Variation in calanoid copepod resting egg abundance among lakes with different acidification histories

The maintenance of species and genetic diversity within zooplankton egg banks may be crucial to the re-establishment of zooplankton communities following historical disturbance, such as anthropogenic acidification which globally caused widespread damage to ecological communities. Despite this, no other study has described basic characteristics of zooplankton egg banks among lakes with different acidification histories, such as variation in resting egg concentration. Theoretically, habitats with frequent periods of harsh environmental conditions are expected to select for resting egg production or prolonged dormancy in zooplankton, which would increase the size of the resting egg bank in lake sediments. In this study, we compared abundances of viable and inviable calanoid copepod resting eggs among three freshwater lakes with different acidification histories. While Swan Lake underwent major chemical and biological changes from acid and metal deposition, Teardrop and Bat lakes were relatively unaffected by historical acidification and had comparatively constant, but different pH over time. We also tested the effect of age on the viability of resting eggs. As predicted, higher numbers of viable resting eggs were found in recent sediments from acid-recovering Swan Lake compared to study lakes with relatively temporally constant environments (Teardrop and Bat lakes) when the total number of eggs was held as a covariate. We detected this result in spite of similar pelagic abundances of Leptodiaptomus minutus, the dominant species in zooplankton communities of these lakes. This pattern did not necessarily hold for inviable egg concentrations since these eggs were more abundant in both Swan and Bat lakes compared to Teardrop Lake in older sediments (1939–1951, 1800s). Within study lakes, the abundance of viable resting eggs declined with increased egg age. Further study is required to test mechanisms underlying these patterns. Handling editor: S. I. Dodson     

Salvelinus namaycush spawning substratum attracts egg predators and opportunists through chemosensory cues

Two separate field experiments were conducted in a series of small boreal lakes to test for the attraction of egg predators to lake trout Salvelinus namaycush spawning shoals and subsequently to determine whether chemosensory cues attract egg predators to these sites. In the first experiment, minnow traps set on spawning sites captured significantly more egg predators than those set on structurally similar non‐spawning sites. Captures of slimy sculpin Cottus cognatus, common shiner Luxilus cornutus, blacknose shiner Notropis heterolepis and virile crayfish Orconectes virilis were more than double on spawning sites relative to non‐spawning sites for the two study lakes. To test whether chemosensory cues could attract egg predators to S. namaycush spawning sites, paired minnow traps were placed on eight to 10 sites in each of the three study lakes; one trap contained visually concealed S. namaycush spawning substratum and the other with visually concealed non‐spawning substratum. Traps containing spawning substratum consistently captured more fish and had higher mean daily catches than those that contained non‐spawning substratum. The combined results demonstrate a greater prevalence of egg predators on S. namaycush spawning shoals that appears to be the result of chemosensory attraction to spawning substratum.     

Nest Site Preference and Fidelity of Chinese Alligator(Alligator sinensis)

Oviparous female reptiles select nesting sites with optimal ecological factors that contribute to egg development. Chinese alligator(Alligator sinensis), an oviparous reptile, is a critically endangered crocodilian with temperature-dependent sex determination. Research on its nesting behavior may facilitate the protection of this species. In this study, we monitored nesting behavior over eight years. We compared selected frequency of nest sites, distance from nest site to water, height from nest site top to the water surface, distance from nest site to human activity region, and canopy density between nest sites on the island and bank. The results showed that 45 nest sites were used by female alligators over eight years and each site was selected from one to 10 times. The selected frequency of nest site occurrence on the island was higher than that on the bank(P 0.001). We observed that 88% of the individual alligators(15/17) showed different degrees of nest site fidelity. However, Chinese alligators might not always be loyal to only one nest site because of environmental changes or interspecific competition at nest sites. Our findings suggest that female alligators prefer to nest at island, which might be because of the nests on the island had a higher canopy density(P = 0.010) and were further from the human activity region(P 0.001) than those on the bank did. It would be beneficial to reduce human activities during the breeding season and protect the vegetation of Chinese alligator habitats in the future. of the beach(Ali et al., 2005; Kamel and Mrosovsky, 2005; Turkozan et al., 2012). Other species such as snakes and lizards select nest sites with more moist substrates, which significantly increases their body size at hatching(Brown and Shine, 2004; Reedy et al., 2013). Nest site preference may be maintained for one or more breeding seasons(Janzen and Morjan, 2001; Kamel et al., 2006). When female chose the same site or area during different years, it is called nest site fidelity(Switzer, 1993). This nest choice behavior may influence offspring survival(Lindeman, 1992) and nest site fidelity has been documented in numerous species such as birds(Lindberg and Sedinger, 1997), turtles(Mitrus, 2006; Walde et al., 2007), tuatara(Refsnider et al., 2010), and crocodilians(Elsey et al., 2008).Chinese alligator(Alligator sinensis) is a critically endangered freshwater crocodilian endemic to China(Thorbjarnarson and Wang, 1999). Recent investigations show that there are no more than 130 Chinese alligators     

