Population contraction of the white-backed woodpecker Dendrocopos leucotos in Finland as a consequence of habitat alteration

Biological Conservation t993, 66, 47-53

POPULATION CONTRACTION OF THE WHITE-BACKED WOODPECKER Dendrocopos leucotos IN F I N L A N D AS A CONSEQUENCE OF HABITAT ALTERATION Raimo Virkkala Department of Zoology, PO Box 17, FIN-O0014, University of Helsinki, Helsinki, Finland

Tapio Alanko H~rMankatu 4 A 8, SF-15210 Lahti, Finland

Timo Laine Nurmelantie 54, SF-35220 ErtijLlrvi, Finland

&

Juha Tiainen Finnish Game and Fisheries Research Institute, PO Box 202, FIN-O0151 Helsinki, Finland

(Received 19 May 1992; revised version received 23 November 1992; accepted 4 December 1992)

modern forestry which generally removes all dead trees. As a result of forest management, the white-backed woodpecker has become an endangered species in Sweden (Aulrn, 1986, 1988) and has also decreased in many other European countries, such as Norway (H~land & Ugelvik, 1990), Poland (Wesolowski & Tomia|ojic, 1986), Germany (Scherzinger, 1990) and Russia (Nazarova, 1977). In Finland, large-scale intensive forestry started in the 1950s. Deciduous forests with dead trees have been cut and replanted with conifers. This has resulted in a considerable decrease and fragmentation of habitats preferred by the white-backed woodpecker during the past 40 years. At present the population is endangered and this woodpecker may disappear as a breeding species in Finland (see Rassi & V~iis/inen, 1987). Merely stochastic demographic or environmental factors, such as exceptional weather conditions, may devastate a small population (Shaffer, 1981; Gilpin & Soulr, 1986). Demographic stochasticity is a chance variation of birth and death. The effects of a cold winter may be the ultimate cause for the extinction of a small population. In Sweden the middle-spotted woodpecker Dendrocopos medias remained for decades as a small isolated population of 15-20 pairs, but became extinct after exceptionally cold winters and low reproductive success (Pettersson, 1984). Although the white-backed woodpecker has been reported to have declined in many countries, detailed quantitative analysis of population trends have not been made. In this paper we describe the contraction of the Finnish population in relation to habitat alteration, and analyse the biological factors affecting this decrease.

Abstract The population decline of the white-backed woodpecker Dendrocopos leucotos in Finland is presented and factors affecting this decrease are analysed. The woodpecker favours mature, deciduous forests which include many dead trees. These forests have been clearcut and thinned heavily from the 1950s. Winter bird counts from the late 1950s to the 1980s show declines of over 90%. The size of the present population is estimated at 30-50 breeding pairs. Fledging success and adult survival rate are not exceptionally low compared to the results of other studies but juvenile mortafity appears to be very high as the recruitment rate is low. The breeding population is also very sparse: the mean distance between nearest nests was 15 kin. In order to preserve the white-backed woodpecker from extinction in Finland a network of deciduous forests favoured by the woodpecker has been proposed.

Key words: Finland, white-backed woodpecker, contraction, forestry. INTRODUCTION

The white-backed woodpecker Dendrocopos leucotos is a species of broadleaved forests (Glutz von Blotzheim & Bauer, 1980; Cramp, 1985). The woodpecker feeds on wood-boring and bark-living insect larvae, mainly beetles and moths, in deciduous trees. Consequently, the species is dependent on the occurrence of dying and dead trees (Aulrn, 1988, 1991), and is very sensitive to Biological Conservation 0006-3207/93/$06.00 © 1993 Elsevier Science Publishers Ltd, England. Printed in Great Britain 47

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Raimo Virkkala et al.

