Forest fragmentation truncates a food chain based on an old-growth forest bracket fungus

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OIKOS 90: 119 – 126. Copenhagen 2000

Forest fragmentation truncates a food chain based on an

old-growth forest bracket fungus

Atte Komonen, Reijo Penttila¨, Mariko Lindgren and Ilkka Hanski

Komonen, A., Penttila¨, R., Lindgren, M. and Hanski, I. 2000. Forest fragmentation truncates a food chain based on an old-growth forest bracket fungus. – Oikos 90: 119 – 126.

We studied the effect of forest fragmentation on the insect community inhabiting an old-growth forest specialist bracket fungus, Fomitopsis rosea, in eastern Finland. Samples of the fungus from large non-isolated control areas were compared with samples from forest fragments in two isolation time classes; 2 – 7 yr and 12 – 32 yr since isolation. Fomitopsis rosea hosted a species-rich community with relatively many specialized old-growth forest insects. The numerically dominant food chain consisted ofF.rosea, the tineid mothAgnathosia mendicellaand the tachinid flyElfia cingulata, a specialist parasitoid of A. mendicella. The frequency of F. rosea on suitable fallen spruce logs and the frequency ofA.mendicellain fruiting bodies were significantly lower in the forest fragments than in the control areas. The median number of trophic levels decreased from three in the control areas to one in the fragments that had been isolated for the longest period of time. The parasitoid was completely missing from the fragments isolated for 12 – 32 yr. Our results show that in boreal forests habitat loss and fragmentation truncate food chains of specialized species in the course of time since isolation.

A. Komonen and I. Hanski, Dept of Ecology and Systematics, Di6. of Population Biology,PO Box17,FIN-00014 Uni6ersity of Helsinki,Finland(present address of AK:Eastern Steppe Biodi6ersity Project,PO Box350,Choibalsan,Dornod,Mongolia

[esbp@magicnet.mn]). – R. Penttila¨, Research Centre of Friendship Park, To¨no¨la¨,

FIN-88900Kuhmo,Finland. –M.Lindgren,Dept of Ecology and Systematics,Di6.of Systematic Biology,PO Box47,FIN-00014Uni6ersity of Helsinki,Finland. Boreal forest is the most extensive terrestrial biome on

earth, covering 10% of all land area and including 45% of all forests (Mooney et al. 1995). As boreal forests are located at high latitudes and dominated by a few tree species only, it is commonly assumed that they have little biodiversity. In reality, boreal forests are surpris-ingly diverse in many taxa, for example, of the esti-mated 300000 species in Canada, 200000 species meet at least part of their ecological requirements in forest ecosystems (Anon. 1993). In Fennoscandian boreal forest landscapes, spruce swamp forests in particular are centres of biodiversity (Ohlson 1990, Kuusinen 1996) with hundreds of fungal, lichen, moss and beetle species occurring primarily in old-growth swamp forests (Esseen et al. 1992, Berg et al. 1994). A key

microhabi-tat for maintaining biodiversity in boreal forests is decaying tree trunks (e.g. Bader et al. 1995, Ohlson et al. 1997). The rate of decay is slow and the amount of fallen and standing decaying wood is consequently high under pristine conditions (Hofgaard 1993). In Fennoscandia, about 1000 species of beetles are depen-dent on decaying wood or wood-decomposing fungi (Esseen et al. 1992). As the amount of decaying wood is greatly reduced in managed forests, the worldwide transformation of virgin boreal forests to managed forests, though not generally leading to deforestation, may lead to a massive wave of extinctions (Hanski and Hammond 1995).

Old-growth forest destruction is manifested in the loss of total area and in the fragmentation of the Accepted 25 November 2000

Copyright © OIKOS 2000 ISSN 0030-1299

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remaining old-growth forests. Environmental condi-tions in small isolated fragments may become so greatly altered that the risk of extinction of local populations is increased (Saunders et al. 1991). Small populations in small fragments have a high risk of extinction also for stochastic reasons (Hanski and Gilpin 1997). For differ-ent taxa the effects of isolation vary depending on the extinction proneness of the species, time since isolation, and degree of connectivity to other fragments and populations.

