Community Assembly and Spatial Ecology of Saproxylic Coleoptera

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List of Papers

This thesis is based on the following papers, which are referred to in the text by their Roman numerals.

I Victorsson, J. Tough neighbourhood ? Density-dependent effects more important in old-growth forest than in recently burnt forest for saproxylic Coleoptera. Manuscript. II Victorsson, J. Priority effects and asymmetric competition in saproxylic cerambycids. Manuscript. III Victorsson, J. Community structure convergence despite different species composition: community assembly in saproxylic Coleoptera. Manuscript. IV Victorsson, J. & Ås, S. What is the matrix ? A sea or a habitat for saproxylic Coleoptera in deciduous forest fragments in boreal forest. Manuscript. V Victorsson, J. & Ås, S. Forest-type dependent response to spatial factors in birch-living saproxylic Coleoptera assemblages. Manuscript.

Contents

Introduction...... 9 Saproxylic Coleoptera: living in dead trees ...... 9 Forest fire: setting the stage...... 10 Community assembly: enter the species ...... 11 Spatial ecology: depending on a patchy and heterogeneous resource...... 12 Fragmentation and forestry...... 12 Study sites and methods...... 14 Study sites ...... 14 Bolts and cages...... 14 Analysis...... 17 Results and discussion ...... 18 Community assembly...... 18 Spatial ecology ...... 23 Conservation in a forestry-dominated landscape ...... 26 Summary in Swedish (sammanfattning)...... 28 Acknowledgement ...... 31 References...... 33

Abbreviations

CWD coarse woody debris ERP ephemeral resource patch BRASSB Brassberget GÅSB Gåsberget NMDS non metric multidimensional scaling ANOSIM analysis of similarity

Introduction

Ecology, as a branch of science, has moved away from the notion of nature as a uniform, deterministic arena for ecological interactions to a new view where nature is perceived as patchy, spatially structured, and often perturbed by natural disturbances. This view of ecological communities has developed from a focus on metapopulations (Hanski and Gilpin 1997), via spatial ecology (Tilman and Kareiva 1997), to metacommunities (Holyoak et al. 2005). The importance of natural disturbance is well known (Pickett and White 1985) but has not been easy to integrate with ecological interactions, such as competition (but see Dayton 1971, Sousa 1985). Ephemeral resource patches (ERPs), consisting of dead organic matter such as coarse woody debris (CWD), dung pads, and carcasses, are often rich in nutrients and energy (Doube 1987). Insects in such habitats complete their larval development within the confined space of the patch and the dis- crete nature of the patches make them suitable to experiments investigating competition (Finn 2001) and community assembly.

Saproxylic Coleoptera: living in dead trees

”Dying and dead wood provides one of the two or three greatest resources for animal species in a natural forest… if fallen timber and slightly decayed trees are removed the whole system is gravely impoverished of perhaps more than a fifth of its fauna” Elton (1966).

Saproxylic Coleoptera (dependent on dead wood, Dajoz 1966) is an important group in the heterotrophic succession in CWD (Fig. 1). After tree-death, insects colonize the thin, nutrient-rich sub-cortical layer (phloem, cambium, and outer sapwood)(Haack and Slansky 1987). Beetle larvae are the main primary consumers (cambivores) with Scolytinae and Cerambycidae being two important groups (Esseen et al. 1992, 1997). The Coleoptera assemblage also includes predators, fungivores, and detritivores. The sub-cortical layer is consumed in less than two years and can be considered the most ephemeral resource in dead trees.

9 1 3 5 7

6 2 4 8

Figure 1. Saproxylic Coleoptera common in my samples. Cerambycidae: 1 and 2) Acanthocinus aedilis; 3 and 4) Rhagium inquisitor. Scolytinae: 5) Pityogenes chal- cographus; 6) Dryocoetes autographus. Curculionidae: 7) Pissodes sp. Staphylini- dae: 8) Placusa sp. who is a predator, all other species are cambivores. Pictures from: Bugwood Network Image Archives (InsectImages.org).

Two common, swedish cerambycids (long-horn beetles) are Acanthocinus aedilis and Rhagium inquisitor (Fig. 1). A. aedilis prefers breeding under the rough bark of the basal part of Scots pine, Pinus sylvestris, and under this bark-type larvae of both species are often found together. R. inquisitor is also common in other species of trees. Larval development time is one year for A. aedilis and two years for R. inquisitor. Adults of the two species are similar in size.

Forest fire: setting the stage Forest fire was the dominant, large-scale disturbance in boreal forests (Esseen et al. 1997), and the interplay between disturbance and biological processes, such as competition, can be important in explaining community structure (Dayton 1971, Sousa 1985). It is estimated that 0.5-2.9 % of the Swedish boreal forest burned annually before anthropogenic fire-suppression began (Zackrisson 1977, Niklasson and Granström 2000). Fire creates recently burnt forest, characterized by large amounts of sun-exposed CWD. CWD in old- growth forest is produced during small-scale mortality events when trees are killed by age, self-thinning, drought, or fungus infestations (Esseen et al.

10 1997, Siitonen 2001). Newly created CWD in old-growth forest is less sun exposed and relatively sparce (at least compared to burnt forest). Fire also restarts forest succession. Early- and mid-succession on mesic, productive soils was dominated by deciduous tree-species, primarily birch, Betula spp., and aspen, Populus tremula (Esseen et al. 1997). This type of post-fire successional decidious forest once covered approximately 8 % of the boreal forest of Sweden (Zackrisson and Östlund 1991). Also in deciduous old-growth forest, there is a steady input of CWD via self-thinning and gap-dynamics.

