Edaphic niche differentiation among Polybotrya ferns in western Amazonia: implications for coexistence and speciation

To study the degree of edaphic specialization in Amazonian plants, the distribution patterns of seven species of Polybotrya ferns were studied in 109 sites in a climatically uniform area of northwestern Amazonia (Colombia, Ecuador and northern Peru). The two most abundant species of Polybotrya were found in about two-thirds of the sites with almost 7000 individuals each, the rarest species occurred in just one site with 40 individuals. Each of the seven species appeared to have a unique realised niche, when niche dimensions were defined by gradients in soil texture, soil cation content, and inundation. The species also differed in how broadly or narrowly they were distributed along each gradient. Some species were practically never found in the same sites, whereas others co-occurred with a high frequency, in spite of showing clearly different abundance patterns among sites. A single site only contains a small part of the edaphic variation present in the landscape, and a small proportion of any species' niche space, so broad-scale studies are needed to adequately describe and compare species' niches and to assess to what degree niche differences promote species coexistence. The distribution patterns in Polybotrya are consistent with, but do not prove, that ecological speciation may have been important in the radiation of the genus. If such a pattern is found to be common in other Amazonian plants, this would indicate that each evolutionary lineage has adapted to the available habitats largely independently of the others.     

Hybridization and ecology of Geum rivale L. and Geum urbanum L. (Rosaceae) at Malham Tarn, Yorkshire

Geum urbanum L. (Rosaceae) at Malham Tarn, Yorkshire. A large population of Geum rivale, G. urbanum and their hybrids at Malham Tarn, Yorkshire were examined morphometrically. Principal components analysis and cluster analysis of various floral and vegetative characters scored from 203 individuals completely separated the parent species. A few hybrids were grouped with G. urbanum , and some were grouped separately from either parent species, but many others were grouped with G. rivale. This confirmed subjective visual examination of the population, which suggested that while hybrids remained distinct from G. urbanum , the delimitation of hybrids from G. rivale was difficult and not practical. Measurements of soil water content and of transmitted photosynthetically active radiation (PAR) indicated that the habitat range of G. urbanum at Malham Tarn was limited to well-drained shady sites, while G. rivale occurred in sites with a very varied degree of shading and soil water content. Hybrids were most frequent and most variable in the well-drained and heavily shaded 'urbanum-type' habitats. Hybrids very similar to G. rivale were found in waterlogged sites, but in unshaded sites only G. rivale occurred, with little evidence of hybridization. The availability of intermediate 'hybrid' habitats was not a major factor in determining the pattern of morphological variation in the hybrid population, and a number of alternative hypotheses are suggested.     

土壤盐分对高寒草甸主要植物生态位的影响

采用Levins生态宽度指数和Cowell生态位相似性公式,计算了巴音布鲁克高寒草甸主要物种在4个盐分梯度上土壤水分、有机质、速效钾3个资源维的生态位宽度及生态位相似性.结果表明：在3个资源维上,随着土壤盐分含量的增加,主要建群种细果苔草的平均生态位由0.4433降至0.1740,伴生种鹅绒委陵菜的平均生态位由0.1263升至0.2215,说明耐盐性较差的物种生态位宽度逐渐减小,耐盐性较好的物种逐渐增大,可能成为重要的演替种.随土壤盐分含量的增加,耐盐性较差物种间的生态位相似性逐渐升高, 而耐盐性较好的物种则逐渐降低.细果苔草和线叶嵩草的生态位相似比例由0.701升至0.842, 而鹅绒委陵菜和山地蒲公英的生态位相似比例由1降至0.708.物种的生物学特性是物种对土壤盐分响应不同的主要原因.     

