Foliage acceptability to browsing ruminants in relation to seasonal changes in the leaf chemistry of woody plants in a South African savanna

Summary We investigated seasonal changes in food selection by hand-reared kudus and impalas in savanna vegetation in northern Transvaal, South Africa. The acceptability of the leaves of woody plants to these animals was compared with leaf concentrations of nutrients, fibre components and old leaf phenophases. No consistently significant correlation was found between acceptability and any single chemical factor. Based on an a priori palatability classification, discriminant function analysis separated relatively palatable species from unpalatable species in terms of a linear combination of protein and condensed tannin concentrations. The high acceptability of certain otherwise unpalatable species during the new leaf phenophase was related to elevation of protein levels relative to condensed tannin contents. Species were added to the diet during the dry season approximately in the order of their relative protein-condensed tannin difference.     

Diel changes in forager size, activity, and load selectivity in a tropical leaf-cutting ant, Atta cephalotes

Abstract. 1. The leaf-cutting ant Atta cephalotes (L.) in a Costa Rican tropical moist forest showed diel changes in foraging activity. In most colonies studied, foraging was primarily nocturnal, although in a few colonies it was primarily diurnal.
2. In all colonies studied, mean forager mass was larger at night than during the day.
3. At night, most foragers carried freshly cut leaf fragments, whereas during the day a large proportion carried dried fragments and other vegetable matter collected from along their trail.
4. