zoogeographical and ecological determinants of collembolan distribution

Zoogeographical and Ecological Determinants of Collembolan DistributionAuthor(s): R. E. Blackith and R. M. BlackithSource: Proceedings of the Royal Irish Academy. Section B: Biological, Geological, andChemical Science, Vol. 75 (1975), pp. 345-368Published by: Royal Irish AcademyStable URL: http://www.jstor.org/stable/20518985 .

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1 345 1

18.

ZOOGEOGRAPHICAL AND ECOLOGICAL DETERMINANTS OF COLLEMBOLAN DISTRIBUTION

By R. E. BLACKITH and R. M. BLACKITH

Department of Zoology, Trinity College, Dublin, 2

(Communicated by J. J. Moore, M.R.IA.)

(Received, 21 SEPTE.MBER 1973. Read, 9 DECEMBER 1974. Published, 18 JULY 1975.]

Irish Contribution to International Biological Programme (No. 6)

ABSTRACT

A Principal Coordinates Analysis of species lists of the Collembola of the world shows that their distribution is limited by broad zoogeographical factors, probably associated with Continental Drift, together with short-range ecologi cal determinants. Some evidence for a Gondwanaland or pre-Gondwanaland origin of the group is adduced. The diversity of Asian Coilembola, considering the size of the area, seems fto be very slight, when compared with ithat of European Collembola.

In all, 70 species lists involving 1,479 species were analysed. Altthough the Isotomidae are distributed somewhat differently, the Poduridae, Onychiuridae,

Entomobryidae and Sminthuridae seem to have much in common in this respect, and can be considered as an entity.

Wiithin the West European zoogeographical province, an over-riding ecologi cal factor seems to be the plant cover, which is normally closely associated

with the nature of 'the soil; however, where the pilant cover is atypical of the soil, it is the former which determines the collembolan distribution.

Introduction

The basic question in zoogeographical studies could be phrased "what

factors determined the absence of a particular species from any given area?".

The choice of "absence" rather tthan "presence" stems from the fact that

presence merely indicates that the animal occurs in the area with sufficient freque'ncy to have been found by the sampling effort actually employed. Absence, on the conitrary, can be attributed to one of two clear propositions; that the animal never reached the area, or that it was unable to maintain

itself there in sufficient numbers to be taken in a sample if it did reach ilt.

This distinction lends itself to quantitative investigationts, because, by analysing the occurrences of the animal over a sufficiently wide range of territory, it should be possible to separate the failure to reach an area from that to

maintain itself once there. These components could be termed the zoo

geographical and ecological determinants of absence, respectively. Lists of ispecies, which are a fairly widespread product of zoological work

even in difficult terrain, form a neglected mine of valuable information whose

exploitation should do much to clarify the zoogeographical and ecological

components of animal distribution. In particular, such lists should be a pro

duct of the national contributions to the International Biological Programme,

witthin which this paper forms part of the Irish contribution. There are many

PROC. R.I.A., VOL. 75, SECT. B [18]



346 Proceedings of the Royal Irish Academy

techniques available for asses;sing the degree of similarity or dissimilarity between two sites, once we are prepared Ito regard the presence or absence of a species as an attribute of the siltes in question. This approach inverts

Hutchinson's (1965) definition of the ecological niche of a species as a set of

points in abstract Cartesian space, the coordinates of which are the environ mental facitors affecting ithe survival of the species. Here, we consider the site as a point in an abstract Riemannian space whose coordinates are thie

abundances (or presence-absence critteria coded as dummy variables) of tlle various species. We use a Riemannian space (that is, one whose reference axes

are curvilinear) instead of a Cartesian one because we wish our axes to be

uncorrelaited, so that distortions of the space may be needed as compared with

the strictly rectilinear Cartesian space of Hutchinson's definition: this, how ever, is a trivial distinction for practical purposes. Our approach to zoo

geography has much in common with ithat expounded by Holloway andl

Jardine (1968). By adopting this approach to zoogeographical problems, their essentially

multivariate nature is laid bare, and much of the methodological confusioni illustrated by Udvardy's (1969) account of the current state of zoogeography is swept away.

The animals chosen for this pilot project were ithe Collembola, isince they are world-wide in their distribution, with many lists of species available, and are a group with which the authors are sufficiently familiar)to avoid most of

the difficulties arising from synonymy, inadequate identifications in the older literature, and unfamiliarity with the ecology of the group. As Salmon (1949) has remarked, ithe importance of these small, isoil-inhabiting animals in any

discussion of zoogeography cannot be too strongly stressed. Any method of assesising the siftes in respect of their collembolan faunas

must meet three criteria: it must give an appreciation of the similarities

between the siites taking into account all the thousand and more species

involved, considered simultaneously; it should assist the experimeniter in decid ing what major axes of variation underlie the observed differences between the siltes; and it should be capable of handling presence or absence data, which are not distributed normally (their distribution is rectangular), and so must not be closely dependent on multivariate norMality for iits theoretical justifica

tion. ' Principal Coordinates analysis meets all three criteria (Blackilth and

Reyment 1971). Because the data matrix is never stored as a whole in ithe core storage of the computer, very large numbers of species can be -handled

simultaneously (up to 9999 in the version used here); the principal coordinate axes enabIe clusters existi"ng in multivariate data to be exhibited in Euclidean space (Gower 1966); and the programme uses the measure of similarity pro posed by Gower (1971) to form 'the association matrix, exhibilting the simi larities between each site and every other silte in respect of all -the 'species used for a particular comparison.

