Confronting two-sex demographic models with data

Most population dynamics models explicitly track the density of a single sex. When the operational sex ratio can vary, two-sex models may be needed to understand and predict population trajectories. Various functions have been proposed to describe the relative contributions of females and males to recruitment, and these functions can differ qualitatively in the patterns that they generate. Which mating function best describes the dynamics of real populations is not known, since alternative two-sex models have not been confronted with experimental data. We conducted the first such comparison, using laboratory populations of the bean beetle Callosobruchus maculatus. Manipulations of the operational sex ratio and total density provided strong support for a demographic model in which the birth rate was proportional to the harmonic mean of female and male densities, and females, males, and their offspring made unique contributions to density dependence. We offer guidelines for transferring this approach to other, less tractable systems in which possibilities for sex ratio manipulations are more limited. We show that informative experimental designs require strong perturbations of the operational sex ratio. The functional form of density dependence (saturating vs. over-compensatory) and the relative contributions of each sex to density dependence can both determine in which direction and at which population densities such perturbations would be most informative. Our experimental results and guidelines for design strategies promote synthesis of two-sex population dynamics theory with empirical data.     

Maternal nest defense reduces egg cannibalism by conspecific females in the maritime earwig <Emphasis Type="Italic">Anisolabis maritima</Emphasis>

Nest predation imposes a major cost to reproductive females, who should therefore take measures to avoid encounters with predators. However, when predators are conspecifics, avoidance can be more difficult and may be a consequence of social or aggregative behaviors. In this study, we measured the consequences of conspecific egg cannibalism on hatching success in the maritime earwig (Anisolabis maritima), which occasionally form aggregations. We hypothesized that conspecific egg cannibalism is a byproduct of aggregation, and that cannibalism rates would increase with aggregation density; however, our results do not support this. We combined field data with a lab experiment to test the effectiveness of maternal nest defense in protecting nests from a conspecific. Nests with a guard had higher hatching success and lower rates of cannibalism than unattended nests in the presence of a conspecific. We also measured body and forcep size to see whether the outcome of contests was determined by relative size. Female guards who were larger relative to the invading conspecific maintained their nest and had higher hatching success than females who were relatively smaller, suggesting that the maritime earwig is under directional selection for larger body and/or forcep size.     

Chemical aspects of mass spawning in corals. II. (-)-Epi-thunbergol,the sperm attractant in the eggs of the soft coral Lobophytum crassum (Cnidaria: Octocorallia)

Sea turtles migrate between feeding and nesting areas that are often geographically separated by hundreds or thousands of kilometers. Observations of their aggregations at sea and at nesting beaches have led to the hypothesis that sea turtles migrate in socially structured groups. While this migratory strategy is common to many marine vertebrates, socially facilitated behavior is not well documented in testudines. In 1990 and 1991, we attached satellite transmitters to olive ridleys (Lepidochelys olivacea Eschscholtz) found ovipositing together during a mass nesting at Nancite Beach, Costa Rica, to determine whether they migrate independently or in groups after they leave the nesting beach. Results showed that the turtles were not spatially associated during the internesting period, were capable of re-establishing themselves as a group during a subsequent nesting emergence, and were not spatially associated during their postnesting migrations to oceanic feeding areas. We suggested that what appear to be socially structured groups of L. olivacea are in fact individual turtles simultaneously using the same habitat.     

Antarctic krill aggregation characteristics from acoustic observations in the Southwest Atlantic Ocean

The distributional features and physical characteristics of 4830 krill (Euphausia superba Dana) aggregations detected acoustically in the Southwest Atlantic between 26 January and 21 February 1981 are described. Results are compared with aggregations detected in the Indian Ocean. Aggregations in the Atlantic were larger, closer to the surface and to each other than in the Indian Ocean. Similar patterns in the distribution of aggregation spacing along survey transects were found in the two areas, although the pattern of spacings in the Atlantic indicates differences in the scale of aggregation. Serial interdependence of aggregation variables was minimal in the Atlantic, with aggregation thickness, length and spacing showing weak inter-relationships. Weak functional association, between water depth and aggregation thickness was evident. Investigation of variability in aggregation structure in relation to prevailing environmental conditions gave equivocal results and no clear association between any aggregation variable and prevailing hydrography was observed. The implications of these results for future studies on krill aggregation are discussed in relation to a conceptual framework which was developed from the present results and aimed at linking krill aggregation characteristics to environmental features.     

