Department of Physics, Chemistry and Biology Master thesis
Boxing for biodiversity: a long-term follow up of an
artificial dead wood environment
Staffan Carlsson
LiTH-IFM- Ex15/3006SE
Handledare: Nicklas Jansson & Per Milberg, Linköpings universitet Examinator: Lars Westerberg, Linköpings universitet
Department for physics, chemistry and biology Linköpings universitet
Rapporttyp Report category Examensarbete D-uppsats Språk/Language Engelska/English Titel/Title:
Boxing for biodiversity: a long-term follow up of an artificial dead wood environment
Författare/Author:
Staffan Carlsson
Sammanfattning/Abstract: Today many saproxylic species are threatened because of habitat decline in Europe.
Hollow trees represent a great part of the habitats that saproxylic species use. Since hollows takes a long time to develop, management actions are needed to prevent the extinction of saproxylic species. The aim of this study was to investigate the succession of saproxylic beetle species in artificial habitats in the form of wooden boxes. Wooden boxes were filled with a potential substrate and placed at different distances (0-1800 m) from oak hollow hot spots. In addition to the start mixture, four different additional substrates were added. In total, 4510 specimens of 114
saproxylic beetle species were sampled in 43 boxes over ten years. The specimens of tree-hollow species, wood rot species and nest species increased with 38% from the fourth to the final year but species richness decline from 47 to 29, respectively. A dead hen had a tendency for attracting more species but the small effect of different added substrates diminished over the years and had no significant effect on species richness after ten years. There was a higher similarity in species richness after ten years between the boxes and real hollow oaks. In conclusion, the artificial habitat developed into a more hollow like environment, with fewer but more abundant wood mould specialists, during ten years. This study clearly shows that the wooden boxes are used as habitats for saproxylic species as the boxes seems to develop into a more hollow-like habitat with time.
ISBN
LITH-IFM-G-EX—15/3006—SE
__________________________________________________ ISRN
__________________________________________________
Serietitel och serienummer ISSN
Title of series, numbering
Handledare/Supervisor Nicklas Jansson & Per Milberg
Ort/Location: Linköping
Nyckelord/Keyword:
Nyckelord: Artificial habitats, saproxylic beetles, wood mould boxes, hollow trees, succession
Datum/Date
2015-05-28
URL för elektronisk version
Institutionen för fysik, kemi och biologi
Department of Physics, Chemistry and Biology
Avdelningen för biologi
Contents
1. Abstract ... 1
2. Introduction ... 1
3. Material and methods ... 3
3.1 Field trial ... 3 3.1.1 Box design ... 3 3.1.2 Study sites ... 5 3.1.3 Colonization time ... 6 3.2 Sampling of invertebrates ... 6 3.3 Data analyses ... 7 4. Results ... 8
4.1 Time and distance effect ... 10
4.2 Substrate effect ... 12
4.3 Species assemblage ... 13
5. Discussion ... 18
5.1 Successional process in the boxes favours tree hollow specialists 18 5.2 Starting mix in boxes is less important... 19
5.3 Distance from core area affects the composition ... 19
5.4 High species richness similarity between boxes and real hollow oaks 20 5.5 Box usage in conservation management ... 21
6. Conclusion ... 21
7. Acknowledgements ... 22
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1. Abstract
Today many saproxylic species are threatened because of habitat decline in Europe. Hollow trees represent a great part of the habitats that saproxylic species use. Since hollows takes a long time to develop, management actions are needed to prevent the extinction of saproxylic species. The aim of this study was to investigate the succession of saproxylic beetle species in artificial habitats in the form of wooden boxes. Wooden boxes were filled with a potential substrate and placed at different distances (0-1800 m) from oak hollow hot spots. In addition to the start mixture, four different additional substrates were added. In total, 4510 specimens of 114
saproxylic beetle species were sampled in 43 boxes over ten years. The specimens of tree-hollow species, wood rot species and nest species increased with 38% from the fourth to the final year but species richness decline from 47 to 29, respectively. A dead hen had a tendency for attracting more species but the small effect of different added substrates diminished over the years and had no significant effect on species richness after ten years. There was a higher similarity in species richness after ten years between the boxes and real hollow oaks. In conclusion, the artificial habitat developed into a more hollow like environment, with fewer but more abundant wood mould specialists, during ten years. This study clearly shows that the wooden boxes are used as habitats for saproxylic species as the boxes seems to develop into a more hollow-like habitat with time. Keywords: Artificial habitats, saproxylic beetles, wood mould boxes, hollow trees, succession
2. Introduction
Conservation biologists struggles to maintain the landscape as a whole, with all the different habitats that are harboured within (Hannah et al. 1995, Kirby et al. 1995). At a landscape level, old or dead trees are one habitat type that is decreasing in frequency (Bryant et al. 1997, Eliasson & Nilsson 2002, Andersson & Östlund 2004, Lindenmayer et al. 2012). As trees grow old the internal woody tissues often get colonised by fungi. With time the tissue rots, birds can split up the tissue and insect larvae consume the softened wood. The hollows that are formed in the process become filled with residual woody debris and get mixed over the years with insect
fragments, dead leaves, bird and mammal remains. This flour and saw-dust like material is called wood mould. Hollows take time to form and are more abundant in old and large trees (Gibbon & Lindenmayer 2002, Ranius
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et al. 2009). A wide range of invertebrates, mammals and birds are
dependant on tree hollows as they provide larval feeding resources, resting or nesting places (Ranius & Wilander 2000, Ranius et al. 2005, Buse et al. 2008, Bryant et al. 2012, Chiari et al. 2012, Manning et al. 2013, Hussain et al. 2013, Taylor & Ranius 2014).
