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1.
Aerial surveys on seagrass (Zostera spp.) indicate a three to fourfold increase in bed area from 1994 to 2006 with up to 100 km2 or 11% of intertidal flats in the Northfrisian Wadden Sea (coastal eastern North Sea), observed at seasonal maximum in August when flying during low tide exposure 300 to 500 m above ground. When viewed from the air, difficulties in distinguishing between seagrass and green algae and a lack of contrast on dark-coloured mudflats are sources of error in areal estimates. Particularly the positioning of beds remote from shores was imprecise. However, the consistency in method over time gives confidence to the inferred positive trend which is opposite to the global pattern. Both, the spatial pattern and a recent decrease in storminess suggest that sediment stability is the key factor for seagrass dynamics in this tidal area. On exposed sand flats, high sediment mobility may be limiting and along the sheltered mainland shore land claim activities with high accretion rates may cause a scarcity of seagrass. The potential area of seagrass beds may be twice as large as the realized maximum in 2006 but eventually the rising sea level will reverse the observed seagrass expansion.  相似文献   

2.
Self-organization and complexity in historical landscape patterns   总被引:3,自引:0,他引:3  
Self‐organization describes the evolution process of complex structures where systems emerge spontaneously, driven internally by variations of the system itself. Self‐organization to the critical state is manifested by scale‐free behavior across many orders of magnitude (Bak et al. 1987, Bak 1996, Solé et al. 1999). Spatial scale‐free behavior implies fractal properties and is quantified by the fractal dimension. Temporal scale‐free behavior is evident in power spectra of fluctuations that obey power laws. Self‐organized criticality is a universal phenomenon that likely produces some of the fractals and power laws observed in nature. We investigated the historical landscape of southern Wisconsin (USA) (60,000 km2) for self‐organization and complexity. The landscape is patterned into prairies, savannas, and open and closed forests, using data from the United States General Land Office Surveys that were conducted during the 19th century, at a time prior to Euro‐American settlement. We applied a two‐dimensional cellular automaton model with one adjustable parameter. Model evolution replaces a cell that dies at random times by a cell chosen randomly from within a circular radius r, where r typically takes values between 1 (local) and 10 units (regional). Cluster probability is used to measure the degree of organization. The model landscape self‐organizes to a realistic critical state if neighborhoods of intermediate size (r=3) are chosen, indicating that (a) no particular time or space scale for the clusters is singled out, i.e. the spatial dependence is fractal, and temporal fluctuations in the cluster probability exhibit power laws; (b) a simple model suffices to replicate the landscape pattern resulting from complex spatial and temporal interactions. Measures of comparison between the observed and the simulated landscape show good agreement: fractal dimensions for simulated (1.6) and observed landscapes (1.64), cluster probabilities for simulated (32.3%) and observed (32.6%) landscapes, and algorithmic complexity for simulated (6792 bytes) and observed (6205 bytes) landscapes. The results are robust towards variation of initial and boundary conditions as well as perturbations.  相似文献   

3.
The majority of field experiments have been carried out on relatively small spatial and short temporal scales, but some of the most interesting ecological processes operate at much larger scales. However, large-scale experiments appropriate to the landscape, often have to be carried out with minimal plot replication and hence reduced statistical power. Here, we report the results of such a large scale, un-replicated field experiment on the Mondego estuary, Portugal, which nevertheless provides compelling evidence of the importance of habitat structure for invertebrate community composition and dynamics. In this estuary, seagrass beds have suffered a dramatic decline over the last 20 years, associated with changes in invertebrate assemblages. In addition, the most abundant species in the system, Hydrobia ulvae, displays distinctly different population structures in those sites. The aim of the field experiment was to test the hypothesis that these differences are related to enhanced survival of snails due to protection from avian or fish predators so that they can grow to larger body sizes in the more complex habitat provided by seagrass. We tested this hypothesis through a large-scale experiment using artificial seagrass beds over a 12-month period. Adult snail densities were higher in the artificial bed plots compared to controls. However, these differences emerged only slowly, related to snail growth rate. This suggests that protection from epibenthic predators can have a significant effect on population structure and hence biomass and productivity of key species in this system. However, the invertebrate assemblage in artificial seagrass plots and the natural seagrass bed, remained statistically separate by the end of the experiment. Handling editor: K. Martens  相似文献   