Community assembly: enter the species Community assembly (Diamond 1975) is the process deciding inclu- sion/exclusion of species as well as determining the relative abundance of those species in a local community (Weiher and Keddy 1999). Community assembly implies change over time as communities develop. It can be viewed as the trajectory, regarding species composition and community structure, a community exhibits over time as new species fail or succeed in establishing (Drake et al. 1999). Studying final states of communities is important in community ecology. However, by ignoring assembly trajectories, the different final states might seem idiosyncratic, especially if the mechanisms acting during assembly are no longer present in the final state (Drake 1991). Most cambial-living saproxylic Coleoptera have a larval development- time of one year or less, with some cerambycid species requiring longer time than that (Ehnström and Axelsson 2002), so the time for community assembly is short and significant assembly events could be expected to be separated by weeks only. Community patterns can be created either by environmental differences between patches or biotic interactions within patches. In this respect community assembly can be viewed as a sequential filtering of species from the species pool (Weiher and Keddy 1999). Abiotic filtering removes species for which the patch does not fulfil environmental niche criteria. Further biotic filtering, i.e. community ecology interactions, such as resource competition and predation, exclude or include later-arriving species. The species composition of a given community can be dependent on the arrival sequence of its species (Drake 1990). This is particularly so for insect communities in ERPs where colonization of patches is an essential part of the species life cycle. If the species that first arrives at an ephemeral patch can lay its eggs before competitors arrive, then its larvae will be larger than later-arriving species. These larger larvae are then able to better exploit the resource and can thus negatively influence the colonization of later-arriving species via exploitative or interference competition. Priority effects are found in ERP-living insects (Kneidel 1983, Rankin and Borden 1991, Shor-

11 rocks and Bingley 1994, Krebs and Barker 1995, Hodge et al. 1996) and can affect both offspring number and quality.

Spatial ecology: depending on a patchy and heterogeneous resource

“In the place where the tree faleth, there it shall be” Ecclesiastes

The assemblages of saproxylic Coleoptera are influenced by factors forming a spatial hierarchy: (1) patch, the individual dead tree; (2) forest stand; and (3) region, a larger area containing multiple stands capable of supporting metacommunities. Saproxylic Coleoptera can probably best be described as habitat-tracking metapopulations (Schroeder et al. 2007) where there is patch turnover as new patches are created and lost. A patch becomes available when a tree dies and is suitable habitat for these species for one to three years, until the cambial layer is consumed (Esseen et al. 1997). The metacommunity concept is an extension of metapopulation ecology. Species living in a metacommunity are affected by both patch quality and regional factors (Leibold et al. 2004). Patch area and isolation, matrix quality, and proportion of habitat in the region – all these regional factors can affect species composition within a given patch. A basic assumption in ecology is positive spatial autocorrelation of biological processes. In general, species assemblages in patches situated close to each other will be similar in species composition whereas increasing inter- patch distances should lead to decreased community similarity (Fortin and Dale 2005). Positive spatial autocorrelation can be created by localized species interactions and dispersal (Pacala and Levin 1997). Dispersal distances are poorly known in saproxylic Coleoptera (Ranius 2006) so we can not use knowledge of dispersal distances to deduce at what scale assemblages of saproxylic Coleoptera show spatial autocorrelation.

Fragmentation and forestry Fragmentation of natural habitats is one of the main causes of the on-going biodiversity crisis. It consists of: (1) habitat loss (lower total amount of primary habitat); (2) decreased patch connectivity (larger inter-patch distances); (3) habitat degradation of remnant patches; and (4) matrix degradation (how similar the matrix is to the original habitat in quality). The theoretical founda- tion of habitat fragmentation studies focus on species dispersal and patch connectivity (Harrison and Bruna 1999) wheras most empirical studies focus on biological interactions within remaining patches (McGarigal and Cushman

12 2002). Our knowledge of habitat fragmentation has been hampered by this focus on habitat patches and the neglect of an empirical evaluation of whether the matrix constitutes a “sea” (as assumed when applying the theory of island biogeography to habitat islands) or actually constitutes a habitat (albeit a possibly poor one) (Haila 2002). Understanding how abiotic and biotic factors in the matrix affect biological processes in remnant patches is important (Saunders et al. 1991), especially in a landscape where an increasing portion of all habitat is managed for economic gain rather than biodiversity.

13 Study sites and methods

Study sites The experiment in papers I and III was performed in recently burnt forest and old-growth forest (Fig. 2). Two pairs of sites of the two forest types were studied. The first burnt-forest site, Eggåsen (B1), was burnt for conservation purposes one day before the experiment started in June 1997. The second burnt-forest site, Galhån (B2), was a pine-dominated, managed forest burnt by wildfire. The corresponding old-growth sites, Rödmyrberget (OG1) and Ryggen (OG2) were both StoraEnso set-asides that had been excluded from forest management because of their old-growth characteristics and high conservation values. In papers IV and V we studied a total of 30 forest sites of three forest types: deciduous forest, clear-cuts and mature coniferous forest (Fig. 2). We compared two regions differing in the level of habitat fragmentation for birch- living Coleoptera: (1) Brassberget (BRASSB) , a less fragmented region; and (2) Gåsberget (GÅSB), a more fragmented region. On a landscape level (satel- lite data, IV) BRASSB have 5.3 % deciduous forest and GÅSB 3.3 %.

Bolts and cages I did all studies using bolts (pieces of the host tree, 0.35-1 m long) of Nor- way spruce, Picea abies (I, III), Scots pine, Pinus sylvestris (II), and birch, Betula pendula, (IV, V). This method short-circuited the normal lifecycle of the beetles (Fig. 3). The adults of the parent-generation laid eggs in the bolts and the larval growth then took place in the cambium. Before the offspring- generation emerged the bolt was put inside a trap for collection of the emerging beetles. The trap consisted of a cotton cylinder with a funnel and a col- lecting device, with a preservative fluid, at the bottom. Patch quality, such as tree species (Palm 1959), decomposer-fungi species (Gibb et al. 2006c), decay stage (Vanderwel et al. 2006), and position (standing or lying down)(Jonsell and Weslien 2003) all affect species composition of saproxylic Coleoptera. Using standardized bolts assured that patch quality was the same at all sites. Interpretation of results can then focus on abiotic and biotic forest types differences.