Most of the empirical work on habitat fragmentation has focused on fragmentation at a relatively small scale (reviewed by Harrison and Bruna 1999). Large-scale empirical work on fragmentation in real landscapes, including the temporal course of the consequences of fragmentation, is badly needed. The existing evidence on the effects of forest fragmentation comes predomi-nantly from tropical and temperate forests (e.g. Love-joy et al. 1986, Saunders et al. 1991, Turner 1996). Studies conducted in boreal forests have mainly com-pared species richness in forests with different manage-ment histories (Va¨isa¨nen et al. 1993, Siitonen and Martikainen 1994, Bader et al. 1995, Pettersson 1996, Dettki and Esseen 1998). These studies have demon-strated severe biological impoverishment of fragmented forests, partly caused by physical edge effects (Lovejoy et al. 1986, Camargo and Kapos 1995, Esseen and Renhorn 1998), but also by decreased availability and quality of decaying wood and other microhabitats (Sii-tonen and Martikainen 1994, Bader et al. 1995). There is also a growing body of evidence showing that frag-mentation may lead to chains of indirect effects (Love-joy et al. 1986, Turner 1996, Didham et al. 1998). These higher-order biological effects, for instance loss of an important predator leading to drastic changes at lower trophic levels, have been observed in a number of ecosystems including tropical forests. In this paper we show that the truncation of food chains and extinction of specialist species at high trophic levels happens also in boreal forests in insect communities inhabiting spe-cific microhabitats.

The fruiting bodies of macrofungi are important microhabitats for many specialized insects, but little is known about the ecology and habitat requirements of the vast majority of old-growth forest species feeding on fungi. Fungal insect communities typically consist of the species consuming the fungal tissue and their para-sitoids and predators. Fungivorous insects are generally assumed to be generalists (Hanski 1989), but many monophagous species exist especially among taxa that inhabit bracket fungi (e.g. Lawrence 1973, Rawlins 1984, Rukke and Midtgaard 1998, Kehler and Bondrup-Nielsen 1999). In addition to their specific fungal associations, some fungus-inhabiting insects have also specialized macrohabitat requirements, that is, they are restricted to old-growth forest although their fungal host occurs also in managed forests (Jonsell

1999). Generally, specialized species at higher trophic levels are more vulnerable to extinction (Pimm and Lawton 1977, Schoener 1989, Pimm 1991) as a result of lower absolute population sizes (Holt 1996), higher population variability (den Boer 1993, Kruess and Tscharntke 1994) and the dependence of higher trophic levels on the populations at lower trophic levels (Schoener 1989, Holt 1996). Consequently, food chain length appears shorter in small habitat fragments (Schoener 1989) and in unpredictable systems (Pimm 1991). The occurrence of fungi and their fruiting bodies is spatially and temporally highly variable (Hanski 1989, Ohlson et al. 1997). Such variability might make the fungal insect communities in small habitat frag-ments especially vulnerable to extinction.

The boreal forests in Finland have about 560 species of polyporoid and corticoid wood-decaying fungi (Aphyllophorales), of which 25% are currently threat-ened (Kotiranta and Niemela¨ 1996). The high level of threat is primarily caused by the dramatic loss in the area of old-growth forests. Presently only 0.1% of the forest land in southern Finland is covered by old-growth, and the average for the entire country is 5% (E. Tomppo unpubl. based on the National Forest Inven-tory of Finland). Because of the vulnerability of bracket fungi to forestry-related extinction, many insect species that are specialized on old-growth fungal species are also likely to be threatened. We have studied an old-growth forest specialist bracket fungus, Fomitopsis rosea(Alb. & Schwein.: Fr) P. Karsten (Polyporaceae), which has greatly declined in Finland due to forestry-caused habitat loss (Kotiranta and Niemela¨ 1996). Here we address one specific question: Does the structure of the insect community inhabiting F. rosea change in old-growth forest fragments in the course of time since isolation?

Material and methods

Fungal species

Fomitopsis roseais a wood-rotting bracket fungus caus-ing brown rot. The global distribution of the species is circumboreal in coniferous forests (Ryvarden and Gilbertson 1994). Fomitopsis rosea is dependent on old-growth spruce swamp forest where suitable Norway spruce trunks for the fungus are continuously available in large numbers. Spruce trunks that are suitable are typically relatively large, hard and partly barkless (Ren-vall 1995). The current distribution of F. rosea in Finland reflects the north-eastern distribution of the remaining old-growth forests (Kotiranta and Niemela¨ 1996). In these forests the species is among the most abundant bracket fungi (Renvall 1995). Fomitopsis roseais a perennial species and its fruiting bodies may last for several years with new hymenia produced every

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year. The fruiting bodies are typically less than 7 cm wide and 3 cm thick.

