What does a threatened saproxylic beetle look like? Modelling extinction risk using a new morphological trait database

Habitat requirements of deadwood-dependent invertebrates that occupy tree hollows

Tree hollows support a specialised species-rich fauna. We review the habitat requirements of saproxylic (= deadwood dependent) invertebrates which occupy tree hollows. We focus on studies quantifying relationships between species occurrence patterns and characteristics of tree hollows, hollow trees, and the surrounding landscape. We also explore the processes influencing species occurrence patterns by reviewing studies on the spatio-temporal dynamics of populations, including their dispersal and genetic structure. Our literature search in the database Scopus identified 52 relevant publications, all of which were studies from Europe. The dominant taxonomic group studied was beetles. Invertebrates in hollow trees were often more likely to be recorded in trees with characteristics reflecting a large amount of resources or a stable and warm microclimate, such as a large diameter, large amounts of wood mould (= loose material accumulated in the hollows mainly consisting of decaying wood), a high level of sun exposure, and with entrance holes that are large and either at a low or high height, and in dry hollows, with entrances not directed upwards. A stable microclimate is probably a key factor why some species of saproxylic invertebrates are confined to tree hollows. Other factors that are different in comparison to downed dead wood is the fact that hollows at a given height from the ground provide shelter from ground-living predators, that hollows persist for longer, and that the content of nutrients might be enhanced by the accumulation of dead leaves, insect frass, and remains from dead insects. Several studies have identified a positive relationship between species occupancy per tree and the amount of habitat in the surrounding landscape, with a variation in the spatial scale at which characteristics of the surrounding landscape had the strongest effect over spatial scales from 200 to 3000 m. We found empirical support for the extinction threshold hypothesis, which predicts that the frequency of species presence per tree is greater if a certain number of trees are aggregated into a few large clusters of hollow trees rather than distributed among many small clusters. Observed thresholds in species occurrence patterns can be explained by colonisation-extinction dynamics, with species occupancy per tree influenced by variation in rates of immigration. Consistent with this assumption, field studies suggest that dispersal rate and range can be low for invertebrates occupying tree hollows, although higher in a warmer climate. For one species in which population dynamics has been studied over 25 years (Osmoderma eremita), the observed population dynamics have characteristics of a "habitat-tracking metapopulation", as local extinctions from trees occur possibly because those trees become unsuitable as well as due to stochastic processes in small populations. The persistence of invertebrate fauna confined to tree hollows may be improved by prolonging the standing life of existing hollow trees. It is also important to recruit new generations of hollow trees, preferably close to existing larger groups of hollow trees. Thus, the spatio-temporal dynamics of hollow trees is crucial for the invertebrate fauna that rely upon them.

Alternative measures of trait-niche relationships: A test on dispersal traits in saproxylic beetles

Functional trait approaches are common in ecology, but a lack of clear hypotheses on how traits relate to environmental gradients (i.e., trait-niche relationships) often makes uncovering mechanisms difficult. Furthermore, measures of community functional structure differ in their implications, yet inferences are seldom compared among metrics. Community-weighted mean trait values (CWMs), a common measure, are largely driven by the most common species and thus do not reflect community-wide trait-niche relationships per se. Alternatively, trait-niche relationships can be estimated across a larger group of species using hierarchical joint species distribution models (JSDMs), quantified by a parameter Γ. We investigated how inferences about trait-niche relationships are affected by the choice of metric. Using deadwood-dependent (saproxylic) beetles in fragmented Finnish forests, we followed a protocol for investigating trait-niche relationships by (1) identifying environmental filters (climate, forest age, and deadwood volume), (2) relating these to an ecological function (dispersal ability), and (3) identifying traits related to this function (wing morphology). We tested 18 hypothesized dispersal relationships using both CWM and Γ estimates across these environmental gradients. CWMs were more likely than Γ to show support for trait-niche relationships. Up to 13% of species' realized niches were explained by dispersal traits, but the directions of effects were consistent with fewer than 11%-39% of our 18 trait-niche hypotheses (depending on the metric used). This highlights the difficulty in connecting morphological traits and ecological functions in insects, despite the clear conceptual link between landscape connectivity and flight-related traits. Caution is thus warranted in hypothesis development, particularly where apparent trait-function links are less clear. Inferences differ when CWMs versus Γ estimates are used, necessitating the choice of a metric that reflects study questions. CWMs help explain the effects of environmental gradients on community trait composition, whereas the effects of traits on species' niches are better estimated using hierarchical JSDMs.

Global change drives phenological and spatial shifts in Central European longhorn beetles (Coleoptera, Cerambycidae) during the past 150 years

Temperature increases and land-use changes induce altered annual activity periods of arthropods. However, sufficiently resolved long-term data sets (> 100 years) are mostly missing. We use a data set of longhorn beetle records (71 species) collected in Luxembourg 1864-2014. Increase of annual temperatures was significantly correlated with an earlier annual appearance. Forty-four species present before and after 1980 appeared on average 8.2 days earlier in the year in the more recent period. Since 1950, the estimated shift was 0.26 days per year. Increase of temperature in spring (March-June) preponed the first appearance of beetles by on average 9.6 days per 1 °C. We found significant changes in the composition of beetle communities, with a net gain in species richness during the last 40 years. Eleven species recorded only after 1997 were characterized by comparatively early annual appearance. Smaller beetles tended to appear earlier in the year in comparison to large-bodied species. Shifts in phenology did not correlate with species Red List status. As also demonstrated by our data, climate change in general affects insect phenologies and changes species composition. However, land-use change has taken place in parallel with climate change. Both aspects of global change are influencing the changes in longhorn beetle occurrences in Luxemburg in their combination. This might be most clearly reflected in the strong decrease of species with continental climate niches dwelling in old-growth deciduous forests that apparently are threatened by the loss of these habitats and increasing spring temperatures.

