Leafhoppers (Hemiptera, Auchenorrhyncha) in Fragmented Habitats 523- 530 © Biologiezentrum Linz/Austria; Download Unter

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Zeitschrift/Journal: Denisia

Jahr/Year: 2002

Band/Volume: 0004

Autor(en)/Author(s): Biedermann Robert

Artikel/Article: Leafhoppers (Hemiptera, Auchenorrhyncha) in fragmented habitats 523- 530 © Biologiezentrum Linz/Austria; download unter www.biologiezentrum.at

Leafhoppers (Hemiptera, Auchenorrhyncha) in fragmented habitats

R. BIEDERMANN

Abstract

The distribution of leafhoppers (inclu- patterns in the habitat patches, in most ding planthoppers, spittlebugs and tree- leafhopper species effects of area and iso- hoppers) largely depends on the distribu- lation on density and occupancy were tion of their host plants. Plants occur reported. Increasing patch area often more or less aggregated and frequently resulted in higher densities and higher form discrete patches. In natural as well as incidences. On the other hand, increasing in cultural landscapes these patches may isolation was found to reduce the inciden- be fragmented to some extent. A review of ce in the patches. The relevance of these existing studies on leafhopper populations results are discussed in the light of recent in fragmented landscapes summarises the metapopulation theory. current knowledge on the role of area and isolation on occurrence and density of Key words: Auchenorrhyncha, host leafhoppers. Whereas little information is plant, habitat fragmentation, area, isola- available on the dynamics of occupancy tion

Denisia 04, zugleich Kataloge des OÖ. Landesmuseums, Neue Folge Nr. 176 (2002), 523-530

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Introduction The analysis of leafhopper communities may yield additional information on, for instance, The overall feature of leafhoppers (inclu- species richness or trophic interactions in rela- ding planthoppers, spittlebugs and treehop- tion to habitat configuration (e.g. DENNO & pers) is their close relation to plants. The host RODERICK 1991). plants provide nutrition (e.g. BACKUS 1985, There a two major theoretical concepts COBBEN 1988), shelter for eggs (e.g. CLARIDGE dealing with the explanation of animal distri- et al. 1977) and transmission channels for bio- butions on islands or habitat patches. The acoustic signals (e.g. MlCHELSEN et al. 1982). theory of island biogeography (MACARTHUR Accordingly, one major requisite of leafhopper & WILSON 1967) predicts species numbers on habitats is the presence of the host plants. In particular in host plant specialists among leaf- islands or habitat patches in relation to isola- hoppers the habitat is principally given by the tion and area. In leafhoppers, the theory of distribution of the host plants. island biogeography was studied, for instance, by NlEDRINGHAUS (1991) on dune islands in Plants frequently occur aggregated to some the North Sea. The application of island bio- extent and form discrete patches. The amount geography theory is confined to communities and physiology of the host plant may largely and seems not suitable for the analysis of determine the quality of such host plant pat- populations of single species (see HANSKI ches (e.g. PRESTIDGE 1982, MOON et al. 2000). 2001). Further, it assumes a mainland as a However, some leafhopper species exhibit source of colonising individuals. In fragmen- more complex habitat requirements. Additio- ted landscapes there is not necessarily a main- nal factors like host plant architecture, vege- land, but a situation with a large number of tation structure or microclimate may be rele- patches of similar area may occur. The corre- vant (e.g. CLARIDGE 1986, DENNO & RODE- sponding theory for populations on regional RICK 1991) and therefore the habitat may be scales is the metapopulation theory (HANSKI not primarily defined by the occurrence of the 1998). Recent metapopulation theory is a spa- host plants. In these species as well as in more tially explicit attempt to describe the dyna- generalist species with a broader range of host mics of a regional population with its members plants, the habitat patches may not be clearly distributed over a set of habitat patches. The delimited in the field. In contrast, in host main feature of metapopulation dynamics is plant specialists among the leafhoppers the the occurrence of extinction and colonisation survey of the distribution of the host plant was events in the habitat patches. found to be a very feasible way to determine all potential habitats of a certain species in a The objective of this review is to make a landscape (DENNO et al. 1981, BIEDERMANN survey on existing studies on leafhopper popu- 2000). lations in fragmented habitats and to discuss In natural as well as in cultural landscapes the results in regard to metapopulation theory. the habitats of leafhoppers are frequently frag- According to recent theory, the occurrence mented to some extent. In the recent two and density of leafhoppers in a particular habi- decades there is increasing concern about tat patch may depend on area, isolation, qua- ongoing habitat fragmentation by human lity and surrounding landscape structure of the activities, especially in cultural landscapes patch. The quality of a patch is the funda- (SETTELE et al. 1996). Fragmentation is sugge- mental factor for the survival and reproduc- sted to be one major cause of the extinction of tion of leafhoppers and is not a particular issue species and thereby declining biodiversity of fragmented habitats. The role of surroun- (MORRIS 1995). Looking at existing results on ding landscape structure on leafhopper distri- leafhoppers in fragmented habitats there are bution in habitat patches is largely unknown two levels of consideration, the population (but see JONSEN & FAHRIG 1997). Thus, in the level and the community level. At the popu- following the current knowledge on the role of lation level the occurrence, abundance and area and isolation on occurrence and density dynamics of particular species is of interest. of leafhoppers will be summarised.