Study sites

This study was conducted in old-growth spruce swamp forests in eastern Finland at 64°N and 29 – 30°E in the middle boreal zone (Ahti et al. 1968). Approximately 9% of all forests in this area are old-growth (Hiltunen et al. 1997). Our study is part of a research project in which various taxa were surveyed in five control areas and in five fragments isolated for 2 – 7 yr and 10 frag-ments isolated for 12 – 32 yr since the cutting of their immediate surroundings. Data on the occurrence and abundance of F. rosea comes from this survey con-ducted for polyporous fungi by two of us, RP and ML. To study the insect community inF.roseawe selected three additional control areas to get a more comprehen-sive picture of the insect community. For logistic rea-sons, we did not sample fruiting bodies from three of the fragments. Therefore, in the insect community study, we compared eight control areas with four and eight fragments that had been isolated for 2 – 7 yr and for 12 – 32 yr, respectively.

The control areas are large, non-fragmented old-growth forests. Isolated fragments were selected in the following manner. First, spruce-dominated, old-growth forest stands were selected from the database main-tained by the Finnish Forest and Park Service. Second, isolation in distance and time since isolation, if not known exactly, were determined using aerial photo-graphs generally taken every fifth year. Isolation dis-tance was estimated as the disdis-tance to the nearest large (tens of ha) unmanaged mature or old-growth forest.

The estimation was done subjectively as the complexity of the landscape did not warrant any other simple and biologically sound measure. Eight of the fragments are surrounded by more than 100 m of clear-cut or saplings in all directions and two fragments by 50 m. Finally, all the stands thus selected were visited and the important forest characteristics were measured to make sure that the forest fragments were of similar quality. Following this detailed procedure only 15 forest stands remained in the state-owned forest area of about 2500 km2 (Hiltunen et al. 1997).

All the selected study sites were of equal quality as measured by the tree species composition, the number of dead trees, and the age of the forest (Table 1 summarizes the relevant information). Some of the selected forest stands have been treated with selective logging of larger trees in the beginning of this century. The forest stands have retained a multi-aged structure with many fallen and standing decaying trunks. The forest stands are mesic, mostlyMyrtillus-type (Cajan-der 1949) spruce-dominated forests with paludified patches where Sphagnum spp. dominate on the forest floor.

Sampling and rearing

We estimated the occurrence and abundance ofF.rosea in each control area by establishing a 9-ha square grid at a randomly selected location. Within the grid all fallen spruce trunks were checked forF.roseafruiting bodies. The sampling of fruiting bodies in control areas was conducted such that mesic patches were chosen from topographical maps, and samples were collected from several trunks in such a manner that the whole Table 1. Comparisons of the study site characteristics in control areas and in isolated fragments (mean (SD)).

Control areas

Variable Isolated fragments P

2–7 yr 12–32 yr (n=10) (n=5) (n=8) 60 251

Fruiting bodies collected 44 per site

32.3 (5.4) 15.0 (5.2) 11.0 (9.4) 1306 (1609) 8.2 (2.2) 6.1 (3.5) Area (ha) nse nse 1.7 (1.0) Isolation (km) – 1.2 (0.5) – 165 (22) Edge-area relationa 272 (114) 0.02e

Living trees m3/hab 236 (23) 215 (13) 229 (61) nsf

Snags/hab,c 63 (14) 68 (28) 59 (22) nsf nsf 135 (59) 129 (50) 113 (39) Logs/hab,c Spruce logs/hac 68 (29) 75 (48) 58 (35) nsf 171 (22) 172 (20)

Age of spruce treesd 171 (28) nsf

aEdge lengths of the old-growth forest fragments were divided by the fragment area. bMeasured for all tree species.

cOnly trunks at least 10 cm in breast height diameter were included. dThe age was measured for 10 trees/study site.

eDifferences between two fragment classes were tested with two samplet-tests;t-test with unequal variances was applied for the comparison of the edge-area relation.

fDifferences were tested with one-way ANOVA. Because of the unequal variances Kruskal-Wallis one-way ANOVA was used for the comparison of the amount of living trees.

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Table 2. Comparison of the insect community inF.roseabetween forest fragments in different isolation-in-time classes (only species with more than four individuals included).

