Plastic responses to competition: Does bacteriocin production increase in the presence of nonself competitors?

Anticompetitor traits such as the production of allelopathic toxins can confer significant competitive benefits but are often costly to produce. Evolution of these traits may be facilitated by environment‐specific induction; however, the extent to which costly anticompetitor traits are induced by competitors is not well explored. Here, we addressed this question using bacteriocins, which are highly specific, proteinaceous anticompetitor toxins, produced by most lineages of bacteria and archaea. We tested the prediction that bacteriocin production is phenotypically plastic and induced by the presence of competitors by examining bacteriocin production in the presence and absence of nonself competitors over the course of growth of a producing strain. Our results show that bacteriocin production is detectable only at high cell densities, when competition for resources is high. However, the amount of bacteriocin activity was not significantly different in the presence vs. the absence of nonself competitors. These results suggest that bacteriocin production is either (a) canalized, constitutively produced by a fixed frequency of cells in the population or (b) induced by generic cues of competition, rather than specific self/nonself discrimination. Such a nonspecific response to competition could be favored in the natural environment where competition is ubiquitous.     

Herbivore-induced resource sequestration in plants: why bother?

Herbivores can cause numerous changes in primary plant metabolism. Recent studies using radioisotopes, for example, have found that insect herbivores and related cues can induce faster export from leaves and roots and greater partitioning into tissues inaccessible to foraging herbivores. This process, termed induced resource sequestration, is being proposed as an important response of plants to cope with herbivory. Here, we review the evidence for resource sequestration and suggest that associated allocation and ecological costs may limit the benefit of this response because resources allocated to storage are not immediately available to other plant functions or may be consumed by other enemies. We then present a conceptual model that describes the conditions under which benefits might outweigh costs of induced resource sequestration. Benefits and costs are discussed in the context of differences in plant life-history traits and biotic and abiotic conditions, and new testable hypotheses are presented to guide future research. We predict that intrinsic factors related to life history, ontogeny and phenology will alter patterns of induced sequestration. We also predict that induced sequestration will depend on certain external factors: abiotic conditions, types of herbivores, and trophic interactions. We hope the concepts presented here will stimulate more focused research on the ecological and evolutionary costs and benefits of herbivore-induced resource sequestration.     

Muscle wasting in the presence of disease,why is it so variable?

Skeletal muscle wasting is a common clinical feature of many chronic diseases and also occurs in response to single acute events. The accompanying loss of strength can lead to significant disability, increased care needs and have profound negative effects on quality of life. As muscle is the most abundant source of amino acids in the body, it appears to function as a buffer for fuel and substrates that can be used to repair damage elsewhere and to feed the immune system. In essence, the fundamentals of muscle wasting are simple: less muscle is made than is broken down. However, although well‐described mechanisms modulate muscle protein turnover, significant individual differences in the amount of muscle lost in the presence of a given severity of disease complicate the understanding of underlying mechanisms and suggest that individuals have different sensitivities to signals for muscle loss. Furthermore, the rate at which muscle protein is turned over under normal conditions means that clinically significant muscle loss can occur with changes in the rate of protein synthesis and/or breakdown that are too small to be measurable. Consequently, the changes in expression of factors regulating muscle turnover required to cause a decline in muscle mass are small and, except in cases of rapid wasting, there is no consistent pattern of change in the expression of factors that regulate muscle mass. MicroRNAs are fine tuners of cell phenotype and are therefore ideally suited to cause the subtle changes in proteome required to tilt the balance between synthesis and degradation in a way that causes clinically significant wasting. Herein we present a model in which muscle loss as a consequence of disease in non‐muscle tissue is modulated by a set of microRNAs, the muscle expression of which is associated with severity of disease in the non‐muscle tissue. These microRNAs alter fundamental biological processes including the synthesis of ribosomes and mitochondria leading to reduced protein synthesis and increased protein breakdown, thereby freeing amino acids from the muscle. We argue that the variability in muscle loss observed in the human population arises from at least two sources. The first is from pre‐existing or disease‐induced variation in the expression of microRNAs controlling the sensitivity of muscle to the atrophic signal and the second is from the expression of microRNAs from imprinted loci (i.e. only expressed from the maternally or paternally inherited allele) and may control the rate of myonuclear recruitment. In the absence of disease, these factors do not correlate with muscle mass, since there is no challenge to the established balance. However, in the presence of such a challenge, these microRNAs determine the rate of decline for a given disease severity. Together these mechanisms provide novel insight into the loss of muscle mass and its variation in the human population. The involvement of imprinted loci also suggests that genes that regulate early development also contribute to the ability of individuals to resist muscle loss in response to disease.     

