Temporal Changes In Density Research Articles

Population size, abundance, habitat relationships and the result of a translocation programme in the Gran Canaria Blue ChaffinchFringilla polatzeki

SummaryThe Gran Canaria Blue ChaffinchFringilla polatzekiis a threatened, endemic, forest‐dwelling bird species of the Canary Islands, whose core population at the end of the 20th century was restricted to the pine forests of Inagua Nature Reserve (38 km2). A translocation programme released birds from a breeding centre into the nearby (<3 km) pine forests of La Cumbre in the years following 2010. From 2015 to 2019 the La Cumbre population was reinforced by translocation of wild juveniles from the source population of Inagua. We estimate the population size, the spatial variation of abundance, and recent temporal changes in density of the species in Inagua and La Cumbre by means of line transects, distance sampling, and habitat suitability modelling using random forests. The average density of the Blue Chaffinch in Inagua Nature Reserve was 10.2 birds/km2in spring 2019, with a population estimated at 362 birds (95% CI: 257–489). The most important variables affecting the distribution of the Blue Chaffinch in Inagua were the amount of precipitation during the summer (July–September), the solar radiation in June, and the northern position in the reserve, highlighting the importance of abiotic factors related to thermal and hydric stress during the breeding season. The density was considerably lower in the translocated population inhabiting 21 km2of pine forests in La Cumbre (3.3 birds/km2), with an estimate of 68 Blue Chaffinches (35–141) breeding freely in the wild. The translocation programme successfully contributed to the establishment of a second viable nucleus, accounting for 16% of the total population within a time span of 10 years. This result reinforces the role of translocations in preventing extinctions of endangered species with very low population sizes restricted to only one isolated area.

Evaluating the potential impact of bird predation on the SW Atlantic fiddler crab Leptuca uruguayensis

The southernmost permanent population of the fiddler crab Leptuca uruguayensis occurs along the Samborombón Bay (36°22′S, 56°45′W, Argentina), an important feeding site for many bird species, including ruddy turnstones (Arenaria interpres), whimbrels (Numenius phaeopus), grey plovers (Pluvialis squatarola), american golden plovers (Pluvialis dominica) and gull-billed terns (Gelochelidon nilotica). Although all these birds are known to prey on many fiddler crab species worldwide, there is no estimation of their joint predation impacts, probably due to the difficulty in conducting experiments on an appropriate spatial scale. In these situations, computer simulation methods are useful tools. By using Monte Carlo methods and field data, we modeled the decrease of a fiddler crab population due to bird predation. The model found that under current bird occurrences and crab densities, birds do not consume more than 0.03% of the studied fiddler crab populations. Birds only consume more than 10% of the population if crab density is below 0.02 crabs m2, or if bird occurrences are at least 3 orders of magnitude higher than currently observed. Both situations are unlikely, as mean crab density is 140 crabs m2, and bird density is never so high. Furthermore, by monitoring three different fiddler crab patches, we found that bird predation cannot account for temporal density changes, suggesting that other population processes are more important than bird predation. In conclusion, even though fiddler crabs may exhibit strong predator-avoidance behavior, direct lethal effects of bird predation are currently small.

Repeat Absolute and Relative Gravity Measurements for Geothermal Reservoir Monitoring in the Ogiri Geothermal Field, Southern Kyushu, Japan

Repeat hybrid microgravity measurements were conducted around the Ogiri Geothermal Field on the western slope of Kirishima volcano, southern Kyushu, Japan. This study was undertaken to detect the short-term gravity change caused by the temporary shutdown of production and reinjection wells for regular maintenance in 2011 and 2013. Repeat microgravity measurements were taken using an A-10 absolute gravimeter (Micro-g LaCoste) and CG-5 gravimeter (Scintrex) before and after regular maintenance. Both instruments had an accuracy of 10 μgal. The gravity stations were established at 27 stations (two stations for absolute measurements and 25 stations for relative measurements). After removal of noise effects (e.g., tidal movement, precipitation, shallow groundwater level changes), the residual gravity changes were subdivided into five types of response. We detected a gravity decrease (up to 20 μgal) in the reinjection area and a gravity increase (up to 30 μgal) in the production area 1 month after the temporary shutdown. Most of the gravity stations recovered after the maintenance. The temporal density changes in the geothermal reservoir were estimated based on these gravity changes.

