Altitudinal zonation of the invertebrate fauna on branches of birch (Betula pubescens Ehrh.)
SIGMUND HAGVAR
Hagvar, S. 1976. Altitudinal zonation of the invertebrate fauna on branches of birch (Betula pubescens Ehr.). Norw. ]. Ent. 23, 61-74. The invertebrate fauna on branches of birch (Betula pubescens Ehrh.) was investi gated during 1967 at Sogndal, western Norway, by sweep-netting at the following altitudes: 10-40 m, 250-300 m, 450-500 m, 650-700 m, and 850-900 m. From the lowest to the highest station, the following numbers of Heteroptera species were recorded: 12, 5, 4, 3, 0; of Cicadidae: 11, 10, 7, 3, 2; of Coleoptera: 21, 16, 18, 13, 8; and of Araneida: 9, 5, 8, 2, 1. The generally low number of species above 500 m accompanies a change from mixed forest to an almost purely birch forest. The density of Araneida, Heteroptera, and Anthocoris nemorum (L.) (Het.) de creased greatly with increasing altitude. Cicadidae showed similar densities up to 500 m, but markedly lower densities above. Psocoptera was most numerous in the middle of the gradient. Predators and parasites collectively decreased greatly in density with increasing altitude. Total invertebrate densities were lowest above 500 m. The material that year was dominated by larvae of Geometridae (Lep.), which had a mass occurrence during 1966-67. Trophic relationships at different altitudes are discussed. S. Hdgvar, Norwegian Forest Research Institute, N-1432 AS-NLH, Norway.
Some invertebrate groups have been investi have been identified in Orthoptera by Prav gated with respect to the altitudinal zonation din (1965) and in Lepidoptera by Kuznetsov of their species composition. Among these (1958). The vertical occurrence of a given investigatitlns are Lindberg's (1945) study of species is usually continuous, but the density the Heteroptera-fauna in the mountains of may vary much over the range of occurrence. Bulgaria, Lindroth's (1949) study of Cara In most such studies, the intention has been bidae (Col.) in Scandinavia, a study of the to record the total number of species at dif same group by Ponomarchuk (1963) in the ferent altitudes. The altitudinal zonation of Carpates, a study of the zonation of Lamelli the total number of species depends on many cornia (Col.) species by Pek (1961) in the factors, among which the altitudinal zonation Kirgisia Mountains north of the Himalayas, of vegetation communities is of high impor and Kopaneva's (1962) work on Orthoptera in tance. However, if the habitat could be kept the Caucasian Mountains. The general trend relatively constant, the changes in species seems to be that, within a given taxonomic composition would mainly be a response to group, the number of species decreases with changing climate. Of course, biotic factors increasing altitude. This is, however, not al such as predators, parasites, or competing ways the case. In a study of Araneida in species may still be relevant. However, main Chile, Zapfe (1961) found the highest num taining constant habitat greatly standardizes ber of species in the middle ranges of the the situation. gradient. In this study, branches of birch (Betula Most species seem to have a vertical oc pubescens Ehrh.) were chosen as the habitat. currence which includes the lowlands and is The field work was performed during 1967 limited upwards. A few species may be on a rather steep mountain slope near Sogn endemic for high altitudes. Species endemic dal, Sogn og Fjordane County, in the western to certain sections of the altitudinal gradient port of South Norway. That year, the density 62 S. Hagvar
Fig. 2. Part of the forest at the second station, 250-300 m a.s.l. Epiphytic lichens on birch are more common than at sea level. Other tree species are Sorbus aucuparia. Populus tremula L., Alnus Fig. 1. Part of the forest at the lowest station, incana (L.), Pinus silvestris L., and Juniperus 10-40 m a.s.l. The most common tree species be communis L. sides birch are Prunus padus L., Sorbus aucuparia L., and Tilia cordata Mill.
