Spatial heterogeneity confounded ozone-exposure experiment in semi-natural grassland

Interpretation of observations from manipulative experiments is often complicated by a multitude of uncontrolled processes operating at various spatial and temporal scales. As such processes may differ among experimental plots there is a risk that effects of experimental treatments are confounded. Here we report on a free-air ozone-exposure experiment in permanent semi-natural grassland that suggested strong ozone effects on community productivity after 5 years. We tested ozone effects and investigated the potential of confounding due to changes in nutrient management. Repeated-measure ANOVA revealed mainly negative temporal trends for frequency of abundant productive plant species. Constrained ordination additionally showed converging trajectories of species compositions for ozone and control treatments with time. Yields sampled prior to the start of the experiment and soil nitrogen concentrations revealed that spatial heterogeneity in the soil nutrient status was not accounted for by the random allocation of treatments to plots with a bias towards less productive patches in the elevated-ozone plots. Re-analysis of yield data using repeated-measure ANOVA with a covariable to account for productivity prior to the start of fumigation revealed effects on the temporal changes in total yield and yield of legumes that cannot be separated between ozone and pre-treatment nutrient status. Changes in species composition favour an ecological interpretation with spatial heterogeneity as the major cause of different yield declines. Although elevated ozone may cause subtle physiological changes with longer term implications, our new results suggest that species-rich mature grassland such as the one studied at Le Mouret may be less sensitive to elevated ozone than previously assumed. In this experiment a confounded design was hidden at the start by transitory effects of a prior change in nutrient treatments.     

Assessment of the functional response parameters of Coccinella septempunctata to varying densities of Acyrthosiphon pisum

The functional response of a predator describes the rate at which it kills its prey at different prey densities and is, therefore, an important tool to determine the efficiency of a predator in regulating the population dynamics of a prey. We investigated the functional response of Coccinella septempunctata L. (Coleoptera: Coccinellidae) to varying densities of Acyrthosiphon pisum (Harris) (Hemiptera: Aphididae). The results revealed that the linear coefficient, P₁ in the reduced logistic model was found significantly negative for the 4^th instar larvae and adult male and female of C. septempunctata (?0.002, ?0.004 and???0.002, respectively) indicating a type II functional response. The parameter estimates of adult male, female and 4^thinstar larvae of C. septempunctata calculated through Holling's disc model revealed that the highest attack rate (a) (1.047?±?0.014), shortest handling time (T_h) (0.0984?±?0.024?h) and largest maximum capture rate (T/T_h) (243.902) was exhibited by female. Parameter estimates calculated through Rogers’s equation also showed the same sequence where maximum attack rate (a) of 0.00256?±?0.0003 was exhibited by female followed by 4^th instar larvae (0.00251?±?0.0005). The shortest handling time (T_h) (0.210?±?0.003?h) and highest maximum capture rate (T/T_h) (114.17) was also exhibited by female. Comparison of functional response curves of adult male, female and 4^th instar larvae revealed that maximum consumption of prey at all the densities offered was shown by female followed by 4^thinstar larvae. The study manifested that C. septempunctata could be an efficient biocontrol agent of pea aphid, A. pisum.     

Spatial and functional heterogeneity of sphingolipid-rich membrane domains

Little is known about the organization of lipids in biomembranes. Lipid rafts are defined as sphingolipid- and cholesterol-rich clusters in the membrane. Details of the lipid distribution of lipid rafts are not well characterized mainly because of a lack of appropriate probes. Ganglioside GM1-specific protein, cholera toxin, has long been the only lipid probe of lipid rafts. Recently it was shown that earthworm toxin, lysenin, specifically recognizes sphingomyelin-rich membrane domains. Binding of lysenin to sphingomyelin is accompanied by the oligomerization of the toxin that leads to pore formation in the target membrane. In this study, we generated a truncated lysenin mutant that does not oligomerize and thus is non-toxic. Using this mutant lysenin, we showed that plasma membrane sphingomyelin-rich domains are spatially distinct from ganglioside GM1-rich membrane domains in Jurkat T cells. Like T cell receptor activation and cross-linking of GM1, cross-linking of sphingomyelin induced calcium influx and ERK phosphorylation in the cell. However, unlike CD3 or GM1, cross-linking of sphingomyelin did not induce significant protein tyrosine phosphorylation. Combination of lysenin and sphingomyelinase treatment suggested the involvement of G-protein-coupled receptor in sphingomyelin-mediated signal transduction. These results thus suggest that the sphingomyelin-rich domain provides a functional signal cascade platform that is distinct from those provided by T cell receptor or GM1. Our study therefore elucidates the spatial and functional heterogeneity of lipid rafts.     