DATA SOURCES AND METHODS There is qualitative information on the Finnish bird fauna from the middle and end of the 19th century (von Wright, 1859; von Haartman et al., 1963-72), which provides a baseline for estimating changes in distribution. Details of the distribution of the whitebacked woodpecker in the 20th century were compiled by von Haartman et al. (1963-72), Virkkala (1988) and Tiainen (1990). However, the species has never been abundant in Finland, it is secretive and detection is therefore poor. Changes in abundance of the white-backed woodpecker were also analysed from bird counts, which have been carried out every winter since 1956/57 (database at Zoological Museum, Finnish Museum of Natural History). The number of census routes has averaged about 500 with a total length of 5000-6000 km annually (Hild6n, 1987; Hild6n et al., 1991). Single midwinter counts are made between 26 December and 10 January when all birds observed along the route are recorded irrespective of their distance from the observer. Such counts cover the whole of Finland but the great majority of census routes are in the south. Habitat characteristics of white-backed woodpecker nesting sites have been quantified (Ikonen, 1989) within plots of 20-m radius (0.126 ha) centred on the nest sites. All trunks over 6 cm in diameter at breast height (DBH) were measured for each tree species and divided into DBH-classes of 6-12 cm and over 12 cm. The number of dead trees within the circle was also counted. Habitat descriptions of the woodpecker woods were carried out in the Forestry Board district of Ita-H~ime (8000 km2), where forests occupy 75% of the area and are dominated by conifers (89%). Deciduous trees only represent 9% of the forested area (Aarne et al., 1990) and only one-third of these were predominantly older than 60 years. The proportions of deciduous forests in the total forest area in southern Finland from the 1920s to the 1980s were obtained from national forest inventories in the official forest statistics. In the main distribution area of the white-backed woodpecker in eastern Finland the amount of forests with mature deciduous trees (over 60 years) predominating are available from three periods: 1964-67, 1977-80 and 1986-89. In 1988-91 a large scale survey of 2500 potential woodpecker forests organized by the World Wide Fund for Nature Finland (WWF) was carried out based on satellite imagery, aerial photographs and earlier records of the species. Mature deciduous forests thus detected were later examined in the field to check whether they were occupied by the woodpecker or considered suitable from the presence of dead and decaying trees. Whitebacked woodpeckers leave identifiable beak marks in wood, which differ from those made by the lesser spotted woodpecker Dendrocopos minor and the black woodpecker Dryocopus martius searching for wood-

boring insects. A forest with many foraging signs of the white-backed woodpecker was carefully searched to locate the individual(s). A colour-ringing programme of white-backed woodpeckers was started in 1988. Both the adults and fledglings were marked with individual combinations of colour rings. Survival rate of adult birds was calculated based on the Jolly-Seber mark-recapture method (Krebs, 1989). RESULTS Habitat characteristics and changes In the nesting woods of the white-backed woodpecker, deciduous trees comprised 93% of all living trees, with birches Betula pubescens and B. pendula (51%), alders Alnus incana and A. glutinosa (23%) and aspen Populus tremula (15%) dominant (n = 37, based on Ikonen, 1989). Birches formed 67% of the medium-sized (DBH 12-24 cm) and large (DBH >24 cm) trees, whereas among the small trees (DBH 6-12 cm) alders (32%) and aspen (19%) were more common (Fig. 1). The mean (+ SD) number of standing and fallen dead trees was 140 + 60/ha, i.e. 15%of the number of living trees. All the woodpecker woods were over 60 years old. Deciduous forests have declined as a percentage of total forested land in the whole of southern Finland from the 1920s to the 1980s as follows (Kuusela, 1972; Aarne et al., 1990): 1921-24, 20.4; 1936-38, 20.9; 1951-53, 17.5; 1960-63, 11-2; 1964-70, 9.6; 1977-84, 7.9. The slight decline between 1938 and 1952 was due to the change in the eastern boundary line of Finland as a

A

] ] ] ] ] ]

BirchBetula spp AlderAlnus spp AspenPopulus tremula Otherdeciduous tree species PinePinus sylvesttfs SprucePicea abies

B

Fig. 1. Tree species distribution around the nests of the white-backed woodpecker (n = 37). A, tree species with a diameter breast height (DBH) of 6-12 cm; B, tree species of over 12 cm DBH.

