Research Article |
Corresponding author: Benoît Dodelin ( benoitdodelin@orange.fr ) Academic editor: Sonke Hardersen
© 2017 Benoît Dodelin, Simon Gaudet, Guillaume Fantino.
This is an open access article distributed under the terms of the Creative Commons Attribution License (CC BY 4.0), which permits unrestricted use, distribution, and reproduction in any medium, provided the original author and source are credited.
Citation:
Dodelin B, Gaudet S, Fantino G (2017) Spatial analysis of the habitat and distribution of Osmoderma eremita (Scop.) in trees outside of woodlands. In: Campanaro A, Hardersen S, Sabbatini Peverieri G, Carpaneto GM (Eds) Monitoring of saproxylic beetles and other insects protected in the European Union. Nature Conservation 19: 149-170. https://doi.org/10.3897/natureconservation.19.12417
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The endangered and specialised saproxylic beetle Osmoderma eremita occurs in veteran trees, a habitat particularly threatened by changes in agricultural landscapes. Its conservation requires information about populations and key features of host trees. Surveys of 8,014 trees (pollarded or in hedgerows) were carried out and analysed, based on habitat description (tree level) and spatial information (hedgerow length and distance to the nearest inhabited tree). A suitable cavity was present in 61% of the trees and O. eremita was detected in 42 trees, mainly in Salix (30 observations), the most common tree amongst those surveyed. A small or absent crown was a significant factor in explaining the beetle’s presence, as was the distance to the nearest inhabited tree. The largest population of O. eremita, 19 inhabited trees, was found in a wide and continuous area formed by trees with suitable cavities, with distances of less than 250m from each another. Seven smaller areas, with 7, 5 or 1 inhabited trees, were also found. When analysing inhabited trees on a 1km² grid, 17km2 hosted O. eremita, corresponding to a dense network of 63km of hedges. The presence of O. eremita significantly increased per km² with increasing length of hedges and this variable was thus used to guide forthcoming investigations directed toward Osmoderma. As the hedgerows existing in 1999 had decreased by 6.1% in 2009, it is concluded that the long term survival of O. eremita is under threat. The preservation of trees outside woodlands is urgent and has already started, in connection with Natura 2000 policies. Regeneration and creation of new hedgerows is also ongoing and can be reinforced both by using Salix and by promoting pruning to increase formation of cavities.
Insect conservation, Habitat quality, Osmoderma eremita, Saproxylic beetle, Hollow tree
Wood pastures, hedgerows with trees and trees outside woodlands (TOW) (
In the northwest of France, a dense hedgerow network persisted over centuries in connection with the traditional system of agriculture. The maximum density of the hedgerow network was reached at the end of the 19th century and decreased from the 1950s onwards, mainly as a result of intensified farming practices (
The Parc Naturel Régional des Boucles de la Seine Normande (PNR-BSN) is partly covered by a dense and extensive network of large trees following the Seine river. This area is also characterised by the presence of two large cities, Rouen and Le Havre. The Seine river, with the major ports of Rouen, Le Havre and Paris, is economically important. This context negatively impacts wetland ecosystems: dredging, drainage and gravel mining are common activities and today, this last activity probably has the most significant negative impact on old trees as it is often carried out inside the hedgerow network. Conflicts between nature protection and economic development are thus a serious problem. Moreover, historic data collected before 1950 suggested that O. eremita could be present in the hedgerow network. The PNR-BSN hence decided to use this species as an indicator of the quality of the network. In accordance with the state services (DREAL), this was integrated into the objectives and action plans of Natura 2000 sites, as well as into the PNR-BSN conservation strategy for 2012–2016 (
The specific aims of this study are to:
a) contribute to the knowledge on the ecology of O. eremita;
b) explore the characteristics of the inhabited trees and their spatial patterns in the hedgerow network in the PNR-BSN;
c) predict favourable areas where O. eremita is likely to occur and contribute to the establishment of guidelines for actions favourable for the protection of the species.
This study is based on fieldwork carried out with prefecture permits dated 19/08/2008 (fieldwork from summer 2008 and earlier) and 30/07/2013 (fieldwork from 2013 to 2016).
