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Home , Gerromorpha, Leafhopper, Mellum, Memmert, Nepomorpha

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Zeitschrift/Journal: Denisia

Jahr/Year: 2002

Band/Volume: 0004

Autor(en)/Author(s): Niedringhaus Rolf

Artikel/Article: (, ) from the perspective of insular biogeography 531-540 © Biologiezentrum Linz/Austria; download unter www.biologiezentrum.at

Leafhoppers (Hemiptera, Auchenorrhyncha) from the perspective of insular biogeography

R. NlEDRINGHAUS

Abstract

Distribution and abundance of leaf- stics, including (1) degree of spe- hoppers (Hemiptera: Auchenorrhyncha) cialization; (2) preference for special were investigated on the chain of the East groups of ; (3) preference for Frisian dune islands situated in the particular stages of landscape develop- from 3 to 13 km off the north ment; (4) degree of niche overlap; (5) western German coast in to explore abundance in the recruitment area. It is the absence or presence of on the demonstrated that the specific characteri- islands and to explain differences in colo- stics of each potential colonist species nization success of species. - A total of combine to a "disposition" regarding their 172 species was assessed from prevailing colonization chances: high dis- the islands by standardized sweepnet position values render a high colonization samplings from 1982 to 1988. Species rate, low disposition values render a low numbers found on the 7 old islands are colonization rate. roughly on level (110 to 139 species), but Key words: Auchenorrhyncha, North differences in species composition are con- West , East , insu- siderable. The species-abundance relations lar biogeography. of the leafhopper assemblages show great similarities to those of dynamic and insta- ble communities. The leafhopper data of the fit the "habitat- diversity hypothesis" more closely than the "area-per-se-hypothesis" of insular biogeography theory. Colonization success of the potential colonists correlates positi- Denisia 04, vely or negatively with certain characteri- zugleich Kataloge des OÖ. Landesmuseums, Neue Folge Nr. xx (2002),

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Introduction results fit current theories of "insular biogeo- graphy"? Islands are particularly suited for analysing (3) Which role do "organisms-related" distribution patterns of species assemblages factors (dispersal ability, body size, hiberna- and their dynamic in space and time. Impor- tion, generation cycle, habitat requirements, tant factors such as size, isolation and age of feeding specificity, etc.) play? Which charac- development are easily determined. Islands teristics can be related to increasing or decrea- are often "more simple" in structure and their sing colonization chances? communities of organisms are more easily to (4) Does the specific set of characteri- survey. stics of each potential colonist convey a It was not until the sixties of the last specific "predisposition" in relation to its colo- century when islands were brought to the nization chances? attention of ecologists by the "equilibrium (5) How far does this explain a species' theory of insular biogeography" of Robert colonization success or failure? MACARTHUR and Edward WILSON (1963, 1967). Their concept, which seeks to explain Due to their biology and distribution leaf- species numbers of organismic assemblages by hoppers are well suited for ecofaunistic studies the parameters size and degree of isolation of on different spatial scales as well as for analy- an island in first place, provoked lively debate ses of colonization processes: and extensive conceptual advances that have - They form diverse assemblages in precipitated in a plethora of publications to virtually all terrestrial . On the East the present day (for reviews see CONNOR & Frisian islands they are only absent from the MCCOY 1979, WILLIAMSON 1981). glasswort fields and barren beaches. The chain of East Frisian dune islands - Leafhoppers complete their life cycle situated off the central European mainland within the same habitat. Compared to other offer all advantages as study sites: groups of population density is The islands emerged independently from mostly very high. Hence, leafhopper assembla- the mainland, so that all organisms had - and ges can be sufficiently surveyed in terms of still have - to immigrate. The islands share quantity and quality. many features, but do also differ in size, degree - More than 90% out of a total of 380 spe- of isolation, landscape structure as well as age cies recorded from north western Germany are and state of development. The islands show a restricted to distinct habitats. Strong preferen- characteristic set of well distinguishable habi- ces for single host species for oviposition tats on a small scale which themselves can be or feeding (more than 85% are mono- to viewed as "islands", due to their mosaic-like oligophagous) is in many cases coupled with a distribution ("habitat patches"). strong preference of a distinct microclimate In the present contribution, which is lar- and, hence, of a distinct habitat (more than gely based on research conducted from 1982 70% are classified stenotopic). In this way, a to 1988 (NlEDRINGHAUS 1991), the coloniza- reliable alignment of habitat data and potenti- tion success of leafhoppers (Hemiptera: al species composition data can be achieved. Achenorrhyncha) on the dune islands is ana- - Notwithstanding a certain "homing lysed by comparing potential colonist and behaviour" and a comparatively small range of assessed species. The following questions are vagrancy of individuals, the dispersal ability of addressed in particular: populations can be regarded as fairly strong: (1) How many and which species have Population waves which occur at times and in successfully colonized the islands and different places provide a steady proportion of migra- habitats? ting or drifting individuals to render coloniza- (2) Which role do "area-related" factors tion from more or less remote areas within a like size, isolation, habitat diversity, age and comparatively short period (e.g. RAATIKAINEN stage of development of an island play in the 1972, RAATIKAINEN & VASARAINEN 1973, process of colonization? How do the present WALOFF 1973).

