Does hydrological fluctuation alter impacts of species richness on biomass in wetland plant communities?

Aims The diversity–productivity relationship is one of the most critical questions in ecology and can be altered by environmental factors. Hydrological fluctuation affects growth of wetland plants, and such effects vary with plant species. Therefore, we hypothesized that hydrological fluctuation changes effects of species richness on productivity of wetland plant communities.Methods We constructed wetland plant communities consisting of three or six wetland plant species and subjected them to hydrological fluctuation (i.e. gradually changing water level) of two frequencies and two ranges, with unchanged water level as the control. We measured height, root and shoot dry mass of each plant at harvest.Important findings Hydrological fluctuation significantly decreased biomass of wetland plant communities, which was due to impacts of fluctuation range, but not those of fluctuation frequency. Community biomass was significantly higher when species richness was higher, and such an effect did not depend on hydrological fluctuation. Therefore, hydrological fluctuation can decrease the productivity of wetland plant communities but may not alter the diversity–productivity relationship.     

Does responsiveness to arbuscular mycorrhizal fungi depend on plant invasive status?

Differences in the direction and degree to which invasive alien and native plants are influenced by mycorrhizal associations could indicate a general mechanism of plant invasion, but whether or not such differences exist is unclear. Here, we tested whether mycorrhizal responsiveness varies by plant invasive status while controlling for phylogenetic relatedness among plants with two large grassland datasets. Mycorrhizal responsiveness was measured for 68 taxa from the Northern Plains, and data for 95 taxa from the Central Plains were included. Nineteen percent of taxa from the Northern Plains had greater total biomass with mycorrhizas while 61% of taxa from the Central Plains responded positively. For the Northern Plains taxa, measurable effects often depended on the response variable (i.e., total biomass, shoot biomass, and root mass ratio) suggesting varied resource allocation strategies when roots are colonized by arbuscular mycorrhizal fungi. In both datasets, invasive status was nonrandomly distributed on the phylogeny. Invasive taxa were mainly from two clades, that is, Poaceae and Asteraceae families. In contrast, mycorrhizal responsiveness was randomly distributed over the phylogeny for taxa from the Northern Plains, but nonrandomly distributed for taxa from the Central Plains. After controlling for phylogenetic similarity, we found no evidence that invasive taxa responded differently to mycorrhizas than other taxa. Although it is possible that mycorrhizal responsiveness contributes to invasiveness in particular species, we find no evidence that invasiveness in general is associated with the degree of mycorrhizal responsiveness. However, mycorrhizal responsiveness among species grown under common conditions was highly variable, and more work is needed to determine the causes of this variation.     

Does herbivore diversity depend on plant diversity? The case of California butterflies

It is widely believed that the diversity of plants influences the diversity of animals, and this should be particularly true of herbivores. We examine this supposition at a moderate spatial extent by comparing the richness patterns of the 217 butterfly species resident in California to those of plants, including all 5,902 vascular plant species and the 552 species known to be fed on by caterpillars. We also examine the relationships between plant/butterfly richness and 20 environmental variables. We found that although plant and butterfly diversities are positively correlated, multiple regression, path models, and spatial analysis indicate that once primary productivity (estimated by a water-energy variable, actual evapotranspiration) and topographical variability are incorporated into models, neither measure of plant richness has any relationship with butterfly richness. To examine whether butterflies with the most specialized diets follow the pattern found across all butterflies, we repeated the analyses for 37 species of strict monophages and their food plants and found that plant and butterfly richness were similarly weakly associated after incorporating the environmental variables. We condude that plant diversity does not directly influence butterfly diversity but that both are probably responding to similar environmental factors.     

Does the terminal-link effect depend on duration or reinforcement rate?

