Growth and fecundity of Daphnia after diapause and their impact on the development of a population

Laboratory and field investigations revealed that the life history traits of exephippial and parthenogenetic generations of Daphnia differ substantially. Daphniids hatching from resting eggs grow faster and their definitive body sizes are bigger than of hatchlings from subitaneous eggs. Size at maturity for exephippial animals is significantly larger. In spite of this, they mature a few days earlier than parthenogenetic females. In this study, the difference was 3–4 days for the laboratory experiments and 1–3 days for the field. Fecundity of the exephippial generation is markedly higher. Here, the clutch size for this generation was up to 3.5–4.0 times as large as for the parthenogenetic generation. Moreover, obtained results suggest that the relationship between clutch size and body length for both generations differ significantly.Estimates of the intrinsic rate of increase for field Daphnia populations demonstrated that life history traits of exephippial animals lead to a two or threefold higher rate of increase in the conditions of invertebrate predation pressure. Under moderate fish pressure, obtained r values for the daphniids hatching from resting eggs were larger than those from subitaneous. High growth rate of exephippial females is disadvantageous only under the conditions of severe pressure by fish. Obtained results suggest that hatchlings from diapausing eggs an acceleration of population increase by several times during the beginning of the development of a population with periodical re-establishment from resting eggs.     

Relative importance of pico-, nano-, and microplankton to the productivity of Mountain Lake, Virginia

Studies of the relative importance of various phytoplankton size classes (pico-, nano-, micro-, and macro-) during thermal stratification at oligomesotrophic Mountain Lake, Giles County, Virginia were conducted from March through October 1994. A detailed time- and depth-series study was conducted to compare ¹⁴C-fixation rates in these phytoplankton size fractions. Data indicated that on average the microplankton were the most important size class in cell number and cell volume and contributed 95% of the total primary productivity. The picoplankton ranked second in importance constituting 5% of total primary productivity, whereas the relatively small numbers of nano- and macroplankton contributed little primary productivity. No obvious relationships were apparent among cell densities, cell volumes, and chlorophyll a values during late summer thermal stratification, possibly due to shifts in community structure at that time. The findings contrast with many other oligo- to mesotrophic ecosystems where the pico- and nanoplankton have demonstrated significantly greater importance, especially in summer.     

Sex ratio regulation in the waspEupelmus vuilleti (crwf.), an ectoparasitoid on bean weevil larvae (Hymenoptera: Pteromalidae)

In Africa, the seeds and/or pods ofVigna unguiculata andVigna radiata (Papilionacea) are attacked in fields and storage structures by bruchid beetlesCallosobruchus maculatus andBruchidius atrolineatus, on which parasitoid communities can develop. One of these parasitoids is the solitary ectoparasitoidEupelmus vuilleti (Eupelmidae). The storage conditions ofV. unguiculata andV. radiata favor the overlapping of all host stages during several months. These conditions suggest that female parasitoids would vary the sex ratio of their offspring according to the different sizes or developmental stages of hosts. The sex ratio ofE. vuilleti was strongly related to the developmental stage of the hostC. maculatus. Under our experimental conditions, where superparasitism is rare, the proportion of daughters varied between 5 and 25% on the third larval stage but reached 70 to 90% on the pupae. The increase in the proportion of daughters was also observed in the absence of superparasitism. In this case, there was an absolute coincidence between the sex ratio of eggs laid and that of emerged adults. Manipulation of the sex of the egg by the females seems to take place at the time of the egg's fertilization. The relation between host weight and egg sex showed that the male eggs are preferentially laid on lighter host larvae and the female eggs on heavier ones.     