Until recently, European Osmoderma were considered as a single species, O. eremita (Scopoli, 1763), but recent evidence regarding morphology and from sequencing of mtDNA cytochrome C oxidase I gene, leads to a division into two distinct lineages. West European populations belong to O. eremita and the eastern ones to O. barnabita Motschulsky, 1845, while in Southern European ancient glacial refuges, there are three others species: O. italicum Sparacio, 2000, O. cristinae Sparacio, 1994 and O. lassallei Baraud & Tauzin, 1991, respectively endemic to the Italian peninsula, Sicily and Greece and European Turkey (
Osmoderma species live in the cavities of large and old broad-leaved trees and, very likely, they all have the same biological requirements. Suitable trees can be in old-growth forests (
In Sweden,
In France, although O. eremita is widely distributed, a large proportion of the known localities are old or very old and many local populations are already extinct (
Fieldwork was carried out in the territory of the PNR-BSN, North-West of Paris, France (49°27'03"N; 00°39'57"E). In this area, hedgerows and orchards still form a dense network in areas such as the Seine and Risle valleys, the Marais Vernier and the southern plateau. The territory of the PNR-BSN has not been studied exhaustively. Based on aerial photographs, study sites were selected that included a dense network of hedges with trees and pollarded trees. In 2007, the studied area covered the Seine river floodplain at the meanders of Jumièges, Anneville/Ambourville, Roumare and the Roumois’ plateau. In 2008, the survey investigated the western part of the PNR-BSN, the Roumois’ plateau, the meander of Brotonne, the Marais Vernier and the Risle valley. Selected sites were studied exhaustively: all suitable trees within each site were surveyed (woodlands excluded).
The field inventory of trees was undertaken during summer and autumn 2005-2015, with most work being done in 2005 and 2007–2008. Over the period from 2012-2015, trees inhabited by Osmoderma or with indices of presence were surveyed (see below) in order to confirm the activity of the species. Altogether, 8,014 trees were investigated (Table
Data sources and description for Osmoderma eremita in the PNR-BSN. Some trees have been surveyed two times or more.
Year of sampling | Type of survey | Trees surveyed | Trees inhabited by Osmoderma eremita | Reference |
---|---|---|---|---|
2005 | Targeted searches (Scale: tree patch) | 4,181 | 8 | ( |
2007 | Full inventory (Presence-Absence data) | 4,378 | 29 | ( |
2008 | Full inventory (Presence-Absence data) | 3,608 | 2 | (ALISE Environnement et al. 2008) |
2012 | Targeted searches (Scale: tree) | 5 | 4 | PNR-BSN |
2013 | Targeted searches (Scale: tree) | 383 | 10 | PNR-BSN |
2014 | Targeted searches (Scale: tree) | 31 | 22 | PNR-BSN |
2015 | Targeted searches (Scale: tree) | 2 | 2 | PNR-BSN |
Total | 8,014 | 42 | This study |
If necessary, ladders (length 4m) were used to reach the trunk cavities. A total of 38 cavities were unreachable. Cavities were explored visually with a lamp and their depth was estimated with a small diameter metal rod. The surface of the wood mould was inspected in order to search for indications of the presence of Osmoderma, those indications being live larvae and imagines, cocoons made with wood mould, remains of exoskeleton and/or larval faecal pellets. When no indications were observed on the surface, a small amount (approximately 1 litre) of wood mould was sieved and spread out on a white sheet in the field for careful inspection. Afterwards, the wood mould was returned to the cavity. The characteristic odour of the male pheromone was also considered as indicating the presence of O. eremita.
The detection of Osmoderma in cavities may be difficult and increasing the sampled volume of wood mould increased the probability of detecting larvae (
During the field work, spatial coordinates of each tree were taken with a GPS. Trees were described and cavities examined. The trees descriptors were: identification to species level, evolution stage of hollow defined according to
All tree variables were pair-tested to confirm the absence of correlations. Values of large discontinuous variables were then grouped into smaller categories to reinforce the explanatory powers of the models: the information on tree species was reduced to the three most common genera plus a fourth category which contained all other genera; the developmental stage of the cavity was grouped into three categories instead of five; crown development was reduced to three categories instead of five (Table
Description of Variables.
