Fire severity and ecosytem responses following crown fires in California shrublands

Chaparral shrublands burn in large high-intensity crown fires. Managers interested in how these wildfires affect ecosystem processes generally rely on surrogate measures of fire intensity known as fire severity metrics. In shrublands burned in the autumn of 2003, a study of 250 sites investigated factors determining fire severity and ecosystem responses. Using structural equation modeling we show that stand age, prefire shrub density, and the shortest interval of the prior fire history had significant direct effects on fire severity, explaining > 50% of the variation in severity. Fire severity per se is of interest to resource managers primarily because it is presumed to be an indicator of important ecosystem processes such as vegetative regeneration, community recovery, and erosion. Fire severity contributed relatively little to explaining patterns of regeneration after fire. Two generalizations can be drawn: fire severity effects are mostly shortlived, i.e., by the second year they are greatly diminished, and fire severity may have opposite effects on different functional types. Species richness exhibited a negative relationship to fire severity in the first year, but fire severity impacts were substantially less in the second postfire year and varied by functional type. Much of this relationship was due to alien plants that are sensitive to high fire severity; at all scales from 1 to 1000 m2, the percentage of alien species in the postfire flora declined with increased fire severity. Other aspects of disturbance history are also important determinants of alien cover and richness as both increased with the number of times the site had burned and decreased with time since last fire. A substantial number of studies have shown that remote-sensing indices are correlated with field measurements of fire severity. Across our sites, absolute differenced normalized burn ratio (dNBR) was strongly correlated with field measures of fire severity and with fire history at a site but relative dNBR was not. Despite being correlated with fire severity, absolute dNBR showed little or no relationship with important ecosystem responses to wildfire such as shrub resprouting or total vegetative regeneration. These findings point to a critical need for further research on interpreting remote sensing indices as applied to postfire management of these shrublands.     

Non-linear ecological processes, fires, environmental heterogeneity and shrub invasion in northwestern Patagonia

The emergent behaviors of nature are not only the sum of interactions among ecosystem parts but also depend on the organization of these interactions. Fire, climate and vegetation patterns produce non-linear fire propagation across the landscape. Environmental heterogeneity, like outcrop presence and hare density, increases landscape patchiness and makes possible the occupation of fire refuges by plants, like Fabiana imbricata shrubs. We monitored shrub recruitment and cover during nine postfire years in northwestern Patagonia grasslands and we studied the long-term population dynamics under different environmental conditions through a matrix model, exploring different fire frequencies and spring precipitation regimes. Both, the field monitoring and the model seem to confirm the relationships between shrub invasion and fire. The climate change forecast predicts an increase in the frequency of El Niño Southern Oscillation phenomena that could causes more coupled fires—rainy springs in northwestern Patagonia, and consequently, more recruitment windows for shrubs, like F. imbricata. The matrix model also indicates that this scenario would be the most favourable for shrub invasion. Our results contribute to the knowledge of the ecosystem properties and processes, providing useful information to improve the grasslands sustainable use.     

Possible linkages between lignite aquifers, pathogenic microbes, and renal pelvic cancer in northwestern Louisiana, USA

In May and September, 2002, 14 private residential drinking water wells, one dewatering well at a lignite mine, eight surface water sites, and lignite from an active coal mine were sampled in five Parishes of northwestern Louisiana, USA. Using a geographic information system (GIS), wells were selected that were likely to draw water that had been in contact with lignite; control wells were located in areas devoid of lignite deposits. Well water samples were analyzed for pH, conductivity, organic compounds, and nutrient and anion concentrations. All samples were further tested for presence of fungi (cultures maintained for up to 28 days and colonies counted and identified microscopically) and for metal and trace element concentration by inductively-coupled plasma mass spectrometry and atomic emission spectrometry. Surface water samples were tested for dissolved oxygen and presence of pathogenic leptospiral bacteria. The Spearman correlation method was used to assess the association between the endpoints for these field/laboratory analyses and incidence of cancer of the renal pelvis (RPC) based on data obtained from the Louisiana Tumor Registry for the five Parishes included in the study. Significant associations were revealed between the cancer rate and the presence in drinking water of organic compounds, the fungi Zygomycetes, the nutrients PO₄ and NH₃, and 13 chemical elements. Presence of human pathogenic leptospires was detected in four out of eight (50%) of the surface water sites sampled. The present study of a stable rural population examined possible linkages between aquifers containing chemically reactive lignite deposits, hydrologic conditions favorable to the␣leaching and transport of toxic organic compounds from the lignite into the groundwater, possible microbial contamination, and RPC risk.     

