Offense and defense in landscape-level invasion control

Biological invasions are multi-stage processes comprising chance demographic events, species interactions, and dispersal. Despite this complexity, simple models can increase understanding of the invasion process. We model the spread of aquatic invasive species through a network of lakes to evaluate the effectiveness of two intervention strategies. The first, which we call offense, contains the invader at sources; the second, which we call defense, protects uninvaded destinations. Deterministic models reveal the effects of these intervention strategies on spread rates. Practical applications involve finite collections of uninvaded lakes, however, and we therefore also present a stochastic model to describe how these strategies affect expected times to important invasion milestones. When the goal is to reduce overall spread rates, both approaches agree that offense is better early in invasions, but that defense is better after 1/2 the lakes are invaded. When the goal is to protect areas of high conservation value, however, defensive site protection always provides lower per site introduction rates. Although we focus on lakes, our results are quite general, and could be applied to any discrete habitat patches including, for example, fragmented terrestrial habitats.     

Optimal defense strategy against herbivory in plants: Conditions selecting for induced defense, constitutive defense, and no-defense

To examine the conditions selecting for induced defense, constitutive defense, and no-defense, we developed a model of plant defense strategy against herbivory. In the model, a plant consists of two modules between which signal inducing defense compounds can be translocated. We assume three strategies: plants produce defense compounds responding to herbivory (induced defense), they have the compounds beforehand (constitutive defense), and they never produce the compounds (no-defense). We found that no-defense is optimal if the amount of biomass lost due to herbivory is small because of the growth cost of having defense compounds. The constitutive defense is optimal if the amount of biomass lost is not so small and the probability of herbivory is high. If the biomass loss is not so small but the probability of herbivory is low, the induced defense or no-defense is optimal. When the induced defense is optimal, the probability of herbivory necessarily increases in plants once herbivory has occurred. If the probability stays the same, no-defense is optimal. Thus, the behavior of herbivores, i.e., whether they remain around a plant and attack it repeatedly, affects the evolution of the induced defense.     

Cyber War Game in Temporal Networks

In a cyber war game where a network is fully distributed and characterized by resource constraints and high dynamics, attackers or defenders often face a situation that may require optimal strategies to win the game with minimum effort. Given the system goal states of attackers and defenders, we study what strategies attackers or defenders can take to reach their respective system goal state (i.e., winning system state) with minimum resource consumption. However, due to the dynamics of a network caused by a node’s mobility, failure or its resource depletion over time or action(s), this optimization problem becomes NP-complete. We propose two heuristic strategies in a greedy manner based on a node’s two characteristics: resource level and influence based on k-hop reachability. We analyze complexity and optimality of each algorithm compared to optimal solutions for a small-scale static network. Further, we conduct a comprehensive experimental study for a large-scale temporal network to investigate best strategies, given a different environmental setting of network temporality and density. We demonstrate the performance of each strategy under various scenarios of attacker/defender strategies in terms of win probability, resource consumption, and system vulnerability.     

SPERM COMPETITION AND THE EVOLUTION OF SEMINAL FLUID COMPOSITION

Male ejaculates include large amounts of seminal fluid proteins (Sfps) that influence male sperm competitive success. In spite of their diverse proximate functions, Sfps involved in sperm competition increase male fitness in one of three ways: (1) “avoidance” proteins help males avoid sperm competition, (2) “defense” proteins help males defend their sperm from displacement by the female's subsequent mate, and (3) “offense” proteins aid males in displacing sperm of preceding males. Here, we present a population genetic model of the evolution of allocation of finite resources by males to the three kinds of Sfps. We analyze the influence of relative efficiencies of different Sfps, of plasticity in resource allocation, and of differences in viability costs of Sfps. We find that in absence of plasticity or different viability costs, equal investment in defense and offense Sfps evolves, irrespective of their relative efficiency. In all cases, males evolve to invest more in avoidance when avoidance proteins are increasingly efficient, and when offense is more efficient than defense. Differences in viability costs result in lower investment in costly proteins, whereas plasticity has complex effects, influencing both the optimal seminal fluid composition and maintenance of variation in investment in these proteins across populations.     

