Guild structures of seed insects in relation to acorn development in two oak species

We investigated patterns of acorn growth in Quercus variabilis Blume and Quercus serrata Thunb., seasonal trends in emergence of seed insects found in the acorns, oviposition periods of the insects and falling periods of insect-infested acorns. In Q. variabilis, two insect guilds were associated with acorn development: (i) the immature acorn feeding (IAF) guild [Curculionidae sp., Poecilips cardamomi (Schaufuss), and Characoma ruficirra (Hampson)]; and (ii) the mature acorn feeding (MAF) guild [Curculio robustus (Roelofs), Curculio sikkimensis (Heller), and Cydia glandicolana (Danilevsky)]. In Q. serrata, there were three guilds: (i) the pistillate flower feeding (PFF) guild (cynipid wasp); (ii) the IAF guild [sap absorption by Mechoris ursulus (Roelofs)]; and (iii) the MAF guild (M. ursulus, C. sikkimensis, Cydia danilevskyi (Kuznetzov), C. glandicolana and Autostichidae sp.). The succession of guilds during acorn development may be a consequence of the use by different species of the limited food resource. The lack of a PFF guild in Q. variabilis that was found in our field site would have a positive effect on IAF guilds in utilizing the acorns.     

Assessment of interspecific interactions among parasitoids on the outcome of inoculative biological control of leafminers attacking chrysanthemum

Indigenous natural enemies occur within field grown crops at varying densities dependent upon a variety of other biotic and abiotic parameters. This natural control often does not provide adequate suppression, which results in the application of other pest management solutions including augmentative biological control. When releasing mass-reared natural enemies into a backdrop of existing natural enemy populations, competitive interactions are likely to occur. To assess the influence of these interspecific interactions on the outcome of such biological control practices studies were conducted in a simulated, field cage grown, cut chrysanthemum production system. Competitive interactions of two commercially available parasitoids were studied both in terms of parasitoid-host population dynamics and the impact of interspecific interactions on crop quality at harvest. The parasitoids Diglyphus isaea and Dacnusa sibirica attacking the leafminer Liriomyza langei were used as the model insect system. Both parasitoids are cosmopolitan and are known to occur in many ornamental production areas. Treatment comparisons included single species releases with complimentary releases of both species either simultaneously or with 2-week time lags, as well as a no release control to measure the background effects of natural mortality. Conclusions drawn from results of population-level studies replicated within and among years were that levels of interspecific competition among parasitoid species were undetectable at leafminer densities typical of field-grown ornamental crops (low densities), and thus, the efficacy of one species released into a backdrop of potentially competing parasitoids did not negatively or positively affect the outcome of the augmentative biological control, nor was there a positive outcome; however, crop quality at harvest was influenced.     

塔克尔莫乎尔沙漠银沙槐群落主要植物的种间关系

根据野外调查的样地数据资料,运用2×2列联表的Fisher精确检验、Pearson相关系数和Spearman秩相关系数检验,并结合DCA排序,分析塔克尔莫乎尔沙漠银沙槐群落主要植物种群之间的种间关系。结果表明:Fisher精确检验有3个种对呈显著正关联,3个种对呈显著负关联;Pearson相关系数检验有1个种对呈显著正相关,12个种对呈显著负相关;Spearman秩相关系数检验,有14个种对存在显著相关,其中负相关13对,占所有种对数的14.29%,正相关1对,占所有种对数的1.1%。银沙槐群落种间关系相对松散,反映出植物群落随着环境变化其物种组成也发生相应变化,正向着相对稳定独立的无关联方向发展。     

Performance of the fir coneworm Dioryctria abietivorella (Grote) as affected by host species and presence or absence of seed cones

1 Larval performance of Dioryctria abietivorella (Grote) (Lepidoptera: Pyralidae) was compared when insects were reared in white spruce, black spruce and Jack pine seed orchards. For each species, half of the insects developed in the presence of cones, while the other half was maintained on branches without cones. 2 Significantly faster development rates were observed on spruce species when compared to Jack pine. Significant differences in survival were also recorded between the different feeding treatments. 3 The presence of seed cones on spruce species significantly increased insect mean weight compared with branches without cones, but no significant differences were noticed between the different tree species with cones. 4 White spruce is the most suitable host tree for fir coneworm feeding and the availability of seed cones plays an important role in determining D. abietivorella larval performance.     

Vertical stratification of figs and fig-eaters in a Bornean lowland rain forest: how is the canopy different?