Species Isolation-in-time-classes Control 2–7 yr 12–32 yr x2a Fomitopsis rosea trunks suitable 529 736 826 0.00 trunks occupied: C vs 2–7 27 38 trunks occupied: C vs 12–32 27 18 8.59** trunks occupied: 2–7 vs 12–32 38 18 10.03** Agnathosia mendicella

fruiting bodies collected 251 60 44

fruiting bodies occupied: C vs 2–7 63 7 5.01*

fruiting bodies occupied: C vs 12–32 63 3 7.20**

fruiting bodies occupied: 2–7 vs 12–32 7 3 0.69

Elfia cingulata

fruiting bodies with host 63 7 3

fruiting bodies occupied 15 2 0 0.07b

Other species

Trichosia sinuata(Dip.) 31/12c 0 0

Montescardia tessulatella(Lep.) 22/13 1/1 0

Cis dentatus(Col.) 20/10 1/1 2/2

Stenomacrus cur6ulus(Hym.) 14/9 2/1 0

Cis glabratus(Col.) 13/4 0 0

adf=1; *PB0.05; **PB0.01.

bMaterial in the two fragment classes pooled for the test.

cFigures are the number of individuals/the number of fruiting bodies in which the species was present.

area of a particular forest stand was covered. In the fragments, we checked all fallen spruce trunks for the F. rosea fruiting bodies, and sampled all the suitable fruiting bodies that could be found. Fruiting bodies of F. rosea grow on the sides of fallen trunks, and are easily sampled. Insects only inhabit fruiting bodies, which are at least partly dead. We collected 251 dead and dying fruiting bodies ofF.roseafrom the non-iso-lated control sites, and 60 and 44 fruiting bodies from the fragments isolated for 2 – 7 and 12 – 32 yr, respec-tively (Table 1). Sampling lasted 10 d starting from 23 May in 1998.

To rear out the insects, fruiting bodies were placed in cloth-covered plastic boxes and kept sheltered in out-door conditions for just longer than a period of one year. Rearings were checked for insects once a month and all the individuals that had emerged were preserved in alcohol or as dry specimens. Fruiting bodies were kept in outdoor conditions for winter, and the rearings were checked for insects for the last time in late July 1999. As far as possible, all adults were identified to species.

Results

We reared 33 insect species from theF.rosea fruiting bodies, including 19 Coleoptera (n=63 individuals), 5 Diptera (n=75), three Lepidoptera (n=194) and six Hymenoptera (n=21) species (summarized in Table 2; detailed discussion of the insect community in A. Komonen unpubl.). Relatively many of the species

reared in this study are classified as rare in Fennoscan-dia and/or are old-growth forest species. The numeri-cally dominant insect species (33% of all the insect individuals) were the microlepidopteran moth Ag -nathosia mendicella(Denis & Schiffermu¨ller) (n=168), larvae of which eat the fungal tissue, andElfia cingulata (Robineau-Desvoidy) (n=37), a parasitic fly appar-ently specializing on A. mendicella in old-growth forests.Elfia cingulatahas not been recorded from any other fungal species in Fennoscandia, and in this study no individuals of the parasitoid were reared from the fruiting bodies in which the other lepidopteran species, Montescardia tessulatella(Lienig & Zeller), was present. The average number (9SD) ofA.mendicella individu-als per occupied fruiting body was 2.792.7 in the control areas and 2.693.5 in the fragments. The corre-sponding figures for the parasitoid were 2.292.1 and 2.091.4, respectively.

The median number of trophic levels in the food chain consisting of F. rosea, A. mendicella and E. cingulatadecreased from three in control areas to one in the most isolated fragments (Fig. 1). The fraction of forest fragments in whichF.roseawas present declined with time since isolation and with larger edge to area relationship, followed by a more severe decrease in the fraction of A. mendicella-occupied forest fragments. The parasitoid was completely absent in the fragments isolated for more than 12 yr. The average number of suitable spruce trunks (\19 cm at breast height diame-ter) per location occupied byF.rosea, and the average number of host individuals per location occupied byA. mendicella and E. cingulata, were lower in the most

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isolated fragments, though variation in abundance was high (Fig. 2). In the fragments, the overall proportion of suitable trunks occupied byF.roseaincreased as the number of suitable trunks increased (logistic regression; deviance=13.04, df=1,PB0.001).

