Mammalian predator scent, vegetation cover and tree seedling predation by meadow voles

Herbivores are thought to respond to the increased risk of attack by predators during foraging activities by concentrating feeding in safe habitats and by reducing feeding in the presence of predators. We tested these hypotheses by comparing tree seedling predation by meadow voles within large outdoor enclosures treated either with scent of large mammalian predators (red fox, bobcat, coyote) or a control scent (vinegar). In addition, we compared the distribution of voles in relation to naturally occurring variation in vegetation cover and the tendency of voles to attack tree seedlings planted in small patches with cover manipulation (intact, reduced or removed cover). Predator scent did not affect the rate or spatial distribution of tree seedling predation by voles, nor did it affect giving up densities (a surrogate of patch quitting harvest rate), survival rates, body size or habitat distribution of voles. In both predator scent and vinegar treatments voles preferred abundant vegetation providing good cover, which was also the site of almost all tree seedling predation. We conclude that large mammalian predator scent does not influence the perception by voles of the general safety of habitat, which is more strongly affected by the presence of cover.     

可获得种子量和种子出苗对草地裸沙斑植被恢复的相对制约

以科尔沁沙化草地的裸斑为研究对象,布设种子捕捉器监测生长季传播到裸斑的种子数量、物种组成,并人工播种监测裸斑中出苗情况,研究可获得种子数量和出苗对裸斑植被恢复的制约.结果表明:沙生植物3种,种子数为71 ～623粒·m-2,非沙生植物9种,种子数为73 ～410粒·m-2;裸斑中沙生植物的出苗数(9.3 ～22.3株·m-2)高于非沙生植物(0 ～3.4株·m-2);沙生植物在裸斑的出苗率最高为沙蓬(12.2％ ～34.1％),其次是乌丹蒿(4.5％ ～10.3％),小叶锦鸡儿(0～0.5％),非沙生植物狗尾草出苗率为0～6％,蒲公英为0 ～0.4％,其余植物种出苗率为0;沙生植物裸斑出苗数(9.3 ～22.3株·m-2)高于植被区(7.7株·m-2),而非沙生植物在裸斑的出苗数(0～3.4株·m-2)低于植被区(4.5株·m-2).可见,裸斑植被恢复中,沙生植物的可获得种子量制约高于出苗制约;非沙生植物的可获得种子量制约与出苗制约相当.     

Native herbivore exerts contrasting effects on fire regime and vegetation structure

Although native herbivores can alter fire regimes by consuming herbaceous vegetation that serves as fine fuel and, less commonly, accumulating fuel as nest material and other structures, simultaneous considerations of contrasting effects of herbivores on fire have scarcely been addressed. We proposed that a colonial rodent, vizcacha (Lagostomus maximus), reduces and increases fire intensity at different stages in its population cycle in the semiarid scrub of Argentina. Specifically, we hypothesized that, when colonies are active, vizcachas create natural fire-breaks through intense grazing, generating over time patches of large unburned shrubs in grazed zones. In contrast, when colonies are abandoned, recovery of fine fuels and previous accumulation of coarse wood on colonies during territorial displays increases fire intensity, creating patches of high shrub mortality. To test these hypotheses, we estimated stem age of the dominant shrub (Larrea divaricata) and measured aboveground biomass in zones actively grazed by vizcachas and in ungrazed zones, and compared densities of live and dead shrubs on abandoned colonies and adjacent zones following fire. In active colonies, age and biomass of shrubs were much greater in grazed than ungrazed zones. In abandoned colonies that had been burnt, density of dead, burned shrubs was higher and density of live shrubs was lower than in adjacent zones. These results support our hypotheses and reveal a new interaction between native herbivores and fire, in which herbivores augment fire intensity by gathering fuel. Our findings indicate that, through opposing effects on fire, native herbivores enhance the heterogeneity of vegetation in woody-dominated ecosystems.     

