Teivainen, Terttu

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METSÄNTUTKIMUSLAITOKSEN TIEDONANTOJA 21 METSÄNSUOJELUN TUTKIMUSOSASTO Terttu Teivainen  GEOGRAPHIC TRENDS IN VOLES IN FINLAND IN THE YEARS 1973-80  MYYRÄTUHOJEN ESIINTYMISALUEEN  SIIRTYMINEN SUOMESSA VUOSINA 1973-80  HERBIVORE-PLANT INTERACTIONS AT NORTHERN LATITUDES  A SYMPOSIUM WORKSHOP AT KEVO FINLAND 14-18 SEPTEMBER 1981 ISSN 0358-4283 1. INTRODUCTION Many field studies on cyclic  fluctuations of vole populations  have been done during recent decades but most  of them consist  of materials collected from rather restricted or local areas.  The availablity  of material covering  long periods  of time has made it possible  to state with some certainty  that local va  riations in the size  of vole stocks exhibit a certain degree  of regular  periodicity,  in which the peaks  generally follow each other at intervals of three or four years,  although  the intervals may sometimes be either shorter or  longer (e.g.  WILDHAGEN 1949, 1952, 1954 cited by MYRBERGET 1973, KALELA 1962, MYRBERGET 1965, LAHTI et ai.  1976).  After analysing  material  covering  vole stocks in different years in Norway,  HAGEN (1956  cited by MYRBERGET 1973) found that the vole peak in southern Norway  occurred one year before that in Central Norway  and two years  before that in northern Norway. According  to MYRBERGET (1965), the vole peak  moved from region  to region  during the three cycles  occurring  in the period  1946-60,  in the same way as described by HAGEN, but not during the fourth cycle. In the 1960 's there was again  one cycle  in Norway (MYRBERGET  1973) in which the peak in the south was one year earlier than in the north and two cycles in which the regional  differences were  random. The asynchronisation  of vole peaks  in different areas has also been observed in Finland. According  to KALELA (1962), there were  three peaks  during the period 1946-61 in northern Lapland  and four in southern Lapland. He stated that the synchronisation  of vole peaks  in different regions  throughout  the country  decreased on moving southwards while at the same time the fluc  tuation in vole populations  becomes less and less pronounced.  The increase in the amount of young plantations  especially  on old fields, has been followed by an increase in the amount of damage caused by voles to forest tree seedlings  in the Nordic Countries (cf. KANERVO & MYLLYMÄKI  1970, TEIVAINEN 1974,  1978,  CHRISTIANSEN 1975, LARSSON 1975). The damage has risen 2 to such proportions  that it  has been considered necessary to carry  out studies into the periodicity  of the variations in  vole abundance by means  of extensive systematic  trapping (MYL  LYMÄKI 1977, MYLLYMÄKI  et ai. 1977). In addition to methods based on direct trapping,  vole density  can also be followed by  inventorying  the damage caused by voles, the frequency  of dam  age thus indicating  the relative abundance of voles (cf. MYLLY  MÄKI 1959)  . Over 200 million seedlings  are planted  annually  in Finland. This means that about 100 000 ha of new planting  areas, which are  more or less  distributed evenly  throughout  the country,  are  established each year. Finland is therefore continuously  covered by a fairly  uniform network of plantations  containing  seedlings  which are only  a few years old. These young stands are  generally  watched and tended rather closely  by professional  foresters or  plantation  owners during the first  few years after  planting. There is therefore good reason  to assume that vole damage,  which is in most cases directed at seedling  stands during  the first  few years after planting and which is often alarmingly  evident,  will be readily  noticed in such stands. Seedling stand damage  can therefore be considered to be a good  indicator of the relative number of voles, to the extent that the different species  of vole are important  damaging agents.  This talk is based on the results  of the nationwide inventory  of damage caused by voles to forest tree seedlings  carried out annually  (TEIVAINEN 1978). However, control trapping and observations on the wintering  signs  of voles are also made annually  in different parts  of Finland. The aim of the re  search work has been to determine the temporal and regional  regularities  of vole damage in order to provide  a basis for making forecasts,  which have been published  every year since 1975, for use in practical  forestation  work (e.g. TEIVAINEN 1975, 1976, 1977  a, 1977b, 1979,1980).  