The seed bank in an estonian calcareous grassland: Comparison of different successional stages

Seed bank species-composition and seed-density were determined in a successional calcareous (alvar) grassland in western Estonia. Three similar study areas were chosen to compare two different successional stages: open alvar grassland and overgrown areas with young pine forest 30 to 40 years old. In both successional stages, the centre and the edge of a relatively uniform stand were examined. Fifteen soil samples (7 cm in diameter, 5 cm deep) were taken from each of twelve sampling sites. The seedling emergence method was used to estimate seeds in the soil samples. A total of 69 species were detected in the seed bank, of which 18 did not occur in the vegetation. Eighty-nine taxa were recorded in the vegetation and of these 38 were not detected in the seed bank. Fifty-one species occurred both in the seed bank and in the vegetation. The three most abundant taxa in the seed bank wereCarex tomentosa, Linum catharticum andPlantago media, which together made up 49% of the seedlings recorded. Differences in the species compositions of seed bank samples from grassland and forest sites were negligible, although the species richness per area of the above-ground vegetation was significantly higher in the open grassland. The only species tending to be lost from forest site vegetation but still occurring in the forest soil seed bank wereArenaria serpylifolia, Cerastium fontanum andLinum catharticum. About half of all the emerged species from all samples belonged to the transient or short-term persistent seed bank. In the grassland sites there were more species which belonged to the transient seed bank than in the forest sites, where the seed bank contained more short-term persistent type seeds. The seed density was significantly higher in forest sites and lower in grassland sites, which may be explained by the better germination conditions in well-illuminated communities. On the basis of the current study it might be assumed that the soil seed banks of overgrown alvar grasslands which include young pine forests can play a certain role in grassland restoration management.     

Drought and aquatic community resilience: the role of eggs and seeds in sediments of temporary wetlands

1. A long‐lived bank of propagules consisting of eggs, seeds and spores is one mechanism that allows aquatic communities to survive drought. A drying (drought) event is, for aquatic organisms in a temporary wetland, a phase from which communities must recover. Such a dry phase is often considered a disturbance but should not be considered adverse or catastrophic for the organisms that have evolved to live in temporarily wet habitats. 2. This paper explores the parallels between the egg bank of zooplankton and the seed bank of aquatic plants as means of survival in temporary wetlands. The resilience of communities in temporary wetland ecosystems is assessed by examining dormancy, hatching, germination, establishment and reproduction of animals and plants from the egg and seed banks of wetlands with a range of wetting and drying regimes. 3. Both the zooplankton and aquatic plants of the temporary wetlands studied rely on their egg and seed banks as a means for surviving drying. These communities recover after the disturbance of drying by means of specific patterns of dormancy, dormancy breakage, hatching, germination, establishment and reproduction. Spatial and temporal patterns of species richness allow resilience through dormancy, as not all species are present at all sites and not all species hatch and germinate at the same time. Multiple generations in the egg and seed bank and complexity of environmental cues for dormancy breakage also contribute to the ecosystem's ability to recover after a drying event. A persistent egg and seed bank allows species‐rich communities to hatch, germinate and develop rapidly once dormancy is broken. Rapid establishment of species‐rich communities that reproduce rapidly and leave many propagules in the egg and seed bank also facilitates community recovery on flooding of a temporary wetland after a drying event. 4. To maintain the diversity of temporary wetland communities through droughts and floods we need to manage the dry and wet phases of wetlands. To conserve a wide range of wetland types, we need to maintain a variety of hydrological patterns across the landscape.     