In southern Sweden, mixed deciduous forests are rapidly decreasing in favour of species poor coniferous forests (Björse & Bradshaw 1998,
Lindblad & Bradshaw 1998). Hollow trees in urban parks, which harbours endangered saproxylic species, may be removed due to safety reasons for the park visitors (Carpaneto et al. 2010). The present landscape – with clear-cut forests, agricultural lands and overgrown pastures and meadows – are habitat poor and fragmented which are negative for many species using old hollow trees (Nilsson & Baranowski 1997, Ranius & Jansson 2000, Buse et al. 2008). Together with habitat loss, fragmentation is the most serious threat to biodiversity today (Fahrig 2003). The spatial distances in a fragmented landscape can influence key ecological processes (Fahrig 2003, Herrera et al. 2011), negatively influence small animals (Ranius 2002a, Götmark et al. 2008) and create possible inbreeding in low dispersal range species (Oleksa et al. 2013, Ranius & Hedin 2001). Saproxylic invertebrate species use hollows with wood mould to a great extent and as a result of the reduced habitat availability in Europe many of the species have become threatened (Hannah et al. 1995, Buse et al. 2008, Gossner et al. 2013). These negative impacts on the saproxylic fauna suggest that management action is justified (Jonsell et al. 1998, Martikainen 2001, Buse et al. 2008). The saproxylic fauna requires dead wood and there are a few viable
management actions to create suitable habitats or manage the spatial distribution of dead wood. Pollarding willows (Salix alba) is a relatively rapid way to create tree hollows (Sebek et al. 2013) and killing deciduous trees, by ring barking the trunks, creates insect habitat (Aulén 1991). Another alternative is to accumulate dead wood in sun exposed areas (Franc & Aulén 2008). One method to increase the amount and spatial distribution of microhabitat is to directly construct them. Several authors have filled wooden boxes with substrates that imitates, or quickly develops into, wood mould (Jansson et al. 2009a, Hilszczański et al. 2014), and they showed that the boxes will act as habitats for wood mould dependent invertebrates and other species that uses these hollows during 1-4 years after it was initiated in field. Although the short-term success of attracting saproxylic beetles to this kind of boxes has been documented, the
knowledge of the succession of species over a longer time span remains unknown. The artificial wood mould in the boxes should decrease over
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time, due to larval consumption and fungal activity. Therefore the species richness and abundance would also decrease.
The overall aim with this study was to investigate the change in species composition of saproxylic species in artificial habitats in the form of
wooden boxes. A number of specific questions were addressed 1) How has the species composition changed during a duration of six years? 2) How has the composition of the categories tree-hollow species, nest species or wood rot species changed? 3) Does the initial composition of artificial substrates affect the colonization of saproxylic beetle species or tree hollow species, nest species and wood rot species? 4) How does the distance from a dispersal source affect the saproxylic beetle species composition in the boxes? 5) How similar is the saproxylic beetle composition in the boxes compared to the composition in living hollow oaks in the region?
3. Material and methods 3.1 Field trial
3.1.1 Box design
The boxes, 48 in total, were made of oak wood, 25 mm thick walls and roof and 50 mm thick bottom. The size of the boxes were 0.70m x 0.30m x 0.30m, which gives a volume of about 60 L. A hole, 80 mm in diameter, was created on the front of the boxes (Figure 1). The roof had a cross milled into it with four drilled holes (8 mm in diameter) at the endpoints of the cross, in order to let some rain water in and resemble the conditions in hollow oaks. The roof and one side of the box could be opened. Behind the side of the box there was a transparent plastic window for eventual wood mould activity studies. To retain some of the moisture the bottom of the box was covered with 50 mm of clay formed as a bowl.