Variable | Description | Categories used in models |
---|---|---|
Tree genera | Salix (n = 3,961). Fraxinus (n = 2,227). Populus (n = 1,286). Carpinus (n = 185). Quercus (n = 74). Other trees + Missing data (n = 281). | Salix (n = 3,961). Fraxinus (n = 2,227). Populus (n = 1,286). Other (n = 540). |
Developmental stage of cavity | Stage 0: no cavity (n = 576). Stage 1: beginning of cavity development, not deep, small amount of mould (n = 2,157). Stage 2: Cavity with mould and a small entrance (n = 1,125). Stage 3: Large cavity with an important volume of mould (>5 litres) and a small entrance, optimum stage for Osmoderma (n = 1,067). Stage 4: Large and open cavity, empty of its mould after natural degradation of the trunk. The cavity soon to be uninhabitable by Osmoderma (n = 2,711). Stage 5: Dead or alive tree with emptied cavity (n = 373). Missing data (n = 5). | Suitable = Stage 2+3+4 (n = 4,903). Unsuitable = Stage 1+5 (n = 2,530). Absent = Stage 0 (n = 576). |
Date of last pruning | Old (>10 yrs). Medium (5-10 yrs). Recent (< 5 yrs). Missing data (n = 9). | Old (n = 5,369). Medium (n = 1,605). Recent (n = 1,031). |
Crown development | Absence of crown either because tree is dead or after a recent, major pruning (n = 351). Small crown (n = 810). Medium crown (n = 1,700). Large crown (n = 1,975). Very large crown (n = 3,170). Missing data (n = 8). | Absent or small (n = 1,161). Medium size = Medium + large (n = 3,675). Very large (n = 3,170). |
Presence of Osmoderma eremita | Observation of larvae, adults, exoskeleton remains and/or faecal pellets of larvae of O. eremita. | Presence (n = 42). Absence (n = 7,972). |
Distance to the nearest tree inhabited by Osmoderma eremita | Minimum distance, in metres, to the nearest tree with presence of O. eremita, calculated after GPS coordinates of the trees. | Metric. |
Length of TOW per 1km² | Length of hedgerow with trees per square of a 1km² grid. | Metric: Min.: 0.0; Median: 2,616; Mean: 2,844; Max.: 7,768. |
Hedgerows with trees were measured using the 1:2,000 and 1:5,000 maps of the PNR-BSN. Ortho-photographs from 1973, 1999 and 2009 were digitalised and classified according to hedgerow types, amongst which were selected only hedgerows composed of trees or containing trees with a minimum length of 15m (forest edges excluded). Solitary trees, small series of pollards and orchards were also included. In this article, the terminology tree outside woodland (TOW) was used to refer to a hedgerow with trees together with solitary and/or pollards and orchards.
Connectivity was measured in two ways. First, the distance to the nearest neighbour tree inhabited by O. eremita was used. There is evidence that this measure is less powerful (
The landscape context of TOW was described by considering groups of trees at the 250m spatial scale. Groups were constructed by applying a buffer zone around any tree having a suitable cavity (i.e. cavity of stage 2, 3 or 4) and then by merging the adjacent polygons. The areas of the resulting zones (in hectares) were then calculated, the number of trees counted and the length of hedges contained in these polygons.
Ranges of 1km² and 250m were chosen because they corresponded respectively to the average dispersal distances already published for O. eremita (see above), as well as to infrequent but longer dispersal distances.
To find the variables that significantly affect the occurrence of O. eremita, two techniques were used. First, univariate analyses were used to assess each variable independently against occurrence of O. eremita. Second, generalised linear models (GLMs) were run with a binomial error distribution and logit link function. One important weakness of GLMs is that they consider the absence data to be certain. However, it is clear that the detection probability of O. eremita in wood mould sampling is well below 100% (
Means of the data issued from TOW measurements in the 1km² grid were compared using one-way ANOVA. Correlations were explored with the non-parametric Spearman’s rank correlations.
All statistical analyses and models were carried out with the R software version 3.1.0, RStudio version 0.99.896 and PAST version 2.17c (
The PNR-BSN comprises 900 squares of 1km², inside of which 70,616 hedgerows and TOW have been mapped, for a total length of 3,898km according to the most recent analysis based on the aerial photographs from 2009. TOW represented 1,258.6km, a number that includes 151.2km of discontinuous hedgerow with trees and at least 5.4km of pollards in rows (pollards were not identified everywhere).