盐碱胁迫对小麦幼苗生长和根际细菌群落结构的影响

【背景】植物可通过改变根际微生物群落结构来减轻胁迫,但盐碱胁迫下小麦生长和根际细菌群落结构变化尚待明晰。【目的】明确盐碱胁迫下小麦幼苗生长及根际细菌群落结构变化。【方法】以添加NaCl和Na2CO3调节Hoagland营养液Na+浓度和pH来模拟盐碱胁迫,通过MiSeq高通量测序分析细菌群落结构。【结果】与无胁迫对照相比,单纯盐胁迫使小麦幼苗地上和地下部干重、叶绿素和根际土壤速效氮含量分别显著降低了25.0%、57.1%、21.2%和42.9%(P<0.05);小麦幼苗生长指标和根际土壤速效氮、速效磷、有机质含量及酶活性(除过氧化氢酶)随碱度升高而降低;所有处理的小麦幼苗根际优势菌门(丰度7.1%-71.5%)均为变形菌门(Proteobacteria)、放线菌门(Actinobacteria)和厚壁菌门(Firmicutes),优势属(丰度0.5%-5.1%)有肠杆菌属(Enterobacter)、嗜多甲基菌属(Methyloversatilis)、固氮菌属(Azotobacter)、假单胞菌属(Pseudomonas)和异根瘤菌属(Allorhizobium)。盐碱胁迫未显著...     

Effects of vegetation structure and floristic diversity on detritivore,herbivore and predatory invertebrates within calcareous grasslands

Calcareous grasslands in Europe have shown wide scale declines in their extent and quality as a result of modern agricultural practices, increased atmospheric eutrophication and lack of management. In addition to being a key habitat for specialist plants, calcareous grasslands are also important for many threatened invertebrates. In this UK based study, we investigated the impact of military vehicle activity, floral species richness and vegetation structure on assemblages of detritivore, herbivore and predatory invertebrates. We also consider the impact that disturbance by military vehicle activity on the proportion of invertebrate species capable of flight, a surrogate for dispersal ability. Sward height was negatively correlated with detritivore, herbivore and predator species richness. Herbivores species richness was positively correlated with both forb and grass species richness. Spatial variation in the number of plant species was negatively correlated with herbivore species richness. Those sites most heavily disturbed by military vehicle activity supported the lowest proportions of flightless invertebrates. Successful management for calcareous grassland invertebrates should aim to maintain short swards with high floristic diversity, in terms of both the forbs and grasses. It should be noted, however, that these findings refer to principally surface rather than sward active invertebrates. While disturbance associated with military vehicle activity was not found to affect invertebrate species richness, it has negative consequences for the structure of invertebrate assemblages by selecting against invertebrates with low dispersal ability. To support invertebrate diversity in calcareous grasslands we emphasise the need for variety in the timing and type of management applied to promote heterogeneity in sward structure.     

Guild mobility affects spider diversity: Links between foraging behavior and sensitivity to adjacent vegetation structure

The movement patterns of species may affect their susceptibility to modified habitat structure. It is likely that sedentary species perceive habitat features at smaller spatial extents compared to mobile species, but there is a lack of experimental research on the effects of fine-scale habitat characteristics on organisms of differing mobility. Spiders display two basic mobility levels based on foraging behavior: web-building species are restricted to specific sites whereas active hunters are mobile. We collected spiders inhabiting sagebrush shrubs with a structurally enhanced, unmodified, or removed understory, to examine (1) whether habitat structure in the immediate vicinity of shrubs affected cursorial and web spiders differently in terms of abundance and species richness and (2) which genera most contributed to changes in community composition. Shrubs without understory had reduced cursorial spider densities and species richness compared to shrubs with added and unmodified understories, whereas web spiders lacked significant responses to treatments. Community-level differences based on relative abundance of genera were detected in cursorial spiders but not in web spiders, despite a strong contribution of the web-building genus Theridion to community dissimilarities. Our results support the hypothesis that sedentary organisms may be sensitive to contiguous habitat at finer spatial scales than cursorial organisms, and highlight the risks associated with only collecting local habitat information when studying mobile species.     