'The similarity s5qk between the ith and jth observation's of 'the variable

k -is given by



BLACKITH AND BLACKITs-Determinants of collembolan distribution 347

wlhere xi and xi are the observaltions an'd Rk is the range of 'the variable k.

When presence or absence data are used, presence is coded +1 and absence

I- so that s?ik is 1 when a species is present in both sites, and zero otherwise. Malmgren and Kennett (1973) have published a study of the distribution

of foraminiferal plankton in the South Pacific which closely parallels the work reported here in ithe statistical treatment of the data. As in our investiga tion, they employ the presence or absence of species, data which are the stuff

of itraditional biological activity at the level of 'simple recording. The

P'COORD programme preserves ithe distances between the sites, as shown by their similarities, unchanged during the analysis, an advantage of Principal

Coordinate analysis over the older and better-known Principle Components analysis.

Such an approach will ordinate the sites along a -small number of axes

of zoogeographical and ecological variation known as oligovariates. These oligovariates are ithe latent vectors (or eigenvectors) of the association matrix

generated by ithe programme. The computer then uses these oligovariates as the axes of a series of charts displaying the inter-relationships between the sites, plotted by (the line-printer. The biological initerpretation of variation

along any one of 'these oligovariates should greatly clarify the reasons why certain species occur on some sites and not on' others. If 'such ordinations

were applied to species lists from all over the world, it should be possible to

discover the factors which matter to the animals in question, rather ithan

to continue arguing about the formal geographical factors which, as investiga tors, we tend to suppose must matter ito ;the animals because they happen Ito

mnatter tto us. As, one zologist has commented (Phipps 1968), ". . 'the

accumulation of a mass of geographical data may have hampered the

clarification of the main fealtures of ecological dependence." Only a proportion of the total number of published collembolan species

lists were used; the' lists were chosen either for their geographic relevance, some rather unsatisfactory lilsts being accepted because they related ito parts

of the world in whose collembolan fauna we were interested, or for 'their relevance -to ecological situations of concern to Tundra Biome of the Inter

national Biological Programme. Many local lists related to areas whose

characteristics were ill defined, from the point of view of their soil inhabitants.

More cogently, pilot studies soon showed that much detailed information could become lost in fully comprehen;sive surveys. Each of the seventy lists eventually used was processed to remove anomalies arising from ithe factors

mentioned at the end of tle introduction, and then the data were transferred

to 80-column punched cards with presences coded as +1 and absences as -1. The data were processed by the PCOORD programme, slightly modified

to take the large number of species involved, using the I.B.M. systems 360/44 computer of Trin;ity College, Dublin. It may be noted in passing that no

problem arises from the widely different assemblages of species recorded in the various lists, since any one species is unequivocally either present or absent in any one place; if a species was not recorded as present in a list, it was scored as absent.




Two-dimensional plots of the positions of the sites when ordinated along the first three oligovariastes are produced automatically by the PCOORD pro. gramme, which was again slightly modified to give plots of the fourth ai

fifth oligovariates.

TABLE 1

Number

Site Symbol species Reference

**Afghanistan A 51 Stach 1960, 1963; Yosii 1963, 1966a, b

*Alaska Al 50 Bohnsack 1968; Hammer 1953a; (Inc. Arctic and McAlpine 1965; Mills and Richards

Boreal Canada, 1953; Oliver 1963 Ellef Ringnes Is., Ellesmere Is.)

* Antarctica An 30 Tilbrook 1967; Wise 1970; Yosii 1959 The Burren, Ireland B 59 Lawrence 1961 Bulgaria Bu 82 Rusek 1965

*Canada (all records) C 109 Hammer 1953 Canada (woodland) Cw 29 Marshall 1967 Camargue Ca 25 Poinsot 1966

**Ceylon Ce 4 Yosii 1966a ** China Ch 47 Stach 1964

Connecticut (all records) 57 Bellinger 1954 young soft woods 39

mature soft woods 32 grassland 50

*Cothill Pen, England Cot 17 Macfadyen 1952 Czechoslovakia Dunger 1970

a sub-alpine Czs 56 b montane Czm 77 c high moor Czh 55 d limestone Czl 23

*Czechoslovakia CzPT 268 Dunger 1970; Nosek 1969; Szeptyck; 1964 1967

+ Poland + Tatras

* Denmark (Jutland) D 74 Petersen 1965 * East Germany EG 25 Palissa 1959

**Formosa F 15 Yosii 1965 Greenland G 54 Hammer 1944; 1953a, 1954;

Jorgensen 1934 Gambia Ga 19 Murphy 1960 Hiddensee H 43 Palissa 1969

* Ireland (all records) I 116 Blackith 1974; Lawrence 1961 a shelter-belt Is 39 Blackith 1974 b grassland Ig 27 Blackith 1974 c virgin bog Iv 46 Blackith 1974



BLACKITH AND BLACKITH-Determinants of collembolan distribution 349

Number of

Site Symbol spectes Reference Iceland Ic 75 Bodvarsson 1966, 1967

* India In 119 Prabhoo 1971a, 1971b; Salmon 1956, 1957, 1963, 1965, 1969, 1970; Singh and Mukharji 971; Stach 1964;

Yosii 1966a Italy It 105 Dallai 1970 Japan J 223 Niijima 1966; Uchida 1954a, b, 1957,