Quantifying and estimating ecological network diversity based on incomplete sampling data

An ecological network refers to the ecological interactions among sets of species. Quantification of ecological network diversity and related sampling/estimation challenges have explicit analogues in species diversity research. A unified framework based on Hill numbers and their generalizations was developed to quantify taxonomic, phylogenetic and functional diversity. Drawing on this unified framework, we propose three dimensions of network diversity that incorporate the frequency (or strength) of interactions, species phylogenies and traits. As with surveys in species inventories, nearly all network studies are based on sampling data and thus also suffer from under-sampling effects. Adapting the sampling/estimation theory and the iNEXT (interpolation/extrapolation) standardization developed for species diversity research, we propose the iNEXT.link method to analyse network sampling data. The proposed method integrates the following four inference procedures: (i) assessment of sample completeness of networks; (ii) asymptotic analysis via estimating the true network diversity; (iii) non-asymptotic analysis based on standardizing sample completeness via rarefaction and extrapolation with network diversity; and (iv) estimation of the degree of unevenness or specialization in networks based on standardized diversity. Interaction data between European trees and saproxylic beetles are used to illustrate the proposed procedures. The software iNEXT.link has been developed to facilitate all computations and graphics. This article is part of the theme issue 'Detecting and attributing the causes of biodiversity change: needs, gaps and solutions'.

Insect decline in forests depends on species' traits and may be mitigated by management

Insects are declining, but the underlying drivers and differences in responses between species are still largely unclear. Despite the importance of forests, insect trends therein have received little attention. Using 10 years of standardized data (120,996 individuals; 1,805 species) from 140 sites in Germany, we show that declines occurred in most sites and species across trophic groups. In particular, declines (quantified as the correlation between year and the respective community response) were more consistent in sites with many non-native trees or a large amount of timber harvested before the onset of sampling. Correlations at the species level depended on species' life-history. Larger species, more abundant species, and species of higher trophic level declined most, while herbivores increased. This suggests potential shifts in food webs possibly affecting ecosystem functioning. A targeted management, including promoting more natural tree species composition and partially reduced harvesting, can contribute to mitigating declines.

Can morphological traits explain species-specific differences in meta-analyses? A case study of forest beetles

Meta-analyses have become a valuable tool with which to synthesize effects across studies, but in ecology and evolution, they are often characterized by high heterogeneity, where effect sizes vary between studies. Much of this heterogeneity can be attributed to species-specific differences in responses to predictor variables. Here, we aimed to incorporate a novel trait-based approach to explain species-specific differences in a meta-analysis by testing the ability of morphological traits to explain why the effectiveness of flight-intercept trap design varies according to beetle species, a critical issue in forest pest management. An existing morphological trait database for forest beetles was supplemented, providing trait data for 97 species, while data from a previous meta-analysis on capture rates of bark or woodboring beetles according to different trap designs were updated. We combined these sources by including nine morphological traits as moderators in meta-analysis models, for five different components of trap design. Traits were selected based on theoretical hypotheses relating to beetle movement, maneuverability, and sensory perception. We compared the performance of morphological traits as moderators versus guild, taxonomic family, and null meta-analysis models. Morphological traits for the effect of trap type (panel vs. multiple-funnel) on beetle capture rates improved model fit (AIC_c ), reduced within-study variance (σ² ), and explained more variation (McFadden's pseudo-R² ) compared with null, guild, and taxonomic family models. For example, morphological trait models explained 10% more of the variance (pseudo-R² ) when compared with a null model. However, using traits was less informative to explain how detailed elements of trap design such as surface treatment and color influence capture rates. The reduction of within-study variance when accounting for morphological traits demonstrates their potential value for explaining species-specific differences. Morphological traits associated with flight efficiency, maneuverability, and eye size were particularly informative for explaining the effectiveness of trap type. This could lead to improved predictability of optimal trap design according to species. Therefore, morphological traits could be a valuable tool for understanding species-specific differences in community ecology, but other causes of heterogeneity across studies, such as forest type and structure, require further investigation.

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.

Local ecological divergence of two closely related stag beetles based on genetic, morphological, and environmental analyses

The process of phenotypic adaptation to the environments is widely recognized. However, comprehensive studies integrating phylogenetic, phenotypic, and ecological approaches to assess this process are scarce. Our study aims to assess whether local adaptation may explain intraspecific differentiation by quantifying multidimensional differences among populations in closely related lucanid species, Platycerus delicatulus and Platycerus kawadai, which are endemic saproxylic beetles in Japan. First, we determined intraspecific analysis units based on nuclear and mitochondrial gene analyses of Platycerus delicatulus and Platycerus kawadai under sympatric and allopatric conditions. Then, we compared differences in morphology and environmental niche between populations (analysis units) within species. We examined the relationship between morphology and environmental niche via geographic distance. P. kawadai was subdivided into the “No introgression” and “Introgression” populations based on mitochondrial COI gene – nuclear ITS region discordance. P. delicatulus was subdivided into “Allopatric” and “Sympatric” populations. Body length differed significantly among the populations of each species. For P. delicatulus, character displacement was suggested. For P. kawadai, the morphological difference was likely caused by geographic distance or genetic divergence rather than environmental differences. The finding showed that the observed mitochondrial–nuclear discordance is likely due to historical mitochondrial introgression following a range of expansion. Our results show that morphological variation among populations of P. delicatulus and P. kawadai reflects an ecological adaptation process based on interspecific interactions, geographic distance, or genetic divergence. Our results will deepen understanding of ecological specialization processes across the distribution and adaptation of species in natural systems.