Effects of seed size and emergence time on tree seedling establishment: importance of developmental constraints

Arafura filesnakes (Acrochordus arafurae) are large (up to 2.5 m, 5 kg) aquatic nonvenomous snakes that feed entirely on fishes. A 10-year field study in the Australian wet-dry tropics revealed strong correlations between rainfall patterns, fish abundance, and snake population dynamics. All of these characteristics showed considerable annual variation. High rainfall late in the wet season (February–March) caused prolonged inundation of the floodplain. Following such years, dry-season sampling revealed that fishes were abundant, filesnakes were in good body condition, and a high proportion of adult female filesnakes were reproductive. Annual variation in recruitment to the population (as judged by the relative abundance of yearling snakes) was also correlated with fish abundance and thus, with rainfall patterns in the late-wet season. Our results fit well with those from other studies on a diverse array of aquatic and terrestrial species within the wet-dry tropics. Annual variation in rainfall patterns, via its effects on prey abundance, may drive the population dynamics of many tropical predators. Received: 28 June 1999 / Accepted: 29 February 2000     

Interaction intimacy affects structure and coevolutionary dynamics in mutualistic networks

The structure of mutualistic networks provides clues to processes shaping biodiversity [1-10]. Among them, interaction intimacy, the degree of biological association between partners, leads to differences in specialization patterns [4, 11] and might affect network organization [12]. Here, we investigated potential consequences of interaction intimacy for the structure and coevolution of mutualistic networks. From observed processes of selection on mutualistic interactions, it is expected that symbiotic interactions (high-interaction intimacy) will form species-poor networks characterized by compartmentalization [12, 13], whereas nonsymbiotic interactions (low intimacy) will lead to species-rich, nested networks in which there is a core of generalists and specialists often interact with generalists [3, 5, 7, 12, 14]. We demonstrated an association between interaction intimacy and structure in 19 ant-plant mutualistic networks. Through numerical simulations, we found that network structure of different forms of mutualism affects evolutionary change in distinct ways. Change in one species affects primarily one mutualistic partner in symbiotic interactions but might affect multiple partners in nonsymbiotic interactions. We hypothesize that coevolution in symbiotic interactions is characterized by frequent reciprocal changes between few partners, but coevolution in nonsymbiotic networks might show rare bursts of changes in which many species respond to evolutionary changes in a single species.     

Time-scale effects in the interaction between a large and a small herbivore

In the short term, grazing will mainly affect plant biomass and forage quality. However, grazing can affect plant species composition by accelerating or retarding succession at longer time-scales. Few studies concerning interactions among herbivores have taken the change in plant species composition into account. In a salt-marsh system, the long-term effects of exclusion of a large herbivore (cattle) on the abundance of a small herbivore (hare) were studied. Excluding cattle grazing for 30 years resulted in large changes in vegetation composition. In general, the cover of tall-growing species increased in the absence of cattle grazing. These long-term changes negatively affected hare grazing intensity. Hares preferentially fed on Festuca rubra and negatively selected tall growing plants, such as Elymus athericus, both in cattle-grazed and long-term ungrazed areas. However, the intensity of hare grazing was not related to the cover of F. rubra. The cover of tall-growing plants (E. athericus, Atriplex prostrata and Juncus maritimus) appeared to be the best predictor and hare grazing intensity decreased sharply with an increase of the cover of tall plants. When cover of tall plants did not increase, hare grazing intensity was not affected. The study shows that the time-scale of the experiment is of prime importance in studying interactions between herbivores. Species that do not seem to influence the abundance of one another or are competing for the same resources on a short time-scale might well be facilitating each other when looking at larger time-scales while taking plant species replacement into account.     