49

White-backed woodpecker and habitat alteration

Old forests dominated by deciduous trees have declined by 60% in the area during this 20-year period.

Changes in distribution and abundance

Fig. 2. The present distribution of the white-backed woodpecker in Finland ( ~ ) , and the northern boundary of the species' breeding range in the first half of the 20th century ( - - ) . /////~, The five Forestry Board districts (50 300 km z, see text). Numbers 1-4 refer to the recorded movement of the four fledglings (see text), consequence of the Second World War. Between then and 1977-84 the area of deciduous forests has been reduced to less than half, particularly in the late 1950s, when clear-felled areas were almost exclusively planted with pine Pinus sylvestris or spruce Picea abies. The decrease of deciduous forests in the whole of southern Finland has become slower in the 1970s and 1980s, as drainage of mires, started in the 1960s, has resulted in the establishment of dense young stands of Betula pubescens, particularly in western Finland. These forests are not suitable for the white-backed woodpecker. The main distribution centre of the white-backed woodpecker in Finland has been in the southeastern part of the country (Fig. 2). Figure 3 presents the decrease of old broadleaf stands (>60 years) in the five Forestry Board districts situated in southeastern Finland (see Fig. 2) from the mid-1960s to the late 1980s (based on Kuusela, 1967; Kuusela & Salovaara, 1968; Kuusela & Salminen, 1980, 1984; Aarne et al., 1990).

The white-backed woodpecker was distributed over most of southern Finland until the early 20th century (Fig. 2). However, the distribution was patchy (von Wright, 1859; von Haartman et al., 1963-72; Tiainen, 1990), the species being absent, for instance, from the barren watershed regions. The centre of distribution was in eastern Finland, where deciduous forests were abundant around the shorelines of many lakes. Deciduous trees also grew on former areas of slash-and-burn cultivation practised in eastern Finland until the late 19th century. Also as a consequence of cattle grazing in forests deciduous trees were dominant as the grazing prevented the growth of coniferous trees. The present population can be divided into two subpopulations: an eastern in the provinces of Karelia and Savo and a western in Itii-Hiime (Fig. 2). There are occasionally also some individual pairs outside the shown distribution. The extent of the western subpopulation is much more accurately known than that of the eastern subpopulation, which has only recently been verified (1988-91, Virkkaia et al., 1992). The population is everywhere very sparse. In the western subpopulation the shortest measured distance between nearest nests is 4-5 km, and the mean (_+ SD) distance between the two nearest nests in 1986-91 was 15 +_ 7 km (21 nest sites, range 4.5-26 km). The size of the western subpopulation is about 15 breeding pairs in a total area of about 6000 km 2 (one pair/300--500 km2). Figure 4 presents the population trend of the whitebacked woodpecker based on winter bird counts. The number of individuals observed annually was small, and results are therefore presented as individuals per 1000 km in five-year periods from the winter of 1956/57 to 1990/91. They show a significant decrease of over 90% from the late 1950s (1956/57-1960/61) to the 1980s (1981/82-1990/91; Kendall rank correlation, ~- -- -0.905, p < 0.01, n -- 7). About 30 individuals were observed in 2.0

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2000

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o

E

~

1.2

:_~

0.8

~

o

1000 -

s4

-

0.4. 6467

7780

8689

Years

Fig. 3. The area (km 2) of over 60-year-old forests dominated by deciduous trees in the area of five Forestry Board districts in eastern Finland (Itii-H~.me, Etel~i-Savo, It~l-Savo, Etel~iKarjala and Pohjois-Karjala) in 1964457, 1977-80 and 1986-89. The decrease (%) in the amount of deciduous forests from the 1960s to the 1970s and from the 1970s to the 1980s is given above the histograms.

0

, 5660

6165

6670

7175

7680

8185

8690

Winters

Fig. 4. Population trend of the white-backed woodpecker in Finland based on winter bird counts. Individuals observed in 1000-km survey transects are presented in five-year periods from winters of 1956/57 to 1990/91. The numbers of woodpecker observations are also presented as indices, the first five-year period having an index value of 100.