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Figure 1. A wooden box with a milled cross and drilled holes on the roof. Bowl-shaped clay on the bottom of the box and a transparent window with a door to the right.
The boxes were filled to 70 % with a mix of oak saw dust (50 %), oak leaves (25 %), hay (10 %), 1 L lucerne flour (2.5 %) (Medicago falcata) and 5 L water (12.5 %), as a potential substrate for saproxylic organisms. In addition, boxes were filled with one of four different substrates: 1) a dead hen (Gallus domesticus), 2) 1 L of chicken dung, 3) 1 L additional lucerne flour plus 1 L of oat flakes and 4) five potatoes (Table 1). The dead hen and chicken dung were used to imitate occupied and/or deserted bird nests. The lucerne flour and oat flakes were to raise the protein content and the potatoes to create a moist environment. The boxes were suspended on oak trunks, approximately 4 m from the ground, with a metallic band that was attached to the backside of the boxes. To minimize differences in microclimate between the boxes and to create a relatively stable
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environment over time the boxes were positioned on the shadiest side of the oaks.
3.1.2 Study sites
The study locations, Bjärka-Säby, Brokind and Grebo, are situated about 15-20 km south, south-east of Linköping, Sweden (Figure 2). The distance between the locations was 10-20 km. These locations were selected
because oak is the dominant tree species among the old hollow trees in the areas. The area harbours a species-rich saproxylic invertebrate fauna living in old hollow oaks (Ranius & Jansson 2000). Another reason for the
selection of locations was the lack of old hollow oaks in one or more directions from their core areas. About 50-100 hollow oaks stood in the core area of each of the three locations.
Figure 2. The locations of the three study sites, south, south-east of Linköping, Sweden (distance in kilometre).
Each study location consisted of a core area and two or three surrounding sites where the boxes were placed. The surrounding sites were situated within a distance of 100 to 1800 m from the core area (Table 1). The surrounding sites were selected in different directions from the core area, which resulted in 100 to 2000 m distances between the sites. The distances used in this study were within the range of observed dispersal distance of saproxylic beetles that are of conservation interest (Ranius 2006). In each
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site the boxes were placed with a distance of 10 to 200 m between them. The boxes in the core areas were placed on old hollow oaks and the boxes in the surrounding sites were placed on younger oaks. To ensure that the core areas were the nearest dispersal source for saproxylic species, the areas around the surrounding sites were thoroughly searched for hollow trees. In and around the surrounding sites there were therefore no hollow trees of any species that may contain large amounts of wood mould (Quercus robur, Fraxinus excelsior, Tilia cordata, Acer platanoides,
Ulmus glabra). A few woodpecker holes on aspen trees were spotted in the
sites but the woodpecker holes are often impossible to investigate.
Furthermore, aspen typically have no or only small volume of wood mould (Carlsson 2013).
Table 1. Number of boxes per distance from sites with hollow oaks and the different added substrates used in the artificial wood mould.
Distances (m)
Dead hen Chicken
dung Lucerne flour and oat flakes Potatoes Total number of boxes 0 3 3 3 3 12 100 2a 2 2 2a 8 200 1a 1 1 1 4 300 1 1 1 1 4 400 1 1 1 1 4 600 2a 2 2 2 8 1800 2 2a 2 2 8
a One of these boxes was broken during the course of the study, and hence
excluded from the analyses
3.1.3 Colonization time
The boxes stayed in the field for ten years, but had different starting years (Brokind: 2002, Bjärka Säby: 2003, Grebo: 2004). Hence the full study continued from 2002 to when the last boxes where taken down permanently in 2013. For financial reasons the boxes at Grebo (n=18) only stayed in the field for nine years and were taken down 2013. Henceforth, boxes from the tenth year, from Brokind and Bjärka Säby, and the Grebo boxes (ninth year) will be referred to as the “final year”. Five boxes were lost over the field period due to broken suspension and thus broken boxes (Table 1).
