Patch connectivity and genetic variation in two congeneric grasshopper species with different habitat preferences
Jes Johannesen
1*, J ¨org Samietz
2, Michael Wallaschek
3, Alfred Seitz
1and Michael Veith
11
Institut f¨ur Zoologie, Abteilung f¨ur ¨ Okologie, Universit¨at Mainz, Saarstraße 21, D-55099 Mainz, Germany
2
Institut f¨ur ¨ Okologie, Universit¨at Jena, Dornburger Straße 159, D-07743 Jena, Germany
3
Institut f¨ur Zoologie, Universit¨at Halle-Wittenberg, Kr ¨ollwitzer Straße 44, D-06099 Halle, Germany
Received 10 November 1998; accepted 20 April 1999
Two congeneric species of grasshopper, Stenobothrus lineatusand S. stigmaticus, are compared in an analysis of genetic structure relative to their observed mobility, and to the spatial structure of their habitat networks. The species differ in their habitat requirements, the latter being rarer and more restricted to isolated patches. We tested for different patch connectivity between the two species in an analysis of genetic variance (based on allozymes) under the assumption that, besides isolation, rarity influences the genetic parameters. Between the species we found no differences in genetic structure as estimated by FST; i.e., no isolation effects and no apparent differences between the species in the potential to move between habitat fragments on either a local or regional scale were found. However, the amount of genetic variation in the more widely distributed and less xerothermic S. lineatuswas significantly higher than in S. stigmaticus. Some consistency with observed philopatry within patches was found (FIS.0), but we consider regular dispersal events of medium and especially long distance to cause the habitat linking.
We conclude that the connectivity between occupied patches inferred by genetic analyses can seldom be derived from low observed life-time movements recorded by conventional marking studies. Consequences of applying observed relative to indirect dispersal estimates for the examination of grasshopper metapopulations are discussed.
Keywords: dispersal, species abundance, xerothermic habitat, Stenobothrus lineatus, Stenobothrus stigmaticus
Introduction
One important factor for the persistence of populations in fragmented landscapes is the ability of individuals to disperse between discretely located habitat patches.
Connectivity of these patches is the basis of meta- population theory where a colonization-extinction equilibrium describes the incidence of species (Levins, 1970; Hanski, 1994). Thus, the ability of an individual to bridge the interspacing distances is generally of great importance for survival in a given landscape.
In grasshoppers diurnal flight has been described as being mainly non-spontaneous and primarily as movements within patches (overview by Farrow, 1990).
Solitary grasshoppers seldom experience population outbreaks which may result in mass migration. In the natural habitat, even good fliers, such as Stenobothrus lineatus, mainly move by walking. Jumping or fly- ing seldom occurs under field population densities (Samietz, 1998).
The most often used methods for inferences of dis- persal ability of non-migratory grasshoppers are (1)
marking studies, mark-recapture and mark-resight studies, respectively; (2) the distribution of neutral genetic markers. Life-time mobility, investigated in terms of movements within patches by marking studies, generally tends to be low in non-migratory grasshoppers, showing individual ranges about 20 to 100 m (e.g., Mason et al., 1995; Samietz and K ¨ohler, 1994; Samietz et al., 1996; Samietz, 1998). Even small barriers such as hedges may hinder movements con- siderably (Gerber and Templeton, 1996; Samietz, 1998).
However, marking studies generally pose methodo- logical problems. They are often conducted within optimum quality patches and may thus miss demo- graphically and/or abiotically induced events that initiate dispersal. Moreover, because mark-recapture studies rely on observations, it is not known if missing individuals have died or have dispersed over long or at least medium distances. Marking with reflective tape (Samietz and Berger, 1997; Samietz, 1998) may eluci- date movements up to a few hundred meters but is unable to investigate movements at the kilometer scale.
Spontaneous or not, long distance flight may be more
* To whom correspondence should be addressed at: Tel: 149 (0)6131 393946; fax: 149 (0)6131 393731; e-mail: Jesjo@hydra.biologie.uni-mainz.de J o u r n a l o f I n s e c t C o n s e r v a t i o n , 3, 2 0 1 – 2 0 9 ( 1 9 9 9 )
common than previously thought: animals have been observed to ascend vertically and drift downwind (Farrow et al., 1982). Farrow et al. (1982) mention that long-winged forms of a short-winged grasshopper species can cover distances of 1 to 3 km within 10 minutes.