4.
Seagrass habitat structure influences epifaunal density, diversity, community composition and survival, but covariation of structural elements at multiple scales (e.g., shoot density or biomass per unit area, patch size, and patch configuration) can confound studies attempting to correlate habitat structure with ecological patterns and processes. In this study, we standardized simulated seagrass shoot density and bed area among artificial seagrass beds in San Diego Bay, California, USA to evaluate the singular effect of seagrass bed configuration (“patchiness”) on the density and diversity of seagrass epifauna. Artificial seagrass beds all were 1 m2, but were composed of a single large patch (“continuous” treatment), four smaller patches (“patchy” treatment), or 16 very small patches (“very patchy” treatment). We allowed epifauna to colonize beds for 1 month, and then sampled beds monthly over the next 3 months. Effects of seagrass bed patchiness on total epifaunal density and species-specific densities were highly variable among sampling dates, and there was no general trend for the effects of fragmentation on epifaunal densities to be positive or negative. Epifaunal diversity (measured as Simpson's index of diversity) was highest in very patchy or patchy beds on two out of the three sampling dates. Very patchy beds exhibited the highest dissimilarity in community composition in the first two sampling periods (August and September), but patchy beds exhibited the highest dissimilarity in the third sampling period (October). Our results indicate that seagrass patch configuration affects patterns of epifaunal density, diversity, and community composition in the absence of covarying bed area or structural complexity, and that patchy seagrass beds may be no less valuable as a habitat than are continuous seagrass beds. The spatial pattern employed when harvesting or planting seagrass may influence epifaunal habitat use and should be a key consideration in restoration plans.  相似文献   

5.
Recent research has identified a need for seagrass habitat management plans to be based on landscape-level approaches as they offer a more appropriate scale for large mobile fauna than smaller scales. Also, conservation decisions are more likely to be a choice between different seagrass beds rather than parts of individual beds. The present study examined the spatial utilisation of subtidal seagrass beds by fish around the coast of Jersey, English Channel (49°N 02°W) with the aim of identifying influential scales of complexity. A hierarchical-scale of landscape configuration and habitat characteristics was measured for eight seagrass beds using aerial photographic analysis (e.g. core area, contiguity and other landscape metrics), digital echo-sounder data (e.g. depth, canopy height) and diver surveys (e.g. epiphyte load). The contributions of these variables as predictors of functional fish groups were explored using multiple linear regression models. Results indicated that more fragmented seagrass beds supported lower numbers of fish species than more homogenous seagrass landscapes (squared semi-partial correlation coefficient, sr2 = − 0.3). Densities of juveniles of larger fish species showed a negative relationship with increased fragmentation of the seagrass (sr2 = − 0.34). At smaller scales of structural complexity the densities of cryptic fish were related positively to canopy height (sr2 = 0.46). At night, fewer patterns could be explained by the independent variables in the model, which was attributed to the greater movement of fish between the seagrass and adjacent habitats to forage, and a breakdown in the association with seagrass habitat as a refuge from predation.  相似文献   