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The sampling was made in the second, third, fourth and final year. Pitfall traps were placed in the boxes the second and third year and in the fourth and final year sampling was made in the boxes by using an eclector trap (Økland 1996). The boxes were covered and sealed with a dark bag made of cloth. A hole was made in the cloth bag approximately in front of the orifice and a white but half transparent plastic bottle was placed at the exit hole. The plastic bottles were filled with a preservative liquid. The liquid consisted of 50% propylene glycol, 50% water and some drops of dish soap to remove the surface tension. The bottle acted as the only exit hole in the cloth bag and the only place where light came into the trap. This made emerging invertebrates attracted to the bottles and thus all emerging insects ended up in the preservative liquid. The bottles were changed every third week. The procedure started in early spring, around March, and continued until there were no more emerging insects, approximately for 4-5 months. In the final year the exact same method was made with the exception that the boxes were brought into lab instead. The data from the years 2-4 have been reported previously in Jansson et al. (2009a).
Most of the invertebrates were identified to species level by Carlsson S., but some difficult genera were identified by experts (Jansson N., Sjödin G., Snäll S. and Andersson R.). The beetle species were classified as obligate saproxylic (species that exclusively lives on dead wood or on saproxylic organisms) or facultative saproxylic (species that may use dead wood or saproxylic organisms but also other organic substrates) or none of the above (beetles that have no saproxylic preference) according to Dodelin et al. (2008). The saproxylic beetle species were also classified after which microhabitat they are associated with (Ranius & Jansson 2000; Ranius & Jansson 2002). The classes are: tree-hollow species (wood mould larvae development; rotten wood exclusively in hollows), wood rot species (rotten wood in any part of the trunk, even on the outside), nest species (nests from birds or other animals in tree hollows), dry wood species (dead, dry wood in trunks) or fungi species (fruiting bodies of saproxylic fungi). The study focused on saproxylic beetles but also includes pseudoscorpions since they also requires hollow trees (Ranius & Wilander 2000).
3.3 Data analyses
All statistics were calculated in R (R team 2013). Comparisons were made between year four and the final year since the same method was used to sample the species these years. However, in the comparisons between boxes and living hollow oaks, the captures from all years in the boxes were used, i.e. year two/three/four and the final year, henceforth regarded as all
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years. Comparisons were made with the total amount of species and specimens.
The effects of time (years), distance from core area (DFCA), and the four different substrates added to the boxes was tested on the beetles with a Generalized Linear Models for Multivariate Abundance Data (manyGLM), with a negative binomial distribution, unadjusted univariate test and 999 permutations using the mvabund package (Wang et al. 2010). DFCA was square root transformed prior to the test and the normality of residuals was checked visually.
In the current study, an ordination method was used to visualize similarities between the species composition in the boxes and in 90 hollow oaks from a study by Ranius & Jansson (2000). Ranius & Jansson (2000) did not
identify all families of saproxylic species in that study, thus to make the two datasets comparable, the families Latrididae, Nitidulidae and most of Staphylinidae were left out from the current data set. A Nonmetric
Multidimensional Scaling (NMDS), with Bray-Curtis dissimilarity index and four dimensions in the vegan package (Oksanen et al. 2014) was used for the comparison.
Finally, species-wise odds ratios were calculated for year four vs. final year (odds of finding species X in a box that year) and boxes vs. hollow oaks. An odds is calculated on the proportion presence-absence for one year (odds_fourth, odds_final) and OR=log(ratio odds_fourth/odds_final) indicate if the species was more frequent in the fourth year (OR>0) or the last year (OR<0) (Szumilas 2010). The odds ratios and 95% confidence intervals were calculated manually and ln transformed. Consequently, species with no year or habitat (box or oak) specificity, would be normally distributed around zero.
4. Results
In total, 4510 specimens of 114 saproxylic beetle species were collected from the boxes during all years (Table 2). The mean value of saproxylic beetle species per box was 17.0 (SD 6.8). The most species-rich box had 30 species and the box with the least amount of saproxylic beetles had six species. Specimens ranged from 17 to 319 per box.
There was a decline from 75 to 42 saproxylic beetle species among the 1089 and 1081 specimens caught in the fourth year and the final year, respectively. The mean species number dropped from 5.7 (SD 4.3) to 5.0
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(SD 2.8) from the fourth to the final year, respectively. In total, 65 different tree-hollow species, wood rot species and nest species were found. The specimens in these categories increased with 38% from the fourth year to the final year. The proportion of tree-hollow, wood rot and nest species of saproxylic beetle species increased with 6% during this period but there was a decline in number of species, 47 to 29 (Table2, Figure 6).
Table 2. A summary of species and specimens found in 43 boxes (total sample [all years]) with artificial wood mould.