Fig trees (Ficus spp; Moraceae) are a common constituent of many tropical forests, where they produce figs that are eaten by a wide range of bird and mammal species. In our Bornean field site six Ficus seed dispersal guilds can be recognised, differentially attracting subsets of the frugivore community. Guild membership appears to be determined by figs' size, colour, crop size and height above ground, and frugivores' size, sensory and locomotory physiology and foraging height. Vertical stratification therefore appears to be an important determinant of fig and frugivore partitioning. The guild structure observed is discussed with respect to implications for seed dispersal and the differences between the canopy and understorey. Regarding figs eaten primarily by birds, larger fruit and crops can be found in the canopy where they are exposed to larger assemblages of potential frugivores than those presented in the understorey.     

Larval consumption rates,interspecific predation,and local guild composition of egg-overwintering Agabus (Coleoptera,Dytiscidae) species in vernal ponds

Summary In vernal ponds in the boreal region, egg-over-wintering Agabus species form a guild that feeds mainly on larvae and pupae of aedine mosquitoes. The regular co-existence of very similar Agabus species indicates local communities not structured by interspecific competition. However, the lower number of species in local guilds than in the regional species pool poses a problem of limited membership. We suggest that the species of this guild display habitat differences mainly with respect to water temperature, pond size and prey density. In this view, habitat selection reflects body size and thermal growth response of the species, mainly in connection with larval development. We present field data from two northern Swedish vernal ponds. Based on these data, feeding experiments were performed to test the hypothesis outlined above. At a high prey density, larvae of all instars of the larger species A. erichsoni Gemm. & Har. had a significantly higher consumption rate than those of the smaller species A. opacus Aubé. At a low prey density the differences were smaller, and only the third instar larvae differed significantly. At 2° C, larvae of A. opacus had a significantly higher consumption rate than those of A. congener (Thunberg). At 15° C, no significant difference was observed. In studies of within-guild interspecific predation, always the larger larvae consumed the smaller ones. Field data show that egg hatching is spread out in time, and show interspecific differences. Consequently, the effects of unexpected droughts differ with species.     

Association of cone thermogenesis and volatiles with pollinator specificity in Macrozamia cycads

Cone traits (volatile components and thermogenesis) of three cycad species in the genus Macrozamia were examined for differences related to their specific insect pollinators, the weevil, Tranes spp., or the thrips, Cycadothrips chadwicki. Linalool (>80% of emissions) dominated cone volatile components of M. machinii (Tranes-pollinated) and beta-myrcene was a minor component (<9% of emissions). Volatiles of M. lucida and M. macleayi cones (Cycadothrips-pollinated) were dominated by beta-myrcene (up to 97% of emissions), but no linalool was detected. Pollinator movement into and out of cones coincided with cone thermogenesis and peak odor emission: around sunset for Tranes, and at mid-day for Cycadothrips. Female cone traits were similar to those of their conspecific male cones. Differences in cone traits between Macrozamia species may thus be responsible for conferring pollinator specificity in areas of sympatry.     

Coexistence of plant species in a biodiversity hotspot is stabilized by competition but not by seed predation

Understanding the mechanisms of species coexistence is a key task for ecology. Recent theory predicts that both competition and predation (which causes apparent competition among prey) can either promote or limit species coexistence. Both mechanisms cause negative interactions between individuals, and each mechanism promotes stable coexistence if it causes negative interactions to be stronger between conspecifics than between heterospecifics. However, the relative importance of competition and predation for coexistence in natural communities is poorly known. Here, we study how competition and apparent competition via pre‐dispersal seed predators affect the long‐term fecundity of Protea shrubs in the fire‐prone Fynbos biome (South Africa). These shrubs store all viable seeds produced since the last fire in fire‐proof cones. Competitive effects on cone number and pre‐dispersal seed predation reduce their fecundity and can thus limit recruitment after the next fire. In 27 communities comprising 49 990 shrubs of 22 Protea species, we measured cone number and per‐cone seed predation rate of 2154 and 1755 focal individuals, respectively. Neighbourhood analyses related these measures to individual‐based community maps. We found that conspecific neighbours had stronger competitive effects on cone number than heterospecific neighbours. In contrast, apparent competition via seed predators was comparable between conspecifics and heterospecifics. This indicates that competition stabilizes coexistence of Protea species, whereas pre‐dispersal seed predation does not. Larger neighbours had stronger competitive effects and neighbours with large seed crops exerted stronger apparent competition. For 97% of the focal plants, competition reduced fecundity more than apparent competition. Our results show that even in communities of closely related and ecologically similar species, intraspecific competition can be stronger than interspecific competition. On the other hand, apparent competition through seed predators need not have such a stabilizing effect. These findings illustrate the potential of ‘community demography’, the demographic study of multiple interacting species, for understanding plant coexistence.     