Local Management of Mangrove Forests in the Philippines: Successful Conservation or Efficient Resource Exploitation?

Recent environmental narratives suggest that local people are effective stewards of forest resources. Local restoration and management of mangrove forests, in particular, are now widely advocated as a solution to achieve both economic and environmental conservation goals. This paper presents findings from a study of 2 coastal sites in the Philippines that are renowned and often showcased as success stories in community-based, mangrove reforestation and management. These cases are especially intriguing because local tree planting and management emerged in both areas long before governments and nongovernment organizations began to promote such activities. These management systems are a successful economic innovation in that planted mangroves protect homes and fish pond dykes from wave and wind damage, and the production of high-value construction wood is dramatically enhanced through intensive plantation management. Mangrove plantations are an efficient alternative to harvesting from unplanted, natural mangroves and their spread may reduce harvesting pressures on existing forests. However, mangrove plantations are structurally and compositionaly very different from unplanted forests, a finding of particular concern given that such plantations are increasingly encroaching into and replacing natural forests. Furthermore, planted forests are not typically viewed by planters in terms of their environmental conservation values and are frequently cut and cleared to make space for alternative uses, especially fish farming and residential settlement. The suggestion that these local mangrove management systems are successful for conservation thus needs to be qualified.     

Correlation between male size and territory quality: consequence of male competition or predation susceptibility?

Territory characteristics correlate with male characteristics in several species. This can result from male competition for the best territories, or from males varying in their ability to pay other costs of territoriality, such as predation risk costs. In a population of threespine sticklebacks, Gasterosteus aculeatus , we found the biggest males to defend the biggest territories with a low structural complexity and a high female encounter rate. By experimentally manipulating competition intensity and habitat structure, we show that both male competition and predation exposure influenced the distribution of territories among males. Males increased the size of their territory when a neighbouring male was removed, whereas they reduced their territory when habitat complexity and cover from predators were reduced, with large males reducing their territory size less than smaller males. This suggests that large males occupy large, open territories both because of their superior competitive ability and because of their either lower predation susceptibility or higher risk-taking. Large, open territories were beneficial in mate attraction and male competition and predation exposure therefore biased mating opportunities towards large males. This suggests that cost of territoriality to males may reduce mate choice costs to females by securing that large males are encountered more often than small males, and by providing an additional cue, territory quality, which indicates which males are worth inspecting.     

The positive effects of negative interactions: Can avoidance of competitors or predators increase resource sampling by prey?

Spatial overlap between predators and prey is key to predicting their interaction strength and population dynamics. We constructed a spatially-explicit simulation model to explore how predator and prey behavioral traits and patterns of resource distribution influence spatial overlap between predators, prey, and prey resources. Predator and prey spatial association primarily followed the ideal free distribution. Departures from this model were intriguing, especially from the interactions of predator and prey behavior. When prey weakly avoided conspecifics, they associated more highly with resources when predators were present. Predators increased the rate of prey movement between patches, which increased their ability to sample their environment and aggregate in patches with high resources. When prey strongly avoided each other, predators decreased prey association with resources. That is, an increased rate of prey movement increased the probability that prey would interact and avoid each other without regard to the distribution of resources. More generally, a more highly clumped distribution of resources acted as a spatial anchor that generally increased prey, predator, and resource association. Prey tended to congregate with resources and predators generally congregated with prey.     

Foraging in surfperches: resource partitioning or individualistic responses?

Synopsis On Naples Reef off southern California, five sympatric species of surfperch (Embiotocidae) eat small animal prey associated with algal turf and other benthos. Although they tend to select different foods in different microhabitats, their different foraging traits are probably individualistic responses (non-interactive or autecological species responses to resources) because the species' pairwise resource overlaps are neither uniformly small nor generally complementary. Mean overlap in both food and foraging microhabitat was significantly greater than random as most species converged in resource use. Only two exploiters of extremes in the available spectra of prey types and microhabitats had significantly narrow overlap. Thus, resource overlaps may simply reflect similarities and differences in the species' independently evolved foraging traits, which were rendered more or less inflexible as different morphological specializations for picking, crunching, or winnowing prey. Therefore, any one species may occur about the reef to its best advantage independently of the others. Its distribution and abundance may simply reflect its tolerance of the local environment. The evidence indicates that the surfperches are not concertedly partitioning resources on Naples Reef, i.e., that the five species had not all coevolved to avoid interspecific competition in the past nor have they all mutually shifted their niches to minimize it in the present.     