Preference in concurrent chains for the richer terminal-link schedule becomes more extreme as the schedule values increase with their ratio held constant, a result known as the terminal-link effect. We report two experiments that attempt to determine whether this effect is related to terminal-link duration or the overall rate of reinforcement. These variables have been confounded in prior studies, but can be separated by comparing variable-duration schedules that end after a single reinforcer has been earned, with constant-duration schedules during which a variable number of reinforcers may be earned. In Experiment 1, the terminal-link effect was obtained with variable-duration schedules when duration and overall reinforcement rate were manipulated, but not with constant-duration schedules when overall reinforcement rate was changed with duration held constant. In Experiment 2, the terminal-link effect was obtained with constant-duration schedules when duration was manipulated with overall reinforcement rate held constant. Taken together, these results show that the terminal-link effect depends on changes in terminal-link duration, not overall reinforcement rate (or equivalently, average time to reinforcement). This accords with the account of the terminal-link effect provided by the contextual choice model [J. Exp. Anal. Behav. 61 (1994) 113] but not delay-reduction theory [J. Exp. Anal. Behav. 12 (1969) 723].     

Does competition drive character differences between species on a macroevolutionary scale?

Sympatric sister species generally have a degree of phenotypic differentiation that allows them to coexist. It has been well documented that phenotypic similarity results, through resource competition, in one of two major outcomes: local extinction of either competitor or character displacement. Limiting similarity suggests that there is a maximum degree of phenotypic niche overlap with which similar species may coexist. Breaching that maximum would result in exclusion. Character displacement, on the other hand, implies that the species differentiate phenotypically so that resource competition is reduced to the point where coexistence is possible. While it has been suggested that these theories have the potential to accelerate (character displacement) or limit phenotypic evolution (competitive exclusion) on microevolutionary time scales, their effects on macroevolution remain under‐studied. If competition accelerates evolution on a macroevolutionary scale, one would expect that phenotypic diversity increases as novel species ‘push aside’ existing species. On the other hand, one might also expect that phenotypic evolution comes to a halt as novel species are trapped in the (ever decreasing) phenotypic space not yet occupied by existing species, except at the extremes of the phenotypic spectrum. Studying the current geographical ranges of more than 3000 extant species representing 29 mammalian families and their respective body masses, I found little evidence of competition accelerating body size differentiation between species.     

Does oligodendrocyte survival depend on axons?

BACKGROUND: We have shown previously that oligodendrocytes and their precursors require signals from other cells in order to survive in culture. In addition, we have shown that about 50% of the oligodendrocytes produced in the developing rat optic nerve normally die, apparently in a competition for the limiting amounts of survival factors. We have hypothesized that axons may control the levels of such oligodendrocyte survival factors and that the competition-dependent death of oligodendrocytes serves to match their numbers to the number of axons that they myelinate. Here we test one prediction of this hypothesis - that the survival of developing oligodendrocytes depends on axons. RESULTS: We show that oligodendrocyte death occurs selectively in transected nerves in which the axons degenerate. This cell death is prevented by the delivery of exogenous ciliary neurotrophic factor (CNTF) or insulin-like growth factor I (IGF-1), both of which have been shown to promote oligodendrocyte survival in vitro. We also show that purified neurons promote the survival of purified oligodendrocytes in vitro. CONCLUSION: These results strongly suggest that oligodendrocyte survival depends upon the presence of axons; they also support the hypothesis that a competition for axon-dependent survival signals normally helps adjust the number of oligodendrocytes to the number of axons that require myelination. The identities of these signals remain to be determined.     

Does biodiversity increase spatial stability in plant community biomass?

We tested the hypothesis that biodiversity decreases the spatial variability of biomass production between subplots taken within experimental grassland plots. Our findings supported this hypothesis if functional diversity (weighted Rao's Q ) was considered. Further analyses revealed that diversity in rooting depth and clonal growth form were the most important components of functional diversity stabilizing productivity. Using species or functional group richness as diversity measures there was no significant effect on spatial variability of biomass production, demonstrating the importance of the biodiversity component considered. Moreover, we found a significant increase in spatial variability of productivity with decreasing size of harvested area, suggesting small-scale heterogeneity as an important driver. The ability of diverse communities to stabilize biomass production across spatial heterogeneity may be due to complementary use of spatial niches. Nevertheless, the positive effect of functional diversity on spatial stability appears to be less pronounced than previously reported effects on temporal stability.     