Discrimination among pinyon pine trees by Clark's Nutcrackers: effects of cone crop size and cone characters

Summary The influences of Colorado pinyon pine (Pinus edulis) cone crop size, cone and seed weight, cone length, number of seeds per cone, number of viable seeds, and percent viable seeds on the foraging behavior of avian seed dispersal agents were examined in field and laboratory settings. In the field, there was a significant positive relationship between cone number per tree and both the absolute number of cones and the percentage of the cone crop from which seeds were harvested. Cone weight and the number of viable seeds were also significantly related to seed harvest intensity. Laboratory experiments examined the relationship between crop size and cone characters on seed harvest by 18 Clark's Nutcrackers (Nucifraga columbiana). Nutcrackers were offered a choice of two tree types: one with 20 cones attached, and another with 10 cones attached. Significantly more birds chose to remove seeds first from the tree with 20 cones than the tree with 10 cones. In timed trials, they also harvested seeds from significantly more cones on the tree with the higher cone density. In the laboratory, cones chosen for seed removal by the nutcrackers had significantly more viable seeds, more seeds, and were longer compared to cones that were not chosen. Such discriminatory foraging behavior may increase avian foraging efficiency and result in differential reproductive success of pinyon pines. This behavior may therefore influence the evolution of pinyon pine reproductive traits.     

Island size, isolation, or interspecific competition? The breeding distribution of the Parus guild in the Danish archipelago

The Parus guild (Parus spp., Sitta, Certhia, and Regulus) is distributed as a complex mosaic within the Danish archipelago, with from one to eight species on different islands. We assessed the roles of island isolation, island size, and interspecific competition in determining the breeding species compositions of this guild on 53 Danish islands. Small, isolated islands supported fewer species than larger, nearshore islands. These effects, however, were largely restricted to a few sedentary species (P. cristatus, P. palustris, S. europaea) that are known to be poor dispersers/colonizers. In some cases, these three species were also absent from large, nearshore islands with suitable habitat, suggesting that habitat availability was not always responsible for the absence of a species. Monte Carlo simulations suggested that the pattern of species presence/absence was not a result of interspecific interactions. Thus, although a number of previous studies have documented interspecific competition among members of the Parus guild, our results suggest that such competition is not responsible for the unusual pattern of species distribution within the Danish archipelago. Received: 28 October 1996 / Accepted: 7 February 1997     

Reproductive potential of balsam fir (Abies balsamea), white spruce (Picea glauca), and black spruce (P. mariana) at the ecotone between mixedwood and coniferous forests in the boreal zone of western Quebec

The reproductive potentials of balsam fir and white spruce (co-dominants in mixedwood forests) and black spruce (dominant in coniferous forests) were studied to explain the location of the ecotone between the two forest types in the boreal zone of Quebec. Four sites were selected along a latitudinal gradient crossing the ecotone. Cone crop, number of seeds per cone, percentage filled seeds, and percentage germination were measured for each species. Balsam fir and white spruce cone crops were significantly lower in the coniferous than in the mixedwood forest, while black spruce had greater crop constancy and regularity between both forest types. Mast years were more frequent for black spruce than for balsam fir in both forest types (mast year data not available for white spruce). The number of seeds per cone was more related to cone size than to forest type for all species. Black spruce produced more filled seeds in the coniferous forest than balsam fir or white spruce. The sum of growing degree-days and the maximum temperature of the warmest month (both for the year prior to cone production) significantly affected balsam fir cone production. The climate-related northward decrease in reproductive potential of balsam fir and white spruce could partly explain the position of the northern limit of the mixedwood forest. This could change drastically, however, as the ongoing climate warming might cancel this competitive advantage of black spruce.     

Obituaries

Abstract

It has been suggested that bryophyte communities differ from those of higher plants, in that species coexistence is not limited by inter-specific competition. To test this hypothesis, bryophyte ‘lawns’ were-sampled at six locations in southern New Zealand. At each site, 625 quadrats, each ca 1 X 1cm, were arranged on a contiguous grid.

Variance in quadrat richness was significantly less than expected on a random basis at four sites, with the same trend in a fifth, i.e. quadrat richness was relatively constant. Since a patch model was used to reduce the effects of micro-environmental variation and spatial autocorrelation, this suggests that competition was restricting coexistence. The degree of restriction was similar to that found previously in grass lawn communities.

Species association was calculated on a patch basis, examining only small spatial-scale deviations from expectations. At four- sites this gave only negative associations. At the other two sites there were both positive and negative associations, but the pattern of associations gave no indication of Root/Pianka-type guilds.

