Spatially heterogeneous refugia and predation risk in intertidal salt marshes

The effect of habitat structure on interactions between predators and prey may vary spatially. In estuarine salt marshes, heterogeneity in refuge quality derives from spatial variation in vegetation structure and in tidal inundation. We investigated whether predation by blue crabs on periwinkle snails was influenced by distance from the seaward edge of the salt marsh and by characteristics of the primary habitat structure, smooth cordgrass ( Spartina alterniflora ). Spartina may provide refuge for snails and interfere with foraging by crabs. Furthermore, predation risk should decline with distance from the seaward edge because landward regions require more travel time for crabs during tidal inundation. We investigated these processes using a comparative survey of snails and habitat traits, an experiment that assessed the crab population and measured predation risk, and a size-structured model that estimated encounter rates. Taken together, these approaches indicated that predation risk for snails was lower where Spartina was present and was lower in a landward direction. Furthermore, Spartina architecture and distance from the seaward edge interacted. The strength of the predation gradient between seaward and landward regions of the marsh was greater where Spartina was tall or dense. These predation gradients emerge because vegetation and distance inland decrease encounter rates between crabs and snails. This study suggests that habitat modification, a process not uncommon in salt marshes, may have consequences for interactions among intertidal fauna.     

Spatial correlation as leading indicator of catastrophic shifts

Generic early-warning signals such as increased autocorrelation and variance have been demonstrated in time-series of systems with alternative stable states approaching a critical transition. However, lag times for the detection of such leading indicators are typically long. Here, we show that increased spatial correlation may serve as a more powerful early-warning signal in systems consisting of many coupled units. We first show why from the universal phenomenon of critical slowing down, spatial correlation should be expected to increase in the vicinity of bifurcations. Subsequently, we explore the applicability of this idea in spatially explicit ecosystem models that can have alternative attractors. The analysis reveals that as a control parameter slowly pushes the system towards the threshold, spatial correlation between neighboring cells tends to increase well before the transition. We show that such increase in spatial correlation represents a better early-warning signal than indicators derived from time-series provided that there is sufficient spatial heterogeneity and connectivity in the system.     

Regime shifts: catastrophic responses of ecosystems to human impacts

Evidence of abrupt changes in ecosystem states, such as sudden eutrophication in lakes, has been increasingly reported in a variety of aquatic and terrestrial systems. Ecosystems may have more than one state with a self-stabilizing mechanism, so that a shift between states does not occur frequently and is not readily reversible. These big changes are termed regime shifts where often one state is preferred over another. Thus, regime shifts are problematic for ecosystem managers, and the need exists for studies that lead to the identification of thresholds of key variables that trigger regime shifts. Regime shifts are currently difficult to predict and in many cases may be caused by the human pursuit of efficiency in land and water productivity in the last few decades. Here I briefly introduce a theoretical approach to predict the shift between a clear-water state and a turbid state in lakes, the best-studied example of regime shifts. This paper also discusses alternative states in other natural systems besides ecosystems to draw more attention to the research currently being performed on regime shifts. Motomi Genkai-Kato is the recipient of the 10th Denzaburo Miyadi Award.     

Spatially heterogeneous stochasticity and the adaptive diversification of dormancy

Diversified bet‐hedging, a strategy that leads several individuals with the same genotype to express distinct phenotypes in a given generation, is now well established as a common evolutionary response to environmental stochasticity. Life‐history traits defined as diversified bet‐hedging (e.g. germination or diapause strategies) display marked differences between populations in spatial proximity. In order to find out whether such differences can be explained by local adaptations to spatially heterogeneous environmental stochasticity, we explored the evolution of bet‐hedging dormancy strategies in a metapopulation using a two‐patch model with patch differences in stochastic juvenile survival. We found that spatial differences in the level of environmental stochasticity, restricted dispersal, increased fragmentation and intermediate survival during dormancy all favour the adaptive diversification of bet‐hedging dormancy strategies. Density dependency also plays a major role in the diversification of dormancy strategies because: (i) it may interact locally with environmental stochasticity and amplify its effects; however, (ii) it can also generate chaotic population dynamics that may impede diversification. Our work proposes new hypotheses to explain the spatial patterns of bet‐hedging strategies that we hope will encourage new empirical studies of this topic.     

