Relationship between horizontal pattern and vertical structure in a chalk grassland

Our objective was to examine the relationships between patterns of vertical structure and species microassociations at various scales in a species-rich chalk grassland.Transect data were analyzed for species microassociations by multiscale ordination of presence data from the Gerendal Nature Reserve, The Netherlands. Results indicated microassociations at scales of 30 cm, 2 m, 3 m, and 10 or more m. The microassociations at each scale comprised different but overlapping constellations of species.For the same transect, profile photographs were taken through the vegetation against a white background, and image analysis was used to provide indices of vegetation vertical cover, height and vertical center of biomass. Pattern analysis of these data indicated a regular pattern at a scale of 3 m.Standardized cross-variograms of the patterns of microassociations and vertical structure revealed only very weak relationships, even though one microassociation pattern and the vertical structure patterns occurred at the scale of 3 m. This is because the two 3 m patterns fell in and out of phase.Abbreviations PCA principal components analysis     

Composition of plant life forms of subtropical shrubland in China and its correlation with temperature and precipitation

Aims Shrubland is one of the most important vegetation types in subtropical region, but studies on its life-form composition and geographic patterns are scarce. In this paper, we addressed the following questions: what are the characteristics of the composition of plant life forms of subtropical shrublands in China? And what are the relation-ships between the life form compositions, geographic patterns of subtropical shrublands and hydrothermal factors?
Methods A total of 688 plots, designed with the stratified random sampling method, were systematically set and investigated in subtropical shrublands in China. Life forms were identified based on Raunkiaer’s (1934) life form systems. Life form spectrums were compiled at each sampling site and within the whole shrubland, respectively. Partial correlation analysis was applied to reveal the latitudinal, longitudinal and altitudinal patterns of each life form. Pearson’s correlation analysis was adopted to evaluate the relationships between each life form and hydrothermal factors. Further, redundancy analysis (RDA) was used to distinguish the effect of temperature and precipitation on geographical variations of life forms.
Important findings (1) In subtropical shrublands in China, 56.37% of the total species were woody species, among which 33.88% were deciduous species and 22.49% were evergreen species. The remaining 43.63% of the total species were herbaceous species, dominated by 35.15% perennials. Generally, phaenerophytes (Ph) was the most dominant component, while the second dominant one was hemicryptophytes (H). The life form spectrum showed a pattern of Ph (55.74%) > H (31.03%) > T (therophytes 8.80%) > Cr (cryptophytes 4.08%) > Ch (chamaephytes 1.34%). (2) Plant life forms mainly exhibited longitudinal and altitudinal patterns. With the increase of longitude, the percentage of Ph tended to increase while H, Cr and T tended to decrease. Furthermore, an increase in altitude would lead to an increase in H and Cr but a decrease in Ph, Ch and T. (3) Among all the investigated climatic factors, the mean temperature of the warmest month (MWT) and the mean precipitation of the coldest month (MCP) had the greatest impact on Ph, H, Cr and T. With an increase of MWT and MCP, Ph would increase while H, Cr and T would decrease. Taken together, MWT and MCP could explain 41.97% of the variation of life form spectrum, among which 28.35% was attributed to their joint-effect. The results suggested that Ph and H were mainly influenced by both temperature and precipitation, and that Cr was more sensitive to temperature gradients. Besides, T was more sensitive to precipitation, and Ch showed little fluctuation along hydrothermal gradients. In conclusion, MWT and MCP are the most important hydrothermic factors that determine the geographic patterns of life forms in subtropical shrublands in China.     

Stream fish assemblage and habitat structure in a tropical African river basin (Nyagui River, Zimbabwe)

Few studies have been carried out on stream ecology in southern Africa although many species are endangered. This study investigated the stream fish assemblage and their habitat associations over a period of 3 months (October 2004 to January 2005), in view of the proposal to build a dam across the Nyagui River. Twenty-four fish species were collected and were separated into groups based on preferred microhabitats. The first group, dominated by Barbus paludinosus , comprised species collected from the upstream stations with slow flow, shallow depth (pools) and fine substrate type. Species associated with riffles, which included Chiloglanis neumanni , Labeobarbus marequensis and Opsaridium zambezense , comprised the second group on the downstream. The last group comprised species preferring pools with rock substrate and slow flow such as Pharyngochromis acuticeps and Pseudocranilabrus philander . The species were consistently associated with their habitat types throughout the sampling period. This relationship may be explained by the fish's morphological adaptations. Species richness increased from nine in the upstream section to twenty in the downstream section and this was related to increasing habitat complexity downstream. The construction of the Kunzvi Dam across the Nyagui River is likely to lead to loss of rheophilic species while cichlids and introduced species may increase.     

Ordinational analysis of benthic communities upstream and downstream of a prairie storage reservoir

Ordinational analysis of 445 benthic samples taken over the years 1975 through 1979 revealed at least three distinct benthic communities; upstream of the reservoir, in the hypolimnion influenced area, and in the recovered downstream areas. In addition to previously reported influences of thermal constancy, Ca/Na ratio increases in the hypolimnion influenced areas showed high correlation with the dominance of molluscs in the cold water area. Increases in Ca/Na ratios allowed greater reproductive success and better ability to regulate ion concentration in molluscan forms. Combined with thermal regime alterations and high algal cover, most aquatic insects were excluded from this area. The hypolimnion release reservoir and associated downstream areas act as barriers, both thermal and geographical, to aquatic insect dispersal resulting in an impoverished benthic community in thermally recovered areas downstream of the reservoir. This barrier effect, as well as the probability of increased availability of detrital material (from increased periphyton growth in hypolimnion influenced areas) and more uniform flows, has caused changes in dominance structures of benthic communities in recovered downstream areas compared to upstream areas.     