6.
Geomorphology and fish assemblages in a Piedmont river basin, U.S.A.   总被引:7,自引:0,他引:7  
1. We investigated linkages between fishes and fluvial geomorphology in 31 wadeable streams in the Etowah River basin in northern Georgia, U.S.A. Streams were stratified into three catchment sizes of approximately 15, 50 and 100 km2, and fishes and geomorphology were sampled at the reach scale (i.e. 20–40 times stream width). 2. Non‐metric multidimensional scaling (NMDS) identified 85% of the among‐site variation in fish assemblage structure and identified strong patterns in species composition across sites. Assemblages shifted from domination by centrarchids, and other pool species that spawn in fine sediments and have generalised food preferences, to darter‐cyprinid‐redhorse sucker complexes that inhabit riffles and runs, feed primarily on invertebrates, and spawn on coarser stream beds. 3. Richness and density were correlated with basin area, a measure of stream size, but species composition was best predicted (i.e. |r| between 0.60–0.82) by reach‐level geomorphic variables (stream slope, bed texture, bed mobility and tractive force) that were unrelated to stream size. Stream slope was the dominant factor controlling stream habitat. Low slope streams had smaller bed particles, more fines in riffles, lower tractive force and greater bed mobility compared with high slope streams. 4. Our results contrast with the ‘River Continuum Concept’ which argues that stream assemblages vary predictably along stream size gradients. Our findings support the ‘Process Domains Concept’, which argues that local‐scale geomorphic processes determine the stream habitat and disturbance regimes that influence stream communities.  相似文献   

7.
To clarify differences in community structures and habitat utilization patterns of fishes in Enhalus acoroides- and Thalassia hemprichii-dominated seagrass beds on fringing coral reefs, visual censuses were conducted at Iriomote and Ishigaki islands, southern Japan. The numbers of fish species and individuals were significantly higher in the E. acoroides bed than in the T. hemprichii bed, although the 15 most dominant fishes in each seagrass bed were similar. Cluster and ordination analyses based on the number of individuals of each fish species also demonstrated that fish community structures were similar in the two seagrass beds. Species and individual numbers of coral reef fishes which utilized the seagrass beds numbered less than about 15% of whole coral reef fish numbers, although they comprised about half of the seagrass bed fishes. Of the 15 most dominant species, 5 occurred only in the two seagrass beds, including seagrass feeders. Ten other species were reef species, their habitat utilization patterns not differing greatly between the two seagrass beds. Some reef species, such as Lethrinus atkinsoni and L. obsoletus, showed ontogenetic habitat shifts with growth, from the seagrass beds to the coral areas. These results indicate that community structures and habitat utilization patterns of fishes were similar between E. acoroides- and T. hemprichii-dominated seagrass beds, whereas many coral reef fishes hardly utilized the seagrass beds.  相似文献   

8.
Predicting Forest Microclimate in Heterogeneous Landscapes   总被引:1,自引:0,他引:1  
Forest microclimate plays an integral role in ecosystem processes, yet a predictive understanding of its spatial and temporal variability in heterogeneous landscapes is largely lacking. In this study, we used regression kriging (RK) to analyze the degree to which physiographic versus ecological variables influence spatio-temporal variation in understory microclimate conditions. We monitored understory temperature in 200 forest plots within a 274 km2 environmentally heterogeneous region in northern California (0.55 obs/km2). For each plot location, we measured four physiographic influences (elevation, coastal proximity, potential solar radiation, topographic wetness index) and three ecological drivers (forest patch size, proximity to forest edge, tree abundance). Temperature observations were aggregated to three time scales (hourly, daily, and monthly) to examine temporal variability in microclimate dynamics and its effect on spatial prediction. The obtained prediction models included both physiographic and vegetative effects, although the relative importance of individual effects varied greatly between the different models. Across time scales, elevation and coastal proximity had the most consistent physiographic effects on temperature, followed by the vegetative effects of forest patch size and distance to forest edge. RK captured significantly more landscape-scale variability in understory temperature than a regression-only approach with considerably better model performance at hourly and daily time scales than at a monthly scale. Using varied sampling density scenarios our results also suggest that predictive accuracy drops considerably at densities less than 0.34 obs/km2. This research illustrates how geospatial and statistical modeling can be used to distinguish physiographic versus ecological effects on microclimate dynamics and elucidates the spatial and temporal scales that these processes operate.  相似文献   

