The role of pollen limitation on the coexistence of two dioecious, wind-pollinated, closely related shrubs in a fluctuating environment

Elucidating the mechanisms of species coexistence is a crucial goal in ecology. Theory suggests that, when resource abundance fluctuates, coexistence can be achieved if each species in a competing pair is better at exploiting resources at opposite extremes of a fluctuating resource spectrum. Nonetheless, the proximal mechanisms allowing coexistence remain largely unexplored. In a previous paper, we showed that the coexistence of two Atriplex species was facilitated by their varying demographic response (in survival, growth and recruitment) to fluctuation in water availability. Here we explore the effect of spatial distribution, and pollen and resource limitation on the reproductive success (production of viable seeds) of the same two species. An analysis of their spatial distribution showed that Atriplex acanthocarpa had a clumped distribution, which is thought to increase the effectiveness of pollination in wind-pollinated plants, while Atriplex canescens had a random distribution, a pattern expected to restrict wind-pollination success. A pollen and resource (water and nutrients) addition experiment implemented through a repeated-measures design demonstrated that seed viability of A. canescens was both pollen and resource limited, but that these effects were negligible in A. acanthocarpa. Under natural conditions, pollen limitation restricted seed number in A. canescens to only one-third of that recorded when manual pollination was performed. By decreasing its fecundity (and consequent potential seedling recruitment), pollen limitation reverses the competitive advantage of A. canescens over A. acanthocarpa when the limiting resource (water) is abundant and seedling recruitment takes place. To our knowledge, our study of this congeneric pair in the Chihuahuan Desert is the first to document a link between pollen limitation and species coexistence.     

Potential mechanisms of coexistence in closely related forbs

The stable coexistence of very similar species has perplexed ecologists for decades and has been central to the development of coexistence theory. According to modern coexistence theory, species can coexist stably (i.e. persist indefinitely with no long‐term density trends) as long as species' niche differences exceed competitive ability differences, even if these differences are very small. Recent studies have directly quantified niche and competitive ability differences in experimental communities at small spatial scales, but provide limited information about stable coexistence across spatial scales in heterogeneous natural communities. In this study, we use experimental and observational approaches to explore evidence for niche and competitive ability differences between two closely related, ecologically similar and widely coexisting annual forbs: Trachymene cyanopetala and T. ornata. We experimentally tested for stabilizing niche differences and competitive ability differences between these species by manipulating species' frequencies, under both well‐watered and water‐stressed conditions. We considered these experimental results in light of extensive field observations to explore evidence of niche segregation at a range of spatial scales. We found little evidence of intra‐specific stabilization or competitive ability differences in laboratory experiments while observational studies suggested niche segregation across pollinator assemblages and small‐scale microclimate heterogeneity. Though we did not quantify long‐term stabilization of coexisting populations of these species, results are consistent with expectations for stable coexistence of similar species via a spatial storage effect allowing niche differences to overcome even small (to absent) competitive ability differences.     

Evolutionary coexistence in a fluctuating environment by specialization on resource level

Microbial communities in fluctuating environments, such as oceans or the human gut, contain a wealth of diversity. This diversity contributes to the stability of communities and the functions they have in their hosts and ecosystems. To improve stability and increase production of beneficial compounds, we need to understand the underlying mechanisms causing this diversity. When nutrient levels fluctuate over time, one possibly relevant mechanism is coexistence between specialists on low and specialists on high nutrient levels. The relevance of this process is supported by the observations of coexistence in the laboratory, and by simple models, which show that negative frequency dependence of two such specialists can stabilize coexistence. However, as microbial populations are often large and fast growing, they evolve rapidly. Our aim is to determine what happens when species can evolve; whether evolutionary branching can create diversity or whether evolution will destabilize coexistence. We derive an analytical expression of the invasion fitness in fluctuating environments and use adaptive dynamics techniques to find that evolutionarily stable coexistence requires a special type of trade-off between growth at low and high nutrients. We do not find support for the necessary evolutionary trade-off in data available for the bacterium Escherichia coli and the yeast Saccharomyces cerevisiae on glucose. However, this type of data is scarce and might exist for other species or in different conditions. Moreover, we do find evidence for evolutionarily stable coexistence of the two species together. Since we find this coexistence in the scarce data that are available, we predict that specialization on resource level is a relevant mechanism for species diversity in microbial communities in fluctuating environments in natural settings.     