What limits herb biomass in grasslands: competition or herbivory?

Competition and herbivory are two of the main forces shaping plant communities. Although several studies have investigated their impact on plant populations separately, few investigations have examined how they might interact. With the purpose of clarifying the combined roles of competition and herbivory on herb biomass in a grassland, we assessed the effects of different herbivores with reduced grass competition. We conducted a field experiment in 2000-2001 in a British acid grassland (Oak Mead), where we experimentally reduced grass biomass and excluded rabbits, insects and molluscs in a factorial design. Removing the grasses from Oak Mead dramatically increased herb biomass and total above-ground biomass. Herbivore exclusions (i.e. rabbits, insects and molluscs) did not affect total above-ground biomass, but they altered relative abundance of several species. Grass removal and rabbit exclusion had positive interactions on biomass of several herb species, and there were some subtle interactions between different herbivore groups: monocots benefitted when both rabbits and molluscs were excluded, and some herb species had greater biomass when insects and rabbits were absent. We then compared the results with a 10-year experiment that manipulated similar variables in neighbouring grassland (Nashs Field). The comparison between Oak Mead and Nashs Field showed that cessation of herbicide application returns the system to its previous state of grass dominance after 3 years. Therefore, even when herbs were more abundant, they could not prevent reinvasion of the grasses once external factors were removed.     

Invasion-resistance in experimental grassland communities: species richness or species identity?

The question as to why some communities are more invasible than others has pro-found implications for conservation biology and land management. The theoretical issues involved go right to the heart of our understanding of species coexistence and community assembly. The experiment reported here indicates that for productive, small-scale grassland plots, species identity matters more than species richness in determining both the number of invading species and the total biomass of invasives.     

Leaf economics spectrum–productivity relationships in intensively grazed pastures depend on dominant species identity

Plant functional traits are thought to drive variation in primary productivity. However, there is a lack of work examining how dominant species identity affects trait–productivity relationships. The productivity of 12 pasture mixtures was determined in a 3‐year field experiment. The mixtures were based on either the winter‐active ryegrass (Lolium perenne) or winter‐dormant tall fescue (Festuca arundinacea). Different mixtures were obtained by adding forb, legume, and grass species that differ in key leaf economics spectrum (LES) traits to the basic two‐species dominant grass–white clover (Trifolium repens) mixtures. We tested for correlations between community‐weighted mean (CWM) trait values, functional diversity, and productivity across all plots and within those based on either ryegrass or tall fescue. The winter‐dormant forb species (chicory and plantain) had leaf traits consistent with high relative growth rates both per unit leaf area (high leaf thickness) and per unit leaf dry weight (low leaf dry matter content). Together, the two forb species achieved reasonable abundance when grown with either base grass (means of 36% and 53% of total biomass, respectively, with ryegrass tall fescue), but they competed much more strongly with tall fescue than with ryegrass. Consequently, they had a net negative impact on productivity when grown with tall fescue, and a net positive effect when grown with ryegrass. Strongly significant relationships between productivity and CWM values for LES traits were observed across ryegrass‐based mixtures, but not across tall fescue‐based mixtures. Functional diversity did not have a significant positive effect on productivity for any of the traits. The results show dominant species identity can strongly modify trait–productivity relationships in intensively grazed pastures. This was due to differences in the intensity of competition between dominant species and additional species, suggesting that resource‐use complementarity is a necessary prerequisite for trait–productivity relationships.     

Does resource availability,resource heterogeneity or species turnover mediate changes in plant species richness in grazed grasslands?