1965, 1969; Yosii 1954, 1955, 2957, 1961b, 1965, 1967, 1969, 1971b

'Jan Mayen JM 21 Gisin 1953 * Korea K 20 Yosii and Lee 1963 * Malaya M 59 Salmon 1951; Yosii 1959

Madeira & Azores MA 101 De Gama 1959, 1961; Paclt and Bodvarsson 1961

* Maghreb Mah 126 Cassagnau 1963; Lawrence 1963a, b Moor House (mineral) Mm 31 Hale 1966

'Moor House (peat) Mp 21 Hale 1966 Moravia Mo 105 Rusek 1959, 1965, 1968 Norway N 71 Altner 1963; Cadwalladr 1969

* Nepal & Himalayas NH 133 Choudhuri 1958; Singhot al. 1956; Yosii 1966c, 1971a

New Mexico NM 143 Scott 1960a, b, 1961a, b, c, 1962a, b, c, 1963a, b, c, 1964a, b, 1965a, b

*' North Vietnam NV 32 Stach 1965 * New York State NY 161 Maynard 1951 New Zealand NZ 73 Salmon 1941

* Oceania 0 18 * Poland (all records) P 158 Szeptycki 1964, 1967

a grassland Pg 41 Szeptycki 1967 b moss Pm 63 Szeptycki 1967 c woodland PW 99 Szeptycki 1967 d xerothermic grassland Px 60 Szeptycki 1967

Portugal Po 76 De Gama 1964, 1968 *Russia R 265 Grinbergs 1960 * Sweden (all records) S 107 Bodvarsson 1961

grassland Sg 64 Bodvarsson 1961 peat Sp 68 Bodvarsson 1961

* South & Central Africa SA 57 Paclt 1967 0 South America SAm 104 Cassagnau 1963b; Izarra 1965, 1970

Solomon Islands Si 19 Lawrence 1969 Silesia Sil 64 Rusek 1963 Skokholm Sk 35 Gough 1971 Spitzbergen Spz 36 Stach 1962 Stromboli St 11 Altner 1961

6* Thailand T 20 Yosii 1961 Tatras Ta 194 Nosek 1969 West Germany WG 50 Hiither 1961

* West Pakistan WP 22 Yosii 1963a, b, 1964; Yosii and Ashraf 1965a, b

* Sites labelled in Fig. 2 ** Sites constituting the "Asian" cluster in Figs. 1-3

PROC. R.I.A., VOL. 75, SECT. B [I 8A]




Results

THE COLLEMBOLA AS A WHOLE

In the first instance all seventy sites were analysed together in respect of their total of 1,479 species. Such an approach assumes ithat Collembola react

more or less alike irrespective of ttheir taxonomic position, an assumption which wa-s later investigated separately. The abbreviated names for the sites represented in Fig. I are clarified and referenced in Table 1. As is to be expected, geographically propinquenit sites share many 'species in common and consequently have high similarities, clustering together on the chart. For instance, Polish, 'Czech, Russian and Bulgarian lists occupy 'the top left-hand corner of the chart, whereas the Asian lists cluster strongly with Antarctica and South America in ithe (top right-hand corner. In the centre (left) are

West European sites, with -some Arctic ones (right) including Jan Meyen, Spitzbergen, Canada and Alaska.

Below 'these come some north-west European sites, including several Irish, and Greenland. In the initial run of the programme, there was a surprising

outlying cluster for the four Connecticut sites which represent a disturbing anomaly; inspection of ithe Connecticut lisits suggested that common ispecies with a virtually world-wide representation were seriously under-represented in these lists, which consist, perhaps as a resulit of unconscious selectioln, mainly

of rarer, possibly more "interesting", species. However this may be, the

Connecticut lists are so widely separated from those of the adjacent state of

New York and of Canada ithat they were reluctantly removed from 'the lists, in case itheir presence distorted the proces;s of extracting axes of variation for reasons which seem likely to bear little relation to the zoogeography of the state.

Fig. I shows the results of running the analysis without the Connecticut lists'. In ithe event the remaining sites were arranged almos't exactly as they

were when the Connecticut sites were included. A *special feature is the absence of iany clear axis corresponding to the contrast between the tropics and the poles. Evidenitly, 'the Asian and peri-Antarctic sites form an entity

which shares many species in common, despite 'the fact that about one-fifth

of the Earth's surface is spanned by these areas. The tropical areas form a

surprisingly tight cluster (shadowed in Fig 1), bu't this is not far from such distinctively non-tropical peri-Arctic sites as Canada, Jan Meyen, and Spiltz

bergen. Essentially, the first axis of variation in Fig. 1 seems to be a contrast

between the combined tropical and peripolar siltes on the one hand, and the temperate sites on the other. The first two axes of variation account for some

10% of the total variation (defined as the trace of the transformed association

matrix) between the 66 isites left after the elimination of Connectticut. The

second axis of variation, in ithis chart, is influenced by the species richness of

the site, since the sites with long lists of presences occupy one end of the scale and those with few species present occupy 'the other. However, 'species richness cannot over-ride species 'similarity since Ceylon, with only 5 species recorded, clusters in the Asian group with Japan which has 180.



BLACKITH AND BLACKI-H-Determinants of collembolan distribution 351

CzPT

Ta

P 4 0

R

Pw

Mo

2

Px Bu ..