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Habitat fragmentation and MANN 1997, BIEDERMANN 1998, ZABEL &. metapopulation theory TSCHARNTKE 1998, BIEDERMANN 2000, BLE- DERMANN unpubl. data). The main focus of Habitat fragmentation describes the these studies was on the dependency of the decrease in number and size of suitable habi- incidence (= probability of occurrence) and tats for species (e.g. HARRISON & BRUNA density of a particular species on area and iso- 1999, HUXEL &. HASTINGS 1999). The proces- lation of patches. In nearly all studies the first ses which lead to habitat fragmentation are step was a complete survey of the host plant decreasing area of patches up to their comple- patches in the study area providing the area te elimination and consequently increasing and isolation of the patches. This step was fol- isolation of patches. The term fragmentation lowed by a survey of the leafhopper occurren- describes both the processes and the outcome. ce or density in these patches. In order to It often implies a loss of habitat due to human quantify extinction or colonisation events in activities. As in natural landscapes suitable some studies the survey for the occurrence of habitat may be patchy without anthropogenic species has been repeated for several years or influence, here the term patchiness may be generations. more appropriate. However, in the following the term fragmentation will be used for both Incidence and area fragmented and patchy habitats. The first study on the relationship bet- Living in such a set of patches animals ween the occurrence of leafhoppers and the build up structured populations (HARRISON area of their habitats was performed by HALK- 1991), with its members distributed over a KA et al. (1971) on the spittlebug Philaenus number of habitats. In recent years the analy- spumarius (Aphrophoridae). They were analysis of spatial population structure has made a sing the distribution of Philaenus spumarius in rapid development, both in theoretical (e.g. island habitats in an archipelago of 135 islands HANSKI 2001) and empirical studies (e.g. in the Baltic Sea. GUTIERREZ et al. 2001). The results suggest A method to analyse the role of patch area that animals build up a wide continuous range on the occurrence of leafhoppers in host plant of spatial population structures from isolated patches is the use of incidence functions patches with virtually no immigration, over metapopulations with a limitation in the (ADLER & WILSON 1985, TAYLOR 1991). Incidence functions quantify the relationship dispersal between patches, up to patchy popu- between species presence and area in a set of lations with high colonisation and thus low patches of various area. In order to calculate extinction rates (HARRISON 1991, THOMAS & incidence functions, in each patch the presence KUNIN 1999). According to metapopulation or absence of the species is recorded. The theory (HANSKI 1998) the dynamics of the resulting occupancy patterns are related to occurrence of a species in its habitat patches is patch area using logistic regression (HOSMER determined by local extinction and colonisa- & LEMESHOW 2000). Logistic regression is a tion. The extinction rate is assumed to decrease statistical method which detects relationships with increasing patch area and the colonisa- between a dependent binary variable and one tion rate depends on the degree of isolation of or more independent variables. Here, the a patch. For the long-term persistence of a dependent variable is the presence-absence metapopulation an equilibrium between pattern in the patches and the independent extinction and colonisation is required. variable is the area of the patches. The resulting incidence functions show the relationship Leafhopper populations in between the probability of occurrence (= inci- fragmented habitats dence) and area (Fig. 1). The distribution of species in a number of In order to obtain the incidence function habitat patches of various area and isolation of Philaenus spumarius a re-analysis of the data was analysed in several studies on leafhoppers (HALKKA et al. 1971) was performed. The (HALKKA et al. 1971, LAWTON 1978, RAUPP & results showed a positive relationship between DENNO 1979, DENNO et al. 1981, MACGARVIN the occurrence of Philaenus spumarius and the 1982, BIEDERMANN & APPELT 1996, BIEDER- suitable habitat area on the islands.