Maternal behavior and clutch manipulation in the ring-legged earwig (Dermaptera: Carcinophoridae)

The expression and maintenance of maternal behavior in the earwig,Euborellia annulipes, was examined through manipulation of clutch size, age, and species and through observations of interactions between brooding females. Females underwent discrete gonadotrophic cycles culminating in oviposition of first clutches that were highly variable in size. Neither the head capsule width nor the age of the mother was correlated with clutch size. Maternal care extended through embryogenesis and for the week following hatching. Clutch removal significantly shortened the interclutch interval, indicating that the presence of brood inhibited the onset of the second gonadotrophic cycle. Brooding females readily accepted replacement clutches of the same age. Thus, mothers did not appear to distinguish their own eggs from those of other females. Experimental doubling of clutch size did not significantly reduce the proportion hatching or fledging. In contrast, reducing clutch size diminished the percentage successfully fledging. Manipulation of clutch age resulted in reduced hatching/fledging success. Placing two females, each with newly laid clutches, in the same cage usually resulted in egg transfer from the nest of one female to that of the other within 12 h. Nests of females with larger forceps were significantly more likely to contain both clutches. When mothers with first clutches were paired with mothers with third clutches, eggs were more likely to be transferred to the nest of the older female.E. annulipes females with newly laid clutches appeared to accept as replacement clutches eggs of the earwigDoru taeniatum. Alien clutches were maintained for the typical duration of embryogenesis; however, noD. taeniatum hatchlings were observed.     

Does the energy equivalence rule apply to intertidal macrobenthic communities?

Energy equivalence assumes equal contribution of large and small species to production and energy flow in communities. As in a double logarithmic plot, physiological rates decline with body weight by –0.25, log biomass should increase by 0.25 and log abundance decline by –0.75 with log species weight, when this concept is valid. This was tested with annual data sets of the macrobenthos of 4 intertidal sites in the German Wadden Sea (Königshafen) and 3 sites in a south Portuguese lagoon (Ria Formosa). Only abundance data from two of these sites displayed significantly negative slopes with mean body size of the species. Biomass and secondary production data were significantly positively correlated with mean body size for all Ria Formosa sites and also for the biomass of a mussel bed in Königshafen. However, high variation in body size of the individuals of a species limits interpretation of these plots.It is preferable to test this concept by body weight classes regardless of its species composition. At Königshafen, biomass and production displayed two distinct peaks. One peak at small body size was caused by browsing species. The other peak at larger body size was caused by animals which potentially extract their food from the water column. This bimodality was only vaguely reflected at one station in the Ria Formosa, possibly because of a dominance of detritus feeding species. In a normalized form (log biomass or production / width of size classvs. log size class), these spectra imply a dominance of small individuals in biomass and production at all sites (except for a mussel bank at Königshafen). This is interpreted as a consequence of permanent disturbances.     

Regressive development in neuronal structure during song learning in birds

We investigated the development of spiny neurons in the lateral magnocellular nucleus of the anterior neostriatum before, during, and after song learning in male zebra finches (Taeniopygia guttata). The frequency of dendritic spines, dendritic field size, and branching characteristics were quantified at different ages in Golgi-stained tissue using a three-dimensional computerized tracing system. During development, overall spine frequencies increase between 3 and 5 weeks and decrease thereafter. In particular, spine frequencies of middle segments decrease significantly by 14% between 5 and 7 weeks posthatching (p = 0.017). A further reduction of 48% occurs between 7 weeks and adulthood (p < 0.001), resulting in a spine reduction of 56% on middle segments between 35 days of age and adulthood. In addition to the reduction of spine frequencies, we find regressive events also on some of the neuronal parameters that we have quantified. In general, dendrites of adult animals terminate closer to the cell body than those of 7-, 5-, or 3-week-old birds. Whereas no changes in segment length of first- and second-order dendrites have been identified, third-order dendrites end 19% closer to the cell body in adults than in younger birds (p < 0.024). Second-order dendrites in adult animals branch less frequently than in 3-week-old animals (35%, p = 0.017). There is also a trend of a smaller number of tertiary branches in adulthood compared with 3-week-old birds (41%, p = 0.060). The morphological changes may be related to the function of this nucleus and the sensitive phase for song acquisition. © 1995 John Wiley & Sons, Inc.     