Examination of the variance in guild proportions, using taxonomic (moss vs liverwort) and morphological (prostrate vs erect) guilds, gave- no indication of significant guild structure for any site (i.e. in the overall analyses for each site, index RV_gp was very close to the null-model expectation of 1.0).

Searches using the Wilson- Roxburgh method failed to reveal any significant intrinsic guild classification. That is, there was no indication of groups of species within a community that tended to be mutually exclusive because of similarity in resource use. Thus, lack of guild proportionality using a priori guilds was not due to the use of an inappropriate guild classification, but to lack of a guild structure in the communities.

It is concluded that there is community structure among bryophytes, in that species exclude each other to the same degree as higher-plants do in their communities. However, there is no evidence of structuring of these bryophyte communities into guilds; it seems that bryophyte species all form one guild.     

The significance of the asymmetric effect of crowding for coexistence in a mixed temperate forest

Abstract. The coexistence of coniferous (mostly Abies homolepis) and broad-leaved tree species (mostly Fagus crenata) in a mixed temperate old-growth forest in Japan was simulated by a size-structure dynamics model incorporating the asymmetrical (one-sided) effect of shading between these two life-form guilds. The model assumes that the crowding effect due to one-sided competition for light on a tree of a given size regulates the rate of size growth and recruitment. The cumulative basal area of trees larger than a given tree in the forest is employed to express the intensity of one-sided competition on that tree. Cumulative basal areas of both guilds negatively affected the growth rate of any tree. The shading effect by conifers on the growth rate of either guild was stronger than that by broad-leaved species. Two types of model were tested for recruitment; an additive and a reciprocal model. A reciprocal model, where basal area density of conifers and broad-leaved species has a negative effect on the recruitment of its own guild but has a positive effect on that of the other guild, fit the observed data better than an additive model where total basal area of the two guilds suppresses recruitment rates. Simulations using these models showed that, within a particular range of the set of recruitment rates, the two guilds could coexist. The tendency for reciprocal replacement, incorporated in the reciprocal model, substantially widened the range of coexistence and shortened the time required for convergence.     

三种外来入侵斑潜蝇种间竞争研究进展

美洲斑潜蝇、南美斑潜蝇、三叶斑潜蝇是危害蔬菜、花卉的世界性大害虫。不同种间相互更替或取代的现象时有发生。根据国内外相关研究报道,综述了对近年来3种斑潜蝇种间竞争的研究。对斑潜蝇种间更替和取代现象的分析显示,生殖干扰、新生物型的出现、天敌、生态位、寄主植物以及抗药性等因素可能是影响它们种间竞争的关键。以期为斑潜蝇种群演替研究及防治策略制定提供参考。     

Structural analysis of the phytophagous insect guilds associated with the roots of Centaurea maculosa Lam. C. diffusa Lam., and C. vallesiaca Jordan in Europe:

Summary During extensive field surveys in central and eastern Europe, 21 herbivorous root insect species were found on Centaurea maculosa ssp. rhenana Boreau, 12 species on C. diffusa Lam. and 11 species on C. vallesiaca Jordan, representing 12 families in 4 orders. The large geographic distribution (species-area function), the high number of Centaurea spp. present (host speciation rate), and the high apparency of the rosettes and the rich food resources offered by the roots during winter, together with their poor accessibility, correlate with the high number of specialist feeders associated with the roots of C. maculosa and C. diffusa. The members of the taxonomically diverse root entomofauna exploit specific structures of the tap root (food niches). Interspecific competition among members of food niches, as well as species-specific responses to different phenological stages (for oviposition) and tissues (for larval development) are thought to be responsible for the high predictability in guild structure. The relatively low levels of host plant attack (two thirds of the roots were unattacked) and the fact that food niches remained unoccupied in most of the regions suggest, however, that the majority of the studied guilds do not represent equilibrium assemblages. Ecological (different habitats), climatic (transitional zone) and historical (ancient pre-Pleistocene communities) factors could account for the highest values of species diversity, infestation levels, species packing and food niche utilization, which are found on C. maculosa in E. Austria/NW. Hungary, compared to other regions. A positive correlation between species packing (number of root-feeding species per population) and infestation rates (percent of roots attacked) was only found for the more stable, semi-natural habitats. A comparative analysis of the regional root insect guilds of C. maculosa with corresponding data for the phytophagous insects associated with the flower heads revealed distinct taxonomical differences, but a high degree of numerical and structural similarity. The different geographical regions are similarly ranked for host plant attack, herbivore pressure, average species packing and level of food niche utilization.     