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Fig. 1: Directly using the technique of logistic was increasing with increasing area of the host Examples of incidence functions of (a) regression, BIEDERMANN & APPELT (1996) cal- plant patches. In a second study of the tree- Gargara genistae and (b) Ribautodel- phax pungens demonstrating the posi- culated incidence functions of five monopha- hopper Gargara genistae in another landscape tive relationship between the inciden- gous leafhoppers occurring in host plant pat- with 237 patches of Sarothamnus scoparius a ce (= probability of occurrence) and ches of various area in fragmented dry grass- similar result was found (BIEDERMANN unpubl. area (= patch size), based on logistic data), the incidence is increasing with increa- regression using data from BIEDERMANN land: the leafhopper Adarrus multinotatus sing patch area (Fig. 1). These results indicate (unpubl.) and BIEDERMANN & APPELT (Cicadellidae), the spittlebug Neophilaenus (1996). a high variability in the incidence-area relati- albipennis (Aphrophoridae) in 506 patches, onship among species, even within one guild. the planthopper Ribautodelphax pungens (Del- In a study of several phytophagous and 1 0- predatory insects on Urtica dioica ZABEL &. TSCHARNTKE (1998) recorded a positive relationship between the area of the host plant pat- 0.8- ches and the incidence of the monophagous / / leafhopper Macropsis scutellata (Cicadellidae). In general, the effects of patch area are of 0.6- a / / b relevance for the explanation of the distribution of leafhoppers as in all species a significant 0.4- relationship exists between incidence and

Incidenc e area. In order to analyse the differences in the incidence-area relationship between species, 0.2- from the existing results area thresholds were calculated. Based on the incidence functions area thresholds were taken as a measure of the 0.0- characteristic of the incidence function. As 10 100 1000 10000 100000 thresholds the area at an incidence of 0.5 and 2 Area [m ] 0.95 were chosen. The comparison shows a high variability among species (Table 1). Table 1: Some species like the leafhopper Adarrus mul- Comparison of the area requirements (in m2) tinotatus or the planthopper Kelisia haupti of eight leafhopper species for two given incidence thresholds (I = 0.5 and I = 0.95), occupy even very small patches of only a few calculated by incidence functions using logi- square meters, whereas in other species like stic regression; references: 1: BIEDERMANN & the spittlebug Neophilaenus albipennis or the APPELT (1996), 2: ZABEL & TSCHARNTKE (1998), 3: recalculated from HALKKA et al. (1971), planthopper Ribautodelphax pungens only large 4: BIEDERMANN (unpubl. data). patches have a considerable incidence.

Species Number of patches Incidence 0.5 0.95 Adarrus multinotatus^ 506 8 Kelisia haupti* 21 10 18 Garqara qenistae* 24 24 79 Philaenus spumarius3 135 56 301 Garqara qenistae* 237 75 212 Macropsis scutellata2 32 493 1229 Neophilaenus albipennis* 506 747 1976 Ribautodelphax pungens* 73 4162 10052

phacidae) in 73 patches of Brachypodium Density and area pinnatum, the planthopper Kelisia haupti The relationship between the density and (Delphacidae) in 21 patches of Carex humilis the area of host plant patches was analysed in and the treehopper Gargara genistae (Membra- several studies on leafhoppers (Table 2). In cidae) in 24 patches of Sarothamnus scoparius. the planthopper Ditropis pteridis (Delphaci- In all species a significant relationship exists dae) a positive relationship was detected between incidence and area. The incidence between density and patch area of the host

