Selfish mothers: reproductive expenditure and resource availability in bighorn ewes

We present a marginal value model explaining intraspecific andinterspecific variation of mating systems in nonterritorialungulates. The model takes into account the simultaneous effectsof spatial and temporal distribution of females, female groupsize, male-male competition, female choice, and the operationalsex ratio (i.e., the proportion of estrous females). The modelpredicts that higher numbers of females per group increasesthe average exploitation time of such groups by males. An increasein female group density, operational sex ratio, and age-specificfighting success of males are predicted to reduce the averageexploitation time of female groups, leading to roving of males(i.e., moving between female groups). In contrast, an increasein die female rejection rate of males and in the time spentby males on nonmating activities (i. e., foraging, lying down,ruminating, migrating) are predicted to increase the averageexploitation time of female groups and to favor staying behaviorof males (i.e., defending a female group over a longer periodof time). Consequently, die model predicts that young maleswill tend to be "stayers," whereas middle-aged and old malesare expected to be "rovers." Model predictions were tested widifield data collected on muskoxen Ovibos moschatus in a naturalpopulation in west Greenland. Observed correlations betweenbull exploitation times of cow groups and the six above-mentionedsocial and environmental parameters were all in die predicteddirection and statistically significant in five of die six cases.Overall, 69% of die observed variation in exploitation timeof cow groups by males was explained by die model predictions.Stepwise regression suggested that, of die six parameters, variationin sex ratio (R² = .56) and time spent on nonmating activities(R² = .35) had the largest effects on male exploitation time.Also, die observed age-specific variation in bull exploitationtime of cow groups was as predicted.     

Effects of resource distribution, patch spacing, and preharvest information on foraging decisions of northern bobwhites

We investigated patch assessment by northern bobwhites (Collinusvirginianus) in an experimental arena where the distributionof resources in patches, preharvest information about thesepatches, and spacing of patches varied. We found that preharvestinformation about patch quality and a bimodal distribution ofpatch rewards allowed birds to selectively exploit patches highin resources. In contrast, uniform distribution of patch qualitiesand lack of preharvest information caused birds to forage nonselectivelyamong patches. Birds distinguished among patches of differentquality when these patches were spaced 13 m apart, but failedto react to patch quality differences when patches were 0 or3 m apart We also found a strong effect of the level of patchdepletion on foraging decisions: as resources in die arena becamescarce, birds increasingly foraged selectively in die most profitablepatches. Foraging decisions of bobwhites are biased by die waythey experience and memorize a spatially and temporally variableenvironment. The relative cost of this cognitive bias (i.e.,lost opportunity) is nonlinearty related to die mean resourcedensity in die environment and to die difference between thismean density and die resource density in die exploited patch.Cognitive bias should be considered when evaluating patch assessmentcapabilities of foragers in complex environments.     

Elevated CO2 concentrations and stomatal density: observations from 17 plant species growing in a CO2 spring in central Italy

Stomatal density (SD) and stomatal conductance ( g _s) can be affected by an increase of atmospheric CO₂ concentration. This study was conducted on 17 species growing in a naturally enriched CO₂ spring and belonging to three plant communities. Stomatal conductance, stomatal density and stomatal index (SI) of plants from the spring, which were assumed to have been exposed for generations to elevated [CO₂], and of plants of the same species collected in a nearby control site, were compared. Stomatal conductance was significantly lower in most of the species collected in the CO₂ spring and this indicated that CO₂ effects on g _s are not of a transitory nature but persist in the long term and through plant generations. Such a decrease was, however, not associated with changes in the anatomy of leaves: SD was unaffected in the majority of species (the decrease was only significant in three out of the 17 species examined), and also SI values did not vary between the two sites with the exception of two species that showed increased SI in plants grown in the CO₂-enriched area. These results did not support the hypothesis that long-term exposure to elevated [CO₂] may cause adaptive modification in stomatal number and in their distribution.     