A simple artificial life model explains irrational behavior in human decision-making

Although praised for their rationality, humans often make poor decisions, even in simple situations. In the repeated binary choice experiment, an individual has to choose repeatedly between the same two alternatives, where a reward is assigned to one of them with fixed probability. The optimal strategy is to perseverate with choosing the alternative with the best expected return. Whereas many species perseverate, humans tend to match the frequencies of their choices to the frequencies of the alternatives, a sub-optimal strategy known as probability matching. Our goal was to find the primary cognitive constraints under which a set of simple evolutionary rules can lead to such contrasting behaviors. We simulated the evolution of artificial populations, wherein the fitness of each animat (artificial animal) depended on its ability to predict the next element of a sequence made up of a repeating binary string of varying size. When the string was short relative to the animats' neural capacity, they could learn it and correctly predict the next element of the sequence. When it was long, they could not learn it, turning to the next best option: to perseverate. Animats from the last generation then performed the task of predicting the next element of a non-periodical binary sequence. We found that, whereas animats with smaller neural capacity kept perseverating with the best alternative as before, animats with larger neural capacity, which had previously been able to learn the pattern of repeating strings, adopted probability matching, being outperformed by the perseverating animats. Our results demonstrate how the ability to make predictions in an environment endowed with regular patterns may lead to probability matching under less structured conditions. They point to probability matching as a likely by-product of adaptive cognitive strategies that were crucial in human evolution, but may lead to sub-optimal performances in other environments.     

Survivor's dilemma: Defend the group or flee?

We consider a survival game of gregarious individuals, in which the aim of the players is survival to reproductive age under predator attacks. The survivor’s dilemma (shortly: SVD) game consists in the following: a group member either surely survives alone by fleeing, while its defensive mate may be killed; or tries to save its mate’s life, risking to get killed. The dilemma is that, in every single attack, fleeing ensures maximal survival probability, but if its mate survives by fighting both, and they remain together, its risk to be killed at the next attack will be lower. We show that, if defense is successful enough, then the one-attack game is a prisoner’s dilemma (PD), where fleeing is the strict ESS. We have additively decomposed the SVD game, according to the survival of the group mate of the focal prey, into two games: the aim of the “collective game” is survival of the group of prey. Counter-wise, the aim of the “hostile game” is survival alone (focal prey survives and its mate is killed by the predator). We obtain the following results: if the attack number is large enough, the multi-attack SVD game is dominated by the “collective game” in the sense that each individual can ensure its own maximal survival probability by maximizing the group survival probability in each attack. In the hostile game, the only strict ESS is the fleeing strategy. In the collective game there are two different cases: either defense is a unique strict ESS, or the collective game is bistable, i.e. fleeing and defense are local strict ESS’s. If defense is the only strict ESS in the collective game, and the attack number is large enough, defense replaces fleeing strategy in the multi-attack SVD game. However, in the bistable case, defense cannot invade into the fleeing population. It is shown that, if the interaction between relatives is frequent enough, than defense can replace fleeing strategy, in spite of the fact that in the well-mixed population the collective game is bistable.     

Differences between winning and defeated top quality basketball teams in final tournaments of European club championship

The goal of this research was to identify parameters among the 12 indicators of situation-related efficiency that differentiated between the winning and defeated top quality teams which played in final tournaments of the European club championships from 1992 to 2000. The differences were confirmed by discriminant analysis, although the canonical correlation was here somewhat lower than in the previous similar research studies done on the so-called regular season games. The probable reason for the smaller differences obtained in the present study may be found in almost equal (high) quality of the teams competing in Final Fours. The highest discriminative power was obtained in the variable defensive rebounds, then in the variables field goal percentage and free throw percentage, whereas the variable assist had evidently smaller impact with regard to the referent studies. The obtained results suggested that the winning teams showed more of tactical discipline and responsibility in controlling inside positions for defensive rebounds, as well as in controlling play on offense and the ball until the required open shot chance, which considerably reduced game risks and resulted in a lower number of turnovers and in a higher shooting percentage. Such a type of decision-making in play require a high degree of reciprocal help of players on both defense and offense and a higher level of concentration and self-confidence when shooting field goals and free throws. The common denominator of the winning teams was a lower number of imbalanced states in their play (the organized style of play on defense and offense implied) and a higher level of collective outplaying the opponents with the controlled system of play, which enabled entire potential of the victorious teams to be expressed.     

Intervention in gene regulatory networks via greedy control policies based on long-run behavior

Background  

A salient purpose for studying gene regulatory networks is to derive intervention strategies, the goals being to identify potential drug targets and design gene-based therapeutic intervention. Optimal stochastic control based on the transition probability matrix of the underlying Markov chain has been studied extensively for probabilistic Boolean networks. Optimization is based on minimization of a cost function and a key goal of control is to reduce the steady-state probability mass of undesirable network states. Owing to computational complexity, it is difficult to apply optimal control for large networks.     