Productivity and complementarity in grassland microcosms of varying diversity

Several researchers have hypothesized that, through various mechanisms, loss of species and functional group richness from a plant community will affect the magnitude and interannual variability of productivity. To test this hypothesis, I conducted a microcosm study of California grassland communities that differed in species richness. I grew cohorts of microcosms that simulated undisturbed grassland (in one year) and gopher-disturbed grassland (in two consecutive years). As the number of species per functional group decreased from 4 to 1, biomass production remained constant in all three cohorts. As species richness decreased from 16 to 1 (or 8 to 1, in either case including a drop in functional group richness), productivity declined in one of the cohorts. In this cohort, productivity of one polyculture marginally exceeded that of the most productive monoculture. Resource complementarity and a type of selection effect may have each contributed to the observed diversity-productivity relationships. Results suggest the existence of a selection effect that involves species that are highly productive in mixtures, rather than in monoculture. Over two seasons, species and functional group richness did not affect the interannual variability of biomass production. Comparisons of interannual changes in the productivity of monocultures and polycultures suggested that, in some polycultures, increased water availability might have relieved interspecific competition more than intraspecific competition. Based on results from this experiment and other manipulative experiments, I develop a framework to explain the relationship between species richness and productivity in terrestrial plant communities. The framework highlights the importance of environmental variation in shaping the diversity/productivity relationship.     

Shape and sources of variations of the functional response of wildfowl: an experiment with mallards, Anas platyrhynchos

Understanding the variations of the functional response of an organism, i.e. the predation rate in relation to prey density, is necessary to understand the interactions between the animal and its food supply. This has received little attention in dabbling ducks so we investigated experimentally the shape of the functional response of mallard feeding on poultry pellets, and assessed the influence of several factors such as the size of food items, sex or individual performance on this functional response. Individual differences in intake rate are of crucial importance in group or gregarious foraging species. We used two approaches of the functional response: 1) the relation between feeding rate (pellets/s) and pellet densities (pellets/m²), and 2) the relationship between instantaneous intake rate (g/s) and biomass density (g/m²). For both approaches, we found that the Type II functional response gave better estimates than a Type I linear functional response but explained only a third of the variance. Our results show that pellet size has a large effect on instantaneous intake rate. The comparison of the functional response parameters suggest that handling time per prey may not reflect the real constraints on intake rate, but that handling time per gram ingested may be more appropriate to integrate the effect of item size in the functional response. We then discuss the possible mechanisms involved. We also found individual variations in the functional response for each of the experiments, with some consistency in the hierarchy regarding feeding efficiency. We did not find any differences between males and females. Our results provide an evaluation of individual variations in intake rate in interference-free conditions, which has rarely been done, and call for more controlled experiments to allow a finer understanding of the mechanisms of food acquisition in dabbling ducks.     

Costs of coexistence along a gradient of competitor densities: an experiment with arvicoline rodents

1. Costs of coexistence for species with indirect resource competition usually increase monotonically with competitor numbers. Very little is known though about the shape of the cost function for species with direct interference competition. 2. Here we report the results of an experiment with two vole species in artificial coexistence in large enclosures, where density of the dominant competitor species (Microtus agrestis) was manipulated. Experimental populations of the subordinate vole species (Clethrionomys glareolus) were composed of same aged individuals to study distribution of costs of coexistence with a dominant species within an age-cohort. 3. Survival and space use decreased gradually with increasing field vole numbers. Thus, responses to interference competition in our system appeared to be similar as expected from resource competition. The total number of breeders was stable. Reproductive characteristics such as the timing of breeding, and the litter size were not affected. In the single species enclosures a proportion of surviving individuals were not able to establish a breeding territory against stronger conspecifics. Under competition with heterospecifics such nonbreeders suffered high mortality, whereas the breeders survived. 4. Combined interference of dominant conspecifics and heterospecifics probably increased the frequency of aggressive interactions, social stress and mortality for the weaker individuals within a homogeneous age cohort of the subordinate competitor population. 5. Our results suggest, that in open systems where bank voles are outcompeted over the breeding season by faster reproducing field voles, animals able to establish a territory may be able to withstand competitor pressure, while nonbreeding bank vole individuals are forced to emigrate to suboptimal forest habitats.     

