A statistical evaluation of non-ergodic variogram estimators

Geostatistics is a set of statistical techniques that is increasingly used to characterize spatial dependence in spatially referenced ecological data. A common feature of geostatistics is predicting values at unsampled locations from nearby samples using the kriging algorithm. Modeling spatial dependence in sampled data is necessary before kriging and is usually accomplished with the variogram and its traditional estimator. Other types of estimators, known as non-ergodic estimators, have been used in ecological applications. Non-ergodic estimators were originally suggested as a method of choice when sampled data are preferentially located and exhibit a skewed frequency distribution. Preferentially located samples can occur, for example, when areas with high values are sampled more intensely than other areas. In earlier studies the visual appearance of variograms from traditional and non-ergodic estimators were compared. Here we evaluate the estimators' relative performance in prediction. We also show algebraically that a non-ergodic version of the variogram is equivalent to the traditional variogram estimator. Simulations, designed to investigate the effects of data skewness and preferential sampling on variogram estimation and kriging, showed the traditional variogram estimator outperforms the non-ergodic estimators under these conditions. We also analyzed data on carabid beetle abundance, which exhibited large-scale spatial variability (trend) and a skewed frequency distribution. Detrending data followed by robust estimation of the residual variogram is demonstrated to be a successful alternative to the non-ergodic approach.     

Bright plumage in the magpie: does it increase or reduce the risk of predation?

Conspicuous color patterns in birds may lead to increased risk of predation. Alternatively, bright birds may be aposematic or unprofitable prey, which leads to decreased predation. During four autumns, I examined whether the black-and-white plumage of (stuffed) magpies Pica pica increases or reduces the risk of attack by migrating goshawks Accipiter gentilis. Attack risk was higher for normal magpies than for cryptic, brown magpies whether the mounts were placed near one another or far apart. However, the brown magpie may have been avoided because of its novelty. For magpies and cryptic jays Garrulus glandarius exposed together, the attack risk was similar. In 2 years, magpie and jay mounts were also exposed far apart. In 1994, with an invasion of migrating jays, attack risk was much higher for jays than for magpies. In 1995, with a normal density of jays, hawks attacked the magpies more often. The results demonstrate frequency-dependent prey selection by goshawks, which would influence any predation cost of bright plumage. The attacks on normally colored magpie mounts suggest that magpies are not aposematic. Trials with photographs and human observers indicated that normal magpies were somewhat easier to detect than jays. The plumage of the magpie possibly increases the risk of predation, but may be favoured by sexual or social selection. Received: 18 January 1996 / Accepted after revision: 6 October 1996     

Uran in Wässern

The study presents 172 measurements of the uranium²³⁸U and²³⁴U isotopes in ground water and 21 measurements of these substances in the rivers of Eastern Germany. The uranium concentration in the ground water ranged from under 0.1 mBq/l to over 1000 mBg/l with a mean of 12 mBq/l. The respective activity relationships of²³⁴U/²³⁸U ranged from 0.85 to 12.6 with a mean of 1.66. A comparison of these values with those in the literature demonstrates an agreement with more recent measurements obtained from water in Thuringia and Saxony which were seen to have a mean value of 19 mBq/l. The activity relationships of²³⁴U/²³⁸U which were seen to be substantially over the equilibrial value of 1.0 and which were found in a quite high proportion of the samples must be taken into consideration when evaluating these. The measurements of river water demonstrated values for the Elbe river which are related to the periods of extensive uranium mining in Saxony and Thuringia. Measurements performed in the catchment regions going into the Baltic Sea revealed substantially higher values than would be expected from the distribution of uranium in these areas. A number of reasons are discussed here in an attempt to explain these findings.     

Seasonal variations in distribution patterns of sympagic meiofauna in Arctic pack ice

During two expeditions of the R.V. Polarstern to the Arctic Ocean, pack ice and under-ice water samples were collected during two different seasons: late summer (September 2002) and late winter (March/April 2003). Physical and biological properties of the ice were investigated to explain seasonal differences in species composition, abundance and distribution patterns of sympagic meiofauna (in this case: heterotrophs >20 µm). In winter, the ice near the surface was characterized by extreme physical conditions (minimum ice temperature: –22°C, maximum brine salinity: 223, brine volume: 5%) and more moderate conditions in summer (minimum ice temperature: –5.6°C, maximum brine salinity: 94, most brine volumes: 5%). Conditions in the lowermost part of the ice did not differ to a high degree between summer and winter. Chlorophyll a concentrations (chl a) showed significant differences between summer and winter: during winter, concentrations were mostly <1.0 µg chl a l^–1, while chl a concentrations of up to 67.4 µmol l^–1 were measured during summer. The median of depth-integrated chl a concentration in summer was significantly higher than in winter. Integrated abundances of sympagic meiofauna were within the same range for both seasons and varied between 0.6 and 34.1×10³ organisms m^–2 in summer and between 3.7 and 24.8×10³ organisms m^–2 in winter. With regard to species composition, a comparison between the two seasons showed distinct differences: while copepods (42.7%) and rotifers (33.4%) were the most abundant sea-ice meiofaunal taxa during summer, copepod nauplii dominated the community, comprising 92.9% of the fauna, in winter. Low species abundances were found in the under-ice water, indicating that overwintering of the other sympagic organisms did not take place there, either. Therefore, their survival strategy over the polar winter remains unclear.Communicated by O. Kinne, Oldendorf/Luhe     

