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

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1 OIKOS 90: Copenhagen 2000 Forest fragmentation truncates a food chain based on an old-growth forest bracket fungus Atte Komonen, Reijo Penttilä, Mariko Lindgren and Ilkka Hanski Komonen, A., Penttilä, R., Lindgren, M. and Hanski, I Forest fragmentation truncates a food chain based on an old-growth forest bracket fungus. Oikos 90: 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 yr since isolation. Fomitopsis rosea hosted a species-rich community with relatively many specialized old-growth forest insects. The numerically dominant food chain consisted of F. rosea, the tineid moth Agnathosia mendicella and the tachinid fly Elfia cingulata, a specialist parasitoid of A. mendicella. The frequency of F. rosea on suitable fallen spruce logs and the frequency of A. mendicella in 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 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, Di. of Population Biology, PO Box 17, FIN Uni ersity of Helsinki, Finland (present address of AK: Eastern Steppe Biodi ersity Project, PO Box 350, Choibalsan, Dornod, Mongolia [esbp@magicnet.mn]). R. Penttilä, Research Centre of Friendship Park, Tönölä, FIN Kuhmo, Finland. M. Lindgren, Dept of Ecology and Systematics, Di. of Systematic Biology, PO Box 47, FIN Uni ersity 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 surprisingly diverse in many taxa, for example, of the estimated species in Canada, 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 microhabitat 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 dependent 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 Printed in Ireland all rights reserved OIKOS 90:1 (2000) 119

2 remaining old-growth forests. Environmental conditions 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 different 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 predominantly from tropical and temperate forests (e.g. Lovejoy et al. 1986, Saunders et al. 1991, Turner 1996). Studies conducted in boreal forests have mainly compared species richness in forests with different management histories (Väisänen et al. 1993, Siitonen and Martikainen 1994, Bader et al. 1995, Pettersson 1996, Dettki and Esseen 1998). These studies have demonstrated 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 (Siitonen and Martikainen 1994, Bader et al. 1995). There is also a growing body of evidence showing that fragmentation may lead to chains of indirect effects (Lovejoy 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 specific 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 parasitoids 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 fragments 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 threatened (Kotiranta and Niemelä 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 oldgrowth, and the average for the entire country is 5% (E. Tomppo unpubl. based on the National Forest Inventory 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 oldgrowth forest specialist bracket fungus, Fomitopsis rosea (Alb. & Schwein.: Fr) P. Karsten (Polyporaceae), which has greatly declined in Finland due to forestrycaused habitat loss (Kotiranta and Niemelä 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 rosea is a wood-rotting bracket fungus causing 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 (Renvall 1995). The current distribution of F. rosea in Finland reflects the north-eastern distribution of the remaining old-growth forests (Kotiranta and Niemelä 1996). In these forests the species is among the most abundant bracket fungi (Renvall 1995). Fomitopsis rosea is a perennial species and its fruiting bodies may last for several years with new hymenia produced every 120 OIKOS 90:1 (2000)

3 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 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 fragments isolated for yr since the cutting of their immediate surroundings. Data on the occurrence and abundance of F. rosea comes from this survey conducted for polyporous fungi by two of us, RP and ML. To study the insect community in F. rosea we selected three additional control areas to get a more comprehensive picture of the insect community. For logistic reasons, 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 yr, respectively. The control areas are large, non-fragmented oldgrowth forests. Isolated fragments were selected in the following manner. First, spruce-dominated, old-growth forest stands were selected from the database maintained by the Finnish Forest and Park Service. Second, isolation in distance and time since isolation, if not known exactly, were determined using aerial photographs generally taken every fifth year. Isolation distance was estimated as the distance 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 km 2 (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, mostly Myrtillus-type (Cajander 1949) spruce-dominated forests with paludified patches where Sphagnum spp. dominate on the forest floor. Sampling and rearing We estimated the occurrence and abundance of F. 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 for F. rosea fruiting 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)). Variable Control areas Isolated fragments P (n=8) 2 7 yr yr (n=5) (n=10) Fruiting bodies collected per site 32.3 (5.4) 15.0 (5.2) 11.0 (9.4) Area (ha) 1306 (1609) 8.2 (2.2) 6.1 (3.5) ns e Isolation (km) 1.2 (0.5) 1.7 (1.0) ns e Edge-area relation a 165 (22) 272 (114) 0.02 e Living trees m 3 /ha b 236 (23) 215 (13) 229 (61) ns f Snags/ha b,c 63 (14) 68 (28) 59 (22) ns f Logs/ha b,c 113 (39) 129 (50) 135 (59) ns f Spruce logs/ha c 68 (29) 75 (48) 58 (35) ns f Age of spruce trees d 172 (20) 171 (22) 171 (28) ns f a Edge lengths of the old-growth forest fragments were divided by the fragment area. b Measured for all tree species. c Only trunks at least 10 cm in breast height diameter were included. d The age was measured for 10 trees/study site. e Differences between two fragment classes were tested with two sample t-tests; t-test with unequal variances was applied for the comparison of the edge-area relation. f Differences 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. OIKOS 90:1 (2000) 121

