Interactions between vegetation development and island formation in the Alpine river Tagliamento

Abstract. Early-successional stages of woody vegetation on gravel bars were studied in an island-braided section of the River Tagliamento in northeastern Italy. We mapped landscape-level changes in the study area (125 ha) by GIS-based analysis of aerial photographs for two time periods (1984–1986, 1986–1991); we surveyed island vegetation, and estimated island age by tree ring analysis. The study area experienced considerable changes between 1984 and 1991 due to at least two major floods in 1987 and 1990. The development of woody vegetation on bars follows three distinct phases: (1) gravel bars plus large woody debris (LWD), (2) pioneer islands, and (3) established islands. Established islands have sections dominated by shrubs of Salix elaeagnos, S. purpurea, S. daphnoides and S. triandra, and tree-dominated sections with Populus nigra, Salix alba and Alnus incana. Large woody debris seems to play a key role for plant colonization on gravel bars. The succession from bars to established islands took about 10 - 20 yr, and the probability of an island being washed away decreased with island age. Erosion produced new LWD which again initiated successional processes in the active zone of the river. Most species were already present in the early-successional stages, although the number of species increased with island development. Established islands were characterized by a distinctive species composition, including an assemblage of species less tolerant of inundation. The results are discussed within the framework of island dynamics and its significance for restoration of early-successional habitats in more regulated rivers.     

Early vegetation succession on gravel bars of Czech Carpathian streams

Rivers with a natural flooding regime and gravel accumulations are an important natural habitat endangered by regulations and other types of human impact. Succession after disturbances by floods creates a mosaic of different vegetation types, some of them containing rare specialist species. We studied vegetation succession and changes in plant diversity on river gravel bars of four streams in the Western Carpathians and their foothills in the eastern Czech Republic. This area experienced extreme 50-year flood event in May 2010. Gravel bar vegetation was destroyed, some of the former bars were covered by sediments, and some new bars arose. We sampled gravel bar vegetation two months after the floods and repeated the sampling on each site during the next three years. Initial vegetation has developed through a sparse and species-rich stage into denser stands with more shade-tolerant species. In the fourth year, tall herbs, such as Urtica dioica, Phalaris arundinacea and the alien Impatiens glandulifera, dominated the communities, but shrub vegetation started to develop only in a few places. Species capable of vegetative dispersal prevailed over species dispersed by seeds only. Altitude and size of gravel/stone particles were identified as important factors affecting vegetation succession. The succession ran faster on gravelly substrates at lower altitudes than on stony substrates at higher altitudes. Although the studied streams are partly influenced by human interventions and host only few gravel bar specialists, they are of considerable conservation importance.     

Report of the Alpine Section

Background: Under climate-change scenarios, rock outcrops, by providing microclimatically diverse habitats, different from those of surrounding zonal vegetation matrix, may serve as climatic refugia and thereby facilitate the persistence of specialist species.

Aim: We tested whether rock outcrops of southern temperate latitudes could act as local refugia for a cold-adapted flora in the face of global warming.

Methods: We related species composition of 50 outcrops and that of their surrounding vegetation to climatic data at local and regional scales to establish whether species distributions reflected differences that could indicate the existence of climatic refugia.

Results: While at a regional scale, species composition of the outcrops across the study area was related to mean annual precipitation and mean annual maximum temperature, locally, southern faces of outcrops had different species and lower maximum temperatures than insolated north faces and surrounding vegetation plots.

Conclusions: South faces of outcrops by providing cool microhabitats, and currently harbouring species not found in the surrounding zonal vegetation matrix, could serve as local refugia for heat-intolerant plants and other microthermic organism. The degree of ‘stability’ of these refugia will depend on the degree of future change in climate. Therefore, it is important to plan protected area networks that maximise local environmental heterogeneity, including the protection of rock outcrops both as refugia for cold-adapted species and as potential stepping stones that would allow dispersal of these species between supportive environments through unfavourable ones.     