Control of the intertidal distribution of Gastroclonium coulteri in monterey bay,California, USA

Transplantation and selective removal experiments were carried out on Gastroclonium coulteri (Harvey) Kylin at Mussel Point, California, USA, between June, 1975 and May, 1979. Transplanted G. coulteri survived higher in the intetidal than the natural population, except during March and April when extreme low tides, and resultant severe desiccation, occurred during the daytime. Transplants survived, but did not grow, in low intertidal tidepools. Transplants to the shallow subtidal survived and increased in overall length. Transplants to the deeper subtidal zone died within 1 month. Transplants from the subtidal to the intertidal survived, but decreased in size. At the upper boundary of G. coulteri, removal of Gigartina papillata and Rhodoglossum affine resulted in a decrease in the percent cover of Gastroclonium coulteri. Presence of the other algae thus had a beneficial rather than competitive effect on G. coulteri. Removal of G. coulteri resulted in an increase in the percent cover of G. papillata and R. affine, demonstrating that the lower boundary of these two species is set by competition with G. coulteri. At the lower boundary of Gastroclonium coulteri, removal of Phyllospadix spp. or Prionitis lanceolata led to an increase in percent cover of G. coulteri. Removal of G. coulteri did not lead to an increase in percent cover of Phyllospadix spp. Thus, the lower boundary of G. coulteri is set by competition with Phyllospadix spp. or Prionitis lanceolata, but the upper boundary of Phyllospadix spp. is not set by competition with G. coulteri. The results support the hypothesis that upper boundaries tend to be set by physical factors (here, exposure at low tide), and lower boundaries tend to be set by biological factors (here, competition).     

Demography of the intertidal brown alga Pelvetia fastigiata in Southern California,USA

The demography of Pelvetia fastigiata was studied from 1973 through 1977 in the intertidal zone near La Jolla, California, USA. Populations were usually small and relatively stable as new recruits replaced older individuals that were lost; however, in 1977 high recruitment substantially increased total abundances. The survivorship of recruits was similar regardless of the season or year of recruitment. Mortality rates decreased with age, and the mean life expectancy continued to increase for at least 1 yr. Losses of juveniles were high; 50% lived 80 d and only 9% survived to the age of first reproduction (about 1.5 yr). Plants present at the start of the study had higher survivorship rates than new recruits, 19 and 7%, respectively, living more than 3 yr. Individuals within aggregations of large plants lived longer than most dispersed individuals, and aggregations remained intact throughout the study in spite of low recruitment. Established individuals had low mortality rates and populations probably relied on these persistent individuals for reproduction.     

Larval abundance and recruitment of the sand dollar Dendraster excentricus in Monterey Bay,California, USA

Seasonal abundance of planktonic larvae of the sand dollar Dendraster excentricus was determined in Monterey Bay, California, USA. Larvae were counted from two offshore stations and also over a coastal sand dollar bed, and these data were compared with settlement in the sand dollar bed, with adult population structure and with adult reproductive condition. These measurements were made during the period July 1978 to October 1980 and in October 1981. Sand dollar larvae were most abundant in the plankton during the summer, a period when phytoplankton productivity tended to be high and currents were relatively slow and variable. In some years, small-scale current variations appeared to prevent many larvae in the open bay from reaching the nearshore sand dollar beds; however, in other years, massive shoreward transport of the larvae evidently did occur, since the adult population in the sand dollar bed exhibited a mode in size, indicating a large settlement. A comparison of settled individuals in 1980 and the adult size-frequency distribution in 1981 gives an estimated mortality rate of 88% yr^-1 for early juveniles of D. excentricus.     