异质生境对荒漠草原植物群落组成和种群生态位的影响

为揭示荒漠草原不同植物种群生态位特征以及对有限资源的利用状况,以围栏内外不同土壤类型下植物群落为研究对象,运用经典生态位理论,探讨其群落组成和种群生态位特征。结果表明：在调查样方中共记录到15科36属42种植物,以豆科、禾本科、菊科和藜科的草本植物为主。风沙土生境中的植被生物量和密度最高,一年生植物生态位宽度值往往较高,以猪毛蒿的重要值最大。灰钙土生境中植被盖度较高,多年生植物生态位宽度值相对较高,以牛枝子和针茅的重要值最大。放牧会减少多年生草本的种类和数量,而一年生草本有所增加,对半灌木数量则影响不大。在不同生境下猪毛蒿生态位宽度和总宽度值均居首,是该区域主要优势种和典型的泛化种。在放牧和短期围栏内猪毛蒿与其他物种的生态位重叠指数均较高,但在长期围封样地中,特别是灰钙土生境下,重叠指数比重有所降低。基岩风化沉积土中物种间的重叠程度最大,而风沙土中则最小。猪毛蒿分布在生态位重叠指数DCA排序图的中心,说明其在群落中占有重要地位,对有限资源的竞争能力和对环境的适应能力较强。综上,生境的异质性是导致群落组成及生态位不同的关键因子,因此在荒漠草原进行植被恢复建设和保护措施时,应考虑不同土壤类型条件下各物种生态适应性,避免生态位功能冲突。     

Diurnal Activity Rhythms and Niche Differentiation in a Carrion Beetle Assemblage (Coleoptera: Silphidae) in Opava, the Czech Republic

Diurnal activity rhythms of carrion beetles (Coleoptera: Silphidae) were investigated in the surroundings of Opava in the Czech Republic. Beetles were collected in automatic pitfall traps at two-hour catch intervals during three one-month periods in spring (May), summer (July) and autumn (October) in forest and meadow habitats. Five Nicrophorinae species: N. vespillo, N. vespilloides, N. humator, N. fossor and N. investigator and three Silphinae species: Thanatophilus sinuatus, T. rugosus and Oiceoptoma thoracica occurred in the study sites. T. sinuatus, T. rugosus and O. Thoracica showed exclusively diurnal activity patterns. N. fossor and N. investigator were active from the late afternoon to the nightfall. The other carrion beetles did not show a restricted pattern of diurnal activity. Patterns of temporal utilisation among species were compared using Levins¹ measure of niche breadth and Horn's index of niche overlap. Lower niche overlap was found between species with different habitat preference. During the autumn period niche release was observed, due to lesser competition for carrion with species that only need feeding before hibernation.     

Seed bank,seed size and dispersal in moisture gradients of temporary pools in Southern France

Plants reduce risk of extinction due to unpredictable rainfall by soil seed banks, dispersal or large seeds. However, seed size also increases independently in dry habitats, and since seed size is in a trade-off with seed number, size of seed banks is expected to increase in moister habitats. Therefore, we wanted to test if seed abundance in soil increases in wet habitats, if seed size increases in dry habitats, and if spread of seeds along the gradient is higher for plants of intermediate habitats in local moisture gradients.We studied 15 temporary pools in three biogeographically separated wetlands in Southern France. For each pool we studied five different moisture levels, totalling 75 local plant communities. We quantified soil seed bank by the seedling emergence method, seed size and an index of spatial spread of seeds in the soil for every species. We also quantified water levels for each plot.We found increasing abundance of seeds in the soil with increasing water levels but lower seed size and higher spread at intermediate water levels. When we controlled for niche position, we found no trade-off between seed size, spread and abundance in the soil seed bank.Type and importance of risk reduction strategies thus appeared to be strongly driven by the plant species’ moisture niche and the spatial arrangement of water levels.     

长白山地区曲尾藓属植物生态分化的排序研究

曲尾藓属(ＤｉｃｒａｎｕｍＨｅｄｗ.)属于顶蒴藓类植物,在长白山地区主要分布于苔原、森林林下和沼泽地中[1,2]。分析曲尾藓属植物对不同生态环境的适应特点和种间生态关系,对揭示该属植物物种分化的生态学机制,有一定学术价值。排序是分析植物群落分布与环境关系的常用方法,以往人们主要是应用ＤＣＡ、ＰＣＡ等方法,因而不同种类与环境关系的解释带有较大的主观性。典范对应分析能够在同一排序图上展示植物种类与环境因子的关系,是当前最有效的排序方法,其结果的生态学解释比较客观。Ｓｌａｃｋ指出应用排序也能够研究植物种间的生态位分…     