Pm Czm NY S i Mah NM*~SAm Wk-\N

Axis 11 MA

NZ,S

0O. . S A

ca,, o Cw*

Czh C* Czl

Sp CZs Po H

SgP o

D Sk Pg JM

WG. Al Spz

-2

B* N

Cot Mp Mm0 EG,00

Is -4 I lv

-4 -2 O 2 4

Axis I

Fxa, 1-The 66 sites ordinated along the first two axes of variation in a Principal Coordinates analysis of 1479 species of Collembola.

The "Asian" cluster is shadowed.




TADLZ 2-Scores along axis III in order of magnitude (for details of sites see Table 1). Score Score

Czechoslovakia a 036 China -001 Czechoslovakia b 0f36 India - 0f02 Poland b 0-34 Canada woodland - 0 03 Czechoslovakia c 0 33 West Pakistan - 0 03 Poland c 0'29 Italy - 0 03 Silesia 0-26 Nepal and Himnalayas - 0 03 Poland d 0-26 Norway _ 0-03 Cothill Fen 0-23 Hiddensee - 0-04 Czechoslovakia d 0-18 Japan -0-04 Moor House peat 0-17 Spitzbergen - 0-08

South America - 0-08 Moor House mineral 0-14 Dnak-f0 Ireland b 0-13 Poland a 0112 Sweden grassland - 010

Camargue 012 ~~~~~~~~Sweden peat - 010 Moravia 0-11 Oena-1 Stromboli 0-11 New Zealand - 0-12 Ceylonbol 0-10 Tatras -

012 Ceylonri ? X ? Czechoslovakia, Poland and Bulgaria 0-08 Tta 1 East Germany 0-08 The Burren - 0-13 Skokholm 0-06 West Germany - 0-13 Jan Meyen 0-05 Greenland -0-16 Formosa 0-05 South and Central Africa - 0-16 Gambia 0-05 Alaska -0)16 North Vietnam 0-04 New Mexico - 0-17 Solomon Islands 0-04 Madeira and Azores - 0-20 Afghanistan 0-03 Maghreb - 0-21 Thailand 0-03 Iceland - 0-22 Poland 0-03 Canada - 0-23 Korea 0-02 Russia - 0-23 Malaya 0-02 Portugal -0-23 Antarctica 0-01 Sweden - 0-24 Ireland a 0-01 New York State - 0-25 Ireland c 0-00 Ireland - 0-36

The third axis (Table 2) has little to do with zoogeographical varialtion, and apparently much to do with ecological differences between the sites; it accounts for a further 3% of the total variation, and the Irish tsiites are strung out along its Iength. The lists for the whole of Ireland, and for the JBurren (a limestone karst or garrigue area) occur in the lower half of the chart,

whereas ithe pealtiland sites occur in the upper half (for a description of these sites, see Blackith 1974). Of the four Czech sites whose Collembola fauna

was published by Dunger (1970) that on limestone differs clearly from the others, high moor, montane and sub-alpine in nature. The fourth and fifth axes necessarily account for even less of the itotal variation, since the axes are arranged in declining order of importance in this respect. The fourth axis

clearly distinguishes the four peri-Arctic sites (Alaska, Spitzbergen, Canada

and Greenland) from the remainder, and the three Czech sites on acid soil cluster close to the four peri-Arctic ones, possibly because they were high altitude sites.



BLACKITH AND BLACKITH-Determinants of collembolan distribution' 353

Poduridae Onychiuridae P.

Mp CzPT

4

2 2 R

Cot 2 L

* CzPT * 0 NY Mah

o Mah R 0 NZ

EX -2 E A* WG C .

-2 SAm2 Al *

~~4SACo

NZo 0-rnia Cordnae -Cotsof6 ie, fwih aiu

*t Y*NY

NZ f Mp, ar*hw o lrt,rpeene yfu aiisoh

EG

0- lembla 0Nt:FrteEtmbydeteplrt fae

-4 -2 0 2 4 -2 0 2 4

Entomobryidae Sminthuridae

Cot 4*P

4 CzPTO

R EG 1 2 G.~~~~~~~~~

NZ ..~~~~ ~ NY Mali5

* * ~~~~~NM Mariat w for t oNZ

* MP"i I*aSA NM0 A So

SAm~~A

EG. Cot W 0 ~~~~~~-24

, . CzPT -4 -.2 0 2 4 -2' 0 2 4

IFIG. 2-Principal Coordinates analyses of 66 sites, of which a max-imum Of 34 are shown for clarity, represented by four families of the Collembola. (Note: For the Entomobryidae the polarity of axes I and II is reversed by the computer compared w-ith the polarity of the same axes in Onychiuridae and Sminthuridae: for the Poduridae the computer has extracted as Axis I (abscissa) the

oligovariate which constitutes Axis II for the other three families.) The "Asian" cluster is shadowed.




DiSTINCTrONS BETWEEN THE FAmiLES OF THE COLLEMBOLA

As men't'ioned above, a zoogeographical analysis involving all the world's

Collembola, in so far as these are represented in the ispecies lists used in this work, implies that all five conventionally recognised families can be regarded as an enitity for such purposes. To examine this concept more closely, five separate runs of the analysis were performed, successively involving 'the

Poduridae (414 species), Onychiuridae (160), Isotomidae (264), Entomobryidae (369) and Sminthuridae (272). In most casies a few ;sites had to be eliminated temporarily for the technical reason that ithey contained no recorded species

of the family in question, and thus induced programme interruptions in the computer. We shall consider the five families in sequence.