The alternative method to estimate dispersal between patches and to clarify patch connectivity is analysis of the distribution of genetic markers. In an island model of population genetic structure among population genetic variance, FST, in genetic drift-migration equi- librium, can be translated into a gene flow estimate, Nem5(1/FST-1)/4, where Nem is the number of migrants per generation. Gene flow measures the level of reproductive input into a habitat patch per genera- tion, whereas observed migration does not. Unfortu- nately, gene flow estimates also have shortcomings.
The distribution of genetic markers are affected by a time-lag, i.e., an analysis cannot distinguish between ongoing and historical gene flow (Slatkin, 1985). In this sense, gene flow estimates give an indirect dispersal estimate over a given period of time. The amount of genetic variation can be taken as an inference of the effective population size (Kimura, 1983). At the species level, the amount of genetic variation is often a good indicator of the relative abundances of congeneric species because the dynamics which usually decrease genetic variation, e.g. habitat and population fluctu- ations, are averaged out when based on many intra- specific populations (Johannesen and Loeschcke, 1996).
When comparing the results of alternative methods to analyse connectivity, gene flow estimates for grass- hoppers tend to contradict the direct observations of limited intrapatch and interpatch movements. Popula- tion genetic studies suggest high levels of dispersal between habitat patches (e.g., Gill, 1981a,b; Schmeller et al., 1996; Nicklas-G ¨orgen, 1997) and may confirm interpatch movements rather than those inferred by marking studies (for Oedipoda caerulescens see Appelt and Poethke, 1997; Nicklas-G ¨orgen, 1997).
In Germany, the two congeneric grasshopper species, Stenobothrus lineatus (PANZER) 1796 and S. stigmaticus (RAMBUR) 1838, occur at the northern limit of their distribution ranges. Both inhabit semi-arid to arid grassland, but their habitat preferences differ. S. lineatus prefers medium vegetation cover at least several centi- metres high. Sparse tufts of the grass Bromus erectus build an optimum structure in central European semi- arid grassslands (Samietz 1996, 1998). S. stigmaticusuti- lizes warmer patches where the vegetation cover is much lower and sparser (not above 1 cm high), and is often characterized by spots of bare soil (Wallaschek,
1996). In central Europe, habitats of S. stigmaticus are generally rarer and restricted to grasslands with tradi- tional land use patterns. In recent years S. stigmaticus has become rare due to changing agricultural practices, and is considered to be endangered on a greater scale.
In the investigated areas, however, both species are locally fairly common.
The aim of the present study was to evaluate the amount and distribution of genetic variation in both Stenobothrusspecies in relation to their current popula- tion status, and to test for different connectivity of their habitat patch networks. Being the less abundant species and inhabiting a narrower ecological range we expected S. stigmaticus to have less genetic variation and its populations to be more structured than S. lineatus. We tested this hypothesis by analysing the two species in a local and a regional framework. Each species was investigated within a local habitat network and com- pared to two geographically distant populations. The latter comparison was undertaken for two reasons:
(1) to evaluate the level of differentiation on a larger scale; (2) to make confident inferences about connectiv- ity of habitat patches for both species, we would need to sample in the same landscape, where abiotic factors co-vary among the two species. In particular, local weather effects have been shown to influence grass- hopper population dynamics (Appelt and Poethke, 1997; Griebeler, 1998). Since we were unable to sample several patches of both species within the same local area, the distant populations are necessary to evaluate possible effects of habitat history (e.g. fragmentation period) on genetic structure and variation. If the genetic estimates within the central populations do not differ from estimates including the distant popula- tions we do not expect different species-related habitat histories to be important for the genetic structure esti- mates. The core areas are situated in the same region of the Saale river basin and experience similar meteoro- logical conditions.
Methods and materials
Individuals of Stenobothrus lineatuswere collected from eight habitat patches around the city of Jena (J1–7, J10), central Saale-river valley, Germany, and from two geo- graphically distant populations for larger-scale com- parisons (Fig. 1). The average distance between patches at Jena was 4725 metres (range: 190–8100 m). The two distant populations were situated at Halle and W ¨urz- burg (WU), 60 km north and 120 km south of Jena, respectively. Stenobothrus stigmaticuswas collected from seven porphyry hills north of Halle, Germany (patches:
I26, II5–23, III39 and IV8). The average distance between patches at Halle was 2743 metres (range:
600–6200 m). The two distant populations were col- lected at Pfarrberg (P) and Bacharach (B) situated 30 km east and 250 km west of Halle, respectively (Fig.1). Both species were collected in fragmented land- scapes, habitat patches separated by urban develop- ment, forests and agricultural land. Average distances between potential habitat, including unsampled patches, were about 200–500 m within the investigated areas of Halle and Jena.