1-5 show representative views ot. each sta tion. The actual 'habitat' is not quite constant of geometrid larvae (Lep.) on birch was throughout the gradient. At the upper part especially high. The data achieved on the of the slope, the trees grow more slowly, are altitudinal zonation of geometrid larvae has smaller, are more covered with epiphytic been published earlier (Hagvar 1972). The lichens, and often have crooked stems because present paper deals with the altitudinal zona of larger seasonal accumulation of snow. tion of the species composition within Ara Invertebrates were collected from branches neida, Heteroptera, Cicadidae, and Cole on the lowest three metres of the trees by a optera. An analysis of the altitudinal zonation sweep-net, as described by Zubareva (1930). of the density is also presented, which covers For each sampling at a given station, a new all invertebrate groups recorded. Some in group of trees was chosen. The net was used formation is given on the trophic structure on all sides of each tree. The effectiveness of of the invertebrate fauna at the respective the sweep-net may be somewhat different for altitudes. different taxonomic groups, but the method gives a picture of the composition of the total invertebrate fauna. Within a given species or MATERIAL AND METHODS group, the method is less selective. At each Five stations were chosen, one at each of the collection, from 100 to 400 sweeps were following altitudes: 10-40 m, 250-300 m, made. The density of animals is given as 450-500 m, 650-700 m and 850-900 m. The number per 100 sweeps. Acarina and Collem timber line is at 900 m. The photos in Figs. bola were not collected. Zonation of invertebrate fauna 63
Fig. 3. Part of the forest at the third station, 450 500 m a.s.!. Among the birches are some Sorbus aucuparia, Prunus padus, Alnus incana, Pinus siluestris, and Juniperus communis. Fig. 4. Part of the forest at the fourth station, which at this altitude, 650-700 m a.s.!', is an al most pure birch forest. A few specimens of Sorbus The weather conditions were favourable aucuparia, Populus tremula, Pinus siluestris and (without wind or rain) during all collections. Juniperus communis occur. The birch stems are often more or less crooked due to rather large Collection dates were: 10-40 m a.s.l.: 14 May, amounts of snow. 15 June, 13 July, 27 July and 1 September; 250-300 m a.s.l.: 15 May, 16 June, 14 July, 28 July and 2 September; 450-500 m a.s.l.: 19 cluding all adults and some of the juveniles. May, 17 June, 15 July, 29 July and 4 Septem No species of Heteroptera and only a few ber; 650-700 m a.s.l.: 24 June, 16 July, I Au species of the other groups were recorded at gust and 5 September; 850-900 m a.s.l.: 17 the timber line. In all groups, the greatest July, 5 August and 6 September. number of species was found at the lowest A total of 9384 invertebrates were collected. station, and the lowest number at the highest station. The number of Heteroptera species re RESULTS corded dropped rapidly with increasing alti Altitudinal zonation in number of species tude. Among Cicadidae, the number of spe Tables I-Ill show at which altitudes the dif cies decreased more gradually, but most ferent species from the four groups men slowly in the beginning. As for Coleoptera tioned were recorded. A few species not and Araneida, the number of species recorded naturally occurring on birch are excluded in at the second lowest station was somewhat the tables. The data is based on the 68 adult lower than at the two neighbouring stations. Heteroptera collected, and some of the larvae, In Coleoptera, the number dropped evenly on the 377 adult Cicadidae, the 497 adult from the middle to the uppermost station. In Coleoptera, and 183 of the Araneida, in- Araneida, the two uppermost stations showed 64 S. Hl1gvar
Table I. The occurrence of different species of Hemiptera (Heteroptera and Homoptera Cicadidae) on branches of birch (Betula pubescens) at different altitudes (m a.s.l.). "sp." denotes that the genus has been recorded.
0 0 0 0 ...0 .., 0 0 .... 0 0 , I '"I ...0 0 b ~ 0 "'N ..."' '" 00"' lleteroptera Anthoeol'i8 nemol'W7l (L.) x x x x P8aUus betuleti (Fall.) x PsaUu.s falleni lEuter x x x Temnostethu8 grooilis HoIvath x x x Stenodema holsatum (Fabr.) x x Lal'icuZa pselaphifornris OJrtis x Lygocoris contaminatUB (FalL) x BZepharidopteru.s angulatus (Fall.) x P8a HUB ambiguus (Fall.) x Acalypta oa:t'inata (Panzer) x Elasrrrucha grisea (L.) x Elasmostethus inte1"8tinatus (L.) x Additional species (.1arvae> 1 Total mnber of species 12
Hatoptera I Cicadidae
Oncapsis flavicoUis (L.) x x x x X Fig. 5. Part of the forest at the fifth and upper OnCOPSj.8 tristis (Zett.) x x x x x station, 850-900 m a.s.l., just below the timber Speudotettix subfuscuLUil (Fall.) x x x X line. The branches and stems may be rather den Oncopsis suhangulatus J. SahIb. x x x sely covered by lichens. Large amounts of snow Linnavuo1"iana decempunctata (Fall.) at this altitude have resulted in markedly crooked x x x Kybos betuZicola w. Wagner stems. Scattered Juniperus communis and Betula sp. x x nana L. occur. Typhlocyba be1"gmani Tullgren x sp. x Alnetoidia alneti (Dahll::an) x x Oncopsis planiscuta ('Ihanson) x Empoasca flavescens (F.) x sp. Cixius cunicularius (L.) x Cixius distinguendus (Kl:Jn.) x low numbers of species compared with the Total ntnber of species 11 10 three lowest stations. Probably the picture of zonation becomes more correct if we assume that the vertical distribution of each species is continuous. By Kybos betulicola and Oncopsis subangulatus filling such empty holes in Tables I-Ill, the (Cicadidae) are new to Norway. number of Coleoptera species from the lowest In addition to the four groups mentioned, to the highest station becomes: 21, 21, 20, 14, small numbers of Psyllidae were also iden 8 and of Araneida species: 9, 8, 8, 2, 1. If we tified. Psylla nigrita (Zett.) and P. sorbi (L.) furthermore assume that all species listed may were recorded at the three lowest stations. P. occur down to sea level, the number of spe corcontum Sulc was found at the medium cies is as follows: Heteroptera: 13, 6, 4, 3, 0; station and P. alni (L.) at the uppermost Cicadidae: 12, 10, 7,3,2; Coleoptera: 31, 27, station. The material indicates that the Psyl 24, 14, 8 and Araneida: 15, 11, 9, 3, 1. lidae species are limited mainly to the three These corrections do not change the zona lowest stations. tion pattern in Heteroptera and Cicadidae. They do, however, indicate that the number Altitudinal zonation of density of Coleoptera and Araneida species is not Figs. 6-10 show how the density of some markedly changed in the lower half of the major groups changes from May to Septem gradient, but clearly lower in the two upper ber at different altitudes (Araneida, Heter most stations. optera, Anthocoris nemorum (L.) (Heterop According to Ossiannilsson (pers. comm.), tera), Cicadidae and Psocoptera). The figures Zonation of invertebrate fauna 65
Table IIa The occurrence of different species of Table IlIa The occurrence of different species of Coleoptera on bcanches of birch ('Btula pvbBBcBns) Araneida on branches of birch (Betula pubeacens) at different altitudes (m •• s.l.). at different altitudes (m a.s.l.).
o 0 0 0 o 0 000 o 0 0 0 ~ M 11') f""'" cr'I o 0 0 0 0 "<2' l"i lJ"I f""'" O'l
o 000 0 rl U1 11') ." 11') o 0 0 0 0 N ..,. \0 CD rl U") Uj 11') Uj N "<2' \0 en
Po lydro8us undatus Fabr. x x x x x AransU8 patagiatus Cl. x Podi.tra piZo8a Payk. x x x x x Lepthyphantes expunctus (Cambr) x x x Antllophagus OIMlinus zett. x x x x x AraneUB eucurbitinus Cl. x x x x Anoplus pZont
40 include larval or juvenile stages. For simplic ity, the altitudes are denoted as 0, 300, 500, 700 and 900 m a.s.l. 30 Characteristic for Figs. 6-8 is a decreasing density with increasing altitude. In no case was the maximum density found to be higher 20 at a given altitude than at any lower station. There is a great difference in maximum density between the lowest and the uppermost 10 station at which a group was recorded. The maximum density of imagines in Heteroptera (Fig. 7, vertical lines) and in A. nemorum (Fig. 8, vertical lines) also decreases with MAY JUNE JULY AUG increasing altitude. Fig. 6. Density of Araneida, given as number per In Cicadidae, both the total density and the 100 sweeps, during summer at different altitudes.