Interspecific nest destruction in the Tropical Kingbird Tyrannus melancholicus: kleptoparasitism or competition?

Bird nest destruction and nest material kleptoparasitism (i.e., the theft of nest material from other bird’s nests) are poorly documented behaviors, and little is known about the parasite species and their hosts. Here, I present the first account of nest material kleptoparasitism in the Tropical Kingbird (Tyrannus melancholicus) followed by nest destruction, which occurred on a Variegated Flycatcher nest (Empidonomus varius). I explore the implications of these behaviors for both the kleptoparasite and the victim species and, due to the lack of basic information on the general behavior of both species, I point out some directions to guide future researches on the subject.

     

Geographic Variation in Populations of Microcebus murinus in Madagascar: Resource Seasonality or Bergmann's Rule?

Gray mouse lemurs (Microcebus murinus) occur in western Madagascar from the evergreen littoral rain forests at the southern tip of the island to the seasonal dry deciduous forests of the west and northwest. The wide geographic distribution allows researchers to investigate whether ecogeographic variations reflect adaptations to ambient temperatures, as a proxy for constraints of thermoregulation, or to rainfall, as a proxy of food availability. We compared body mass, number of litters/yr, and longevity for 3 populations: 1 from the evergreen littoral rain forest of the south (Mandena: annual mean temperature 23°C, 1600 mm rain/yr), 1 from the dry deciduous forest of the west (Kirindy/CFPF: 25°C, 800 mm), and 1 from the dry deciduous forest of northwestern Madagascar (Ampijoroa: 27°C, 1200 mm). Body mass decreases with increasing ambient temperature from south to north (Mandena > Kirindy/CFPF > Ampijoroa). The number of litters/yr was highest in the littoral rain forest (2 or 3 litters/yr) and decreased with decreasing rainfall (Mandena > Ampijoroa > Kirindy/CFPF). Life expectancy is lowest in the littoral forest (13% recaptures of mouse lemurs between years) and high (ca. 30–40% recaptures between years) in the dry forests (Mandena < Kirindy/CFPF and Ampijoroa). We postulate that constraints of thermoregulation result in the latitudinal gradient of body mass. Reduced resource productivity or seasonality is reflected in differences in reproductive rates, which seemed to be traded off against longevity. Thus body mass, longevity, and reproductive parameters respond differently to ambient conditions.     

Induction of phlorotannin production in a brown alga: defense or resource dynamics?

Increase of phenolic secondary metabolites, phlorotannins, in brown algae due to gastropod grazing has been interpreted as an anti-herbivore adaptation. Here we tested whether such a response could be due to changes in truly available resources for the alga, not by the grazing activity of snails as such. We allowed two species of snails, Theodoxus fluviatilis and Physa fontinalis to graze on Fucus vesiculosus . These species feed on epibiota and particulate matter on the thallus but do not eat the thallus of F. vesiculosus . We further simulated snail grazing by nutrient enhancement, removal of epibiota and by a combination of the two. Manipulations of nutrient and light availability revealed the crucial role of epibiota in mediating resource availability for F. vesiculosus . Nutrient enhancement alone increased epibiota and decreased phlorotannins. Cleaning the thallus resulted in increased growth, and together with nutrient enhancement also in a trade-off with phlorotannins. Presence of T. fluviatilis on the thallus induced phlorotannin production, a response differing from the simulations of snail grazing. However, we suggest that the increase in phlorotannins may not be an induced defense but rather a consequence of a specific way of resource manipulation by this snail species. T. fluviatilis removes hyaline hairs that facilitate nutrient uptake. P. fontinalis did not remove hyaline hairs and the response of the alga to its grazing was similar to the treatment where we mechanically removed epibiota suggesting that cleaning of the thallus is the major mechanism how this snail species affects F. vesiculosus . Genetic variation in phlorotannin concentrations highly exceeded the induced responses of simulated or real snail grazing. This casts doubt for the efficiency of induced phlorotannin production to act as a defense, but is not contradictory with the interpretation of phlorotannins responding to variation in resource availability.     

Abiotic factors,competition or predation: what determines the distribution of young crayfish in a watershed?