Fishes of floodplain habitats of the Fly River system, Papua New Guinea, and changes associated with El Niño droughts and algal blooms

Biological monitoring of the Fly River system in Papua New Guinea has shown that floodplain habitats (oxbow lakes, blocked valley lakes and seasonally inundated grassed floodplain) support diverse and abundant populations of freshwater fishes. Since monitoring first commenced in the early 1980's, a total of 66 fish species representing 33 families has been sampled, with gillnets and rotenone, from a range of sites located on the floodplains of the Fly and Strickland rivers. The fish fauna was dominated by catfishes in the families Ariidae (11 species) and Plotosidae (7 species), with aquatic invertivores being the dominant feeding group. Herring species (Nematalosa spp.) were often very abundant in the oxbow lakes, forming 66% of the total catch in all the floodplain habitats. The fish communities in the oxbow lakes and blocked valley lakes were distinctly different, with several of the smaller fish species being more abundant in the blocked valley lakes, while the oxbow lakes supported more of the larger predatory species. Catches in the oxbow lakes were also generally higher and more diverse than the blocked valley lakes or grassed floodplain sites. Since the commencement of monitoring, catches from the floodplain sites have varied considerably, both spatially and temporally. Reduced catches seen at some sites are probably associated with natural climatic factors, particularly El Niño-induced droughts and algal blooms. Introduced species and increased commercial and artisanal fishing may also be affecting stocks of native fish. Fish populations were shown to recover only slowly in lakes affected by severe drought conditions, with extensive mats of floating grasses hindering fish recolonisation. Fish stocks in a lake affected by an algal bloom recovered more quickly than stocks in lakes affected by drought.     

河水漫溢对塔里木河下游荒漠河岸林地表植被与土壤种子库的影响

采用种子萌发法,研究了河水漫溢对塔里木河下游荒漠河岸林地表植被与土壤种子库的影响.结果表明：塔里木河下游漫溢区地表植被分属8科13属14种,非漫溢区为10科21属26种植物;与非漫溢区相比,漫溢区地表植被中出现了一些浅根系和喜湿的草本植物;漫溢区单位面积物种数、植被盖度、植株密度、物种多样性指数和丰富度指数均比非漫溢区有明显增加.漫溢区土壤种子库中有物种19种,比非漫溢区增加了5种;漫溢区土壤种子库总密度比非漫溢区增加了3.94倍;与非漫溢区相比,漫溢区1年生草本植物种子的比例增加了23.07%,而灌木植物种子比例减少了20.99%;多年生草本的变化则不明显;河水漫溢提高了土壤种子库的生物多样性.漫溢区和非漫溢区土壤种子库与地表植被的共有物种分别为18和9种,土壤种子库与地表植被的相似性系数分别为0.842和 0.667.     

Diversity,composition and guild structure relationships between soil-stored seed banks and mature vegetation in alien plant-invaded South African fynbos shrublands

We investigated vegetation-seed bank relationships at three fynbos sites on the Cape Peninsula, South Africa, and the impacts to these sites of invasion by the alien tree Acacia saligna. Soil-stored seed banks in uninvaded fynbos were of a similar density to those previously measured in fynbos (ca. 1100–1500 seeds m^-2) and were dominated by mostly short-lived species. Lack of similarity between mature vegetation and seed banks, suggests that seed banks are poor predictors of mature vegetation composition and structure in fynbos. This lack of correspondence was attributed to the ephemerals (present only in the soil seed bank) and the dominance of serotinous (aerial seed bank) and sprouting (soil seed bank low to absent) species, in mature vegetation. Long-lived seeders were among the 10 most abundant species in the seed banks at all sites and at two sites shrub species contributed more to seed bank richness than any other growth form. Soil-stored seed banks, therefore, boost species richness and diversity both in early post-fire and later seral stages.There was a decline in fynbos species richness, diversity and abundance both in the standing vegetation and seed banks with increasing duration of invasion by the alien tree, Acacia saligna. However, the rate of decline was higher for the vegetation than the seed banks, suggesting that many fynbos species have long-term persistent seed banks. At two sites, there was no obvious shift in community composition associated with Acacia invasion: invaded sites were depauperate versions of the uninvaded site. However, at a third site, the vegetation composition shifted towards a community dominated by bird-dispersed thicket species and its seed bank shifted towards a community dominated by wind-dispersed perennials. Community composition of the soil seed banks under dense, recent Acacia was very similar to that of the corresponding uninvaded fynbos at all sites, indicating that there is good potential to return to species-rich fynbos vegetation after removal of the alien Acacia. Most seed bank species persisted in the soil seed bank of the long-invaded fynbos at low frequency and density, indicating high seed longevity in many species. We suggest that either a thick Acacia litter layer or a deep (>5 cm) burial moderated the fire and ambient temperature effects, preventing these seeds from germinating after fire and thus preventing loss from the seed bank.     

