Extinction conditions for isolated populations affected by environmental stochasticity

We determine the critical patch size below which extinction occurs for populations living in one-dimensional habitats surrounded by completely hostile environments in the presence of environmental fluctuations. The population dynamics is reformulated in terms of a stochastic reaction–diffusion equation and is reduced to a deterministic equation that incorporates the systematic contributions of the noise. We obtain bifurcation diagrams and relations for the mean population density at the stationary state, the critical patch size, and the mean number of individuals in the habitat. The effect of the noise differs, depending on whether it affects the net growth rate or the intraspecific competition term. Fluctuations in the net growth rate decrease the critical patch size, whereas fluctuations in the competition term do not change the critical patch size. We compare our analytical results with numerical solutions of the stochastic partial differential equations and show that our procedure proves useful in dealing with reaction–diffusion equations with multiplicative noise.     

The distribution of coalescence times and distances between microsatellite alleles with changing effective population size

We investigate the effects of past changes of the effective population size on the present allelic diversity at a microsatellite marker locus. We first derive the analytical expression of the generating function of the joint probabilities of the time to the Most Recent Common Ancestor for a pair of alleles and of their distance (the difference in allele size). We give analytical solutions in the case of constant population size and the geometrical mutation model. Otherwise, numerical inversion allows the distributions to be calculated in general cases. The effects of population expansion or decrease and the possibility to detect an ancient bottleneck are discussed. The method is extended to samples of three and four alleles, which allows investigating the covariance structure of the frequencies f(k) of pairs of alleles with a size difference of k motifs, and suggesting some approaches to the estimation of past demography.     

Morphology, body size and behaviour of recently-metamorphosed sea lampreys, Petromyzon marinus, from the lower River Severn, and their relevance to the onset of parasitic feeding

Several hundred recently-metamorphosed Petromyzon marinus were caught during heavy freshwater discharge in the River Severn on the night of 30 November/1 December 1988. The total lengths of a subsample of 42 males and 82 females ranged from 155 to 218 mm, with a mean of 182 mm in both sexes. The buccal funnel, eyes, fins and the teeth on the suctorial disc and tongue-like piston were very well-developed. When placed in full-strength sea water, the lampreys fed on bass ( Dicentrarchus labrax ), leaving prominent scars on these hosts. The morphology, body size and time of capture of the River Severn sea lampreys provide very strong circumstantial evidence that these lampreys had been feeding for some weeks. Since they were caught when moving downstream and were capable of feeding in sea water, they were presumably about to begin their parasitic marine life.     

Cholecystokinin and bombesin act independently to decrease food intake in the rat

The satiating effects of cholecystokinin-octapeptide (CCK-8) and bombesin (BBS) when injected alone and in combination were compared in intact rats. When injected alone, both CCK-8 and BBS elicited a dose-related decrease of 30-minute food intake. Injections of BBS were less potent than the equivalent doses of CCK-8 in producing satiety. BBS reached an asymptotic level of suppression of approximately 40 percent at a dose of 2 micrograms/kg, whereas injections of 4 micrograms/kg of CCK-8 resulted in a 72 percent suppression of food intake. When the two peptides were administered in a single injection, the resulting suppression of food intake was equivalent to that which would be predicted if their effects were completely additive. These results support the hypothesis that CCK-8 and BBS act via independent mechanisms to induce satiety. A preliminary model of peptidergic satiety, based on this hypothesis, is proposed.     

More food,less habitat: how necromass and leaf litter decomposition combine to regulate a litter ant community

1. In brown food webs of the forest floor, necromass (e.g. insect carcasses and frass) falling from the canopy feeds both microbes and ants, with the former decomposing the homes of the latter. In a tropical litter ant community, we added necromass to 1 m² plots, testing if it added as a net food (increasing ant colony growth and recruitment) or destroyer of habitat (by decomposing leaf litter). 2. Maximum, but not mean, colony growth rates were higher on +food plots. However, neither average colony size, nor density was higher on +food plots. In contrast, +food plots saw diminished availability of leaf litter and higher microbial decomposition of cellulose, a main component of the organic substrate that comprises litter habitat. 3. Furthermore, necromass acted as a limiting resource to the ant community only when nest sites were supplemented on +food plots in a second experiment. Many of these +food +nest plots were colonised by the weedy species Wasmannia auropunctata. 4. Combined, these results support the more food–less habitat hypothesis and highlight the importance of embedding studies of litter ant ecology within broader decomposer food web dynamics.     