Grazing by large ungulates often increases plant species richness in grasslands of moderate to high productivity. In a mesic North American grassland with and without the presence of bison (Bos bison), a native ungulate grazer, three non-exclusive hypotheses for increased plant species richness in grazed grasslands were evaluated: (1) bison grazing enhances levels of resource (light and N) availability, enabling species that depend on higher resource availability to co-occur; (2) spatial heterogeneity in resource availability is enhanced by bison, enabling coexistence of a greater number of plant species; (3) increased species turnover (i.e. increased species colonization and establishment) in grazed grassland is associated with enhanced plant species richness. We measured availability and spatial heterogeneity in light, water and N, and calculated species turnover from long-term data in grazed and ungrazed sites in a North American tallgrass prairie. Both regression and path analyses were performed to evaluate the potential of the three hypothesized mechanisms to explain observed patterns of plant species richness under field conditions. Experimental grazing by bison increased plant species richness by 25% over an 8-year period. Neither heterogeneity nor absolute levels of soil water or available N were related to patterns of species richness in grazed and ungrazed sites. However, high spatial heterogeneity in light and higher rates of species turnover were both strongly related to increases in plant species richness in grazed areas. This suggests that creation of a mosaic of patches with high and low biomass (the primary determinant of light availability in mesic grasslands) and promotion of a dynamic species pool are the most important mechanisms by which grazers affect species richness in high productivity grasslands.     

Does cancellous screw insertion torque depend on bone mineral density and/or microarchitecture?

During insertion of a cancellous bone screw, the torque level reaches a plateau, at the engagement of all the screw threads prior to the screw head contact. This plateau torque (T_Plateau) was found to be a good predictor of the insertion failure torque (stripping) and also exhibited strong positive correlations with areal bone mineral density (aBMD) in ovine bone. However, correlations between T_Plateau and aBMD, as well as correlations between T_Plateau and bone microarchitecture, have never been explored in human bone.     

Effects of temporal heterogeneity of water supply and nutrient levels on plant biomass growth depend on the plant’s relative size within its population

Temporal heterogeneity of water supply can alter the biomass growth of plants, even when the same total amount of water is provided. Most studies of heterogeneous watering have focused on responses of whole populations rather than individuals in a population. The effects of water supply heterogeneity may also depend on nutrient levels. Thus, we investigated the integrated effects of water supply heterogeneity and nutrient levels on plants within a population. Six plants of Perilla frutescens per pot were grown under different combinations of frequency of water supply and nutrient level. The effects on yield per pot, individual biomass, and allocation to roots were analyzed after a 44-day watering regime. A homogeneous water supply resulted in a greater yield per pot and greater biomass of individual plants than a heterogeneous supply. However, the interaction between water supply heterogeneity and nutrient level was significant only in larger individuals, not in smaller plants or at the p. Water supply heterogeneity affected the growth of all plants, but the effects differed among individuals depending on their relative size within their population. It is therefore important to focus not only on whole-population characteristics such as yield but also on individuals in a population in order to reveal the detailed effects of water supply heterogeneity.     

Sensor types in signal transduction pathways in plant cells responding to abiotic stressors: do they depend on stress intensity?

Despite the fast growing knowledge about the components of the signalling pathways involved in the activation of drought‐, cold‐, osmotic stress‐ and salt‐responsive genes, relatively little is known about the sensor types responsible for the induction of the pathways. It is thought that different signals have their own cognate receptors which independently, or in cooperation, initiate a downstream signalling cascade ( Xiong and Zhu 2002 ). On the other hand, the stress level‐dependent activation of different receptors has been proposed in plants responding to osmotic stress ( Munnik and Meijer 2001 ). The question arises as to whether the activation of different signalling systems will depend on the nature of the sensors or on the stress‐induced primary event, which may differ in cells subjected to moderate or severe stress. In this article, a discussion is given of the available literature data concerning this controversial question. It is proposed that, in plants responding to mild stress, a disturbance of water balance is the primary stress‐induced event affecting the cell wall–plasma membrane interactions, resulting in the activation of receptor‐like kinases, including wall‐associated kinases, cytoskeleton‐related mechanosensors, stretch‐dependent ion (calcium) channels and redox‐mediated systems. The mild stress‐sensing systems, assisted by an increased supply of abscisic acid, seem to be involved in the activation of the pathways that enable the adjustment of plant growth and metabolism to the stressful conditions, i.e. allowing acclimation. Severe or suddenly acting stressors are sensed by membrane destabilization (membrane depolarization, alterations in ion transport systems), which results in the triggering of phospholipid signalling. This may lead to the increased production of reactive oxygen species, the accumulation of H₂O₂, lipid peroxidation and increased synthesis of hormones such as jasmonates and ethylene. These are characteristic features of the alarm situation, which may result in irreversible injury and cell death, or in cell recovery, depending on stress impact.     