5 - Norw. Journal of Ent. 66 S. Hagvar
NUMBER PER NUMBER PER lOO SWEEPS 100 SWEEPS 40 90
80
30 70 60 ",+ .•... 20 ~ ~// -":- 501 ~/ ..t ...,:" ..... "''''~/ 40 ...... " ,?'7~. . :-- >:..... // "" 10 30 ...... ,/ "" ----(~ " . 20 " 700 M " - .S.O.O.M.···· 700 10 MAY JUNE JULY AUG Fig. 7. Density of Heteroptera, given as number MAY JUNE JULY AUG per 100 sweeps, during summer at different alti tudes. Heteroptera were not recorded at 900 m Fig. 9. Density of Cicadidae (Homoptera), given a.s.!. Length of vertical lines denotes the density as number per 100 sweeps, during summer at dif of imagines. ferent altitudes. Length of vertical lines denote the density of imagines.
The seasonal variations in density at dif Neuroptera, Trichoptera, Hymenoptera sym ferent altitudes of the other groups recorded phyta, insect larvae and pupae). are shown in Table IV, together with the total The maximum density of Aphidae recorded catches of invertebrates. Some groups are at the four lowest stations is very similar so sparsely represented in the actual habitat (11-16 animals per 100 sweeps), but a much that the material gives no good picture of higher density (112) was recorded at the their altitudinal zonation, but merely their uppermost station in July. This may have been presence at certain altitudes (viz. Opiliones, a local phenomenon. Some three weeks later Ephemeroptera, Plecoptera, Thysanoptera, the density was again low, but the density of Formicidae was then high (48), probably as
NUMBER PER a response to the peak in aphid density. 100 SWEEPS
15 NUMBER PER 250100 SWEEPS
200 10 OM :il/ 0.' ~/ 150 .t. / ....., ,300 M I "\ :/"" , ~I \ , 5 .i' , i· , lOO d \ /'.. '. 500.~ ...,y·l'r / ... r \« .. / : /7 50 • I ~', .... _-- - -1.' 700 M '/o~.i~~::. MAY JUNE JULY AUG Fig. 8. Density of Anthocoris nemorum (Heterop MAY JUNE JULY AUG tera), given as number per 100 sweeps, during summer at different altitudes. The species was not Fig. 10. Density of Psocoptera, given as number recorded at 900 m. Length of vertical lines denote per 100 sweeps, during summer at different alti the density of imagines. tudes. Table IV. Number of invertebrates collected per 100 sweeps with a sweep-net on birch (Betula pubescens) during difterent months (from May to September) and at different altitudes. The actual collecting dates are given in "Material and methods" s ::!!.:;. The following groups have been excluded in the table, as their densities are presented elsewhere: Araneida (Fig. 6), ~-
Trophic relationships at different altitudes NUMBER PER Although the effectiveness of the method 100 SWEEPS 70 may be somewhat different for different groups, it is of interest to analyze roughly the catches in view of the trophic position of the 60 OM invertebrates. The recorded fauna can be divided into· three trophic groups: 50 1. Birch leaf or sap feeders: Thysanoptera, Homoptera, Heteroptera except the preda tory species mentioned below, the coleopter 40 ous families Curculionidae, Elateridae and Chrysomelidae, Lepidoptera larvae, larvae of 30 Hymenoptera symphyta and probably a few SOOM. other insect larvae recorded. 2. Feeders on fungi, lichen, excrements or detritus and non 20 .... feeders on birch: Diptera adults; Psocoptera; Trichoptera; Ephemeroptera; Plecoptera; the coleopterous families Cisidae, Ptiliidae, prob 10 ably the actual Staphylinidae, Cantharidae and Heteromera; imagines of Lepidoptera and of Hymenoptera symphyta; Formicidae MAY JUNE JULY AUG (probably eating excrements of aphids); Fig. 11. Density of invertebrate predators and larvae of Dermaptera and insect pupae. 