Linking habitat modification to catastrophic shifts and vegetation patterns in bogs

Paleoecological studies indicate that peatland ecosystems may exhibit bistability. This would mean that these systems are resilient to gradual changes in climate, until environmental thresholds are passed. Then, ecosystem stability is lost and rapid shifts in surface and vegetation structure at landscape scale occur. Another remarkable feature is the commonly observed self-organized spatial vegetation patterning, such as string-flark and maze patterns. Bistability and spatial self-organization may be mechanistically linked, the crucial mechanism being scale-dependent (locally positive and longer-range negative) feedback between vegetation and the peatland environment. Focusing on bogs, a previous model study shows that nutrient accumulation by vascular plants can induce such scale-dependent feedback driving pattern formation. However, stability of bog microforms such as hummocks and hollows has been attributed to different local interactions between Sphagnum, vascular plants, and the bog environment. Here we analyze both local and longer-range interactions in bogs to investigate the possible contribution of these different interactions to vegetation patterning and stability. This is done by a literature review, and subsequently these findings are incorporated in the original model. When Sphagnum and encompassing local interactions are included in this model, the boundaries between vegetation types become sharper and also the parameter region of bistability drastically increases. These results imply that vegetation patterning and stability of bogs could be synergistically governed by local and longer-range interactions. Studying the relative effect of these interactions is therefore suggested to be an important component of future predictions on the response of peatland ecosystems to climatic changes.     

Spatially resolved microrheology of heterogeneous biopolymer hydrogels using covalently bound microspheres

Characterization of the rheological properties of heterogeneous biopolymers is important not only to understand the effect of substrate elasticity on cell behaviors, but also to provide insights into mechanical changes during cellular remodeling of the environment. Conventional particle-tracking microrheology (PTM) techniques are compromised by probe–network slippage and cage-hopping problems, and require a priori knowledge of network mesh size in order to determine a suitable probe size. We demonstrated here the usefulness of covalently bound probes for PTM of biopolymers to overcome the above limitations. We showed that, in a well-defined system like polyacrylamide gels, surface-modified probe particles using a zero-length crosslinker provided more reliable measurements of network mechanics as compared to standard carboxylated probes. We further demonstrated that appropriate surface modification of microspheres for PTM circumvented the requirement of using microspheres larger than the network mesh, an approach typically considered to be ideal. Using the method presented in this study, we found the local network at the leading edge of a typical C6 glioma cell to be stiffer as compared to the side. Our findings established that permanent interaction between the probe and network is crucial to reliably measure the local network mechanics in reconstituted, heterogeneous networks using PTM.     

Spatially heterogeneous impact of climate change on small mammals of montane California

Resurveys of historical collecting localities have revealed range shifts, primarily leading edge expansions, which have been attributed to global warming. However, there have been few spatially replicated community-scale resurveys testing whether species'' responses are spatially consistent. Here we repeated early twentieth century surveys of small mammals along elevational gradients in northern, central and southern regions of montane California. Of the 34 species we analysed, 25 shifted their ranges upslope or downslope in at least one region. However, two-thirds of ranges in the three regions remained stable at one or both elevational limits and none of the 22 species found in all three regions shifted both their upper and lower limits in the same direction in all regions. When shifts occurred, high-elevation species typically contracted their lower limits upslope, whereas low-elevation species had heterogeneous responses. For high-elevation species, site-specific change in temperature better predicted the direction of shifts than change in precipitation, whereas the direction of shifts by low-elevation species was unpredictable by temperature or precipitation. While our results support previous findings of primarily upslope shifts in montane species, they also highlight the degree to which the responses of individual species vary across geographically replicated landscapes.     

A synthetic peptide from Helix aspersa raises arterial pressure in rats

New FMRF-amide like peptide, pGlu-Asp-Pro-Phe-Leu-Arg-Phe-NH2, originally isolated from the ganglia of Helix aspersa, was synthetized. Intravenous injections of this peptide (40-300 micrograms/kg) produce rapid dose dependent increase in the blood pressure and heart rate of anaesthetised rats. Since alpha-adrenoreceptor blockade with prazosin eliminated pressor response, it is concluded that the increase in the arterial pressure is mediated by sympathetic activation.     

