Regulation of Crustacean Molting: A Multi-Hormonal System

SYNOPSIS. In order to increase in size, arthropods must firstmolt (shed) their confining exoskeleton. This molting processis under the immediate control of the steroid molting hormone20-hydroxyecdysone (20-HE). Both the initial rise in circulatinghormone concentration and a coordinated decline are necessaryfor successful molting. Synthesis and/or release of ecdysone,the precursor to 20-HE, is regulated by the neuropeptide molt-inhibitinghormone (MIH). We have determined the primary amino acid sequenceof MIH in the lobster, Homarus americanus. This peptide hasa high degree of identity with the lobster hyperglycemic hormone.Another endocrine factor that appears to be involved in moltingis the juvenile hormone-like terpenoid methyl farnesoate (MF).We have characterized hemolymph MF binding proteins during themolt cycle. In addition, recent data indicate that MF may stimulatethe secretion of 20-HE in vivo and in vitro.     

A test for negative frequency-dependent mating success as a function of male colour pattern in the bluefin killifish

Rare male mating advantage (a form of negative frequency dependence) is frequently proposed as a mechanism for the maintenance of genetic variation within populations. This hypothesis is attractive for systems with pronounced male colour polymorphism because it can maintain particularly high levels of variation. We tested for negative frequency-dependent mating success between yellow and red male colour patterns in bluefin killifish, Lucania goodei . Lucania goodei populations harbour substantial colour pattern polymorphism, and a large proportion of this variation has a genetic basis. We established outdoor mesocosms with red and yellow males in three different ratios: yellow rare (one yellow ♂ : five red ♂), even (three yellow ♂ : three red ♂), and red rare (five yellow ♂ : one red ♂). We obtained eggs and used microsatellites to determine paternity. By contrast to expectations, we found no support for a rare male mating advantage. Red males had slightly higher spawning success than yellow males, particularly in replicates with large clutches and when red males were rare. However, yellow males did not have higher mating success when rare. We discuss alternative mechanisms for the maintenance of the polymorphism as well as the potential reasons for the lack of a rare male mating advantage.  © 2009 The Linnean Society of London, Biological Journal of the Linnean Society , 2009, 98 , 489–500.     

Physiological and environmental regulation of interannual variability in CO2 exchange on rangelands in the western United States

For most ecosystems, net ecosystem exchange of CO₂ (NEE) varies within and among years in response to environmental change. We analyzed measurements of CO₂ exchange from eight native rangeland ecosystems in the western United States (58 site‐years of data) in order to determine the contributions of photosynthetic and respiratory (physiological) components of CO₂ exchange to environmentally caused variation in NEE. Rangelands included Great Plains grasslands, desert shrubland, desert grasslands, and sagebrush steppe. We predicted that (1) week‐to‐week change in NEE and among‐year variation in the response of NEE to temperature, net radiation, and other environmental drivers would be better explained by change in maximum rates of ecosystem photosynthesis (A_max) than by change in apparent light‐use efficiency (α) or ecosystem respiration at 10 °C (R₁₀) and (2) among‐year variation in the responses of NEE, A_max, and α to environmental drivers would be explained by changes in leaf area index (LAI). As predicted, NEE was better correlated with A_max than α or R₁₀ for six of the eight rangelands. Week‐to‐week variation in NEE and physiological parameters correlated mainly with time‐lagged indices of precipitation and water‐related environmental variables, like potential evapotranspiration, for desert sites and with net radiation and temperature for Great Plains grasslands. For most rangelands, the response of NEE to a given change in temperature, net radiation, or evaporative demand differed among years because the response of photosynthetic parameters (A_max, α) to environmental drivers differed among years. Differences in photosynthetic responses were not explained by variation in LAI alone. A better understanding of controls on canopy photosynthesis will be required to predict variation in NEE of rangeland ecosystems.     

Iron limits primary productivity during spring bloom development in the central North Atlantic

We present in situ biophysical measurements and bioassay experiments that demonstrate iron limitation of primary productivity during the spring bloom in the central North Atlantic. Mass balance calculations indicate that nitrate drawdown is iron (Fe)-limited and that aeolian Fe supply to this region cannot support maximal phytoplankton growth during the bloom. Using a simple simulation model, we show that relief of Fe limitation during the spring bloom can increase nitrate drawdown and, hence, new primary production, by 70%. We conclude that the episodic nature of iron supplied by dust deposition is an important factor controlling the dynamics of the spring bloom. From this, we hypothesize that variability in the timing and magnitude of the spring bloom in response to aeolian Fe supply will affect carbon drawdown and food web dynamics in the central North Atlantic.     

Temperature‐related increases in grass growth and greater competition for food drive earlier migrational departure of wintering Whooper Swans

Understanding the departure decisions of migratory birds is critical for determining how changing climatic conditions will influence subsequent arrival times on the breeding grounds. A long‐term dataset (1972–2008) of Whooper Swan Cygnus cygnus departure dates from a wintering site in Ireland was used to assess the factors determining the timing of migration. Early and late migrating swans showed different departure patterns. Earlier wintering ground departure was more pronounced for the first 50% of the population than the last 10% of departing individuals. Earlier departure was associated with an increase in February temperatures at the wintering site for all departure phases except the date when the last individual departed. The date by which the first 50% of Swans had departed was earlier with increasing numbers of wintering Swans, suggesting that competition on the wintering grounds may further influence the timing of departure. The results also suggested that departure is mediated by the influence of spring temperature on food resources, with increased February grass growth in warmer years enabling earlier departure of migrating Swans. To determine why arrival dates in the breeding ground have altered, environmental conditions in the wintering grounds must be taken into account.     