EFFECTS OF SOIL TYPE,PLOT CONDITION,AND SLOPE ON RUNOFF AND INTERRILL EROSION OF TWO SOILS IN THE LAKE TAHOE BASIN1

ABSTRACT: Few studies have addressed sediment discharge due to interrill erosion from natural and minimally disturbed alpine and subalpine forested watersheds. Infiltration, runoff, and surface erosion of two Tahoe Basin soils under several conditions were investigated using rainfall simulation. A significant three-way interaction among soil type, plot condition, and slope was identified. Although high erodibiity was commonly associated with disturbance and/or high slope, this was not always the case. Soil type, plot condition, slope, and duration of the event were all found to be important factors in determining the amount of erosion. Decreased water clarity in Lake Tahoe has been partly attributed to increased algal growth associated with surface runoff and erosion from adjacent watersheds. Interpretive evaluation for resource management planning should be event based and carefully delineated on a sitespecific basis.     

Development of a simulator for training of fetoscopic myelomeningocele surgery

Objective

To develop a realistic simulation model for laparotomy-assisted fetoscopic spina bifida aperta (SBa) surgery, to be used for training purposes and preoperative planning.

Methods

The predefined general requirement was a realistic model of an exteriorized uterus, allowing all neurosurgical steps of the intervention. The uterus was modelled using ultrasound and MRI images of a 25 weeks’ gravid uterus, consisting of flexible polyurethane foam coated with pigmented silicone. The fetal model, contained an opening on the dorsal side for a customizable spinal insert with all the aspects of a SBa, including a cele, placode, and myofascial and skin layer. The model was assessed in a series of validation experiments.

Results

Production costs are low, uterus and fetus are reusable. Placental localization and the level and size of the spinal defect are adjustable, enabling case-specific adaptations. All aspects of the simulator were scored close to realistic or higher for both appearance and functional capacities.

Conclusions

This innovative model provides an excellent training opportunity for centers that are starting a fetoscopic SBa repair program. It is the first simulation model with adjustable spinal defect and placental localisation. Further objective validation is required, but the potential for using this model in preoperative planning is promising.     

Fish Assemblage Response to a Small Dam Removal in the Eightmile River System,Connecticut, USA

We examined the effects of the Zemko Dam removal on the Eightmile River system in Salem, Connecticut, USA. The objective of this research was to quantify spatiotemporal variation in fish community composition in response to small dam removal. We sampled fish abundance over a 6-year period (2005–2010) to quantify changes in fish assemblages prior to dam removal, during drawdown, and for three years following dam removal. Fish population dynamics were examined above the dam, below the dam, and at two reference sites by indicator species analysis, mixed models, non-metric multidimensional scaling, and analysis of similarity. We observed significant shifts in fish relative abundance over time in response to dam removal. Changes in fish species composition were variable, and they occurred within 1 year of drawdown. A complete shift from lentic to lotic fishes failed to occur within 3 years after the dam was removed. However, we did observe increases in fluvial and transition (i.e., pool head, pool tail, or run) specialist fishes both upstream and downstream from the former dam site. Our results demonstrate the importance of dam removal for restoring river connectivity for fish movement. While the long-term effects of dam removal remain uncertain, we conclude that dam removals can have positive benefits on fish assemblages by enhancing river connectivity and fluvial habitat availability.     

Modelling Tundra Vegetation Response to Recent Arctic Warming

The Arctic land area has warmed by >1 °C in the last 30 years and there is evidence that this has led to increased productivity and stature of tundra vegetation and reduced albedo, effecting a positive (amplifying) feedback to climate warming. We applied an individual-based dynamic vegetation model over the Arctic forced by observed climate and atmospheric CO₂ for 1980–2006. Averaged over the study area, the model simulated increases in primary production and leaf area index, and an increasing representation of shrubs and trees in vegetation. The main underlying mechanism was a warming-driven increase in growing season length, enhancing the production of shrubs and trees to the detriment of shaded ground-level vegetation. The simulated vegetation changes were estimated to correspond to a 1.75 % decline in snow-season albedo. Implications for modelling future climate impacts on Arctic ecosystems and for the incorporation of biogeophysical feedback mechanisms in Arctic system models are discussed.