3. parasites, given as number per 100 sweeps, during Predators and parasites: Araneida, Opiliones, summer at different altitudes. Hymenoptera parasitica, probably the few Neuroptera imagines and larvae collected, and the following Heteroptera species: Antho and the rest of the first trophic category. coris nemorum, T emnostethus gracilis, Lori This remainder always constituted less than cula pselaphiformis and Blepharidopterus 50(J/o of the total catches. When the geometrid angulatus. larvae had disappeared or was only found The highest density of birch leaf or sap incidentally, this percentage remained fairly feeders, 1836 animals per 100 sweeps, was constant throughout the gradient, varying recorded in June at the second lowest station, between 17 and 29. where the density of geometrid larvae was It is a general feature of all altitudes that highest. In the actual sample, geometrid the second trophic group becomes dominant larvae made up 97.6(J/o of this trophic cate when the density of geometrid larvae has gory. dropped. Psocoptera and Nematocera dominated the Due to a rather stable density of predators second trophic group (Table IV). The highest and parasites at each altitl,lde from July to density of this category, 393 animals per 100 September (Fig. 11) and to a general decline sweeps, was found at the middle station in in total invertebrate density at all stations July. In this sample, Psocoptera represented during the last half of the summer (Table 63(J/o of the group. IV), the relative dominance of predators and The density of predators and parasites was parasites increased at all stations during the always highest at the lowest level and de latter period (Fig. 12). The highest percentage creased with increasing altitude (Fig. 11). At achieved by this trophic group, 37(J/o, was all stations, the density changes from July recorded at the lowest station in September. to September were rather small. Araneida Of the total season's catches at each alti dominated this trophic group. tude, the first trophic group was represented Fig. 12 shows the relative dominance of by the following percentages, listed from the each trophic group in the different catches. lowest to the highest station: 56, 74, 64, 43, Because of the dominance of geometrid larvae and 72(J/o. Corresponding values for geometrid in early summer over the whole gradient, larvae alone were 33, 64, 53, 31, and 53%, there is a distinction made between this group, and for the rest of this trophic group 23, 10. 70 S. Hagvar
100 gradient during the season, the mean value for the first trophic group was 65% (geo metrid larvae 51% and other 14% ), for the second group 29%, and for the third group ;i 501W~ 6%. Altitudinal zonation of development In spring, the birch leaves developed about 100 four days later for every 100 m increase in I ~ altitude. It is reasonable to infer that this E delay also retards the start of development 0 0 of the invertebrate species at different alti 50 ~·.\.I I I I I ..... I tudes. In addition, the lower mean tempera 0 tures occurring at higher altitudes will slow '"ID a § \ill!JJj...llilllJ down the developmental rate. Anthocoris nemorum (Heteroptera) may 100 serve to illustrate the altitudinal zonation of ~~I I~. development. The species overwinters as E imago and lays eggs in young leaves. As 0 0 shown in Fig. 13, the development of the 501111111.: I===:j'.\j I I I I '"I species is delayed one larval stage between 0
'"~ 86 t::::::J ···@!!j··... lillllJ
100 1.Q.0 .C"'. i:--' n=13 ~~. ~ f:::: n=6 E t:: t:: 0 15 July F:: ~ 29July 0 r- 50 '" ...... I ~ f::: 0 f::: 500 m '"N I"':~ r-....