Holocene forest development and maintenance on different substrates in the Klamath Mountains, northern California, USA

The influence of substrate on long-term vegetation dynamics has received little attention, and yet nutrient-limited ecosystems have some of the highest levels of endemism in the world. The diverse geology of the Klamath Mountains of northern California (USA) allows examination of the long-term influence of edaphic constraints in subalpine forests through a comparison of vegetation histories between nutrient-limited ultramafic substrates and terrain that is more fertile. Pollen and charcoal records spanning up to 15000 years from ultramafic settings reveal a distinctly different vegetation history compared to other soil types. In non-ultramafic settings, the dominant trees and shrubs shifted in elevation in response to Holocene climate variations resulting in compositional and structural changes, whereas on ultramafic substrates changes were primarily structural, not compositional. Fire activity was similar through most of the Holocene with the exception of declines over the last 4000 years on ultramafic substrates, likely due to the reduction of understory fuels and cooler wetter conditions than in the middle Holocene. These results suggest that the tree and shrub distributions were more responsive to past climate changes on non-ultramafic substrates compared to those on ultramafic substrates. The combination of these dynamics may help explain high levels of plant diversity in the Klamath Mountains and provide insights for managing these complex ecosystems.     

Domoic acid in benthic flatfish on the continental shelf of Monterey Bay,California, USA

Within Monterey Bay, California, USA, the food web transfer of domoic acid (DA), a neurotoxin produced by diatoms of the genus Pseudo-nitzschia, has led to major mortality events of marine mammals and birds. Less visible, and less well known, is whether invertebrates and fish associated with the benthos are also affected by blooms of DA-producing Pseudo-nitzschia spp. This study examines the presence of DA in benthic flatfish offshore of Davenport, California, (37°0′36″N, 122°13′12″W) and within Monterey Bay, California (36°45′0″N, 122°1′48″W), including species that feed primarily in the sediment (benthic-feeding) and species that feed primarily in the water column (benthopelagic-feeding). Flatfish caught between 10 December 2002 and 17 November 2003 at depths of 30–180 m had concentrations of DA in the viscera ranging from 3 to 26 μg DA g⁻¹ of viscera. Although the DA values reported are relatively low, benthic-feeding flatfish were frequently contaminated with DA, especially as compared with the frequency of contamination of flatfish species that feed in the water column. Furthermore, on days in which both benthic-feeding and benthopelagic-feeding flatfish were collected, the former had significantly higher concentrations of DA in the viscera. Curlfin turbot, Pleuronicthys decurrens, the flatfish with both the highest level and frequency of DA contamination, are reported to feed exclusively on polychaetes, suggesting that these invertebrates may be an important vector of the toxin in benthic communities and may pose a risk to other benthic-feeding organisms. Electronic supplementary material The online version of this article (doi:) contains supplementary material, which is available to authorized users.     

Environmental stresses and intertidal assemblages on hard substrates in the port of Long Beach,California, USA

Nine intertidal stations within the Port of Long Beach, California, USA, were sampled during September and October 1975 using the random point method and the scraping technique. A total of 96,168 individuals belonging to 55 taxa were recorded in the quantitative survey; qualitative scrapings included 136 taxa. The protected outer coast intertidal assemblages were found at all but three stations along the shoreline. The shoreline, being situated in between the more environmentally stressed inner harbor and the less stressed open ocean, supported assemblages characterized by an instability in species composition. The upper midlittoral zone was typically dominated by Chthamalus dalli, C. fissus and Balanus glandula. The lower midlittoral was colonized either by Anthopleura elegantissima-Prionotis lanceolata facies along the outer breakwater or by A. elegantissima-Tetraclita squamosa rubescens facies along the inner breakwater. The mussel bed, which is the characteristic assemblage of the Californian lower midlittoral zone was obscured by the competitors P. lanceolata, T. squamosa rubescens and A. elegantissima, and was thinned out by predators, of which Pisaster ochraceus was the most voracious. Suspension feeders were dominant in the upper midlittoral and producers and grazers in the lower midlittoral. Predators were few in number and restricted to levels below mean lower low water. There was a gradual increase in numbers of intertidal species and individual abundance from level +5 ft (1.5 m) to level +1 ft (0.3 m) and from shoreline to outer breakwater. This may be attributable to the decrease in environmental stresses in the lower water levels and the improvement in water quality from the inner to outer harbor area.     