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Figure 2. Map of Sweden with study areas indicated. The priority effect study (II) was done in Uppsala. All other study areas were in the middle boreal zone of Swe- den (medium gray area in map). Burnt forest (B) and old-growth forest (OG) (I, III). Study regions Brassberget (BRASSB) and Gåsberget (GÅSB) ( IV, V). Distance between BRASSB and GÅSB was 55 km. Forest sites indicated in inset maps are: D: deciduous forest; C: clear-cut; M: mature coniferous forest. Ovals in inset maps indicate two of Swedens finest areas of deciduous forest fire remnants, Brassberget and Gåsberget proper.

15 Growth, resource- competition in bolt

Hatching

Life cycle

Dispersal and egglaying

Figure 3. Life cycle of saproxylic Coleoptera and illustration of the method used. Adult beetles (parent generation) lay their eggs in bolts (pieces of tree bole, 0.35 - 1 m long), larval development takes place in the bolt. Bolts are hanged, individually in emergence traps where the offspring generation is collected.

The bolts were cut from trees felled 1 day-1 month before the experiment. I enclosed the bolts in mesh cages (mesh size 1mm) when I wanted to prevent further colonization. The cage consisted of a nylon mesh bag with wire frames and wooden supports that kept the mesh clear of the bolt.

In the different studies I used an increasing level of “naturalness”. • In the study on priority effects (the first data-set: II) the bolts were kept in mesh cages during the whole experiment and no natural colonization was allowed. Individuals of two cerambycid species were collected in the field and transferred, as pairs in copula, to the particular bolt. The total number of parent beetles released on each bolt was four in all treatments, either two pairs of A. aedilis and two pairs of R. inquisitor, or four pairs of only one of the species. Pairs of beetles were released on the bolts on two occasions with a 2-week delay between releases. The pairs released at the first occasion therefore had a 2-week priority over the pairs released at the second occasion.

16 • In the studies comparing old-growth forest to burnt forest (second data- set: I, III) all bolts were initially exposed to natural colonisation by beetles at the sites. The bolts were then enclosed in mesh cages after 0.5, 1, 2, 4, or 8 weeks. Coleoptera reared from the bolts offer “snapshot” views of the developing sub-cortical assemblage, providing quantitative data of community structure and species composition during community assembly. • In the studies of birch-living Coleoptera (third data-set: IV, V) I used natural colonization where the bolts were exposed at the forest sites and allowed to be colonized by insects in the area for 1-3 seasons.

Analysis Density, the number of saproxylic Coleoptera individuals emerging per bolt, as well as standard community ecology measures such as, evenness, dominance, and species richness (Magurran 2004) were calculated for each bolt or site. In the priority effect study (II), mean offspring number was calculated as number of offspring produced in a bolt divided by the number of parent-generation females of that species in that particular treatment. Off- spring quality (larval weight or adult size) was also measured for the two species. In the community ecology studies (III, IV, V) I also analysed guild proportions. Species were assigned to guilds using data from the literature (Palm 1951, 1959, Martikainen et al. 1999, Dajoz 2000, Lieutier et al. 2004, Johansson et al. 2007) and classified as: cambivores, fungivores, predators or detritivores. For all univariate variables, general(ized) linear (mixed) models were fited to data using standard techniques. When needed, the response variable was transformed to improve normality and homoscedasticity of residuals. Additional tests were then performed to answer specific questions. All these tests were performed using SAS version 9.2.(SAS institute, Cary, NC). The community level analyses (III, IV, V) involved multivariate anlyses on bolt (or site) by species matrices. Non-metric multidimensional scaling (NMDS) was the method of choice with a follow-up analysis of similarity (ANOSIM) to test for differences between groups. Redundancy analysis (RDA) and (partial) canonical correspondence analysis (CCA) were used to answer questions primarily when explanatory variables were continous. I used software PRIMER (Clarke and Warwick 2001) and CANOCO (ter Braak and Šmilauer 1997-2003). For an introduction to these multivariate methods see McCune and Grace (2002).

17 Results and discussion

Community assembly Research questions: • Do Coleoptera colonizing CWD in burnt forest and old-growth forest experience forest-type specific density-dependent effects? • Is there a priority effect (i.e. does it matter for a species if it arrives before or after a competitor) ? • Are there forest-type specific final states and assembly trajectories in burnt forest and old-growth forest?

The small-scale, continous, production of CWD in old-growth forest stands in sharp contrast to the large-scale, instant, production of CWD during a forest fire. During a forest fire a large proportion of the saproxylic beetles already present in the forest stand is probably killed whereas in an old- growth forest there are many such internal colonists. It is easy to imagine that these differences in substrate-dynamics and number of internal colonists could affect the intensity of density-dependent effects. I found that the number of Coleoptera emerging from the bolts showed a significant concave-down linearity in density over time in the old-growth sites and a concave-up, continuous increase in the burnt-forest sites (I, Fig. 4). Density in the bolts was similar in the two forest types up to and includ- ing the 2-week exposure. At longer exposure times the density was much higher in the old-growth forest bolts, on average 11 times higher in the 4- week exposure and 2 times higher in the 8-week exposure. Evidence of a concave-down pattern in density is also found in the scoly- tids Tomicus piniperda, (Sauvard 1989), and Ips typographus (Anderbrant et al. 1985). Dajoz (2000) interprets this pattern as an effect of larval resource competition that offsets any increase in number of colonizers. Experiments on saproxylic beetles also reveal effects of larval resource competition where a higher number of colonizing adults negatively affect the number of offspring emerging (Coulson et al. 1980, Anderbrant et al. 1985, Rankin and Borden 1991, Schlyter and Anderbrant 1993, Schroeder and Weslien 1994, Weslien 1994, Reeve et al. 1998). In light of this knowledge, I think that the results (I) suggests that competition was more intense in the old-growth sites than in the burnt sites. This effect of forest fire on intensity of competition could create forest-type specific community patterns in saproxylic Coleoptera (III).

