Character convergence under competition for nutritionally essential resources

Resource competition is thought to drive divergence in resource use traits (character displacement) by generating selection favoring individuals able to use resources unavailable to others. However, this picture assumes nutritionally substitutable resources (e.g., different prey species). When species compete for nutritionally essential resources (e.g., different nutrients), theory predicts that selection drives character convergence. We used models of two species competing for two essential resources to address several issues not considered by existing theory. The models incorporated either slow evolutionary change in resource use traits or fast physiological or behavioral change. We report four major results. First, competition always generates character convergence, but differences in resource requirements prevent competitors from evolving identical resource use traits. Second, character convergence promotes coexistence. Competing species always attain resource use traits that allow coexistence, and adaptive trait change stabilizes the ecological equilibrium. In contrast, adaptation in allopatry never preadapts species to coexist in sympatry. Third, feedbacks between ecological dynamics and trait dynamics lead to surprising dynamical trajectories such as transient divergence in resource use traits followed by subsequent convergence. Fourth, under sufficiently slow trait change, ecological dynamics often drive one of the competitors to near extinction, which would prevent realization of long-term character convergence in practice.     

Intraspecific competition for host resources in a parasite

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Base composition of deoxyribonucleic acid isolated from mycobacteria

Guanine plus cytosine values of deoxyribonucleic acid derived from 30 cultures representing 14 mycobacterial species or varieties are presented. These data provide impressive reasons for maintaining the separation between the genera Corynebacterium and Mycobacterium; no conclusions can be arrived at from these data with respect to the Nocardia-Mycobacterium relationship. A bimodal clustering, in terms of guanine plus cytosine composition, is apparent within the genus Mycobacterium. In general, all members of any single phenetic species appear to fit into one or another of these clusters. The phenetic separation of species is, in some cases, confirmed by separation in terms of guanine plus cytosine values. The bimodal separation of guanine plus cytosine values within the genus Mycobacterium does not correspond to a division of the species into slow and rapid growers; it thus provides no justification for splitting Mycobacterium into two genera, composed of slow and rapid growers. This is not to say that such a split would not be useful, only that these data do not contribute to such a decision. Any further attempts to correlate phenetic classification with properties of mycobacterial deoxyribonucleic acid will require more specific techniques, such as molecular hybridization.     

Transient behavior of population density with competition for resources

In this paper we derive a perturbation expansion of some nonlinear diffusion equations proposed by J. G. Skellam and M. Kimura. The solution is derived by converting the differential equations into integral equations by means of the Green's function for the diffusion equation. This research was supported in part by the Office of Naval Research under Contract Nonr-595(17).     

Population structure inhibits evolutionary diversification under competition for resources

A model is presented that explores how population structure affects the evolutionary outcome of ecological competition for resources. The model assumes that competition for resources occurs within groups of a finite number of individuals (interaction groups), and that limited dispersal of individuals between groups (according to Wright's island model of population structure) results in genetic structuring of the population. It is found that both finite-sized interaction groups and limited dispersal can have substantial effects on the evolution of resource exploitation strategies as compared to models with a single, infinitely large, well-mixed interaction group. Both effects, in general, tend to select for less aggressive competitive strategies. Moreover, both effects also tend to reduce the likelihood of the evolutionary diversification of resource exploitation strategies that often occurs in models of resource competition with infinite populations. The results are discussed in the context of theories of the evolutionary diversification of resource exploitation strategies and speciation.     

Bottom-up derivation of population models for competition involving multiple resources

This paper provides first-principles derivations of population models for competition involving multiple resources with different competition types, based on resource partitioning between individuals. The following two cases are investigated. The first is the case in which the resource competed for and its competition type change depending on life stages from scramble to contest competition, or from contest to scramble competition. The second is the case in which individuals compete for two resources simultaneously with scramble and contest types, respectively. In both cases, population models are derived analytically, and in particular, the Hassell model is derived in the second case. The nature of reproduction curves and the stability properties of three population models derived are compared with each other. These models provide three representative models for competition involving both scramble and contest types.     

Oscillations and chaos generated by competition for interactively essential resources

Recent theory shows that: (i) competition between multiple species for multiple resources may generate oscillations and chaotic fluctuations in species abundances; and (ii) these non-equilibrium dynamics may favor a high biodiversity. These findings were based on Liebigs Law of the Minimum, which assumes that each species is limited by only one resource at a time. In reality, however, resources can have interactive effects on growth. Here, we investigate whether competition for interactively essential resources may generate oscillations and chaos as well. Our results show that competition for interactively essential resources may, indeed, exhibit dynamics of similar complexity. This illustrates the wide potential for non-equilibrium dynamics generated by multispecies competition, and suggests that competitive chaos may occur on a wide variety of different resource types.     

Base composition of DNA from symbiotic dinoflagellates: a tool for phylogenetic classification

DNA of eight endosymbiotic dinoflagellates (zooxanthellae) from seven different host species has been analyzed as to its thermal characteristics and base composition by means of spectrophotometry and high performance liquid chromatography. All algae under investigation contain both methylcytosine and hydroxymethyluracil in addition to the bases typical of nuclear DNA. As a result, melting temperatures are decreased, suggesting lower contents of guanine plus cytosine than actually present. True percentages of guanine plus cytosine plus methylcytosine range from about 43 to 54 mol%. They are unique for the symbionts from different hosts, indicating phylogenetic separation of the taxa comparised within the genus Symbiodinium.Abbreviations dA deoxyadenosine - dC deoxycytidine - dG deoxyguanosine - dT deoxythymidine - m5dC 5-methyldeoxycytidine - hmdU 5-hydroxymethyldeoxyuridine - rC ribocytidine - Br8G bromine-80guanosine - A adenine - C cytosine - G guanine - T thymine - m5C 5-methylcytosine - hmU 5-hydroxymethyluracil - G+C guanine plus cytosine plus 5-methylcytosine - HPLC high performance liquid chromatography - T _m temperature at the midpoint of hyperchromic shift - CTAB N-cetyl-N,N,N-trimethyl-ammonium bromide - EDTA ethylenediamine-tetraacetic acid, disodium salt - TRIS tris-(hydroxymethyl)-aminomethane - 1×SSC standard saline citrate (0.15 M NaCl+0.015 M trisodium citrate, pH 7.0)     

Effects of metabolic rate and sperm competition on the fatty‐acid composition of mammalian sperm

The sperm membrane is a key structure affecting sperm function and thus reproductive success. Spermatozoa are highly specialized and differentiated cells that undergo a long series of processes in the male and female reproductive tracts until they reach the site of fertilization. During this transit, the sperm membrane is prone to damage such as lipid peroxidation. The characteristics and performance of the sperm membrane are strongly determined by the fatty‐acid composition of membrane phospholipids. Polyunsaturated fatty‐acids (PUFAs) are the most prone to lipid peroxidation. Lipid peroxidation and other types of oxidative damage increase with higher metabolism and with higher levels of sperm competition due to the increased ATP production to fuel higher sperm velocities. Consequently, we hypothesized that, in order to avoid oxidative damage, and the ensuing impairment of sperm function, sperm cells exhibit a negative relationship between PUFA content and mass‐specific metabolic rate (MSMR). We also hypothesized that higher sperm competition leads to a reduction in the proportion of sperm PUFAs. We performed a comparative study in mammals and found that high MSMR and high levels of sperm competition both promote a decrease in the proportion of PUFAs that are more prone to lipid peroxidation. The negative relationship between MSMR and these PUFAs in sperm cells is surprising, because a positive relationship is found in all other cell types so far investigated. Our results support the idea that the effects of MSMR and sperm competition on sperm function can operate at very different levels.     

Global analysis of competition for perfectly substitutable resources with linear response

We study a model of the chemostat with two species competing for two perfectly substitutable resources in the case of linear functional response. Lyapunov methods are used to provide sufficient conditions for the global asymptotic stability of the coexistence equilibrium. Then, using compound matrix techniques, we provide a global analysis in a subset of parameter space. In particular, we show that each solution converges to an equilibrium, even in the case that the coexistence equilibrium is a saddle. Finally, we provide a bifurcation analysis based on the dilution rate. In this context, we are able to provide a geometric interpretation that gives insight into the role of the other parameters in the bifurcation sequence. Funding was provided by the National Science Foundation-funded ADVANCE Institutional Transformation Program at New Mexico State University, fund # NSF0123690. Research partially supported by the Natural Science and Engineering Research Council of Canada.     

Direct competition results from strong competition for limited resource

We study a model of competition for resource through a chemostat-type model where species consume the common resource that is constantly supplied. We assume that the species and resources are characterized by a continuous trait. As already proved, this model, although more complicated than the usual Lotka–Volterra direct competition model, describes competitive interactions leading to concentrated distributions of species in continuous trait space. Here we assume a very fast dynamics for the supply of the resource and a fast dynamics for death and uptake rates. In this regime we show that factors that are independent of the resource competition become as important as the competition efficiency and that the direct competition model is a good approximation of the chemostat. Assuming these two timescales allows us to establish a mathematically rigorous proof showing that our resource-competition model with continuous traits converges to a direct competition model. We also show that the two timescales assumption is required to mathematically justify the corresponding classic result on a model consisting of only finite number of species and resources (MacArthur in, Theor Popul Biol 1:1–11, 1970). This is performed through asymptotic analysis, introducing different scales for the resource renewal rate and the uptake rate. The mathematical difficulty relies in a possible initial layer for the resource dynamics. The chemostat model comes with a global convex Lyapunov functional. We show that the particular form of the competition kernel derived from the uptake kernel, satisfies a positivity property which is known to be necessary for the direct competition model to enjoy the related Lyapunov functional.     