The clam Neilonella sulculata in San Jorge Gulf (Argentina, Southwest Atlantic): spatial distribution pattern and inverse relationship between size and density

ABSTRACTNeilonella sulculata is a dominant bivalve on muddy bottoms of the continental shelf and gulfs off Argentina (Southwest Atlantic). Two benthic surveys performed in 2007 and 2009 in San Jorge Gulf, a depositional area characterized by silt/clay sediments, gave us the opportunity to test whether the population of this clam displays a significant spatial structure and to analyse if there is an inverse relationship between size and density. Density, biomass and size–frequency distribution displayed significant spatial structure, being positively autocorrelated at the smallest spatial scales (c. 1.7–14/17 km). Biomass also showed spatial contagion at scales of 25–33 km. A substantial increase in density and biomass occurred between 2007 and 2009. Empty valves in the sediment showed that the maximum size attained by the species may exhibit considerable variation at relatively short temporal scales. Regardless of temporal changes in density and biomass, the spatial structure of these variables remained stable. In densely populated areas, shell size decreased with increasing density, suggesting a density-dependent control of growth. A clear north–south increase in density and biomass was detected, which might be related to a gradient in food availability caused by a thermohaline frontal system associated with wind-related upwelling.

Regional distribution and temporal changes in density and biomass of Didymosphenia geminata in two Mediterranean river basins

This study aimed to strengthen the knowledge of Didymosphenia geminata distribution in Spain, and to determine which environmental variables were related to its regional distribution and temporal changes in growth and production in two Ebro River tributary basins (Iregua and Najerilla¸ La Rioja Region). Living cells of D. geminata were present in 40% of the studied sites of both basins but only four of the sites developed massive growths. The diatom is widely distributed only in mountain areas of both basins (above 690 masl), which have low average annual air temperature (8.27 ± 0.28 oC) and low water Soluble Reactive Phosphorus (SRP) concentration (average value 0.024 ± 0.007 ppm). Massive growths occur in sites with high light intensity, hydrological regulation of river flow and the lowest measured SRP concentrations (below 0.018 ppm SRP). We carried out an intensive spatio-temporal survey in the Lumbreras River (Iregua basin), where the first massive growth was detected in La Rioja Region. The highest cell densities (1.6 × 105 cells cm−2) and biomass values (480 gDWm−2) were recorded in late summer. An increase in solar radiation and slight rise in water temperature were associated with an increase in biomass over time, but always in waters with low SRP concentration, the key factor that enhances massive growth development. The alteration of the natural streamflow in the Lumbreras River by the Pajares Reservoir and the fact that water is discharged from the hypolimnion (which decreases the summer river water temperature) are important factors in understanding why D. geminata shows such an increase in biomass in a Mediterranean river. This paper provides new insights into the importance of different factors controlling D. geminata massive proliferations in the Mediterranean climate zone, where it is assumed that the species should not develop such massive growths, compared to temperate zones where the majority of previous studies have been carried out. We suggest that changing the reservoir management strategy would be a very important contribution to the control of such significant developments.

Three-dimensional microdistribution ofChironomus balatonicuslarvae (Chironomidae, Diptera) in soft sediments from the Vistula Lagoon (South Baltic Sea)

Despite the worldwide distribution and ecological importance of Chironomus larvae in both freshwater and brackish water ecosystems, patterns of their spatial distribution have been rarely studied. This study was conducted in summer, spring and autumn (2010–2011) in the deep-water zone of the polymictic, strongly eutrophic Vistula Lagoon. Its objective was to test whether temporal changes in density of the larvae affect their horizontal microdistribution and burrowing depth. Horizontal distribution, as inferred from indices of dispersion and patchiness, was density-dependent. The larvae spread more regularly with increasing density. The decreased density was associated with an increase in clustered distribution. The larvae burrowed into sediments up to a depth of 25 cm. The maximum burrowing depth positively correlated with total density, but also depended on the size of the larvae. The smallest individuals (<10 mm) did not occur deeper than 5 cm. Larger larvae were encountered throughout the sediment profile. The largest larvae did not always burrow the deepest. In spring before pupation, the larvae gathered below the sediment surface.