Do isolation and local habitat jointly limit the structure of stream invertebrate assemblages?

1. Both local and regional processes simultaneously control species assemblages depending on spatial habitat configuration. In dendritic networks like streams, the unique spatial arrangement of habitats produces various combinations of local habitat size and isolation. Stream invertebrate assemblages could therefore be controlled by different combinations of local and regional processes, depending on their location in the network. 2. Using quantile regression, we investigated how local habitat size, local environmental conditions and spatial isolation influenced variation in assemblage composition. Adult Trichoptera and benthic macroinvertebrate assemblages were represented by non‐metric multidimensional scaling (NMDS) ordination scores, as were local environmental conditions, in four headwater stream networks in New Zealand. 3. With increasing local habitat size, there was a decrease in variation in assemblage composition (NMDS scores) of both adult Trichoptera and benthic macroinvertebrates. This relationship between habitat size and assemblage variation was related to local habitat conditions at the upper limit of assemblage variability and spatial isolation at the lower limit of assemblage variability, for both adult Trichoptera and benthic assemblages, indicating joint local and regional controls on stream invertebrate assemblages. 4. The relationships between local assemblages and their neighbours, based on community similarity scores, differed between benthic macroinvertebrates and adult Trichoptera. For benthic assemblages, the larger the stream, the more similar assemblages were to neighbouring assemblages, whereas there was no consistent relationship between assemblage similarity and stream size for adult Trichoptera. This difference in structuring could be attributed to contrasting spatial influences linked to the different dispersal modes of adults and larvae. However, because adult and benthic assemblages are not independent, the influence of life stage on spatial distribution is difficult to determine (i.e. it is essentially a ‘chicken and egg’ argument). 5. Overall, our approach using quantile regression to evaluate limit responses, rather than regressions on means, has highlighted the joint importance of local habitat and spatial processes in structuring stream invertebrate assemblages. Furthermore, we have provided evidence for the importance of the spatial network arrangement and interactions between life stages and dispersal processes, in structuring stream assemblages.     

Range dynamics of small mammals along an elevational gradient over an 80‐year interval

One expected response to observed global warming is an upslope shift of species elevational ranges. Here, we document changes in the elevational distributions of the small mammals within the Ruby Mountains in northeastern Nevada over an 80‐year interval. We quantified range shifts by comparing distributional records from recent comprehensive field surveys (2006–2008) to earlier surveys (1927–1929) conducted at identical and nearby locations. Collector field notes from the historical surveys provided detailed trapping records and locality information, and museum specimens enabled confirmation of species' identifications. To ensure that observed shifts in range did not result from sampling bias, we employed a binomial likelihood model (introduced here) using likelihood ratios to calculate confidence intervals around observed range limits. Climate data indicate increases in both precipitation and summer maximum temperature between sampling periods. Increases in winter minimum temperatures were only evident at mid to high elevations. Consistent with predictions of change associated with climate warming, we document upslope range shifts for only two mesic‐adapted species. In contrast, no xeric‐adapted species expanded their ranges upslope. Rather, they showed either static distributions over time or downslope contraction or expansion. We attribute these unexpected findings to widespread land‐use driven habitat change at lower elevations. Failure to account for land‐use induced changes in both baseline assessments and in predicting shifts in species distributions may provide misleading objectives for conservation policies and management practices.     

Lagged effects of experimental warming and doubled precipitation on annual and seasonal aboveground biomass production in a tallgrass prairie

Global climate change is expected to result in a greater frequency of extreme weather, which can cause lag effects on aboveground net primary production (ANPP). However, our understanding of lag effects is limited. To explore lag effects following extreme weather, we applied four treatments (control, doubled precipitation, 4 °C warming, and warming plus doubled precipitation) for 1 year in a randomized block design and monitored changes in ecosystem processes for 3 years in an old‐field tallgrass prairie in central Oklahoma. Biomass was estimated twice in the pretreatment year, and three times during the treatment and posttreatment years. Total plant biomass was increased by warming in spring of the treatment year and by doubled precipitation in summer. However, double precipitation suppressed fall production. During the following spring, biomass production was significantly suppressed in the formerly warmed plots 2 months after treatments ceased. Nine months after the end of treatments, fall production remained suppressed in double precipitation and warming plus double precipitation treatments. Also, the formerly warmed plots still had a significantly greater proportion of C₄ plants, while the warmed plus double precipitation plots retained a high proportion of C₃ plants. The lag effects of warming on biomass did not match the temporal patterns of soil nitrogen availability determined by plant root simulator probes, but coincided with warming‐induced decreases in available soil moisture in the deepest layers of soil which recovered to the pretreatment pattern approximately 10 months after the treatments ceased. Analyzing the data with an ecosystem model showed that the lagged temporal patterns of effects of warming and precipitation on biomass can be fully explained by warming‐induced differences in soil moisture. Thus, both the experimental results and modeling analysis indicate that water availability regulates lag effects of warming on biomass production.