Global dynamics of microbial competition for two resources with internal storage

We study a chemostat model that describes competition between two species for two essential resources based on storage. The model incorporates internal resource storage variables that serve the direct connection between species growth and external resource availability. Mathematical analysis for the global dynamics of the model is carried out by using the monotone dynamical system theory. It is shown that the limiting system of the model basically exhibits the familiar Lotka-Volterra alternatives: competitive exclusion, coexistence, and bi-stability, and most of these results can be carried over to the original model. B. Li’s research was partially supported by NSF grants DMS 0211614 and DMS 0616445. H. L. Smith’s research was supported in part by NSF grant DMS 0414270.     

Aggregated distribution of resources creates competition refuges for rainforest dung beetles

This study examines the effects of resource distribution on colonisation, intra- and interspecific aggregation, and the occurrence of low-density, competition refuges for tropical dung beetles. In field experiments from central Peru, using dung pats (resource patches) of different volumes, the numbers of interacting species and total beetle biomass at individual pats increased with increasing pat volume. In two of three separate experiments (including an experiment that also varied patch density), this represented a decrease in the biomass of beetles per unit volume (biomass-density) at larger patches. The numbers of interacting tunneller species and tunneller biomass-density were also related to the distance between pats (patch density) in one of two experiments with constant numbers of pats. Closely positioned pats had generally fewer interacting species and a lower biomass-density of beetles. For the most abundant Dichotomius species, interspecific associations increased as distances increased between dung pats. The numbers of interacting species and biomass-density declined linearly under the combined effects of increasing patch density and local patch abundance in 25 m² plots. In experimentally placed grids with large numbers of pats, colonisation of pats at the edge of the grids was generally higher than at the centre of the grids for tunnellers and Eurysternus spp. but not for ball rollers; however, at least tunnellers did not readjust to avoid patches with high densities of competitors. These results indicate that an aggregated distribution of dung and natural variability in patch size contribute to species coexistence by creating low-density refuges for weaker competitors.     

Correcting contraceptive failure rates for sample composition and sample selection bias

Abstract

There is reason to believe that in the short run marriages are becoming more stable in some Western African countries such as Cameroon. One of the crucial questions facing these countries is whether fertility rates can be expected to increase or decrease due in part to the increased stability of marriages. Analyzing 1978 Cameroon World Fertility Survey data and using a multivariate regression model which compares the fertility rate of women who have had at least one marital disruption with that of continuously married women, we studied the relationship between marital instability and fertility. The results show that fertility rates for women married more than once are significantly lower than those for continuously married women even before the end of their first marriage. Furthermore, marital disruption significantly reduces fertility rates after the dissolution of the first marriage. Finally, even after the length of reproductive time lost is controlled, there is an inverse relationship between the number of marriages and fertility. The results are discussed in the context of economic development, modernization, and urbanization.     

Altered metabolic fluxes result from shifts in metabolite levels in sucrose phosphorylase-expressing potato tubers

As reported in a previous paper (Plant, Cell and Environment 24, 357–365, 2001), introduction of sucrose phosphorylase into the cytosol of potato results in increased respiration, an inhibition of starch accumulation and decreased tuber yield. Herein a more detailed investigation into the effect of sucrose phosphorylase expression on tuber metabolism, in order to understand why storage and growth are impaired is described. (1) Although the activity of the introduced sucrose phosphorylase was low and accounted for less than 10% of that of sucrose synthase its expression led to a decrease in the activities of enzymes of starch synthesis relative to enzymes of glycolysis and relative to total amylolytic activity. (2) Incubation of tuber discs in [¹⁴C]glucose revealed that the transformants display a two‐fold increase of the unidirectional rate of sucrose breakdown. However this was largely compensated by a large stimulation of sucrose re‐synthesis and therefore the net rate of sucrose breakdown was not greatly affected. Despite this fact major shifts in tuber metabolism, including depletion of sucrose to very low levels, higher rates of glycolysis, and larger pools of amino acids were observed in these lines. (3) Expression of sucrose phosphorylase led to a decrease of the cellular ATP/ADP ratio and energy charge in intact growing tubers. It was estimated that at least 30% of the ATP formed during respiration is consumed as a result of the large acceleration of the cycle of sucrose breakdown and re‐synthesis in the transformants. Although the absolute rate of starch synthesis in short‐term labelling experiments with discs rose, starch synthesis fell relative to other fluxes including respiration, and the overall starch content of the tubers was lower than in wild‐type tubers. (4) External supply of amino acids to replace sucrose as an osmoticum led to a feed‐back inhibition of glycolysis, but did not restore allocation to starch. (5) However, an external supply of the non‐metabolizable sucrose analogue palatinose – but not sucrose itself – stimulated flux to starch in the transformants. (6) The results indicate that the impaired performance of sucrose phosphorylase‐expressing tubers is attributable to decreased levels of sucrose and increased energy consumption during sucrose futile cycling, and imply that sucrose degradation via sucrose synthase is important to maintain a relatively large sucrose pool and to minimize the ATP consumption required for normal metabolic function in the wild type.