Offspring number in pygmy marmosets, Cebuella pygmaea *, in relation to group size and the number of adult males

In many species of birds and mammals with a co-operative breeding and rearing system, offspring survival is positively related to the number of helpers. In the New World callitrichine primates (marmosets and tamarins), adult males are considered as particularly valuable helpers, and female reproductive success may depend strongly on the males' contribution to infant care. We analysed the number of offspring (infants, juveniles) in groups of wild pygmy marmosets, Cebuella pygmaea (Callitrichinae, Cebidae, Primates), in relation to the number of adult males and to the number of adult and subadult group members. In contrast to other callitrichines with a co-operative system of infant care, no relationship was found between the number of adult males and the number of infants and offspring. However, there was a significant positive relationship between the number of juveniles and the number of adult and subadult group members. The lack of a relationship between infant and adult-male number is interpreted as resulting from the reduced importance of adult males as helpers in pygmy marmosets in comparison to other callitrichines, probably due to the reduced costs of infant care. The relationship between the number of juveniles and the number of adult and subadult group members is in accordance with increased offspring survival in larger groups, as observed in other primates. Received: 1 February 1999 / Received in revised form: 5 June 1999 / Accepted: 29 June 1999     

Light-saturated photosynthesis by phytoplankton size fractions in the New York bight,USA

The relationships between netplankton and nanoplankton assimilation numbers, temperature, and major nutrient concentrations were studied and evaluated in the context of seasonal patterns in the biomass of these phytoplankton size fractions. Netplankton and nanoplankton blooms typically occur during late winter (2° to 8°C) and summer (18° to 24°C), respectively. Variations in nanoplankton and netplankton assimilation numbers were not statistically related to the development or collapse of specific blooms based on weekly sampling, but assimilation numbers were higher during the bloom periods than during transition periods of rapid temperature change (8° to 18°C). Differences in the assimilation numbers between size fractions could account for the dominance of the nanoplankton fraction during the summer bloom period but not for the dominance of netplankton during the winter bloom period. Nanoplankton and netplankton assimilation numbers were exponential functions of temperature between 8° and 24°C and 8° and 20°C, respectively. Below 8°C the assimilation numbers of both fractions were higher than expected on the basis of temperature. Above 20°C netplankton assimilation numbers declined with temperature. Netplankton and nanoplankton assimilation numbers were occasionally correlated with dissolved inorganic nitrogen concentrations from less than 1.0 to more than 15 g-at l^-1. Under these conditions, nanoplankton growth rates (calculated from assimilation number and carbon:chlorophyll) were higher and increased more rapidly with dissolved inorganic nitrogen than netplankton growth rates.     

Presence of Alexandrium catenella and paralytic shellfish toxins in finfish, shellfish and rock crabs in Monterey Bay, California, USA

The central California coast is a highly productive, biodiverse region that is frequently affected by the toxin-producing dinoflagellate Alexandrium catenella. Despite the consistent presence of A. catenella along our coast, very little is known about the movement of its toxins through local marine food webs. In the present study, we investigated 13 species of commercial finfish and rock crabs harvested in Monterey Bay, California for the presence of paralytic shellfish toxins (PSTs) and compared them to the presence of A. catenella and PSTs in sentinel shellfish over a 3-year period. Between 2003 and 2005, A. catenella was noted in 55% of surface water samples (n = 307) and reached a maximum concentration of 17,387 cells L⁻¹ at our nearshore site in Monterey Bay. Peak cell densities occurred in the month of July and were associated with elevated shellfish toxicity in the summers of 2004 and 2005. When A. catenella was present, particulate PSTs were detected 71% of the time and reached a maximum concentration of 962 ng STXeq L⁻¹. Of the 13 species tested, we frequently detected PSTs in Pacific sardines (Sardinops sagax; maximum 250 μg STXeq 100 g⁻¹), northern anchovies (Engraulis mordax; maximum 23.2 μg STXeq 100 g⁻¹), brown rock crabs (Cancer antennarius; maximum 49.3 μg STXeq 100 g⁻¹) and red rock crabs (C. productus; 23.8 μg STXeq 100 g⁻¹). PSTs were also present in one sample of Pacific herring (Clupea pallas; 13.3 μg STXeq 100 g⁻¹) and one sample of English sole (Pleuronectes vetulus; 4.5 μg STXeq 100 g⁻¹), and not detected in seven other species of flatfish tested. The presence of PSTs in several of these organisms reveals that toxins produced by A. catenella are more prevalent in California food webs than previously thought and also indicates potential routes of toxin transfer to higher trophic levels. Electronic supplementary material  The online version of this article (doi:) contains supplementary material, which is available to authorized users.     