Structural and functional heterogeneity in an insect muscle.

Singing muscles of the katydid, Neoconocephalus robustus (Insecta, Tettigoniidae) are neurogenic, yet perform at contraction-relaxation frequencies as high as 212 Hz (Josephson and Halverson, '71). The mechanical and electrical responses of different bands of one of these muscles (the dorsal longitudinal muscle, DLM) has been examined with respect to ultrastructural features of each part which may be related to muscle performance. The DLM is composed of three bands and is innervated by four motoneurones. The cell bodies of three of these motoneurones occur ipsilaterally in the prothroracic ganglion; the cell body of the other motoneurone is contralateral in the mesothoracic ganglion. Three of the motoneurones (as yet unidentified fast axons) initiate extraordinarily fast twitches (rise time equal 7.3 msec, half duration equals 14.3 msec, 25 C), the fourth (an unidentified slower axon) evokes twitches which are considerably slower (rise time equals 18.9 msec, half duration equals 5.10 msec). Whereas the ventral and medial bands of the muscle are innervated only by fast axons (some fibers of the medial band are doubly innervated), the dorsal band is innervated by both a fast axon and the slower axon. A few fibers of the dorsal band are doubly innervated. The structure of fibers from the ventral and medial bands is very similar, with short sarcomeres (4.0 and 4.3 mum, respectively) and thin strap-like myofibrils delineated by well-developed sarcoplasmic reticulum (SR). Twenty-four percent of the volume of ventral band fibers is SR and the diffusion distance from SR to the center of the adjacent myofibril averages 0.083 mum. Twenty percent of the medial band fiber volume is SR, with a diffusion distance of 0.118 mum. Ventral and medial band fibers contain about 40% mitochondria, and 33% myofibrils. The dorsal band fibers have longer sarcomeres (9.5 mum), and only 10% of the fiber volume is SR. The muscle fibrils of the dorsal band are larger and consequently the diffusion distance is greater (0.227 mum) than in the ventral and medial bands. Mitochondria comprise 23% of the volume of dorsal band fibers. Most dorsal band mitochondria are aggregated into distinct clumps. Although some dorsal band fibers are innervated by a fast axon and some by the slower axon, the dorsal band fibers are structurally homogeneous, suggesting that neurotrophic effects are not important in maintaining the structure of dorsal band fibers. The mechanical-electrical performance and ultrastructure of the ventral and medial bands suggest their roll as fast, metabolically active but weak muscles, used in singing; the dorsal band as a slower but stronger muscle, perhaps involved in postural movements of the wing during singing.     

Spatial heterogeneity in size and parasitism: How it arises in an estuarine snail population

Populations of Ilyanassa obsoleta, a snail abundant in estuaries along the Atlantic coast of North America, exhibit much unobvious spatial heterogeneity. Lifetime dispersal of snails stands to be great, as they live for decades and can make daily moves of tens of meters. They could distribute uniformly, but in many populations there are characteristic spatial distributions of differently sized and trematode-infected snails. This is the case where this work was done in the Savages Ditch habitat of Rehoboth Bay, Delaware, U.S.A.. The study was designed to explain how the spatial heterogeneity in that habitat arises as a consequence of snail movements. Individually marked snails were tested for trematode infections and released from five well-separated positions. Snails released from each position were in three groups: (1) natives, collected from the position area, and (2) large and (3) small “outlanders”, collected from a site in the habitat that was separated from all release positions, had enhanced tidal currents, and where both size classes were available. Locations of found, released snails were noted for up to four years. Depending on collection site, many native snails were parasitized. In any case, they always tended to disperse in the vicinity where initially collected. Large outlanders (∼ 21% infected) dispersed variably, moving back toward their source area from certain release positions, but not from others. Small outlanders (∼ 4% infected), almost regardless of where released, tended to disperse back toward their source area. Dispersals of these snail groups from the five release positions explains much about how the pattern of spatial heterogeneity seen in the habitat arises.     