18 Old-growth forest

Burnt forest Density (emerging Coleoptera/bolt)

Exposure: time for colonization (weeks) Figure 4. The relationship between density (Coleoptera reared per bolt) and exposure time. The shape and slope of the two lines differ according to an ANCOVA (I). Means and 95 % confidence limits shown. n= 14.

In the priority effect study I found a priority effect in both study species. In A. aedilis offspring number was affected, while in R. inquisitor offspring quality was affected (II, Fig. 5). The effect of priority was large. A. aedilis females produced on average two more offspring when they had priority over the other species, and R. inquisitor larvae were on average 1.8 times heavier when they had priority. Differences of this magnitude have large effects on fitness. The effect of interspecific competition was asymmetric such that A. aedilis was superior and produced higher offspring number when reared together with the other species. In R. inquisitor offspring number was not affected by the presence of the other species but R. inquisitor had lower offspring quality together with the other species. It is important to note that a priority effect was found also in the species that suffered from interspecific competition. To my knowledge, this is the first time that priority effects in cerambycids have been shown in an experiment. My results suggest that exploitative competition in the larval stage is the most likely mechanism. Priority effects can affect community assembly (Jenkins and Buikema 1998, Price and Morin 2004). Communities of ERP-living insects are probably best viewed as metacommunities (sensu Leibold et al. 2004). Environ- mental heterogeneity creates differences between patches in metacommunities, explaining some inter-patch variation in community composition. In the metacommunity setting, priority effects could add a level of variation – creating community differences where there are no environmental differences.

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factor competition: alone alone together together factor delay: early late early late Figure 5. Priority effect in two cerambycids. A priority effect was found for both species, on mean number of offspring for Acanthocinus. aedilis and on offspring quality for Rhagium. inquisitor. Means and measures of dispersion for simple effects from the corresponding ANOVA models (II). The asterisks denote the important difference (significant at p < 0.05) between simple effects together early and together late. This means that together with the other species the focal species has higher fitness when it arrives early than when it arrives late (after a 2-week delay) – a priority effect. n=13

If community structure and composition can be recorded non-invasively then community assembly can be studied directly. However, insects living in CWD complete their larval development within the confined space of the resource patch and then leave the substrate as adults (or late-stage larvae). Therefore species interactions are hidden inside the habitat patch and in- specting the community by removing the bark would destroy the process under study. In the community assembly experiment (III) I therefore exposed bolts of Norway spruce to natural colonization for increasingly longer times. Coleoptera reared from the bolts offer “snapshot” views of the developing sub-cortical assemblage, providing quantitative data of community structure and species composition. The final-state assemblages in the two forest types diverged regarding species composition but converged regarding community structure (III). Eight of the 20 most abundant species were only reared from one forest type, possibly indicating environmental filtering. For example, one fungivorous species, Atomaria pulchra, was only reared from burnt-forest bolts. This species could be fire adapted since Johansson et al (2007) found that it prefers burnt logs over unburnt logs.

20 Final states in both forest types were dominated by cambivores (III). Five common cambivorous species made a large contribution to the difference in species composition. Old-growth bolts had a higher abundance of Dryoco- etes autographus and Tetropium castaneum, two species that are more common in closed forest than in sun-exposed forest (Ehnström and Axelsson 2002). D. autographus is also common in managed forest. The higher abundance of Pissodes gyllenhalii in burnt-forest bolts is intriguing. This species is normally rare and local but can reach high abundance when there is locally large amounts of CWD (e.g after storm fellings: Ehnström and Axelsson 2002) creating an opportunity for weak competitors to invade patches with relatively low density of potential competitors. Density-dependent effects were lower in the burnt-forest bolts in this experiment (I) and that could explain the high abundance of P. gyllenhalii. Both species composition, guild proportions and community structure showed forest-type specific trajectories. The fact that the assemblages in the two forest types followed different assembly trajectories regarding species- composition is noteworthy (III, Fig. 6). Given the similar patch quality in both forest types and possibly good dispersal capabilities of these “habitat- tracking” saproxylic Coleoptera (Wikars 1997, Gibb et al. 2006b, Schroeder et al. 2007) different species composition was not a self-evident outcome. As community density per bolt increases, interacting species will be forced into more intense interactions where inherent competitive ability will shape the resulting assemblage. Since the community-level carrying capacity was reached in the 4- and 8-week bolts in old-growth forest but not in the corresponding bolts in burnt forest (I) it seems likely that species interactions constrained the late-trajectory assemblages to a larger degree in old- growth than in burnt forest. Species composition within exposure treatment also became increasingly similar along the assembly trajectory in old-growth forest but not in burnt forest (III). Chase (2007) distinguish two community types on the basis of intensity of species interactions. First, niche-assembled communities should be constrained by species interactions in the patch. Competitive hierarchies and species traits will determine community assembly. Increased community similarity along the assembly trajectory is pre- dicted for this type of community. Second, dispersal-assembled communities where colonization and extinction are unaffected by species competitive traits (such as in the neutral theory of Hubbell 2001). Such communities should have high patch-to-patch variation in species composition. My results for old-growth bolts indicate that those assemblages conform to the niche- assembled type of community whereas the assemblages in burnt-forest bolts, on the other hand, conform more to a dispersal-assembled community (III).

21 2D Stress: 0.01 B8

OG4 B1 B4 OG8 B2

OG2

Distance B0.5 0.02 OG1 0.09 OG0.5 0.19 0.31

Figure 6. Species-composition assembly trajectories are different in old-growth forest and burnt forest (III). I manipulated number of colonizing saproxylic Coleop- tera by exposing bolts to colonization for increasingly longer times (0.5, 1, 2, 4, and 8 weeks) with longer exposure times allowing more colonizers to find the bolt. This is a non-metric multi dimensional scaling (NMDS) ordination of R-values from analysis of similarity (ANOSIM) of all treatments (using Bray-Curtis similarity, 4th root transformed abundances). Arrows indicate assembly trajectory in “species composition space”. Euclidian distance represents R-values (ranging from 0 to 1) a measure of how different treatments are from each other in species composition. Circles with R-value distances (from cluster analysis on the same data) added to increase interpretability. B: burnt-forest bolts; OG: old-growth forest bolts.