Poduridae (Fig. 2)

The plot of ithe 66 sites on the first 'two axes of variation is essentially the same as that obtained from the whole of the Collembola. There is a switch of axes, as Is not uncommon in multivariate analyses, axis I of the "whole"

plot (as in Fig. 1) becoming axis H1 of the "podurid" plot, and conversely.

This 'switch means no more than that the magnitude of the component of variance taken up by the first axis has fallen below that taken up by the

second, and this is a trivial change. New World sites again fall into the

agglomeration of Asian and tropical lists. Turning to the third axis, the Irish sites are again 'strung out, Swedish grass and peatland sites are well separated but the comparable si;tes at Moor House occur in the reverse order, with

the peat site having a higher iscore than the mineral soil isite, an exception to the rule 'that mineral soil or grassland sites have higher scores along this

"ecological" axis than pealtland sites.

Onychiuridae (Fig. 2)

There are no importanit differences for the 160 species of Onychiuridae used here as compared with 'the totality of the Collembola; the Moor House

sites revert to ithe usual configuration along axis III with the mineral 'soil site

having a higher score than the peatland one, but only by a small margin. However, these 'two sites are, unusually, well separated along axis I. Again, the limestone site from Czechoslovakia is ctearly distinguished from the remainder in that country.

Isotomidae (Fig. 3)

The I-sotomidae (264 species involved) are -the one family to differ from

the general picture obtained by this series of multivariate analyses. There is a reversal of the signs of the scores along axis I, again a itrivial matter, and a

separatlon of the peatland and grassland sites in a well-defined way for Ireland, Sweden and the English sites at Moor House. This separation occurs along axis II and, more markedly, along the "ecological" axis III. Consistent with

the "whole Collembola" plot is the separation of the limestone site in




CzPTe

Ta.

4 Re

Pe

S* 2 * MA

Cw SAm N

AxisU ~ ~ ~ ~ Bu oE Cx z

0J Maa NY Pp O

Mmo

j. Ca, NM. SC S

W..

Si- G OIC

NZ* Al

H Pg, Czs, B

PwO No

Czm.

PM

BU,,G Pxm Czh.

JG 3*cot Spa

MM.

-4~~~~- p

CzI.

Sk,

-4 ~~-2 024

Axis 1

FIG. 3-Principal Coordinates analysis of the same sites, using records for

the Isotomidae. .The "Asian" cluster is shadowed.




Czechoslovakia from the remainder, again along axes II and III. It seenms as if, for these Isotomidae, the "ecological" axis, the one that is reflecting sensitivity -to the nature of the site rather than its location on the globe, has

become oriented obliquely, instead of being essentially parallel with one of

the major axes of variation extracted by the computer. This feature may

welI have come about because, in the plots for the Isotomidae, ithe lists for East and West Germany fall at one extreme of axis III whereas those for

Czechoslovakia fall at the other extreme. This is a surprising finding in view

of 'the propinquity of fthese countries, and its dominant influence in defining the third axis extracted by the computer for Isotomidae has over-ridden the

influence of the other ecological factors responsible for the third axis in other

plots for individual families.

In certain plots the Asian cluster of sites was strung out along one or other of the axes of variation when 'the Isotomidae were listed, but not when any

of the other families were. At first sight this difference appears striking, but

it loses much of itis impact when we note later on -that this cluster splays out

freely, irrespective of the family studied, as soon as the preponderance of non-Asian sites is excluded from the analysis; it is likely that the plots involving the Isatomidae are allowing some of this latent variation of the Asian sites to

become manifest.

Entomobryidae (Fig. 2)

For this family the Asian sites revert to their clustered positions, and there are few differences from the "whole Collembola" plots except that for the

Entomobryidae (369 species) the Canadian sites and Denmark separate along axiis II from the remaining sites, Alaska being their nearest site in the sense

of having the highest similarity to the Canadian ones.

Sminthuridae (Fig. 2)

There are few inconsistencies of note when -the plots for the Sminthuridae

(272 species) are compared with ithose for the Collembola as a whole.

CONTRASTS BETWEEN EAST AND WEST EUROPEAN FAUNAS

One feature of Fig. I which was unexpected is the marked east-west polarisation of Europe along the second axis of variation. Czechoslovak (including Moravian), Russian, Polish and Silesian sites have high scores on

this axis, despite the very diverse ecological natures of the sites from which the Collembola were collected. Irish, Dan-ish, Swedish, English, German and

Portuguese sites have low scores, though there is some overlap of the Swedish sites with those of Czechoslovakia and Poland. We chose to include several of these lists because the nature of the site was described by the original

authors (e.g. woodland, high moor, etc.) but these factors seem of little con

sequence so far as this oligovariate is concerned. We must look for some more

general factor to explain the polarisation, and the degree of continentality of

the site seems as likely a candidate as any.




)istribution within a Province

European Soils and Vegetation

Iit is often maintained that soils, rather than the vegetation cover, deter

mine the distribution of Collembcla, although Blackith (1974) has shown

that the validity of this proposition may be restricted to arable soils in

temperate climates, and that when tundra-group soils are considered the

converse situation may obtain. Naturally, there is a generally close association

between the soil type and the vegetation which it supports.

We analysed the data for a number of European localities which are

known, or plausibly suspected, of having predominantly peatland or pre

dominantly mineral soils. We included a few non-European species lists to

give some idea of the scale on which any ecological determinant was working.