Geographic signatures in species turnover: decoupling colonization and extinction across a latitudinal gradient

High latitude communities have low species richness and are rapidly warming with climate change. Thus, temporal changes in community composition are expected to be greatest at high latitudes. However, at the same time traits such as body size can also change with latitude, potentially offsetting or increasing changes to community composition over time. We tested how zooplankton communities (copepods and cladocerans) have changed over a 25–75 year time span by assessing colonization and extinction rates from lakes across an 1800 km latitudinal gradient, and further tested whether species traits predict rates of community change over time. Lake‐level dissimilarity, measured with Sorenson distance, decreased at higher latitudes. This decrease was due to higher colonization rates of cladocerans in lower latitude lakes and consistent extinction rates across the latitudinal gradient. At the species level, colonization increased with regional occupancy, and tended to be higher for smaller bodied, locally abundant, species. Local extinction rates were negatively correlated with local abundance and regional occupancy, but were not influenced by body size. None of these species‐specific characteristics changed predictably with latitude. Contrary to our expectations, low‐latitude zooplankton communities changed more rapidly than high‐latitude communities by becoming more species rich, not by losing species that were historically present. Moreover, colonization and extinction trends suggest that lakes have become increasingly dominated by species with smaller body sizes and that are already common locally and regionally. Together, these findings indicate that rates of species turnover in freshwater lakes across a latitudinal gradient are not predicted by rates of temperature change, but that turnover is nonetheless resulting in trait‐shifts that favour small, generalist species.     

Intermittent instability is widespread in plankton communities

Chaotic dynamics appear to be prevalent in short-lived organisms including plankton and may limit long-term predictability. However, few studies have explored how dynamical stability varies through time, across space and at different taxonomic resolutions. Using plankton time series data from 17 lakes and 4 marine sites, we found seasonal patterns of local instability in many species, that short-term predictability was related to local instability, and that local instability occurred most often in the spring, associated with periods of high growth. Taxonomic aggregates were more stable and more predictable than finer groupings. Across sites, higher latitude locations had higher Lyapunov exponents and greater seasonality in local instability, but only at coarser taxonomic resolution. Overall, these results suggest that prediction accuracy, sensitivity to change and management efficacy may be greater at certain times of year and that prediction will be more feasible for taxonomic aggregates.     

The river domain: why are there more species halfway up the river?

Biologists have long noted higher levels of species diversity in the longitudinal middle‐courses of river systems and have proposed many explanations. As a new explanation for this widespread pattern, we suggest that many middle‐course peaks in richness may be, at least in part, a consequence of geometric constraints on the location of species’ ranges along river courses, considering river headwaters and mouths as boundaries for the taxa considered. We demonstrate this extension of the mid‐domain effect (MDE) to river systems for riparian plants along two rivers in Sweden, where a previous study found a middle‐course peak in richness of natural (non‐ruderal) species. We compare patterns of empirical richness of these species to null model predictions of species richness along the two river systems and to spatial patterns for six environmental variables (channel width, substrate fineness, substrate heterogeneity, ice scour, bank height, and bank area). In addition, we examine the independent prediction of mid‐domain effects models that species with large ranges, because the location of their ranges is more constrained, are more likely to produce a mid‐domain peak in richness than are species with small ranges. Species richness patterns of riparian plants were best predicted by models including both null model predictions and environmental variables. When species were divided into large‐ranged and small‐ranged groups, the mid‐domain effect was more prominent and the null model predictions were a better fit to the empirical richness patterns of large‐ranged species than those of small‐ranged species. Our results suggest that the peak in riparian plant species richness in the middle courses of the rivers studied can be explained by an underlying mid‐domain effect (driven by geometric constraints on large‐ranged species), together with environmental effects on richness patterns (particularly on small‐ranged species). We suggest that the mid‐domain effect may help to explain similar middle‐course richness peaks along other rivers.     