3  2. MATERIAL AND METHODS The material presented  here concerning  the damage  caused by  voles to forest tree seedlings  has been collected during 1973-80 using  methods which have been described elsewhere (TEIVAINEN  1978). The material consists  of the 2121 cases  of vole damage  occurring  in seedling  stands during a period  of eight  years,  damaging a total of 7,4 million seedlings.  Vole control trapping  was usually  carried out in the autumn and wintering  signs  were examined in the spring  during the snow  melt. These field studies were carried out in order to check  and control the picture  given by the damage survey about the areas where vole stocks  are high.  3. DISTRIBUTION OF DAMAGE AND REGIONAL RHYTHM The areas where vole damage occurred have varied every  year. In 1972/7  3 they were concentrated in the central parts of south  ern Finland (Fig. 1). The next. year the damage epicentre  moved further to the east  and the north, extending  right  upto the western part of Lapland  (Fig. 2). However,  the amount of dam  age along the south-western and southern coasts increased at the same time in comparison  to the previous  year's damage (Fig.9C).  In 1974/75 the trapping figures  indicated that the vole peak  had now  reached the eastern and northern parts  of Lapland  al  though no damage  had occurred there. On the other hand, the damage centre in Central Finland was  confined to a small area  (Fig. 3). In 1975/76 damage occurred over a wider area in Central Finland,  on the southern and eastern side of the pre  vious year's epicentre  (Fig. 4). The damage centre in the following  year had moved, on the one hand, to the east and to  the north, and on the other, to the south-west and to the south (Fig. 5), the most serious damage occurring  in almost the same area as  in 1973/74,  but more  to the south. In 1977/78 the damage centre had moved to Lapland, further north and more to the east of the previous  year's damage 4 centre (Fig. 6). Simultaneously  the vole stocks in south  western Finland and along the western half of the coast  of the Gulf  of  Finland were high and on the increase in Central Finland  (Fig.9C).The  next year (in 1978/79)  the damage area  spread,  start  ing from the concentration in Central Finland,in  the same way as  it had done three years earlier, to the south and to the east (Fig. 7). In 1979/80 the damaged areas were  restricted to a dense cluster touching the eastern border (Fig. 8)  but  damage  did not spread to western Lapland  as had happened  three and six  years earlier. At the present  time, there are large plenty of voles in Kainuu and in western Lapland,  i.e. the area which lies to the north of the previous  concentration. During these years, the damage centre appeared  to move in a rather regular  fashion from one  area to the next in successive  years. If the starting point is taken as the rather restricted  area  in Central Finland,  in the second year the damage centre spread  to the south and to the east. In the third year it  moved even further to the east and to the north, right  up to the southern and western parts of Lapland  (Fig. 9A) . The peak  ap  peared in the eastern and northern parts of Lapland in the fourth year, while at the same time a new cycle  was starting in Central Finland. The spread  of the damage wave towards the western and southern coasts  of South Finland was  not so clear as  in the eastern and northern directions (Fig. 9 B and C) . However, the clear but  small  damage peaks  along the south-western and southern coastal regions  during  the study  period  occurred  at the same time as those in the North-Karelian -  North Ostrobothnian zone. The results  thus indicate that the spread  of the damage wave,  which started in Central Finland,  spread  out more or less in all directions. However, the actual  progression  of the wave  followed, on the one hand, the distribution of fertile land and,  on the other, was more apparent  in areas  where there is slight  variation in the snow  conditions in different years.  The rhythm of cyclic  fluctuation in the vole stocks  in Central  Finland is, according  to MYLLYMÄKI (1977), triennial. Accord  5 ing to our material, too, the damage peaks in that area  occurred at intervals of three years. In Finland's  northern  most corner, at  Kilpisjärvi,  the study (LAHTI  et ai. 1976)  carried out using the material which KALELA et al. collected during  a period of 25 years showed that the vole peaks occurred  at intervals of 4 -  5 years. The longer cycle,  characteristic  of northern Finland,  was also evident in the damage survey  material as both successive peaks at four-year intervals, and as a gradual  shift  in the area of the damage centre to the south calculated according  to three year cycles.  