Parental effort of kestrels (Falco tinnunculus) in nest defense: effects of laying time, brood size, and varying survival prospects of offspring

We studied the nest defense behavior of Eurasian kestrels (Falcotinnunculus) towards a stuffed pine marten (Martes martes) througha 3-year vole cycle (1990–92) in western Finland. Survivalprobability of offspring decreases with a later start of breeding,and, therefore, early breeders should protect their offspringmore than late ones. We found this true for males during theincubation period, but not for females. In addition, we expectedthe nest defense intensity to increase with offspring number.During the incubation period, this was true for females, butnot for males. During the nestling phase, parents did not adjusttheir defense effort to natural or manipulated (by one to twoyoung) brood size. Survival prospects of kestrel offspring werehighest in the increasing vole year 1991 and lowest in the decreasingvole year 1992, and, therefore, we expected the defense activityof kestrels to follow the same trend. However, the oppositeresult appeared true for females with a similar tendency formales. Most hypotheses predicting avian nest defense behaviorwere not supported by our data. Temporally heterogeneous environmentand low degree of nest-site tenacity of migratory kestrels maymake them unfamiliar with environmental variation and survivalprospects of their offspring. Therefore, fitness benefits ofparental care are not predictable, and kestrels may thus adjusttheir parental effort to their own future reproductive potential(i.e., number of future breeding attempts), rather than to somecurrent investment indicator, like offspring age and number.     

The influence of habitat structure on arthropod diversity in Argentine semi-arid Chaco forest

Abstract. The Argentine Chaco is a mosaic of grassland and open forest habitats maintained by natural disturbance activities such as fire. Since the introduction of domestic livestock and other human activities, the balance of this mosaic has been significantly altered, both in plant species and structural composition. This study focuses on the impact of such changes on the diversity of ground-dwelling arthropods within semi-arid Chaco forest. Quantitative measures of habitat structure and arthropod diversity were taken in forest areas previously subjected to grazing, logging and ploughing. Results indicated that arthropod diversity was smaller on sites with reduced structural complexity, with marked changes in arthropod family composition. The habitat components relating to plant architectural and vertical diversity were particularly influential on arthropod diversity. The guild size ratio of predatory to non-predatory arthropods also differed significantly between habitats suggesting a change in the resource base available to some groups. The latter suggests a shift in the functional organisation of the forest ecosystem which could have important repercussions for the diversity of other trophic levels.     

Acorn mass and seedling growth in Quercus rubra in response to elevated CO2

Abstract. In order to explore whether seed size affects plant response to elevated CO₂, plants grown from red oak (Quercus rubra L.) acorns were studied for differences in their first year response to CO₂ concentrations of 350 and 700 μl/l. Overall, at final harvest, total biomass of plants grown in elevated CO₂ were 47 % larger than that of plants grown in ambient CO₂. There were significant interactions between CO₂ treatments and initial acorn mass for total biomass, as well as for root, leaf, and stem biomass. Although total biomass increased with increasing initial acorn mass for both high and ambient CO₂ plants, high CO₂ plants exhibited a greater increase than ambient CO₂ plants, as indicated by a steeper slope in high CO₂ plants. However, CO₂ levels did not affect biomass partitioning traits, such as root/shoot ratio, leaf, stem, and root weight ratios, and leaf area ratio. These results suggest that variation in seed size or initial plant size can cause intraspecific variation in response to elevated CO₂.     

Modelling the effect of treatment and behavioral change in HIV transmission dynamics

In this paper we analyze a model for the HIV-infection transmission in a male homosexual population. In the model we consider two types of infected individuals. Those that are infected but do not know their serological status and/or are not under any sort of clinical /therapeutical treatment, and those who are. The two groups of infectives differ in their incubation time, contact rate with susceptible individuals, and probability of disease transmission. The aim of this article is to study the roles played by detection and changes in sexual behavior in the incidence and prevalence of HIV. The analytical results show that there exists a unique endemic equilibrium which is globally asymptotically stable under a range of parameter values whenever a detection /treatment rate and an indirect measure of the level of infection risk are sufficiently large. However, any level of detection/ treatment rate coupled with a decrease of the transmission probability lowers the incidence rate and prevalence level in the population. In general, only significant reductions in the transmission probability (achieved through, for example, the adoption of safe sexual practices) can contain effectively the spread of the disease.