Grasshoppers were stored in liquid nitrogen until electrophoretic analysis. Genetic variation was invest- igated by means of cellulose acetate electrophoresis (Hebert and Beaton, 1993). Hind femur muscle was homogenised in 70 ml Pgm-buffer (Harris and Hopkin- son, 1978) by ultrasound and centrifuged at 14,000 rpm for two minutes. A total of sixteen enzymes could be scored in both species (Table 1). However, in S. lineatus the heterozygotes of the polymorphic enzyme, Pk, did not segregate properly and an unambiguous genotype scoring was possible in only two populations. For reasons of falsification we chose to leave out the
locus from further analysis of S. lineatus population structure.
Deviations from Hardy–Weinberg expectations were tested using the Louis–Dempster (1987) exact test with the program GENEPOP (Raymond and Rousset, 1995), and allele distributions were tested for homogeneity among all localities within Jena (S. lineatus) and Halle (S. stigmaticus) applying a G-test (Sokal and Rohlf, 1981). Genetic differentiation among samples was quantified by F-statistics using the algorithms of Weir and Cockerham (1984) applying the program BIOSYS (Swofford and Selander, 1989). Standard deviations were obtained by jack-knifing over loci. Genetic differ- entiation among populations was tested at two levels.
First, for an analysis of patch connectivity and local scale genetic structure we considered all samples of S.
lineatus at Jena, and all samples of S. stigmaticus at Halle as separate populations. In the second analysis we pooled the allozyme data for Jena (S. lineatus) and Halle (S. stigmaticus) samples respectively, and treated each area as a single patch. An area’s genetic variation was analysed relative to the two distant populations for larger-scale differentiation. Genotypic disequilibrium was estimated for all samples and for the pooled data sets.
Results
On the basis of the investigated loci S. lineatushad sig- nificantly more polymorphic loci (most common allele below the 95% level) (P,0.001) (Table 2), and a higher expected heterozygosity, He, (P,0.001) (Table 3) than S. stigmaticus. For this comparison we have left out the polymorphic locus Pk for S. lineatus. The analysis is therefore conservatively biased towards lesser genetic variation in S. lineatus than is actually present, thus strengthening the difference between the species. Pk variation could be estimated in the S. lineatussamples J2 (3 alleles He50.64, Ho50.60) and J7 (3 alleles He50.60, Ho50.78). The remaining samples indicated the same degree of polymorphism.
All samples of both species showed Hardy–Weinberg proportions across all loci. However, in S. lineatus the locus Pgmshowed a heterozygote deficit in all popula- tions except for W ¨urzburg. Pgm had four alleles and heterozygotes of allele 2 were always missing. Even at W ¨urzburg, where no overall heterozygote deficit was found, heterozygotes of allele 2 were missing from the expected result. Thus, the locus Pgm in S. lineatus showed a significant deviation from expected Hardy–
Weinberg proportions, P,0.001, across all samples.
When removing allele 2 from analysis, neither Pgm Figure 1. Sampling locations of Stenobothrus lineatus and S.
stigmaticus.
across all samples nor the total Jena population (pooled data), deviated significantly from Hardy–Weinberg proportions, P.0.10. Among localities all polymorphic loci showed homogeneous allele frequencies.
No samples of S. stigmaticus deviated significantly from Hardy–Weinberg proportions when calculated across all loci. In all but two samples there was a lack of Me heterozygotes, and in one sample, IV8, an excess (not significant) of Idh homozygotes was found. These sample-related heterozygote deficiencies caused the total Halle population (pooled data) to deviate signifi- cantly from Hardy–Weinberg expectations, P,0.05.
However, Idhand Meshowed homogeneous allele fre- quencies across all localities. This suggests that the Hardy–Weinberg deviation of the pooled data was not caused by a Wahlund effect in the total Halle popula- tion, but rather by sampling errors within single patches. The locus Pkdid not have a homogeneous dis- tribution across all localities, P,0.05 (Bonferroni cor- rected). This was mainly caused by one sample, IV-30A (pairwise locus comparisons; GENEPOP Raymond and Rousset, 1995).