Changes in the flora of some Aegean islets 1968–2000

The flora of 17 islets was studied during the period 1968–2000, with thorough inventories from 1971. At each visit the species present and their population sizes were noted. The flora of the smallest islets of hard rock consisted mainly of perennials, and remained unchanged in terms of species present throughout the period of study. In the larger and more complex islets the major part of the flora consisted of islet-specialised taxa staying for long periods of time. To them are added some other mostly casual species from the close large islands, introduced by human influence, birds or winds and standing for most of the turnover in the islets flora. Few changes were observed in the plant communities of rock crevices, the spray zone of the shores and other constant habitats. The dominant perennial species forming layer of shrubs or subshrubs also mostly remained the same. Most of the changes concerned soil spaces opened by animals or erosion. Most annual species were found to survive by an extreme seed bank strategy. Many taxa common on the neighbouring islands were lacking or casual on the islets. Some others reached dominance or high frequencies that have not been observed on islands or in mainland areas. The conditions for the flora on the Aegean islets are considered to differ fundamentally from that on large isolated islands because they are situated comparatively close among each other and to islands and mainland areas. They are also vulnerable towards catastrophic events and therefore frequently presenting opened targets for invasions. The islet-specialised taxa are found to retain good ability for dispersal over moderately long distances. The grazing of the islets is stated to be an important factor for the evolution of islet adapted and specialised taxa.     

Nutrient transfer between parafluvial interstitial water and river water: influence of gravel bar heterogeneity

1. Nutrient concentrations in interstitial water springs at the downstream ends of two gravel bars in the River Rhône were measured in order to estimate the importance of nutrient inputs [nitrate, dissolved organic carbon (DOC), biodegradable and refractory fractions of DOC] from the parafluvial zone (saturated sediments adjacent to the wetted channel, i.e. interstitial habitats of gravel bars) to the river as well as seasonal variations in these inputs.
2. Compared with surface water, parafluvial water had lower concentrations of DOC (especially the refractory fraction) and generally higher concentrations of nitrate. These differences were at their lowest during winter.
3. The influence of gravel bar geomorphological heterogeneity (grain size and spatial distribution of fine sediments) on parafluvial inputs was studied in terms of nutrient content and biofilm characteristics along transects across the gravel bars.
4. A gravel bar located in a stable area of the river had low transversal heterogeneity and acted as a sink for DOC and as a source of nitrate. The low fine sediment content and the high oxygenation of interstitial water must have enabled aerobic processes such as mineralization of organic matter and nitrification to occur.
5. In contrast, gravel bars located in a degradation zone of the river had a spatially variable structure, acting as a sink for DOC and nitrate because localized accumulation of fine sediment and ensuing hypoxic conditions induced anaerobic processes such as denitrification.
6. This study highlights the important influence of geomorphological heterogeneity in gravel bars for nutrient transformations as well as nutrient exchanges between parafluvial interstitial systems and the adjacent river over a seasonal cycle.     

Spatial and temporal segregation of spawning habitat by catostomids in the Savannah River, Georgia and South Carolina, U.S.A.

Spawning aggregations of five species of catostomids were observed on the two mid-channel gravel bars of the Savannah River, Georgia and South Carolina, in 2004 and 2005 to assess the degree of spatial and temporal overlap in the use of this habitat and determine the habitat preferences leading to segregation. Spawning catostomids showed a considerable amount of temporal overlap in their use of these mid-channel gravel bars. The observed temporal overlap was consistent between 2004 and 2005 and corresponded to temperatures at which species were present. The distribution of catostomids was not uniform at the upstream gravel bar. Carpsuckers Carpiodes sp., spotted sucker Minytrema melanops and robust redhorse Moxostoma robustum both demonstrated some spatial overlap with notchlip redhorse Moxostoma collapsum ; however, their overall distributions were different from one another. Northern hogsucker Hypentelium nigricans was present across the gravel bars, apparently as an egg predator. Spawning catostomids segregated based on flow, depth, slope and substratum size. Whether due to limited habitat availability or changes in the timing of reproduction due to altered cues, temporal and spatial overlap occurs between spawning catostomids despite the apparent partitioning of available spawning habitat. It is unclear, however, if this overlap results in excessive mortality in the early life-history stages of these species. Results suggest spatial overlap among catostomid species was minimized due to species spawning in areas within a narrow range of conditions. Intraspecific interactions such as nest site superimposition or disturbance may be a concern.     