If the observed regional  order in the movement of the damage wave  takes  place regularly,  as the material collected upto now indicates,  the difference in the duration of the cycles  in southern and northern Finland caused a regional  multiple  rhythm  of longer  duration. Thus the similar regional  appearance of a vole peak would recur  between intervals longer than one  cycle. Respectively,  the areal extent of vole peak  will vary  between successive  cycles.  4. THE REGIONAL RHYTHM IN RELATION TO BIOMASS PRODUCTION  Periodiocity  is a very common  phenomenon  in nature. The change  over from day to night  causes  photoperiodicity , alternation of seasons  produce  changes  in day-length,  winter and summer, coldness and warmth. The further north we go, the greater  the amplitude  of the variation. The length of the growing season  shortens,  but the intensity  of plant biomass formation in  creases  as a result of  the increased length  of the daylight  per iod . This periodicity,  the intensive but short period  of plant  biomass production  and the long  period  of winter dormancy aan  not be considered to have had no effect on the development  of a  fluctuating  type of ecosystem.  In order that the food cycle  of this ecosystem  from green plant  biomass  to mammalian herbivores and to carnivores could have evolved under these conditions,  plant  production  will have had to have been utilized  as  efficiently  as possible  and the transfer of energy, too, 6 fixed in the plant  biomass as  efficiently  as possible.  As,  however,  the bearing  capacity  of the first  trophic  level and regeneration  is the weaker, the further north the location is, the energy flow from plants to herbivores and to predators  has had to adapt  to the basic  rhythm, while at the same time, however,  adapting to exploitation  of plant biomass as  effi  ciently  as possible. The evolutionary  selection value of  the variation in the size of the vole population  can be inter  preted from this by assuming that it consists  of two components  -  temporal rhythm  (length  of the cycles)  and geographical  rhythm (movement  of the vole peak  epicenters  from area to area  in a specific  order). The interaction  between these two components  permits the vegetation  to be exploited  as  effi  ciently  as possible  right up to over-grazing-level , as  a result  of which the vole population  increases  to overdense levels  and then collapses.  However,  destruction of the vegetation  cover  is not permanent  as there is a period  lasting  for three years at  least before the peak  again  develops  in the same area, and hence the vegetation  has time to recover. In the north where regeneration  is slower, the time interval between peaks  is longer.  As a matter of fact the movement of the vole peak  epicenter  from one area to another in a specific  order is a rather similar phenomenon  to the nomadic grazing  of large herbivores according  to the season. However,  movement of the density  center of the vole population  from one  area  to another occurs  in successive  years, in different generations  and in different populations.  The system represents,  on the one hand, retention of the trophic  level occupied  by the voles, and on the other,  a continued supply of food to predators,  from the point  of view of successfull  nesting. According  to this study,  the distance between successive  density  centers is so short that the movement of many predators  with specialised  food require  ments, especially  predacious  birds,  along  with the density  centers to new nesting grounds is  completely  possible.  As a  matter of fact it follows from this that variations in the size of the populations  of the prey and predators  with special  food requirements  are partly,  at least, synchronised,  these predators  "riding  on the crest of the vole peak wave". 7 REFERENCES CHRISTIANSEN, E. 1975. Field rodent damage in Norway. -  Ecol. Bull. 19: 37-45. KALELA,  0. 1962. On the fluctuations in the numbers of arctic  and boreal small rodents as a problem of production  biology. - Ann. Acad. Sci. Fenn.,  Ser . Biol. 66: 1-38. KANERVO,  V. & MYLLYMÄKI, A. 1970. Problems caused by the field vole, Microtus agrestis (L. ) r in Scandinavia.  