Contributions of airborne dispersal and dormant propagule recruitment to the assembly of rotifer and crustacean zooplankton communities in temporary ponds

相似文献   

Variation in soil seed bank species composition of a dry coniferous forest: spatial scale and sampling considerations

In order to assess the effects of disturbance on soil seed bank spatial structure, variation in seed bank species composition was analyzed at two spatial scales in mature (undisturbed) and recently disturbed stands of Interior Douglas-fir forest in south-central British Columbia. Coarse-scale analysis among stands 10–100 km apart using Multi-Response Permutation Procedures showed that individual mature stands were mainly characterized by unique seed bank species compositions. Disturbance had an homogenizing effect on seed bank species composition at the coarse scale––there was less variability among stands following disturbance by both low and high severity fires, and by non-salvage logging. In contrast, finer scale analysis among sampling units c. 10 m apart showed that heterogeneity in seed bank species composition was greater on severely burned and logged sites, commensurate with greater levels of soil disturbance, than on lightly burned and undisturbed sites. Despite the high intensity of seed bank sampling (sixty 25 cm² soil samples from each of 16 sites), species-area curves leveled off only when infrequent species were removed from the dataset. The number of seed bank samples required to account for the common species ranged from 84 on severely burned sites, to 196 on undisturbed sites; more than the 240 samples collected per disturbance class in this study would have been required to account for the remaining infrequent species. Overall, this study highlights the importance of sampling intensively within multiple stands to capture the variation in species composition inherent to these dry coniferous forest soil seed banks.     

Variation of scavenger richness and abundance between sites of high and low iceberg scour frequency in Ryder Bay,west Antarctic Peninsula

Physical disturbance, particularly from iceberg scour, is a major structuring force in polar benthic communities at shelf depths. Scouring kills and damages benthic organisms providing food for the abundant scavenging fauna of coastal Antarctic waters. This trophic group is likely to be strongly affected by changes in iceberg scouring. A baited underwater camera system was used to examine the distribution of scavenging fauna in relation to the spatial variation in exposure to iceberg impacts experienced at different iceberg scouring conditions and depths within Ryder Bay. The results indicate that the relationships between depth and scavenger abundance and assemblage composition differed between high and low scour sites. Scavenger abundance increased with depth at high scour sites and fell with depth a low scour sites. There was also significant difference in community composition between sites within each scouring condition. Scavenger species richness also exhibited an increase with depth at most sites consistent with the established pattern of declining iceberg scouring frequency with depth. Shannon–Wiener diversity increased with depth but significantly more steeply in highly scoured sites. Our results suggest that depth and exposure to icebergs interact to shape the scavenger community. The significant differences within the high and low scour groups suggest that other factors remain to be investigated and that there is probably a nonlinear relationship between scouring intensity and the favourability of a site for scavengers.     

Interaction between patch area and shape: lakes with different formation processes have contrasting area and shape effects on macrophyte diversity

Although both patch area and shape are key factors driving biodiversity in fragmented terrestrial landscapes, researchers have had limited and mixed success in documenting the effects of these two factors on aquatic ecosystems. Here we examined the effects of lake area and shape on macrophyte species richness in a lowland floodplain by considering the differences in lake types (i.e. marsh, oxbow, man-made lakes). We surveyed species richness of native macrophytes in 35 lakes including 11 marshes, 11 oxbows and 13 man-made lakes with various complex shapes covering from 0.25 to 46.3 ha. Model selection clearly supported the existence of interaction between area and shape effects: large-circular and small-complex lakes supported a higher macrophyte species richness, while it was lower in large-complex and small-circular lakes. Among the three lake types, marsh lakes were more circular and man-made lakes had more complex shapes, while oxbow lakes were intermediate between these two. Also, marsh lakes had positive species–area relationships, while man-made lakes had negative relationships. Our results suggest the opposing shape complexity and species–area relationships of these two contrasting lake types are the result of the interactions between lake area and shape. These results indicate that different lake types result in variations in their conservation value for preserving macrophyte diversity. We suggest that small complex-shaped patches (especially oxbow lakes), which are often given the lowest conservation priority in terrestrial ecosystems, cannot be disregarded when conserving macrophyte biodiversity in aquatic ecosystems.     