河南登封方家沟遗址的石料产地

方家沟遗址主文化层出土了大量石制品,以石片石器工业为特征,年代约在距今5～3万年前。遗址周边是一个石料资源相对丰富的区域,可以通过原料分布、质量和可获性了解当时人类为何选择特定原料。遗址的原料既包括石英砂岩、石英岩和脉石英砾石,又包括脉石英岩块。石料调查显示,遗址附近丘岗顶部的中更新世砾石层可能是砾石原料的来源;西北距遗址约7～11 km处的嵩山东南麓元古宙地层中多见脉石英出露,最有可能是脉石英岩块的采集地。遗址周边区域并不缺乏燧石等质量更好的原料,但极少被当时人类所利用,这说明当地石片石器工业的长期延续不能用缺乏优质原料来解释。     

Arrival times,laying dates,and reproductive success of Snowy Plovers in two habitats in coastal northern California

ABSTRACT Habitat quality, as indexed by the reproductive success of individuals, can greatly influence population growth, especially for rare species near the limits of their range. Along the Pacific coast, the Snowy Plover (Charadrius alexandrinus nivosus) is a threatened species that, in recent years, has been breeding on both riverine gravel bars and ocean beaches in northern California. From 2001 to 2009, we compared the habitat characteristics, breeding phenology, reproductive success, and abundance of Western Snowy Plovers occupying these two habitats. Similar percentages of yearling and adult plovers returned to gravel bars and beaches, but plovers breeding on gravel bars arrived and initiated first clutches 2–3 weeks later than those breeding on beaches. Despite this delay, however, the mean annual fledging success of plovers on gravel bars (1.4 ± 0.4 [SD]) was double that on beaches (0.7 ± 0.3). Differences in cumulative reproductive success produced a stronger pattern. By their sixth year, males on gravel bars had fledged 14.5 ± 2.1 chicks, more than four times the number of young fledged by males on beaches (3.3 ± 3.1). Over 9 years, local population size decreased by about 75%, coincident with a shift in breeding distribution away from high‐quality gravel bars to ocean beaches. This unexpected population decline and shift to poorer quality beaches may have been related to occasional low survival of plovers that over‐winter exclusively on beaches in our study area. Consistently low productivity of plovers breeding on ocean beaches suggests the need for intensified management to ameliorate the negative impacts of predation and human activity on the recovery of this population.     

Gravel Bars Can Be Critical for Biodiversity Conservation: A Case Study on Scaly-Sided Merganser in South China

Gravel bars are characteristic components of river landscapes and are increasingly recognized as key sites for many waterbirds, though detailed studies on the ecological function of gravel bars for waterbirds are rare. In this study, we surveyed the endangered Scaly-sided Merganser Mergus squamatus along a 40 km river section of Yuan River, in Central China, for three consecutive winters. We derived the landscape metrics of river gravel bars from geo-rectified fine resolution (0.6 m) aerial image data. We then built habitat suitability models (Generalized Linear Models—GLMs) to study the effects of landscape metrics and human disturbance on Scaly-sided Merganser presence probability. We found that 1) the Scaly-sided Merganser tended to congregate at river segments with more gravel patches; 2) the Scaly-sided Merganser preferred areas with larger and more contiguous gravel patches; and 3) the number of houses along the river bank (a proxy for anthropogenic disturbance) had significantly negative impacts on the occurrence of the Scaly-sided Merganser. Our results suggest that gravel bars are vital to the Scaly-sided Merganser as shelters from disturbance, as well as sites for feeding and roosting. Therefore, maintaining the exposure of gravel bars in regulated rivers during the low water period in winter might be the key for the conservation of the endangered species. These findings have important implications for understanding behavioral evolution and distribution of the species and for delineating between habitats of different quality for conservation and management.     

Cyanobacteria and Cyanobacterial Lichens from Inselbergs of the Ivory Coast,Africa

This study focuses on the saxicolous lichens and cyanobacteria of the open, exposed rock surface of inselbergs. Twenty-three species of cyanobacteria and 17 cyanobacterial lichen species (“cyanolichens”) from several inselbergs and other rocky outcrops of three major climatic regions, savanna, transition zone and rain forest, are reported from the Ivory Coast. Inselbergs are isolated and frequently mountains consisting of Precambrian granites or gneisses that abruptly rise from the surrounding plains. Cyanobacteria were found to be the dominating organisms on all rock surfaces. The lichens found mainly belong to the family Peltulaceae and a few were present from the family Lichinaceae. Nine species of the cyanolichens and most of the cyanobacteria are new for the Ivory Coast. A gradient in total species number (cyanolichens and cyanobacteria) occurs from the savanna to the rain forest, with a decrease in species number towards the rain forest. Saxicolous cyanobacterial lichens reached a higher species number in the savanna type ecosystem (11) than on inselbergs in the rain forest (7). The cyanolichens and cyanobacteria found are characteristic for larger, light-exposed rock surfaces and species like P. congregate, P. lingulata, P. tortuosa and P. umbilicata preferentially occur on the granite or sandstone of inselbergs.     