Raimo Virkkala et al.

50

both five-year periods in the 1960s but only a few birds (in total six) were seen in the last two five-year periods in the 1980s. The total Finnish white-backed woodpecker population was roughly estimated at 1000 pairs in 1950 (Lehtonen, 1955). For instance, population estimates for the great spotted woodpecker Dendrocopos major and the black woodpecker based on line transect censuses in 1941-56 were 100 000 and 15 000 pairs, respectively (Merikallio, 1958). These species have always been much more abundant in Finland than the whitebacked woodpecker. The large-scale survey in 1988-91 estimated 30--50 breeding pairs. Besides the 22 nests that were found in 1991 another eight territories with pairs were known (Virkkala et al., 1992). The decrease based on winter bird counts (about 95%) agrees with the estimated change in the population size from 1950 (about 1000 pairs) to 1988-91 (30-50 pairs). During the past two decades the western subpopulation (see Fig. 2) held 14 pairs around the communes of Lahti and Sysm~i in an area of 1500 km 2 in the mid 1970s, but only two were present in 1990-91 (T. Alanko & K. Reinikainen, pers. comm.).

Breeding data The mean (+SD) number of fledglings per nest during 1970-87 was 2.6 + 0.8/nest (n -- 20) and for 1988-91 was 2.7 + 1.0/nest (n = 42) with no significant betweenyear difference (Fig. 5, Kruskall-Wallis one-way analysis of variance, H = 2.49, d.f. = 3, ns). In the western subpopulation in It~i-Hame all the nests of known pairs were found in 1990-91 and the nesting was successful, all the pairs producing fledglings (MOiler & Alanko, 1991; Alanko & MOiler, 1992). Nestlings died in two unsuccessful nests in 1988-89 (Fig. 5). Clutch size and nesting success of the Finnish woodpeckers were recorded only in 1988 due to difficulties in recording all the eggs in the nest cavity. The mean clutch size was 3.2 (3 or 4 eggs, n = 9) and nesting success (% of young fledging of eggs laid) 72%. Altogether 118 fledglings were ringed in 1987-91, but by summer 1992 only four individuals had been

10-

8

= "5

6

E=

4

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1988

[]

1989

[]

1990

[]

1991

z 2

0

1

2

3

4

Number of fledglings

Fig.

5.

Number of Finnish white-backed woodpecker fledglings per nest in 1988-91.

recorded after leaving their natal territory (Fig. 2) and only two had been recruited into the population as breeding birds (recruitment rate 1.7%). A bird hatched in 1987 was found breeding in 1990 at a distance of 80 km from its natal site (No. 1 in Fig. 2). This bird was observed 60 km south of its natal site in winters 1987/88-1989/90 and was recorded as nesting 130 km from the wintering area. Another nestling ringed in 1991 was breeding in spring 1992 at a distance of 30 km from the natal site (No. 2 in Fig. 2). The other two birds were found dead at the age of one year 120 km (No. 3) and 20 km (No. 4 in Fig. 2) from their natal sites. Many white-backed woodpeckers are regularly observed at feeding stations. Adult breeding birds have been colour-ringed in the western subpopulation at these winter feeding sites from 1987/88: altogether 28 (17 males, 11 females) to 1990/91. Annual survival rate (+SE) of adults in the four-year period 1988-92 was 0.80 (+0-10). All such adults have been recorded less than 15 km from their original ringing site. DISCUSSION

