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PubMed2005

Cost considerations and applicant characteristics for the Pediatric Surgery Match.

BACKGROUND/PURPOSE: Formal training in pediatric surgery is highly competitive. The limited number of accredited positions has historically favored applicants with basic science experience, numerous publications, national presentations, and exposure to well-known pediatric surgeons. This review analyzes characteristics of successful applicants and cost associated with the Match. METHODS: A survey was e-mailed to 45 applicants after the 2003 Match. Geographic provenance, demographics, qualifications, costs, and valued program characteristics were assessed. Statistics were formulated by chi2 and Student's t test. RESULTS: Thirty-six applicants (80%) responded. Successful characteristics for matched vs unmatched included number of publications, 11.2 vs 5.7 (P < .01); first-author designation, 6.4 vs 3.1 (P = .02); basic science papers, 5.7 vs 1.7 (P < .01); national presentations, 5.8 vs 2.4 (P = .02); and presentations at pediatric surgical meetings, 2.0 vs 0.6 (P = .04). Ninety percent of matched applicants took time off to perform basic science research (P < .01). Average candidate expense was $6974, which represented 14% of pretax salary. Forty-one percent of applicants noted that cost limited the number of interviews taken. Fifty percent preferred a regional interview process to limit expense. Candidates ranked case diversity, volume, and mentor's advice as the most valued program characteristics. Successful applicants matched at their fifth rank on average. Eighty-six percent of unsuccessful applicants will reapply. CONCLUSIONS: Results of this study are important to those interested in the future of pediatric surgery. Successful applicants were shown to have several national presentations and multiple scientific publications, especially in basic sciences. Applicant costs are high, totaling more than $236,000 for survey respondents.

Adult↗

PubMed2006

Changes in athlete burnout over a thirty-week "rugby year".

Controversy over a negative experiential state among players, discussed as "burnout" in media reports, stimulated the New Zealand Rugby Union to commission research (including the present study) into players' experiences. Athlete burnout research to date has largely been limited to studies featuring cross-sectional designs. The purpose of this study was to examine changes in the key characteristics of burnout over a "rugby year", a 30-week competitive period involving two or more independent competitions. Players completed the Athlete Burnout Questionnaire at three different times during the 30-week competitive rugby year. Some statistically significant changes were observed in key characteristics of burnout across the competitive rugby year. Specifically players reported that their feelings of reduced accomplishment increased pre-competition to in-competition phases. Changes in exhaustion over time were associated with playing position. Burnout was also associated with injury, non-selection, rugby experience and team membership. Overall the results reflect that burnout is a dynamic experience and indicate that factors such as playing position, injury, selection and starting status are worthy of further investigation.

Adult↗

PubMed2004

The effects of interaction compartments on stability for competitive systems.

The interactions between species are unlikely to be randomly arranged, and there is increasing evidence that most interactions occur within small species sub-groups, or compartments, that do not strongly interact with one another. We examine whether arranging the interactions of a competitive system into compartments influences the system properties of linear stability, feasibility, reactivity, and biomass stability, thereby altering the likelihood of species persistence. Model Lotka-Volterra systems of diffuse competition were analysed with interactions arranged randomly and in compartments. It was found, using a variety of dynamical measures, that arranging interactions into compartments enhances the likelihood of species persistence. Since many natural competitive systems appear to have interactions arranged within compartments, this may be an outcome of the positive attributes that this form of organization offers.

Competitive Behavior↗

PubMed2004

Obligate vertebrate scavengers must be large soaring fliers.

Among extant vertebrates, only the 23 species of vulture are obligate scavengers. We use an energetic modelling approach to explore the constraints imposed by an obligate scavenging lifestyle, and to ask whether obligate scavengers must always be avian and generally large-bodied users of soaring flight. Our model found that aerial scavengers always out-competed postulated terrestrial ones, mainly because flight allows area to be searched much more rapidly for carrion. Soaring was favoured over flapping flight because the reduction in flight speed (and so rate of area search) was more than compensated for by the decrease in the costs of transport. Large individual size is selected for if carrion is available in large packages, when obligate scavenger feed only infrequently, and so must be able to survive on body reserves in the periods between discovering food falls. In the absence of avian radiation, an obligate terrestrial scavenger seems energetically feasible, but we argue that such a beast is unlikely to have evolved. In birds, in order to become exclusive scavengers, vultures have needed to specialize for efficient soaring flight as a low energy form of travel, and as a consequence they have lost the agility needed to kill prey. In mammals, however, no comparable trade-off occurs. So for terrestrial carnivores there is probably no strong selection pressure towards being an exclusive scavenger. Indeed it will perhaps always be more advantageous to retain the flexibility of obtaining food by either predation or scavenging.

Animals↗

PubMed2004

Predation, competition, and nutrient recycling: a stoichiometric approach with multiple nutrients.