Year four (SD) Year ten (SD) All years (SD)
Total number of saproxylic beetle species 75 42 114
Total number of saproxylic beetle specimens 1089 1081 4510
Mean number of saproxylic beetle species per box 5.7 (4.3) 5.0 (2.8) 17.0 (6.8) Mean number of saproxylic beetle specimens per box 25.3 (43.2) 25.1 (36.0) 104.9 (83.2)
Total number of HWNb species 47 29 65
Total number of HWNb specimens 669 922 3094
Mean number of HWNb species per box 4.1 (3.3) 3.5 (2.2) 11.4 (4.6)
Mean number of HWNb specimens per box 15.5 (18.7) 21.4 (31.3) 72.0 (61.5)
Proportion HWNb of saproxylic species/specimens 63%/61% 69%/85% 57%/68%
Pseudoscorpion species / specimens 3 / 4 3 / 61 5 / 86
bHWN = the classes tree-hollow, wood rot species and nest species.
The mean number of saproxylic beetle species and specimens per box and the mean number of tree-hollow/wood rot/nest species and specimens were higher in the boxes than in the hollow oaks (Table 3). The proportion of tree-hollow, wood rot and nest species of saproxylic beetle specimens was higher for the boxes than the hollow oaks.
There was a high increase of pseudoscorpion specimens from the fourth to final year but the species count remained unchanged (Table 2). The total amount of pseudoscorpion species and specimens were lower in the boxes than in the hollow oaks (Table 3).
Table 3. A summary of species and specimens from 43 boxes (total sample [all years]) with artificial wood mould and from 90 living hollow oaks in the region. The families Latrididae, Nitidulidae and most of Staphylinidae were removed from the box data before comparison.
Boxes All years (SD)
Living hollows oaks (SD)c
Total number of saproxylic beetle species 85 98
Total number of saproxylic beetle specimens 2829 2535
Mean number of saproxylic beetle species per box or tree 12.1 (5.2) 10.3 (4.8) Mean number of saproxylic beetle specimens per box or tree 65.8 (61.5) 28.2(19.6)
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Total number of HWNd specimens 2648 2159
Mean number of HWNd species per box or tree 10.1 (4.3) 8.7 (4.2)
Mean number of HWNd specimens per box 61.6 (60.5) 24.0 (17.6)
Proportion HWN of saproxylic species/specimens 67%/94% 76%/85%
Pseudoscorpion species / specimens 5 / 86 7 / 116
cRanius & Jansson (2000). dHWN = the classes tree-hollow, wood rot species
and nest species.
4.1 Time and distance effect
Both time and DFCA had significant effects on the saproxylic species assemblage but not the interaction between time and DFCA (P = 0.011, P = 0.002 and P = 0.276, respectively; manyGLM, data not shown).
Approximately 17 species contributed to 50% of the effects of both time and distance from core area (DFCA; Table 4, 5). The composition of species in the boxes was more similar to the hollow oaks when the boxes where set close to the dispersal source (Figure 3).
Table 4. The top 17 saproxylic species that contributed with approximately 50% of the time effect together with their contribution to the DFCA effect. Partial results from manyGLM (Generalized Linear Models for Multivariate Abundance Data). Estimate equals positive (+) or negative (-) relationship to effects.
% Contribution to time effect (P value; estimate) % Contribution to DFCA effect (P value; estimate) Habitat type (Saproxylic type)*
Scraptia fuscula 5.2 (0.024; + ) 0.3 (0.832; + ) nest (O)
Liocola marmorata 4.8 (0.028; + ) 0.1 (0.958; + ) hollow (O)
Quedius brevicornis 4.3 (0.053; - ) 0.8 (0.551; + ) nest (O)
Prionychus ater 4.2 (0.077; + ) 3.5 (0.003; - ) hollow (O)
Ptinus rufipes 3.4 (0.070; + ) 0.5 (0.618; - ) rot (O)
Bisnius subuliformis 3.4 (0.108; - ) 2.2 (0.122; + ) nest (O)
Atheta nigricornis 3.1 (0.210; - ) 2.8 (0.078; + ) (F)
Euplectus karsteni 3.0 (0.138; - ) 0.4 (0.792; + ) hollow (F)
Gnathoncus buyssoni/nannetensis 3.0 (0.164; - ) 3.3 (0.028; + ) nest (F)
Ctesias serra 3.0 (0.029; + ) 0.0 (0.996; - ) rot (O)
Phyllodrepa melanocephala 3.0 (0.160; - ) 0.2 (0.910; - ) (F)
Hypebaeus flavipes 2.9 (0.002; + ) 0.4 (0.675; + ) rot (O)
Corticaria serrata 2.8 (0.143; - ) 1.2 (0.341; - ) (F)
Anaspis marginicollis 2.6 (0.061; + ) 0.2 (0.795; - ) (O)
Latridius nidicola 2.6 (0.001; + ) 1.5 (0.062; + ) (F)
Anaspis thoracica 2.5 (0.099; + ) 1.3 (0.260; - ) (O)
Dasytes cyaneus 2.5 (0.027; + ) 0.3 (0.667; - ) (O)
*Nest = Nest species, hollow = tree-hollow species, rot = wood rot species, O = obligate saproxylic speices and F = facultative saproxylic species.