4 Table 2. Comparison of the insect community in F. rosea between 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 yr 2a Fomitopsis rosea trunks suitable trunks occupied: C vs trunks occupied: C vs ** trunks occupied: 2 7 vs ** Agnathosia mendicella fruiting bodies collected fruiting bodies occupied: C vs * fruiting bodies occupied: C vs ** fruiting bodies occupied: 2 7 vs Elfia cingulata fruiting bodies with host fruiting bodies occupied b Other species Trichosia sinuata (Dip.) 31/12 c 0 0 Montescardia tessulatella (Lep.) 22/13 1/1 0 Cis dentatus (Col.) 20/10 1/1 2/2 Stenomacrus cur ulus (Hym.) 14/9 2/1 0 Cis glabratus (Col.) 13/4 0 0 a df=1; * P 0.05; ** P b Material in the two fragment classes pooled for the test. c Figures 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 of F. rosea from the non-isolated control sites, and 60 and 44 fruiting bodies from the fragments isolated for 2 7 and yr, respectively (Table 1). Sampling lasted 10 d starting from 23 May in To rear out the insects, fruiting bodies were placed in cloth-covered plastic boxes and kept sheltered in outdoor 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 As far as possible, all adults were identified to species. Results We reared 33 insect species from the F. 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 Fennoscandia and/or are old-growth forest species. The numerically dominant insect species (33% of all the insect individuals) were the microlepidopteran moth Agnathosia mendicella (Denis & Schiffermüller) (n=168), larvae of which eat the fungal tissue, and Elfia cingulata (Robineau-Desvoidy) (n=37), a parasitic fly apparently specializing on A. mendicella in old-growth forests. Elfia cingulata has 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 ( SD) of A. mendicella individuals per occupied fruiting body was in the control areas and in the fragments. The corresponding figures for the parasitoid were and , respectively. The median number of trophic levels in the food chain consisting of F. rosea, A. mendicella and E. cingulata decreased from three in control areas to one in the most isolated fragments (Fig. 1). The fraction of forest fragments in which F. rosea was 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 diameter) per location occupied by F. rosea, and the average number of host individuals per location occupied by A. mendicella and E. cingulata, were lower in the most 122 OIKOS 90:1 (2000)

5 isolated fragments, though variation in abundance was high (Fig. 2). In the fragments, the overall proportion of suitable trunks occupied by F. rosea increased as the number of suitable trunks increased (logistic regression; deviance=13.04, df=1, P 0.001). 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. rosea and 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 difference 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 that F. rosea had a significantly lower frequency on suitable trunks in the fragments isolated for yr than in the control areas and in recently isolated fragments. Agnathosia mendicella had significantly lower frequencies in the two fragment classes than in the control areas. There was no significant difference in the frequency of A. mendicella in the two isolation-in-time classes or in the frequency 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 by F. rosea, fruiting bodies occupied by A. mendicella, and host-occupied fruiting bodies occupied by E. cingulata per location in control areas and in the fragments isolated for 2 7 yr and 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. Fig. 1. The number of trophic levels in the food chain consisting of the fungus F. rosea, the moth A. mendicella and the parasitoid E. cingulata in control areas and in the fragments isolated for 2 7 yr and yr. Discussion The effect of forest fragmentation was clear for all the common species, and especially for the numerically dominant food chain consisting of F. rosea, A. mendicella, 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 also F. rosea. It is important to notice that the two numerically dominant parasitoid species, E. cingulata and the wasp Stenomacrus cur ulus (Thompson) (Ichneumonidae), were completely absent from the forest fragments isolated for the longest period of time. Stenomacrus cur ulus probably parasitizes the fly Trichosia sinuata Menzel & Mohrig (Sciaridae) as it was exclusively OIKOS 90:1 (2000) 123