Resource availability determines the importance of niche‐based versus stochastic community assembly in grasslands

Niche‐based selection and stochastic processes can operate simultaneously to generate spatial and temporal variation in species composition. Yet, the conditions under which ecological dynamics are dominated by niche‐based versus stochastic processes are poorly understood. Using a field experiment in early‐successional temperate grassland and null models of beta diversity, this study investigates the effects of soil nutrient supply on the relative importance of niche‐based selection versus stochastic dynamics for variation in species composition among sites. Nutrient availability was manipulated experimentally, individual seed mixtures with 25 species were sown in each experimental plot, and then stochastic and deterministic niche‐based assembly processes were allowed to happen. We found that compositional variation among grassland plots with low nutrient supply was driven by stochastic immigration and extinctions. In contrast, nutrient enrichment reduced the importance of stochasticity and imposed a deterministic environmental filter that homogenized communities through the selection of few species with greater competitive ability for light. This demonstrates that soil nutrient availability is a critical environmental feature that dictates the degree to which terrestrial plant communities are controlled by niche‐based selection versus stochastic assembly processes. Our study shows further that alternative states of eutrophic grasslands emerge from initial stochastic variation in the composition of a particular functional group of species that can become dominant at high nutrient supply. We discuss potential mechanisms underlying the shift from stochastic to niche‐driven dynamics along soil nutrient gradients.     

Water availability,fundamental niches and realized niches: A case study from the Cape flora

The ability of plants to survive drought or waterlogging constitutes an important niche parameter, which might be particularly significant in explaining species coexistence in the species‐rich and seasonally dry Cape Floristic Region of South Africa. However, the degree of physiological adaptation and specialization to these eco‐hydrological parameters (the fundamental niche) cannot be readily inferred from correlative studies based on species distributions and spatial variation in environmental parameters (the realized niche). We used an ex situ greenhouse experiment to compare the fundamental hydrological niches (different mean annual precipitation, rainfall seasonality and soil drainage) of six eco‐hydrologically divergent African Restionaceae species. Juvenile plants were subjected to six different watering treatments, ranging from no watering to waterlogging, to determine drought and waterlogging susceptibility and optimal growth conditions. We used the rate of biomass accumulation and survival rate as response measures. We found that species from dry and mesic (but well‐drained) habitats had optimal or near‐optimal growth at benign conditions (under which most restio species grow well). All species performed worse when droughted and died when not watered. Species from dry habitats tended to perform better (assessed in growth) than species from wet habitats under droughting. Species from wet habitats performed best when waterlogged, whereas species from dry habitats performed very poorly when waterlogged – thus showing that realized and fundamental niches covaried at the wet end of the hydrological gradient. We conclude that eco‐hydrological parameters are part of the fundamental niche, and fundamental and realized species niches are approximately correlated along them. The distribution of wet habitat species appears not to reflect their drought tolerance, suggesting that it may not be predicted by bioclimatic variables, but rather by soil drainage characteristics.     

Morphological and physiological differentiation of seedlings between dry and wet habitats in a tropical dry forest

A common observation in tropical dry forests is the habitat preference of tree species along spatial soil water gradients. This pattern of habitat partitioning might be a result of species differentiation in their strategy for using water, along with competing functions such as maximizing water exploitation and tolerating soil water stress. We tested whether species from drier soil conditions exhibited a tolerance strategy compared with that of wet-habitat species. In a comparison of 12 morphophysiological traits in seedlings of 10 closely related dry and wet-habitat species pairs, we explored what trade-offs guide differentiation between habitats and species. Contrary to our expectations, dry-habitat species showed mostly traits associated with an exploitation strategy (higher carbon assimilation capacity, specific leaf area and leaf-specific conductivity and lower water-use efficiency). Strikingly, dry-habitat species tended to retain their leaves longer during drought. Additionally, we detected multiple strategies to live within each habitat, in part due to variation of strategies among lineages, as well as functional differentiation along the water storage capacity-stem density (xylem safety) trade-off. Our results suggest that fundamental trade-offs guide functional niche differentiation among tree species expressed both within and between soil water habitats in a tropical dry forest.     