Effects of habitat changes The rapid decline of the white-backed woodpecker in Finland is associated with the effects of habitat alteration and loss mainly caused by forestry. Mature deciduous forests preferred by the woodpecker decreased in eastern Finland by 60% from the mid-1960s to the late 1980s. Between the early 1950s and mid-1960s deciduous forests had already declined by nearly half, mainly as a consequence of extensive silvicultural practises to convert economically low-productive deciduous forests to coniferous stands. Thus, the total decrease of mature deciduous forests over the last 40 years is probably in the order of 80% in the species' main range. In the official forestry statistics, deciduous forests include all stands with over 50% deciduous trees. However, in the forests inhabited by white-backed woodpeckers the proportion of deciduous trees is over 90%. The decrease of pure deciduous forests is probably much greater than in mixed forests with deciduous trees predominating. Pure deciduous stands comprise only a tiny part of the forests dominated by deciduous trees. According to both Aulrn (1988) in Sweden and Scherzinger (1990) in Germany, a pair of white-backed woodpeckers needs at least 50-100 ha of deciduous forest to survive. In Finland no large uniform deciduous forests remain; they are nowadays highly fragmented and most of the white-backed woodpecker's nesting woods are only about 10-20 ha. To survive, the woodpeckers have to move over a very large area with several patches of small deciduous woods included in the home range. Moreover, most of the forests with deciduous trees predominating are nowadays managed stands from which practically all the dead and dying trees have been removed. The white-backed woodpecker is a species of mediumaged forests in the successional series. In boreal forests

White-backed woodpecker and habitat alteration of North Europe the spruce gradually outcompetes the deciduous trees (see Sir6n, 1955). Forest fires create new areas in which deciduous trees can grow. Many of the Finnish forests inhabited by the white-backed woodpecker have originated as a consequence of fire, slash-and-burn cultivation. 'Natural' forest fires caused by lightning occur in Fennoscandia in a given site usually once every 100-200 years (Zackrisson, 1977; Haapanen & Siitonen, 1978; Engelmark, 1987). After a forest fire, deciduous trees dominate for decades and fires also leave part of the trees as dead snags, thus providing good foraging and nesting trees for the white-backed woodpecker, which was therefore originally probably dependent on the effects of forest fires. During the 20th century forest fires have effectively been prevented in Finland. Another endangered species, the North American red-cockaded woodpecker Picoides borealis, is also dependent on the effects of fire. This species inhabits open pine woodlands which earlier were maintained by regular fires (Ligon et al., 1986; Waiters, 1991). Population biology Adult white-backed woodpeckers are sedentary, living all year in or near their nesting territory. Birds may remain and nest in the breeding habitat after the forest has been cut. Although there are observations of nests in a single tree in the middle of a clear-cut, this is due to the time lag effect of habitat selection (see Wiens, 1985); birds may stay in their territory although the habitat is totally altered. Nests found in clear-cuts were occupied by the woodpeckers before cutting. However, when one or both of the mates die the area is no longer inhabited by the woodpeckers. Offspring production (measured as number of fledglings/nest) of the white-backed woodpecker seems to be fairly good, and there is no evidence that it has decreased during the past 20 years. In western Norway the number of fledged young/nest was 3.1 (n = 7) in the 1970s (Bringeland & Fj~ere, 1981) and 2.8 (n = 12) in the late 1980s (Stenberg, 1990). Offspring production does not differ between Finland and Norway (MannWhitney U-test, z = 0-99, ns). In western Norway the population has been stable with 700-900 pairs (H~land & Ugelvik, 1990) in contrast to eastern Norway. The clutch size of the white-backed woodpecker is between 3 and 5 (Cramp, 1985); for instance, in Norway the mean clutch size was 3.7 (n = 12, based on Bringeland & Fj~ere (1981); Stenberg (1990)). As the mean number of fledglings/nest was 2.7 in Finnish white-backed woodpeckers, overall nesting success of the woodpecker is probably also fairly good. The survival rate of adult, breeding birds in Finland is not particularly low. Most of the breeding birds receive supplementary food during winter, in the form of fat provided at feeding stations. This feeding may enhance adult survival and breeding success as birds are probably in good condition in spring when breeding starts. In general, there are very few survival estimates of woodpeckers (excluding communally