Disturbance affects spatial patterning and stand structure of a tropical rainforest tree

The distribution and spatial patterns of plant populations in natural ecosystems have recently received much attention; yet the impacts of human‐induced disturbances on these patterns and underlying processes remain poorly understood. We used the sub‐canopy tree, Ryparosa kurrangii (Achariaceae), to explore the possible effects of such disturbances on stand structure and spatial patterning in an Australian tropical rainforest. We studied three populations that differed in their extent of habitat modification: anthropogenic disturbance (proximate settlement and roads) and internal damage by an invasive alien species, the feral pig (Sus scrofa). Populations were mapped, characterized, and three size cohorts (seedlings, saplings, trees) were analysed using a suite of spatial point pattern analyses (univariate: Diggle's G and F and Ripley's K; bivariate: Diggle's G and Ripley's K). Ryparosa kurrangii has a typical stand structure for a sub‐canopy tree species, but occurs at high densities locally (>400 stems ha^?1). At all sites, the tree cohort were randomly distributed and saplings were spatially aggregated at distances of up to 2–3 m. Between sites there were distinct differences in the size structure and spatial pattern of seedlings, the cohort most affected by recent habitat modification. That is, the least disturbed site had no aggregation among seedlings, the site with the greatest anthropogenic disturbance had many small, clustered seedlings that were spatially associated with trees, and the site with pig damage had clustered seedlings that had no spatial relationship with trees. We propose that habitat modification by anthropogenic and pig disturbance disrupts seed dispersal and establishment regimes, which leads to altered seedling spatial patterns. These disturbances could have long‐term implications for the population structure and health of R. Kurrangii.     

Endozoochory of a dry‐fruited tree aids quarry passive restoration and seed soaking further increases seedling emergence

As practitioners promote passive restoration as a complementary approach to technical reclamation, it is imperative to know its drivers. Although the consequences of endozoochory can be crucial to passive restoration success, few experimental studies assess the use of heavily disturbed sites by seed dispersers such as carnivores and how the seeds they bring in emerge and survive. Using an indoor sowing experiment conducted in a quarry located within a natural park in Portugal, we examined for the first time how carnivore endozoochorous seeds collected in the quarry potentially influence its passive restoration, through effects on plant emergence and survival. Also, we tested whether sowing date and water soaking, relevant factors when sowings are to be carried out, would affect seedling emergence and mortality rates when compared with the effect of endozoochory. Our target species were included in the revegetation plan of the quarry, of which endozoochorous seeds of Carob tree (Ceratonia siliqua) were collected in sufficient number for analysis. Irrespective of the carnivore species, endozoochorous carob seeds performed similarly to untreated seeds regarding emergence rates. Endozoochorous carob seedlings showed greater mortality rates but the net result for the plant can still be the colonization of recently vacant habitats by a large proportion of viable seeds. Concerning sowing date, the later carob seeds were sown over the fruit‐ripening season the faster seedlings emerged. Water soaking increased emergence rate by 6.5 times. Broadly, sowings with previous soaking and carnivore‐mediated seed dispersal of this dry‐fruited tree can jointly enhance quarry restoration.     

Early-season effects of supplemented solar UV-B radiation on seedling emergence, canopy structure, simulated stand photosynthesis and competition for light

Mixtures and monocultures of wheat (Triticum aestivum) and wild oat (Avena fatua), a common weedy competitor of wheat, were exposed to enhanced solar UV-B radiation simulating a 20% reduction in stratospheric ozone to assess the timing and seasonal development of the UV-B effects on light competition in these species. Results from two years of field study revealed that UV-B enhancement had no detectable effect on the magnitude or timing of seedling emergence in either species. End-of-season measurements showed significant UV-B inhibition of leaf insertion height in wild oat in mixture and monoculture in the second year (irrigated year) but not in the first year (drought year). Leaf insertion height of wheat was not affected by UV-B in either year. The UV-B treatment had no detectable effect on monoculture or total (combined species) mixture LAI but did significantly increase (5–7%) the fractional contribution of wheat to the mixture LAI after four weeks of growth in both years. In addition, the UV-B treatment had subtle effects on LAI height profiles with early season mixtures showing significant reductions in wild oat LAI in lower canopy layers in both years while midseason Year 2 mixtures showed significant reductions in wild oat LAI in upper canopy layers. The changes in canopy structure were found to significantly increase (6–7%) the proportional simulated clear sky canopy photosynthesis and light interception of wheat in mixture. These findings, and others, indicate that the effects of UV-B enhancement on competition are realized very early in canopy development and provide additional support for the hypothesis that UV-B enhancement may shift the balance of competition between these species indirectly by altering competitive interactions for light.     