Effects of strength and timing of harvest on seasonal population models: stability switches and catastrophic shifts

Population abundance of many species is controlled by a combination of density-dependent processes during different periods of the annual cycle. In the context of population exploitation or conservation programs, sequential density dependence has the potential to dramatically change population responses to harvesting. Looking for a better understanding of the potential effects of harvesting on the dynamics of seasonal populations, we carry out a theoretical analysis of a discrete model for a semelparous population with an annual cycle involving three discrete density-dependent events: breeding, natural mortality, and harvesting. Our study reveals how the interplay between the model parameters determines the importance of harvest timing on stability and population abundance, especially when two nontrivial stable equilibria coexist. We address the possibility for compensatory mortality and report different forms of the hydra effect, including non-smooth ones due to catastrophic shifts. These drastic switches may include hysteresis, which has important implications for conservation goals. Regarding variability, we show that increasing the harvesting effort may either stabilize or destabilize the population, and these effects strongly depend on harvest timing and natural mortality rates. Our results also emphasize the importance of sampling populations after every discrete event occurs during one cycle. Indeed, though the dynamics are not affected by census timing, the model shows that changes in population abundance in response to changes in harvesting pressure are substantially different depending on when population is sampled. Thus, a manager would receive different (and sometimes contradictory) messages depending on census time, which could lead to managing mistakes.     

Equilibrium shifts in enzyme reactions at high pressure

The effect of pressure on the equilibrium of a reaction was studied. Theoretical equilibrium constants and product concentrations have been calculated at elevated pressures. The theory is illustrated with an example of l-malate synthesis catalyzed by a fumarase. To study shifts in the equilibrium relatively low pressures can be applied (50–200 MPa), but our calculations show that for process optimisation much higher pressures (up to 1000 MPa) have to be used.

At these higher pressures, more stable enzymes are needed. We performed experiments with the hyperthermophilic β-glycosidase from Pyrococcus furiosus as a catalyst. Oligosaccharides were synthesized from glucose in an equilibrium reaction at pressures from 0.1 to 500 MPa. The enzyme remained active at 500 MPa. The equilibrium of the reaction was influenced by pressure and shifted towards the hydrolysis side, decreasing final oligosaccharide concentrations with increasing pressure. This pressure dependence of the final product concentration and the equilibrium constant could be described with a positive reaction volume of 2.4 mol/cm³.     

Regime shifts and heterogeneous trends in malaria time series from Western Kenya Highlands

Large malaria epidemics in the East African highlands during the mid and late 1990s kindled a stream of research on the role that global warming might have on malaria transmission. Most of the inferences using temporal information have been derived from a malaria incidence time series from Kericho. Here, we report a detailed analysis of 5 monthly time series, between 15 and 41 years long, from West Kenya encompassing an altitudinal gradient along Lake Victoria basin. We found decreasing, but heterogeneous, malaria trends since the late 1980s at low altitudes (<1600 m), and the early 2000s at high altitudes (>1600 m). Regime shifts were present in 3 of the series and were synchronous in the 2 time series from high altitudes. At low altitude, regime shifts were associated with a shift from increasing to decreasing malaria transmission, as well as a decrease in variability. At higher altitudes, regime shifts reflected an increase in malaria transmission variability. The heterogeneity in malaria trends probably reflects the multitude of factors that can drive malaria transmission and highlights the need for both spatially and temporally fine-grained data to make sound inferences about the impacts of climate change and control/elimination interventions on malaria transmission.     