Twenty-four species lists were used, covering 1479 species of Collembola, and

the results are shown in Fig. 4.

Evidently, there is a clear separation between those sites with essentially

mineral soils and those with essentially peatland soils, but this separation is,

equally evidently, not attributable directly to the soils themselves, because of

the positions itaken up by the Peatland Grass plot (maintained by An Foras

Talutntais as part of the Irish I.B.P. activities, where deep peat has been

levelled, partly drained, and sown ito grass) and the poor, sandy, Danish soils

at Thy in Jutland. The peatland grass plot falls neatly into the mineral soil

group, showing that it is the grass, or the cultivation practices associated with

its growth, rather than the peaty soil, which determines the suite of Collembola

inhabiting the plot. Again, the Danish soill has a higher rating on axis III

than any peatland soil, whereas ithe mineral soils generally rate lower on this

axis, but the soils at Thy are probably even poorer nutritionally than the peaty

soils and covered with an impoverished vegetation (Petersen 1965).

The contrast between the North American localities and those strung out

through Portugal, Madeira and 'the AMores, the Maghreb countries, South

Africa, Nepal and the Himalayas, and Oceania affords some indication of the

magnitude of this ecological determination when assessed against the zoo

geographic determination of collembolan distribution.

The picture of collembolan distribution gained from Fig. 4 is much

cleaxer than that afforded by Fig. I in which many other non-European sites

were included. Such inclusions may distort local relationships because they

orient the axes to carry mainly, in this case, zoogeographical information

within which framework the ecological determinant becomes partly obscured,

just as the distribution of the Isotomidae was obscured by the inclusion of a

preponderance of other Collembola. It is often a matter of experience and

judgement on the part of the multivariate analyst to decide when further to

subdivide the material to hand, and -this kind of decision may depend on a certain "flair" for erecting creative hypotheses as well as on such mundane

matters as the cost of computer runs.




NM

NY*

II.

Axis Ill Sp0 Is SV IC Cot

MPO,

Sg* 0

WGo

.2 PoO Mah

MSA 0

00 SA NH

-4

2 0 2 4 6

Axis, II

FIG. 4-Ordination of West European and other sites along second and third axes of variation, using families of Collembola. (Note: Iv, Is, Sp, Cot and Mp are mainly peatland sites: Sg, I, Mm, B, and

Wg are mainly mineral soil sites: Ig is a peatland site with grass cover.)



BLACKITH AND BLACKiTH-Determinants of collembolan distribution 359

''he Asian-Antarctic-Tropical Group of Sites

In aLl plots, except for that showing the distribution of the Isotom-idae,

there has been a distinct cluster of sites mainly concerned with Asian and

peri-Antarctic regions, together wiith some tropical regions in Africa. As noted above, there is always a doubt as whether any clustering is genuine, or

is a result of the inclusion of strongly disparate sites which force less distinct

ones inito a somewhat artificial cluster, a matter to which little attention has

been paid in multivariate studies. This doubt increases whenever the cluster

formed seems 'to contain heteroclite elements, as it does in this instance. We therefore ran the programme so ithat only the elements of ithe "Asian etc."

cluster were used in the analysis -but involving all 1479 species as before. The

results were striking, and are shown in Fig. 5. The sites are strung out along a line which is oblique to both the first two axes of variation, and, with some

anomalies, this line makes good sense from a zoogeographical poirnt of view. There is a clear north-south trend in the positions of the sites on the plot,

with Korea and China at the top and the Solomon Islands and Antarctica at

the bottom. Formosa and Afghanistan seem to fall too far down the series,

New Zealand too far up, in relation to itheir actual geographic positions.

Again, we have clear evidence that apparent clusters in a multivariate

analysis may open out to give biologically meaningful arrangements of their elemenits once the preponderance of other types of element have been re

moved from the analysis. No doubt, in theory, such arrangements could be

detected on late axes of variation, but in practice we were not able to discover

the one just menitioned on any of the first nine axes of variation when all 66

sites were included in the analysis, although nearly 16% of the variation

between 'the elements of the "Asian etc." group was taken up by -the two axes

of Fig. 5.

Disussion

Several somewhat disparate matters deserve to be discusised. Some ecolo

gists might like to see work isuch as 'this carried out usling the abundances of

the Collembola rather than presence-absence data. The PCOORD programme

is quite capable of inc;luding abundances, but the data are lacking on any

sufficient scale. It is difficult even to define the abundance of any animal

when speaking 'of its distribution over some substantial area because a species

reistricted to, say, woodland, will have an abundance in any particular country

which is closely determined by Ithe amount of woodland present. Another

factor intervenes at this point, namely the tendency for data which might be

thought more precise to perform less well, in analyses such as this, ithan simple

presence or absence data. The reason seems to be that the abundances would

be influenced by a wide range of irrelevant sampling errors, whereas these

have been eliminated from the presence-absence data; it is uncertain, a priori, whether the gain outweighs the loss, but on balance Blacki-th and Reyment

(1971) consider that the use of dichotomous data actually benefits the analysis

in a wide range of instances.




K.

Ch.

4

NV.

WP J o

2

NH.

Axis 11

0

In.

NZ.

A.

-2 " To

Me

Ce 0 An

oF

-4 -2 0 2

Axis I

FIG. 5-Ordination along Ist and 2nd axes of variation showing the essentially north-south arrangement of sites forming the "Asian' cluster in Fig. 1.