t2...0 n =18 t-< 75 nz18 r- 14 JUly ~ 50 28 Jul y E o t:: 25 ~ - 300 m I f;:t' ~" o t:- t=::" .. ~ ~ j : 1.Q.0 ~ : : : : Ii j n=37 ~ : ~ 1 75 n=30 MAY JUN. JUL. J.lAU. SEP. : : ~ ~ ~ ~ ~ ~ 13 July 50 27 July Fig. 12. Relative composition (in % of total num . ber) of the invertebrate fauna on branches of birch : .~ 25 during summer at different altitudes. 1: Larvae - Om of Geometridae. 2: Other leaf or sap feeders. 3: i~ Feeders on fungi, lichen, excrements or detritus and non-feeders on birch. 4: Predators and para :12:3:4:5: % :lb:3:4:5: sites. Further explanation in text. stage stage
11, 12, and 190/0. For the second trophic group, these mean values were 29, 21, 33, 49, and 14 days 27%. For the third trophic group, they were Fig. 13. Percentage distribution of larval stages in Anthocoris nemorum (Heteroptera) at different 14, 5, 3, 8, and 2%. altitudes. Data from two samplings in July, 14 If we pool all catches over the whole days apart, at each station. tJ. = number of larvae. Zonation of invertebrate fauna 71 the first and second station and between the tified, except the geometrid larvae (Hagvar second and third station. This is evident both 1972), showed a higher number of species in when stage four dominates at sea level, and the lower part of the gradient than in the when stage five dominates fourteen days upper part. The groups respond differently, later. however, to the gradient, the number of spe cies being reduced most rapidly among Heter optera. Lindberg (1945) also found a strong decline in the total number of Heteroptera species over the altitudinal gradient from the DISCUSSION deciduous forests (silva frondosa) in the Bul The results clearly demonstrate that the in garian mountains, through the coniferous vertebrate fauna on branches of birch changes forest belt (silva acerosa) to the areas above with altitude. The change can be character the timber line (regio alpina). Among 180 ized by the species number and species com species recorded by him, only one species was position; the presence, density or absence of endemic for regia alpina. certain groups; the preferred altitude of The number of Cicadidae species decreased groups or single species; the relative domi rather evenly over the gradient, whereas with nance of groups at different altitudes; and Coleoptera and Araneida the most marked by the total density of invertebrates. difference occurred between the three lowest No invertebrate orders clearly preferred and the two uppermost stations. A general the upper part of the gradient, neither by decline in the number of Carabidae species their level of highest density nor by their (Col.) with increasing altitude has been pre level of highest number of species when these viously demonstrated in Scandinavia by were identified. Among single species, O. Lindroth (1949). The relative low number of autumnata occurred mainly in the upper half Coleoptera species in alpine habitats has also of the gradient and had its highest density at been demonstrated by Brundin (1934), Fri the uppermost station. The preference for den (1968 and 1971), 0stbye (1969), and the timber line level is typical for the species Fjellberg (1972). (Tenow 1972). No other species identified, The drop in the number of species occur with the possible exception of A. alpinus, ring from the three lowest to the two highest showed any clear preference for the upper stations is correlated with a change in the two stations. composition of the forest. While the forest Psocoptera are characterized by mainly in the lower part of the gradient contains a inhabiting the middle part of the gradient. mixture of many tree species, it becomes an Being to a high degree dependent on lichens almost purely birch forest above 500-600 m. as food (Landin 1967), the group would This change may exclude species that prefer favour the higher altitudes where there is a more varied habitat than found in the greater lichen growth on the trees. As climate, purely birch forests. however, probably becomes less favourable Just as the number of species within a with increasing altitude, it seems reasonable given animal group generally decreases with that a central range of the gradient may offer increasing altitude, the species number also Psocoptera the most optimal environmental usually decreases with increasing latitude, conditions. i.e., increasing distance from equator (Fisher A. omalinus, occurring at all altitudes, also 1960). Usually, there is a correlation between prefers the centre of the range. This may, at altitudinal and latitudinal distribution of least partly, be a result of competition with single invertebrate species. Examples of such A. alpinus on the upper half, and with A. studies are those of Lindberg (1945) and caraboides on the