Intraspecific niche flexibility facilitates species coexistence in a competitive community with a fluctuating environment

We analyzed the role of niche usage flexibility (i.e. niche width) in promoting species coexistence in competitive communities in a one-dimensional niche space. We included two types of stochasticity, namely, a random sampling effect of community founding and environmental fluctuation. Fluctuation was further divided into two categories: niche-independent fluctuation (synchronized over the niche space) and niche-dependent fluctuation (variable among individual niche positions). In the analysis, two types of genetic and inheritance systems of individual niche position were considered, i.e. sexual reproduction with multiple loci and asexual reproduction with phenotypic plasticity. We found that niche usage flexibility promoted species diversity only under restricted situations when the environment was constant, but it generally promoted diversity when the environment fluctuated. In particular, under niche-independent fluctuation, niche usage flexibility significantly enhanced species diversity. In contrast, the analysis also predicted that when niche flexibility was constant, species diversity decreased with increasing environmental correlation between neighboring niches.     

DRG represents a family of two closely related GTP-binding proteins

In a previous publication we identified a novel human GTP-binding protein that was related to DRG, a developmentally regulated GTP-binding protein from the central nervous system of mouse. Here we demonstrate that both the human and the mouse genome possess two closely related drg genes, termed drg1 and drg2. The two genes share 62% sequence identity at the nucleotide and 58% identity at the protein level. The corresponding proteins appear to constitute a separate family within the superfamily of the GTP-binding proteins. The DRG1 and the DRG2 mRNA are widely expressed in human and mouse tissues and show a very similar distribution pattern. The human drg1 gene is located on chromosome 22q12, the human drg2 gene on chromosome 17p12. Distantly related species including Caenorhabditis elegans, Schizosaccharomyces pombe and Saccharomyces cerevisiae also possess two drg genes. In contrast, the genomes of archaebacteria (Halobium, Methanococcus, Thermoplasma) harbor only one drg gene, while eubacteria do not seem to contain any. The high conservation of the polypeptide sequences between distantly related organisms indicates an important role for DRG1 and DRG2 in a fundamental pathway.     

Mathematical model allowing the coexistence of closely related competitors at the initial stage of evolution

The mathematical model presented here aims to elucidate the essential mechanisms of coexistence of species, especially those of closely related forms, as a result of competition in the same environment. It describes a system where the fate of the competitors or mutants is observed at the initial stage of evolution. The model encompasses both the external variables and the internal state of the competitors, which differ only in one of the metabolic rate constants. Results of simulations, even with the simplified form of the model, show that stable coexistence of closely related forms in a uniform environment is possible. In addition, the model allows the analysis of the limitations on the level of differences and similarities among the competitors for achieving a state of coexistence. The essential mechanisms for the coexistence of closely related competitors are proposed to be the involvement of the metabolic network in allowing the same growth rate of competitors which have different internal states, and the interplay between the internal states of the competitors and the external variables of their environment.     

Spatial patterns and coexistence mechanisms in systems with unidirectional flow

River ecosystems are the prime example of environments where unidirectional flow influences the dispersal of individuals. Spatial patterns of community composition and species replacement emerge from complex interplays of hydrological, geochemical, biological, and ecological factors. Local processes affecting algal dynamics are well understood, but a mechanistic basis for large scale emerging patterns is lacking. To understand how these patterns could emerge in rivers, we analyze a reaction-advection-diffusion model for two competitors in heterogeneous environments. The model supports waves that invade upstream up to a well-defined "upstream invasion limit". We discuss how these waves are produced and present their key properties. We suggest that patterns of species replacement and coexistence along spatial axes reflect stalled waves, produced from diffusion, advection, and species interactions. Emergent spatial scales are plausible given parameter estimates for periphyton. Our results apply to other systems with unidirectional flow such as prevailing winds or climate-change scenarios.     