Effects of soil erosion on the floristic composition of plant communities on marl in northeast Spain

Abstract. This study explored the validity of three responses of vegetation to increased soil erosion: reduction of vegetation cover, number of species and reduced substitution of species. 201 relevés, including edaphic and geomorphological data, were surveyed in the intensely eroded Eocene marls of the Prepyrenees (NE Spain). Changes in plant species’ presence in relevés from different degradation stages were compared. The level of vegetation degradation was defined as the total phanerogam cover which, in the studied area, was correlated to the degree of soil erosion. The considered trends were validated. Reduction of phanerogam cover and species number were gradual from low to high‐eroded areas. Vegetation degradation explained 48% of the species number variance. In the later stages of degradation a significant substitution of species was not observed, only a lower frequency of occurrence of several species that appeared in the whole set of relevés. Through the process of degradation, 47% of species displayed significantly reduced frequencies as degradation increased, none showed a significant increase in frequency. It is concluded that there are no characteristic species in these plant communities that survive in the severely eroded marls. Among the few species that had increased in frequency, most only colonised favourable micro‐environments.     

Lost and found: Short-term dynamics of the flora on 100 small islands in the White Sea

The aim was to uncover factors that influence short-term (decade) flora dynamics and species richness of northern marine islets characterized by poor flora and weak anthropogenic pressure. The study used presence–absence data of vascular plant species on 100 small uprising islets of the Kandalaksha Gulf of White Sea (Northern Karelia, Russia). We investigated the influence of islands' attributes on species richness and rates of flora dynamics. Two island types were analyzed separately: younger, stone-like and older, islet-like (which generally are larger and have higher diversity of habitats). Sampled islands were studied via classical biogeographical per island approach and metapopulation per species approach. Stone-like islands had noticeably poorer flora with higher rates of immigration and extinction when compared to those of islet-like islands. The species number for islet-like islands correlated positively with number of habitats, abundance of different habitat types and island area. Species richness of stone-like islands correlated positively only with number of habitat types. Plant species associated with birds, crowberry thickets and coastal rocks were the most stable, and the species of disturbed habitats were significantly less stable. Floristic changes that have occurred have been caused by the massive establishment of new species rather than the extinction of pre-existing taxa. Thus, most of these islands are still in the colonization (assortative) stage. While we found no relationship between island area and species number for stone-like islands, this relationship was seen on islet-like islands.     

Potential influence of river engineering in two West Carpathian rivers on the conservation management of Calamagrostis pseudophragmites

This article focuses on assessing the habitats characteristics of endangered Calamagrostis pseudophragmites in selected localities in the Czech Republic with different river engineering measures, which can potentially influence the occurrence of this species. Furthermore, we give some general recommendations for management practices based on our results to promote the conservation of Calamagrostis pseudophragmites.Our research shows that there is a high correlation between Calamagrostis pseudophragmites and natural or near natural sunny biotopes on gravel bars or straight reaches near the water level that consist of non-silted gravel substrates with a higher concentration of pebbles or cobbles, which are typical of piedmont and mountain-valley areas. Many of these areas are rapidly disappearing due to intensive anthropogenic pressure. We also found that Calamagrostis pseudophragmites is temporarily capable of surviving among other competitively stronger plant communities.To assess the potential influence of river engineering we selected two different types of rivers where this species is present – one was heavily altered by both the construction of a water reservoir in its upper reach and channelization of the stream, the other was less severely regulated by the construction of several weirs. Our results show differences in the occurrence of Calamagrostis pseudophragmites between the two rivers, being found at three times as many localities in the less regulated river than in the more regulated one. This corresponds with a higher number of gravel bars in the less regulated river.Based on our results, our general recommendations for the management of this species include only the partial removal of gravel bars (to ensure flood control in populated areas), blocking vegetation succession by removing woody vegetation and aggressive invasive species, the restoration of natural river dynamics, and connecting existing biotopes where this species is found.     