A modern pollen–climate calibration set based on lake sediments from the Tibetan Plateau and its application to a Late Quaternary pollen record from the Qilian Mountains

Aim Fossil pollen spectra from lake sediments on the Tibetan Plateau have been used for qualitative climate reconstruction, but no modern pollen–climate calibration set based on lake sediments is available to infer past climate quantitatively. This study aims to develop such a dataset and apply it to fossil data. Location The Tibetan Plateau, between 30 and 40° N and 87 and 103° E. Methods We collected surface sediments from 112 lakes and analysed them palynologically. The lakes span a wide range of mean annual precipitation (P_ann; 31–1022 mm), mean annual temperature (T_ann; −6.5 to 1 °C), and mean July temperature (T_July; 2.6–19.7 °C). Redundancy analysis showed that the modern pollen spectra are characteristic of their respective vegetation types and local climate. Transfer functions for P_ann, T_ann and T_July were developed with weighted averaging partial least squares. Model performance was assessed by leave-one-out cross-validation. Results The root mean square errors of prediction (RMSEP) were 104 mm (P_ann), 1.18 °C (T_ann) and 1.17 °C (T_July). The RMSEPs, when expressed as percentages of the gradient sampled, were 10.6% (P_ann), 15.7% (T_ann) and 11.9% (T_July). These low values indicate the good performance of our models. An application of the models to fossil pollen spectra covering the last c. 50 kyr yielded realistic results for Luanhaizi Lake in the Qilian Mountains on the north-eastern Tibetan Plateau (modern P_ann 480 mm; T_ann−1 °C). T_ann and P_ann values similar to present ones were reconstructed for late Marine Isotope Stage 3, with minimum values for the Last Glacial Maximum (c. 300 mm and 2 °C below present), and maximum values for the early Holocene (c. 70 mm and 0.5 °C greater than present). Main conclusions The modern pollen–climate calibration set will potentially be useful for quantitative climate reconstructions from lake-sediment pollen spectra from the Tibetan Plateau, an area of considerable climatic and biogeographical importance.     

Should ecologists prefer model‐ over distance‐based multivariate methods?

Ecological data sets often record the abundance of species, together with a set of explanatory variables. Multivariate statistical methods are optimal to analyze such data and are thus frequently used in ecology for exploration, visualization, and inference. Most approaches are based on pairwise distance matrices instead of the sites‐by‐species matrix, which stands in stark contrast to univariate statistics, where data models, assuming specific distributions, are the norm. However, through advances in statistical theory and computational power, models for multivariate data have gained traction. Systematic simulation‐based performance evaluations of these methods are important as guides for practitioners but still lacking. Here, we compare two model‐based methods, multivariate generalized linear models (MvGLMs) and constrained quadratic ordination (CQO), with two distance‐based methods, distance‐based redundancy analysis (dbRDA) and canonical correspondence analysis (CCA). We studied the performance of the methods to discriminate between causal variables and noise variables for 190 simulated data sets covering different sample sizes and data distributions. MvGLM and dbRDA differentiated accurately between causal and noise variables. The former had the lowest false‐positive rate (0.008), while the latter had the lowest false‐negative rate (0.027). CQO and CCA had the highest false‐negative rate (0.291) and false‐positive rate (0.256), respectively, where these error rates were typically high for data sets with linear responses. Our study shows that both model‐ and distance‐based methods have their place in the ecologist's statistical toolbox. MvGLM and dbRDA are reliable for analyzing species–environment relations, whereas both CQO and CCA exhibited considerable flaws, especially with linear environmental gradients.     

Effects of temperature on spring bloom dynamics of epilithic diatom communities in the Gulf of Bothnia

Abstract. In order to forecast consequences of climatic changes for littoral algae in coastal ecosystems, the effects of temperature increase on spring bloom dynamics of epilithic diatom communities were studied in two ways: (1) Communities were compared from sites receiving different amounts of cooling water discharge from a nuclear power plant. (2) Communities from the exceptionally warm spring of 1989 were compared with those from the same sites in the preceding normal years. The studies were carried out in and around the Forsmark Biotest Basin, an artificial waterbody that receives brackish cooling water from the Forsmark nuclear power plant on the Swedish east coast. Species composition and biomass data from 200 epilithic diatom samples taken in the period January-May from 1983 to 1989 were analysed. Multiple regression analysis was used to show the responses of diatom cover, community diversity and abundances of individual taxa to water temperature and other environmental factors. Constrained ordination (CCA) was used to display the relationship of overall community composition to environment. Increased water temperature resulted in higher biomass through a chain of ecological effects. Strong reduction of ice cover was crucial in this chain, resulting in reduced loss of diatom cells by reduced abrading and higher primary production by higher availability of light, nutrients and substrate. The conclusion is that large blooms of fast-growing epilithic diatoms in large colonies can occupy the niche that emerges when the ice-free season is prolonged in the northern Baltic Sea and areas similar in salinity and climatic conditions. Dramatic species shifts may only be expected if the winter ice cover is totally absent.