Among‐species overlap in rodent body size distributions predicts species richness along a temperature gradient

Temperature is widely regarded as a major driver of species richness, but the mechanisms are debated. Niche theory suggests temperature may affect richness by filtering traits and species in colder habitats while promoting specialization in warmer ones. However, tests of this theory are rare because niche dimensions are challenging to quantify along broad thermal gradients. Here, we use individual‐level trait data from a long‐term monitoring network spanning a large geographic extent to test niche‐based theory of community assembly in small mammals. We examined variation in body size among 23 communities of North American rodents sampled across the National Ecological Observatory Network (NEON), ranging from northern hardwood forests to subtropical deserts. We quantified body size similarity among species using a metric of overlap that accounts for individual variation, and fit a structural equation model to disentangle the relationships between temperature, productivity, body size overlap, and species richness. We document a latitudinal gradient of declining similarity in body size among species towards the tropics and overall increase in the dimensions of community‐wide trait space in warmer habitats. Neither environmental temperature nor net primary productivity directly affect rodent species richness. Instead, temperature determines the community‐wide niche space that species can occupy, which in turn alters richness. We suggest a latitudinal gradient of trait space expansion towards the tropics may be widespread and underlie gradients in species diversity.     

Water source niche overlap increases with site moisture availability in woody perennials

Classical niche partitioning theory posits increased competition for and partitioning of the most limiting resource among coexisting species. Coexisting plant species may vary in rooting depth, reflecting niche partitioning in water source use. Our goal was to assess the soil water partitioning of woody plant communities across northern Arizona along an elevational moisture gradient using stem and soil water isotopes from two sampling periods to estimate the use of different water sources. We hypothesized that niche overlap of water sources would be higher and monsoon precipitation uptake would be lower at sites with higher moisture availability. Pairwise niche overlap of coexisting species was calculated using mixing model estimates of proportional water use for three sources. Across the moisture gradient, niche overlap increased with site moisture index (precipitation/potential evapotranspiration) across seasons, and site moisture index explained 37% of the variation in niche overlap of intermediate and deeper sources of water. Desert trees utilized more winter source water than desert shrubs, suggesting the partitioning of water sources between functional groups. However, seasonal differences in surface water use were primarily found at intermediate levels of site moisture availability. Our findings support classical niche partitioning theory in that plants exhibit higher overlap of water sources when water is not a limiting resource.     

Conservation and possible reintroduction of an endangered plant based on an analysis of community ecology: a case study of Primulina tabacum Hance in China

The distribution of the rare and endangered perennial herb Primulina tabacum Hance is restricted to eight karst caves in southern China. To conserve P. tabacum and to evaluate possible reintroduction, we studied its historical distribution and conducted field surveys of both its biotic and physical environment. We used detrended correspondence analysis and canonical correspondence analysis to investigate the plant community structure and to identify the major environmental factors associated with its presence at eight sites and absence from three other sites where it formerly grew. The results revealed differences in community structure among sites and close correlations between species composition and environmental conditions. The data indicate that P. tabacum has special ecological requirements, including alkaline soil, low soil content of N, P, K and organic matter, low light, a high soil water content and high relative humidity, and a high atmospheric CO₂ concentration. Soil K content, relative humidity and atmospheric CO₂ concentration were the three variables most strongly associated with the vegetation composition and structure of the 11 sites. It appears that as a result of reductions in humidity, some of the historical localities of P. tabacum may no longer be suitable for reintroduction. Pilea notata , Pteris cretica var. nervosa and two moss species, Heteroscyphus coalitus and Gymnostomiella longinervis , were strongly associated with P. tabacum . The first two plants could be useful as indicators of suitable environments for P. tabacum , and the moss species could be useful as nurse plants for the reintroduction of P. tabacum into wild habitats.     