There was no overall treatment effect on dominance, but when D. autographus was bolt-dominant (species with the highest abundance in a particular bolt), bolts had higher values of dominance, at least in old-growth forest (III). Much fewer bolts were dominated by D. autographus in burnt forest and this could have contributed to a more open community in burnt-forest bolts. The greater diversity of bolt-dominating species in burnt forest could also indicate that the assemblages in burnt-forest bolts had a less absolute competitive hierarchy. If this reasoning is correct, then it supports the hypothesis (Esseen et al. 1992, Wikars 1992) stating that burnt forest act as competition-relaxed “havens” for saproxylic Coleoptera. If burnt forest hosts dispersal-assembled communities then species that are poor competitors could reach high abundances in that forest type. On a landscape level, burnt forest could act as source patches for such species. Predation can structure communities during assembly (Warren et al. 2003, Price and Morin 2004). Predators, in both forest types, comprised a larger proportion of all individuals during assembly than in the final states (III). They made up ca 10 % of all individuals during parts of the assembly trajec-

22 tory but only 1-3 % in the final state. It seems likely that predation was an important factor early in the assembly trajectory but later along the trajectory predators could not track the increase in prey species. The high proportion predators during assembly had become “erased” in the final state and would not have been detected in a study of final states only (as discussed in Drake 1991).

Conclusions: • Density-dependent effects were more intense in the old-growth forest bolts than in the burnt-forest bolts. • I found a priority effect in two cerambycid species. Different fitness components were affected in the two species: offspring number in A. aedilis and offspring quality in R. inquisitor. • The final-state assemblages in the two forest types diverged regarding species composition but converged regarding community structure. Along the assembly trajectory both species composition, guild proportions and community structure showed forest-type specific patterns.

Spatial ecology Research questions: • Are host-tree patches in matrix forest perceived as matrix or as habitat for the assemblage of birch-living saproxylic Coleoptera? • How much of the variation in speces composition in these Coleoptera can be explained by habitat and spatial factors respectively ? • At what scale is there spatial autocorrelation in species composition ?

The birch-living Coleoptera reflected the underlying regional difference in habitat fragmentation (IV). In the less fragmented region (BRASSB) the Coleoptera assemblages in deciduous remnants and mature coniferous forest were largely similar in species composition, guild proportions, species richness, evenness, and dominance. The clear-cuts in this region differed from the two tree-bearing forest types in all these community measures. In the more fragmented region (GÅSB) there was a different pattern. In that region, the deciduous remnants were different from the matrix forest types, both mature coniferous sites and clear-cuts, in species composition, guild proportions, and evenness whereas the two matrix forest types had largely converged regarding all these community measures. The effects of increasing habitat fragmentation was primarily found in mature coniferous matrix forest. Regional comparisons of forest types, showed that there was forest type convergence in guild proportions in both deciduous remnants and in clear- cuts but regional divergence in mature coniferous sites

23 A change in guild proportions can indicate changes in ecological processes (Schowalter 2006) and in the present study the pattern in species composition can best be understood by looking at guild proportions (Fig. 7, IV). Guild proportions of cambivores and fungivores separated the sites into two groups: (1) deciduous sites in both regions and mature coniferous sites in BRASSB were characterized by a low proportion of cambivores (< 25 %) and a high proportion of fungivores (~ 20 %); (2) clear-cuts in both regions and coniferous sites in GÅSB were characterized by a high proportion cambivores (~ 55 %) and a small proportion of fungivores (~ 6 %). A large part of multivariate variance in species composition in the sites could be explained by habitat variables (22.9 %) or spatial variables (15.8 %) (V). When the forest types were tested separately, saproxylic assemblages in deciduous sites responded to distance apart, those in coniferous sites to region, whereas assemblages in clear-cuts did not respond to any spatial variables. Thus, the 15.8 % of variation explained by spatial factors could mainly be attributed to spatial effects in two of the forest types: deciduous and mature coniferous sites. It is likely that forest stands harbour individual populations of saproxylic species (as assumed by Schroeder et al. 2007) that are connected by adults dispersing by flight before egg-laying. This means that the scale of these metapopulations are largely unknown since dispersal distances are poorly known in saproxylic Coleoptera (Ranius 2006). Many studies find that species composition differ between different forest types (Kaila et al. 1997, Martikainen et al. 2000, Wikars 2002, Gibb et al. 2006a). This is important in relation to spatial pattern, since it indicates that the dispersal distances for these Coleoptera in general are long enough to allow species sorting. It probably means that most species as dispersing adults are able to sample enough of the surrounding forest to allow individuals to reach suitable habitat and make a habitat choice. Dispersal rates high enough to allow species sorting would indicate that regional factors can affect local community dynamics and could justify using a metacommunity perspective for these species. In mature coniferous sites faunal composition was different in the two regions (V). This agrees with results where there was regional difference in guild proportions in this forest type (IV). This result was probably not an effect of spatial autocorrelation but rather an effect of different levels of habitat fragmentation in the two regions. For the saproxylic Coleoptera in deciduous sites there was distance decay over long spatial distances and the assemblages showed some positive spatial autocorrelation up to a distance of ca 80 km (V). At longer distances there was first no correlation then negative correlation at distances longer than 90 km. A conclusion from these results is that a metacommunity approach could be useful for understanding the community dynamics in the deciduous sites. The scale was surprisingly large with positive autocorrelation at inter-site distances of 80 km. The assemblages in clear-cuts were unaffected by all measured spatial variables (V).