526 © Biologiezentrum Linz/Austria; download unter www.biologiezentrum.at plant Pteridium aquilinum (LAWTON 1978). In dence was significantly affected by isolation, a guild of leafhoppers on Spartina patens RAUPP increasing distance to the next occupied patch &. DENNO (1979) and DENNOetal. (1981) repor- was reducing the probability of occurrence in ted high variability in the response of species the patches. density on patch area. A positive relationship In contrast, in other leafhopper species no between area and density was found in the effect of isolation could be detected. The spa- planthoppers Tumidagena minuta (Delphaci- tial analysis of the distribution of the leafhop- dae), Delphacodes detecta (Delphacidae) and per Adarrus mulanotatus showed no effect of Aphelonema simplex (Issidae), whereas the isolation on the occupancy in the patches of densities of the planthopper Megamelus lobatus the host plant Brachypodium pinnatum (BlE- (Delphacidae) and the leafhoppers Amplice- Table 2: Summary of the results on DERMANN 1997). In a set of 506 patches even phalus simplex (Cicadellidae) and Destria bisig- the relationship between leafhopper patches with large distances to other patches density and patch area, +: positive, nata (Cicadellidae) were not affected by area. had a high incidence. In the leafhopper 0: no relationship; references: In a study of the density of the spittlebug Macropsis scutellata isolation had no effect on 1: LAWTON (1978), 2: RAUPP & DENNO Philaenus spumarius in 24 patches of Chameri- (1979), 3: DENNO et al. (1981), 4: MAC- the incidence in 32 patches of Urtica dioica on angustifolium no significant relation could GARVIN (1982), 5: BIEDERMANN (1997), (ZABEL & TSCHARNTKE 1998). 6: BIEDERMANN (unpubl. data). be detected between area and density (MAO GARVIN 1982). In the leafhopper Adarrus mul- Species Number of patches Density-area tinotatus no correlation was found between relationship density and patch area in 14 patches of Ditropis pteridis'1 9 + 2 Brachypodium pinnatum (BIEDERMANN 1997). Delphacodes detecta 25 + Tumidagena minuta3 25 + A similar result was obtained in the treehop- 3 Aphelonema simplex 25 + per Gargara geniswe. There was no relations- Amplicephalus simplex3 25 0 3 hip between density and patch area in 17 pat- Destria bisiqnata 25 0 Meqamelus lobatus3 25 0 ches of Sarothamnus scoparius (BIEDERMANN Philaenus spumarius* 24 0 unpubl. data). Adarrus multinotatus5 14 0 Gargara genistae6 17 0

Incidence and isolation Species Number of patches Incidence-isolation Among the existing investigations on the relationship spatial distribution of leafhoppers in habitat Philaenus spumarius^ 135 patches no study was available on the relati- Neophilaenus albipennis* 506 Gargara genistae^ 237 _ onship between density and isolation. Howe- Adarrus multinotatus2 506 0 ver, there are a few studies testing the effect of Macropsis scutellata3 32 0 isolation on the occupancy of patches (Table Table 3: Summary of the results on 3). The analysis of Philaenus spumarius on Incidence, area and isolation the relationship between leafhopper small islands in the Baltic Sea (HALKKA et al. incidence and patch isolation, -: nega- The simultaneous analysis of both area tive, 0: no relationship; references: 1971) showed a significant effect of isolation and isolation of the patches was performed in 1: HALKKA et al. (1971), 2: BIEDERMANN on the distribution of the spittlebug. In a stu- (1997), 3: ZABEL & TSCHARNTKE (1998), three species. In the spittlebug Neophilaenus dy of the spittlebug Neophilaenus albipennis 4: BIEDERMANN (2000), 5: BIEDERMANN albipennis both factors have a significant effect (unpubl. data). BIEDERMANN (1997) was using logistic regres- on the occurrence in the host plant patches. sion (as the independent variable the distance These two factors yet account for 27.2 % of to the next occupied patch was used as a measure of isolation) to detect effects of isolation the variance in the occurrence when analysed on the incidence of the spittlebug in the 506 in a multiple logistic regression analysis (BlE- patches of its host plant Brachypodium pinna- DERMANN 1997). That is, even without consi- turn. The results clearly state that the inciden- dering habitat quality of the patches a rather ce of Neophilaenus albipennis was reduced in high amount of variance was explained by the more isolated patches. Another study these two factors of spatial configuration. In using logistic regression was performed in the the treehopper Gargara genistae the relation- treehopper Gargara genistae in a set of 237 pat- ship between occurrence and these two factors ches of the host plant Sarothamnus scoparius was similar. The amount of variance explained (BIEDERMANN unpubl. data). Again, the inci- was 25.4 % (BIEDERMANN unpubl. data).