Salt marsh vegetation of the Western Mediterranean desert of Egypt

The extensive salt marshes in the plains and depressions of the western Mediterranean desert of Egypt were classified into three habitat types: A with shallow water table and high salinity, B with relatively deep water table and high salinity, and C transitional habitats in which salinity and water table are no controlling factors. Fourteen vegetation types were distinguished, each dominated by one or two species.The dominating life forms are chamaephytes in sites of high salinity, and therophytes in sites of low salinity. Spatial and temporal variations in the standing crop biomass were pronounced. The accumulation of material started during spring and reached a maximum in autumn, when photosynthetic activity was maintained to account for transpiration losses.There was a general trend of increasing salinity and concentration of different ions from habitat type A to habitat type C through habitat type B. The periodical variation in the water table was insignificant, while a significant drop in salinity and the concentration of different ions was detected in spring, which was attributed to the diluting effect of rain water during that season.Most species exhibited clear distribution patterns and their a bundance varied significantly along gradients of different factors. Simple correlations between the compositional gradients or the distributional behaviour of species and salinity or the concentration of individual ions were generally low, while correlations with combinations of ions in the form of ratios (notably sodium and potassium adsorption ratos) were higher.During the early stages of succession the building up of soil and the decrease in salinity are the most important factors while at more advanced stages, soil texture and calcium carbonate content become more decisive.Nomenclature follows Täckholm (1974)     

Cold tolerance of micro-arthropods from Alaskan taiga

Abstract. Mean supercooling points for a variety of soil and litter arthropods including mites, springtails, a heteropteran and immature spiders from a central Alaskan taiga site ranged from -6.3 to -28.5°C during autumn. Variation in supercooling ability of five species of cryptostigmatid mites occurred throughout the year with increased cold tolerance in autumn and early winter concomitant with the temperature pattern of the habitat. No correlation between the level of supercooling and water content of the mites was evident. Changes in the frequency distribution of individual supercooling points occurred in autumn, winter, spring and summer samples which were species specific. All arthropods tested were susceptible to freezing, and the mites utilize supercooling to avoid freezing.     

An estimation of the natural value of dune habitats using Empidoidea (Diptera)

The dune landscape along the Belgian coast was evaluated on the basis of its Empidoidea fauna by investigating the faunal composition in different habitat types. The sites selected for sampling were marram dunes, dune grassland, dune slack, scrubby vegetations and three different dune woodlands. White water traps at soil surface level were used to sample five sites from the end of April until November 1989, and four sites from April to August 1992. Data on the most abundant Dolichopodidae, Empididae and Hybotidae were analysed by means of the following multivariate analyses: detrended correspondence analysis (DCA), canonical correspondence analysis (CCA) and two-way indicator species analysis (TWINSPAN). Most sites showed a distinctive species composition, and several species were confined to particular habitat types. Empidoid communities from canopied and open habitats showed the largest contrasts during 1989. Six environmental variables were measured in July and August 1992, and correlations indicated that aerial humidity and, to a lesser extent, soil moisture were probably the main factors explaining dolichopodid distributions. In Empididae, on the contrary, the presence of canopy by scrub or trees appeared to be important for the occurrence of many species. Thus species diversity and abundance increased from dry to humid sites in dolichopodids and from open to canopied sites in empidids. On the other hand, canopied sites appeared to function as home-bases from which certain empidid species colonized more sun-exposed areas. With the Dolichopodidae it was possible to examine the recorded distribution of all species throughout Belgium. Nearly 75% of the dolichpodids appeared to be typical lowland species and in eight species, largest Belgian populations were established in the dune region. Moreover, seven species could be considered as true coastal species as they were almost entirely restricted to this region. These results clearly stress that not merely the coastal dunes themselves but the full diversity of the associated habitats needs to be maintained in order to protect the characteristic dune empidoid communities.     

Effect of soil humidity on the survival of Solenopsis invicta Buren workers

The effect of soil humidity on the survival of Solenopsis invicta (Buren) workers was evaluated in this study. The study showed that the relative soil water (RSW) content inside the mound contained less variation than the surrounding soil. At a depth of 5 cm underground, the RSW was 7.6% in the mound, while it was 29% in the surrounding soil. At a depth of 20 cm underground, the RSW was 99% in the surrounding soil, while in the mound, the RSW did not reach 98% until it was 45-cm deep. The soil humidity affected the survival rate of S. invicta. For workers, the survival rate decreased when they were exposed to a higher RSW. The spring population had a higher tolerance to a high RSW than the autumn population, while the drought tolerance was the opposite. In extreme RSW, the longer that S. invicta was exposed, the lower their survival rate. The drought tolerance of the fire ant workers could be improved if they were pre-exposed to a low non-lethal RSW for a short period of time. The water content of the workers changed after acclimation to humidity. After a low RSW treatment, the water content of the acclimated workers was higher than the control workers. This suggests that this species is able to maintain a certain water content after acclimation, and that the water content of workers increases in accordance with the RSW.     