A model for two competing prey species and one predator is formulated in which three essential nutrients can limit growth of all populations. Prey take up dissolved nutrients and predators ingest prey, assimilating a portion of ingested nutrients and recycling or respiring the balance. For all species, the nutrient contents of individuals vary and growth is coupled to increasing content of the limiting nutrient. This model was parameterized to describe a flagellate preying on two bacterial species, with carbon (C), nitrogen (N), and phosphorus (P) as nutrients. Parameters were chosen so that the two prey species would stably coexist without predators under some nutrient supply conditions. Using numerical simulations, the long-term outcomes of competition and predation were explored for a gradient of N:P supply ratios, varying C supply, and varying preference of the predator for the two prey. Coexistence and competitive exclusion both occurred under some conditions of nutrient supply and predator preference. As in simpler models of competition and predation these outcomes were largely governed by apparent competition mediated by the predator, and resource competition for nutrients whose effective supply was partly governed by nutrient recycling also mediated by the predator. For relatively small regions of parameter space, more complex outcomes with multiple attractors or three-species limit cycles occurred. The multiple constraints posed by multiple nutrients held the amplitudes of these cycles in check, limiting the influence of complex dynamics on competitive outcomes for the parameter ranges explored.

Animal Nutritional Physiological Phenomena↗

PubMed2004

Population models of sperm-dependent parthenogenesis.

Organisms that reproduce by sperm-dependent parthenogenesis are asexual clones that require sperm of a sexual host to initiate egg production, without the genome of the sperm contributing genetic information to the zygote. Although sperm-dependent parthenogenesis has some of the disadvantages of sex (requiring a mate) without the counterbalancing advantages (mixing of parental genotypes), it appears amongst a wide variety of species. We develop initial models for the density-dependent dynamics of animal populations with sperm-dependent parthenogenesis (pseudogamy or gynogenesis), based on the known biology of the common Enchytraeid worm Lumbricillus lineatus. Its sperm-dependent parthenogenetic populations are reproductive parasites of the hermaphrodite sexual form. Our logistic models reveal two alternative requirements for coexistence at density-dependent equilibria: (i) If the two forms differ in competitive ability, the form with the lower intrinsic birth rate must be compensated by a more than proportionately lower competitive impact from the other, relative to intraspecific competition, (ii) If the two forms differ in their intrinsic capacity to exploit resources, the sperm-dependent parthenogen must be superior in this respect and must have a lower intrinsic birth rate. In general for crowded environments we expect a sperm-dependent parthenogen to compete strongly for limiting resources with the sexual sibling species. Its competitive impact is likely to be weakened by its genetic uniformity, however, and this may suffice to cancel any advantage of higher intrinsic growth rate obtained from reproductive investment only in egg production. We discuss likely thresholds of coexistence for other sperm-dependent parthenogens. The fish Poeciliopsis monacha-lucida likewise obtains an intrinsic growth advantage from reduced investment in male gametes, and so its persistence is likely to depend on it being a poor competitor. The planarian Schmidtea polychroa obtains no such intrinsic benefit because it produces fertile sperm, and its persistence may depend on superior resource exploitation.

Animals↗

PubMed2005

Spatial dynamics of invasion: the geometry of introduced species.

Many exotic species combine low probability of establishment at each introduction with rapid population growth once introduction does succeed. To analyse this phenomenon, we note that invaders often cluster spatially when rare, and consequently an introduced exotic's population dynamics should depend on locally structured interactions. Ecological theory for spatially structured invasion relies on deterministic approximations, and determinism does not address the observed uncertainty of the exotic-introduction process. We take a new approach to the population dynamics of invasion and, by extension, to the general question of invasibility in any spatial ecology. We apply the physical theory for nucleation of spatial systems to a lattice-based model of competition between plant species, a resident and an invader, and the analysis reaches conclusions that differ qualitatively from the standard ecological theories. Nucleation theory distinguishes between dynamics of single- and multi-cluster invasion. Low introduction rates and small system size produce single-cluster dynamics, where success or failure of introduction is inherently stochastic. Single-cluster invasion occurs only if the cluster reaches a critical size, typically preceded by a number of failed attempts. For this case, we identify the functional form of the probability distribution of time elapsing until invasion succeeds. Although multi-cluster invasion for sufficiently large systems exhibits spatial averaging and almost-deterministic dynamics of the global densities, an analytical approximation from nucleation theory, known as Avrami's law, describes our simulation results far better than standard ecological approximations.

Animal Migration↗

PubMed2005

Coexistence of competing species by the oscillation of polymorphisms.