EPPO Publ . Ser. A 58: 11-26. LAHTI, S., TAST, J. & UOTILA, H. 1976. Pikkujyrsijöiden  kan- nanvaihteluista Kilpisjärvellä  vuosina 1950-75. - Luon  non Tutkija  80: 97-107. LARSSON,  T.-B. 1975. Damage  caused by small rodents in Sweden. -  Ecol. Bull. 19: 47-55. MYLLYMÄKI, A. 1959. Bedeutung und Ursachen der Mausefrass  schäden in Finnland. - Valtion maatalouskoetoiminnan jul  kaisuja  178: 75-100. 1977. a program for control of  damage by  the field vole,  Microtus agrestis  (L. ), in seed orchards of forest trees.  - EPPO Bull. 7 (2): 523-531. -"- ,  CHRISTIANSEN,  E. & HANSSON, L. 1977. Five-years  surveillance of small mammal abundance in Scandinavia. EPPO Bull. 7 (2): 385-396. MYRBERGET,  S. 1965. Vekslinger  in bestandsstorrelsen hos norske smägnagere  in ärene 1946-60. - Medd. Stat. Viltunders. 2 (19): 1-54. 1973. Geographical  synchronism  of cycles  of small rodents in Norway. - Oikos 24: 220-224. TEIVAINEN, T. 1974. Talvella 1973/74 myyrien  aiheuttamien tuhojen  valtakunnallisen  inventoinnin tuloksista. - Met  sänviljelyn  koeaseman tiedonantoja,  Suonenjoki  12: 27-33. 1975. Myyräkanta  kehittymässä  huippuunsa  Keski-Suomessa.  Metsälehti 1975: 31. 1976. Raportti  myyrien  v. 1975/76 metsäpuiden  taimille aiheuttamista vahingoista  ja ennuste tulevasta kehityksestä  Moniste Metsäntutkimuslaitos.  8 TEIVAINEN, T. 1977 a. Myyrätuho  ei  ole yllätys. - Pellervo 18: 52-53. 1977 b. Vahva myyräkanta  kiertää Suomen sydäntä. - Metsä  lehti 1977: 13. 1979. Myyräraportti.  Metsäpuiden taimien myyrätuhot  1977/78 ja ennuste myyräkannan tulevasta kehityksestä.  -  Metsä ja Puu 1979: 5.  1980. Myyräraportti. Eniten tuhoja Savossa ja Keski  suomessa. - Metsä ja  Puu 1980: 1. Fig. 1-8. Total area (ha) of plantations  damaged  by voles during 1973-80 in different communes. Fig. 9. Cyclical  rhythm of vole damage in different parts  of Finland during 1973-80. Kuvat 1-8. Istutettujen  metsäpuiden  taimistojen  myyrätuhot  vuosina 1973-80 kunnittain. Kuva 9. Myyrätuhojen  rytmi Suomen eri osissa ilmaistuna kunkin osa-alueen tuhojen  kokonaismäärän prosenttisena  ja  kautumana eri vuosien osalle. 17 Fig. 9. Cyclical rhythm of vole damage  in different parts of Finland during 1973-80. Kuva 9. Myyrätuhojen  rytmi Suomen eri osissa ilmaistuna kunkin osa-alueen tuhojen kokonaismäärän prosenttisena  jakau  tumana eri vuosien osalle. 18 TIIVISTELMÄ  Kahdeksan vuoden aikana (1973-80) kootun tiedusteluaineiston mu  kaan, joka käsittää tiedot 2 121 istutetusta taimistosta, myyrät  vioittivat  tai tuhosivat 7,4 miljoonaa tainta, yhteensä  4 000 ha. Tuhokeskittymä  oli peräkkäisinä  vuosina eri alueilla ja siirtyi  huomattavan säännönmukaisessa järjestyksessä  alueelta toiselle. Jos lähtökohtana pidetään  suppeata  aluetta Keski-Suomessa,  laajeni  tuhokeskittymä toisena vuonna  toisaalta kohti Hämettä, toisaalta kohti Savon ja Karjalan  eteläosia,  kolmantena vuonna Pohjois-  Karjalaan,  Pohjois-Savoon,  Kainuuseen ja/tai Pohjanmaalle.  Saman  aikaisesti  Lapin länsi- ja eteläosissa oli  myyräkanta  nousussa.  Neljäntenä  vuonna tuhokeskittymä  oli Lapissa  ja samanaikaisesti  uusi sykli  oli alkamassa Keski-Suomessa.  Tuhoaallon eteneminen länsi- ja  etelärannikolle ei ollut yhtä  selvää kuin  itä- ja  pohjoissuuntaan.  Kuitenkin myös näillä alueil la oli havaittavissa kohti lounasta etenevä "myyräaalto". Tuho  huippu  lounaisrannikolla oli samaan aikaan kuin Lapissa.  Tuhoaalto Keski-Suomen keskuksesta  alkaen eteni siis enemmän tai vähemmän säteittäisesti  eri suuntiin. Aallon eteneminen noudatti viljavien  ja viljeltyjen  maiden levinneisyyskuvaa  ja huiput ilme  nivät selvimmin alueilla, missä ilmasto oli mantereisinta. Keski-Suomessa tuhohuiput  seurasivat toisiaan kolmen vuoden vä  lein, mutta Pohjois-Suomessa  harvemmin. Tämä Pohjois-Suomelle  luonteenomainen pitempisyklisyys  näkyi  aineistossa tuhokeskittymä  alueen vähittäisenä siirtymänä  etelään päin kolmivuotisjaksojen  mukaan laskien. Mikäli todettu tuhoaallon etenemisen alueittainen järjestys  to  teutuu säännönmukaisesti,  niin kuin tähänastisen aineiston mukaan näyttää,  syklien  pituusero  Etelä- ja Pohjois-Suomessa  aiheuttaa pitempijaksoisen  alueellisen kerrannaisrytmin,  mikä ilmenee myyrä  huipun esiintymisalueen  samankaltaisena toistumisena kolmea neljää  vuotta pitemmin  aikavälein,  samalla kun  esiintymisalueen  laajuus  vaihtelee peräkkäisten  syklien  vastinvuosina. 

Myyrätuhojen esiintymisalueen siirtyminen Suomessa vuosina 1973-80.

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