6 reared from the same fruiting bodies and the species is known to parasitize dipteran larvae (R. Jussila pers. comm.). Our results indicate that ecological specialization 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 experienced 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 by A. 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 duration of the perennial F. rosea fruiting bodies, the present situation reflects environmental conditions even tens of years ago. Some of the existing fungal populations 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 (Soulé 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 populations of the fungal species (Högberg 1998, Lindgren 1999, Penttilä 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 polyporous fungi apparently belong to species with narrow microclimatic optima (Rayner and Boddy 1988, Renvall 1995) and are hence affected by edge effects (Snäll 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. rosea and the associated species. However, as F. rosea occasionally occurs on sun-exposed trunks (Kotiranta and Niemelä 1996), is longlived, and the fragmentation in this study has occurred relatively recently, edge-effects may not have had a major influence on the fungal species. Also, microlepidopteran 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 that F. rosea and species inhabiting it were more abundant in the fragments with larger supplies of suitable substrate, the availability of the substrate is probably the most important factor affecting the occurrence of F. rosea and the associated species (see also Bader et al. 1995). Second, limited dispersal range, making recolonization 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 (Nordén 1999, Penttilä 1999), though they mainly disperse to the close vicinity of the fruiting body (Penttilä 1999). The forest fragments in this study were isolated by an average of 1.5 km (Table 1), were surrounded by clearcuts 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 microlepidopteran moths is likely to be a few hundred metres, and much shorter in comparison with the recorded flight distances of butterflies and larger moths. Parasitoids 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 consequently likely to be poor dispersers. In this study, the frequency of the parasitoid E. cingulata was not lower in the fragments than in the control areas. Nevertheless, the parasitoid was missing from the most isolated fragments. 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 redlisted insect species reared from the related fungal species F. pinicola showed 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 environmental changes. In this study, we overcame this problem by studying fragments with dissimilar times since isolation (for another similar study see Soulé etal. 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 124 OIKOS 90:1 (2000)

7 and isolated old-growth forest fragments cannot maintain the most specialized species in a managed forest landscape. Species loss appears mainly caused by decline 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 relatively 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 Niemelä 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 Várgonyi, 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. References Ahti, T., Hämet-Ahti, L. and Jalas, J Vegetation zones and their sections in northwestern Europe. Ann. Bot. Fenn. 5: Anon The State of Canada s Forests, Canadian Forest Service, Catalogue Fo1-6/1994-E. Ottawa. Bader, P., Jansson, S. and Jonsson, B. G Wood-inhabiting fungi and substratum decline in selectively logged boreal spruce forests. Biol. Conserv. 72: Berg, A., Ehnström, B., Gustafsson, L. et al Threatened plant, animal, and fungus species in Swedish forests: distribution and habitat associations. Conserv. Biol. 8: den Boer, P. J Are the fluctuations of animal numbers regulated or stabilized by spreading of the risk. Oikos 72: Cajander, A. K Forest types and their significance. Acta For. Fenn. 56: Camargo, J. L. C. and Kapos, V Complex edge effects on soil moisture and microclimate in central Amazonian forest. J. Trop. Ecol. 11: Dettki, H. and Esseen, P.-A Epiphytic macrolichens in managed and natural forest landscapes: a comparison at two spatial scale. Ecography 21: Didham, R. K., Lawton, J. H., Hammond, P. M. and Eggleton, P Trophic structure stability and extinction dynamics of beetles (Coleoptera) in tropical forest fragments. Philos. Trans. R. Soc. Lond. B 353: Esseen, P. and Renhorn, K Edge effects on an epiphytic lichen in fragmented forests. Conserv. Biol. 12: Esseen, P. A., Ehnström, B., Ericson, L. and Sjöberg, K Boreal forests the focal habitats of Fennoscandia. In: Hansson, L. (ed.), Ecological principles of nature conservation. Elsevier, pp Hanski, I Fungivory: fungi, insects and ecology. In: Wilding, N., Collins, N. M., Hammond, P. M. and Webber, J. F. (eds), Insect-fungus interactions. Academic Press, pp Hanski, I. and Hammond, P Biodiversity in boreal forests. Trends Ecol. Evol. 10:. Hanski, I. and Gilpin, M. E. (eds) Metapopulation biology. Ecology, genetics, and evolution. Academic Press. Hanski, I., Moilanen, A. and Gyllenberg, M Minimum viable metapopulation size. Am. Nat. 147: Harrison, S. and Bruna, E Habitat fragmentation and large-scale conservation: what do we know for sure? Ecography 22: Hiltunen, V., Kytövuori, T., Siira, J. et al Kainuun alueen luonnonvarasuunnitelma. Metsähallituksen metsätalouden julkaisuja 8, Vantaa. Hofgaard, A yr of change in a Swedish boreal old-growth Picea abies forest. J. Veg. Sci. 4: Högberg, N Population biology of common and rare wood-decay fungi. Ph.D. thesis, Swedish Univ. of Agricultural Sciences, Uppsala. Holt, R. D Food webs in space: an island biogeographical perspective. In: Polis, G. A. and Winemiller, K. O. (eds), Food webs: integration of patterns and dynamics. Chapman & Hall, pp Jonsell, M Insects on wood-decaying polypores: conservation aspects. Ph.D. thesis, Swedish Univ. of Agricultural Sciences, Uppsala. Kehler, D. and Bondrup-Nielsen, S Effects of isolation on the occurrence of a fungivorous forest beetle, Bolitotherus cornutus, at different spatial scales in fragmented and continuous forests. Oikos 84: Kotiranta, H. and Niemelä, T Threatened polypores in Finland, 2nd ed. Finnish Environment Inst., Edita, Helsinki. Kruess, A. and Tscharntke, T Habitat fragmentation, species loss, and biological control. Science 264: Kuusinen, M Importance of spruce swamp-forests for epiphyte diversity and flora on Picea abies in southern and middle boreal Finland. Ecogaphy 19: Lawrence, J. F Host preference in Ciid beetles (Coleoptera: Ciidae) inhabiting the fruiting bodies of basidiomycetes in North America. Bull. Mus. Comp. Zool. 145: Lindgren, M Polypore (Basidiomycetes) species richness and community composition in old-growth boreal forests of northeastern Finland and adjacent Russian Karelia. MSc. Thesis, Dept of Ecology and Systematics, Univ. of Helsinki. Lovejoy, T. E., Bierregaard, R. O., Jr., Rylands, A. B. et al Edge and other effects of isolation on Amazon forest fragments. In: Soulé, M. E. (ed.), Conservation biology: the science of scarcity and diversity. Sinauer, pp Mooney, H. A., Lubchenco, J., Dirzo, R. and Sala, O. E Biodiversity and ecosystem functioning: ecosystem analyses. In: Heywood, V. H. (ed.), Biodiversity assessment. Global UNEP, Cambridge Univ. Press, pp Nordén, B Dispersal ecology of some wood-decaying fungi. In: Abstract volume, Nordic symposium on the ecology of coarse woody debris in boreal forests, 31 May 3 June Umeå Univ., Umeå. Ohlson, M Dikning av näringsrik sumpskog ett hot mot våra mest artrika skogsekosystem. Skogsfakta Flora, fauna, miljö, 14. Swedish Univ. of Agricultural Sciences, Uppsala. OIKOS 90:1 (2000) 125