24 BRASSB (n=6) GÅSB (n=4)

*** *** M

camb Denticollis Synchita pred detr fung unknown

Figure 7. Guild proportions in the birch-living assemblage of saproxylic Coleoptera in remnant patches of deciduous forest and in two matrix forest-types: mature coniferous forest and clear-cuts (IV). We exposed standardized birch bolts to natural colonization in a total of 30 forest sites and then collected Coleoptera emerging from those bolts (8 bolts per site). Here the proportion of individuals belonging to four guilds are seen: cambivores, predators, detritivores, and fungivores. Two individual species, Denticollis linearis and Synchita humeralis, that were classified as both cambivores and predators are indicated as individual species. Significant regional forest-type differences indicated for each guild (as from ANOVAs, IV). *: p < 0.05; **: p < 0.01; ***: p < 0.001. C: clear-cut sites; M: mature coniferous; D: deciduous.

25 Conclusions: • There was forest-type convergence in guild proportions in both deciduous remnants and in clear-cuts but regional divergence in mature coniferous sites. Thus, the effects of increasing habitat fragmentation was primarily found in mature coniferous forest. • Host-tree patches in matrix forest were perceived as matrix in the more fragmented region but as habitat in the less fragmented region. • A large part of variance in species composition could be explained by habitat variables (22.9 %), such as forest type, or spatial variables (15.8 %), such as distance apart. • The assemblages in deciduous sites responded to distance apart and showed some positive spatial autocorrelation up to a distance of 80 km whereas assemblages in clear-cuts did not respond to any spatial variables.

Conservation in a forestry-dominated landscape All study sites were embedded in a highly disturbed landscape dominated by forestry. From a conservation point of view, the high dominance of D. autographus – a species common in managed forest (Ehnström and Axelsson 2002) – in the old-growth sites is problematic (III). These sites had been left as set-asides because of their high conservation values. If CWD also in such sites are invaded by common species, thriving in managed forests, there is a real risk that old-growth sites become “empty shells” of forest, only retain- ing old-growth characteristics regarding CWD but with a saproxylic Coleop- tera assemblage hard to distinguish from managed forest. In this respect it is inspiring that the burnt-forest bolts were less dominated by D. autographus and that a (relatively) simple conservation measure – creating CWD by burning – can affect community processes for early-successional saproxylic Coleoptera also in as trivial a substrate as unburnt spruce (III). Judging from the results in the assembly study it seems imperative that conservation burning creates dispersal-assembled communities that can develop with a high evenness during assembly. Only then could resulting assemblages be dominated by other species than those already common in managed forest. Species able to maintain populations in clear-cuts (generalist species, toler- ant of sun exposure) can become common and abundant on a landscape scale and can subsequently invade also high priority conservation habitats, such as deciduous remnants (Ås 1999). This was seen in the most abundant species in the birch bolts, Schizotus pectinicornis, a cambivore. This species reliably indicated degraded forest but nonetheless occurred in six out of ten deciduous sites (IV). In a forestry-dominated landscape species closely adapted to the deciduous stage become rare, even if they may have been common before man’s alteration of the forest. Such shifts in species commonness and rarity can have profound effects on our attempts to conserve biodiversity.

26 The assemblages in clear-cuts were unaffected by all measured spatial variables (V).The present study offers no possibility to distinguish between the two hypothesis that could create this lack of spatial effects in the clear- cuts: (1) the assemblages in clear-cuts contain more species with good dispersal abilities; or (2) clear-cuts are a habitat with high connectivity. The first explanation invokes the good dispersal ability of species thriving in clear-cuts (Schroeder et al. 2007). If species breeding in CWD in clear-cuts indeed are good dispersers then increased biotic homogenization would occur. The second explanation acknowledges the fact that approximately 1 % of the Swed- ish boreal forest is treated by clear-felling each year (Esseen et al. 1997). Given that a clear-cut contains increased amounts of CWD up to seven years after cutting (Schroeder et al. 2007) this type of habitat could have high connectivity. Species able to breed in the relatively small amounts of CWD in a modern clear-cut would not need to be good dispersers since they are living in the most common habitat in a forestry-dominated landscape. It is well known that forestry removes structures vital for biodiversity conservation. For example, the average amount of CWD in forestry- dominated landscapes are 90-98 % lower than in natural boreal forest (Siitonen 2001). In Sweden ca 7000 species depend on that CWD (Dahlberg and Stokland 2004) and it can come as no surprise that ca 40 % of our 1257 saproxylic Coleoptrea are red-listed (Gärdenfors 2000). A hypothetical sce- nario is that forestry not only removes vital structures from the forest (CWD, deciduous trees, old-growth forest) but that it also alters the community dynamics of saproxylic beetles. Forest types that were formerly common, such as burnt forest and successional deciduous forest developing after fire, are now rare wheras a totally artificial habitat, clear-cuts (with it’s sun-baked, low-diameter, scarce CWD) is the most common “forest”-type in any landscape. Our forest data from satelite imaging indicates that ca 52 % of the forested area in the two regions BRASSB and GÅSB are clear-cut or young forest (IV) – forest types lacking all structures necessary to sustain forest biodiversity (Esseen et al. 1997). My results indicate that the situation is even worse than that figure indicate since also areas designated for conservation are invaded by species thriving in a forestry-dominated landscape. The solution is conceptually very simple. Stop logging all existing old- growth forest and create new forest with high conservation values. Creation can involve prescribed burning of standing forest and allowing mature coniferous sites to age into old-growth stands. Some of the deciduous sites I studied (IV, V) were deciduous forest in moist areas along creeks and mires. Also these sites, that are not forest fire remnants, can be of considerable conservation value and should be excluded from logging.