Long-Term Competitive Dynamics of Two Cryptic Rotifer Species: Diapause and Fluctuating Conditions

Life-history traits may have an important role in promoting species coexistence. However, the complexity of certain life cycles makes it difficult to draw conclusions about the conditions for coexistence or exclusion based on the study of short-term competitive dynamics. Brachionus plicatilis and B. manjavacasare two cryptic rotifer species co-occurring in many lakes on the Iberian Peninsula. They have a complex life cycle in which cyclical parthenogenesis occurs with diapausing stages being the result of sexual reproduction. B. plicatilis and B. manjavacasare identical in morphology and size, their biotic niches are broadly overlapping, and they have similar competitive abilities. However, the species differ in life-history traits involving sexual reproduction and diapause, and respond differently to salinity and temperature. As in the case of certain other species that are extremely similar in morphology, a fluctuating environment are considered to be important for their coexistence. We studied the long-term competitive dynamics of B. plicatilis and B. manjavacas under different salinity regimes (constant and fluctuating). Moreover, we focused on the dynamics of the diapausing egg bank to explore how the outcome of the entire life cycle of these rotifers can work to mediate stable coexistence. We demonstrated that these species do not coexist under constant-salinity environment, as the outcome of competition is affected by the level of salinity—at low salinity, B. plicatilis excluded B. manjavacas, and the opposite outcome occurred at high salinity. Competitive dynamics under fluctuating salinity showed that the dominance of one species over the other also tended to fluctuate. The duration of co-occurrence of these species was favoured by salinity fluctuation and perhaps by the existence of a diapausing egg bank. Stable coexistence was not found in our system, which suggests that other factors or other salinity fluctuation patterns might act as stabilizing processes in the wild.     

Hatching and viability of rotifer diapausing eggs collected from pond sediments

1. Planktonic rotifers inhabiting variable environments produce diapausing eggs that accumulate in the sediment of lakes and ponds, forming egg banks that may withstand adverse periods. A common assumption in zooplankton diapausing egg bank studies is to count as viable all eggs in the sediment that look healthy. This assumption should be challenged by asking how effectively ‘healthy‐looking’ eggs represent viable eggs. 2. In this study, viability of more than 1100 ‘healthy‐looking’ diapausing eggs belonging to the Brachionus plicatilis species complex was assessed in a laboratory hatching experiment. Eggs were collected at different depths from sediment cores obtained from 15 ponds located in coastal and inland areas of Eastern Spain. 3. Only approximately one half of the ‘healthy‐looking’ diapausing eggs hatched after incubation in experimental conditions. Almost all the hatchlings (99.4%) survived to maturity. The proportion of ‘healthy‐looking’ diapausing eggs that hatched varied among areas and among ponds within area, and substantially declined with sediment depth. Most of the hatchlings (88%) were obtained from the uppermost 2 cm of sediment. ‘Healthy‐looking’ eggs from upper sediment layers hatched after significantly shorter incubation times than eggs recovered from deeper layers. 4. Both decreased hatching success and increased incubation time for hatching with sediment depth suggest that older ‘healthy‐looking’ eggs are less responsive to hatching stimuli and could become unviable. However, the strong correlation found between the number of ‘healthy‐looking’ eggs and the number of hatchlings indicates that the abundance of ‘healthy‐looking’ eggs is a good index of egg bank viability.     