8 Ohlson, M., Söderström, L., Hörnberg, G. et al Habitat qualities versus long-term continuity as determinants of biodiversity in boreal old-growth swamp forests. Biol. Conserv. 81: Penttilä, R Dispersal of Phlebia centrifuga, a wood-rotting fungus specialized on old-growth forests. In: Abstract volume, Nordic symposium on the ecology of coarse woody debris in boreal forests, 31 May 3 June Umeå Univ., Umeå. Pettersson, R. B Effect of forestry on the abundance and diversity of arboreal spiders in the boreal spruce forest. Ecography 19: Pimm, S The balance of Nature? Chicago Univ. Press. Pimm, S. L. and Lawton, J. H Number of trophic levels in ecological communities. Nature 268: Rawlins, J. E Mycophagy in Lepidoptera. In: Wheeler, Q. and Blackwell, M. (eds), Fungus-insect relationships. Columbia Univ. Press, pp Rayner, A. D. M. and Boddy, L Fungal decomposition of wood. Its biology and ecology. John Wiley & Sons. Renvall, P Community structure and dynamics of woodrotting Basidiomycetes on decomposing conifer trunks in northern Finland. Karstenia 35: Roland, J. and Taylor, P. D Insect parasitoid species respond to forest structure at different spatial scales. Nature 386: Rukke, B. A. and Midtgaard, F The importance of scale and spatial variables for the fungivorous beetle Bolitophagus reticulatus (Coleoptera, Tenebrionidae) in a fragmented forest landscape. Ecography 21: Ryvarden, L. and Gilbertson, R. L European polypores 1. Abortiporus-Lindtneria. Synopsis Fungorum 6. Saunders, D. A., Hobbs, R. J. and Margules, C. R Biological consequences of ecosystem fragmentation: a review. Conserv. Biol. 5: Schoener, T. W Food webs from the small to the large. Ecology 70: Siitonen, J. and Martikainen, P Occurrence of rare and threatened insects living on decaying Populus tremula: a comparison between Finnish and Russian Karelia. Scand. J. For. Res. 9: Snäll, T. and Jonsson, B. G Edge effect on polyporous fungi in forest fragments. In: Abstract volume, Nordic symposium on the ecology of coarse woody debris in boreal forests, 31 May 3 June Umeå Univ., Umeå. Soulé, M. E Thresholds for survival: maintaining fitness and evolutionary potential. In: Soulé, M. E. and Wilcox, B. A. (eds), Conservation biology: an evolutionary-ecological perspective. Sinauer, pp Soulé, M. E., Alberts, A. C. and Bolger, D. T The effects of habitat fragmentation on chaparral plants and vertebrates. Oikos 63: Turner, I. M Species loss in fragments of tropical rain forest: a review of the evidence. J. Appl. Ecol. 33: Väisänen, R., Biström, O. and Heliövaara, K Sub-cortical Coleoptera in dead pines and spruces: is primeval species composition maintained in managed forests? Biodiv. Conserv. 2: OIKOS 90:1 (2000)