27 Summary in Swedish (sammanfattning)

När ett träd dör i skogen finns det gott om skalbaggar som gärna tillbringar sin tid som larver under barken. Den näringsrika innerbarken är bra mat för långhorningar, barkborrar, vivlar, praktbaggar och andra skalbaggar. Skal- baggar kommer flygande för att lägga sina ägg i det döda trädet och under barken utvecklas ett samhälle. Vilka arter kommer att finnas i detta samhälle och hur många individer av varje art ? Det finns många arter att välja på, 1257 arter av vedlevande skalbaggar finns det i Sverige. Samhället under bark utvecklas allteftersom nya arter anländer och andra försvinner. En del arter kan inte etablera sig eftersom konkurrensen är för hård eller för att kva- litén på det döda trädet inte är rätt. I min avhandling har jag undersökt en del av de faktorer som påverkar hur samhällen av vedlevande skalbaggar ser ut. För att undersöka skalbaggsfaunan använde jag korta stockar (0.35 – 1 m långa) som lades ut i olika skogstyper, både naturliga och brukade. Genom att använda denna metod så vet jag att värdträdets kvalitet är likadan i mina undersökta områden. Därmed kan jag vara säker på att de skillnader jag hit- tat beror på skogstyp – inte på vedtyp. Jag jämförde hur vedskalbaggsfaunan i granstockar utvecklas i två skogstyper som var mycket vanliga innan människan började bruka skogen: bränd skog och barrdominerad naturskog. Jag fann att tätheterna av skalbaggar i varje stock var mycket högre i naturskogen än i den brända skogen. Detta innebär att skalbaggar i bränd skog upplever mindre av konkurrens om maten än vad de gör i naturskogen. Naturskogen kräver helt enkelt arter som är tuffare konkurrenter. En anledning till detta kan vara att det finns så mycket mera död ved i den brända skogen, den har ju brunnit och då skapas i ett enda slag massor av döda träd. Artsammansättningen skilde sig också mellan de båda skogstyperna. I den brända skogen hittade jag till exempel fler arter som lever av svamp som växer under barken. Fungivorer, svampätare, kallas skalbaggar som livnär sig så. Hur kom svampen dit undrar du kanske ? Antingen tog den sig dit för egen maskin, svampar sprider sig med miljontals små sporer, eller också fraktades den dit av en hjälpsam skalbagge. Många vedlevande skalbaggar har små fickor på kroppen där de transporterar svampar till nya träd. Svam- pen bryter ned innerbarken och frigör näringsämnen som skalbaggen annars skulle haft svårt att tillgodogöra sig. Under den säsong när mina försöks- stockar låg ute i de båda skogstyperna utvecklades samhällena olika. Sam- hällena i den brända skogen var också under tillblivelsen mera öppna sam-

28 hällen där många olika arter kunde etablera sig. Stockarna i naturskogen blev nästan alltid dominerade av en särskild art, håriga barkborren. Den dominerade faktiskt i 74 % av stockarna i naturskogen men bara i 33 % i den brända skogen. I den brända skogen blev många fler arter vanliga, till exempel gran- lågeviveln. Denna art trivs främst när det inte är så hård konkurrens om maten – ytterligare ett exempel på att den brända skogen är en ganska välkom- nande miljö. Att håriga barkborren var så vanlig i naturskogen kan vara ett problem. De båda naturskogsområden jag studerade var undantagna från skogsbruk just för sina höga naturvärden. Då är det ju illa om en bagge som håriga barkborren, som klarar sig alldeles utmärkt i den brukade skogen, äter upp maten för dom hotade arter som vi vill bevara genom att avsätta naturreservat. När ett ekologiskt samhälles utvecklas kan det spela stor roll om man kommer före eller efter sina värsta konkurrenter. Ty i skalbaggsvärlden blir först ofta också störst. Den skalbaggshona som kan lägga sina ägg under barken före sina konkurrenter ger sina avkomlingar ett försprång som är svårt att ta igen. Dessutom är en del långhorningar inte bara snälla vegetaria- ner, de är rovdjur också. Även om stapelfödan är innerbark så smakar det gott med andra skalbaggslarver också. Samtidigt minskar ju konkurrensen en smula. Jag gjorde ett försök där jag egenhändigt placerade äggläggande skalbaggshonor på tallstockar. Jag samlade in hanar och honor av två arter långhorningar: timmerman och barrträdslöpare. När dom hade parat sig så fick dom lägga ägg på mina stockar. För att testa om det spelade någon roll om man kom först eller sist så gjorde jag om alltihop efter två veckor. Då hade alltså dom nykläckta larverna från första omgången haft två veckor på sig att växa sig stora och starka. Och se, det spelade roll. När timmermännen fick komma först så blev barrträdslöparna mycket mindre än när ordningen var den omvända. Hos timmermännen var det inte storleken utan antalet som påverkades. När de fick komma först blev de fler än när de kom sist. Om detta beror på ren konkurrens om maten eller om det beror på att de äter också på varandra återstår att ta reda på. I Dalarna finns landets finaste lövbränna, Gåsberget. Skogen är olikt allt som skogsbruket kan prestera. Helt dominerad av lövträd, björk, asp, al, sälg, rönn, precis som en riktig skog ska se ut. Lövbrännor uppkommer efter skogsbränder när skogen får sköta sig själv utan att människan lägger sig i. Då bildas detta lövrika stadium efter cirka 50 år. Skogen är naturligtvis full av arter som är hotade av skogsbruket. I Dalarna och Hälsingland brann stora skogsområden år 1888 och efter dessa bränder finns en del små fragment av lövbrännor kvar. Jag undersökte Gåsberget med omgivningar förstås men även Brassberget som ligger 55 km norr därom. Kring Gåsberget har skogsbruket varit mycket intensivt och därför finns mycket lite lövträd i den brukade skogen. Här har man aktivt dödat lövträd genom fickning, som man gjorde under 80-talet. Den brukade skogen har förvandlats till monokulturer av gran och tall, bra för skogsägarnas plånböcker, mindre bra för den biolo-