Advantage of storage in a fluctuating environment

We will elaborate the evolutionary course of an ecosystem consisting of a population in a chemostat environment with periodically fluctuating nutrient supply. The organisms that make up the population consist of structural biomass and energy storage compartments. In a constant chemostat environment a species without energy storage always out-competes a species with energy reserves. This hinders evolution of species with storage from those without storage. Using the adaptive dynamics approach for non-equilibrium ecological systems we will show that in a fluctuating environment there are multiple stable evolutionary singular strategies (ss's): one for a species without, and one for a species with energy storage. The evolutionary end-point depends on the initial evolutionary state. We will formulate the invasion fitness in terms of Floquet multipliers for the oscillating non-autonomous system. Bifurcation theory is used to study points where due to evolutionary development by mutational steps, the long-term dynamics of the ecological system changes qualitatively. To that end, at the ecological time scale, the trait value at which invasion of a mutant into a resident population becomes possible can be calculated using numerical bifurcation analysis where the trait is used as the free parameter, because it is just a bifurcation point. In a constant environment there is a unique stable equilibrium for one species following the "competitive exclusion" principle. In contrast, due to the oscillatory dynamics on the ecological time scale two species may coexist. That is, non-equilibrium dynamics enhances biodiversity. However, we will show that this coexistence is not stable on the evolutionary time scale and always one single species survives.     

Strain-specific differences in the genetic control of two closely related mycobacteria

The host response to mycobacterial infection depends on host and pathogen genetic factors. Recent studies in human populations suggest a strain specific genetic control of tuberculosis. To test for mycobacterial-strain specific genetic control of susceptibility to infection under highly controlled experimental conditions, we performed a comparative genetic analysis using the A/J- and C57BL/6J-derived recombinant congenic (RC) mouse panel infected with the Russia and Pasteur strains of Mycobacterium bovis Bacille Calmette Guérin (BCG). Bacillary counts in the lung and spleen at weeks 1 and 6 post infection were used as a measure of susceptibility. By performing genome-wide linkage analyses of loci that impact on tissue-specific bacillary burden, we were able to show the importance of correcting for strain background effects in the RC panel. When linkage analysis was adjusted on strain background, we detected a single locus on chromosome 11 that impacted on pulmonary counts of BCG Russia but not Pasteur. The same locus also controlled the splenic counts of BCG Russia but not Pasteur. By contrast, a locus on chromosome 1 which was indistinguishable from Nramp1 impacted on splenic bacillary counts of both BCG Russia and Pasteur. Additionally, dependent upon BCG strain, tissue and time post infection, we detected 9 distinct loci associated with bacillary counts. Hence, the ensemble of genetic loci impacting on BCG infection revealed a highly dynamic picture of genetic control that reflected both the course of infection and the infecting strain. This high degree of adaptation of host genetics to strain-specific pathogenesis is expected to provide a suitable framework for the selection of specific host-mycobacteria combinations during co-evolution of mycobacteria with humans.     

Advantage of storage in a fluctuating environment

We will elaborate the evolutionary course of an ecosystem consisting of a population in a chemostat environment with periodically fluctuating nutrient supply. The organisms that make up the population consist of structural biomass and energy storage compartments. In a constant chemostat environment a species without energy storage always out-competes a species with energy reserves. This hinders evolution of species with storage from those without storage. Using the adaptive dynamics approach for non-equilibrium ecological systems we will show that in a fluctuating environment there are multiple stable evolutionary singular strategies (ss's): one for a species without, and one for a species with energy storage. The evolutionary end-point depends on the initial evolutionary state. We will formulate the invasion fitness in terms of Floquet multipliers for the oscillating non-autonomous system. Bifurcation theory is used to study points where due to evolutionary development by mutational steps, the long-term dynamics of the ecological system changes qualitatively. To that end, at the ecological time scale, the trait value at which invasion of a mutant into a resident population becomes possible can be calculated using numerical bifurcation analysis where the trait is used as the free parameter, because it is just a bifurcation point. In a constant environment there is a unique stable equilibrium for one species following the “competitive exclusion” principle. In contrast, due to the oscillatory dynamics on the ecological time scale two species may coexist. That is, non-equilibrium dynamics enhances biodiversity. However, we will show that this coexistence is not stable on the evolutionary time scale and always one single species survives.     

Stationary gene frequency distribution in the environment fluctuating between two distinct states

Summary A general method is given to obtain a stationary distribution in a stochastic one-dimensional dynamical system in which an environmental parameter specifying the dynamical system is a stationary Markov process with only two states. By applying this method, the exact stationary gene frequency distribution is obtained for a genic selection model in the environment fluctuating between two distinct states. Several limiting stationary distributions are obtained therefrom, and one of them is shown to coincide with a stationary solution of the diffusion equation heuristically derived by us for more general cases. Discussion is given on the relationship between the diffusion equations obtained by various authors starting from discrete, non-overlapping generation models.     

