A parametric model for the interval censored survival times of Acacia mangium plantation in a spacing trial

Survival times for the Acacia mangium plantation in the Segaliud Lokan Project, Sabah, East Malaysia were analysed based on 20 permanent sample plots (PSPs) established in 1988 as a spacing experiment. The PSPs were established following a complete randomized block design with five levels of spacing randomly assigned to units within four blocks at different sites. The survival times of trees in years are of interest. Since the inventories were only conducted annually, the actual survival time for each tree was not observed. Hence, the data set comprises censored survival times. Initial analysis of the survival of the Acacia mangium plantation suggested there is block by spacing interaction; a Weibull model gives a reasonable fit to the replicate survival times within each PSP; but a standard Weibull regression model is inappropriate because the shape parameter differs between PSPs. In this paper we investigate the form of the non-constant Weibull shape parameter. Parsimonious models for the Weibull survival times have been derived using maximum likelihood methods. The factor selection for the parameters is based on a backward elimination procedure. The models are compared using likelihood ratio statistics. The results suggest that both Weibull parameters depend on spacing and block.

Parametric accelerated failure time models with random effects and an application to kidney transplant survival

Accelerated failure time models with a shared random component are described, and are used to evaluate the effect of explanatory factors and different transplant centres on survival times following kidney transplantation. Different combinations of the distribution of the random effects and baseline hazard function are considered and the fit of such models to the transplant data is critically assessed. A mixture model that combines short- and long-term components of a hazard function is then developed, which provides a more flexible model for the hazard function. The model can incorporate different explanatory variables and random effects in each component. The model is straightforward to fit using standard statistical software, and is shown to be a good fit to the transplant data. Copyright (C) 2004 John Wiley Sons, Ltd.

Bayesian analysis of an inverse Gaussian correlated frailty model

In survival analysis frailty is often used to model heterogeneity between individuals or correlation within clusters. Typically frailty is taken to be a continuous random effect, yielding a continuous mixture distribution for survival times. A Bayesian analysis of a correlated frailty model is discussed in the context of inverse Gaussian frailty. An MCMC approach is adopted and the deviance information criterion is used to compare models. As an illustration of the approach a bivariate data set of corneal graft survival times is analysed. (C) 2006 Elsevier B.V. All rights reserved.

Competition and dispersal in the pea aphid: clonal variation and correlations across traits

1. The presence of an across-species trade-off between dispersal ability and competitive ability has been proposed as a mechanism that facilitates coexistence. It is not clear if a similar trade-off exists within species. Such a trade-off would constrain the evolution of either trait and, given appropriate selection pressures, promote local adaptation in these traits. 2. This study found substantial levels of heritable variation in competitive ability of the pea aphid, Acyrthosiphon pisum Harris (Homoptera: Aphididae), measured in terms of relative survival when reared with a single clone of the vetch aphid, Megoura viciae Buckton (Homoptera: Aphididae). 3. Pea aphids can move to new patches by either flying (longer distance dispersal) or walking (local dispersal) from plant to plant. There was considerable clonal variation in dispersal ability, measured in terms of the proportion of winged offspring produced, and ability to survive away from their host plant. 4. Winged individuals showed longer off-plant survival times than wingless forms of the same pea aphid clone. 5. There was no evidence of a relationship between clonal competitive ability and either measure of dispersal ability, although the power of the test is limited by the number of pea aphid clones used in the trial. 6. However, there was a positive correlation between clonal fecundity and the proportion of winged offspring produced. Although speculative, it is suggested that clones that are more likely to either overwhelm their host plant or attract higher numbers of natural enemies as a result of having higher fecundity are more likely to produce winged morphs.

Supplementary feeding of wild birds indirectly affects the local abundance of arthropod prey

Providing supplementary food for wild birds is a globally popular past-time; almost half of the households in many developed countries participate and billions of US dollars are spent annually. Although the direct influence of this additional resource on bird survivorship and fecundity has been studied, there is little understanding of the wider ecological consequences of this massive perturbation to (what are usually) urban ecosystems. We investigated the possible effects of wild bird feeding on the size and survivorship of colonies of a widespread arthropod prey species of many small passerine birds, the pea aphid [Acyrthosiphon pisum (Harris); Hemiptera: Aphididae], in suburban gardens in a large town in southern England. We found significantly fewer aphids and shorter colony survival times in colonies exposed to avian predation compared to protected controls in gardens with a bird feeder but no such differences between exposed and protected colonies in gardens that did not feed birds. Our work therefore suggests that supplementary feeding of wild birds in gardens may indirectly influence population sizes and survivorship of their arthropod prey and highlights the need for further research into the potential effects on other species.

Effects of soil conditions and drought on egg hatching and larval survival of the clover root weevil (Sitona lepidus)

Soil-dwelling insect herbivores are significant pests in many managed ecosystems. Because eggs and larvae are difficult to observe, mathematical models have been developed to predict life-cycle events occurring in the soil. To date, these models have incorporated very little empirical information about how soil and drought conditions interact to shape these processes. This study investigated how soil temperature (10, 15, 20 and 25 °C), water content (0.02 (air dried), 0.10 and 0.25 g g−1) and pH (5, 7 and 9) interactively affected egg hatching and early larval lifespan of the clover root weevil (Sitona lepidus Gyllenhal, Coleoptera: Curculionidae). Eggs developed over 3.5 times faster at 25 °C compared with 10 °C (hatching after 40.1 and 11.5 days, respectively). The effect of drought on S. lepidus eggs was investigated by exposing eggs to drought conditions before wetting the soil (2–12 days later) at four temperatures. No eggs hatched in dry soil, suggesting that S. lepidus eggs require water to remain viable. Eggs hatched significantly sooner in slightly acidic soil (pH 5) compared with soils with higher pH values. There was also a significant interaction between soil temperature, pH and soil water content. Egg viability was significantly reduced by exposure to drought. When exposed to 2–6 days of drought, egg viability was 80–100% at all temperatures but fell to 50% after 12 days exposure at 10 °C and did not hatch at all at 20 °C and above. Drought exposure also increased hatching time of viable eggs. The effects of soil conditions on unfed larvae were less influential, except for soil temperature which significantly reduced larval longevity by 57% when reared at 25 °C compared with 10 °C (4.1 and 9.7 days, respectively). The effects of soil conditions on S. lepidus eggs and larvae are discussed in the context of global climate change and how such empirically based information could be useful for refining existing mathematical models of these processes.