Mixed messages across multiple trophic levels: the ecology of bark beetle chemical communication systems

Summary. Chemical, physiological and behavioral components of pheromone communication have been described for a number of bark beetle species, yet our understanding of how these signals function under natural conditions remains relatively limited. Development of ecologically based models is complicated by the multiple functions and sources of variability inherent in bark beetle semiochemistry. This discussion addresses four ecological issues of chemical signaling in bark beetles: the effects of aggregation on individual fitness, the possibility of cheating, how plants can defend themselves against herbivores that employ aggregation pheromones, and the implications of variability in chemical communication systems to predator avoidance. An analysis of published data from thirteen scolytid – conifer systems indicates that the net benefit and optimal colonization density vary with host condition and beetle species. When beetles attack live trees, the benefit of cooperative host procurement exceeds losses due to competition for the limited substrate, at least up to moderate densities. When beetles colonize dead tissue, however, the effect of subsequently arriving beetles on initial colonizers is almost entirely negative. This suggests that aggregation originated as exploitation of senders, but evolved into manipulation of receivers. It is also proposed that the optimal colonization density which typifies each species or population may offer a more objective and less value–laden index of behavior than current labels such as “aggressiveness”. Beetles can maximize the relative benefits of group attack by incorporating instantaneous measures of host resistance into their colonization behavior, and by adjusting oviposition with colonization density. This system may provide opportunities for cheating. However a number of factors may select against a fixed strategy of cheating, including the linkage between tree allelochemistry and beetle semiochemistry, the reduced quality of substrate available to late arrivers, the short adult lifespans of most bark beetles, differential exposure to some predators, the difficulty of locating signalers during extensive endemic periods, and the low costs incurred during host assessment. However, the possibility that beetles employ flexible, density – dependent strategies deserves heightened attention. The ability of bark beetles to collectively exhaust host defenses poses a particular problem for plant defense. It is argued here that the ideal defense should include both direct resistance mechanisms against invading beetles, and indirect mechanisms that inhibit chemical communication. Evidence for the latter mechanism is explored. The ability of predators to efficiently exploit aggregation pheromones as kairomones in prey finding poses significant risk to bark beetles. It is proposed that minor alterations in pheromone components may provide colonizers with partial escape from such natural enemies while maintaining intraspecific functionality. Traditional interpretations emphasized the fidelity and consistency of pheromones, but under natural conditions chemical signals are modified by unpredictable features of the biotic and abiotic environment. Although we typically view variation in pheromonal signals as experimental noise or simple deviations from a population norm, such variation may reflect evolutionary dynamics. Complex ecological interactions may impose trade-offs between the clarity versus diversity of their signals. Received 3 July 2000; accepted 8 January 2001     

Effect of blue tit song syntax on great tit territorial responsiveness – an experimental test of the character shift hypothesis

A previous study of geographic variation in blue tit (Parus caeruleus) song structure showed that changes in blue tit song syntax (i.e. presence/absence of a trill) are correlated with the breeding density of a close competitor, the great tit (P. major), and are not correlated with other environmental factors such as vegetation structure or blue tit breeding density. We tested the hypothesis that blue tit trilled song represents a character shift that evolved because it reduced territorial interactions with more dominant great tits. We conducted five sets of playback trials in three study populations (mainland southern France, Corsica and Denmark) presenting male great tits with blue tit trilled and untrilled songs and great tit songs. We found that great tits respond equally strongly to both blue tit untrilled songs and to great tit songs, but show a significantly weaker response to blue tit trilled songs. These findings are the first experimental evidence that interspecific competition may play an important role in macrogeographic variation of bird song. Received: 10 October 1999 / Received in revised form: 2 April 2000 / Accepted: 15 April 2000     

Correlations of food distribution and patch size with agonistic interactions in female vervets (Chlorocebus aethiops) and patas monkeys (Erythrocebus patas) living in simple habitats