Field calibration of soil-core microcosms: Ecosystem structural and functional comparisons

Microcosms containing intact soil-cores are a potential biotechnology risk assessment tool for assessing the ecological effects of genetically engineered microorganisms before they are released to the field; however, microcosms must first be calibrated to ensure that they adequately simulate key field parameters. Soil-core microcosms were compared with the field in terms of ecological response to the introduction of a large inoculum of a rifampicin-resistant rhizobacterium,Pseudomonas sp. RC1. RC1 was inoculated into intact soil-core microcosms incubated in the laboratory at ambient temperature (22°C) and in a growth chamber with temperature fluctuations that mimicked a verage field values, as well as into field lysimeters and plots. The effect of the introduced bacterium on ecosystem structure, including wheat rhizoplane populations of total and fluorescent pseudomonads, total heterotrophic bacteria, and the diversity of total heterotrophic bacteria, was determined. Fluorescent pseudomonads were present on the rhizoplane in significantly lower numbers in soil inoculated with RC1, in both microcosms and the field. Conditions for microbial growth appeared to be most favorable in the growth chamber microcosm, as evidenced by higher populations of heterotrophs and a greater species diversity on the rhizoplane at the three-leaf stage of wheat growth. Ecosystem functional parameters, as determined by soil dehydrogenase activity, plant biomass production, and¹⁵N-fertilizer uptake by wheat, were different in the four systems. The stimulation of soil dehydrogenase activity by the addition of alfalfa was greater in the microcosms than in the field. In general, growth chamber microcosms, which simulated average field temperatures, were better predictors of field behavior than microcosms incubated continuously at 22°C.     

Gammadelta T cells: functional plasticity and heterogeneity

Gammadelta T cells remain an enigma. They are capable of generating more unique antigen receptors than alphabeta T cells and B cells combined, yet their repertoire of antigen receptors is dominated by specific subsets that recognize a limited number of antigens. A variety of sometimes conflicting effector functions have been ascribed to them, yet their biological function(s) remains unclear. On the basis of studies of gammadelta T cells in infectious and autoimmune diseases, we argue that gammadelta T cells perform different functions according to their tissue distribution, antigen-receptor structure and local microenvironment; we also discuss how and at what stage of the immune response they become activated.     

Plasmid introduction in metal-stressed,subsurface-derived microcosms: plasmid fate and community response

The nonconjugal IncQ plasmids pMOL187 and pMOL222, which contain the metal resistance-encoding genes czc and ncc, were introduced by using Escherichia coli as a transitory delivery strain into microcosms containing subsurface-derived parent materials. The microcosms were semicontinuously dosed with an artificial groundwater to set a low-carbon flux and a target metal stress (0, 10, 100, and 1,000 micro M CdCl(2)), permitting long-term community monitoring. The broad-host-range IncPalpha plasmid RP4 was also transitorily introduced into a subset of microcosms. No novel community phenotype was detected after plasmid delivery, due to the high background resistances to Cd and Ni. At fixed Cd doses, however, small but consistent increases in Cd(r) or Ni(r) density were measured due to the introduction of a single pMOL plasmid, and this effect was enhanced by the joint introduction of RP4; the effects were most significant at the highest Cd doses. The pMOL plasmids introduced could, however, be monitored via czc- and ncc-targeted infinite-dilution PCR (ID-PCR) methods, because these genes were absent from the indigenous community: long-term presence of czc (after 14 or 27 weeks) was contingent on the joint introduction of RP4, although RP4 cointroduction was not yet required to ensure retention of ncc after 8 weeks. Plasmids isolated from Ni(r) transconjugants further confirmed the presence and retention of a pMOL222-sized plasmid. ID-PCR targeting the RP4-specific trafA gene revealed retention of RP4 for at least 8 weeks. Our findings confirm plasmid transfer and long-term retention in low-carbon-flux, metal-stressed subsurface communities but indicate that the subsurface community examined has limited mobilization potential for the IncQ plasmids employed.     