Collapse of a Regional Frog Fauna in the Yosemite Area of the California Sierra Nevada, USA

There has been much concern about widespread declines among amphibians, but efforts to determine the extent and magnitude of these declines have been hampered by scarcity of comparative inventory data. We resurveyed a transect of the Sierra Nevada mountains in western North America that was carefully studied in the early 1900s. Our comparisons show that at least five of the seven frog and toad species in the area have suffered serious declines. One species has disappeared from the area entirely and a second species, formerly the most abundant amphibian in the area, has dwindled to a few small remnant populations. These declines have occurred in a relatively undisturbed, protected area and show some of the same patterns noted in other reports of amphibian declines. Introduced predatory fish, possibly interacting with drought-induced loss of refuge habitats, have contributed to the decline of some species. However, the overall cause of these dramatic losses remains unknown.     

Screening and bioassays for biologically-active substances from forty marine sponge species from San Diego,California, USA

Biologically-active substances were investigated from 35 demosponge species and 5 calcareous sponge species collected from intertidal or shallow subtidal habitats near San Diego, California, USA, from 1978 to 1980. Crude methanolic extracts of each species were tested for suppression of growth by bacteria (7 species) and a yeast. Antimicrobial activity was found in 26 Demospongiae and 2 Calcarea. Strong activity was found in 11 demosponges and, subsequently, 38 natural products with antimicrobial activity were isolated from 8 of these species (Aplysina fistularis, Dysidea amblia, Leiosella idia, Euryspongia sp., Toxadocia zumi, Axinella sp., Haliclona?cinerea and ?Pachychalina lunisimilis). Twenty-eight of these natural products (usually as pure compounds) were assayed for: (1) suppression of growth of marine fungi (3 spp.) and a red alga; (2) behavior modifications of invertebrate adults (4spp.); (3) toxicity to a goldfish; (4) inhibition of sexual reproduction of a brown alga; (5) inhibition of settlement and/or metamorphosis of late larvae or invertebrate juveniles (4spp.). Many of the natural products were also incorporated into pelleted fish food and tested for feeding-behavior modifications of fishes (5 spp.). Three of the compounds from Dysidea amblia were inactive in all tests. All other natural products were active in at least one assay, although none was active in all assays. The discussion relates the possession of biologically-active substances to the ecology of each sponge species; for example, sponges with antimicrobial substances are rarely overgrown.     

Age-specific survivorship in relation to clutch size and fledging success in California gulls

Summary Data from a natural population of California gulls (Larus californicus) demonstrated that increasing reproductive effort with age was associated with reduced survivorship. Number of offspring fledged but not clutch size was inversely related to adult survivorhip indicating that reproductively induced mortality resulted from the cumulative effects of the entire breeding season. Agerelated increases in fledging success were correlated with increased adult mortality. Young gulls fledged few offspring and had high survival rates. Old gulls tended to fledge more offspring and had low survival rates. However, those old gulls fledging few offspring survived as well as young gulls. Data also invalidate the assumption that survivorship is age-constant in this species.     

Male crickets increase sperm number in relation to competition and female size

There is evidence to suggest that males of various species can respond to the threat of sperm competition by varying the amount of sperm transferred during copulation. We tested this in two species of cricket, Acheta domesticus and Gryllodes supplicans (Orthoptera: Gryllidae) by varying the apparent threat of intermale competition experimentally. The results showed that males of both species increased the amount of sperm transferred as apparent competition increased and that male A. domesticus transferred more sperm when encountering larger females. The results also showed that male G. supplicans produced a larger spermatophylax when a larger ampulla was transferred, a relationship consistent with a sperm protection function. Received: 5 May 1995/Accepted after revision: 27 January 1996     

