Exploring spatiotemporal patterns in early stages of primary succession on former lignite mining sites

Questions: 1. Does random colonization predominate in early stages of primary succession? 2. Do pioneer species facilitate the establishment of later arriving species? 3. Does an initially random distribution change to an aggregated pattern with ongoing succession? Location: Lignite mining region of Lower Lusatia, eastern Germany. Methods: Individual plants were mapped along a 2 m × 28 m transect during three successive years and classified into two groups (1) the pioneer Corynephorus canescens and (2)‘all other species’. Using the pair‐correlation function, univariate point pattern analysis was carried out by applying a heterogeneous Poisson process as null model. Bivariate analysis and a toroidal shift null model were applied to test for independence between the spatial patterns of the two groups separately for each year, as well by exploring spatiotemporal patterns from different years. Results: In the first year Corynephorus and ‘all other species’ showed an aggregated pattern on a spatial scale > 40 cm and in the second and third years a significant attraction for distances between 4 and 12 cm, with an increasing radius in the third year. The analyses of interspecific spatiotemporal dynamics revealed a change from independence to attraction between distances of 4 cm and 16 cm when using Corynephorus as focal species. However, applying ‘all other species’ as focal points results in a significant attraction at distances up to 60 cm in the first year and a diminishing attraction in the second and third years with distances ≤ 6 cm. Conclusions: Facilitative species‐species interactions are present in early stages of primary succession, resulting mainly from pioneer species acting as physical barriers and their ability to capture diaspores being drifted by secondary dispersal along the substrate surface. However, due to gradual establishment of perennial species and their ability of lateral extension by vegetative dispersal, facilitation may influence spatial pattern formation predominantly on short temporal and fine spatial scales.     

A point process driven multiple change point model: A robust resistant approach

Helicases are a class of enzymes involved in Ribonucleic Acid (RNA) metabolism. In this work a statistical method is proposed to analyze the individual mechanistic cycle of these motor proteins which are crucial to the understanding of their cellular functions. The RNA unwinding by NS3 helicase is hypothesized to occur in a series of discrete steps and the steps themselves occurring in accordance to an underlying point process. A point process driven multiple change point model is proposed to model the RNA unwinding mechanism. The methods are portable to other areas of applications as well. Algorithms based on robust-resistant statistical procedures are proposed to detect the change points. Both sequential and a posteriori change point models are considered. The relevant parameters of interest are estimated using a maximum likelihood approach. Simulations are performed to assess the performance of the methodology.     

Tests of significance for point-biserial rank correlation coefficients in the presence of ties

This paper deals with exact and asymptotic tests of significance of the point-biserial rank correlations of the KENDALL and SPEARMAN tests when ties are present and mid-ranks are assigned to tied values.     

A Note on Robust Intensity Estimation for Point Processes

A robust intensity estimator based on independent marking is derived. A simulation study is made to convince that the new estimator works also in such cases where the usual estimators based on the distance methods do not work. Some truncated distributions are derived.     

Hierarchical analysis of habitat use by 0+ juvenile fish in Hungarian/Slovak flood plain of the Danube River

Synopsis To address the lack of information on the distribution and habitat use of 0+ juvenile fishes in the Hungarian/Slovak flood plain of the middle Danube River, we undertook the first cross-border ichthyological investigation, examining three levels of ecological perception (hydrosystem, macrohabitat, microhabitat) during August 1992 using ‘Point Abundance Sampling’ by electrofishing. Being that the Gabćófcovo hydroscheme was about to begin diverting most of the river's discharge away from the flood plain during the winter of 1992, the present investigation represented the last chance to record the distribution and microhabitat use of 0+ fishes within the flood plain. At each sampling point, numerous environmental variables were measured quantitatively, or as percentages. At the hydrosystem level, 25 species of 0+ fishes were captured in the 1170 point samples collected from 52 sites (27 in Hungary, 25 in Slovakia), ranging from 10 to over 200 mm standard length (i.e. pikeEsox lucius). No significant differences were found between the Hungarian and Slovak specimens with respect to standard length (ANOVA, p> 0.31), nor in the relative densities (ind.m⁻²) of 0+ fish (Student's t-test: df 24, t = 0.601, p = 0.553). A typology of macrohabitats using principal components analysis of the sites X species data matrix in absence/presence revealed three groupings of sites: (1) lotic channels, weirs and wing-dams; (2) partially-abandoned channels; (3) abandoned channels; the results corroborated our assumption that weirs of the anabranch systems represent a quasi-lotic refuge for rheophilous 0+ fishes of the flood plain during late summer. At the microhabitat level, an empirical model of microhabitat use was generated using canonical correspondence analysis and association analysis (based on chi-square probabilities). Water velocity was the most influential variable, with the O+ juveniles ordinated along the first canonical axis according to their increasing rheophily. The second most influential microhabitat variable was water transparency, followed by the percentage abundance of macrophytes and substrate composition.     

Notes in Case-Control Studies with Matched Pairs under Inverse Sampling

In case-control studies with matched pairs, the traditional point estimator of odds ratio (OR) is well-known to be biased with no exact finite variance under binomial sampling. In this paper, we consider use of inverse sampling in which we continue to sample subjects to form matched pairs until we obtain a pre-determined number (>0) of index pairs with the case unexposed but the control exposed. In contrast to use of binomial sampling, we show that the uniformly minimum variance unbiased estimator (UMVUE) of OR does exist under inverse sampling. We further derive an exact confidence interval of OR in closed form. Finally, we develop an exact test and an asymptotic test for testing the null hypothesis H₀: OR = 1, as well as discuss sample size determination on the minimum required number of index pairs for a desired power at α-level.