A kinesin medley: biochemical and functional heterogeneity

Over the past decade, a remarkable number and diversity of molecular motors have been described in eukaryotic cells. In addition to the identification of novel forms of myosin and dynein, the kinesins have been defined as an entirely new family of molecular motors. There may be as many as 30 different genes in a single organism encoding members of the kinesin superfamily. Why is such diversity in molecular motors needed? The biochemical and functional diversity of the originally defined form of kinesin provides some insights into the roles of molecular motors in cellular dynamics.     

Morphology of living cells cultured on nanowire arrays with varying nanowire densities and diameters

Vertical nanowire arrays are increasingly investigated for their applications in steering cell behavior. The geometry of the array is an important parameter, which influences the morphology and adhesion of cells. Here, we investigate the effects of array geometry on the morphology of MCF7 cancer cells and MCF10A normal-like epithelial cells. Different gallium phosphide nanowire array-geometries were produced by varying the nanowire density and diameter. Our results show that the cell size is smaller on nanowires compared to flat gallium phosphide. The cell area decreases with increasing the nanowire density on the substrate. We observed an effect of the nanowire diameter on MCF10A cells, with a decreased cell area on 40 nm diameter nanowires, compared to 60 and 80 nm diameter nanowires in high-density arrays. The focal adhesion morphology depends on the extent to which cells are contacting the substrate. For low nanowire densities and diameters, cells are lying on the substrate and we observed large focal adhesions at the cell edges. In contrast, for high nanowire densities and diameters, cells are lying on top of the nanowires and we observed point-like focal adhesions distributed over the whole cell. Our results constitute a step towards the ability to fine-tune cell behavior on nanowire arrays.     

Spatial and temporal heterogeneity in nucleotide sequence evolution

Models of nucleotide substitution make many simplifying assumptions about the evolutionary process, including that the same process acts on all sites in an alignment and on all branches on the phylogenetic tree. Many studies have shown that in reality the substitution process is heterogeneous and that this variability can introduce systematic errors into many forms of phylogenetic analyses. I propose a new rigorous approach for describing heterogeneity called a temporal hidden Markov model (THMM), which can distinguish between among site (spatial) heterogeneity and among lineage (temporal) heterogeneity. Several versions of the THMM are applied to 16 sets of aligned sequences to quantitatively assess the different forms of heterogeneity acting within them. The most general THMM provides the best fit in all the data sets examined, providing strong evidence of pervasive heterogeneity during evolution. Investigating individual forms of heterogeneity provides further insights. In agreement with previous studies, spatial rate heterogeneity (rates across sites [RAS]) is inferred to be the single most prevalent form of heterogeneity. Interestingly, RAS appears so dominant that failure to independently include it in the THMM masks other forms of heterogeneity, particularly temporal heterogeneity. Incorporating RAS into the THMM reveals substantial temporal and spatial heterogeneity in nucleotide composition and bias toward transition substitution in all alignments examined, although the relative importance of different forms of heterogeneity varies between data sets. Furthermore, the improvements in model fit observed by adding complexity to the model suggest that the THMMs used in this study do not capture all the evolutionary heterogeneity occurring in the data. These observations all indicate that current tests may consistently underestimate the degree of temporal heterogeneity occurring in data. Finally, there is a weak link between the amount of heterogeneity detected and the level of divergence between the sequences, suggesting that variability in the evolutionary process will be a particular problem for deep phylogeny.     

Spatial heterogeneity in parasite infections at different spatial scales in an intertidal bivalve