Prescribed Burning Has Limited Long-Term Effectiveness in Controlling Trembling Aspen (&lt;em&gt;Populus tremuloides&lt;/em&gt;) Encroachment into Fescue Grassland in Prince Albert National Park

Encroachment into grassland by woody species is a global ecological phenomenon, and it is of particular concern in remnant fescue (Festuca) prairie at the aspen parkland–boreal forest ecotone. Fire suppression is thought to encourage encroachment; however, prescribed burning as a means of controlling encroachment and restoring system structure, function, and composition has had variable success. The objective of this study was to determine the effects of different season of burn, number of annual burns, and number of years after burning treatments on Trembling Aspen (Populus tremuloides) encroachment into the fescue grasslands within Prince Albert National Park, Saskatchewan. Temporal changes in density and cover of Trembling Aspen in grassland and grassland–forest transition plant communities were evaluated using data from a prescribed burn study conducted in Prince Albert National Park from 1975 to 2010. The effect of year (indicating varying amounts of time relative to prescribed burning) and the interaction between spring burning and year reflect a stimulatory effect of burning on Trembling Aspen suckering. Increased Trembling Aspen cover in the forest transition community with more annual burns, burning in the fall, and the interaction between year and number of annual burns and increased Trembling Aspen cover in the grassland community with year indicate that none of the treatments had lasting control of Trembling Aspen encroachment. Ongoing Trembling Aspen encroachment despite prescribed burning may be due to important missing interactions between fire and grazing. A change in the use and expectation of prescribed burning is needed when attempting to suppress Trembling Aspen encroachment into the fescue grasslands.

WE‐G‐BRA‐07: Biological Modeling and Global Normal Lung Tissue Radiological Response in Patients Treated with Stereotactic Body Radiation Therapy for Lung Tumors

Purpose: To model the probability of normal lung tissue density change and obtain dose‐response curves restricted to observed change areas for patients with lung tumors who received stereotactic body radiation therapy (SBRT). Methods: 129 follow‐up CT scans for 36 patients who received SBRT between 2003 and 2009 with a maximum of 5 fractions and a median total dose of 54 Gy (range, 30–60) were analyzed. RT‐induced lung density changes were evaluated after fusion of planning CT scans with post‐RT follow‐up scans corresponding to interval periods of approximately 3, 6, 12, 18, and 24 months after treatment. Each follow‐up scan was manually reviewed and areas of visible density change were contoured. Scans were divided into regions receiving specific 10Gy dose‐bins. Voxel density changes belonging to contoured areas were averaged over each dose‐bin to obtain dose‐response curves (DRC). The occurrence of visible density change depending on the maximum dose to the tumor was also chosen as an end point to model the normal tissue complication probability (NTCP). Results: With an analysis restricted to area of visible change, the density change magnitude appeared to not depend much on the dose bin. The average density increase was smaller at 3 month (214HU), peaked at 6 months (322HU) and slightly decays thereafter without totally vanishing (300HU). This observed trend was similar using rigid or deformable registration. The maximum target dose for a 50% complication probability for changes &gt;200HU was 80Gy according to the NTCP model. Conclusion: Although local correlations between dose and density increase have been reported for conventional and SBRT lung treatments, our analysis revealed a relatively uniform and stronger density response, while temporal density changes followed a previously reported density change trend (Palma 2010, IJROBP). These discrepancies were likely due to the restriction of the analysis to areas with actual density increase.

In vitro measurements of flow using multislice computed tomography (MSCT)

To examine in vitro whether an assessment of flow in normal and obstructed vessels is essentially possible using modern multislice CT-scanners. An experimental model allowed known stenoses to be perfused at defined flow rates. Aorta and coronary arteries were simulated by silicone tubes. A pulsatile pump was used to perfuse water through the system with intermittent injection of a bolus of radio-opaque contrast agent. CT-measurements were carried out with slice orientation perpendicular to the tubes. 50-90% concentric stenoses were examined 5 times at 4 different stenosis slice distances. A mathematical algorithm calculated the temporal density changes within a ROI in the tube cross-sections. Quantitative assessment of the data simultaneously acquired with the 16-slice system for the "coronary" and "aortal" time-density curves showed that the model allowed for exclusion of a≥80% stenosis grade with a 99% probability when the slopes of the density increase quotient was>0.79; a stenosis grade of≥90% could be excluded when the slopes of the density increase quotient was>0.52. A Quotient>0.94 for "peak density" was associated with a 99% probability of a stenosis grade≥70%. The 64-slice system allowed stenosis grades of≥80% to be discriminated from lower grades. The general feasibility of the in vitro approach was verified in an in vivo model. The spatial, contrast and temporal resolution of CT scanners with at least 16 detector rows enables qualitative and semiquantitative assessment of stenotic changes in flow.