Ozon in Waldökosystemen aus atmosphärenchemischer und pflanzenphysiologischer Sicht

The trend of rising ozone concentrations in forest ecosystems and the phytotoxicity of ozone demand a realistic risk assessment according to an internationally accepted and flux-based quality standard. Ozone fluxes within the canopy are influenced by chemical gas-phase reactions with nitrogen oxide and biogenic hydrocarbons and by surface deposition processes. Therefore, a differentiation of the ozone flux within the canopy is needed between stomatal uptake and other transport pathways. The Eddy Covariance technique is the method of choice for the determination of trace gas fluxes relevant for ozone chemistry. This method is also used for stomatal conductance measurements based on evapotranspiration fluxes and for emission measurements of biogenic hydrocarbons by PTR-MS. Although considerably research efforts were directed to canopy measurements in recent years, the underlying processes are not fully understood yet. Thus, major differences occur in the ratios of stomatal ozone uptake, non-stomatal deposition and gas-phase chemistry between different studies. Furthermore, the vertical concentration gradients within the canopy measured at several forest sites are rather inconsistent and the existing deposition models do rarely account for chemical transformation and detoxification processes. Only a simultaneous measurement of all photochemically relevant trace gases, plant physiological parameters at different sites and forest species over entire vegetation periods, and model parameterization according to the measurement results from the experimental sites will contribute to the clarification of the canopy processes and will ensure realistic risk assessments.     

Cycle synchrony and probability of conception in female hamadryas baboonsPapio hamadryas

We investigated the factors affecting conception in a captive population ofPapio hamadryas. We collected data on reproductive states from 16 females over 12 years. Probability of conception was related to the outcome of the immediately preceding pregnancy and the fate of the preceding offspring. After abortions or when an infant survived more than 6 months, a female needed more cycles to conceive than when an infant died within the first 6 months post partum. The degree of estrus synchrony within, but not between, one-male units influenced the probability of conception. Females experiencing conceptive estrus showed less synchrony than those experiencing nonconceptive estrus. The number of females simultaneously in estrus within a one-male unit was negatively correlated with the probability of conception. Within our captive population, group size and male and female age had no influence on the probability of conception. The results indicate that sperm may be a limited resource in the one-male reproductive units of hamadryas baboons. Female-female competition for conception may exist and should influence the demography of one-male units.     

Nontransitive patterns of historecognition phenomena in the Red Sea hydrocoral Millepora dichotoma

Allogeneic assays were conducted in situ on the Red Sea hydrocoral Millepora dichotoma. Forty-five pairwise combinations among ten colonies and ten control isografts were set up in four replicates each (180 and 40 pairs, respectively) and followed for up to 8 mo in the Gulf of Eilat, Red Sea in 1992–1993. In 42 allogeneic combinations we recorded a reproducible and unilateral tissue and skeleton overgrowth which developed within the first 10 wk up to 20 mm. Following the development of these primary overgrowths, four types of secondary responses were observed among most incompatible combinations: reversals in overgrowth directionality, tissue necroses, stand-offs and abnormal growth patterns. Secondary responses within a given set of replicates of most allogeneic combinations were characterized by high variability in type and intensity of response. Based on the outcomes of primary overgrowths, a complex nontransitive hierarchy was constructed for the ten colonies. All isografts and three allogeneic combinations fused within 3 wk. Fusion pattern between the three allogeneic combinations was nontransitive: one M. dichotoma colony repeatedly fused with two other colonies (four assays each). However, these two colonies not only did not fuse with each other, but one of them repeatedly overgrew its confrére. In the third compatible combination, the most superior and the most inferior colonies in the network of hierarchies among the ten colonies, fused in all tested assays. These results are compared with allogeneic outcomes in other enidarians, where nontransitive fusion between allogenic colonies have never been recorded.