Plant Effects on Spatial and Temporal Patterns of Nitrogen Cycling in Shortgrass Steppe

Because of the water-limited nature and discontinuous plant cover of shortgrass steppe, spatial patterns in ecosystem properties are influenced more by the presence or absence of plants than by plant type. However, plant type may influence temporal patterns of nutrient cycling between plant and soil. Plants having the carbon-3 (C₃) or carbon-4 (C₄) photosynthetic pathway differ in phenology as well as other attributes that affect nitrogen (N) cycling. We estimated net N mineralization rates and traced nitrogen-15 (¹⁵N) additions among plant and soil components during May, July, and September of 1995 in native plots of C₃ plants, C₄ plants, or mixtures of C₃ and C₄. Net N mineralization was significantly greater in C₃ plots than in C₄ plots during both July and September. C₃ plots retained significantly more ¹⁵N in May than did mixed and C₄ plots; these differences in ¹⁵N retention were due to greater ¹⁵N uptake by C₃ plants than by C₄ plants during May. There were no significant differences in total ¹⁵N retention among plant communities for July and September. Soil ¹⁵N was influenced more by presence or absence of plants than by type of plant; greater quantities of ¹⁵N remained in soil interspaces between plants than in soil directly under plants for July and September. Our results indicate that plant functional type (C₃ versus C₄) can affect both the spatial and the temporal patterns of N cycling in shortgrass steppe. Further research is necessary to determine how these intraseasonal differences translate to longer-term and coarser-scale effects of plants on N cycling, retention, and storage. Received 8 December 1997; accepted 6 May 1998.     

The significance of variable and density-independent post-recruitment mortality in local populations of reef fishes

Abstract In this paper I focus on how post-settlement mortality may modify initial patterns of settlement in reef fish. Infrequent recruitment surveys may underestimate the role of early post-settlement mortality as most mortality in reef fishes occurs shortly after settlement. Consequently, results from infrequent recruitment surveys shed little light on the mechanisms producing patterns of abundance because these surveys ignore early post-settlement mortality. Variation in density-independent mortality may be a common mechanism that can prevent a positive relationship between larval settlement and subsequent population abundance. Although density-dependent mortality is the most commonly recognized mechanism that can disrupt the correlation between settlement and adult abundance, density-independent mortality’ can also destroy this correlation if the variance associated with post-settlement mortality is greater than variance in settlement. This point is illustrated with a simulation model in which I modelled two populations: a piscivorous fish population that was recruitment-limited with constant mortality, and a prey population that had variable recruitment and mortality that was a function of the size of the predator population. The results of this model indicate that even when mortality of prey is density-independent, predation can determine prey abundance when variation in piscivore recruitment is high relative to prey recruitment. Thus, initial patterns of prey settlement can be modified by a recruitment-limited predator population.     

我国东、南沿海地区AM真菌群落生态分布研究

在我国东、南沿海地区AM真菌资源调查的基础上研究了AM真菌的群落生态分布和特征。证明球囊霉属在该地区的出现频度和相对多度都最高,其次是硬囊霉属。该区内南四省AM真菌丰度、密度和物种多样性指数均比北三省高。聚类分析结果和上述相似,说明我国东、南沿海各省AM真菌的丰度、频度、密度、多度和多样性特征由北向南呈规律性变化。     

Genetic and biochemical evidence for distinct key functions of two highly divergent GAPDH genes in catabolic and anabolic carbon flow of the cyanobacterium Synechocystis sp. PCC 6803

Cyanobacterial genomes harbour two separate highly divergent glyceraldehyde-3-phosphate dehydrogenase (GAPDH) genes, gap1 and gap2, which are closely related at the sequence level to the nuclear genes encoding cytosolic and chloroplast GAPDH of higher plants, respectively. Genes gap1 and gap2 of the unicellular cyanobacterium Synechocystis sp. PCC 6803 were cloned and sequenced and subsequently inactivated by insertional mutagenesis to understand their metabolic functions. We obtained homozygous gap1^- mutants which have lost the capacity to grow on glucose under dim light while growth on organic acids as well as photosynthetic growth under CO₂ and high light is not impaired. Homozygous gap2^- mutants show the reciprocal phenotype. Under dim light they only grow on glucose but not on organic acids nor do they survive under photosynthetic conditions. Measurements of the anabolic activities (reduction of 1,3-bisphosphoglycerate) in extracts from wild type and mutant cells show that Gap2 is a major enzyme with dual cosubstrate specificity for NAD and NADP, while Gap1 displays a minor NAD-specific GAPDH activity. However, if measured in the catabolic direction (oxidation of glyceraldehyde-3-phosphate) Gap2 activity is very low and increases three- to fivefold after gel filtration of extracts over Sephadex G25. Our results suggest that enzymes Gap1 and Gap2, although coexpressed in cyanobacterial wild-type cells, play distinct key roles in catabolic and anabolic carbon flow, respectively. While Gap2 operates in the photosynthetic Calvin cycle and in non-photosynthetic gluconeogenesis, Gap1 seems to be essential only for glycolytic glucose breakdown, conditions under which the catabolic activity of Gap2 seems to be repressed by a specific low-molecular-weight inhibitor.     