Fast time domain reflectometry (TDR) measurement approach for investigating the liquefaction of soils

Time domain reflectometry (TDR) was used for the spatial observation of transient density changes in a laboratory shake-box apparatus. Shake-box experiments are conducted to investigate the liquefaction of soils due to seismic excitation. As a sensor, a flat ribbon cable was placed within a water saturated sand column two metres in height. In the experiment, the soil column was excited at its base with a sinusoidal signal with low frequencies and moderate amplitudes. During the tests, TDR measurements were conducted with a repetition rate of 30 s−1. Temporal density changes during the liquefaction and reconsolidation process of the quartz sand along the sensor could be shown through signal analysis. The mean porosities calculated from the relative dielectric permittivity using a soil-specific calibration function could be determined with an RMSD-value of 0.02 compared to the porosities determined from the height of the sample. With the metrological set-up for the TDR measurements being used, fast temporal changes in the density profile could be qualitatively visualized indicating the movement of the reconsolidation front during the liquefaction.

Field site selection: getting it right first time around.

The selection of suitable field sites for integrated control of Anopheles mosquitoes using the sterile insect technique (SIT) requires consideration of the full gamut of factors facing most proposed control strategies, but four criteria identify an ideal site: 1) a single malaria vector, 2) an unstructured, relatively low density target population, 3) isolation of the target population and 4) actual or potential malaria incidence. Such a site can exist in a diverse range of situations or can be created. Two contrasting SIT field sites are examined here: the desert-flanked Dongola Reach of the Nile River in Northern State, Sudan, where malaria is endemic, and the island of La Reunion, where autochthonous malaria is rare but risk is persistent. The single malaria-transmitting vector at both sites is Anopheles arabiensis. In Sudan, the target area is a narrow 500 km corridor stretching from the rocky terrain at the Fourth Cataract - just above the new Merowe Dam, to the northernmost edge of the species range, close to Egypt. Vector distribution and temporal changes in density depend on the Nile level, ambient temperature and human activities. On La Reunion, the An. arabiensis population is coastal, limited and divided into three areas by altitude and exposure to the trade winds on the east coast. Mosquito vectors for other diseases are an issue at both sites, but of primary importance on La Reunion due to the recent chikungunya epidemic. The similarities and differences between these two sites in terms of suitability are discussed in the context of area-wide integrated vector management incorporating the SIT.

Diet–shifts by an estuarine goby ( Pomatoschistus microps) in the face of variable prey availability

Ontogenetic shifts in diet provide a mechanism for maximising fitness throughout development and are common where predators exhibit large increases in size. In order to maximise their fitness throughout development, benthic feeding fish can show diet–shifts that centre on the transition from meiofaunal to macrofaunal prey. Here we assessed whether such a shift was influenced by natural variation in prey–size availability by comparing the sizes of prey consumed by naturally foraging common gobies ( Pomatoschistus microps). We tested explicitly for the presence of an ontogenetic shift by analysing the length of prey consumed and an index for prey importance for gobies of different lengths. We also tested the match between actual diets and those predicted by a foraging model. The goby size at which the diet–shift occurred was consistent among locations that differed in their availability of prey and through temporal changes in densities and types of prey. The mean sizes of ingested prey increased for gobies > 35 mm in length and the relative importance of macrofauna increased at 30 mm. The foraging model predicted that gobies > 30 mm would eat larger prey than would smaller gobies which differed from the observed changes in prey–size at 35 mm. Availability of prey did not appear to influence the lengths at which gobies changed diet but did affect the size of prey taken after the diet–shift. A relatively large abundance of large–bodied chironomids at two sites was reflected in the mean size of prey consumed by gobies > 30 mm at these sites. Our study indicates that intrinsic mechanisms can be more important than fluctuating environments in determining prey–choice and shifts in diet, although for the common goby, variability in prey–size may have implications for prey–choice later in ontogeny.