Food distribution is hypothesized to be important in determining the nature of female relationships within social groups of primates. When food limits female reproductive success, spatially clumped foods are expected to produce strong, linear dominance hierarchies within groups, whereas more spatially dispersed foods are expected to produce weaker or non-existent dominance hierarchies. The association between food distribution and competitive relationships presumably occurs because clumped foods are usurpable but dispersed foods are not. We examined the spatial distribution of food patches (trees) and patch size relative to feeding behavior and agonistic interactions in vervets and patas monkeys, two closely related and sympatric species that nonetheless differ in the strength of the female dominance hierarchy. Food patches of both patas monkeys and vervets were small in size and randomly distributed in Acacia drepanolobium habitat. In contrast, in A. xanthophloea woodland, the habitat type that was exclusively used by vervets, food patches were larger and more spatially clumped. These similarities and differences between and within species were correlated with similarities and differences in the strength and linearity of their dominance hierarchies. Patas monkeys and vervets in A. drepanolobium habitat had dominance hierarchies that were weakly defined because there were relatively few agonistic interactions between females. By contrast, in A. xanthophloea habitat, vervets had a stronger, linear dominance hierarchy characterized by a higher rate of agonistic interactions over food. The covariation of agonistic interactions with patch size is discussed in relation to depletion time, another characteristic that may covary with food distribution, and resource renewal rate, an important determinant of agonistic interactions in insectivorous birds, fishes, insects, and mammals. Received: 18 February 2000 / Revised: 5 September 2000 / Accepted: 26 September 2000     

Local mate competition and precise sex ratios in Telenomus fariai (Hymenoptera: Scelionidae), a parasitoid of triatomine eggs

Telenomus fariai is a gregarious endoparasitoid of the eggs of several species of Triatominae (Hemiptera) with a high degree of sibmating: males fertilize their sisters inside the host egg before emergence or emerge first and copulate with their sisters as these emerge. Our results show that, when laying alone, T. fariai behaves adaptively, minimizing offspring mortality and conforming to the prediction of local mate competition (LMC) theory by laying a single male, which is sufficient to fertilize all the sisters. When more than one wasp was placed with one host, sex ratios still conformed to LMC predictions but, despite the decreasing number of eggs laid per wasp, clutch size could not be completely adjusted to avoid mortality. This is not surprising, as superparasitism is rare in the field. Offspring production was independent of the contacts between conspecifics but was affected by the number of mothers laying on a single host egg. The sex of the progeny was precisely determined: a female produced one male per clutch when laying on both unparasitized or previously parasitized hosts. On the other hand, a mother produced less daughters when superparasitizing. Under crowded conditions, the number of eggs laid per female wasp and per host decreased as the number of mothers increased. Developmental mortality also increased with the number of T. fariai eggs per host, determining a maximum of approximately 14 emerged adults. Host resources per individual affected male and female adult size with similar intensity, and male adult mortality was slightly higher than that for females. These results, and previous findings, suggest that T. fariai attains Hamiltonian sex ratios by laying one male and a variable number of females, and that the detection of chemical marks left by conspecifics provides information on the number of foundresses sharing a patch. Received: 4 February 2000 / Received in revised form: 19 April 2000 / Accepted: 20 May 2000     

The response of the woodpigeon (Columba palumbus) to relaxation of intraspecific competition: A hybrid modelling approach

The recent rapid growth of the woodpigeon population in the British Isles is a cause for concern for environmental managers. It is unclear what has driven their increase in abundance. Using a mathematical model, we explored two possible mechanisms, reduced intraspecific competition for food and increased reproductive success. We developed an age-structured hybrid model consisting of a system of ordinary differential equations that describes density-dependent mortality and a discrete component, which represents the birth-pulse. We investigated equilibrium population dynamics using our model. The two hypotheses predict contrasting population age profiles at equilibrium. We adapted the model to examine the impacts of control measures. We showed that an annual shooting season that follows the period of density-dependent mortality is the most effective control strategy because it simultaneously removes adult and juvenile woodpigeons. The model is a first step towards understanding the processes that influence the dynamics of woodpigeon populations.     

鹤山针阔叶混交林的种群竞争初步研究

通过对鹤山针阔叶混交林种群竞争的研究发现,10年生同龄大叶相思—马尾松群落中,大叶相思生长良好,马尾松种群生长不良和死亡率高达72％;异龄大叶相思—湿地松群落内两种群均生长良好;马尾松、湿地松和大叶相思等纯 林内个体高度和胸径的分化差异小,而混交林内各种群个体高度和胸径分化差异大,尤其是大叶相思比针叶树种更明显;在考虑种群竞争指标时,个体枝叶的生物量或单位面积的生物量比植株数和构件更好;大叶相思—湿地松混交林总的生物量为69.091/hm~2,大叶相思—马尾松混交林为55.282/hm~2,这两个群落的生物量均低于7年生大叶相思纯林;营造针阔叶混交林时应以异龄块状混交方式为主。     

THE VALUES OF GOODS AND SERVICES–IMPLICATIONS FOR A FLEXIBLE NATIONAL WATER POLICY1

ABSTRACT: This paper evolved from a concern that water resource administrators and planners do not have a consistent definition of the value of goods and services of outputs from projects and programs. A more flexible approach to planning and evaluation is presented that recognizes the interaction of production and consumption through the use of commodity demand curves. Use of the concept of consumer surplus permits measurement of the differential impacts on producers' and consumers' welfare. Quadratic programming is suggested as a consistent means of quantifying these differential impacts.     