Using the same statistical procedure in ano- reduce the extinction risk due to environ- ther spittlebug, Phiiaenus spiimariiis, in the mental stochasticity (LANDE 1993). Additio- joint analysis of area and isolation of patches nally, the resource concentration hypothesis (data from HALKKA et al. 1971) these two fac- (ROOT 1973, CONNOR et al. 2000) predicts tors even account for 57.5 % of the variance higher densities in larger patches as a result of in the distribution. e.g. greater reproduction, reduced edge effect or reduced predator density. Dynamics of incidence On the other hand, dispersal could obscu- Surveys of the distribution of species in re the correlation between density and area. In several subsequent years or generations yield species with a high dispersal ability like Ampli- the dynamics of patch occupancy. The species cephalus simplex or Adarrus multinotatus, all may get extinct in some patches and other patches could be reached with high probabili- previously empty or new patches may be (re-) ty. Thus, initial differences in densities due to colonised. Metapopulation theory suggests reduced performance in small patches may be that the extinction rate of species depends on levelled off by dispersing individuals. As a patch area. In fact, in the spittlebug Neophila- result, small patches may be sink-habitats enus albipennis the extinction rate is positively (PULLIAM 1996). The outcome of such a pro- correlated with patch area (BIEDERMANN cess would be an equal density in all patches 2000). The extinction rate is high in small irrespective of its area. In fact, for the guild of patches and decreases with increasing patch leafhoppers on Spartina patens DENNO et al. area. The mean yearly rate of extinction was (1981) suggest an association between positi- estimated 3.9 %. The colonisation rate was ve area-density relationship and low dispersal comparable, the yearly rate was 4-9 %. The ability of species (e.g. measured by the fraction treehopper Gargara genistae showed a compa- of macropterous individuals). rable turnover in patch occupancy (BIEDER- The dispersal ability differs between spe- MANN unpubl. data) with a shifted balance cies and may be correlated with the effect of between the yearly extinction rate (11.1 %) isolation on the distribution in fragmented and the yearly colonisation rate (1.9 %). In landscapes. Dispersing individuals may (re-) the spittlebug Cercopis sanguinolenta in a colonise patches which were empty or newly three-years period no extinction was recorded emerged. For instance, in the spatial analysis in ten local populations (BIEDERMANN 1998). of the distribution of the leafhopper Adarrus Obviously, the yearly rate of extinction varies multinotatus no effect of isolation on patch among leafhopper species. However, the data- occupancy could be detected. In the same base is to weak to deduce general conclusions landscape a parallel investigation of the popu- from these results. lation genetics of Adarrus multinotatus was conducted. The results showed a concordant Synthesis result, a high gene flow is suggested to exist between local populations at the regional sca- The fragmentation of habitats may have le (VEITH et al. 1996). In the spittlebug Neo- severe consequences on animal distribution philaenus albipennis which was studied in the (HARRISON & BRUNA 1999). Fragmentation same set of habitat patches, the distribution leads to reduced patch area and as a result the was significantly affected by isolation. As extinction probability of local populations is expected, in a mark-recapture experiment in high in small patches. Below a certain thres- Neophilaenus albipennis low dispersal rates were hold a patch may be too small to maintain via- recorded (BIEDERMANN 1997). ble populations. The comparison of studies on The existing case studies on leafhoppers leafhoppers demonstrated that the area requi- clearly demonstrate the effects of area and iso- rements vary in a broad range. This variability lation on distribution and density in fragmen- may be attributed to parameters of population ted habitats. A set of hypotheses is available to dynamics or life-history traits. Species which explain these static patterns. However, these are able to build up high densities in their pat- studies give only an idea of the importance of ches could reach sufficient population sizes to dynamic spatial processes. In particular, stu-