Spatiotemporal density patterns of the pest predator Rhynchium haemorrhoidale (F.) along a land-use gradient in cacao agroforestry systems

Tropical insect species show year-round breeding activity due to favourable climatic conditions. However, most species also display seasonal reproductive peaks, but little is known about underlying causes of temporal density changes. We investigated population dynamics of the pest predator Rhynchium haemorrhoidale (F.) (Hymenoptera: Eumenidae) and its natural enemies in relation to season, climate and varying shade tree composition in cacao agroforestry systems in Central Sulawesi (Indonesia). Nesting of R. haemorrhoidale showed clear seasonality with highest densities in the wet season and lowest in the dry season, which was not related to changes in temperature. Wasp densities increased with land-use intensity, presumably because less-shaded areas offer more favourable climatic conditions and higher densities of the major prey, the cacao pest Agathodes caliginosalis (Lepidoptera: Pyralidae). Pupal body size was higher in June than in November indicating increasing intraspecific competition due to high wasp densities and food shortage at the beginning of the wet season. Body size between habitat types was similar, thus season appeared to be more important than habitat in terms of food supply. High wasp densities in the wet season were associated with high diversity of the altogether seven parasitoid species. In conclusion, we found a pronounced seasonality of the wasp R. haemorrhoidale and a preference for little shaded agroforestry, which may be due to enhanced temperature and density of its prey. Our data indicate a high potential for biological control of a major cacao pest by management of nesting sites of R. haemorrhoidale.

Assessment of Effectiveness of Protection Strategies in Tanzania Based on a Decade of Survey Data for Large Herbivores

Considerable controversy surrounds the effectiveness of strictly protected areas that prohibit consumptive resource use. For Tanzania we compared temporal changes in densities of large herbivores among heavily protected national parks and game reserves, partially protected game-controlled areas, and areas with little or no protection. Comparisons based on surveys conducted in the late 1980s and early 1990s versus the late 1990s and early 2000s showed three consistent patterns across the country. First, significant declines in the densities of large herbivores between these two snapshots in time overwhelmingly outnumbered significant increases in all protection categories. Second, more species fared well (increased significantly or showed no significant change) in strictly protected national parks than in areas with partial or no protection and in heavily protected game reserves relative to areas with no protection. Third, significantly more species fared poorly (densities declined or were too low to detect a decline) than fared well in areas with partial or no protection. Our results show that although heavy protection was generally more effective in maintaining large herbivore populations than partial or no protection, continued long-term monitoring is needed in Tanzania to inform managers whether large herbivores are experiencing declining population trends even within heavily protected areas.

An experimental assessment of the potential impacts of longline mussel farming on the infauna in an open coastal embayment

Abstract The existence of perceived ecological impacts and development of adaptive management solutions to mitigate these problems are important issues in sustainable aquaculture. This paper examines the general impacts of two newly established trial longline installations on the infauna in Twofold Bay, a large, coastal embayment in south‐east Australia. We hypothesized that the physical presence of these longline installations and the biological activities of the mussels they supported would result in temporal changes in densities of infaunal taxa below installations differing from those at undisturbed control sites. We also predicted different patterns of variability in infauna between longline and control sites from before to after the longlines were stocked. These hypotheses were tested by using a beyond‐BACI sampling design and asymmetrical analyses of variance to compare changes in densities of taxa at several different spatial scales below the proposed longline sites with those at two adjacent control sites, before and after the longlines were stocked. After 18 months of longline operations, there was no evidence of any impact on total number of taxa, nor densities of individual taxa. Short‐term temporal trends in densities in plots at control sites from April to May 2001 were often as different from each other as from those at the longline site. This indicates that densities of taxa at the farm site were within the range typically found at undisturbed sites, so there were no ecological impacts from the farm. These results do not concur with previous studies on impact of mussel farming in semi‐enclosed coastal waters. Differences in location, scale of production, duration of operation and assimilative capacity of the environment probably contributed to this discrepancy. The infauna in Twofold Bay either do not respond to this form of disturbance or have not yet been exposed to disturbance of a sufficient magnitude, or for a sufficient period of time, to elicit a detectable response. Better definition of the potential ecological impacts associated with aquaculture, plus their scale and magnitude in different environments is needed to design experiments and monitoring programmes to detect specific impacts. This activity can only be considered sustainable once we know that these impacts are localized, reversible and short‐term. Copyright © 2006 John Wiley &amp; Sons, Ltd.

Variability of Ruppia cirrhosa in two coastal lagoons with differing anthropogenic stresses

The dynamics of Ruppia cirrhosa were studied over two years in two coastal lagoons on the Corsican coast (France, Mediterranean Sea). The lagoons differed in type of eutrophication: (1) Biguglia lagoon (urban and industrial effluent, agriculture, runoff from catchment area) and (2) Santa Giulia lagoon (tourist pressure in summer). Spatio-temporal variability of R. cirrhosa occurrence was monitored on permanent transects. We also monitored temporal changes in density, aboveground/belowground biomass and organic matter. Most of the parameters studied along the transects show variations with season and site. Density and aboveground biomass of R. cirrhosa in Biguglia lagoon were lower when Ulva species were present. This may be related to differences in nutrient availability. During the first year of the study, rainfall was greater with concomitantly higher nutrient inputs, which may account for the higher values of measured parameters in the first year. The results suggest that environmental parameter variations affect the functioning of R. cirrhosa meadows.