In large-scale multivariate surveys involving presence or absence data, the proportion of the total variation which the first few axes of variation can

be expected to account for is generally low, of the order of 20%. Experience shows that when a Principal Coordinates analysis of quantitative (continuously

distributed) data is performed, as in a morphometric analysis, some 50%/O of

the total variation may be expressed by variation along the first three oligo

variates. This percentage varied in the work reported here, which is on an

unusually large scale, from 14 (for the whole world) to 24 (for the more

restricted sets of sites in Europe or Asia). Broadly speaking, the wider the

scope of ithe enquiry, the lower is the signal/noise ratio in the ana1lysis. Within fairly wide limits, the Collembola do seem to be distributed as if

they formed an ecologically definable entity; the partial exception to this

conformity is the Isotomidae. Since Collembola are recorded from rocks as

early as the Middle Devonian, they must have evolved substantially during

the early Palaeozoic era, at a time when the continental plates came together

to form the pre-Gondwanaland super-continent (Valentine 1973). So very ancient a group could well have spread throughou't the later (Mesozoic) super

Continent of Pangaea, giving rise to the observation from Fig. 1 that the New

World sites dluster together with the Asian and Gondwanaland sites, and

vividly justifying Jeannel's (1943) insistence on Continental Drift as a major determinant of collembolan distribution at a time when such ideas were far from fashionable. Richards (1968) considers that most sminithurid genera had evolved by the close of the Cretaceous.

The position of China with respect ito Gondwanaland is ithe subject of

*some current uncertainty. It has been suggested that Eastern Asia split off

from Gondwanaland during the early part of that continent's fragmentation, travelling northwards towards its present position much as India appears to

have done. For some animal groups which evolved during the Palaeozoic, such an hypothesis accounts for zoogeographical findings that are otherwise quite inexplicable (Blackith 1973), and it would be strange indeed to find no trace of Continental Drift in the present disposition of the Collembola, all the

more so as Ghilarov (1956) has suggested that -soil insects are in some ways

living in a medium which has kept its general properties substantially un changed since early on in the Earth's history.

When viewed in 'the light of 'these remarks, the finding that for the

Collembola as a whole China, Afghanistan, Japan and Korea cluster tightly

together with Antarctica, South America, India, and the Solomon Islands takes on a new -significance. This could in principle be due to a preponderance

of short species lists from Asian countries, but it is not in fact so; for instance,

Japan has the third longest list of all those used here. Are we facing the

results of a diffusion of the Collembola throughout Gondwanaland during the intact period of that continent? Denis (1949) has expressed the view tha-t

some groups evolved on the Gondwanaland -super-Continent and more recently Wallwork (1973) has shown that a isubstantial relict collembolan fauna in

Antarctica can be attributed to Gondwanaland origins. Pushing speculation still further, can we match the finding that a primitive group of Pterygote




insects, the Eumastacidae (Orthoptera) appear to have evolved on Gondwana

land territory (Blackith 1973) to suggest ithat the Insecta as a whole may

have evolved 'there?

There can be no doubt that the present distribution of the Collemnbola is

marked by a tendency to group siltes together in'to what might be called

provinces. The elemeents of some of these provinces share few climatic or

geographical features at 'the present time. One of them is the Asian-New

World-pern-Antaretic province. Another is the Wes.t European province, which embraces Ireland, England, Scandinavia, Portugal and the Maghreb.

A third, which often remains surprisingly clearily delineated from the West European province, is the East European one including Russia, Poland, and

Czechoslovakia. A major ecological determinant may well be a:t work here; the degree of continentality of the climate is consilstently grCater for the East

European province, and if this were indeed the determinant *the fact that

Scandinavian lists fall into 'the West European province finds a ready explanation in 'terms of the relatively maritime nature of the Scandinavian

climate. The finding that the plant cover, rather than the nature of the soil,

determines the suite of Collembola inhabiting a site conforms with recent

work showing that in poor soils:, each species of plant has its own assemblage of Collembola, which may differ more dramatically from plant to pllant in

one site 'than between widely separalted sites (Blackith 1974). Such a finding suggesits that 'tundra-group isites will have essentialily ithe same Collembola, and

indeed Alaska, Jan Meyen and Spitzbergen tend to cluster near one another on almost all axes of variation. These may be considered to represent the

extreme Arctic fauna, whereas Iceland, Ireland, Greenland, Moor House

(upland northern England), Norway and Sweden 'tend to form another

broader group represenrting a less extremely peri-Arctic fauna. In the case of

some very short lists, for instance Stromboli with only eight species, the position of the site on the plots is apt to be misleading, being partly determined by the large number of apparently absent species shared with some other sites.

Such short lists are perhaps undesirable in this kind of work, and one lesson

we have learnt is that their omission would have been preferable. Whatever

the strategy of a multivariate analysis may be, an interesting tactical matter

emerges from the resolution of some apparently tight dlusters once these are

separated from 'the remalning material in the analysis. The very large analysis

has its own perils for the analyst.