Biodiversity and vegetation of small-sized inselbergs in a West African rain forest (Taı, Ivory Coast)

The vegetation of small granitic rock outcrops (geomorphologically small-sized inselbergs) which do not reach the canopy was studied in the Taı rain forest (southwestern Ivory Coast) under aspects of species diversity and phytogeographical affinities. Rock outcrops form edaphically arid (due to absent or very sparse soil cover) and microclimatologically xeric (i.e. low air humidity, temperature regularly exceeding 50°C) islands with cryptogamic crusts, succulents and poikilohydric vascular plants as characteristic elements of their vegetation which differs totally from the surrounding forest. Altogether sixty-six species of vascular plants out of twenty-nine families occur, the number of species correlates positively with inselberg size. Compared with large inselbergs the microclimatic attributes of small-sized rock outcrops are less pronounced. This is accompanied by a decrease of typical inselberg taxa (i.e. species mainly occurring on inselbergs). Low beta diversity between inselbergs indicates deterministic influences as important regulators of species composition. Annual Poaceae and Cyperaceae are richly represented. It can be hypothesized that inselbergs may represent natural growing sites of widely distributed tropical weeds today. Inselbergs might provide habitat resources for savanna elements in rain forest zones.     

Plant species diversity and ecological districts of the Sinai desert

Summary Lists of vascular plant species for each of the 12 ecological districts of Sinai were prepared. In all 78,000 observations on altogether 812 species were analyzed. The number of districts where each species occurred was also counted. Linear regressions were calculated for the log-transformed values of species/area, species/altitude and multiple regression of species on both area and altitude.The regression of species on area for the 12 ecological districts of Sinai gave the equation: log S = 1.0309+0.3 log A The value of z=0.3 is higher than 0.222—the measured overall value for the continents and higher than the values for the Sahara, California mainland, the British Isles, and the Netherlands. However, it is lower than the values for the Galápagos Archipelago and the California Islands.The number of species supported in districts characterized by fissured limestone, gravel plains, chalk, marl, sandstones, sands or fissured magmatic and metamorphic rocks is close to the regression of species on area, whereas large outcrops of smooth-faced rocks are relatively richer in total number of species as well as in stenotople species.It is suggested that the high gamma diversity of Sinai as compared with other parts of the world is primarily due to its environmental heterogeneity. Sinai being a meeting place of three phytogeographical regions and to past climatic changes. The effect of smooth-faced rock outcrops as conducive to providing refugia must also be taken into account.This work is based on a comprehensive study of the vegetation of Sinai directed by Prof. G. Orshan, with the collaboration of Dr. A. Shmida, the present author and the late Prof. N.H. Tadmor and Dr. G. Halevy.     

Patch distribution of interstitial communities: prevailing factors

1. Community structure of interstitial invertebrates was studied in the sediments of two gravel bars in a bypass section of the Rhone River (France), in relation to hydrological patchiness and to major geomorphological characteristics of the sampling areas. Hydrological patchiness was characterized by upweiling or downwelling zones while geomorphological characteristics were concerned mainly with position towards the head or tail of each bar, which presented different environmental conditions. 2. Longitudinal profiles of the two bars, at 0.5 and 1.0m below the substrate surface, were compared. At each station 10-l samples were taken using mobile standpipes and a Bou—Rouch pump. During low-water conditions, replicate samples were taken in both the head-bar and tail-bar regions to confirm the results. Physicochemical parameters were used to differentiate surface water from ground water (conductivity, alkalinity, silica, nitrates, sulphates and temperature) and to detect hydrological patterns. 3. Two different situations were observed within the gravel bars. In the first one, the upweiling zone was situated in the tail bar and downwelling zone in the head bar. The reverse was observed in gravel bar 2 where the head-bar region formed the upweiling zone and the tail-bar region formed the downwelling zone. 4. In the first situation, the distribution of interstitial assemblages corresponded with the observed hydrological patterns: epigean fauna characterized the downwelling zone and stygofauna characterized the upweiling zone. The interstitial fauna seemed to be strictly linked to the mode of water movement within the sediments. Hydrological patchiness therefore appeared to be an important factor for the structure of interstitial assemblages. 5. In the second case there was no relationship between the hydrological and the biological pattern. Epigean fauna and hypogean fauna were scattered all along the gravel bar. These results suggest that geomorphology is a second important factor governing the composition and structure of interstitial assemblages. A precise structure was obtained when hydrological and geomorphological characteristics were superimposed (case 1). When they were opposed (case 2) all the species responded according to their individual ecological requirements.     

