Adaptive dynamic resource allocation can cause tragedy of the commons in plants with nutrient competition.

SummaryIt has recently been suggested that not only nutrients but also presence of a competitor can influence competitive ability in plants, for example, by strategic changes in allocation to roots (a ‘tragedy of the commons’ effect). Such strategic changes might occur also in pollen, because this has potential to increase siring success during pollen competition in the pistil.I tested the new hypothesis that pollen competitive ability – a trait often strongly affected by resources of the pollen parent – was influenced by presence of a root competitor using glasshouse‐grownViola tricolorin 2 years. Plants of two maternal families were combined in pairs with their roots either separated or intermingled in the same amount of resources.Maternal families varied in response to root competition, with pollen performance increasing in some families, decreasing in others and unchanged in others. An increase did not mirror a decrease in the competing family, suggesting an explanation beyond differential ability to gain resources. The responses to competitive environment were often consistent across three independent competitors.There was a positive correlation between family responses in pollen performance and family responses in plant size to root competitors. Larger plants did not produce better pollenper se, indicating that the change in pollen performance was not a pure side effect of altered plant size. There was no support for a ‘tragedy of the commons’ effect on root production. With the experimental design used, an effect of rooting volume could not be completely ruled out.The currently unknown mechanism of the below‐ground interactions between plants ofV. tricolorwas strong enough to change relative pollen competitive ability. These interactions might thus promote variation in the outcome of pollen competition.Synthesis.The results of this study indicate the presence of a competitor on pollen competitive ability beyond the effect of nutrients. Even though the underlying mechanism needs to be explored further, the detected link between pollen competition and soil competition suggests a phenomenon well worth investigating. Further studies may lead to increased understanding of the evolutionary consequences of selection operating in response to interactions with neighbours.

The explanation for the continued existence of sex, despite its many costs, remains one of the major challenges of evolutionary biology. Previous experimental studies have demonstrated that sex increases the rate of adaptation in novel environments relative to asexual reproduction. Whereas these studies have investigated the impact of sex on adaptation to stressful abiotic environments, the potential for biotic interactions to influence this advantage of sex has been largely ignored. Species rarely exist in isolation in natural conditions, so the impact of sex on adaptation to a stressful abiotic environment may be altered by the interactions between coexisting species. To investigate the interplay of sex and competition on adaptation to deteriorating conditions, we allowed populations of the unicellular alga (Chlamydomonas reinhardtii) to evolve in an environment to which they were initially poorly adapted. We manipulated both their mode of reproduction and the presence of a competitor, and monitored population size and proportion of evolutionary rescue events for each mode of reproduction. The results indicate that sex may be the beneficial strategy in the presence of the competitor. Sexual populations had highest probability of evolutionary rescue irrespective of the presence of the competitor. The overall advantage of sex was also manifested through higher level of adaptedness of survived sexual populations relative to asexual populations. Since competitive interactions are commonplace in nature, one of the explanations for the maintenance of sex by natural selection may be the increased rate of adaptation of sexual populations both in the presence and absence of competitors.

The allocation of resources among roots and shoots represents the largest flux of resources within a plant and therefore should have been selected to maximize benefits to plants. Yet, it is unclear why some species like temperate grasses have such high root length density (RLD). Either the slow rate of diffusion of inorganic N in soils or interplant competition could explain the high RLD of temperate grasses. Using a fine-scale model of nutrient dynamics in the soil and plant growth, a cost–benefit approach was used to assess optimal allocation rates for plants that accounted for value of both carbon and nitrogen. In the absence of interplant competition, resource benefits are maximized with very little root length except in extremely dry soils for ammonium. In the presence of a competitor, optimal allocation of N to roots is much greater and increases as ability of competitors to produce root length increase. Competition for inorganic nitrogen generates a classic aspect of the tragedy of the commons, the “race for fish”, where plants must allocate more resources to acquisition of the limiting resource than is optimal for plants in the absence of competition. As such, nutrient competition needs to be directly addressed when understanding plant- and ecosystem-level resource fluxes as well as the evolution of root systems.