Highly heterogeneous residual malaria risk in western Thailand

Over the past decades, the malaria burden in Thailand has substantially declined. Most infections now originate from the national border regions. In these areas, the prevalence of asymptomatic infections is still substantial and poses a challenge for the national malaria elimination program. To determine epidemiological parameters as well as risk factors for malaria infection in western Thailand, we carried out a cohort study in Kanchanaburi and Ratchaburi provinces on the Thailand-Myanmar border. Blood samples from 999 local participants were examined for malaria infection every 4 weeks between May 2013 and Jun 2014. Prevalence of Plasmodium falciparum and Plasmodium vivax was determined by quantitative PCR (qPCR) and showed a seasonal variation with values fluctuating from 1.7% to 4.2% for P. vivax and 0% to 1.3% for P. falciparum. Ninety percent of infections were asymptomatic. The annual molecular force of blood-stage infection (_molFOB) was estimated by microsatellite genotyping to be 0.24 new infections per person-year for P. vivax and 0.02 new infections per person-year for P. falciparum. The distribution of infections was heterogenous, that is, the vast majority of infections (>80%) were found in a small number of individuals (<8% of the study population) who tested positive at multiple timepoints. Significant risk factors were detected for P. vivax infections, including previous clinical malaria, occupation in agriculture and travel to Myanmar. In contrast, indoor residual spraying was associated with a protection from infection. These findings provide a recent landscape of malaria epidemiology and emphasize the importance of novel strategies to target asymptomatic and imported infections.     

Temperature-induced shifts in selective pressure at a critical developmental transition

Selective mortality within a population, based on the phenotype of individuals, is the foundation of the theory of natural selection. We examined temperature-induced shifts in the relationships among early life history traits and survivorship over the embryonic and larval stages of a tropical damselfish, Pomacentrus amboinensis. Our experiments show that temperature determines the intensity of selective mortality, and that this changes with ontogeny. The size of energy stores determined survival through to hatching, after which egg size became a good indicator of fitness as predicted by theoretical models. Yet, the benefits associated with egg size were not uniform among test temperatures. Initial egg size positively influenced larval survival at control temperature (29 °C). However, this embryonic trait had no effect on post-hatching longevity of individuals reared at the higher (31 °C) and lower (25 °C) end of the temperature range. Overall, our findings indicate that the outcome of selective mortality is strongly dependent on the interaction between environment conditions and intrinsic developmental schedules.     

Effect of lower-body positive pressure on postural fluid shifts in men

To quantify the effect of 60 mm Hg lower-body positive pressure (LBPP) on orthostatic blood-volume shifts, the mass densities (+/- 0.1 g.1-1) of antecubital venous blood and plasma were measured in five men (27-42 years) during combined tilt table/antigravity suit inflation and deflation experiments. The densities of erythrocytes, whole-body blood, and of the shifted fluid were computed and the magnitude of fluid and protein shifts were calculated during head-up tilt (60 degrees) with and without application of LBPP. During 30-min head-up tilt with LBPP, blood density (BD) and plasma density (PD) increased by 1.6 +/- 0.3 g.1-1, and by 0.8 +/- 0.2 g.1-1 (+/- SD) (N = 9), respectively. In the subsequent period of tilt without LBPP, BD and PD increased further to + 3.6 +/- 0.9 g.1-1, and to + 2.0 +/- 0.7 g.1-1 (N = 7), compared to supine control. The density increases in both periods were significant (p less than 0.05). Erythrocyte density remained unaltered with changes in body position and pressure suit inflation/deflation. Calculated shifted-fluid densities (FD) during tilt with LBPP (1006.0 +/- 1.1 g.1-1, N = 9), and for subsequent tilt after deflation (1002.8 +/- 4.1 g.1-1, N = 7) were different from each other (p less than 0.03). The plasma volume decreased by 6.0 +/- 1.2% in the tilt-LBPP period, and by an additional 6.4 +/- 2.7% of the supine control level in the subsequent postdeflation tilt period. The corresponding blood volume changes were 3.7 +/- 0.7% (p less than 0.01), and 3.5 +/- 2.1% (p less than 0.05), respectively. Thus, about half of the postural hemo-concentration occurring during passive head-up tilt was prevented by application of 60 mm Hg LBPP.     