For both species there were no indications of geno- typic linkage disequilibrium in either the samples or
the pooled Jena and Halle populations. Summary F- statistics are presented in Table 4. Three variable loci in S. lineatus, Apk, Ldh and Got, and one locus in S.
stigmaticus were not polymorphic at 95% level in any sample, and are omitted from further analyses of genetic differentiation. For S. lineatus, there was no indication of substructuring within Jena, FST50.005, nor among the regional samples, FST50.007, neither were significantly different from zero. The results did not differ when totally omitting Pgm, FST(Jena)50.012, FST(regions)50.023. When removing the Pgm-allele 2, FST(Jena)5 20.007 was not significant, but FST(regions) 50.016 was very slightly greater than zero. The inbreeding estimate FIS was influenced by the Pgm- allele 2, which created a high among-locus variance, leading to a non-significant estimate.
FST50.028 for S. stigmaticus within Halle was non- significant. The estimate of regional subdivision was lower than the estimate within Halle, FST50.017. This finding was probably caused by the pooling of the Halle samples into one sample, thereby averaging out the variation among Halle samples relative to the two distant populations. Neither FIS50.234 nor FIT50.255 were significantly greater than zero. The high variances Table 1. Enzymes and electrophoresis running conditions for Stenobothrus lineatus and S. stigmaticus
Enzyme EC number
Run time (minutes)
S. lineatus S. stigmaticus Voltage
Buffer (pH)
Aat 2.6.1.1 40 40 250 TB (8.9)
Ak 2.7.4.3 30 30 200 TC (8.2)
Apk 2.7.3.3 20 20 250 TG (8.5)
Fum 4.2.1.2 40 40 250 TB (8.9)
G-3-Pdh 1.2.1.12 40 40 200 PP (7.0)
G-6-Pdh 1.1.1.49 30 30 200 TC (8.2)
Idh 1.1.1.42 40 40 200 PP (7.0)
Ldh 1.1.1.27–28 50 40 200 PP (7.0)
Mdh 1.1.1.37 30 30 200 TB (8.9)
Me 1.1.1.40 50 40 200 TM (7.0)
Mpi 5.3.1.8 30 30 250 TM (7.0)
Pep-A (alanine-leucine) 3.4.11 or 13 30 30 250 TG (8.5)
Pgi 5.3.1.9 40 40 200 TM (7.0)
Pgm 5.4.2.2 40 40 200 TM (7.0)
Pk 2.7.1.40 30 30 200 TB (8.9)
Tpi 5.3.1.1 40 40 250 TB (8.9)
Buffers: TB 8.9 (0.13M Tris-borate, buffer E), TC 8.2 (0.1M Tris-citrate, buffer J), and PP 7.0 (0.02M Phosphate, buffer B) (Richardson et al., 1986); TM 7.0 (0.05M Tris-Maleic acid pH57.8 adjusted with maleic acid to pH57.0, Richardson et al., 1986); TG 8.5 (0.1M Tris-Glycine, Hebert and Beaton, 1993).
were caused by rare Idhand Mehomozygotes found in single patches (e.g., two homozygotes of Idh were found in the sample IV8).