Assessing the potential for an ongoing arms race within and between the sexes: selection and heritable variation

In promiscuous species, sexual selection generates two opposing male traits: offense (acquiring new mates and supplanting stored sperm) and defense (enforcing fidelity on one's mates and preventing sperm displacement when this fails). Coevolution between these traits requires both additive genetic variation and associated natural selection. Previous work with Drosophila melanogaster found autosomal genetic variation for these traits among inbred lines from a mixture of populations, but only nonheritable genetic variation was found within a single outbred population. These results do not support ongoing antagonistic coevolution between offense and defense, nor between either of these male traits and female reproductive characters. Here we use a new method (hemiclonal analysis) to study genomewide genetic variation in a large outbred laboratory population of D. melanogaster. Hemiclonal analysis estimates the additive genetic variation among random, genomewide haplotypes taken from a large, outbred, locally adapted laboratory population and determines the direction of the selection gradient on this variation. In contrast to earlier studies, we found low but biologically significant heritable variation for defensive and offensive offspring production as well as all their components (P1, fidelity, P2, and remating). Genetic correlations between these traits were substantially different from those reported for inbred lines. A positive genetic correlation was found between defense and offense, demonstrating that some shared genes influence both traits. In addition to this common variation, evidence for unique genetic variation for each trait was also found, supporting an ongoing coevolutionary arms race between defense and offense. Reproductive conflict between males can strongly influence female fitness. Correspondingly, we found genetic variation in both defense and offense that affected female fitness. No evidence was found for intersexual conflict in the context of male defense, but we found substantial intersexual conflict in the context of male offensive sperm competitive ability. These results indicate that conflict between competing males also promotes an associated arms race between the sexes.     

Defense strategies used by two sympatric vineyard moth pests

Natural enemies including parasitoids are the major biological cause of mortality among phytophagous insects. In response to parasitism, these insects have evolved a set of defenses to protect themselves, including behavioral, morphological, physiological and immunological barriers. According to life history theory, resources are partitioned to various functions including defense, implying trade-offs among defense mechanisms. In this study we characterized the relative investment in behavioral, physical and immunological defense systems in two sympatric species of Tortricidae (Eupoecilia ambiguella, Lobesia botrana) which are important grapevine moth pests. We also estimated the parasitism by parasitoids in natural populations of both species, to infer the relative success of the investment strategies used by each moth. We demonstrated that larvae invest differently in defense systems according to the species. Relative to L. botrana, E. ambiguella larvae invested more into morphological defenses and less into behavioral defenses, and exhibited lower basal levels of immune defense but strongly responded to immune challenge. L. botrana larvae in a natural population were more heavily parasitized by various parasitoid species than E. ambiguella, suggesting that the efficacy of defense strategies against parasitoids is not equal among species. These results have implications for understanding of regulation in communities, and in the development of biological control strategies for these two grapevine pests.     

Changes in cytokinins are sufficient to alter developmental patterns of defense metabolites in Nicotiana attenuata

Plant defense metabolites are well known to be regulated developmentally. The optimal defense (OD) theory posits that a tssue's fitness values and probability of attack should determine defense metabolite allocations. Young leaves are expected to provide a larger fitness value to the plant, and therefore their defense allocations should be higher when compared with older leaves. The mechanisms that coordinate development with defense remain unknown and frequently confound tests of the OD theory predictions. Here we demonstrate that cytokinins (CKs) modulate ontogeny‐dependent defenses in Nicotiana attenuata. We found that leaf CK levels highly correlate with inducible defense expressions with high levels in young and low levels in older leaves. We genetically manipulated the developmental patterns of two different CK classes by using senescence‐ and chemically inducible expression of CK biosynthesis genes. Genetically modifying the levels of different CKs in leaves was sufficient to alter ontogenic patterns of defense metabolites. We conclude that the developmental regulation of growth hormones that include CKs plays central roles in connecting development with defense and therefore in establishing optimal patterns of defense allocation in plants.     