Increased propensity for aerial dispersal in disturbed habitats due to intraspecific variation and species turnover

Animal dispersal depends on multiple factors, such as habitat features and life‐history traits of the species. We studied the propensity for ballooning dispersal in spiders under standardized laboratory conditions. The 1269 tested individuals belonged to 124 species and originated from 16 sites with wide variation in habitat type. Spiders from disturbed habitats ballooned 5.5 times more than spiders from stable habitats. In Meioneta rurestris , for which we had enough data for a single‐species analysis, individuals were most dispersive if they originated from highly disturbed habitats. While the data for the other species were not sufficient for single‐species analyses, a hierarchical model that included the data simultaneously on all species suggested that dispersal propensity generally increases within species with the level of habitat disturbance. Dispersal probability showed a trend to increase with niche width, but the higher commonness of species with wide niches provides an alternative explanation for this pattern. As the prevalence of especially dispersive species was highest in disturbed habitats, variation in dispersal propensity was influenced by both inter‐ and intraspecific factors. We conclude that the positive correlation between niche width and dispersal propensity enables generalist species to utilize highly disturbed habitats, whereas the persistence of specialist species with restricted dispersal ability requires the conservation of stable habitats.     

砒砂岩区不同立地类型人工油松林下草本种群生态位特征及其环境解释

黄土高原砒砂岩区是我国水土流失最为严重的地区之一,油松(Pinus tabuliformis)是该区水土保持与水源涵养的优势乔木。以几种不同立地类型人工油松林为对象,应用Levins生态位宽度指数、Pianka生态位重叠指数和冗余度分析(RDA),研究林下草本群落生态位特征及其与环境的相互响应。结果表明:所调查的150个油松林下草本样方中,共发现31种草本植物,隶属于11科24属,阳性旱生植物居多。不同立地类型人工油松林下草本群落生态位特征不同,生态位宽度指数及生态位重叠指数均表现阳坡大于阴坡,平坡好于斜缓坡,但整体指数均较小,表明物种对环境资源利用程度较低,种间竞争度不大,群落处于相对稳定状态。通过对比典范对应分析(CCA)及冗余度分析(RDA),RDA排序更能较好揭示生态位特征与环境因素的关系,得出乔灌层郁闭度、土壤有机质、土壤含水率和坡度是林下草本群落生态位特征各异的主导环境因子。     

Macrohabitat associations of fishes in shallow waters of the Detroit River

Seasonal variation in large‐scale habitat selection by fishes in shallow, Canadian waters of the Detroit River was examined. Fish communities were compared among three river segments (upstream, middle and downstream) consisting of areas of shallow water habitat separated by wide hydrologic barriers of deep, flowing water and between inshore and offshore areas. In spring, the most unique, diverse and abundant fish assemblages were found at inshore sites in the middle segment where the largest remaining wetland habitats are located. Fishes used inshore habitat to spawn and probably avoided offshore areas because macrophyte cover was not available in spring. In summer, juvenile gizzard shad Dorosoma cepedianum and white bass Morone chrysops were observed in high densities in the upstream segment, probably migrating downstream from Lake St Clair. There was little difference in the fish assemblage among macrohabitats in autumn. The upstream segment appeared to be the most degraded, because it contained no species that were not found in the other segments, had a paucity of uncommon species and had significantly more non‐native species. This state was attributed to the infilling of coastal wetlands by urban land use and a resulting loss of habitat heterogeneity. The middle segment, with the only remaining wetland habitats, had the greatest occurrence of uncommon species and the only species at risk found in this study. Conservation and restoration efforts should be greatest for wetlands; however, shallow offshore areas provide important fish habitat in summer and autumn.