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Table 5. The top 17 saproxylic species that contributed to approximately 50% of the DFCA effect together with their contribution to the time effect. Partial results from manyGLM (Generalized Linear Models for Multivariate Abundance Data). Estimate equals positive (+) or negative (-) relationship to effects.
% Contribution to DFCA effect (P value; estimate) % Contribution to time effect (P value; estimate) Habitat type (Saproxylic type)*
Ptinus fur (♀) 6.2 (0.001; - ) 0.9 (0.674; + ) hollow (F)
Cerylon histeroides 3.9 (0.001; + ) 1.8 (0.186; - ) rot (O)
Prionychus ater 3.5 (0.003; - ) 4.2 (0.077; + ) hollow (O)
Gnathoncus buyssoni/nannetensis 3.3 (0.028; + ) 3.0 (0.164; - ) nest (F)
Trox scaber 3.3 (0.020; + ) 0.2 (0.908; - ) nest (F)
Atomaria morio 3.2 (0.036; - ) 2.3 (0.042; + ) nest (F)
Haploglossa villosula 3.1 (0.008; + ) 2.3 (0.217; - ) nest (F)
Haploglossa marginalis 2.9 (0.005; + ) 0.0 (0.349; + ) nest (F)
Atheta nigricornis 2.8 (0.078; + ) 3.1 (0.210; - ) (F)
Thamiaraea hospita 2.7 (0.064; + ) 0.3 (0.861; - ) (O)
Euplectus nanus 2.6 (0.038; + ) 0.0 (0.396; + ) hollow (F)
Euplectus mutator 2.3 (0.116; + ) 1.7 (0.465; + ) (O)
Bisnius subuliformis 2.2 (0.122; + ) 3.4 (0.108; - ) nest (O)
Hapalaraea nigra 2.1 (0.136; + ) 0.9 (0.308; - ) fungi (F)
Cryptophagus confusus 1.9 (0.149; + ) 0.1 (0.714; - ) hollow (F)
Anthrenus museorum 1.8 (0.079; - ) 0.4 (0.817; + ) nest (F)
Ptinus subpilosus 1.7 (0.158; - ) 1.3 (0.308; + ) hollow (O)
*Nest = Nest species, hollow = tree-hollow species, rot = wood rot species, fungi = beetles associated with fruiting bodies of fungi, O = obligate saproxylic speices and F = facultative saproxylic species.
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Figure 3. A NMDS (Bray-Curtis dissimilarity index) describing the compositional similarity of saproxylic beetles in 90 old hollow oaks and 43 boxes with artificial wood mould mounted at different distances from areas with hollow oaks after ten years.
4.2 Substrate effect
Boxes with a dead hen were somewhat richer in species than boxes with the other substrates (Figure 4), although neither substrate nor the interaction between substrate and time had significant effects on the species richness (manyGLM, data not shown).
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Figure 4. The mean number of species per added substrate in 43 boxes (total sample [all years]) with artificial wood mould. Bars show 95% confidence interval.
4.3 Species assemblage
Of the 91 saproxylic beetle species that were sampled in the eclector traps, 61.5% and 30.7% had higher frequency of occurrence in the fourth and final year, respectively (Figure 5). Species richness decreased in all
categories from the fourth year to the final year but the abundance of tree-hollow species, wood rot species and nest species increased and the proportion of same species between the years were low (Figure 6).
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Figure 5. Odds ratios for 91 saproxylic beetle species found in boxes with artificial wood mould (n=43) of year four and the final year. Bars show 95% confidence interval, the zero reference line indicates no difference, while e.g. positive values indicate that the species was more common in the last year.
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Figure 6. Species richness and abundance data from wooden boxes with artificial wood mould (n=43). a) Species richness from the fourth and final year of each category of saproxylic beetles and the number of species that were sampled in both years. b) Mean changes in abundance of all saproxylic beetle species amongst the different categories from in the fourth year to the final year.