9.
Photosynthesis of most seagrass species seems to be limited by present concentrations of dissolved inorganic carbon (DIC). Therefore, the ongoing increase in atmospheric CO2 could enhance seagrass photosynthesis and internal O2 supply, and potentially change species competition through differential responses to increasing CO2 availability among species. We used short‐term photosynthetic responses of nine seagrass species from the south‐west of Australia to test species‐specific responses to enhanced CO2 and changes in HCO3?. Net photosynthesis of all species except Zostera polychlamys were limited at pre‐industrial compared to saturating CO2 levels at light saturation, suggesting that enhanced CO2 availability will enhance seagrass performance. Seven out of the nine species were efficient HCO3? users through acidification of diffusive boundary layers, production of extracellular carbonic anhydrase, or uptake and internal conversion of HCO3?. Species responded differently to near saturating CO2 implying that increasing atmospheric CO2 may change competition among seagrass species if co‐occurring in mixed beds. Increasing CO2 availability also enhanced internal aeration in the one species assessed. We expect that future increases in atmospheric CO2 will have the strongest impact on seagrass recruits and sparsely vegetated beds, because densely vegetated seagrass beds are most often limited by light and not by inorganic carbon.  相似文献   

10.
11.
Material exchange, biodiversity and trophic transfer within the food web were investigated in two different types of intertidal seagrass beds: a sheltered, dense Zostera marina bed and a more exposed, sparse Z. noltii bed, in the Northern Wadden Sea. Both types of Zostera beds show a seasonal development of above-ground biomass, and therefore measurements were carried out during the vegetation period in summer. The exchange of particles and nutrients between seagrass beds and the overlying water was measured directly using an in situ flume. Particle sedimentation [carbon (C), nitrogen (N) and phosphorus (P) constituents] from the water column prevailed in dense seagrass beds. In the sheltered, dense seagrass bed, a net particle uptake was found even on windy days (7–8 Beaufort). Dissolved inorganic N and orthophosphate were mainly taken up by the dense seagrass bed. At times of strong winds, nutrients were released from the benthic community to tidal waters. In a budget calculation of total N and total P, the dense seagrass beds were characterised as a material sink. The seagrass beds with sparse Z. noltii were a source of particles even during calm weather. The uptake of dissolved inorganic N in the sparse seagrass bed was low but significant, while the uptake of inorganic phosphate and silicate by seagrasses and their epiphytes was exceeded by release processes from the sediment into the overlying water. Estimates at the ecosystem level showed that material fluxes of seagrass beds in the Sylt-Rømø Bight are dominated by the dense type of Zostera beds. Therefore, seagrass beds act as a sink for particles and for dissolved inorganic nutrients. During storms, seagrass beds are distinct sources for inorganic nutrients. The total intertidal area of the Sylt-Rømø Bight could be described as a sink for particles and a source for dissolved nutrients. This balance of the material budget was estimated by either including or excluding seagrass beds. Including the subtidal part, the function of the ecosystem as a source for particles increased, supposing that all seagrass beds were lost from the area. During the vegetation period, seagrass beds act as a storage compartment for material accumulated in the living biomass of the community. There was great biodiversity among the plant and animal groups found in intertidal seagrass beds of the Sylt-Rømø Bay, representing 50–86% of the total number of species investigated, depending on the particular group. Since most species are not exclusively seagrass residents, the loss of intertidal seagrass beds would be of minor importance for biodiversity at the ecosystem level. Food web structure in seagrass beds is different from other intertidal communities. Primary production and detritus input is high, but secondary production is similar to that of unvegetated areas, although the relative importance of the trophic guilds is different. The loss of seagrass beds leads to profound alterations in the food web of the total ecosystem. Historical as well as recent changes in material fluxes and energy flow due to man-made alterations to the ecosystem are discussed.  相似文献   

