Analysis of Dispersion Effects in Unreplicated Factorial Designs

Methods for estimating and testing hypotheses of dispersion effects in two-level unreplicated factorial designs are studied. Under a particular = normal model, some linear combinations of the response variables are constructed in order to make a simple inference. One method uses an F-test after eliminating all but one dispersion parameter, while another method is based on dependencies between linear combinations. The consequences of performing fractional experiments are discussed. The problems that arise in this case are treated either by working with several variance ratios or by using the covariance structure. Often, the quality of the methods differ. It might even happen that some methods work while others are not possible to construct. Comparative calculations of power functions are carried out and illustrations from industry are presented

Analysis of Dispersion Effects in Unreplicated Factorial Designs

Methods for estimating and testing hypotheses of dispersion effects in two-level unreplicated factorial designs are studied. Under a particular = normal model, some linear combinations of the response variables are constructed in order to make a simple inference. One method uses an F-test after eliminating all but one dispersion parameter, while another method is based on dependencies between linear combinations. The consequences of performing fractional experiments are discussed. The problems that arise in this case are treated either by working with several variance ratios or by using the covariance structure. Often, the quality of the methods differ. It might even happen that some methods work while others are not possible to construct. Comparative calculations of power functions are carried out and illustrations from industry are presented

Foraging mode of spiders affects risk of predation by birds

Avian insectivores are top predators of arboreal arthropods in different forest ecosystems. The selective effects of bird predation in relation to foraging behaviour in canopy-living spiders were studied in a 2-year field experiment using exclosures in a spruce forest in southern Sweden. Three different hunting strategies – free-hunting, two-dimensional web, three-dimensional web – were included in the analysis. Comparisons of bird predation rate (ratio ln (abundance net-enclosed branch/abundance control)) showed considerable variation between spider groups. Free-hunting spiders suffered most from avian insectivores and predation rate was significantly higher than in spiders with two-dimensional webs. Spiders with three-dimensional webs were exposed to a predation rate in between those of the two other hunting strategies. Generally, the experimental effect was significantly higher in spring samples than in autumn, suggesting a stronger predation pressure in winter. The high variation in susceptibility to predation by insectivores implies that selection on behaviour of spider individuals is strong. Web building in itself is probably part of the protective mechanism, suggesting that webs have dual functions. We conclude that the risk of bird predation is a selective force on foraging behaviour of spiders in a forest canopy system

Density and fluctuations of a nest-pocket breeding population of the Treecreeper Certhia famililaris over a 28-year period

The number of breeding Treecreeper pairs was estimated from 1982 to 2009 in a 2.7 km2 study area located in south-western Sweden (57\ub039\ub4 N; 12\ub04\ub4 E). Most of the area, which was provided with 205 man-made nest pockets, is covered by broad-leafed forest. The number of first clutches varied between 5 and 21 with an annual average of 14\ub14.2 (SD) breeding pairs (CV 30%). The density of breeding Treecreepers varied from 1.9 to 7.8 pairs/km2 with a mean of 5.1\ub11.86 pairs/km2. The population did not show any statistically significant density trend over the 28 years. The between-year variation in the return rate of ringed adult females that bred after wintering was significantly negatively related to the temperature and precipitation means of the preceding winter. Thus, fewer females returned after milder winters with higher precipitation. The statistical tests pertaining to the variation in the whole breeding population indicate that the species can cope with moderate fluctuations in winter weather, thus preventing significant changes in the number of breeders in the study area