Farm Households and Land Use in a Core Conservation Zone of the Maya Biosphere Reserve,Guatemala

This paper employs cross-tabular analysis, and multivariate and logistic regression to explore demographic, political-economic, socioeconomic, and ecological patterns of farm households and land use outcomes in an emergent agricultural frontier: the Sierra de Lacandón National Park (SLNP)-a core conservation zone of the Maya Biosphere Reserve (MBR), Petén, Guatemala. Data were obtained from a 1998 probability sample of 241 farm households, the first large detailed household land use survey in Guatemala’s Selva Maya-the largest lowland tropical forest in Central America. Virtually all settler households were poor maize farmers who colonized the SLNP in search of land for subsistence. While they faced similar ecological and economic conditions, land use strategies and patterns of forest clearing varied with demographic, household, and farm characteristics. Findings support and refute elements from previous frontier land use theory and offer policy implications for conservation and development initiatives in the Maya Forest specifically, and in tropical agricultural frontiers in general.

Testing mechanisms of coexistence among two species of frugivorous primates

1. We examined mechanisms of coexistence between two congeneric species of frugivorous primates, the blue monkey (Cercopithecus mitis) and the red-tailed monkey (C. ascanius). 2. We used giving-up densities (the amount of food which animals leave in a patch) in fruit trees to measure foraging efficiency and to evaluate possible mechanisms of coexistence. Animals with higher giving-up densities are less likely to persist in the company of those with lower giving-up densities because the former are not able to exploit food patches used previously by the latter. We climbed trees to estimate giving-up densities by counting the fruit which primates left behind. 3. We tested five possible mechanisms of coexistence. Three mechanisms proposed that each frugivorous species has a lower giving-up density than the other in at least one of the following: (1) different tree species, (2) within-tree foraging zones or (3) seasons. The fourth mechanism predicted that the socially dominant species exploits resources first and that the subordinate species has lower giving-up densities. The final mechanism predicted that one species would find resources more quickly than the other, which would in turn have a lower giving-up density. 4. Four of the five mechanisms received no support from our data. Only a trade-off between interspecific dominance and giving-up densities was supported. 5. We discuss the generality of our results and possible interactions with other factors.     

CLUE-S模型在长白山自然保护区外围规划中的应用

受自然保护区旅游业快速发展影响,长白山自然保护区外围土地利用变化加剧。因此,对区域土地利用布局进行科学规划具有十分重要的意义。以长白山自然保护区外围30 km区域为研究对象,探讨了CLUE-S模型在小尺度土地利用规划中的应用。在分析研究区1991—2007年土地利用变化的驱动力基础上,根据区域规划预案,模拟2020土地利用布局。利用模拟结果划定空间管制区和乡镇布局,并与现有规划进行了对比。结果表明,基于CLUE-S模型的土地规划明显抑制了区域景观破碎化进程,减弱了人为活动对景观的影响,该方法可以为长白山区域土地利用规划提供有力的技术手段和科学支撑。     

Aspartate isomerization in the complementarity-determining regions of two closely related monoclonal antibodies

The aspartic acid residues (Asp) present in the complementarity-determining regions (CDRs) of the light chains of two recombinant monoclonal antibodies (MAbs), MAb I and MAb II, are highly susceptible to isomerization due to the presence of glycine residues (Gly) on their C-terminal ends. Asp isomerization in these MAbs leads to formation of the isoaspartate (IsoAsp) and the cyclic imide (Asu) variants of these MAbs. Both MAb I and MAb II, employed in this study, elicit their pharmacological responses through binding human IgE. The formation of the MAb variants as a result of Asp isomerization significantly reduces the binding affinities of these antibodies to IgE, thereby reducing their potencies. Here we report on significant differences in the susceptibility of the MAb I and the MAb II to Asp isomerization. The molecular basis for these differences in rates of Asp isomerization was elucidated. The effect of primary sequence on Asp isomerization was evaluated using pentapeptide models of the MAbs, which included the labile Asp residues and their neighboring amino acid residues. The separation of the parent MAbs and pentapeptides from their isomerization products was achieved using hydrophobic interaction chromatography (HIC) and rp-HPLC, respectively. Structural characterization of the MAbs was performed using differential scanning calorimetry (DSC), circular dichroism (CD), and X-ray crystallography. Our investigations demonstrate that the differences in the Asp isomerization rates between MAb I and MAb II can be attributed to structural factors including the conformational flexibility and the extent of solvent exposure of the labile Asp residue.     