From 1973 to 2009, an increase of the total length of hedge length was found: +30.9% between 1973 and 1999 and +6.2% between 1999 and 2009 (Figure
Of the 8,014 trees studied, 4,903 (61%) presented a cavity suitable for O. eremita and 42 were identified as being inhabited by O. eremita. Four trees hosted live adults; the others contained adult remains and/or faecal pellets of larvae. The data indicated a hermit beetle population mostly confined along the meander of Jumièges, Anneville/Ambourville and Roumare.
The surveys recorded a majority of trees belonging to Salix, Fraxinus and Populus. O. eremita was clearly abundant in Salix, with 30 inhabited trees identified. However, the ratio between the numbers of trees surveyed versus trees inhabited by O. eremita was highest for Quercus, with 4.05% of the trees inhabited. In contrast, this value was lower for Salix (0.76%) and even lower for Fraxinus (0.22%).
Most of the willows had been pruned and developed a suitable cavity in 74.6% of cases, a ratio which represents a higher percentage of possessing a cavity when compared to ash (51.6% of trees with cavities) and poplars (37.0% of trees with cavities) (Table
Count of inventoried trees according to the presence of suitable cavities and observations of O. eremita. †: refers exclusively to unidentified trees species.
Trees genera | Number of trees | Trees with cavities in stage 2, 3 and 4 | Presence of Osmoderma eremita (% of the number of trees) |
---|---|---|---|
Acer | 63 | 40 | 0 |
Alnus | 63 | 53 | 2 (3.17%) |
Carpinus | 185 | 147 | 0 |
Fraxinus | 2,227 | 1,149 | 5 (0.22%) |
Populus | 1,286 | 476 | 0 |
Quercus | 74 | 29 | 3 (4.05%) |
Salix | 3,961 | 2,953 | 30 (0.76%) |
Tilia | 73 | 25 | 0 |
Castanea, Crataegus, Malus, Ulmus, etc., plus unidentified | 82 | 31 | 2 (5.26%)† |
Total | 8,014 | 4,903 | 42 |
The best fitting GLM (AIC = 426.39) identified Tree genera, Developmental stage of the cavity, Crown development and Distance to the nearest tree inhabited by O. eremita as the most important variables. Date of last pruning and Length of TOW per 1km² were excluded during the selection process (Table
Coefficients and probabilities associated with the variables of the best fitting GLM predicting the presence of O. eremita. Significance thresholds: 0.001 % (***), 0.01 % (**), not-significant (ns). GLM deviances: Null deviance = 524.79 on 8,004 degrees of freedom; Residual deviance = 408.39 on 7,996 degrees of freedom; AIC = 426.39.
Variables | Estimate | Standard error | z | Pr (>|z|) |
---|---|---|---|---|
All variables | -23.70 | 1,718 | -0.014 | 0.98899 |
Tree genera - Other | 2.143 | 0.5943 | 3.606 | 0.00031 *** |
Tree genera - Populus | -13.71 | 974.9 | -0.014 | 0.98878 |
Tree genera - Salix | 1.353 | 0.4915 | 2.752 | 0.00592 ** |
Crown development - Medium | 1.094 | 0.6221 | 1.759 | 0.07858 |
Crown development - Absent or small | 2.016 | 0.6333 | 3.183 | 0.00146 ** |
Evolution stage of hollow - Suitable | 17.38 | 1,718 | 0.010 | 0.99193 |
Evolution stage of hollow - Unsuitable | 17.70 | 1,718 | 0.010 | 0.99178 |
Distance to the nearest tree inhabited by Osmoderma eremita | -0.001163 | 0.0003581 | -3.248 | 0.00116 ** |
Significant values for the variables selected in the GLM involved trees other than Fraxinus and Populus and underlined the importance of Salix, inside of which most of the observations of O. eremita were made. A small crown or its absence also played a significant and positive role in the model. Finally, a greater distance to the nearest inhabited tree negatively affected the model.