Summary Theory predicts that consumers can increase their energy intake by spending more time within resource‐rich areas and/or by avoiding areas where local competitor densities are high. A consumer whose movements are described by a simple random walk can achieve these objectives by adjusting its turn frequency and/or speed. We recorded movements, as series of steps, by 180 individual rotifers (Brachionus calyciflorus) in glass capillary tubes to test the influence of resource density, competitor density and their statistical interaction on movement parameters. Four treatments contrasted opposing levels of algae (resource) and conspecific density in a 2 × 2 factorial design. Our results indicate that density‐ and resource‐dependent behaviours act through different mechanisms to shape patterns of rotifer movement. Turn frequency increased up to twofold in resource‐rich treatments, depending on the presence or absence of competitors. In contrast, swimming speed was 50% greater in the presence of competitors under all treatments, but was only slightly depressed by the presence of resources alone. We show how these two different movement mechanisms may be integrated into predictions of consumer population spread as resource and competitor densities vary. We discuss implications of the contrasting and complementary nature of these different movement mechanisms and their possible adaptations to different environmental stimuli.

AtNOS1 was previously identified as a potential nitric-oxide synthase (NOS) in Arabidopsis thaliana, despite lack of sequence similarity to animal NOSs. Although the dwarf and yellowish leaf phenotype of Atnos1 knock-out mutant plants can be rescued by treatment with exogenous NO, doubts have recently been raised as to whether AtNOS1 is a true NOS. Moreover, depending on the type of physiological responses studied, Atnos1 is not always deficient in NO induction and/or detection, as previously reported. Here, we present experimental evidence showing that AtNOS1 is unable to bind and oxidize arginine to NO. These results support the argument that AtNOS1 is not a NOS. We also show that the renamed NO-associated protein 1 (AtNOA1) is a member of the circularly permuted GTPase family (cGTPase). AtNOA1 specifically binds GTP and hydrolyzes it. Complementation experiments of Atnoa1 mutant plants with different constructs of AtNOA1 show that GTP hydrolysis is necessary but not sufficient for the physiological function of AtNOA1. Mutant AtNOA1 lacking the C-terminal domain, although retaining GTPase activity, failed to complement Atnoa1, suggesting that this domain plays a crucial role in planta. cGTPases appear to be RNA-binding proteins, and the closest homolog of AtNOA1, the Bacillus subtilis YqeH, has been shown to participate in ribosome assembly and stability. We propose a similar function for AtNOA1 and discuss it in the light of its potential role in NO accumulation and plant development.

Resource allocation in tragedy of the commons game in plants for belowground competition

Background and Aims Plants proliferate roots in order to acquire nutrients, typically contending with heterogeneous resources and competing neighbours. A mathematical model was developed to identify optimal root proliferation strategies in patchy nutrient environments. The impact of joining mycorrhizal networks was also assessed. Methods A simple model of growth and competition in one spatial dimension was implemented within a genetic algorithm to obtain optimal proliferation strategies under different scenarios of resource distribution, and in the presence or absence of local competition and large-scale mycorrhizal networks. Results A strong proliferation response emerged for isolated plants in heterogeneous environments with low resources, and also for plants growing in competition. Even in statistically homogeneous environments, the presence of competition conferred a selective advantage to plants proliferating in the direction of the most recently acquired patch. In the presence of mycorrhizal networks, the optimal strategy switched from symbiosis to proliferation driven growth as the relative cost of acquiring resources through the networks increased. Conclusions The optimal proliferation response in a given scenario was governed by a hierarchy of factors: resource levels and distribution; the presence or absence of competition; and the marginal benefit of obtaining resources via symbiotic relationships with mycorrhizas.

Studies of risk-sensitive foraging have so far focused only on the effect of food demand on choice of feeding site. We suggest that competition is likely to be another factor influencing risksensitivity. A choice experiment with common shrews showed that, in the absence of competition, risk-aversion increased with increasing food intake relative to requirement. When apparent competitors were present, however, shrews were risk-indifferent regardless of their estimated requirement. The switch to risk-indifference in the presence of competitors appears to be an all-or-nothing rule of thumb which is not modified by experience with reward probability distributions.