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Of the 124 saproxylic beetle species that were sampled in the boxes from all years and in the hollow oaks, 50.8% had a higher frequency of
occurrence in the boxes (Figure 7). Of the eight pseudoscorpion species found in the boxes (all years) and in living hollow oaks, five species had higher frequency of occurrence in the boxes (Figure 8).
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Figure 7. Odds ratios for 124 saproxylic beetle species found in boxes (total sample [all years]) with artificial wood mould (n=43) and living hollow oaks (n=90). The families Latrididae, Nitidulidae and most of Staphylinidae were
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removed from the box data before comparison. Bars show 95% confidence interval and the zero reference line indicates that it was equally abundant.
Figure 8. Odds ratios for all pseudoscorpions found in boxes (n=43) with
artificial wood mould and in living hollow oaks (n=90). a species that were found in the final and/or fourth year, b species that were found in living hollow oaks and in the boxes (total sample [all years])
5. Discussion
5.1 Successional process in the boxes favours tree hollow specialists
We hypothesized that since the artificial wood mould in the boxes should decrease over time, due to larval consumption and fungal activity, richness and abundance would also decrease. Studies have shown that population sizes of some saproxylic species is proportional to wood mould volume (Ranius 2007). The wood mould decreased with almost 10 % every year, with 15-30 % the first three years (Jansson et al. 2009a) and finally after ten years the wood mould had decreased with 73-83 % (95% CI). The hypothesis was confirmed for richness, but not abundance, as the boxes experienced a decrease in species richness between year four and the final year but had more specimens per species on average in the final year (Table 2). All categories experienced a decrease in species richness from the
fourth to the final year but surprisingly the specimens in the categories tree-hollow species, wood rot species and nest species increased with almost 40 % (Figure 6, Table 2). Almost 9 out of 10 specimens in the final year were from the three categories which are strongly associated with wood mould hollows (Ranius & Jansson 2000; Ranius & Jansson 2002). One
explanation for their dominance in the later stages of the succession could be that the artificial wood mould becomes more similar to real wood mould over the years. Another possible mechanism is that as the volume wood mould decreases over the years so does the species richness but the remaining hollow tree specialists takes advantage of the niche to the full extent and increases in abundance. Consequently, the boxes develops from
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a varied to a more specific wood mould habitat. The surrounding biome is also important for saproxylic species richness (Martikainen et al. 2000, Lassauce et al. 2011). However, the surroundings have not changed much in 5-6 years and thus the boxes and the succession of the wood mould should be solely responsible for the changes in richness and abundance. Some of the hollow tree specialised species that increased with time were
Cryptophagus quercinus, Hypabaeus flavipes, Liocola marmorata and Osmoderma eremita (Figure 5). O. eremita is an umbrella species, which
requires quite specific habitats and often indicate high species richness (Ranius 2002b, Jansson et al. 2009b). The fact that O. eremita used the boxes for larval development in the later stages (one and three individuals in two boxes, respectively) clearly shows the boxes great potential. It also strengthen the idea that the artificial wood mould becomes more similar to real wood mould over the years.
5.2 Starting mix in boxes is less important
There was no noticeable effect of the different additives to artificial wood mould or the interaction between time and additives on species richness and abundance of saproxylic beetle species in the manyGLM. Jansson et al. (2009a) showed significant effects of the substrate on some groups of
species after four years in the field. However, since the boxes were open for other animals as well, such as birds, wasps, ants or mammals, the substrate effect might have diminished over the years. Unfortunately, data for these factors were never recorded for the time after the fourth year and could not be accounted for. Nonetheless, there was a weak tendency for the dead hen to attract more species than the other substrates (Figure 4).
Amongst the four different substrates that were added, a dead hen probably resembles the real conditions in a hollow better than the other substrates as hollows are often colonized by birds and some of their offspring are often left for dead.
5.3 Distance from core area affects the composition
The composition of beetles in the boxes where more similar to the real hollow oaks when the boxes were set close to the core areas with hollow oaks (Figure 3). This is consistent with previously studied dispersal limitations of the beetles (Ranius & Hedin 2001, Ranius 2006, Jansson et al. 2009a). However, many of the high contributing species showed a positive relationship to increasing DFCA (Table 5) and the most species rich boxes were situated 100 m from the core area (data not shown). One explanation could be that boxes in the proximity of hollows oaks (0 m) are
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rejected in favour of the hollows and boxes remote from hollow oaks (100-1800 m) acts as the best alternative were there are no hollow trees.
Therefore, boxes in the range of a few hundred meters will act as the best habitat for dispersing species which in turn might for some species show up in the data as positively affected from DFCA.