12.
Genetic structure and diversity can reveal the demographic and selective forces to which populations have been exposed, elucidate genetic connections among populations, and inform conservation strategies. Beds of the clonal marine angiosperm Zostera marinaL. (eelgrass) in Chesapeake Bay (Virginia, USA) display significant morphological and genetic variation; abundance has fluctuated widely in recent decades, and eelgrass conservation is a major concern, raising questions about how genetic diversity is distributed and structured within this metapopulation. This study examined the influence of bed age (<65years versus<6years) and size (>100ha versus<10ha) on morphological and genetic (allozyme) structure and diversity within Chesapeake Bay eelgrass beds. Although both morphology and genetic diversity varied significantly among individual beds (F ST=0.198), neither varied consistently with bed age or size. The Chesapeake eelgrass beds studied were significantly inbred (mean F IS=0.680 over all beds), with inbreeding in old, small beds significantly lower than in other bed types. Genetic and geographic distances within and among beds were uncorrelated, providing no clear evidence of isolation by distance at the scale of 10's of km. These results suggest that local environmental conditions have a greater influence on plant morphology than do bed age or size. They support the hypotheses that eelgrass beds are established by multiple founder genotypes but experience little gene flow thereafter, and that beds are maintained with little loss of genetic diversity for up to 65 years. Since phenotypic and genotypic variation is partitioned among beds of multiple ages and sizes, eelgrass conservation efforts should maximize preservation of diversity by minimizing losses of all beds.  相似文献   

13.
海草是分布在全球海岸带的沉水被子植物,与周围环境共同形成的海草床生态系统是三大典型海洋生态系统之一,具有十分重要的生态功能。20世纪以来,全球海草床衰退严重,研究海草床的生态修复迫在眉睫,现有修复方法未能足够重视微生物在海草床中的重要作用。本文综合阐述了微生物在海草床生态系统有机物矿化和营养流动过程中起到的作用,分析了微生物驱动下的海草床水体与沉积物之间的元素循环,提出了人类活动引起海草床退化的原因,总结了海草床微生物的系统研究方法,并在此基础上提出从微生物生态的角度修复海草床的新思路。  相似文献   

14.
《Aquatic Botany》1987,27(1):3-14
The greatest decline of seagrass beds that is known is certainly the almost simultaneous breakdown of the North Atlantic populations of Zostera marina L., due to the “wasting disease”. Several explanations have been presented for this ecological disaster; however, none of those is satisfactory. It appears that for several localities a local explanation can be given.Apart from this general decline, it appears that, at least in Western Europe, there is a great variation in the temporal and spatial development cycles of the intertidal Zostera-beds. These variations can be ascribed to a number of factors, such as winter temperature (frost or no frost), grazing by birds (grazing or no grazing) and the balance between sedimentation (including sanding) and erosion. The combination “frost-grazing” leads to an annual cycle of the biomass, and a homogeneous bed structure. The combination “no frost-no grazing” causes raising of the seagrass bed, due to sedimentation up to a level where the environmental conditions become less favourable to seagrass; then breakdown sets in. These beds are heterogeneous and show a pluriannual developmental cycle.  相似文献   

15.
Jan Plue  Sara A. O. Cousins 《Oikos》2018,127(6):780-791
Metacommunity theory emphasizes that seed dispersal not only limits but equally maintains plant diversity, though the latter receives little empirical attention. Discerning the temporal and spatial components of seed dispersal and understanding how their interaction shapes fragmented communities and maintains their diversity may be pivotal to further our ecological understanding of spatial and temporal seed dispersal and its implications for landscape‐scale conservation management. To investigate the relative importance of spatial and temporal seed dispersal and their roles in maintaining plant diversity, the herb layer and seed bank of grassland communities were inventoried in 77 sites across abandoned and intact rotational grazing networks in a 100 km2 fragmented grassland landscape in the Stockholm archipelago (Baltic Sea, Sweden). Besides analysing alpha‐ and beta‐diversity patterns, nestedness analyses connect deterministic community changes and diversity losses with dispersal‐related life‐history traits and habitat specialization to identify the mechanism driving community changes and maintaining local diversity. The loss of rotational grazing networks caused community diversity declines via non‐random extinctions of spatially and temporally poor dispersers, particularly among grassland specialists. Temporal seed dispersal halted further community disassembly, maintaining diversity in the abandoned grazing networks. Spatial dispersal within the intact grazing networks was found to be an overriding, homogenizing agent conserving diversity in both the herb layer and seed bank. This empirical evidence establishes how spatial and temporal seed dispersal interact to maintain diversity in fragmented landscapes. Poorly connected grasslands appear limited by spatial dispersal, yet are maintained by temporal seed dispersal. In fragmented landscapes where grazing networks are rarely present, temporal rather than spatial seed dispersal may be more important in maintaining species diversity, since effective spatial dispersal may be significantly diminished. The grazing network's efficacy at boosting spatial dispersal and upholding community diversity presents a powerful management tool to conserve local and regional species diversity.  相似文献   