How NaCl raises blood pressure: a new paradigm for the pathogenesis of salt-dependent hypertension

Excess dietary salt is a major cause of hypertension. Nevertheless, the specific mechanisms by which salt increases arterial constriction and peripheral vascular resistance, and thereby raises blood pressure (BP), are poorly understood. Here we summarize recent evidence that defines specific molecular links between Na(+) and the elevated vascular resistance that directly produces high BP. In this new paradigm, high dietary salt raises cerebrospinal fluid [Na(+)]. This leads, via the Na(+)-sensing circumventricular organs of the brain, to increased sympathetic nerve activity (SNA), a major trigger of vasoconstriction. Plasma levels of endogenous ouabain (EO), the Na(+) pump ligand, also become elevated. Remarkably, high cerebrospinal fluid [Na(+)]-evoked, locally secreted (hypothalamic) EO participates in a pathway that mediates the sustained increase in SNA. This hypothalamic signaling chain includes aldosterone, epithelial Na(+) channels, EO, ouabain-sensitive α(2) Na(+) pumps, and angiotensin II (ANG II). The EO increases (e.g.) hypothalamic ANG-II type-1 receptor and NADPH oxidase and decreases neuronal nitric oxide synthase protein expression. The aldosterone-epithelial Na(+) channel-EO-α(2) Na(+) pump-ANG-II pathway modulates the activity of brain cardiovascular control centers that regulate the BP set point and induce sustained changes in SNA. In the periphery, the EO secreted by the adrenal cortex directly enhances vasoconstriction via an EO-α(2) Na(+) pump-Na(+)/Ca(2+) exchanger-Ca(2+) signaling pathway. Circulating EO also activates an EO-α(2) Na(+) pump-Src kinase signaling cascade. This increases the expression of the Na(+)/Ca(2+) exchanger-transient receptor potential cation channel Ca(2+) signaling pathway in arterial smooth muscle but decreases the expression of endothelial vasodilator mechanisms. Additionally, EO is a growth factor and may directly participate in the arterial structural remodeling and lumen narrowing that is frequently observed in established hypertension. These several central and peripheral mechanisms are coordinated, in part by EO, to effect and maintain the salt-induced elevation of BP.     

Blood pressure and heart rate variability in workers of 8-hour shifts

This study examined the effects of shiftwork on the cardiovascular system. The blood pressure (BP) and heart rate variability (HRV) of 134 male workers, who worked 8-hour shifts with rapid rotation of shifts at 3-day intervals, were examined for all the three shifts. In addition, the job stress was measured by Karasek's JCQ 49-item questionnaire and the circadian type was assessed by the morningness-eveningness questionnaire. The smoking and alcohol drinking habits, marital status and past medical history were also obtained. The method of analyzing the measured data based on a mixed model was used to illustrate the association between the shiftwork duration and the BP or HRV. The average age of workers was 29 years (between 25-44). Among them, 77.9% were current smokers, 50% showed the passive type of job strain in Karasek's model. The mean shiftwork duration was 5.21 years (range 5.4 months--10 years). In the circadian type, none of them belonged to a definitely morning type or a definitely evening type. In the multivariate analysis adjusted by age, job strain, shift, circadian rhythm and smoking, the blood pressure showed significantly increasing trends according to shiftwork duration in both the systolic and diastolic BP. The heart rate variability also showed a significantly decreasing trend according to the shiftwork duration in both the parasympathetic and sympathetic functions (p < 0.05). These results suggests that there are negative health effects arising from shiftwork on the cardiovascular system.     

Codon-substitution models for heterogeneous selection pressure at amino acid sites

Comparison of relative fixation rates of synonymous (silent) and nonsynonymous (amino acid-altering) mutations provides a means for understanding the mechanisms of molecular sequence evolution. The nonsynonymous/synonymous rate ratio (omega = d(N)d(S)) is an important indicator of selective pressure at the protein level, with omega = 1 meaning neutral mutations, omega < 1 purifying selection, and omega > 1 diversifying positive selection. Amino acid sites in a protein are expected to be under different selective pressures and have different underlying omega ratios. We develop models that account for heterogeneous omega ratios among amino acid sites and apply them to phylogenetic analyses of protein-coding DNA sequences. These models are useful for testing for adaptive molecular evolution and identifying amino acid sites under diversifying selection. Ten data sets of genes from nuclear, mitochondrial, and viral genomes are analyzed to estimate the distributions of omega among sites. In all data sets analyzed, the selective pressure indicated by the omega ratio is found to be highly heterogeneous among sites. Previously unsuspected Darwinian selection is detected in several genes in which the average omega ratio across sites is <1, but in which some sites are clearly under diversifying selection with omega > 1. Genes undergoing positive selection include the beta-globin gene from vertebrates, mitochondrial protein-coding genes from hominoids, the hemagglutinin (HA) gene from human influenza virus A, and HIV-1 env, vif, and pol genes. Tests for the presence of positively selected sites and their subsequent identification appear quite robust to the specific distributional form assumed for omega and can be achieved using any of several models we implement. However, we encountered difficulties in estimating the precise distribution of omega among sites from real data sets.     