lower half of the mountain Lindroth (1949). The altitudinal distribution slope. of the present material has been compared That the maximum observed density level with the known latitudinal distribution of of Operophthera sp. was in the lower half of the relevant species of Heteroptera, Cicadi the birch wood gradient confirms earlier ob dae, Curculionidae, and Staphylinidae. The servations (Tenow 1972). correlation is, however, not good. This may All groups in which the species were iden be explained by the fact that the direction 72 S. Hagvar of the isotherms in western Scandinavia is cult to compare directly the composition of NE to SW, probably due to the Gulf Stream. the herbivorous trophic group with Koponen's Brinck & Wingstrand (1949) found that (1973) data. However, a pronounced differ the subarctic Scandinavian birch forests just ence exists because of the fact that Psyllidae below the timber line have few endemic in and larvae of Hymenoptera symphyta to vertebrate species and should be regarded as tally dominated this trophic group on birch a transition zone between the lower forests at Kevo, while the groups were almost ab and the treeless mountain areas. The present sent over the altitudinal gradient at SogndaL material confirms this view. The rather high catch of Psyllidae at sea level At the three uppermost stations, no collec in July probably reflects swarming activity. tions were made just after the birch leaves In this investigation, Cicadidae was generally were developed in 1967. However, samplings the dominant herbivore group next to geo during that same phase the next spring at 450 metrid larvae. If geometrid larvae were dis 500 m and 650-700 m did not reveal ad regarded, Cicadidae would represent 63-98010 ditional species, indicating that the species of the herbivores recorded at the four lowest composition recorded in the upper part of stations in May and June. At the uppermost the gradient is representative. station, however, the density of Cicadidae It is difficult to estimate the effect of the was always very low. In this last respect, the high densities of geometrid larvae on the present results correspond with the condi other part of the invertebrate fauna. It is, tions at Kevo, where the occurrence of Cica however, reasonable to assume that the spe didae was also quite insignificant. cies composition has been little if at all af It is interesting to note that the changes in fected. The population density of certain relative dominance of the three trophic groups leaf-eating species or groups may have been followed a very similar pattern throughout seriously affected in the zones of most heavy the summer at all altitudes, although the defoliation (Hagvar 1972). Predators and absolute densities were different. It is also parasites of geometrid larvae may have been a remarkable feature of the fauna of this positively affected. habitat that a major part of it belonged to Koponen (1973) studied the density and the second trophic group, i.e. either non composition of the invertebrate fauna of the feeders on birch, or species whi.ch obtained mountain birch B. tortuosa Led. (synonymous their food from other sources than fro~ birch with B. pubescens in the present study) at leaves or by predation/parasitizing. This Kevo, Finnish Lapland. Birch leaf or sap trophic group became dominant at all sta feeders represented 78.7'010 of the total ma tions when the geometrid larvae left the terial; fungi, lichen, detritus or non-feeders habitat. The main reason for this dominance 16.0'0/0, and predators and parasites 5.4°/0. is the consistently rather high densities of In the present material, there was generally lichen-feeding Psocoptera and non-feeding a somewhat lower percentage of the first Diptera. trophic group and a higher percentage of The very clear decrease in density of pre the second group. The differences are partly dators/parasites with increasing altitude, due to a relatively low occurrence of Psoc shown in Fig. 11, may partly be a response optera in Koponen's (1973) material (aver to lower density of prey or host specimens, aging 1.8010 during 1971-1972). In the present although this cannot be ascertained from this material, Psocoptera were well represented materiaL even on the 'mountain birch' at the two upper As the vertical range covered by each stations, viz. 16'0/0 of the total catches at species is anticipated to be quite constant 650-700 m and 6% at 850-900 m. The mean from year to year, even during heavy out percentage of parasites and predators is breaks- of geometrid larvae, the picture roughly the same in the two investigations, achieved regarding the altitudinal zonation both over the gradient as a whole, and when in species number is most probably repre the two