Foraging segregation between two closely related shearwaters breeding in sympatry

Trophic segregation has been proposed as a major mechanism explaining the coexistence of closely related animal taxa. However, how such segregation varies throughout the annual cycle is poorly understood. Here, we examined the feeding ecology of the two subspecies of Cory''s shearwater, Calonectris diomedea diomedea and Calonectris diomedea borealis, breeding in sympatry in a Mediterranean colony. To study trophic segregation at different stages, we combined the analysis of isotope values (δ ¹⁵N, δ ¹³C) in blood obtained during incubation and in feathers moulted during chick-rearing and wintering periods with satellite-tracking data during the chick-rearing period. Satellite-tracking and stable isotope data of the first primary feather revealed that C. d. borealis foraged mainly in the Atlantic whereas C. d. diomedea foraged exclusively in the Mediterranean. This spatial segregation could reflect the foraging behaviour of the C. d. borealis individuals before they arrived at the Mediterranean colony. Alternatively, greater wing loading of C. d. borealis individuals may confer the ability to fly across the strong winds occurring at the at the Gibraltar strait. Isotope values of the eighth secondary feather also support segregation in wintering areas between the two forms: C. d. diomedea wintered mainly in association with the Canary current, whereas C. d. borealis wintered in the South African coast. Overall, our results show that spatial segregation in foraging areas can display substantial variation throughout the annual cycle and is probably a major mechanism facilitating coexistence between closely related taxa.     

Distinct male reproductive strategies in two closely related oak species

Reproductive strategies of closely related species distributed along successional gradients should differ as a consequence of the trade‐off between competition and colonization abilities. We compared male reproductive strategies of Quercus robur and Q. petraea, two partly interfertile European oak species with different successional status. In the studied even‐aged stand, trees of the late‐successional species (Q. petraea) grew faster and suffered less from intertree competition than trees of the early‐successional species (Q. robur). A large‐scale paternity study and a spatially explicit individual‐based mating model were used to estimate parameters of pollen production and dispersal as well as sexual barriers between species. Male fecundity was found to be dependent both on a tree's circumference and on its environment, particularly so for Q. petraea. Pollen dispersal was greater and more isotropic in Q. robur than in Q. petraea. Premating barriers to hybridization were strong in both species, but more so in Q. petraea than in Q. robur. Hence, predictions based on the competition–colonization trade‐off are well supported, whereas the sexual barriers themselves seem to be shaped by colonization dynamics.     

Aerobic Heterotrophic Bacterial and Fungal Communities in the Topsoil of Omo Biosphere Reserve in Southwestern Nigeria1

As a part of the surveillance effort to monitor the ecological status of Omo Biosphere Reserve in the southwestern region of Nigeria, the aerobic heterotrophic bacterial and fungal communities of the topsoil were investigated in March 1995 and April 1996, before the onset of the rainy season. Four distinct wood‐tree plantations, a core strict nature reserve (SNR) area, and a buffer zone were sampled. The topsoil samples (7.5 cm depth), including the litter, were taken with an auger (8 cm diameter) and transported to the laboratory in polyethylene bags. One‐gram dry weight equivalent of sample was suspended in 10 ml sterile water, and serial dilutions from it were used for the estimation of bacterial and fungal densities. The bacterial and fungal densities ranged in the order of 106 and 103 cfu/g, respectively. Out of the 18 bacterial and 16 fungal species that were obtained, 13 and 12, respectively, were isolated from the core SNR. About 46 to 69 percent of the bacteria and 50 to 83 percent of the fungi species found in the SNR were absent in different combinations in the plantations and the buffer zone; these variations were significant among the sites monitored. The bacterial and fungal species compositions were significantly different between the SNR and each of the other sites. Proportional distributions within the sites were significant only for the bacterial communities. It would appear that plantation and human activities have caused significant changes in the distribution and species richness of the heterotrophic bacterial and fungal communities relative to the undisturbed SNR area of the Omo Biosphere Reserve.