Analysis of the isopentenyl diphosphate isomerase gene family from Arabidopsis thaliana

Two Arabidopsis thaliana cDNAs (IPP1 and IPP2) encoding isopentenyl diphosphate isomerase (IPP isomerase) were isolated by complementation of an IPP isomerase mutant strain of Saccharomyces cerevisiae. Both cDNAs encode enzymes with an amino terminus that may function as a transit peptide for localization in plastids. At least 31 amino acids from the amino terminus of the IPP1 protein and 56 amino acids from the amino terminus of the IPP2 protein are not essential for enzymatic activity. Genomic DNA blot analysis confirmed that IPP1 and IPP2 are derived from a small gene family in A. thaliana. Based on northern analysis expression of both cDNAs occurs predominantly in roots of mature A. thaliana plants grown to the pre-flowering stage.     

Plant Functional Type Effects on Trace Gas Fluxes in the Shortgrass Steppe

Plant community structure is expected to regulate the microbial processes of nitrification and denitrification by controlling the availability of inorganic N substrates. Thus it could also be a factor in the concomitant release of NO and N2O from soils as a result of these processes. C3 and C4 plants differ in several attributes related to the cycling of nitrogen and were hypothesized to yield differences in trace gas exchange between soil and atmosphere. In this study we estimated fluxes of NO, N2O and CH4 from soils of shortgrass steppe communities dominated by either C3 plants, C4 plants or mixtures of the two types. We collected gas samples weekly from two sites, a sandy clay loam and a clay, throughout the growing seasons of 1995 and 1996. Plant functional type effects on gas fluxes at the clay site were not apparent, however we found several differences among plant communities on the sandy clay loam. CH4 uptake from atmosphere to soil was significantly greater on C4 plots than C3 plots in both years. NO fluxes were significantly greater from C4 plots than from C3 plots in 1995. NO fluxes from C3 and mixed plots were not significantly different between 1995 and 1996, however fluxes from C4 plots were significantly greater in 1995 compared to 1996. Results indicate that under certain environmental conditions, particularly when factors such as moisture and temperature are not limiting, plant community composition can play an important role in regulating trace gas exchange.     

Optimal sampling and spatial distribution of Ixodes pacificus, Dermacentor occidentalis and Dermacentor variabilis ticks (Acari: Ixodidae)

A common method for sampling tick populations is flagging, which is a method of dragging a white cloth over a plant substrate for a fixed distance along a transect. Flagging over rough physical surfaces or using long subtransect lengths could lead to the underestimation of tick densities. Using estimates of the drop-off rates of adult Ixodes pacificus in flag sampling, optimal sampling schemes (the length and number of subtransects) were examined using the relationships between the tick drop-off rates (c), the tick density and distribution patterns and the roughness of the sampled plant substrate. It was found that the optimal number of subtransects and Lmax, the longest subtransect length which did not significantly underestimate the tick density from c, were affected by the tick density, substrate roughness and tick distribution pattern. This study also showed that the density and distribution of I. pacificus and Dermacentor occidentalis varied greatly over time in the populations sampled, while the Dermacentor variabilis densities were low and showed no significant changes over time. Both I. pacificus and D. occidentalis had clumped distributions along trails and these clumps were aggregated. However, the clump sizes (or individual clump areas) changed significantly over time because of density fluctuation or the movement of ticks. Finally, a positive association between the number of I. pacificus and D. occidentalis adults was observed from 2 m subtransect flag collections in March 1995; no relationship was found in 1994. © Rapid Science Ltd. 1998