Scale-eating cichlids in Lake Tanganyika exhibit genetically determined lateral asymmetry, especially in their mouth-opening. Frequencies of the morphs oscillate due to strong frequency-dependent selection caused by the switching of prey's attention, and its delayed effect by their growth period. Two scale-eaters coexist in similar densities at south shore of the lake, with their morph frequencies oscillating in phase. We investigated the effect of the oscillation in morph frequencies to the coexistence of competing species. If the difference of two species' growth period is large, the oscillation facilitates the coexistence of the two species, while small difference of growth periods hinders their coexistence. In the latter case, the species with shorter growth period drives the other species to the extinction.

Animals↗

PubMed2005

The evolution of stability in a competitive system.

The characteristics governing the dynamics of populations can evolve and this evolution can either be towards stability or chaos. Yet it is not obvious how or why such population characteristics can evolve through selection on individuals. In this paper we construct a mathematical model, inspired by experimental results, illustrating the dynamics of a population of competing Drosophila. We demonstrate how selection of life history characteristics and stability influence one another as a population interacts with its environment. We generalize this result and show that population stability can evolve as a consequence of selection on individuals.

Animals↗

PubMed2005

Evolution of extraordinary female-biased sex ratios: the optimal schedule of sex ratio in local mate competition.

Female-biased sex ratio in local mate competition has been well studied both theoretically and experimentally. However, some experimental data show more female-biased sex ratios than the theoretical predictions by Hamilton [1967. Science 156, 477-488] and its descendants. Here we consider the following two effects: (1) lethal male-male combat and (2) time-dependent control (or schedule) of sex ratio. The former is denoted by a male mortality being an increasing function of the number of males. The optimal schedule is analytically obtained as an evolutionarily stable strategy (ESS) by using Pontrjagin's maximum principle. As a result, an ESS is a schedule where only males are produced first, then the proportion of females are gradually increased, and finally only females are produced. Total sex ratio (sex ratio averaged over the whole reproduction period) is more female-biased than the Hamilton's result if and only if the two effects work together. The bias is stronger when lethal male combat is severer or a reproduction period is longer. When male-male combat is very severe, the sex ratio can be extraordinary female-biased (less than 5%). The model assumptions and the results generally agree with experimental data on Melittobia wasps in which extraordinary female-biased sex ratio is observed. Our study might provide a new basis for the evolution of female-biased sex ratios in local mate competition.

Aggression↗

PubMed2005

A stochastic model of the distribution of unequal competitors between resource patches.

We present a stochastic model of individuals' movements between two patches of resources. The population is made up of two types of individual with differing competitive abilities, and two types of movements occur, with individuals moving either to increase their intake rate or at random. Several previous models have used simulations to evaluate the likely distribution of individuals. We instead derive equations for the equilibrium distribution of the population, which can be solved numerically. This avoids the need to choose an initial distribution for the population, and enables us to obtain the probability with which rare events occur. This may not be possible when simulations are used, since a rare event may not occur at all. We find that when random movements are rare, an increase in the rate of random movements out of a patch can increase the number of individuals on that patch. We consider an approximation to the model with rare random movements, which provides an explanation for this phenomenon.

Competitive Behavior↗

PubMed2005

Density-dependence as a size-independent regulatory mechanism.

The growth function of populations is central in biomathematics. The main dogma is the existence of density-dependence mechanisms, which can be modelled with distinct functional forms that depend on the size of the population. One important class of regulatory functions is the theta-logistic, which generalizes the logistic equation. Using this model as a motivation, this paper introduces a simple dynamical reformulation that generalizes many growth functions. The reformulation consists of two equations, one for population size, and one for the growth rate. Furthermore, the model shows that although population is density-dependent, the dynamics of the growth rate does not depend either on population size, nor on the carrying capacity. Actually, the growth equation is uncoupled from the population size equation, and the model has only two parameters, a Malthusian parameter rho and a competition coefficient theta. Distinct sign combinations of these parameters reproduce not only the family of theta-logistics, but also the van Bertalanffy, Gompertz and Potential Growth equations, among other possibilities. It is also shown that, except for two critical points, there is a general size-scaling relation that includes those appearing in the most important allometric theories, including the recently proposed Metabolic Theory of Ecology. With this model, several issues of general interest are discussed such as the growth of animal population, extinctions, cell growth and allometry, and the effect of environment over a population.

Animals↗

PubMed2005

Omnivory and the stability of food webs.

The ecological concept of omnivory, feeding at more than a single trophic level, is formulated as an intermediate stage between any two of three classical three-dimensional species interaction systems-tritrophic chain, competition, and polyphagy. It is shown that omnivory may be either stabilizing or destabilizing, depending, in part, on the conditions of the parent systems from which it derives. It is further conjectured that the tritrophic to competition gradient cannot be entirely stable, that there must be an instability at some level of intermediate omnivory.

Animals↗

PubMed2005

When should signals of submission be given?-A game theory model.