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Study area Material and methods

Situated in the southern part of the North Primarily employed was the method of Sea off the German mainland the arc of East quantitative sweepnet sampling that proved to Frisian islands extends some 90 km along the be particularly reliable for sampling leafhop- Wadden Sea (Fig. 1). Some 2000 years ago the pers from herb and tree/shrub strata, respec- tively (see e.g. EMMRICH 1966, SCHIEMENZ Fig. 1: dune islands emerged independently from the Geographical position of the East 1969, NlEDRINGHAUS & BRÖRING 1986, 1989; mainland from a process of sedimentation and Frisian islands along the southern coast WlTSACK 1975; for details on sampling effi- subsequent formation of dunes aided by sand- of the . cacy see NlEDRINGHAUS 1991). binding plants and since then have been sub- ject to considerable changes in position, size and structure (STREIF 1990). In addition to these 7 "old" islands the investigations were extended to the two smaller islands of Mem- mert and which were only formed about a century ago.

On the 7 old islands there are larger villa- ges of several thousand inhabitants each. Increased tourism and related activities has encroached upon the whole natural environ- ment. This appears the more serious, since these islands are part of one of the last pristi- ne landscapes of northern central Europe. Since 1985 the chain of East Frisian islands is largely part of the Niedersächsisches Watten- meer (Lower Saxon Wadden Sea) National Park. surface area isolation landscape- number of plant (km*) km diversity species Due to similar conditions during their 25.6 10.5 2.747 594 history and their linear position along the 1.7 13.0 2.560 388 edge of the Wadden Sea, the 7 old islands 9.2 8.0 2.713 455 Nordernev 17.8 3.0 2.542 342 developed characteristic sea-land clines of 4.7 4.5 2.617 384 landscape elements and, thus, show a similar Lanqeooq 14.3 5.0 2.565 359 basic landscape pattern. In contrast, landscape Spiekerooq 9.6 6.5 2.571 363 Wanqerooqe 5.9 6.5 2.000 135 elements developed to a much lesser degree on Mellum 3.0 6.0 1.615 200 the two younger islands. The largely patchy distribution and conspicuous variation of The quantitative sweepnet samplings were Tab. 1: habitats on a small scale leads to an extremely repeatedly conducted in representative sam- Surface area (irrespective of beach areas and glasswort mud flats), high heterogeneity and a high potential of pling sites under suitable weather conditions degree of isolation (distance from resources which is unrivalled in most main- (not too hot, dry, slight wind) about every four mainland), diversity of landscape ele- land areas. weeks from end of May until mid September ments (number of habitats per 25ha fields) and number of plant species of With respect to size (notwithstanding of the years 1982 to 1988 (for details see the islands under investigation (see beach areas and glasswort mud flats) and NlEDRINGHAUS 1991). Selection of sampling NIEDRINGHAUS 1991 for further details). degree of isolation from the mainland the sites was guided from considering (1) surface islands show considerable differences (Tab. 1): area and distribution of habitat classes; Area of the smallest island (Memmert) is (2) achieving as much area coverage as possi- merely 7% of the largest (Borkum); the most ble with sampling sites most randomly distri- remote islands are 4 times farther off mainland buted; (3) sufficient representation of hetero- than the nearest. Habitats and plant life are geneity within habitat classes; (4) in single most diverse on both of the largest islands of cases potential habitats or certain host plants Borkum and , but least rich on the of expected leafhopper species were particular- two smallest islands of Memmert and Mellum. ly surveyed.