土壤种子库的分类系统和种子在土壤中的持久性

对国际上已经发表的10个土壤种子库分类系统的内容进行了总结和阐述,并对土壤种子库分类系统进行了评述,其中Thompson ＆ Grime在1979年提出的把土壤种子库分为短暂土壤种子库（Transient soil seed bank）和持久土壤种子库（Persistent seed bank）的二元分类系统以及Thompson等人提出的把土壤种子库分为（1）短暂土壤种子库,（2）短期持久土壤种子库（Short term persistent seed bank）,（3）长期土壤种子库（Long termp ersistent seed bank）的三元分类系统在生态学文献中已被广泛采用。在此分类的基础上产生了植物种子在土壤中的持久性（Persistence）概念,持久性是指植物的一种特性,是指植物的种子在土壤中能够存活超过1a的特性;植物种子的持久性被认为是一种对环境的进化适应,它可以在多个生长季节萌发从而分担环境震荡的风险,持久土壤种子库不仅在不稳定的环境里占有优势;即使在稳定的环境里,也被认为能够减少种内和种间的竞争;造成持久性的原因可分为环境因子和种子本身的特性比如休眠等两个方面,持久土壤种子库的出现使得土壤种子库的研究与进化生物学结合起来,使得土壤种子库的研究进入一个新的领域,更易激发人们的兴趣。关于种子的大小、形状及持久性的关系问题已经引起了相当的争论,基本上有4种格局：一是种子大小和形状与种子在土壤中的持久性有关,小而圆或扁的种子在土壤易存活持久;二是种子大小与种子在土壤中的持久性有关,小种子在土壤中易存活持久,但种子形状与持久性无关;三是种子大小、形状与种子在土壤中的持久性无关;四为较大的种子在土壤易存活持久,而种子形状与种子在土壤中的持久性无关。影响种子在土壤中的持久性因子比较复杂,总结过去的文献发现主要有以下几个因子：①种子的散布方式,②捕食,③植被的物种组成,④风,⑤土壤基质,⑥火,⑦干扰等。通过比较分析和研究,提出影响种子大小和在土壤中的持久性关系格局的关键因子是气候,特别是生态系统所在地的雨量;湿润气候下容易产生前两种格局,而干旱环境下的生境容易产生后两种格局。     

Role of ancient lakes in genetic and phenotypic diversification of freshwater snails

Endemic organisms of ancient lakes have been studied as models to understand processes of speciation and adaptive radiation. However, it remains unclear how ancient lakes play roles in genetic and phenotypic diversity of freshwater mollusks. In the present study, we focus on viviparid freshwater snails in the ancient lakes of East and Southeast Asia (Japan and China) to address this question. Using molecular phylogenetic analyses based on mitochondrial (COI, 16S) and nuclear genes (18S, 28S, H3), we show that patterns of species diversification in viviparid lineages. Colonization to ancient lakes occurred independently in China and Japan at least four times, with subsequent diversification into more than two species within each lake group. Morphological analyses of fossil related viviparids suggest parallel phenotypic evolution occurred in the different lakes and ages. Each lake contained a single lineage which was phenotypically diversified relative to those from other sites. Using genome‐wide SNPs obtained by MIG‐seq, we also examined the genetic structure of three Japanese viviparids, including two endemic species of ancient Lake Biwa. The results suggest that these two species diversified from the population of the third species living in wetlands surrounding the lake. These findings suggest that rapid diversification of lineages and phenotypic divergence can occur in ancient lakes compared to other habitats. Formation of large lakes probably promotes speciation and phenotypic divergence as a result of adaptation into different microhabitats. High numbers of ancient lakes could be a driver of species diversity in Asian viviparid snails.     

Flooding, soil seed bank dynamics and vegetation resilience of a hydrologically variable desert floodplain

1. This paper explores soil seed bank composition and its contribution to the vegetation dynamics of a hydrologically variable desert floodplain in central Australia: the Cooper Creek floodplain. We investigated patterns in soil seed bank composition both temporally, in response to flooding (and drying), and spatially, with relation to flood frequency. Correlations between extant vegetation and soil seed bank composition are explored with respect to flooding. 2. A large and diverse germinable soil seed bank was detected comprising predominantly annual monocot and annual forb species. Soil seed bank composition did not change significantly in response to a major flood event but some spatial patterns were detected along a broad flood frequency gradient. Soil seed bank samples from frequently flooded sites had higher total germinable seed abundance and a greater abundance of annual monocots than less frequently flooded sites. In contrast, germinable seeds of perennial species belonging to the Poaceae family were most abundant in soil seed bank samples from rarely flooded sites. 3. Similarity between the composition of the soil seed bank and extant vegetation increased following flooding and was greatest in more frequently flooded areas of the floodplain, reflecting the establishment of annual species. The results indicate that persistent soil seed banks enable vegetation in this arid floodplain to respond to unpredictable patterns of flooding and drying.