Western Snowy Plovers Charadrius alexandrinus nivosus select nesting substrates that enhance egg crypsis and improve nest survival

Predation is an important cause of nest failure for many birds and has shaped the life‐history characteristics of many species, especially ground‐nesting shorebirds. We examined nesting success, causes of clutch failure and nest survival in relation to variation in substrate characteristics in a colour‐marked population of Western Snowy Plovers Charadrius alexandrinus nivosus breeding on riverine gravel bars in coastal northern California. Plovers experienced higher nesting success on gravel bars than on nearby beaches, which were characterized by more homogeneous, sandy substrates. On gravel bars, Plovers nested in habitats characterized by large, heterogeneous substrates, with more egg‐sized stones, compared with random sites. Egg crypsis, as indexed by time required of a naïve observer to detect a nest, increased with number of egg‐sized substrates. Nest survival correlated negatively with heterogeneity of substrates and positively with the number of egg‐sized stones. Consistently high nesting success of Plovers on gravel bars indicates that this high‐quality habitat deserves special management considerations given the species’ threatened status.     

Aquatic plant diversity in four river floodplains: a comparison at two hierarchical levels

The present study focuses on the role ofenvironmental factors in plant diversity and community organization at both water-body and river floodplain levels. The cover of each plant species was measured at 841 sampling plots along 63 cut-off channels located in four river floodplains. Environmental variables were documented either at river level (suspended matter, water physico-chemistry, river slope) or at cut-off channel level (channel capacity, slope, substrate grain-size). At both river and cut-off channel levels, increasing slope (i.e. increasing erosion during floods) and decreasing nutrient-content of the water raise species richness and uniqueness (number of species found only in one river), according to hypotheses on the combined role of disturbances and productivity in biodiversity. Rivers that are not eroded during floods are nutrient-rich and present the lowest richness and uniqueness at the river level, but dewatering combined with high connectivity increase richness at the cut-off channel level.     

Flora and vegetation of rocky outcrops/cliffs near the Hyrcanian forest timberline in the Mazandaran mountains,northern Iran

Due to their numerous endemic/sub‐endemic and rare plant species, rocky outcrops and cliffs, particularly those occurring at the timberline and forest/steppe ecotones of the Hyrcanian area, are considered as among the most important and fragile ecosystems in Iran. As a representative of such ecosystems, one of the last remnants of relatively undisturbed timberline and ecotone areas of the central Hyrcanian region was selected for phytogeographical, floristic and vegetation analyses. The flora and vegetation of rocky outcrops and crevices at altitudes of 2500–3000 m a.s.l. was surveyed by 40 relevés of 25 m² surface area. A total of 215 vascular plants belonging to 55 families and 150 genera were identified in the different vegetation types. Hemicrytophytes (55.8%) were the dominant life form and Euro–Siberian/Irano–Turanian biregional plants (28.2%) were the most common chorotype at the study sites. A phytogeographical analysis indicated that Euro–Siberian elements (including Hyrcanian endemics) precede other uniregional chorotypes in the timberline and upper‐mountain parts of the central Hyrcanian area. The Caucasus, an important biodiversity hotspot close to Iran, has a considerable number of taxa in common with the Irano–Turanian floristic regions. Five vegetation types were identified using a modified TWINSPAN procedure and subsequently analysed by indirect gradient analysis applying both species‐related (species richness, life form and phytogeographical elements) and topographic/bedrocks features of the studied relevés. The analysis revealed that the vegetation of the area is mainly affected by altitude, heat index, northness, slope inclination and bedrock features.     

Plant population dynamics and species turnover on small islands near Karpathos (South Aegean,Greece)

The vascular flora of small offshore islets around Karpathos (Dodecanesos, Greece) has been surveyed and monitored during six consecutive years. The present study focuses on four islets that are inhabited by the islet specialist Silene holzmannii (Caryophyllaceae), a species that has a widely scattered distribution in the Aegean where it exclusively occurs on very small islands. Repeated census of its populations, partly following controlled extirpation, reveals extreme variation in number of individuals. Sowing experiments involving various species demonstrate the low probability of successful new colonizations. Differences in the amount of apparent turnover of species as well as the occurrence of both stable micro-populations and mass-populations are discussed.