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A total of 927 sampling sites (including Some 25% of the species recorded from 264 permanent plots) and 2098 single sam- the islands (nearly 40 species) are less abun- plings were analysed. In addition, pitfall traps dant to rare in the remaining parts of north were used on each island and in all habitats to western Germany, but are mostly indigenous assess the primarily ground-dwelling (epigeic) and partly common on the islands. Within species. The material analysed includes north western Germany 9 species are exclusi- 210.000 specimens. Nymphs were usually con- vely found on the islands or in the adjacent sidered from 3. stage onwards. coastal area, namely Gravesteinietta boldi. Ana- ceratagallia frisia, aestuannus, Psam- Catch efficiency on the level of individual motettix mantimus, sabulicola in islands is very high and lies between 89 and coastal dunes and limicola, 94% (according to trend-analysis/forecasting artemisiae, horvathi, Psammotettix procedure and jackknife procedure after putoni in salt marshes. HELTSHE & FORRESTER 1983). The East Frisian islands harbour an extre- mely rich fauna of more than 10.000 species Results and discussion (BRÖRING et al. 1991), including a high The leafhopper fauna of the East proportion of rare and endangered as well as Frisian islands highly specialized thermophilic species. For terrestrial bugs indigenous to these islands the A total of 172 leafhopper species has been proportional representation out of the north assessed from the chain of islands (species list western German fauna is 46% (BRÖR1NG in NlEDRINGHAUS 1991). This amounts to 1991), even exceeding 70% for aquatic and nearly 50% of all leafhopper species known semi-aquatic bugs (BROR1NG & NlEDRlNGHAUS from the north western German lowland 1988). Aculeate and sawflies (NIEDRINGHAUS & OLTHOFF 1993). The two are represented with 40 %, each (HAESELER largest and structurally most diverse islands of 1985, RlTZAU 1995), and spiders with 51% Borkum and Norderney are also most diverse (SCHULTZ 1995). However, the question in species, harbouring 129 and 139 native spe- remains, whether and to which extend the cies, respectively. Species numbers of the species characteristic to the islands become remaining old islands are more or less on a endangered irrespective of the proclamation level (Juist 106, Baltrum 110, 114, of the National Park, in that they are banished 112, 118). Irrespec- by organisms being directly (e.g. plantations of tive of great similarities in their resource various trees and shrubs) or indirectly favou- potentials, the islands differ conspicuously in red by human activities. With respect to leaf- species composition (Fig. 2) and dominance hoppers such species already contribute one pattern. In the much smaller and - in terms of third of the present assemblage. landscape elements and resources - less diver- Fig. 2: se islands of Memmert and Mellum 67 species Leafhoppers as indicator orga- Species diversity and distributional pattern of leafhoppers of the East Fri- (62 indigenous) and 59 species (53 indg.), res- nisms of insular dynamics pectively, were recorded. sian islands. Even 2000 years after their formation the islands in parts are still subject to a strong 180 dynamic. On the one side, this is due to natu- 160 T ral processes (dislocation of sand by the action 140 of seawater and wind, periodic and non-peri- £ 120 H y sint^e island • 2-» islands odic floodings of certain insular parts etc.), | 100 1 • 1 H • • 1 • ^«islands but, on the other side, is further occasioned by I " • all 7 old islands human activities (punctual and large-scale n. g 60 alterations of the landscape) which greatly 40 increased during the last decades. 20 II 1 1 1 II 1 This situation is reflected by the present 0 • 1 1I 1 1 18 1 results on the leafhopper fauna from the East I I Frisian islands. The frequency distributions (species - abundance pattern) of the islands

534 © Biologiezentrum Linz/Austria; download unter www.biologiezentrum.at and biotopes show parallels to those found in colonizing plant and species had to species assemblages of unstable and dynamic immigrate from outside. With distances of 1 to ecosystems (e.g. BAZZAZ 1975): 11 kilometres off mainland and between each The observed distribution patterns in other their degree of isolation is fairly low. some cases (islands and biotopes) approxima- Although most investigations on insular bio- te the so-called log-series distribution (more geography focus on the bird life of islands of or less linear graph, FlSHER, COLBERT & much greater isolation (of up to several thou- Fig. 3a-c: Distribution patterns of the leafhop- WILLIAMS 1944, Fig. 3a) which is assumed to sand kilometres), the available analyses of be largely governed by chance effects. This per assemblage (represented from assemblages usually pertain to most common to the rarest species). pattern is found at the beginning of a colo- nization phase when the successively immigra- 100000 ting species are each exploiting a maximum Mellum proportion of a given resource, thus, leaving 10000 only "leftovers" for later colonists. Interspeci- 1000 fic relationships are supposed to play no role at 100 this stage. "* 10 Most islands and biotopes show a distribu- •••• tion pattern intermediate between log-normal 1 distribution (slightly S-shaped graph, cf. 0 20 40 60 80 PRESTON 1948, 1962, Fig. 3b) and broken stick order of rank 1-52 distribution (distinctly S-shaped graph repre- senting many medium-common species, cf. 100000