16.
Changes in the seascape often result in altered hydrodynamics that lead to coinciding changes in sediment dynamics. Little is known on how altered sediment dynamics affect long-term seagrass persistence. We studied the thresholds of sediment dynamics in relation to seagrass presence by comparing sediment characteristics and seagrass presence data of seven separate seagrass meadows. All meadows had a long-term (>20 years) presence. Within these meadows, we distinguish so-called “hotspots” (areas within a meadow where seagrass was found during all mapping campaigns) and “coldspots” (with infrequent seagrass presence). We monitored static sediment characteristics (median grain size, bulk density, silt content) and sediment dynamics (that is, bed level change and maximum sediment disturbance depth), bioturbation (that is, lugworm densities and induced fecal pit and mound relief), and seagrass cover. We statistically analyzed which sediment characteristic best explains seagrass cover. Densely vegetated hotspots were shown to have lower sediment dynamics than sparsely vegetated hotspots and coldspots, whereas static sediment characteristics were similar (grain size, bulk density). The vegetation cover was either low (2–15%) or high (>30%) and sediment dynamics showed a threshold for vegetation cover. From this correlative finding, we postulate a self-sustaining feedback of relatively dense seagrass via sediment stabilization and accordingly a runaway feedback once the seagrass cover becomes too sparse. The sensitivity for sediment dynamics shown in our study implies that future existence of seagrass meadows may be at risk as ongoing climate change might directly (increased environmental extremes) or indirectly (changing seascapes) negatively affect seagrass beds.  相似文献   

17.
Questions: We asked several linked questions about phenology and precipitation relationships at local, landscape, and regional spatial scales within individual seasons, between seasons, and between year temporal scales. (1) How do winter and summer phenological patterns vary in response to total seasonal rainfall? (2) How are phenological rates affected by the previous season rainfall? (3) How does phenological variability differ at landscape and regional spatial scales and at season and inter‐annual temporal scales? Location: Southern Arizona, USA. Methods: We compared satellite‐derived phenological variation between 38 distinct 625‐km2 landscapes distributed in the northern Sonoran Desert region from 2000 to 2007. Regression analyses were used to identify relationships between landscape phenology dynamics in response to precipitation variability across multiple spatial and temporal scales. Results: While both summer and winter seasons show increases of peak greenness and peak growth with more precipitation, the timing of peak growth was advanced with more precipitation in winter, while the timing of peak greenness was advanced with more precipitation in summer. Surprisingly, summer maximum growth was negatively affected by winter precipitation. The spatial variations between summer and winter phenology were similar in magnitude and response. Larger‐scale spatial and temporal variation showed strong differences in precipitation patterns; however the magnitudes of phenological spatial variability in these two seasons were similar. Conclusions: Vegetation patterns were clearly coupled to precipitation variability, with distinct responses at alternative spatial and temporal scales. Disaggregating vegetation into phenological variation, spanning value, timing, and integrated components revealed substantial complexity in precipitation‐phenological relationships.  相似文献   