During contests, losing animals often show signals communicating submission. From an evolutionary viewpoint, however, it is not so obvious why the losing individual gives such a signal instead of running away and why the winning individual accepts the signal instead of inflicting more severe damage. We investigated factors influencing the evolution of signals of submission using a numerical ESS model. The present analysis reveals that there is much space for the evolution of signals of submission, even when the winner of an escalated contest gets some extra benefit. In most cases, signals of submission are given by animals which are slightly or moderately weaker than the opponent. Signals of submission are expected to occur frequently (1) when the value of contested resource does not differ greatly from the cost of injury, (2) when the extra benefit of winning an escalated contest is small, (3) when the opportunity for safe retreat by the losing animal is small, and (4) when the estimation of the difference in the resource holding potential (RHP) between the combatants is accurate but not perfect.

Animal Communication↗

PubMed2006

Evolution of the maturation rate collapses competitive coexistence.

Most theoretical studies on character displacement and the coexistence of competing species have focused attention on the evolution of competitive traits driven by inter-specific competition. We investigated the evolution of the maturation rate which is not directly related to competition and trades off with the birth rate and how it influences competitive outcomes. Evolution may result in the superior competitor becoming extinct if, initially, the inferior competitor has a lower, and the superior one a higher, maturation rate at the coexistence equilibrium. This counterintuitive result is explained by an explosive increase in the adult population of the inferior competitor as a result of the more rapid evolution of its maturation rate, which is caused by differences in the intensity and direction of selection on the maturation rates of the two species and in their adult densities, which are related to differences in their life histories. Thus, a life history trait trade-off with a competitive trait may cause a competitive ecological coexistence to collapse.

Animals↗

PubMed2006

The evolution of altruism between siblings: Hamilton's rule revisited.

This paper explores the validity of Hamilton's rule in the case of other-only altruism in which the benefits are shared by other members of the sibling group excluding the donor. It presents a model of competition between two alleles which code for different kinds of altruism. It derives a simple replicator equation for allele frequencies under conditions of strong selection. This equation does not depend on the size of the sibling group. In mathematical form, the equation is similar to Hamilton's original rule in the case of inbreeding, although the causal mechanism is different. The paper derives a simple criterion to determine whether there will be a polymorphism in which both alleles coexist permanently. Such an event is rare and victory will normally go to the allele with the higher value of 1/2b-c, where b is the total benefit which an offspring confers on its siblings and c is the cost to the donor. The paper also considers how an offspring will behave in particular circumstances. Using a specialized version of the basic model, it shows how, in the absence of polymorphism, natural selection should take the system towards the point of 50% marginal altruism. With this type of altruism, an offspring will perform any act for which the expected cost to the donor is at most half the expected benefit to its siblings. Acts which do not satisfy this criterion are not performed. This accords with Haldane's quip that he would sacrifice his own life for two of his brothers, but not for less. Numerical simulation is used to explore these issues in greater depth. The paper also examines briefly the implications of heterozygote advantage for Hamilton's rule. It concludes with a brief discussion of the connection between other-only altruism and whole-group altruism, in which the donor gains some benefit from its actions.

Altruism↗

PubMed2006

Population biology of multispecies helminth infection: competition and coexistence.

The role that interspecific interactions play in shaping parasite communities is uncertain. To date, models of competition between helminth species have assumed that interaction occurs through parasite-induced host death. To our knowledge, there has been no theoretical exploration of other forms of competition. We examine models in which competition acts at the point of establishment within the host, and at the time of egg production by the adult worm. The models used are stochastic and we allow hosts to vary in their rate of exposure to infective larvae. We derive the Lotka-Volterra model of competition when exposure is homogenous and thus demonstrate that two helminth species cannot coexist on a single limiting resource. We show that coexistence of species is promoted by heterogeneity in host exposure provided that the rates of exposure to the two species are not perfectly correlated, and, if they are positively correlated, provided that the degree of heterogeneity in host exposure is similar for the two competing helminth species. These results are robust to the mechanism of competition.

Animals↗

PubMed2006

Shedding light on plant competition: modelling the influence of plant morphology on light capture (and vice versa).

A plant's morphology is both strongly influenced by local light availability and, simultaneously, strongly influences this local light availability. This reciprocal relationship is complex, but lies at the heart of understanding plant growth and competition. Here, we develop a sub-individual-based simulation model, cast at the level of interacting plant components. The model explicitly simulates growth, development and competition for light at the level of leaves, branches, etc., located in 3D space. In this way, we are able to explore the manner in which the low-level processes governing plant growth and development give rise to individual-, cohort-, and community-level phenomena. In particular, we show that individual-level trade-offs between growing up and growing out arise naturally in the model, and robustly give rise to cohort-level phenomena such as self-thinning, and community processes such as the effect of ecological disturbance on the maintenance of biodiversity. We conclude with a note on our methodology and how to interpret the results of simulation models such as this one.

Algorithms↗