Hide and seek on two spatial scales – vegetation structure effects herbivore oviposition and egg parasitism

Habitat parameters as well as host density and distribution play an important role in host-parasitoid interactions. Vegetation structure can affect both, herbivorous insects searching for places for egg deposition and foraging egg parasitoids. Herbivores might escape egg parasitism by depositing their eggs on sites with vegetation factors unfavourable for host searching parasitoids. The importance of a particular vegetation parameter for the host-parasitoid relationship, however, might depend on the spatial scale. In this study the influence of vegetation structure on oviposition of the polyphagous tansy leaf beetle Galeruca tanaceti L. (Coleoptera: Chrysomelidae) and on egg parasitism by the specialist egg parasitoid Oomyzus galerucivorus was investigated on two spatial scales in the field. Factors like vegetation height and coverage, presence of host plants and microclimate were measured and correlated with presence of herbivore eggs as well as incidence or rate of parasitism. On a larger scale 25 sites (patch size = 300–2000 m²) of suitable semi-arid grassland in different successional stages were studied. The probability that egg clutches of G. tanaceti were present at a site increased with a decreasing area covered by shrubs. Parasitism rates on the macro-site scale showed the same trend and were also higher at sites with a lower percentage of shrubs covering the area.On a smaller scale vegetation structure was investigated using 40 patches (patch size = 1 m²). While the best predictor for the presence of beetle egg masses on this scale was the complexity of vegetation, parasitism was influenced by the presence of a host plant, Achillea millefolium, and a sunny microclimate.In Wirt-Parasitoid Interaktionen spielen Wirtsdichte und -verteilung, aber auch Habitat-Parameter eine wichtige Rolle. Die Vegetationsstruktur kann sowohl herbivore Insekten auf der Suche nach Ei- ablageplätzen als auch ihre Eiparasitoide auf der Suche nach Wirten beeinflussen. Herbivore Insekten könnten einer Eiparasitierung dadurch entgehen, dass sie ihre Eier an Orte legen, die für Parasitoide ungünstige Vegetations-Parameter aufweisen. Die Bedeutung eines Vegetations-Parameters auf die Wirt-Parasitoid Beziehung kann von der räumlichen Skala abhängen, die untersucht wird. Diese Arbeit untersucht den Einfluss der Vegetationsstruktur auf die Eiablage des polyphagen Rainfarnblattkäfers Galeruca tanaceti L. (Coleoptera: Chrysomelidae) und auf die Parasitierung durch den spezialisierten Ei-Parasitoiden Oomyzus galerucivorus auf zwei räumlichen Skalen im Freiland. Verschiedene Vegetations-Parameter wie die Vegetationshöhe, der Deckungsgrad, das Wirtspflanzenvorkommen und das Mikroklima wurden gemessen und mit dem Vorhandensein von Eigelegen und Parasitierung korreliert. Auf der großflächigen Skala wurden 25 Flächen (Größe = 300–2000 m²) semiariden Grasslands in verschiedenen Sukzessionsstadien untersucht. Die Wahrscheinlichkeit, dass Eigelege von G. tanaceti auf einer Fläche gefunden wurden nahm mit sinkendem Verbuschungsgrad zu. Die Parasitierungsrate auf der großflächigen Skala nahm ebenfalls mit sinkendem Verbuschungsgrad zu.Kleinflächig wurde die Vegetationsstruktur in 40 Quadraten (Größe = 1 m²) untersucht. Während die Eiablage des Käfers am stärksten von der Komplexität der Vegetation beeinflusst wurde, wurde die Parasitierungsinzidenz durch das Vorkommen der Wirtspflanze Achillea millefolium und durch ein sonniges Mikroklima bestimmt.     

贺兰山地区植被冠层物候与树干形成层物候的关系

基于贺兰山地区98棵油松树轮样本的宽度数据、植被归一化指数(NDVI)数据以及土地覆被数据,采用VS-oscilloscope模型模拟的油松径向生长过程,研究植被冠层与树干形成层物候之间的联系.结果表明:林地冠层与油松形成层生长结束期(EOS)显著相关,且高于草地与形成层之间的相关.油松生长开始期(SOS)和EOS分别...     