Summary Individual variation in feeding rate in the absence of competitors (absolute feeding rate) and change in feeding rate in the presence of competitors (susceptibility to interference) are crucial parameters used in individual‐based population models that predict the distribution of animals across patches. There are, however, few data on how these parameters are dependent on local foraging conditions, which will vary widely in natural systems. We tested the hypotheses that absolute feeding rate and change in feeding rate with group size were independent of patch conditions. We observed 27 European blackbirdsTurdus merulafeeding in seminatural experimental patches that were either open (where prey was easily visible) or covered with leaves (where prey was invisible until the leaves had been moved aside by the bird). Absolute feeding rate and susceptibility to interference were measured by recording feeding rates of blackbirds alone and in the presence of increasing numbers of competitors, respectively. There was significant individual variation in absolute feeding rate dependent on patch type, with some birds showing relatively large increases in open patches. There was no significant correlation between absolute feeding rate in open and leaf patches. There was significant variation in individual susceptibility to interference dependent on patch type. There was no correlation between an individual’s susceptibility to interference in open and leaf patches. Mathematical models that seek to make predictions of the distribution of animals across heterogeneous patches should incorporate measures of competitive ability over a wide range of patch conditions, or use more complex parameters that incorporate individual changes in density dependent pay‐offs dependent on patch conditions.

Simple SummaryStudies on species’ trophic niches are essential to understand the characteristics of species’ ecology and life traits, as well as to improve conservation strategies. In the absence of competitors, species realize their trophic niche including in their diet the most profitable food resources. In the presence of competitors, species modify their preferences to reduce competition and maintain the highest benefits at the same time. In this study, we assessed the trophic niche of two species of salamanders coexisting in a forested area of Italy and evaluated which might be the mechanisms that these two species adopted to reduce competition. We found that the Italian cave salamander (Speleomantes italicus) mostly consumed flying prey with a hard cuticle, while the fire salamander (Salamandra salamandra) preferred worm-like and soft-bodied prey. In conclusion, we hypothesize that in our case, the two species of salamanders did not have to change their prey preference in order to avoid competition, but divergences in metabolism and behavioral traits likely worked as natural deterrent.The trophic niche of a species is one of the fundamental traits of species biology. The ideal trophic niche of a species is realized in the absence of interspecific competition, targeting the most profitable and easy-to-handle food resources. However, when a competitor is present, species adopt different strategies to reduce competition and promote coexistence. In this study, we assessed the potential mechanisms that allow the coexistence of two generalist salamanders: the Italian cave salamander (Speleomantes italicus) and the fire salamander (Salamandra salamandra). We surveyed, in April 2021, a forested area of Emilia-Romagna (Italy) during rainy nights. Analyzing the stomach contents of the captured individuals, we obtained information on the trophic niche of these two sympatric populations. Comparing our results with those of previous studies, we found that the two species did not modify their trophic niche, but that alternative mechanisms allowed their coexistence. Specifically, different prey preferences and predator metabolisms were likely the major factors allowing reduced competition between these two generalist predators.

Head-to-head running race simulation alters pacing strategy, performance, and mood state

The American mink, Neovison vison, is an established, alien invasive species in the United Kingdom that originally colonized the country at a time when two native mustelids (otters, Lutra lutra, and polecats, Mustela putorius) were largely absent. Both of these species are now recovering their populations nationally. We compared the relative abundance and the behavior of mink in the 1990s and in the 2000s in an area of southern England where both otters and polecats were absent in the 1990s but reappeared in the intervening years. We found that mink were still abundant in the 2000s in the presence of otters and polecats, but that they appeared to have altered some aspects of their behavior. In accordance with previous studies, we found that mink consumed fewer fish in the presence of otters. We also found that mink were predominantly nocturnal in the 1990s (in the absence of competitors) but were predominantly diurnal in the 2000s (in the presence of competitors). We hypothesize that this temporal shift may be an avoidance mechanism allowing the coexistence of mink with the otter and the polecat, although we are unable to attribute the shift to one or the other species. We also found that mink in the presence of competitors weighed less but remained the same size, suggesting the possibility of a competitor-mediated decline in overall body condition. This is one of very few field studies demonstrating a complete temporal shift in apparent response to competitors. The implications of this study are that recovering otter populations may not lead to significant and long-term reductions in the number of invasive mink in the United Kingdom as has been suggested in the media, although we cannot exclude the possibility of a decline in mink in the longer-term.