TOW with a suitable cavity (thereafter named STOW), with a distance of less than 250m from other STOW, were distributed in the PNR-BSN within 75 areas disconnected from each another. Only a few of these areas were wide and included many STOW, whereas all other areas were small and included fewer STOW (Figure
Distribution map of inhabited trees in the PNR-BSN according to the TOW length in each 1km² square. Spatial analysis: 1km² grid.
Only 96 squares from a total of 900 were included in the tree inventory. These represented 238km of TOW (19% of the total length). Trees inhabited by O. eremita belonged to 17 squares, including 63km of TOW (5% of the total length) (Table
Frequencies of 1km² squares according to the TOW length. Three sample groups are presented: PNR-BSN (n = 900 squares); Sampled trees (n = 96 squares); Trees inhabited by O. eremita (n = 17 squares). All differences between groups are significant at the threshold of 0.001% (ANOVA).
Lengths of TOW in the 1km² squares of the PNR-BSN. Data from 2009.
Data per square | PNR-BSN (n = 900 squares) | Sampled trees (n = 96 squares) | Trees inhabited by O. eremita (n = 17 squares) |
TOW length (km) | 1,340.90 | 238.00 | 62.66 |
Average length (km) of TOW | 1.40 | 2.48 | 3.69 |
Median length (km) of TOW | 1.19 | 2.27 | 3.86 |
With the GIS system, it was possible to identify squares with a total length of TOW equal to or greater than that of the squares with trees inhabited by O. eremita. In this manner, two large areas were delimited which may have been occupied by the hermit beetle i.e. the meander of Roumare, in the west of the city of Canteleu and the lower part of the Risle valley, plus the plateau of Saint-Pierre-du-Val (Figure
Osmoderma species are known to live in the cavities of many tree species: Quercus (many studies), Tilia and Alnus (
The GLM model used showed a weakness as it considered absence data to be certain. This is unrealistic with Osmoderma, as it is a “hard-to-find” species in many situations (
In this study, the used model indicated that the date of last pruning was irrelevant but showed that a small or absent crown was a significant factor. As, in most cases, pruning is the main cause for a reduced size or an absence of canopy, this result seems incongruous at first. Moreover, in an agricultural landscape, willows had generally been pollarded to avoid the growth of large branches subject to breakage over the medium term (
Inhabited trees were found to be more abundant in the larger areas established by applying a 250m buffer zone around any TOW with a suitable cavity. No link with the density of hollow trees per ha was however found. The areas with the highest densities of STOW were searched without success. The observed densities in areas with O. eremita were at most 2.46 STOW/ha.
This study confirmed the presence in the PNR-BSN of the endangered and highly specialised saproxylic beetle O. eremita. This confirmation was often based only on traces of its presence, but also on live specimens. It seems likely that O. eremita survived because the landscape of the studied area had not dramatically changed. Moreover, this area benefited from a favourable situation with about 10 times more suitable hollow trees than observed in a nearby site which comprised similar numbers of trees surveyed and trees inhabited by O. eremita (
This study also showed that the length of TOW existing in 1999 had decreased by 6.1% in 2009. Thus, if no protection measures are undertaken, the long term survival of O. eremita in the PNR-BSN is under threat. In the upper Elbe valley,
An action plan for the preservation of old trees is already in progress in the PNR-BSN. It includes several approaches: financial help for pruning, a dedicated commission, an awareness campaign for the public and schools, a reflection on the economic role of pollarded trees in relation to firewood and, wherever possible, an integration of measures for the conservation of pollarded trees into urban planning documents. This is in agreement with the proposal of
In the medium term, it is important to work to establish O. eremita as a flagship species for the public, even if this status will not necessarily confer protection to all other taxa associated with the flagship species (
We would like to thank all the people involved in this work, especially Elodie Brunet, Amélie Garcia-Chaib, Jaime Jimenez, Nicolas Moulin, Mégane Skrzyniarz, Lionel Valladares, Vincent Vignon. We are also grateful to Gaëlle Darmon for her help with statistics and modelling, to Keith Alexander and to the two referees for their relevant remarks and suggestions for improvement of our initial submission. This study was funded by the DREAL Normandie, the Normandie region and "l'Eau Seine Normandie" agency.
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