Discussion
Differentiation among patch populations and a dis- tance effect with respect to genetic variation was not found in either species, and the distant populations dif- fered little from the main investigation areas. However, the common species S. lineatus had significantly more genetic variation than the rarer S. stigmaticus. This is in
accordance with theoretical equilibrium expectations (Kimura, 1983) relative to the population sizes (Wal- laschek, 1996). Thus, there seems to be (i) no differences between the species in the ability to reach neighbouring patches, and (ii) both species have experienced similar habitat histories. Since there was no difference in genetic structure among the two species, the different amount of genetic variation was not caused by unequal drift effects related to different habitat histories. If populations were isolated and had experienced fluctu- ating population sizes the amount of genetic variation would be expected to vary among patches. The present Table 2. Allele frequency table for Stenobothrus lineatusand S. stigmaticus. N5number of individuals sampled. The locus Pkwas polymorphic but not scorable in all populations S. lineatus, and has been omitted
Stenobothrus lineatus S. Stigmaticus
Locus J1 J2 J3 J4 J5 J6 J7 J10 WU Halle Locus I26 II5 II15 II23 III39 IV8 IV30A P B Ak 1 0.95 0.98 1.00 0.87 1.00 1.00 1.00 1.00 1.00 0.89 Idh 1 0 0.03 0 0 0 0 0 0 0
2 0.05 0 0 0.13 0 0 0 0 0 0.11 2 1.00 0.92 1.00 1.00 1.00 0.93 1.00 1.00 1.00
3 0 0.02 0 0 0 0 0 0 0 0 3 0 0.05 0 0 0 0.08 0 0 0
Apk 1 1.00 0.98 1.00 1.00 1.00 1.00 1.00 1.00 1.00 1.00 Ldh 1 1.00 0.97 1.00 0.98 1.00 1.00 1.00 1.00
2 0 0.02 0 0 0 0 0 0 0 0 2 0 0.03 0 0.02 0 0 0 0 0
Got 1 1.00 1.00 1.00 1.00 1.00 0.96 1.00 1.00 1.00 1.00 Me 1 0.79 0.87 0.70 0.80 0.85 0.98 0.75 0.96 0.88
2 0 0 0 0 0 0.04 0 0 0 0 2 0.21 0.13 0.30 0.20 0.15 0.03 0.25 0.04 0.12
Idh
Ldh
Mdh
Mpi
Pep
Pgi
Pgm
N 1 2 1 2 1 2 1 2 3 1 2 3 1 2 1 2 3 4
1.00 0 1.00 0 0.90 0.10 0.90 0.10 0 1.00 0 0 1.00 0 0 0.10 0.50 0.40 10
0.95 0.05 1.00 0 0.95 0.05 0.90 0.10 0 1.00 0 0 0.95 0.05 0 0.24 0.33 0.43 21
1.00 0 0.96 0.04 0.93 0.07 0.93 0.07 0 1.00 0 0 1.00 0 0 0.32 0.36 0.32 14
0.97 0.03 1.00 0 0.92 0.08 0.89 0 0.11 0.94 0.06 0 0.89 0.11 0 0.05 0.42 0.53 19
0.98 0.03 1.00 0 0.98 0.03 0.98 0.03 0 0.93 0.08 0 0.98 0.03 0 0.36 0.31 0.33 18
0.96 0.04 1.00 0 1.00 0 0.96 0.04 0 1.00 0 0 1.00 0 0 0.25 0.25 0.50 14
0.95 0.05 1.00 0 0.85 0.15 0.95 0 0.05 1.00 0 0 0.94 0.06 0 0.05 0.50 0.45 10
0.83 0.17 1.00 0 0.94 0.06 0.89 0.06 0.06 0.94 0 0.06 0.94 0.06 0 0.33 0.33 0.33 9
1.00 0 1.00 0 0.98 0.02 0.95 0.05 0 1.00 0 0 1.00 0 0.17 0.10 0.26 0.48 21
0.86 0.14 1.00 0 1.00 0 1.00 0 0 1.00 0 0 0.95 0.05 0 0.18 0.36 0.45 33
Mpi
Pgm
Pk
N 3 1 2 1 2 1 2 3
0 1.00 0 0.92 0.08 0.04 0.96 0 12
0 1.00 0 1.00 0 0.21 0.79 0 19
0 1.00 0 1.00 0 0.25 0.75 0 10
0 1.00 0 0.93 0.07 0.04 0.96 0 23
0 0.90 0.10 1.00 0 0.05 0.95 0 10
0 1.00 0 1.00 0 0.22 0.75 0.03 20
0 1.00 0 0.98 0.02 0.05 0.89 0.07 22
0 1.00 0 0.92 0.08 0.04 0.96 0 13
0 1.00 0 0.82 0.18 0.18 0.82 0 17
findings indicate that rarity does not mean limited dis- persal power (see Nicklas-G ¨orgen, 1997 for grass-
hoppers of the genus Oedipoda). It is more likely that the rarity of S. stigmaticusrelative to S. lineatusreflects more specialist habitat requirements.
Based on the FST-differentiation estimates, we argue for the Stenobothrus species investigated in the present study that, on a local scale, neither species experiences isolation effects with habitat patches interspersed at distances of up to 8 km, and probably in the range of more than 100 km. Two alternative explanations other than high dispersal rates may cause lack of genetic differentiation. First, equal selection pressures may homogenize allele frequencies among patches. This was hypothesized for lack of structure among British populations of Chorthippus brunneus (Gill, 1981a). The lack of Pgm allele-2 across all S. lineatus populations could suggest selection at this locus and therefore higher levels of gene flow than are actually present.