• MACARTHUR 1957, 1960, Fig. 3c). Log-nor- 10000 mal distribution patterns are often found in Langeoog, sattmarshes 1000 areas where succession has reached a more or less mature state, but which are still subject to 100 moderate dynamic processes. A broken-stick 10 distribution appears where competition 1 between the species of an assemblage prevails 20 40 60 80 over other more or less independent abiotic order of rank 1-43 factors and resource partitioning and niche separation have advanced. 100000

In most cases the distribution patterns 10000 Wangerooge, dune area appear "torn", i.e. there are more or less irre- 1000 gular deviations from the theoretically expec- 100 ted pattern (particularly regarding the medi- —•% .^ um-common species). These indicate conspi- 10 cuous qualitative and especially quantitative 1 fluctuations within the species assemblages. 20 40 60 80 Permanent changes in the availability of order of rank 1-80 resources (caused by natural and human impact) impede the formation of distinct and stable species assemblages. islands as little isolated as those in the present study (e.g. As 1984, NiEMELÄ et al. 1985, Leafhoppers from the per- spective of insular biogeo- BOOMSMA et al. 1987, NlLSSON et al. 1988). In graphy some cases distances to the source areas amo- unted to a few to some hundred meters, only Colonization success of the leaf- (e.g. WILSON & SIMBERLOFF 1969, REY 1981). hoppers from the East Frisian islands Leafhoppers, as a comparatively vagrant Since - unlike the North Frisian "Geest- group of , are not impeded to bridge core" islands - the East Frisian dune islands distances of a few kilometres (cf. e.g. STEARNS were never connected to the mainland, all & Me CREARY 1938, HOLZAPFEL & PERKINS

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1969, WOLF et al. 1986), at least within a lon- tion rates found on the younger islands of ger colonization period. It should, therefore, Memmert and Mellum (67 and 70%) are in not surprise that no relation could be establis- first line attributable to a high proportion of hed between numbers of species and degree of eurytopic species and species being common isolation (Fig. 4a), nor between colonization within the recruitment area which, therefore,

Fig. 4: success and dispersal ability (Fig. 4b). experience higher colonization chances. (a) Relation between numbers of spe- Naturally, not all of the 380 leafhopper Whether such colonization rates are to be cies and degree of isolation, considered high or low and which conclusions (b) relation between colonization suc- species known from the north western Ger- to draw cannot be settled here in want of fur- cess and dispersal ability. man lowland are able to establish themselves ther studies on additional groups.

2.4 .

90 - • - - Factors affecting the colonization 22 F oo ~ fc 70 success ° 2 —•—-___^ c 1 —— g fin Leafhopper species numbers of the East S » 50 Frisian islands are more closely related to their Ji Q. 30 habitat diversity (R = 0,974, p < 0.01) than 1« o size (R = 0,835, p < 0.01; Fig. 5a,b). Hence, M r = -0.402 c ,0

•S M. ti o I 1 the present data do better fit the "habitat- 0» b Q. tn Q. diversity hypothesis" (WILLIAMS 1964: species 1.2 1 2 numbers of isolated areas dependent on its diversity of landscape elements) than the 0 4 0.6 0.S 1 12 I • dispersal ability high Q dspersal ability bw degree of Isolation (logl 0) I "area-per-se-hypothesis" (MACARTHUR & WILSON 1963, 1967: species numbers of isola- ted areas dependent on its size). 2,4 Conversely, species numbers of bugs from 22 NO WM the East Frisian islands are more closely rela- bb BA SP ^^**^ 2 Ol 10 ) ted to island size than diversity of landscape MT ML _ . 1,8 • elements (BRÖRING 1991): In this group island a r = 0,974 ML r = 0,335 p. 0 01 p<0.01 1.6 . be r (lo g size explains more than 72% of the variances