Finally, we may consider the potential of this, technique for investigating

zoogeographical problems. The raw material, species lists, -is abundantly

available for most reasonably well-worked groups of organisms. The process

ing of the data for the computer is routine. The biometrician should at least

collaborate with a biologist who knows the pitfalls of species lists for the group

under investigation, or preferably have had experience wlth that group

himself. Otherwise, any worker can collect the Iiisits, which are usually avail

able in the literature and gain access to a computer for which the PCOO'RD




programme can be adapted. A comprehensive discussion of the uses of multi variate analysis for the faunal classification of sites (inland waters) by Sheldon (1972) spares us from reitera'ting the points made by that author, whose paper is well worth reading by anyone concerned with large-scale zoogeographical

work. We can end by quoting his comment that "many biologists would agree that the organisms are the best measure of the environment" in relation to our earlier remark that we are concerned to discover the factors which

matter to the animals we are studying, and not the factors which matter so much to us that we feel thalt the animals are bound to respond to them.

No geographical information whatever is fed into the calculations de scribed, so that the conclusions are based entirely on the observed distribution

CzPT* BuO R

OP It

Y* NM SAm . ........ 511 77.*77~ Mah

NZL MA SA. o

0 s~~~~

Po

Al ,SOpz

N WG

FIG. 6-Projection of the first three Principal Coordinates on to a sphere. "Asian" cluster shadowed. If the sphere were the world as it is at present, the point O3,0' would probably lie near the South Pole, the sites forning a belt roughly between the Tropic of Capricorn and the Equator.

of the Collembola. As Sneath and McKenzie (1973) comment, it is possible to transform the coordinates of each site on the multivariate charts to project them on to a sphere, and -to examine the concordance with existing land

masses. Sites whose fauna is partly determined by Continental Drift will show an approx.imation to the ancient, rather than to the present arrangement.

We have followed Sneath and McKenzie in projecting our Principal Coordinates on to a sphere, as shown in Fig. 6; we have not, however,




attempted tto draw a map of the world as it now is within the framework of Fig. 6, because it is at once evident that the area of the sphere's surface "covered" by the Colilembola is only about one-third of the whole. In other

words the existing distribution of Collembola corresponds to their having evolved on a continental mass occupying something like one third of the present surface of the globe, a result fully compatible with their evolution on the super-continent of Pangaea, during the early Palaeozoic. It is likely -that the centre of the projection shown in Fig. 6 (the point 00,00) should be sited not far from 'the existing South Pole; the present Northern Hemisphere sites form a ring round the centre, with the Southern Hemisphere sites within the ring. The "Asian" cluster is shaded, and includes India, Nepal, Malaya, Solomon Islands, Thailand, North Vietnam, West Pakistan, Korea, Gambia, China, Afghanistan, Ceylon and Formosa as well as Antarctica. Although we have already shown that this cluster will open out when suitably analysed, its

reality seems established, and may imply thatt Eastern Asia had stronger links with Gondwanaland than had been thought likely in the past.

The sharp contrast between west and east European sites is brought out very clearly, Bulgaria, Czechoslovakia, Russia, Poland, Moravia, Silesia and Italy falling to one side of the Asian cluster, whereas another group of sites comprising Norway, Sweden, Jan Meyen, East and West Germany, Iceland,

Denmark, Portugal and Ireland fall on the other. The New World sites, consisting of New York State, New Mexico, South America, New Zealand, and, more distantly, Canada and Alaska, form a loose cluster beside 'the European sites.

Anomalies are only to be expected, and we have no convincing explanation for the fact that Gambia clusters with the Asian-Antarctic sites, we merely report 'that it does so. Readers familiar with the pitfalls of multivariate analysis and its representation in two dimensions may forgive us for reiterating that clustering is not necessarily evidence of similarity; the ends of a dumb-bell seem close together when viewed end-on, whereas when viewed side-on they seem well separated. Sites which cluster on Fig. 6 may be, and no doubt usually are, faunistically similar, but they might be clearly distinct along one or other of the axes of variation extracted by the computer later than the three used for the projection on to a sphere.

We should also note that 'the location of any given site on a multivariate chart (including projections such as Fig. 6) is not an average of the locations of any sub-sites into which it may be divided. Thus Ireland, regarded as a

single site, is always distinct from subsites such as those denoted by Iv, Ig and Is. On Fig. 6, for instance, the pine shelter belt on modified peat at Glenamoy (Is on other diagrams) would falI at the point -83.60, -11.4o, latitude given first, but if this fact has any zoogeographical significance lt is that the species

which have thrived in that habitat are those of the peri-Arctic component of the Irish fauna rather 'than of its Lusitanian element, reflected in the proximity

of Ireland and Portugal on Fig. 6. One might speculate that most countries are so rich in habitats for the Collemb6la that, even if their gross climatology changes as it would during Continental Drift, most species could maintain




themselves by -transferring to a new habitat which has come to resemble their

old one's original state. By avoiding pressures towards extinction in this way, the Collembola have maintained the appearance of a very slowly evolving group, in which the present distribution is still marked by events which took place during the Palaeozoic era.

Copies of the computer programmnes PCOORD (for the Principal Coordinates Analysis) and WORLD (for projecting the three-dimensional coordinates on to a sphere) are available from the authors: each is written

in FORTRAN IV.

ACKNOWLEDGEMENTS

We are grateful for stimulating discussions with Mr. P. N. Lawrence of

the British Museum (Natural History) and for the use of the extensive collec tions of literature on the Collembola kindly made available to us by Mr. H. E.

Goto of Imperial College, London, and Dr. Henning Petersen of the Mols

laboratoriet, Femm0ller, Denmark.

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zoogeographical and ecological determinants of collembolan distribution

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