Fomitopsis rosea might be absent by chance only in some small forest fragments with a small number of trunks of fallen spruce trees, and the moth and the parasitoid might be similarly absent by chance in forest fragments with a small number of F. rosea. To test whether the frequencies of F.roseaand A.mendicella are significantly lower in the forest fragments than in the control areas we pooled the material in the control areas and in the two classes of fragments with a differ-ence in the time since isolation. Because of the low expected frequency of the parasitoid in the fragments we pooled the material for the two fragment classes. The results in Table 2 show thatF.roseahad a signifi-cantly lower frequency on suitable trunks in the frag-ments isolated for 12 – 32 yr than in the control areas and in recently isolated fragments. Agnathosia mendi -cella had significantly lower frequencies in the two fragment classes than in the control areas. There was no significant difference in the frequency ofA.mendi -cellain the two isolation-in-time classes or in the fre-quency of the parasitoid in the control areas and in the fragments. Table 2 also compares the occurrence of five other insect species between the control areas and the

Fig. 2. The fraction of suitable spruce trunks occupied byF.

rosea, fruiting bodies occupied byA.mendicella, and host-oc-cupied fruiting bodies ochost-oc-cupied byE.cingulataper location in control areas and in the fragments isolated for 2 – 7 yr and 12 – 32 yr.

Fig. 1. The number of trophic levels in the food chain consist-ing of the fungus F.rosea, the moth A. mendicella and the parasitoidE.cingulata in control areas and in the fragments isolated for 2 – 7 yr and 12 – 32 yr.

fragments. It is apparent that many species were absent or occurred in very low numbers in the fragments, but no statistical test can adequately test for a response due to the small numbers of individuals.

Discussion

The effect of forest fragmentation was clear for all the common species, and especially for the numerically dominant food chain consisting ofF.rosea,A.mendi -cella, and E. cingulata (Table 2, Figs 1, 2). Even the habitat generalist species (e.g. M. tessulatella), which occurred in relatively large numbers in the control areas, were missing from F. rosea in the fragments isolated for the longest period of time (Table 2). If these species should occur in the fragments in some other host fungus, they might be expected to inhabit alsoF. rosea. It is important to notice that the two numerically dominant parasitoid species,E.cingulataand the wasp Stenomacrus cur6ulus (Thompson) (Ichneumonidae), were completely absent from the forest fragments iso-lated for the longest period of time. Stenomacrus cur6ulus probably parasitizes the fly Trichosia sinuata Menzel & Mohrig (Sciaridae) as it was exclusively

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reared from the same fruiting bodies and the species is known to parasitize dipteran larvae (R. Jussila pers. comm.). Our results indicate that ecological specializa-tion at all trophic levels makes species vulnerable to extinction resulting from forest fragmentation. Smaller areas sustain smaller populations of all species, but increasingly so at the higher trophic levels. The effects of fragmentation are, therefore, most keenly experi-enced higher up in the food chain where population sizes are inherently smaller (Holt 1996). Below, we discuss the two main causes that may explain the absence of species from the fragments.

First, extinction of the lower trophic level accounted for half of the population extinctions at higher trophic levels in the forest fragments. Agnathosia mendicella was present, though generally less frequent than in the control areas, in all the fragments that had at least 13 dead and decaying fruiting bodies of F. rosea. Elfia cingulata was only present in the fragment with the highest number of trunks occupied by F. rosea and fruiting bodies occupied byA.mendicella, respectively. These results imply that each species in the food chain is very much dependent on the substrate availability at lower trophic level. However, because of the long dura-tion of the perennial F. rosea fruiting bodies, the present situation reflects environmental conditions even tens of years ago. Some of the existing fungal popula-tions can be considered to be ‘‘living deads’’, that is, the small and isolated local populations are doomed to extinction with a time-delay, because of intrinsic and extrinsic stochastic and deterministic factors (Soule´ 1980, Hanski et al. 1996). The occurrence of fruiting bodies, even in the case of perennial species, is spatially highly variable (Hanski 1989, Ohlson et al. 1997), and hence large areas are required to maintain viable popu-lations of the fungal species (Ho¨gberg 1998, Lindgren 1999, Penttila¨ unpubl.) and consequently of the insect species associated with them.