i/i of deviation (partial correlation with diversity 1.4 1,4 maintained constant), but merely 23% in leaf- icie s nu m Q. hoppers (NlEDRINGHAUS 1991:86). Diversity 1 . 1 of landscape elements explains only 56% in 1,5 2 2.5 3 2,5 3 3 5 T bugs, but 82% in leafhoppers. These findings habitat diversity area (Iog10) may be explained by the following argument: Fig. 5: on the chain of islands. Considered as poten- 1. Compared to bugs leafhoppers generally (a) Relation between species number tial colonists are only those species from the show higher population densities and repro- and island habitat diversity (a) and recruitment area (i.e. roughly the mainland island area (b). ductive rates and presumably also a greater region within a radius of some 150 km from vagrancy particularly as to larger distances. the islands; see NlEDRINGHAUS 1991 for This ultimately renders a faster and more details) the resource requirements of which "complete" colonization of potential habitats. are sufficiently met on the islands. Given this, 2. There are comparatively numerous 236 species are regarded as potential colonists members of leafhoppers exerting minimal for the chain of islands as a whole (between areas of a few square meters, only. Since the 212 and 236 spp. each for the old island, 92 potential habitats are usually much larger on and 76 spp., respectively, for the young islands the islands, insufficient habitat size would not of Memmert und Mellum). Hence, the "colo- play a role as a prerequisite for a successful nization rate" is about 71%, in other words, colonization in such cases. 29% of the potential colonists currently failed 3. On all old islands the potential of to establish themselves. For the old islands resources increased through human activities colonization rates range between 49 % (Juist) (e.g. shrub and tree plantations) within the and 59 % (Norderney). The higher coloniza- last decades. In this way, species spectra of

536 © Biologiezentrum Linz/Austria; download unter www.biologiezentrum.at leafhoppers increased considerably on the are characterized by very low disposition islands (by some 50 to 60% each). These new- values (CD 25 to < 28; very small colonizati- habitats being independent of island size are on chances), 56 species (24%) by low (CD 28 fast colonized by leafhoppers (see point 1.). to < 31), 46 species (20%) by high (CD 34 to While according to the area-per-se < 37) and 13 species (6%) by very high approach exclusively quantitative aspects (CD >37) disposition values. 109 species are Tab. 2: have an effect (island area and species num- indifferent in this respect, i.e. their colonizati- Relation between colonization success bers of colonists), the importance of qualitati- and various factors (deviations of cha- on chances appear to be neither increased nor ve aspects (number and specificity of potenti- racteristics-fractioned rates of success decreased (Fig. 6). from mean rates). al habitats, species numbers of colonists and composition of species assemblies) is stressed 2. preference for grasses sedges/rushes herbs woody plants by the habitat-diversity approach. Thus, the special groups of 8±3 -1 ±9 3± 5 -10 ±5 analysis of colonization success of the species host plants under study demands the evaluation of those 6. degree of habitat highly eurytopic eurytopic stenotopic highly stenotopic specialization 29 ±7 3*2 -23 ±6 8±5 characteristics which may affect the coloniza- 7. potential length of very long long short very short tion process, particularly with respect to the colonization period (ca. 2000 years) (ca. 100 years) questions: "Why are certain species absent 27 ±5 11 ±2 -9 ±3 31 ± 5 from the islands ?" and "Why are certain colo- 8. degree of niche high medium low overlap -11 ±4 -1 ±9 11 ±9 nist species more successful than others?" 9. abundance in the very abundant abundant rare very rare For the group of leafhoppers the following recruitment area 35 ± 14 10 ±9 -12 ±6 -10 ±10 characteristics were considered as potentially affecting the colonization chances: 1. dispersal ability; 2. preference for speci- 100 -, * *t

