positive relation with robustness. These results highlight that woodland complexity goes hand in hand with biotic and ecological complexity of saproxylic networks, and together exhibited positive effects on network robustness.

PASSIFOR: A reference library of DNA barcodes for French saproxylic beetles (Insecta, Coleoptera)

Saproxylic beetles – associated with dead wood or with other insects, fungi and microorganisms that decompose it – play a major role in forest nutrient cycling. They are important ecosystem service providers and are used as key bio-indicators of old-growth forests. In France alone, where the present study took place, there are about 2500 species distributed within 71 families. This high diversity represents a major challenge for specimen sorting and identification.

The PASSIFOR project aims at developing a DNA metabarcoding approach to facilitate and enhance the monitoring of saproxylic beetles as indicators in ecological studies. As a first step toward that goal we assembled a library of DNA barcodes using the standard genetic marker for animals, i.e. a portion of the COI mitochondrial gene. In the present contribution, we release a library including 656 records representing 410 species in 40 different families. Species were identified by expert taxonomists, and each record is linked to a voucher specimen to enable future morphological examination. We also highlight and briefly discuss cases of low interspecific divergences, as well as cases of high intraspecific divergences that might represent cases of overlooked or cryptic diversity.