A mammographic image analysis method to detect and measure changes in breast density

We present an image analysis method that can detect and measure breast density from digitised mammograms. We present initial results on applying our method to characterise breast changes, in particular, changes due to Hormone Replacement Therapy (HRT). It has been established that long-term use of certain hormone replacement therapies can increase the risk of breast cancer, a fact that encourages the notion that objective measures of tissue density can be an important development in breast cancer image analysis. A set of 59 temporal pairs of mammograms of patients undergoing HRT (two images per patient) were used. The clinician’s assessment of density changes constituted the ground truth for evaluating the proposed quantitative measures of density change. The measures we developed are based on the Standard Mammogram Form (SMF) representation of interesting tissue and their performance (agreement with the expert’s description) is also compared to the “interactive thresholding” method that has been used in the past to characterise mammographic density. The results clearly indicate that present methods for measuring mammographic density fail to characterise temporal changes while the proposed measures have the potential to aid the radiologist in assessing temporal density changes both on a global and a local basis.

The Social Environment of an Aggregating, Ant-Attended Treehopper (Hemiptera: Membracidae: Vanduzea arquata)

Many membracid treehoppers are attended by honeydew-harvesting ants. Ant mutualism often favors group living, which will in turn influence social interactios and communication. I investigated aspects of life history that underlie the social behavior of an aggregating, ant-attended treehopper. The number of adults, and their patterns of distribution, changes dramatically over the course of a season. Despite the relatively low vagility and high persistence in the same clump of host plants, individuals encounter a wide range of social environments. This aggregating species differs from solitary species in the clumped distribution of females, and possibly in the intensity of acoustic competition among males, but both aggregating and solitary species exhibit large temporal changes in density. A high degree of temporal and spatial variation in the social environment is probably characteristic of many insects and may be an important source of selection on insect communication.

Spontaneous Formation of Regular Defect Array in Water-in-Cholesteric Liquid Crystal Emulsions

We have investigated the spontaneous formation of the regular quadrilateral defect array in cholesteric emulsions, composed of water, surfactants, and cholesteric liquid crystals. Both at the substrate surface and at the surfactant-coated droplet surface, a homeotropic anchoring is enforced to the adjacent liquid crystal. We observe nucleation of numerous point defects, in the cholesteric planar texture immediately after termination of shear to the emulsion, which leads to the formation of the regular defect array. We examined the temporal changes in density, size, and the positional correlation of point defects through the formation process of the defect array by means of two-dimensional fast Fourier transformation (2D FFT) image analysis. Two distinct stages were identified in the formation process; the early stage is the nucleation and growth process of point defects, and the late stage is the rearrangement process of point defects to the regular defect array. Water droplets destabilize the arrangement of cholesteric helix, which induces the transition from the shear-induced planar alignment to the homeotropic orientation.

Spatio–temporal differentiation in the population structure of Hydrobia ulvae on an intertidal mudflat (Marennes-Oléron Bay, France)

Spatio–temporal changes in density, breeding cycle, growth and population structure of Hydrobia ulvae (Gastropoda: Prosobranchia) were studied by monthly sampling of a wide bare mudflat in Marennes-Oléron Bay. Four stations, located along a cross-shore transect and characterized by different geomorphological structures, were investigated from March 2000 to February 2001. Hydrobia ulvae was mainly distributed in the upper half part of the mudflat and no individuals were found in the lower part. The breeding cycle extended from March to December and showed two annual peaks, in spring and in autumn. Three cohorts were recruited during the year and showed high growth rates during summer; the parameters of the von Bertalanffy model describing the growth curves were equal to kmax=0·47±0·5 mm month−1 and L∞=5·4±0·2 mm. The snail population had similar size–frequency structure along the transect at the beginning of the survey but summer recruitment initiated spatial differentiation. Reproduction occurred in the middle part of the mudflat but recruits mainly settled down at the upper level of the mudflat; new cohorts appeared with increasing individual densities. The middle part of the mudflat was rather dominated by adult individuals which showed large density fluctuations.