淮河中上游参考作物腾发量趋势变化及其成因

参考作物腾发量(ET0)是由平均气温(Tm)、实际水汽压(ea)、风速(u2)和净辐射(Rn)等多个气候要素驱动的,因此,气候变化对ET0的影响是显而易见的。分析ET0和各气候要素的时空变化特征可以获得气候变化对ET0的影响。利用Mann-Kendall(MK)检验分析了淮河中上游流域1959—2009年ET0及各气候要素的变化趋势。并且利用偏相关分析和贡献分析获得ET0和主要气候要素之间的定性和定量关系。结果显示,大部分站点的年ET0表现出显著的下降趋势。在夏季,所有站点的ET0都表现出显著的下降趋势,且西北部地区尤其显著。其他3个季节显著下降的ET0趋势主要分布在淮河中上游流域的西北部地区。年及夏季的ET0在1970年以后表现出下降的趋势并且在1975年以后变的显著,其余3个季节ET0并无明显变化特征,说明夏季ET0的显著减少是年ET0显著减少的主要原因;实际水汽压与ET0之间表现出显著的负相关关系,而平均温度、净辐射和风速则表现出显著的正相关关系;无论从定性的偏相关关系还是定量的贡献分析都可看出虽然年及四季的平均气温是显著上升的,并且导致了ET0的增加,但是整个流域显著下降的风速和净辐射直接导致了年及夏季ET0的显著减少。     

Spatial and temporal variations in the population size structure of three lanternfishes (Myctophidae) off Oregon,USA

Size-frequency distributions were determined for 3 common lantern-fishes (Stenobrachius leucopsarus, Diaphus theta, and Tarletonbeania crenularis) off Oregon in the summer. The fishes were caught mainly in sound-scattering layers by a large pelagic trawl with 5 opening-closing nets. Changes in depth distribution and diel vertical migration with growth were evident for all 3 species. The size of S. leucopsarus increased markedly with depth both at 0 to 90 m at night and 250 to 500 m during the day. Larger D. theta were also found deeper during the day (between 250 and 450 m), but neither D. theta nor T. crenularis demonstrated size segregation in the upper 90 m at night. Large D. theta and small T. crenularis did not appear to migrate into surface waters at night. Age-Group O (15 to 20 mm) S. leucopsarus were most abundant in deep water (400 to 480 m) in the daytime and did not migrate into near-surface waters at night. Age-Group I (30 to 40 mm) S. leucopsarus were common at about 300 m by day and within the upper 30 m at night. Age-Group II–III (50 to 60 mm) apparently followed the evening ascent of Age-Group I fish and most resided at 75 to 90 m at night, beneath Age-Group I fish. Age-Group III+fish (70 to 80 mm) were associated with Age-Group O at 400 to 480 m by day and usually did not migrate above 200 m at night. The size structure of S. leucopsarus differed among the nets of a single tow at one depth, or between two tows that fished the same depths on successive nights, indicating horizontal patchiness in age structure. D. theta demonstrated low within-tow variability in size composition which indicated a spatially more uniform age structure on a scale of kilometers. The size structures of these 3 lanternfishes were different in the same area and the same season during two different years, suggesting variable survival of year classes or horizontal patchiness of age composition in the area sampled.     

Microsatellite variation, effective population size, and population genetic structure of vermilion snapper, Rhomboplites aurorubens, off the southeastern USA

Vermilion snapper (Rhomboplites aurorubens) were collected from four sites off the Atlantic coast of the USA and one site in the Gulf of Mexico to evaluate effective population size and genetic stock structure. Previous studies had suggested geographic variation in the ratio of males to females, so this population characteristic was explored in conjunction with the genetic analysis. Sex ratio varied greatly among the five sample sites; males comprised 57% of samples in the Gulf of Mexico, while within the South Atlantic Bight they comprised between 36% (Morehead City, North Carolina) and 53% (Carolina Beach, North Carolina) of samples. No clear geographic trends in the sex ratio emerged; instead, it was found to vary with fish length, the percentage of males decreasing with increasing size. Allelic variation assessed at seven dinucleotide microsatellite loci was large; gene diversities ranged from 0.43 to 0.95 and allelic counts from 7 to 39. Estimates of the effective population size ranged from 24 500 (based on the infinite-alleles model) to 150 500 (based on the stepwise-mutation model). There was evidence for excess homozygosity within samples: estimates of F _IS (the correlation of alleles within individuals) ranged from 0.01 to 0.03 among the seven loci, and three estimates were significantly greater than zero. Differentiation among localities was very weak, as estimates of F _ST (the correlation of alleles within populations) were on the order of 0.001 to 0.002 and genetic distance estimates between localities were not related to geographic distances. This suggested that vermilion snapper in the South Atlantic Bight (Cape Hatteras, North Carolina to Cape Canaveral, Florida) and Gulf of Mexico are likely to consist of one genetic stock. Despite the overall homogeneity, there were indications of a temporally dynamic local structure that would bear further examination. Received: 6 July 1998 / Accepted: 9 February 1999