18.
Reuter, M., Piller, W.E., Harzhauser, M., Kroh, A., Rögl, F. & ?ori?, S. 2010: The Quilon Limestone, Kerala Basin, India: an archive for Miocene Indo‐Pacific seagrass beds. Lethaia, Vol. 44, pp. 76–86. The facies of the fossiliferous Quilon Limestone in SW India is described for the first time in detail at the Padappakkara‐type locality. Facies (fossiliferous, micrite‐rich, bioturbated sediment with intercalated sand pockets) and faunal composition (epiphytic foraminifers, seagrass feeding Smaragdia gastropods, bioimmuration of celleporiform bryozoan colonies) indicate a seagrass environment. The large discoidal archaiasin foraminifer Pseudotaberina malabarica, in particular, is considered as a proxy for seagrass communities. Recent seagrasses have their centre of generic richness in the Indo‐Pacific where they cover wide areas in the tidal and shallow sub‐tidal zones. However, their geological record is only fragmentary and their palaeobiogeographic distribution has a big stratigraphical gap in the Miocene Western Indo‐Pacific region. The described nannoplankton flora and planktonic foraminifers from the Quilon Formation demonstrate that the deposition of the studied seagrass bed occurred in nannoplankton biozone NN3. This timing suggests formation during the closure of the Tethyan Seaway. The Quilon Limestone is thus an early Western Indo‐Pacific seagrass bed and an important step in reconstructing the history of seagrass communities. □Quilon Formation, Pseudotaberina malabarica, seagrass facies, Burdigalian, Indo‐Pacific.  相似文献   

19.
Two plant fossil‐bearing beds from the middle Barremian of Belgium were analysed to ascertain how experimental designs affect conclusions regarding palaeodiversity at a local scale. We analysed eight lateral samples per bed taken regularly every 3 m using an exhaustive sub‐sampling method. The Clench equation was used to evaluate the completeness of the taxonomic inventory of the samples and the sampling effort needed to obtain a reliable representation of diversity. The number of replicates needed to obtain the same representation of diversity from different nearby lateral samples of the same bed ranged from 5 to 19. Richness (S), Evenness (J) and the number of equiprobable taxa (2H’) greatly varied between samples from the same bed, even over short distances. Only one of the studied samples was representative of the taxonomic inventory of its bed. Our study shows that 1) the selection bias of the sampling area is reduced by increasing the number of lateral samples taken in a bed, enabling more reliable conclusions about local‐scale diversity; 2) intense sub‐sampling methods are needed to account for statistically independent observations of detailed lateral variation; and 3) sampling methods in palaeodiversity analyses must look for a similar degree of representativeness in samples rather than a homogeneous sample size. Using a sampling effort analysis provides evidence for the completeness of the data set, adjusting the amount of work required. Implementing the Clench equation in palaeodiversity analyses improves the performance of data acquisition in palaeoecological studies and provides a quality test of the data sets derived from them.  相似文献   

20.
El Niño‐Southern Oscillation (ENSO) events can cause dramatic changes in marine communities. However, we know little as to how ENSO events affect tropical seagrass beds over decadal timescales. Therefore, a diverse array of seagrass (Thalassia hemprichii) habitat types were surveyed once every 3 months for 16 years (January 2001 to February 2017) in a tropical intertidal zone that is regularly affected by both ENSO events and anthropogenic nutrient enrichment. La Niña and El Niño events had distinct effects on the biomass and growth of T. hemprichii. During La Niña years, higher (a) precipitation levels and (b) seawater nitrogen concentrations led to increases in seagrass leaf productivity, canopy height, and biomass. However, the latter simultaneously stimulated the growth of periphyton on seagrass leaves; this led to decreases in seagrass cover and shoot density. More frequent La Niña events could, then, eventually lead to either a decline in intertidal seagrass beds or a shift to another, less drought‐resistant seagrass species in those regions already characterized by eutrophication due to local anthropogenic activity.  相似文献   

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