In the headwaters of the New River in western North Carolina, the dominant crayfish species changes with increasing stream size. One transition occurs between third- and fourth-order streams. Cambarus chasmodactylus is the dominant species in third-order tributaries. Orconectes cristavarius is the dominant species in the fourth-order South Fork of the New River. While adult C. chasmodactylus are present in the South Fork, the young-of-the-year (YOY) of this species are absent despite evidence of reproduction. In this study we evaluated the factors that may be responsible for the absence of C. chasmodactylus YOY from the South Fork. A field experiment was used to evaluate the role of abiotic factors and competition with YOY O. cristavarius. There was no significant effect of either of these factors on mortality or growth of C. chasmodactylus YOY. The growth rate of O. cristavarius was 3× faster than that of C. chasmodactylus in this experiment. Since neither abiotic factors nor competition appeared responsible for the exclusion of C. chasmodactylus YOY from the South Fork we evaluated the potential importance of selective predation by rock bass (Ambloplites rupestris), a fish species that is common in the South Fork but virtually absent in the tributaries. In a laboratory experiment, C. chasmodactylus YOY experienced significantly higher mortality than O. cristavarius YOY in the presence of rock bass. Field observations and a laboratory experiment suggest that the two crayfish species differ in their anti-predator behaviors. Cambarus chasmodactylus was less likely to swim when initially disturbed and swam shorter distances than O. cristavarius. The differences in escape behavior and growth rate may contribute to the differences in the vulnerability of the two species to rock bass predation. Our results suggest that intense predation pressure exerted by the rock bass may contribute to the virtual exclusion of C. chasmodactylus YOY from the fourth-order South Fork. Handling editor: J. Trexler     

Sex Allocation in California Oaks: Trade-Offs or Resource Tracking?

Trade-offs in sex resource allocation are commonly inferred from a negative correlation between male and female reproduction. We found that for three California oak species, aboveground annual net productivity (ANP) differences among individuals were primarily correlated with water availability and soil fertility. Reproductive biomass increased with ANP, but the relative allocation to reproduction was constant, indicating that reproduction tracked productivity, which in turn tracked site quality. Although there was a negative correlation between male and female reproduction, this was not the result of a resource investment trade-off, but rather a byproduct of the positive correlation between female reproductive biomass and ANP combined with the greater overall resource allocation to female, compared to male, function. Thus, we reject the hypothesis of a trade-off between these key life-history components within individuals of these species. For long-lived individuals, a plastic resource tracking response to environmental fluctuations may be more adaptive than directly linking life-history traits through trade-offs.     

Does quasi-local competition lead to pattern formation in metapopulations? An explicit resource competition model

In metapopulations, competitive interactions may extend beyond the confines of the local population such that members of neighbouring habitat patches affect each other adversely (quasi-local competition). We derive a model for quasi-local competition from first principles, assuming that individuals compete for shared resources and members of a population spend a certain fraction of their foraging time in the adjacent populations. Contrary to the results of Doebeli and Killingback [2003. Theor. Popul. Biol. 64, 397-416], our model does not produce spatial patterns of population densities in homogeneous environments. Quasi-local competition nevertheless contributes to pattern formation by amplifying the effect of heterogeneities in the external environment, and this amplification can be extremely strong when dispersal is absent. We discuss why apparently similar models lead to contrasting results.     

What limits herb biomass in grasslands: competition or herbivory?

Competition and herbivory are two of the main forces shaping plant communities. Although several studies have investigated their impact on plant populations separately, few investigations have examined how they might interact. With the purpose of clarifying the combined roles of competition and herbivory on herb biomass in a grassland, we assessed the effects of different herbivores with reduced grass competition. We conducted a field experiment in 2000-2001 in a British acid grassland (Oak Mead), where we experimentally reduced grass biomass and excluded rabbits, insects and molluscs in a factorial design. Removing the grasses from Oak Mead dramatically increased herb biomass and total above-ground biomass. Herbivore exclusions (i.e. rabbits, insects and molluscs) did not affect total above-ground biomass, but they altered relative abundance of several species. Grass removal and rabbit exclusion had positive interactions on biomass of several herb species, and there were some subtle interactions between different herbivore groups: monocots benefitted when both rabbits and molluscs were excluded, and some herb species had greater biomass when insects and rabbits were absent. We then compared the results with a 10-year experiment that manipulated similar variables in neighbouring grassland (Nashs Field). The comparison between Oak Mead and Nashs Field showed that cessation of herbicide application returns the system to its previous state of grass dominance after 3 years. Therefore, even when herbs were more abundant, they could not prevent reinvasion of the grasses once external factors were removed.     

Apoptosis in the heart: when and why?