Spatial heterogeneities in the abundance of free-living organisms as well as in infection levels of their parasites are a common phenomenon, but knowledge on parasitism in invertebrate intermediate hosts in this respect is scarce. We investigated the spatial pattern of four dominant trematode species which utilize a common intertidal bivalve, the cockle Cerastoderma edule, as second intermediate host in their life cycles. Sampling of cockles from the same cohort at 15 sites in the northern Wadden Sea (North Sea) over a distance of 50 km revealed a conspicuous spatial heterogeneity in infection levels in all four species over the total sample as well as among and within sampling sites. Whereas multiple regression analyses indicated the density of first intermediate upstream hosts to be the strongest determinant of infection levels in cockles, the situation within sites was more complex with no single strong predictor variable. However, host size was positively and host density negatively correlated with infection levels and there was an indication of differential susceptibility of cockle hosts. Small-scale differences in physical properties of the habitat in the form of residual water at low tide resulted in increased infection levels of cockles which we experimentally transferred into pools. A complex interplay of these factors may be responsible for within-site heterogeneities. At larger spatial scales, these factors may be overridden by the strong effect of upstream hosts. In contrast to first intermediate trematode hosts, there was no indication for inter-specific interactions. In other terms, the recruitment of trematodes in second intermediate hosts seems to be largely controlled by pre-settlement processes both among and within host populations.     

Spatial heterogeneity and niche differentiation in oceanic zooplankton

Selected epifaunal and infaunal species from three sites of differing substrate were examined over a 12 month period. The characteristic species comprising the epifauna at each site were found to be dictated by substrate type whereas the infaunal amphipods and tanaids were relatively similar between sites. No distinct seasonality of abundance was noted for the macroscopic species; however, the infaunal amphipods and tanaids exhibited distinct seasonal cycles of abundance. These temporal changes in population size can be correlated with the seasonal cycle of primary producers. Females, of each of the infaunal species that were monitored, exhibited brood protection throughout the winter period and released juveniles at times that coincided with the period of high primary productivity. All species exhibited properties characteristic of K-strategists; a result of the very stable and predictable nature of the antarctic benthic environment.     

Allee effects can both conserve and create spatial heterogeneity in population densities

In order to determine conditions which allow the Allee effect (caused by biparental reproduction) to conserve and create spatial heterogeneity in population densities, we studied a deterministic model of a symmetric two-patch metapopulation. We proved that under certain conditions there exist stable equilibria with unequal population densities in the two patches, a situation which can be interpreted as conserved heterogeneity. Furthermore, the Allee effect can lead to instability of the equilibrium with equal population densities if some degree of competition is assumed to occur between the subpopulations (non-local competition). This indicates the potential of the Allee effect to create spatial heterogeneity. Neither of these effects appear under biologically realistic parameter values in a model where uniparental reproduction is assumed. We proved that both the between-patch migration intensity and the degree of non-local competition are decisive in determining boundaries between these types of behaviour of the spatial system with Allee effect. Therefore, we propose that the Allee effect, migration intensity, and non-local competition should be considered jointly in studies focusing on problems like pattern formation in space and invasions of spreading species.     

Spatial heterogeneity in the timing of birch budburst in response to future climate warming in Ireland

In order to predict the impact of future climate warming on trees it is important to quantify the effect climate has on their development. Our understanding of the phenological response to environmental drivers has given rise to various mathematical models of the annual growth cycle of plants. These models simulate the timing of phenophases by quantifying the relationship between development and its triggers, typically temperature. In addition, other environmental variables have an important role in determining the timing of budburst. For example, photoperiod has been shown to have a strong influence on phenological events of a number of tree species, including Betula pubescens (birch). A recently developed model for birch (DORMPHOT), which integrates the effects of temperature and photoperiod on budburst, was applied to future temperature projections from a 19-member ensemble of regional climate simulations (on a 25 km grid) generated as part of the ENSEMBLES project, to simulate the timing of birch budburst in Ireland each year up to the end of the present century. Gridded temperature time series data from the climate simulations were used as input to the DORMPHOT model to simulate future budburst timing. The results showed an advancing trend in the timing of birch budburst over most regions in Ireland up to 2100. Interestingly, this trend appeared greater in the northeast of the country than in the southwest, where budburst is currently relatively early. These results could have implications for future forest planning, species distribution modeling, and the birch allergy season.     

Within-tree response of Nantucket pine tip moth,Rhyacionia frustrana [Lep.: Tortricidae] Parasitoids to varying host densities

The numerical response of Nantucket pine tip moth,Rhyacionia frustrana (Comstock) parasitoids to varying host densities is described. Response surface analysis showed that high rates of parasitism (greater than 50 %) occurs more frequently at low host densities. The implication in using response surface analysis in biological control research is discussed.