Seasonal changes in resource allocation within an individual offspring of the wolf spider, Pardosa pseudoannulata (Araneae: Lycosidae)

Abstract.  Seasonal changes in resource investment into individual offspring are well documented, but no attention has been paid to the allocation of the invested resource among the body parts of the offspring. In the present study, seasonal changes in the absolute and relative sizes in a spiderling of the wolf spider Pardosa pseudoannulata Boes. et Str. are investigated, and the relationship between spiderling body size and the ability to moult during a food shortage is clarified. Spiderlings that emerge in November have a significantly larger cephalothorax and abdomen than those that emerge in June. In addition, the abdomen–cephalothorax size ratio is significantly greater in November than in June offspring. Under limited food availability conditions, only 40% of spiderlings moult. Nymphs that do moult have a significantly larger cephalothorax, abdomen and abdomen–cephalothorax size ratio than nymphs that do not moult. Thus, both the quantity of resources invested in the cephalothorax and abdomen of a spiderling and the proportional allocation of resources between the two body parts change seasonally in Pa. pseudoannulata ; alteration of the resource allocation occurs in late autumn. Larger spiderlings of Pa. pseudoannulata that emerge in late autumn would be able to develop into advanced instars even during food shortage, and therefore may have better overwintering survival rates.     

Nest groups of wild bonobos at Wamba: selection of vegetation and tree species and relationships between nest group size and party size

We examined the location of nest groups, spatial distribution of nests within a nest group, and attributes of individual nests of wild bonobos at Wamba, Democratic Republic of Congo. We also examined the seasonal factors influencing nesting behavior and compared the nest group size with the 1 hr party size during daytime. We defined a nest group to be a cluster of nests that were built in the same evening and found within 30 m from the other nearest nest. Examination of the largest gap within a nest group suggested that 30 m was an acceptable cutoff value. Monthly rainfall or fruit abundance did not significantly influence the monthly mean nest group size. Nests were built in the swamp forest for as many as 13% observation days, suggesting the need for reevaluation of the use of swamp forest by bonobos. The use of swamp forest was influenced not by seasonal rainfall or fruit abundance, but by the fruiting of specific species. Preferred tree species for building nests accounted for 19.8% of standing trees, which suggested that the selection of sleeping sites was not largely restricted by the distribution of specific species. The mean 1 hr party size was almost identical through the day and was similar to the mean nest group size. Parties of bonobos sometimes split into smaller nest groups, especially when feeding on non‐preferred fruits during fruit scarcity. By contrast, when feeding on preferred fruits while ranging in large parties, they often aggregated to form even larger nest groups. When sleeping in small‐ or middle‐sized nest groups, they tended to aggregate the next morning. These tendencies may reflect the gregarious nature of bonobos who prefer to range or sleep together as far as circumstances allow. Am. J. Primatol. 72:575–586, 2010. © 2010 Wiley‐Liss, Inc.     

Internal pilots for observational studies

Study planning often involves selecting an appropriate sample size. Power calculations require specifying an effect size and estimating “nuisance” parameters, e.g. the overall incidence of the outcome. For observational studies, an additional source of randomness must be estimated: the rate of the exposure. A poor estimate of any of these parameters will produce an erroneous sample size. Internal pilot (IP) designs reduce the risk of this error ‐ leading to better resource utilization ‐ by using revised estimates of the nuisance parameters at an interim stage to adjust the final sample size. In the clinical trials setting, where allocation to treatment groups is pre‐determined, IP designs have been shown to achieve the targeted power without introducing substantial inflation of the type I error rate. It has not been demonstrated whether the same general conclusions hold in observational studies, where exposure‐group membership cannot be controlled by the investigator. We extend the IP to observational settings. We demonstrate through simulations that implementing an IP, in which prevalence of the exposure can be re‐estimated at an interim stage, helps ensure optimal power for observational research with little inflation of the type I error associated with the final data analysis.