Simultaneous evolution of competitiveness and defense: induced switching in Arabis drummondii

Optimality theory for plant defense against herbivores predicts an evolutionary tradeoff between the abilities to compete and defend. We tested this hypothesis by studying the effects of genetic variation in competitiveness on defense expression. Two closely related and differentially competitive congeners were compared for levels of resistance, tolerance, and secondary metabolite production. In a growth room experiment, plants of Arabis drummondii and A. holboellii were grown in the presence and absence of the common bunch grass Boutelloua gracilis, the specialist herbivore Plutella xylostella, and generalist herbivore Trichoplusia ni. Tolerance to competition, measured as growth next to the grass relative to controls in the absence of grass, was greatest for A. drummondii, the species that occurred in communities with higher densities of inter-specific neighbors. Measures of defense (resistance to herbivores, tolerance to damage, and concentrations of glucosinolates) varied inconsistently between the Arabis, species, depending on type of herbivore, competition level, and type of defense. The better competitor A. drummondii was more resistant to specialist herbivores, as in the field, and exhibited greater herbivore- and competition-induced changes in glucosinolate profiles. Further, when plants of A. drummondii were fed upon in competitive environments, the induced glucosinolate response was reduced while tolerance levels increased in an apparent switching of induced strategies. We suggest that competitiveness and defense responses are sometimes positively correlated because some defensive traits also function as competitive traits. A competitive function for defenses may also explain why defenses were affected by competition. Alternatively, since the induced response did not increase estimates of total glucosinolate content significantly, minimal defense costs might also allow the simultaneous evolution of competitiveness and defense. Finally, when faced with both herbivory and competition, some competitive species, such as A. drummondii, may switch to growth-based rather than toxin-based strategies as recent theoretical models predict.     

Using direct amplification and next-generation sequencing technology to explore foliar endophyte communities in experimentally inoculated western white pines

Fungal endophytes can influence survivability and disease severity of trees. Here we characterized the endophyte community in Pinus monticola (western white pine), an important species in the northwest USA, largely decimated by pathogenic fungi. We also assessed the ability to successfully inoculate seedlings with desirable endophytes, with the long-term goal of providing a protective microbiome and added defense from pathogens. P. monticola seedlings were inoculated in the field with potential pathogen antagonists and fungi isolated from healthy mature trees. Following inoculations direct amplification and next generation sequencing were used to characterize fungal endophyte communities, and explore interspecific competition, diversity, and co-occurrence patterns in needle tissues. Negative co-occurrence patterns between inoculated fungi and potential pathogens, as well as many other species, suggest early competitive interactions. Our study explores early endophyte community assemblage and shows that fungal inoculations may influence tree growth.     

Effects of parental survival on clutch size decisions in fluctuating environments

Whereas in constant environments parental survival has no effect on optimal clutch size in the absence of trade-offs between juvenile and parental survival, the situation is drastically different in fluctuating environments. We consider a model in which, with respect to reproduction, parents and offspring are equivalent at the start of the next breeding season. When generations are non-overlapping, the clutch size maximizing geometric mean surviving number of offspring is optimal among all pure clutch size strategies. We prove that, as parental survival increases relative to that of the offspring, the optimal clutch size converges to the arithmetic mean maximizing clutch size (the so-called ‘Lack clutch size’). We also give a numerical procedure for calculating optimal mixed strategies and we show that, as environmental variance increases and/or parental survival decreases, mixed rather than pure strategies become optimal. Furthermore, we explain how to estimate fitness from empirical data under the assumptions of our model. This revised version was published online in July 2006 with corrections to the Cover Date.     

Optimal growth pattern of defensive organs: the diversity of shell growth among mollusks

Mollusks show a diversity of shell growth patterns. We develop a model for the dynamic resource allocation to defense organs and analyze it with the Pontryagin maximum principle. A typical optimal growth schedule is composed of the initial phase of soft-body growth without shell followed by a simultaneous growth of shell and soft body and finally the reproductive phase without growth (simultaneous shell growth). If the defensible predation risk is low or if the cost of defense is high, the optimal strategy is to have no shell (shell-less growth). If defensible predation pressure or general mortality differs before and after maturation, an additional three strategies, characteristic of the exclusive growth of shell or soft body, can be optimal (sequential shell growth, additional body-expansion growth, and additional callus-building growth). These optimal strategies are in accord with the patterns observed for mollusks. In particular, the growth strategies with exclusive growth phase of external shells are preferred when durophagous predation pressure after maturation is higher than that before maturation. This result explains the observation that many tropical gastropods with thickened shell lips spend their vulnerable juvenile phase in sheltered habitats.     

A model for the resistance of tumor cells to cancer chemotherapeutic agents

A mathematical model of tumor resistance to chemotherapy based on a stochastic process of change is presented. The probability of no resistant cells is utilized as a fundamental quantity of interest, and the effects of various therapeutic strategies on it are explored. Situations where one or two drugs are available are treated in detail and extrapolation made to the n-drug case. The situation where two drugs may not be given simultaneously is examined, and it is found that sequential alternation of drugs satisfies certain optimality criteria when both drugs are equally effective. From this it is inferred that the simultaneous administration of all available active agents is optimal where this is permissible.     