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other. Whether these findings are indicative of landschaftlichen Diversität als durch die der chance phenomena or further unknown fac- Flächengröße erklären. Die Daten zur Zika- tors remains to be settled. denfauna dieser Inseln sind insofern eher mit In contrast to the approaches and models der "habitat-diversity-Hypothese" als mit der of insular biogeography relying on purely "area-per-se-Hypothese" in Einklang zu brin- quantitative parameters (numbers of species of gen. Die Gründe dafür sind sowohl in spezifi- isolated areas as a variable to be explained) the schen Eigenheiten der Zikaden zu finden (z.T. present study aims at the individual species starke Lebensraum-Spezialisierung und damit and their biological-ecological characteristics zusammenhängend effiziente und schnelle as a starting point of reasoning. The focus lies Besiedlung potentieller Habitate, z.T. sehr on qualitative aspects (the specific combina- geringe Minimallebensräume) als auch in spe- tion of species as a variable to be explained) zifischen Eigenheiten der Inseln (permanente and, thus, on the uniqueness of the studied Dynamik hinsichtlich der landschaftlichen species assemblage. Entwicklung, Entkopplung von Inselgröße und Anzahl potentieller Habitate durch künstliche Acknowledgements Diversifizierungen der Inseln). Es besteht ein Zusammenhang zwischen Special thanks are due to Dr. Michael dem Kolonisationserfolg und bestimmten Struck-Gerbaulet, Phyllobolic Institute, Dan- Merkmalen der potentiellen Kolonisten; ent- nenberg/Elbe, for substantially improving the scheidende Komponenten dabei sind: der Spe- English text and helpful comments on an ear- zialisationsgrad hinsichtlich Lebensraum, die lier draft of the manuscript. Nahrungspräferenz für bestimmte Pflanzen- gruppen, die Präferenz für bestimmte land- Zusammenfassung schaftliche Entwicklungsstufen auf den Inseln, die Einnischungsfähigkeit gegenüber Konkur- Von 1982-1988 wurden auf den im renten, die Häufigkeit im Rekrutierungsareal. Wattenmeer der südlichen Nordsee gelegenen Weitere mögliche Komponenten wie Disper- 7 alten Ostfriesischen Inseln Borkum, Juist, sionsfähigkeit, Grad der Nahrungsspezialisation, Norderney, Baltrum, Langeoog, Spiekeroog Überwinterung, Generationszyklus oder Kör- und Wangerooge sowie auf den beiden jungen pergröße spielen hinsichtlich der Kolonisa- Inseln Memmert und Mellum inselbiogeogra- tionschancen keine nennenswerte Rolle. phische Untersuchungen zur Zikadenfauna Jeder einzelne potentielle Kolonist verfügt (Hemiptera, Auchenorrhyncha) durchgeführt. aufgrund seiner spezifischen Merkmalskombi- Auf der Inselkette wurden insgesamt nation über eine bestimmte „Disposition", 172 Arten nachgewiesen. Die Artenzahlen der gemäß der seine Kolonisationschancen steigen alten Inseln liegen annähernd auf gleichem oder fallen. Die Quantifizierung dieser „Kolo- Niveau (110-139 Arten). Die Artenzusam- nisationsdisposition" ergibt: 3% der potentiel- mensetzungen zeigen trotz starker Ähnlichkei- len Kolonisten zeichnen sich durck sehr nied- ten der Ressourcenpotentiale der Inseln aller- rige Dispositionswerte (stark verminderte dings erhebliche Unterschiede. Die Muster der Kolonisationschancen), 22% durch geringe, Häufigkeitsverteilungen zeigen in vielen 21% durch hohe und 9% durch sehr hohe Dis- Fällen ausgeprägte Parallelen zu Artenzusam- positionswerte aus. Fast die Hälfte aller poten- mensetzungen dynamischer und nicht ausge- tiellen Kolonisten (46%) erweist sich als reifter Landschaften. m.o.w. neutral, so daß bei ihnen weder von Der Isolationsgrad der Inseln ist zu gering, erhöhten noch von verminderten Kolonisa- als daß der Kolonisationserfolg der Zikaden tionschancen auszugehen ist. Ist die Kolonisa- davon beeinflußt wird: Weder zwischen tionsdisposition jedoch gerichtet, so werden - Artenzahl und Isolationsgrad noch zwischen wie die Prüfung der tatsächlichen Erfolgswerte Kolonisationserfolg und Dispersionsfähigkeit ergibt - für 90% der entsprechenden potentiel- ergeben sich Korrelationen. Die Zikaden- len Kolonisten die Erwartungen hinsichtlich Artenzahlen der Ostfriesischen Inselkette las- ihres Kolonisationserfolges bzw. Mißerfolges sen sich besser durch die Variablen der auf der Inselkette erfüllt.

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Address of the author:

Dr. Rolf NlEDRlNGHAUS Department of Biology, Earth and Environmental Sciences University of Oldenburg P.O. Box 2503 26111 Oldenburg Germany

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