Soil steaming effects on weed seedling emergence under the influence of soil type,soil moisture,soil structure and heat duration

Soil steaming applied in bands is a new technology with the potential to radically lower the burden of hand‐weeding intra‐row weeds in non‐herbicidal vegetable cropping. Preliminary studies with band‐steaming have shown effective control of viable weed seeds when the maximum soil temperatures reach 60–80°C. This temperature range has a particular agronomic interest, and the present study aimed at investigating the influence of soil factors and heat duration on weed seed mortality of soil steaming targeting 60–80°C. Two soil types (sand versus sandy loam) and two moisture levels (moist versus dry) were studied in one experiment (expt) while two levels of structure of a sandy loam (coarse versus fine) were included in a second experiment. A third experiment was focussing on the significance of heat duration expressed as the speed of cooling‐down after steaming had been stopped. Weed control efficacy was generally greater in sand than in sandy loam and soil irrigation further improved weed control. Steam application to the finely structured soil improved weed control efficacy relative to that obtained with the coarse soil of larger aggregate size. The rapidity of cooling from the maximum temperature did not affect the efficacy of the treatment on weed seed mortality. Based on these experiments, a maximum soil temperature of 80°C should ensure satisfactory weed control under moist soil conditions, especially if the soil is cultivated prior to steaming.     

不同光环境下枫香幼苗的叶片解剖结构

落叶阔叶树种枫香是常绿阔叶林的优势种之一。对比研究了重庆丰都世坪林场不同光环境下枫香幼苗叶片的解剖特征,结果表明:1)旷地枫香叶片的厚度大,气孔个体较大且排列紧密,栅栏组织发达,具有明显的旱生叶特征;而林下的枫香叶片明显变薄,气孔小且排列稀疏,海绵组织发达,具有一定的阴生叶特征;2)不同光环境下,气孔密度、栅栏组织的厚度和层数、栅栏细胞长度以及栅栏组织与海绵组织厚度比(P/S)等具有较高的可塑性,表明这些因素对于枫香适应不同光环境具有较大的作用;3)从叶片解剖特征与功能的统一来看,阳性树种枫香幼苗更适合生长在旷地等高光生境中,而不易生长在郁闭的林下;4)落叶阔叶树种在中亚热带常绿阔叶林群落恢复与演替中具有重要作用。     

Phenological interactions between tree and understory herbaceous vegetation of a sahelian semi-arid savanna

This study was conducted during the 1990 planting cycle and focused on interactions between trees and the herbaceous layer caused by their phenological behaviour in a Sahelian semi-arid savanna.The results show that (1) the plant phenological development (tree and grassy group) was synchronous; (2) the tree phenological behaviours were most clearly differentiated because reproduction, flowering and fructification phases were spread out throughout the year and (3) the herbaceous species cycle was then essentially determined by the short biologically damp period, 60 days in 1990. The foliage stage of the ligneous species spread on the considered period, with a reduced realization rate (30 to 40%) due to an invasion of locusts.At the grassy group level, the considerable growth rate in every development stage indicated that there was apparently no competition for soil water. Both trees and grass used the available soil water with grass only exploiting the upper soil layer.Mots-clés: Arbre, Compétition, Couvert, Sahel, Stades phénologiques, Strate herbacéeRésumé Cette étude se propose d'établir les interactions Herbe/Arbre à travers les stades phénologiques des principales espèces ligneuses et de la strate herbacée établie sous l'arbre à partir d'observations menées au cours du cycle 1990.Les résultats montrent une superposition des cycles phénologiques du peuplement ligneux et du groupement herbeux. La phase de feuillaison des espèces ligneuses étudiées s'étale sur toute la période considérée. Le taux d'accomplissement des différentes phénophases est généralement faible (30 à 40%) en raison de l'invasion par des criquets.Le taux d'accomplissement des phénophases au niveau du groupement herbeux indique qu'il n'y a apparemment pas de compétition pour l'eau; le partage de l'eau du sol entre l'herbe et l'arbre se faisant principalement dans l'espace.