Removing Pgmallele-2, or the locus completely, did not alter the gene flow estimate. Thus, the lack of differ- entiation suggests high levels of patch connectivity. The second explanation for no differentiation among populations could be that fragmentation is a recent event and that a reduction in population sizes has not yet manifested itself on the genetic variance. If this is true it means that the populations under study must have remained stable since the time of fragmentation, hence the local populations are momentarily not en- dangered. Otherwise, one would expect differentiated populations due to fluctuating population sizes. The apparent substructuring of patches of S. stigmaticusat Halle, as seen by a Hardy–Weinberg disequilibrium, Table 3. Expected and observed heterozygosity, Heand Ho,
and number of alleles per locus for Stenobothrus lineatus(15 loci, the polymorphic, but unscorable, locus Pk was not included) and S. stigmaticus (16 loci) samples. Population average and the mean overall populations. The mean He, Ho, and number of alleles per population was significantly greater for S. lineatus than for S. stigmaticus, P,0.001. The hetero- zygote deficit of S. lineatus was partly caused by the Pgm allele 2 (see Table 4)
S. lineatus S. stigmaticus
POP He Ho
Allele
no. POP He Ho
Allele no.
J1 0.078 0.064 1.357 I26 0.037 0.031 1.188 J2 0.087 0.071 1.571 II5 0.049 0.043 1.313 J3 0.075 0.051 1.357 II15 0.053 0.031 1.125 J4 0.107 0.087 1.571 II23 0.036 0.024 1.250 J5 0.073 0.073 1.500 III39 0.035 0.038 1.188 J6 0.062 0.046 1.357 IIV8 0.037 0.031 1.250 J7 0.082 0.057 1.429 IV30A 0.040 0.031 1.250 J10 0.111 0.103 1.571 P 0.019 0.019 1.188 WU 0.059 0.061 1.357 B 0.051 0.037 1.188 Halle 0.083 0.071 1.357
Mean 0.082 0.068 1.443 0.040 0.032 1.216
Table 4. Summary F-statistics for Stenobothrus lineatus and S. stigmaticus. F- statistics of S. lineatus were calculated omitting and including Pgm-allele 2, and Pgm. Each species was analysed among patches within a core area (Jena or Halle) and between the core area and two distant populations (regions)
S. lineatus S. stigmaticus
FXY Jena Regions Halle Regions
FIS 0.18360.037 0.23960.151 0.23460.174 0.24360.137
FIS (-2) 0.12160.059 0.11860.078
FIS (-Pgm) 0.01560.025 0.00460.035
FIT 0.18860.036 0.24360.140 0.25560.156 0.25560.130
FIT (-2) 0.11560.060 0.13160.072
FIT (-Pgm) 20.00360.033 0.02660.034
FST 0.00560.012 0.00760.011 0.02860.016 0.01760.022
FST (-2) 20.00760.011 0.01660.007
FST (-Pgm) 0.01260.009 0.02360.012
may be influenced by within-patch philopatric behav- iour which can give rise to a sampling artefact (see below).
One of the most confounding observations in non- migratory grasshoppers is the discrepancy between low observed life-time movements and high habitat patch connectivity measured by indirect gene flow esti- mates. Nevertheless, genetic studies of grasshopper breeding structure may to some extent corroborate low intrapatch life-time movements. Several studies of saltatorians show positive FISinbreeding estimates, i.e., a Wahlund effect within subpopulations (Schmeller et al., 1996; Nicklas-G ¨orgen, 1997; Hamrick and Hamrick, 1989; present study). Positive FIS estimates may indi- cate that the single subpopulations themselves are sub- structured, and may be caused by the philopatric nature of many species coupled to non-random sam- pling relative to intrapatch distribution: e.g., sampling sibs in one patch and non-sibs in other patches. This was probably the case with the S. stigmaticus loci Idh and Me, whose homozygotes caused an apparent Wahlund effect within the total Halle population. How- ever, the homogeneity of Idhand Meallele frequencies among samples indicated that not a substructure of the Halle population but a sampling error within samples caused the Wahlund effect. High variances causing non-significance of FIS (and FIT) showed that the sampling error was limited to single samples.