Spatially variable propagule pressure and herbivory influence invasion of chaparral shrubland by an exotic grass

Although numerous studies have identified mechanisms that either resist or facilitate biological invasions, few studies have explicitly tested how resisting and facilitating mechanisms interact to drive invasion success. In California, USA, undisturbed Mediterranean-type shrublands have resisted invasion by the perennial tussock grass Cortaderia jubata. In some cases, however, this resistance has been spectacularly breached even in the absence of large-scale disturbance. I tested the hypothesis that these invasions are facilitated by local reductions in the strength of biotic resistance. I evaluated invasive success using C. jubata seed and seedling additions at different microhabitats: the edge of a chaparral stand, under shrub canopy at different distances from the stand edge, and in canopy gaps within the stand. When left exposed to mammalian herbivores, seedling survivorship decreased sharply from nearly 40% on the stand edge to zero just 10 m into the stand. When transplants were protected from herbivory, however, distance from the edge had no significant influence on transplant survivorship. Seedling emergence was also greater on the edge and in canopy gaps than under the canopy, but these differences were not caused by differences in herbivory. The flux of invasive propagules reaching the soil surface was immense and greater along the edge and within gaps than under the stand canopy. Mirroring these patterns, naturally occurring seedling abundance declined dramatically with distance from the stand edge, and seedlings were far more common within stand gaps than would be expected given gap frequency within the stand. Despite strong biotic resistance to invasion within the stand, the cover of C. jubata has increased 20% over the last 9 years. These results suggest that the relative amount of susceptible edge habitat and the supply of invasive propagules can facilitate invasion even in the face of strong local biotic resistance.     

L-NAME raises systolic blood pressure in the pithed rat by a direct adrenal epinephrine releasing action

It is generally thought that inhibition of nitric oxide synthase leads to blood pressure elevation largely through reduction in vascular levels of the vasodilator nitric oxide. However, there are several reports suggesting that NO synthase inhibitors cause adrenal epinephrine (E) release by both central and peripheral mechanisms. We investigated the role of adrenal E in the pressor effects of the nitric oxide synthase inhibitor L-NAME in the pithed rat to help distinguish central from peripherally mediated actions. L-NAME (10 mg/kg) raised both systolic and diastolic BP by about 30 mm Hg (P < .01) in the absence of exogenous electrical stimulation of sympathetic nerves. During stimulation at 10 V and frequencies of 1 or 2 Hz, systolic BP was about 70 mm Hg higher in L-NAME treated rats than in drug free stimulated rats. This enhancement of systolic BP by L-NAME was less pronounced at 5 or 10 Hz stimulation frequencies. Following these types of electrical stimulations of pithed rats, both plasma norepinephrine (NE) and E levels were dramatically elevated above resting plasma levels. L-NAME pretreatment of these electrically stimulated rats increased plasma E levels by an additional 60% and decreased NE by 18%. Acute adrenalectomy dramatically reduced plasma E levels and abolished the ability of L-NAME to enhance the pressor effect of sympathetic stimulation. In contrast, acute adrenalectomy of unstimulated pithed rats did not significantly reduce the pressor response to L-NAME. We conclude that adrenal E release may mediate much of the systolic pressor response of L-NAME in the stimulated pithed rat, but the magnitude of this effect varies with stimulation frequency. Since pithing disrupts central pathways, this induction of adrenal E release by L-NAME is a peripheral effect.