upper 'mountain' stations are con sentative. cerned. However, the altitudinal zonation in total The unusual high density of geometrid invertebrate density, and in the relative larvae in the present material makes it diffi dominance of different-taxonomic groups, Zonation of invertebrate fauna 73 should be regarded as quite flexible from Fisher, A. G. 1960. Latitudinal variations in or year to year. This is due to the occurrence ganic diversity. Evolution 14, 64-81. Fjellberg, A. 1972. Coleoptera from Hardanger of several 'unstable' groups in the habitat, vidda. Fauna of the Hardangervidda 1, 1-74. which during special years, or during shorter Friden, A. 1968. Weitere Beitrag zur Kenntnis periods (and sometimes rather locally) may der Verbreitung und Okologie von Kiifern im occur in great numbers. Typical examples of zentralen Siidnorwegen. Norsk ent. Tidsskr. 15, 99-104. this are geometrid larvae, Psyllidae, Aphidae Friden, A. 1971. Beetle fauna on the borders of and probably Psocoptera, or species which some Scandinavian glaciers. N orsk ent. Tidsskr. may migrate from other habitats, such as 18, 29-32. 8warming Diptera. Koponen (1973) found Hagvar, S. 1972. Aititudinal zonation of geo metrid larvae (Lep.) during a mass occurrence marked differences in the relative composition on birch at Sogndal, western Norway, 1967. of the invertebrate fauna on birch in two Ent. scand. 3, 69-74. successive years. Due to the mass occurrence Kopaneva, L. M. 1962. Orthoptera of the main of geometrid larvae during the present study, Kavkaz mountain ridge in the upper course of Teberda River and their vertical distribution. the total invertebrate densities were most Zool. Zh. 41, 378-383. (In Russian with Eng certainly higher than in 'normal' years. lish summary.) It may, however, be suggested that the Koponen, S. 1973. Herbivorous invertebrates of total invertebrate density in most years will the mountain birch at Kevo, Finnish Lapland. Rep. Kevo subaretic Res. Stat. 10, 20-28. be lower at altitudes above 500-600 m than Kuznetsov, K. I. 1958. Zonal distribution of Lepi in the lower part of the mountain slope. This doptera and the formation of forest and garden highest part of the gradient, being an almost pests in the mountains of western Kopet Dag. purely birch forest, does not seem to be pre Uchen. Zap. leningr. gos. Univ. 240, 122-147. Landin, B.-D. 1967. Fiiltfauna, Insekter I. 394 pp. ferred by any of the typical 'eruptive' taxo Natur och kuitur, Stockholm. nomic groups. Lindberg, H. 1945. Zur Frage der vertikalen und horisontalen Verbreitung der europiiischen He teropteren. Ergebnisse von Einsammlungen im Rila-Gebirge in Bulgarien. Notul. ent. 25, 118 129. Lindroth, C. H. 1949. Die fennoskandischen Cara ACKNOWLEDGEMENTS bidae. Ill. Goteborgs K. Vetensk. o. Vitterh I would like to thank Pro£. Arne Semb-]o Samh. Handl. Ser B4, 1-911. 0stbye, E. 1969. Records of Coleoptera from the hansson for valuable advice during the study. Finse area. Norsk ent. Tidsskr. 16, 41-43. I am indebted to the following persons for Pek, L. V. 1961. Contributions to questions on the help with the identification of the material: vertical distribution of lamellicorn beetles from Pro£. Frej Ossiannilsson (Hemiptera), Dr. the Issyk-Kul'basin. Izv. Akad. N auk kirgiz. Ser. BioI. Nauk 3, 185-194. phil. h. c. Andreas Strand (most of the Cole Ponomarchuk, V. J. 1963. On the vertical distri optera), Mr. Eivind Sundt (Col., Ptiliidae), bution of Carabids (Coleoptera). Zool. Zh. 42, and Mr. Per F. Waaler (Araneida). Mr. 1485-1493. (In Russian with English summary.) William N. Thompson kindly improved the Pravdin, F. N. 1965. Patterns in the vertical di stribution of orthopteroid insects (Orthopteroi English. The field work was supported by deal in the Adriatic part of the Balkan Pe grants from Collett's legat. ninsula. Ent. Obozr. 43, 260-267. (In Russian with English summary.) Tenow, O. 1972. The outbreaks of Oporinia au tumnata Bkh. and Operophthera spp. (Lep., Geometridae) in the Scandinavian mountain REFERENCES chain and northern Finland 1862-1968. Zoo!' Bidr. Uppsala, Suppl. 2, 1-107. Brinck, P. & Wingstrand, K. G. 1949. The moun Zapfe, C. H. 1961. Altitude distribution of tain fauna of the Virihaure area in Swedish Aranea in the valley of Rio Mapocho. Invest Lappland. I. General account. Acta Univ. Lund, nes zool. chil. i, 129-132. Avd. 2, Bd. 45, No. 2, 69 pp. Zubareva, S. 1930. On the accuracy of quantita Bmndin, L. 1934. Die Coleopteren des T orne tive sweeping with an insect sweep-net. Izv. triiskf!,ebietes. Ein Beitrag zur Okologie und bioI. nauchn:o-issled. Inst.biol. Sta.perm. gosud. Geschichte der Kiiferwelt in: Schwedish-Lapp Univ. Perm 7, 89-104. (In Russian with Eng land. 436 pp. Lund. lish summary.)
Received 15 December 1975