51

breeding species). The annual survival rate of Swedish white-backed woodpeckers was estimated as 0-77 (Aul6n & Carlson, 1990). In the North American downy woodpecker Picoides pubescens the annual survival rate was calculated as 0.54 (Forde & Sloan, 1984) and 0-64 (Karr et al., 1990), and in the redbellied woodpecker Melanerpes carolinus as 0.66 (Karr et al., 1990) The relatively low offspring production of the white-backed woodpecker compared to the great spotted woodpecker, for example, is probably compensated by high adult and juvenile survival (see Glutz yon Blotzheim & Bauer, 1980). The few observations of juvenile Finnish whitebacked woodpeckers after post-fledging dispersal suggest that juvenile mortality is high or that the birds have possibly dispersed over a very large area, including northern Russia. However, almost all the fledglings (73/79) ringed in 1987-90 were from the western subpopulation which is situated 100-150 km from the Russian border. In Sweden the white-backed woodpecker's recruitment rate, 12.5% (seven of 56 fledglings recruited to the population as breeding birds, Carlson & Aul6n, 1992), is significantly higher than in Finland (1.7%, difference: ~ -- 9.0, p < 0.01). In Sweden juvenile survival was estimated at about 50% per year (Aul6n & Carlson, 1990). If the woodpecker has such a high juvenile survival and recruitment rate, there should be many more observations of the young individuals, especially since the western subpopulation is nowadays thoroughly studied with most of the nesting and wintering birds being colour-ringed. However, the numbers of recruited individuals will probably increase in Finland in the next few years. In the endangered spotted owl Strix occidentalis of North America, the low juvenile survival (10-20%) is probably a key factor affecting the decline of the species (Dawson et al., 1987). In Finland juvenile white-backed woodpeckers appear to have difficulties in finding suitable wintering habitats. It is highly probable that the increased juvenile mortality and/or dispersal has been the most important population biological factor affecting the drastic decline of this species in Finland during recent decades. The Finnish white-backed woodpecker population is not totally isolated, as it is probably connected with those in north Russia. For instance, in autumn 1987 there was an immigration of white-backed woodpeckers in Finland as a consequence of which the number of individuals observed during the winter 1987/88 more than doubled compared to the previous winter (Tiainen & Alanko, 1989). However, the number of breeding pairs did not increase in the breeding season of 1988, and the number of woodpeckers in the winter 1988/89 dropped to the same level as before immigration. The Finnish population is nowadays so sparse that individuals may have difficulty in finding a mate. There are observations of a white-backed woodpecker copulating with a great spotted woodpecker (D. Forsman, pers. comm; personal observations). Moreover, in the summer of 1992 a hybrid juvenile between these two species was observed

52

Raimo Virkkala et al.

(T. Laine, unpublished data). Demographic stochasticity, chance variation of survival and death of a few individuals, is thus a serious threat to the Finnish whitebacked woodpecker population.

Implications for conservation Without the protection of the last forest fragments preferred by the woodpecker, the extinction of the breeding white-backed woodpecker population in Finland is highly probable in the next 10-15 years. WWF Finland therefore completed a conservation plan for the species in 1992. This includes a recommendation to protect 199 important forest areas, the total size of which is about 50 km 2, consisting of a network of habitats in the main distribution area of the species (a similar proposal for the northern spotted owl Strix occidentalis is described by Dawson et al. (1987) and Thomas et aL (1990)). The ultimate goal is to provide opportunities for the population to increase in size to 100-150 pairs. The habitat network includes about 50 areas in which the woodpecker has nested during the past decade and others where single birds have been observed. It should facilitate movement of birds and assist juveniles to survive the first winters of their life, as this may be the crucial factor for the survival of the population.

ACKNOWLEDGEMENTS We acknowledge all the persons who have been involved in the woodpecker project. We are particularly grateful to the following who did a large part of the field work: T. Hokkanen, R. Ikonen, P. Koskimies, K. Martiskainen, J. Miettinen, M. Mtiller, E. Niinivirta, R. Pakarinen, K. Reinikainen and S. Sarkanen. The comments of two anonymous referees are acknowledged. The study has been financed by the World Wide Fund for Nature Finland (WWF), the Ministry of Environment in Finland and the Finnish Academy (to RV).

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