Large areas are needed to minimize edge effects for old-growth taxa that require particular microclimatic conditions (Esseen and Renhorn 1998). Many poly-porous fungi apparently belong to species with narrow microclimatic optima (Rayner and Boddy 1988, Ren-vall 1995) and are hence affected by edge effects (Sna¨ll and Jonsson 1999). In this study, the fragments isolated for the longest period of time had significantly more edge in relation to the fragment area than the younger fragments, and, therefore, edge effects may have had a negative effect on F.roseaand the associated species. However, as F. rosea occasionally occurs on sun-ex-posed trunks (Kotiranta and Niemela¨ 1996), is long-lived, and the fragmentation in this study has occurred relatively recently, edge-effects may not have had a major influence on the fungal species. Also, microlepi-dopteran moths inhabiting perennial bracket fungi can generally tolerate extensive drought and are easily reared in laboratory conditions (L. Kaila pers. comm.).

Based on the evidence discussed above, and on the fact thatF.roseaand species inhabiting it were more abun-dant in the fragments with larger supplies of suitable substrate, the availability of the substrate is probably the most important factor affecting the occurrence ofF. roseaand the associated species (see also Bader et al. 1995).

Second, limited dispersal range, making recoloniza-tion of isolated forest fragments unlikely, is another likely factor contributing to our results. There is little information on dispersal ability of fungi and associated insects, but indirect evidence, such as the absence of species from isolated habitat fragments, indicates that their dispersal ability is limited. Spores of old-growth fungal species may disperse at least up to 1 km (Norde´n 1999, Penttila¨ 1999), though they mainly disperse to the close vicinity of the fruiting body (Penttila¨ 1999). The forest fragments in this study were isolated by an average of 1.5 km (Table 1), were surrounded by clear-cuts and saplings, and may therefore have received little spore dispersal of F.rosea. To some extent, isolation was also likely to prevent the colonization of the moth A. mendicella. The dispersal ability of microlepi-dopteran moths is likely to be a few hundred metres, and much shorter in comparison with the recorded flight distances of butterflies and larger moths. Para-sitoids are generally assumed to be affected more by forest fragmentation than their hosts as they are at a higher trophic level, and are small in size and conse-quently likely to be poor dispersers. In this study, the frequency of the parasitoidE.cingulatawas not lower in the fragments than in the control areas. Nevertheless, the parasitoid was missing from the most isolated frag-ments. This suggests that the loss of the lower trophic level is the main factor causing the observed loss of the parasitoid. However, the actual causes of extinction remain largely unknown, and limited dispersal ability may have had an effect (see also Roland and Taylor 1997). According to Jonsell (1999), many Swedish red-listed insect species reared from the related fungal speciesF.pinicolashowed significantly higher frequency in less managed forests and the most specialized species were not able to colonize distinct forest islands.

Long time series of population abundances are rarely available to document the responses of species to envi-ronmental changes. In this study, we overcame this problem by studying fragments with dissimilar times since isolation (for another similar study see Soule´ et al. 1992). However, entirely identical fragments are hard to find, and as a result empirical studies of this type can be criticized for lack of control. Nonetheless, the absolute sampling of all fruiting bodies in this study revealed the true species composition in the isolated fragments and compensates for the slight differences that may have existed among the fragments. Although we can only speculate on the actual processes causing species loss from the fragments, our results clearly show that small

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and isolated old-growth forest fragments cannot main-tain the most specialized species in a managed forest landscape. Species loss appears mainly caused by de-cline in the availability of substratum. The long-term survival of specialized species depends on the species’ characteristics and on various environmental variables, but our results indicate that great changes in the species composition can occur in short time periods in rela-tively small forest fragments. If many of the old-growth forest fungi, and other specific microhabitats, have similar specialized insect communities than F. rosea, forest fragmentation has already initiated a massive extinction wave in boreal forests. Widespread extinction is already well-documented for several old-growth forest taxa in Fennoscandia (e.g. Berg et al. 1994, Siitonen and Martikainen 1994, Kotiranta and Niemela¨ 1996). If these results are representative for managed boreal forests worldwide, they suggest that great changes in forestry are required to prevent a wave of extinction cascades.

Acknowledgements– Our thanks are due to the specialists who identified particular taxa in the material: Marko Mutanen (Lepidoptera), Juha Siitonen (Coleoptera), Hans-Peter Tschorsnig (Tachinidae), Gergely Va´rgonyi, Veli Vikberg, Reijo Jussila (Hymenoptera) and Pekka Vilkamaa (Sciaridae). Steve Matter, Bob O’Hara and Tomas Roslin kindly provided comments on the manuscript. This study is part of the Finnish Biodiversity Research Program (FIBRE) and was supported by the research grant to the project Biodiversity in Boreal Forests.

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