Ignoring the intrapatch point of view when compar- ing the alternative methods to estimate connectivity, the results of marking studies and gene flow estimates obviously seem to contradict one another. For example, the postglacial expansion of Chorthippus parallelus in Europe was estimated to 300–500 m/year whereas life- time movements indicated only 30 m/year (Cooper et al., 1995 and references therein). Population genetic studies in general have found little impact of fragmen- tation on genetic diversity in grasshoppers. Virtually no genetic differentiation between populations for both allozymes and mtDNA have been observed for the migratory grasshopper Melanoplus sanguinipes in open landscape mosaics of North America cultivated prairie (Chapco and Bidockca, 1986; Chapco et al., 1992). Lack of differentiation between populations has also been found for most non-migratory species: Chorthippus brunneus, C. albomarginatus, C. parallelus, Myrmeleotettix maculatus, Omocestus viridulus (Gill, 1981a,b); Oedipoda caerulescens(Nicklas-G ¨orgen, 1997 but not O. germanica);
within chromosomal races of Caledia captiva(Daly et al., 1981; Kohlmann, 1996); Schistocerca pallens(Silveira et al., 1998 (RAPD)); Trimerotropis pallidipennis (Confalonieri et al., 1998 (mtDNA)). Daly et al. (1981) found that
genetic differentiation of one population was caused by isolation of a dividing rainforest, while other distant populations were genetically very similar. Even the relatively isolated populations of the North American alpine grasshopper Aeropedellus clavatus sampled from different mountain ranges showed FST50.082, indicat- ing an average of 2.8 migrants per generation between any patch (Hamrick and Hamrick, 1989).
In this study we also observed a discrepancy between life-time movements recorded by marking studies and gene flow estimates. There was no genetic structure with FSTabout zero indicating a high degree of interpatch connectivity. On the other hand, the life- time movements of Stenobothrus lineatus fall within ranges stated for the majority of non-migrating grass- hoppers. Median values of the activity radius have been found between 10 and 25 m in males and between 5 and 15 m in females (Samietz et al., 1996; Samietz 1998). Life-time movements of Stenobotrus stigmaticus have not been quantified sufficiently but observations suggest that they fall within a similar range (M. Wal- laschek, unpubl.). The discrepancy arises because the two approaches deal with different time-scales. Move- ment studies are most often restricted to annual investigations, whereas gene flow estimates deal with averaged (unobserved) perennial events. Based on the observational data alone, the Jena samples of S. lineatus which are divided by the Saale-river valley, could not have exchanged individuals recently. The river valley, at least 1 km wide, would have been impossible for grasshoppers to cross, considering the valley has built a non-habitat zone for xerothermophilous open-land grasshoppers for hundreds of generations. From our analyses and searching the literature data, we empha- size on the contrary, that the habitat patches are con- nected by mechanisms that will hardly ever be discovered by conventional marking studies. Zoochory, e.g. by huge sheep flocks (cf. Fischer et al., 1996), can be excluded as a transport mechanism due to the rapid changes in agricultural managment during the last 50 years. Rather, we suppose long and medium distance dispersal to be a common feature of grasshopper mobility. If this is true, connectivity of grasshoppers’
habitat patches in fragmented landscapes appears in a different light. Especially concerning conservation issues, far-dispersing individuals must not be over- looked. Simulation models based on empirical mobility values (e.g. Grimm et al., 1994; Stelter et al., 1997;
Samietz et al., 1996; Samietz, 1998) investigating sur- vival probability of grasshopper metapopulations would thus have to be reconsidered to take into account medium and long distance dispersal. However,
with genetic studies we see only the result of connect- ivity and not the mechanisms that act in grasshopper populations to bridge interpatch areas. Therefore em- pirical investigations which elucidate the mechanisms and the factors influencing dispersal events are encouraged.
Acknowledgements
We thank Christian Freund, Andreas Wagner and Renate Schmuck for laboratory help. G. K ¨ohler pro- vided help and support with sampling. The collection permissions were obtained from the Regierung- spr¨asidium Halle, Landesverwaltungsamt Th ¨uringen, Bezirksregierung Koblenz and the Regierung von Unterfranken. The study was funded by the German Federal Ministry of Science and Technology (BMBF grant nos. 339519A and 339524A, FIFB-project). J.
Samietz was supported by the German Research Foundation (DFG grant no. Ko 1994/1–1 and Ko 1994/1–2).
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