Cost-effectiveness evaluation of different control strategies for Clonorchis sinensis infection in a high endemic area of China: A modelling study

Clonorchiasis is an important food-borne parasitic disease caused by Clonorchis sinensis infection. The evaluation of long-term cost-effectiveness of control strategies is important for disease control and prevention. The present study aimed to assess the cost-effectiveness of the three recommended strategies (i.e., WHO, Chinese and Guangdong strategies) and different combinations of commonly used measures (i.e., preventive chemotherapy, information, education, and communication (IEC) and environmental improvement) on clonorchiasis. The study area, Fusha town in Guangdong Province, was a typical high endemic area in China. The analysis was based on a multi-group transmission model of C. sinensis infection. We set the intervention duration for 10 years and post-intervention period for 50 years. The corresponding costs and DALYs were estimated. Strategies with incremental cost-effectiveness ratios (ICERs) less than 1/5 of the willingness-to-pay threshold were identified as highly cost-effective strategies. The optimal control strategy was obtained using the next best comparator method. The ICERs of Guangdong strategy were $172 (95% CI: $143-$230) US for praziquantel and $106 (95% CI: $85-$143) US for albendazole, suggesting the highest cost-effectiveness among the three recommended strategies. For praziquantel, 470 sets of control strategies were identified as highly cost-effective strategies for achieving infection control (prevalence<5%). The optimal strategy consisted of chemotherapy targeted on at-risk population, IEC and environmental improvement, with coverages all being 100%, and with the ICER of $202 (95% CI: $168-$271) US. The results for transmission control (prevalence<1%) and albendazole were obtained with the same procedures. The findings may help to develop control policies for C. sinensis infection in high endemic areas. Moreover, the method adopted is applicable for assessment of optimal strategies in other endemic areas.     

Identifying Stride-To-Stride Control Strategies in Human Treadmill Walking

Variability is ubiquitous in human movement, arising from internal and external noise, inherent biological redundancy, and from the neurophysiological control actions that help regulate movement fluctuations. Increased walking variability can lead to increased energetic cost and/or increased fall risk. Conversely, biological noise may be beneficial, even necessary, to enhance motor performance. Indeed, encouraging more variability actually facilitates greater improvements in some forms of locomotor rehabilitation. Thus, it is critical to identify the fundamental principles humans use to regulate stride-to-stride fluctuations in walking. This study sought to determine how humans regulate stride-to-stride fluctuations in stepping movements during treadmill walking. We developed computational models based on pre-defined goal functions to compare if subjects, from each stride to the next, tried to maintain the same speed as the treadmill, or instead stay in the same position on the treadmill. Both strategies predicted average behaviors empirically indistinguishable from each other and from that of humans. These strategies, however, predicted very different stride-to-stride fluctuation dynamics. Comparisons to experimental data showed that human stepping movements were generally well-predicted by the speed-control model, but not by the position-control model. Human subjects also exhibited no indications they corrected deviations in absolute position only intermittently: i.e., closer to the boundaries of the treadmill. Thus, humans clearly do not adopt a control strategy whose primary goal is to maintain some constant absolute position on the treadmill. Instead, humans appear to regulate their stepping movements in a way most consistent with a strategy whose primary goal is to try to maintain the same speed as the treadmill at each consecutive stride. These findings have important implications both for understanding how biological systems regulate walking in general and for being able to harness these mechanisms to develop more effective rehabilitation interventions to improve locomotor performance.     

Harvesting strategies for conserving minimum viable populations based on World Conservation Union criteria: brown bears in Norway

The World Conservation Union (IUCN) defines populations as vulnerable if the probability of extinction is larger than 10\% within the next 100 years. With the objective of minimizing problems with predation on domestic livestock and, at the same time, conserving a viable population, we consider different threshold harvesting strategies for a small population of brown bear, based on a population dynamics model with growth rate and demographic and environmental variances estimated from the present Swedish population. Taking into account uncertainties in present estimates of the demographic parameters and in population size, we show that the population can be harvested when the population size exceeds 34 female bears,aged one year and older, if the entire population exceeding the threshold is harvested. To minimize the expected long-term population size, however, we show that it is optimal to harvest only a proportion equal to 35% of the population exceeding a lower threshold of 12 female bears. This strategy gives an expected long-term population size of around 20 female bears. If the growth rate of the population is reduced by ca. 3%, the threshold must, under some conditions, be doubled. We argue that the small thresholds are mainly a result of the high intrinsic growth rate of the population considered in the present paper. However, the analysis also suggests that IUCN''s criterion might allow a rate of extinction that is too high.