Since mammalian cardiac myocytes essentially rely on aerobic energy metabolism, it has been assumed that cardiocytes die in a catastrophic breakdown of cellular homeostasis (i.e. necrosis), if oxygen supply remains below a critical limit. Recent observations, however, indicate that a process of gene-directed cellular suicide (i.e. apoptosis) is activated in terminally differentiated cardiocytes of the adult mammalian heart by ischemia and reperfusion, and by cardiac overload as well. Apoptosis or programmed cell death is an actively regulated process of cellular self destruction, which requires energy and de novo gene expression, and which is directed by an inborn genetic program. The final result of this program is the fragmentation of nuclear DNA into typical nucleosomal ladders, while the functional integrity of the cell membrane and of other cellular organelles is still maintained. The critical step in this regulated apoptotic DNA fragmentation is the proteolytic inactivation of poly-[ADPribose]-polymerase (PARP) by a group of cysteine proteases with some structural homologies to interleukin-1-converting enzyme (ICE-related proteases [IRPs] such as apopain, yama and others). PARP catalyzes the ADP-ribosylation of nuclear proteins at the sites of spontaneous DNA strand breaks and thereby facilitates the repair of this DNA damage. IRP-mediated destruction of PARP, the supervisor of the genome, can be induced by activation of membrane receptors (e.g. FAS or APOI) and other signals, and is inhibited by activation of anti-death genes (e.g. bcl-2). Overload-triggered myocyte apoptosis appears to contribute to the transition to cardiac failure, which can be prevented by therapeutic hemodynamic unloading. In myocardial ischemia, the activation of the apoptotic program in cardiocytes does not exclude their final destiny to catastrophic necrosis with release of cytosolic enzymes, but might be considered as an adaptive process in hypoperfused ventricular zones, sacrificing some jeopardized myocytes to regulated apoptosis, which may by less arrhythmogenic than necrosis with the primary disturbance of membrane function.     

Does competition or facilitation regulate migrant ungulate populations in the Serengeti? A test of hypotheses

Summary Interspecific competition and facilitation have both been proposed as processes promoting species separation and co-existence in African ungulates. In one group of grazers on the Serengeti plains, comprising wildebeest (Connochaetes taurinus), zebra (Equus burchelli), and Thomson's gazelle (Gazella thomsoni), these processes have also been suggested to regulate the populations. Censuses of these populations over 20 years have shown changes that allow a test of which, if either, process regulates population numbers. Wildebeest numbers have levelled off as a result of intraspecific competition for food following a five-fold increase due to release from disease and an increase in food supply. Zebra numbers have remained stationary throughout the same period. Gazelle numbers have declined in the last 10 years. These results are contrary to the facilitation hypothesis, which predicts that wildebeest numbers should not have increased if those of zebra did not, and that gazelle numbers should have increased since the wildebeest population increased. The gazelle results are consistent with the interspecific competition hypothesis, but the zebra results are contrary to it. We propose an alternative hypothesis that predation regulates the zebra population, and we suggest ways of testing this.     

Vibratory courtship in a web-building spider: signalling quality or stimulating the female?

Courtship behaviour in spiders in the form of premating vibrations by males may function (1) as a male identity signal used for species recognition, (2) in suppression of female aggressiveness, (3) to stimulate female mating behaviour, or (4) as a quality signal used in female choice. We investigated the function of web vibration by male Stegodyphus lineatus in a series of experiments. Regardless of vibratory performance, all males mated successfully with virgin females but only 56.4% of males mated with nonvirgin females. Vibratory performance did not influence male mating success, but heavier males had a higher probability of mating with mated females. Males vibrated less often and produced fewer vibrations when introduced on the web of a mated female. Males that vibrated webs of virgin females mated faster than nonvibrating males, but there was no effect of vibration rate or body mass. There was no effect of male vibratory effort or vibration rate on female reproductive success measured as time to egg laying, clutch size, number of hatched young, number of dispersed young and offspring body mass after a single mating. Males vibrated on abandoned virgin female webs but the response decreased with increasing duration of female absence, suggesting that females produce a web-borne pheromone, which elicits male vibrating behaviour. Mated females were less receptive and not stimulated by male vibrating behaviour. We conclude that male premating vibrations in S. lineatus do not function as a male quality signal selected via female choice. Rather, the primary function of this behaviour may be to stimulate a receptive female to mate. Copyright 2003 Published by Elsevier Ltd on behalf of The Association for the Study of Animal Behaviour.