5.4 High species richness similarity between boxes and real hollow oaks
The purpose of the boxes was that they should mimic, or least partly, the saproxylic beetle assemblage found on hollow oaks. For that purpose we compared the data from this study with an already published data set from living hollow oaks (Ranius & Jansson 2000). It is worth stressing that the boxes were sampled four years (year two, three, four and final year) while the hollow oaks were sampled only one year. However, the hollow oaks stood in open stands of at least five hollow oaks, which is beneficial for saproxylic insects (Ranius 2002c). In contrast, the boxes were situated up to 1800 m away from any hollow tree. The samplings in the study by Ranius & Jansson (2000) were done with pit fall traps, the same method as year two and three in this study. However, the species from the fourth and the final year in this study were all sampled with eclector traps, which means that the species used the boxes for larval development and winter hibernation. Despite risk for sampling bias, compared to the hollow oaks, the boxes had higher mean richness and abundance per box and almost the same number of species in total (Table 3). Surprisingly there were more species that had higher frequency of occurrence in the boxes than in the hollow oaks (Figure 7, 8). Species like Ampedus nigroflavus, Liocola
marmorata, Anthrenochernes stellae and Hypebaeus flavipes were more
likely to be sampled in the boxes than the hollow oaks. Nevertheless, a couple of wood mould specialists were more likely to be found in the oaks than in the boxes, Osmoderma eremita and Cryptophagus quercinus were sampled less often in the boxes and Elater ferrigineus and Allecula morio were never sampled in the boxes. As stated previously, the boxes seems to develop, over time, towards a hollow like habitat. For example, O. eremita and C. quercinus were more frequent in the oaks but both increased in the boxes with time. This study shows that the boxes clearly can act as a substitutes for the environment in hollow oaks.
After the sampling was complete the remaining wood mould inside the boxes was thoroughly examined for dead insects and insect fragments. Noteworthy is that the wood mould contained a surprisingly large amount of dead/dried larvae of Cetoniidae, Prionychus sp./Pseudocistela sp. and even some fragments of Elater ferrigineus (data not included in this study).
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Since E. ferrigineus is a predator on the O. eremita larvae and relies on the occurrence of that larvae, the occurrence of E. ferrigineus in the boxes further strengthen the boxes potential as artificial habitats.
5.5 Box usage in conservation management
Five boxes were destroyed during the ten year field period (Table 1). The metallic band where only attached to the back side of the box and thus the whole side could come loose. Therefore, if used in conservation
management, the suspension of the boxes must be improved so that all sides are connected with the metallic band. Also, the bottom of the boxes should be covered with some material (preferably a plastic sheet) that insects and other animals do not consume. Due to larvae consumption, wasp gnawing and/or other reasons, a couple of the 43 boxes had almost no bottom left and had begun to leak wood mould (personal observations). The boxes could also benefit from being larger, for a more stable
(temperature and moisture) oak-hollow environment. Finally, addition of extra artificial wood mould should be performed at minimum every decade so that there is sufficient amount of wood mould for larval development. The placement of the boxes should also be considered. The boxes in this study were placed on the shadiest side of the trunks, to ensure a similar as well as stable condition for all boxes. This could be a disadvantage for colonization since sun-exposed trees have a greater species richness and species frequency (Ranius & Nilsson 1997, Ranius & Jansson 2000). Furthermore, large stands of hollow oaks comprise a greater species richness (Ranius 2002c, Ranius & Hedin 2004). Consequently, the
conservation value of a single box may be best in a sun-exposed placement and when arranged in a cluster.
6. Conclusion
In conclusion, the fact that 114 saproxylic beetle species and five pseudoscorpion species were detected to use the boxes of this ten year study, clearly shows that the boxes are sufficient as artificial habitats. Added substrates to the start mix had little influence over time. With altered placement, changes in box design and refilling of artificial wood mould the boxes could well be used as temporal and/or spatial stepping stones in a fragmented, habitat poor landscape prior to the development of real tree hollows.
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7. Acknowledgements
I would like to thank my supervisor Nicklas Jansson for help with identification of species, knowledge about species and writing. Also, professor Per Milberg has helped me a lot with writing and comments during this work and Lars Westerberg for help with understanding different statistical analyses. I would like to thank entomologists Gunnar Sjödin, Stanislav Snäll and Rickard Andersson for help with the identification of difficult species families. I would also like to thank Stiftelsen Oscar och Lili Lamms mine and Eklandskapsfonden i Linköpings kommun for financial support.
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