The effect of glyphosate on the growth and competitive effect of perennial grass species in semi-natural grasslands

Biodiversity within European semi-natural biotopes in agro-ecosystem is declining, and herbicide drift from neighbouring fields is considered as an important factor for the decline. The aim of the present study was to investigate whether the growth and competitive interactions in a model system of two perennial grass species, Festuca ovina and Agrostis capillaris, are affected by sub-lethal doses of glyphosate in field margins. In a glasshouse experiment with ample nitrogen, the interspecific competitive interactions were found to be significantly affected by glyphosate; the competitive effect of F. ovina on A. capillaris increased and the competitive effect of A. capillaris on F. ovina decreased with increasing doses of glyphosate. Furthermore, the importance of interspecific competition increased with the glyphosate dose. The results of the study of competitive interactions are in agreement with the observed plant community dynamics at the field site where F. ovina was found to be more dominant in plots treated with a relatively high dose of glyphosate. Importantly, the effects of glyphosate on the plant community dynamics critically depended on the effect of glyphosate on the plant competitive interactions. The study concludes that the current practice in the environmental risk assessment of non-target effects of herbicides, where single species are tested in the greenhouse, may be inadequate for assessing the effect of herbicides in semi-natural plant communities. The presented methods can be used for assessing the importance of competitive interactions for the sensitivity of non-target plants to herbicides in risk assessment.     

单质硫自养短程反硝化耦合厌氧氨氧化强化脱氮

通过在厌氧氨氧化(ANAMMOX)连续流反应器中添加单质硫,试图引入单质硫自养短程反硝化(short-cut S~0-SADN)来强化ANAMMOX过程中NO~-_3-N的去除.在温度为(33±2)℃,pH为7.8～8.2条件下,探讨不同的进水NH~+_4-N/NO~-_2-N比对耦合系统中氮素转化以及NO~-_2-N竞争特性的影响.结果表明,在不同的进水NH~+_4-N/NO~-_2-N比(1∶1.3、 1∶1.5、 1∶1和1∶1.1)下,耦合系统的TN平均去除率分别达到了96.78%、 97.21%、 94.68%和97.72%,均远远大于ANAMMOX理论TN最高去除率89%.其中,在进水NH~+_4-N/NO~-_2-N比为1∶1或1∶1.1条件下,耦合系统能够实现单质硫自养短程反硝化耦合ANAMMOX深度脱氮的稳定运行.在最佳进水NH~+_4-N/NO~-_2-N比1∶1.1、NH~+_4-N和NO~-_2-N浓度分别为240mg·L~(-1)和265mg·L~(-1)条件下,TN去除速率达到1.50kg·(m~3·d)~(-1),ANAMMOX和S~0-SADN途径的TN去除率分别稳定在(95.68±1.22)%和(2.04±0.77)%.在整个运行过程中,ANAMMOX在底物NO~-_2-N的竞争过程中一直占据着绝对的优势,ANAMMOX菌的活性(以NH~+_4-N/VSS计)稳定在(0.166±0.008)kg·(kg·d)~(-1).     

不同温度下铜绿微囊藻和斜生栅藻的最佳生长率及竞争作用

以温度为主要控制因子,研究了单独培养和按不同接种密度比混合培养下铜绿微囊藻(Microcystis aerugino-sa)和斜生栅藻(Scenedesmus obliquus)的生长状况。结果表明,在温度为7～35℃条件下,微囊藻比增长率随着温度的升高而增大,而栅藻增长率随着温度的升高先增大后减小。竞争体系对2种藻的比增长率均有影响,微囊藻在微囊藻与栅藻接种比例为10∶1的混合培养体系中增长最慢,栅藻则在微囊藻与栅藻接种密度比为1∶10的混合培养体系中增长最慢。不同温度条件下在2种藻不同接种密度比的混合培养体系中,微囊藻对栅藻的竞争抑制能力均大于栅藻对微囊藻的竞争抑制能力。藻种间的竞争抑制能力因温度的变化而得到不同程度的强化或减弱,铜绿微囊藻具有更强的竞争能力。