Growth of Staphylococcus aureus at 15°C, with and without addition of representative spoilage bacteria, was studied in cooked, whole chicken meat and chicken broth. In the absence of competitors, the organism grew better in broth culture than on whole meat, but multiplied more slowly in broth when other organisms were present, even from twice the previous level of inoculum. The presence of competitors had no marked effect on the growth of Staph. aureus on whole meat. Enterotoxin A was not produced at 15°C on either whole meat or in broth, and occurred at 20°C only in pure culture. At 30° and 37°C, toxin was produced whether or not competitors were present. Toxin production by Staph. aureus appeared to be influenced more by growth temperature than by bacterial competition.

Plant – fungal endophyte interactions are common in nature and they can shape the ecology of plants. Vertically transmitted endophytes are hypothesized to serve as mutualists, protecting plants from herbivores. If this hypothesis is true, then we expect endophytes to be most abundant in the presence of herbivores and least abundant in their absence, assuming endophytes incur a cost to their host. We tested this prediction by studying the effects of intense rabbit (Oryctolagus cuniculus Linnaeus) grazing on grass–endophyte interactions at Silwood Park, UK. We examined seeds of red fescue (Festuca rubra L.) collected from 15 natural populations that were protected from rabbits for 0.3–21 years. Contrary to our prediction, the mean proportion of seeds with endophytes increased 1.84×, from 0.45 to 0.83, following 21 years of rabbit exclusion. To better understand the mechanisms driving this increase in frequency, we conducted a fully factorial greenhouse experiment where we manipulated the presence or absence of endophyte infection, intraspecific competition, and simulated grazing on F. rubra plants. In both damaged and undamaged treatments, infected plants produced approximately twice as much biomass as uninfected plants, and endophytes did not influence tolerance to herbivory. These results suggest that endophytes directly change plant growth but not compensatory responses to damage. In the absence of competitors, infected plants produced 2.17× more biomass than uninfected plants, whereas in the presence of competitors, infected plants produced only 1.55× more biomass than uninfected plants. This difference suggests that intraspecific competition might lessen the benefits of endophyte infection. Our results do not support the defensive mutualism hypothesis, but instead suggest that endophyte-induced plant growth is important in shaping the costs and benefits of endophytes in our system.

Summary Individual variation in feeding rate in the absence of competitors (absolute feeding rate) and decline in feeding rate in the presence of competitors (susceptibility to interference) are frequently used as measures of competitive ability or fitness in short‐term foraging studies. They are also crucial parameters of individual‐based population models that predict the distribution of animals across patches. There are, however, few data on whether absolute feeding rate and susceptibility to interference competition remain constant for individuals between years. The hypothesis that absolute feeding rate and susceptibility to interference were similar between years was tested by observing 25 European blackbirdsTurdus merulafeeding in seminatural experimental patches, during January to March over a 2–4‐year period 1995–98. Absolute feeding rate was measured as the feeding rates of solitary blackbirds. Susceptibility to interference was measured as the change in feeding rate of a focal bird when it fed in the presence of different numbers of competitors. Individuals changed significantly in their feeding rate between years, but most individuals did not change in their feeding rate relative to others in the population. The absolute feeding rate of an individual was significantly positively correlated with its feeding rate in the subsequent year. There was no significant variation in susceptibility to interference for individuals between years. Only two of 27 blackbirds showed a significant change in susceptibility to interference between years. Relative absolute foraging rate and susceptibility to interference competition were reasonably similar between years in blackbirds. Relative fitness measures derived from short‐term measures of foraging ability may therefore be valid over long periods. In ideal free models that incorporate individual competitive ability, rather than population averages, temporal changes in competitive ability can possibly be ignored.