528 © Biologiezentrum Linz/Austria; download unter www.biologiezentrum.at dies on the dynamics of the distribution BACKUS E.A. (1985): Anatomical and sensory mecha- (extinction and colonisation) in the habitat nisms of leafhopper and planthopper feeding behavior. In: NAULT L.R. & J.G. RODRIGUEZ, The patches would be required in order to enhan- leafhoppers and planthoppers. — Wiley & Sons, ce our knowledge of spatial processes (e.g. tur- New York, pp. 163-194. nover rates) at the regional scale. Effects of BIEDERMANN R. S M. APPELT (1996): Invertebrates and spatial configuration of patches on incidence area size in the porphyry landscape of Halle. In: and density as well as spatial dynamics of inci- SETTELE )., MARGULES C, POSCHLOD P. & K. HENLE, Species survival in fragmented landscapes. — dence are expected to be scale-dependent Kluwer, Dordrecht, pp. 183-186. (WEBB & THOMAS 1994, KOENIG 1999, HE &. BIEDERMANN R. (1997): Populationsökologische Unter- GASTON 2000). Thus, future studies on sever- suchungen an Zikaden (Homoptera, Auchenorr- al, hierarchically nested scales would assess hyncha): zum Einfluß von Habitatqualität, Habi- the importance of scale on the identification tatgröße und Isolation auf das Vorkommen und Überleben von Populationen. — PhD thesis, of effects of area and isolation. The considera- University of Mainz, pp. 104. tion of dynamic habitat patches (e.g. mosaic BIEDERMANN R. (1998): Populationsökologie der Blut- cycles) would incorporate an additional time zikade Cercopis sanguinolenta (SCOPOLI, 1763) scale into the analysis of regional leafhopper (Homoptera, Cercopidae). — Beiträge zur Zika- populations. denkunde 2: 57-66. BIEDERMANN R. (2000): Metapopulation dynamics of the froghopper Neophilaenus albipennis (F., Zusammenfassung 1798) (Homoptera, Cercopidae) - what is the minimum viable metapopulation size? — Jour- Die Verbreitung von Zikaden ist in star- nal of Insect Conservation 4: 99-107. kem Masse abhängig von der Verbreitung CLARIDGE D.W. (1986): The distribution of a typhlocy- ihrer Wirtspflanzen. Pflanzen treten meist agg- bine leafhopper, Ribautiana ulmi (Homoptera, Cicadellidae) on a specimen wych elm tree. — regiert auf und bilden oft gut abgrenzbare Ecological Entomology 11: 31-39. Bestände. Diese Bestände können sowohl in CLARIDGE M.F., REYNOLDS W.J. & M.R. WILSON (1977): Natur- als auch in Kulturlandschaften zu Oviposition and food plant discrimination in einem gewissen Grad fragmentiert sein. Ein leafhoppers of the genus Oncopsis. — Ecologi- Überblick über die vorhandenen Untersu- cal Entomology 2: 19-25. chungen an Zikaden in fragmentierten Land- COBBEN R.H. (1988): What do we really know about schaften stellt die Ergebnisse zum Einfluss von host selection in Auchenorrhyncha? In VIDANO C. & ARZONE A. Proceedings of the 6th Auchenorr- Flächengrösse und Isolation der Wirtspflan- hyncha meeting, Turin, Italy, September 7-11, zenbestände auf das Vorkommen und die 1987. — Consiglio Nazionale delle Ricerche - Abundanz von Zikaden zusammen. Während IPRA, Rome, pp. 81-92. nur wenige Informationen zur zeitlichen CONNOR E.F., COUTNEY A.C. & J.M. YODER (2000): Indivi- Dynamik der Verbreitungsmuster verfügbar duals-area relationships: the relationship between animal population density and area. — sind, konnte in den meisten untersuchten Ecology 81: 734-748. Zikadenpopulationen ein Effekt von Flächen- DENNO R.F., RAUPP M.J. & D.W. TALLAMY (1981): Orga- grösse und Isolation auf die Dichte und das nization of a guild of sap-feeding insects: equi- Verbreitungsmuster in den Wirtspflanzenbe- librium vs. nonequilibrium coexistence. In: DEN- ständen nachgewiesen werden. Zunehmende NO R.F. & H. DINGLE, Insect life history patterns: habitat and geographic variation. — Springer, Flächengröße resultiert häufig in höheren New York, pp. 151-181. Dichten und höheren Inzidenzen, mit zuneh- DENNO R.F. & G.K. RODERICK (1991): Influence of patch mender Isolation hingegen sinkt die Inzidenz. size, vegetation texture, and host plant archi- Die Bedeutung dieser Ergebnisse wird im Hin- tecture on the diversity, abundance, and life blick auf die aktuelle Metapopulationstheorie history styles of sap-feeding herbivores. In: BELL S.S., MCCOY E.D. & H.R. MUSHINSKY, Habitat struc- diskutiert. ture: The physical arrangement of objects in time and space. — Chapman and Hall, London, References pp. 169-196. GUTIERREZ D., LEÖN-CORTES J.L., MENENDEZ R., WILSON R.J., ADLER G.H. & M.L. WILSON (1985): Small mammals on COWLEY M.J.R. & CD. THOMAS (2001): Metapopu- Massachusetts islands: the use of probability lations of four lepidopteran herbivores on a functions in clarifying biogeographic relations- single host plant, Lotus corniculatus. — Ecology hips. — Oecologia 66: 178-186. 82: 1371-1386.

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