Reproductive traits of two closely related species-pairs on adjacent,different soil types in South African Fynbos

High levels of edaphic endemism and soil-related beta-diversity in Agulhas Plain fynbos communities suggest that reproductive traits of plants growing on different fynbos soils would be related to differences in soil regime. We investigated reproductive traits in two closely related Proteaceae species-pairs growing on adjacent soil types: Protea obtusifolia and Leucadendron meridianum occurring in shallow pockets of limestone-derived soils, and P. susannae and L. coniferum on the adjacent, uniformly deep colluvial sands.We found that species growing on the limestone soil comprised smaller plants, with fewer cones and seeds per plant, than species on the colluvial sands. These differences suggest that the small soil pockets of limestone soil limit plant size, in turn limiting the number of reproductive structures. Annual variation in cones and seed was the same in all species. There were no consistent trends in degree of serotiny, or sex allocation across soil types. The higher cone and seed predation levels of both limestone species than the colluvial sands species were ascribed to the higher plant densities of the former leading to lower insect search times. It was concluded that there were no overall patterns in reproductive traits that could be ascribed to differences in soil regime, other than through size-related effects. Fire regime is likely to have played a more important role in determining reproductive traits.Department of Statistical Sciences, University of Cape Towm     

Species packing and the competition function with illustrations from coral reef fish

This paper concerns the contrast between guilds whose species show resource partitioning and those that show extensive overlap. Using a Lotka-Volterra model, the ease of invasion by a third species into a guild already containing two species is examined for various shapes of resource utilization curves. I show that (a) a guild is more easily invasible and allows tighter packing if its member species have leptokurtic (thick-tailed) resource utilization curves than if they have platykurtic (thin-tailed) curves; (b) the distribution of niche separation distances is bimodal in a “thin-tailed” guild and is unimodal in a “thick-tailed” guild; (c) there are three-species guilds such that removal of one particular species leaves a two-species system in which one of the remaining species excludes the other. In this context, competition pressure is a force maintaining species diversity.Groupers (Serranidae) appear to be a thin-tailed guild, and Parrotfish (Scaridae) and Surgeonfish (Acanthuridae) together appear to be a thick-tailed guild, and these guilds show many properties predicted by the model. I conjecture that thick-tailed guilds form when the constituent species are selected to be generalists and apply this idea to tropical fruit- and flower-feeding birds.     

Effects of different insect species on seed quantity and quality in Norway spruce

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Influence of body size on coexistence of bird species

Theory suggests that body size is an important factor in determining interspecific competition and, ultimately, in structuring ecological communities. However, there is a lack of pragmatic studies linking body size and interspecific competition to patterns in ecological communities. The objective of the present study was to determine the effect of body size (mass) on competitive interactions between bird pairs and to investigate the influence of food guilds. Point-counts were carried out in nine sites every month from November 2002 to November 2003 in the Cuetzalan Region, Mexico, and we used presence/absence and abundance data for the analyses. To calculate the strength of competition we used the Angle Frequency Method to extract form factors from 20 pairwise interactions. A prototype competition interaction and random pairs were also constructed. We used clustering techniques (PCA) to calculate the dissimilarity scores (distances, D) of each of the pairwise interactions to the prototype competition and random pairs and one-way ANOVA to test for differences between the means of the random and competitive pairs. The ratio in body mass (lnBM) for each of the interacting pairs was calculated, and the association between the lnBM ratio and the strength of competition (D) was tested using a Pearson product-moment correlation coefficient. To test for the influence of foraging guilds we used a univariate general linear model. Our results demonstrate a significant negative relationship between bird body mass ratio and competition strength – i.e. competition strength increased when the body masses of the birds became more similar. We did not find a significant influence of foraging guild on the relationship between body mass ratio and competition strength. On the basis of these results, we suggest that high variation in body sizes amongst sympatric species promotes coexistence in communities.     

CONE COLOR POLYMORPHISM ASSOCIATED WITH ELEVATION IN WHITE FIR,ABIES CONCOLOR,IN SOUTHERN COLORADO

Morphological variation in color of female cones in white fir, Abies concolor, is postulated to have adaptive value. The purple variant is only found in one part of the range of white fir where it grows at high elevations. In addition, the frequency of purple cones increases with increasing elevation, suggesting a thermoregulatory function for the purple variant in the cold habitats found at high elevations. Temperature measurements support this hypothesis: purple cones attain higher internal temperatures than do green cones of comparable size. Cone diameter may, in fact, be correlated with cone color but definitive evidence is lacking. Alternative hypotheses are offered which may also be important in maintaining the presence of the purple variant at high elevations. It is suggested that other characters associated with purple cones may place them at a competitive disadvantage at lower elevations.