29 giska mångfalden. Skogen kring Brassberget har klarat sig bättre, här finns ett större lövinslag i den brukade skogen. Den ekologiska termen för detta är fragmentering. Med avseende på lövträd så är skogen kring Gåsberget mera fragmenterad än den kring Brassberget. Jag och min medförfattare till denna raport, Stefan Ås, undersökte dessa områden, denna gång genom att använda björkstockar, det handlar ju om skalbaggar i lövbrännor nu. Vi fann att lövskogsområden är lika i de båda regionerna. Denna skogstyp var dominerad av fungivorer, svampätarna, och av rovlevande skalbaggar. Kalhyggena däremot var helt dominerade av in- nerbarksätande skalbaggar. Så långt alltså, stor skillnad mellan kalhyggen och lövskog, knappast någon överraskning. Men när vi kom till den avverk- ningsmogna barrskogen blev vi förvånade. Artsammansättningen i den skogs- typen skilde sig åt mellan regionerna. I den minst fragmenterade regionen, skogen kring Brassberget, påminde samhällena mest om lövskog. Där var samma höga andel fungivorer och rovdjur. Men i den mera fragmenterade regionen, skogen kring Gåsberget, så var barrskogen mest lik kalhyggen med en total dominans av innerbarksätande skalbaggar. Detta innebär att för en stackars björklevande skalbaggshona, beroende av den skuggiga och stabila miljön i en lövbränna, som letar efter ett gott hem åt sina larver så är döda träd i barrskogen inget alternativ kring Gåsberget. Döda björkar i barrskog i denna region kunde lika gärna ligga på ett kalhygge. Om skalbaggshonan däremot lever i skogen kring Brassberget så ser det ljusare ut, här kan hon glatt lägga sina ägg i döda björkar även i barrskogen. På en landskapsnivå ser alltså framtiden ljusare ut för dessa skalbaggar i Brassberget än i Gåsberget. Ingen vet riktigt hur långt vedlevande skalbaggar kan flyga. Detta kan vara användbar kunskap att ha om man ska planera naturvårdåtgärder. Genom att se hur avstånd mellan våra skogsområden påverkade artsammansättningen kunde vi se att det finns en skillnad mellan samhällena i lövskog och samhäl- lena på kalhyggen. Samhällena i lövskogsfragmenten uppvisade likheter över en stor skala, ca 80 km, medan samhällena på kalhyggena inte påverkades av hur långt det var till närmaste andra kalhygge. Man behöver göra flera lik- nande studier innan man kan dra några säkra slutsatser av detta men ett är säkert, för att bevara dessa arter krävs ett landskapsperspektiv. Om skalbag- garna lever i samhällen som uppvisar likheter över så stora avstånd som 80 km så måste naturvårdsplaneringen ske över samma stora skala.

30 Acknowledgement

My main conclusions from this study was … Naah, will not do that, but this would be a good place though. First of all my supervisor, Stefan Ås. After all these years I am still amazed at how you can turn my doubt over weird results into fascination and motivation. It’s been a pleasure to learn ecology and statistics from you and you have been a good and supportive friend. Thank you. I am grateful to my supervisor Mats Björklund. Thank you for much encouragement and support, also through tough times, and for constructive comments on papers and analyses. I am grateful to Staffan Ulfstrand for accepting me as a grad student and for much encouragement ever since. Thanks to Lasse Wikars for introducing me to field work and handing me the ideas on priority effects and competitive relaxation in burnt forest. Thanks to Anna Sandberg for all constructive input to my studies, from planning to analysis. My research questions asked and methods used bear a heavy stamp of three excellent PhD courses. Thanks to Riccardo Bommarco (Spatial ecology), Nicholas Gotelli and Anne Magurran (Species richness), Ulf Grandin and Leonard Sandin (Multivariate analysis). My career choice owes a great deal to four undergraduate professors. Lars Hedström shared his passion for insects and made the choice of study organisms real easy. Håkan Rydin taught me the joy to be found in ecology. Per Sjögren-Gulve and Torbjörn Ebenhardt taught me the value of data-backed, hard science, especially when it comes to conservation biology. Field workers of the world, unite and take over. Thanks to Anna Sand- berg, Niklas Gyllenstrand, Maria Forslund, Lina Nittérus, Thomas Persson- Vinnersten, Per Olof Hedgren, and Annika Lydänge, Göran Arnqvist introduced me to biometry and patiently answered statistical questions. Martina Schäfer generously did the GIS-programming. Bengt Ehnström, Martin Schroeder, and Åke Lindelöw generously shared their entomological expertise. So did Stig Lundberg who determined the beetles. Uppsala University and StoraEnso provided bolts (0.35-1m long) The study was supported by grants from the Swedish Environmental Protection Agency, Stiftelsen för Zoologisk Forskning, and The Royal Swedish Acad- emy of Science.

31 Ann-Britt Florin, Olle Håstad and Anders Ödeen made my stay at the department so much better. It is always good to know where to turn when the urge to discuss sword fighting (Ann-Britt), WWII aviation (Anders) or X- men trivia (Olle) becomes overwhelming. I am also grateful to Anders and Olle for showing me just how far a “pizza-project” might lead. I am grateful to everybody at the department of Animal Ecology over the years. This is a truly great place for a grad student and I have enjoyed my stay tremendously. Johannes Sandberg my friend and outdoor companion from “20-minute bush-walk” in Småland to camel-riding in India. I would not be where I am, and I would not be who I am, without our time together. Lasse Perk and Roland Jansson and the late Christer Hemborg for showing me, at an early stage, that biology is actually something you can pursue as a career. Thanks to Gunilla and Marcello Aimeta-Iderström for much support on the “home front”. Thanks to my in-laws, Karin and Karl-Erik Sandberg, for much encouragement and baby sitting. The love of nature that was instilled in me from an early age by my late father, Åke, and my mother Evy can not be overemphasized. I am grateful for your never-ending support Evy! Finally, love and light of my life, Anna Sandberg and Timja.

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