Does proliferation capacity of lymphocytes depend on human blood types?

In vitro human lymphocyte culture methodology is well established yet certain confounding factors such as age, medical history as well as individual’s blood type may potentially modulate in vitro proliferation response. These factors have to be carefully evaluated to release reliable test report in routine cytogenetic evaluation for various genetic conditions, radiation biodosimetry, etc. With this objective, the current study was focused on analyzing the proliferation response of lymphocytes drawn from 90 individuals (21-29 years) with different blood types. The proliferation response was assessed in the cultured lymphocytes by cell cycle, mitotic index (MI), and nuclear division index (NDI) after stimulation with phytohaemagglutinin (PHA). To investigate the toxic effect on proliferation, MI was calculated in representative samples of each blood type were X-irradiated. The results showed that there was no significant difference among the cell cycle phases of lymphocytes in different blood types (P > 0.05). Similarly, both MI and NDI of lymphocytes derived from different blood types also did not show significant difference ( P > 0.05). The extensive interindividual variation within and among the blood types is likely responsible for the lack of significant difference in lymphocyte proliferation. Although spontaneous proliferation efficiency of lymphocytes of different blood types after PHA stimulation was grossly similar, the MI observed after radiation exposure showed a significant difference ( P < 0.05) indicating a differential proliferation response among the blood types. Our results suggest that the blood types did not have any impact on PHA-induced proliferation; however, a specific differential lymphocyte proliferation observed after radiation exposure needs to be considered.     

Does insect herbivory on oak depend on the diversity of tree stands?

Studies on the effects of plant diversity on insect herbivory have produced conflicting results. Plant diversity has been reported to cause positive and negative responses of herbivores. Explanations for these conflicting responses include not only various population-level processes but also changes in plant quality that lead to changes in herbivore performance. In a tree diversity experiment, we investigated the effects of tree diversity on insect herbivory on oak in general and whether the effects of tree diversity on herbivore damage are reflected by the performance (leaf consumption, growth) of the generalist herbivore Lymantria dispar. Our study showed that the feeding damage caused by naturally occurring herbivores on oak trees decreased with increasing diversity of tree stands. The performance of L. dispar on oak leaves was not affected by tree diversity, neither in field nor laboratory experiments. Our results can be explained by the various processes behind the hypothesis of associational resistance.     

Variation in species richness on small grassland quadrats: niche structure or small-scale plant mobility?

Abstract. In this study we make clear that the significance of deviations from an expected variance in species richness as demonstrated in alvar grassland, is a function of spatial pattern at the scale richness was measured, i.e. 10 cm². If corrected for spatial dependence, more than half of the significant cases become nonsignificant. As regards cases of significant variance deficit, which has been interpreted as an indication of niche limitation, we suggest that there may be a simple physical limitation to the number of species on the scale of observation in the form of a low number of plant units which can find a place because of the modal plant size of the species involved. In most cases of significant variance deficit the modal size of the plant units involved was probably bigger than average. Insofar as a significant variance deficit can be demonstrated, the resulting species richness distribution curve should be analyzed and we propose a skewness test, enabling us to differentiate between significantly left-skewed curves (‘niche limitation’), significantly right-skewed curves (‘niche facilitation’) and symmetrical curves. We present results obtained with the G-test, a log-likelihood ratio goodness of fit test. Only few cases of significantly left-skewed curves and a majority of symmetrical curves were found. Attempts to demonstrate guild proportionality in grasslands suffer from the heterogeneity of usually distinguished guilds, such as annuals vs. perennials or graminoids vs. dicotyledons. We observe that niche limitation may occur in alvar grassland, but indications for niche facilitation are stronger. Finally, we conclude that deviations in species richness variance are interesting indications of community structure, but only of spatial structure. Niche structure resulting from assembly rules should be investigated through experiments.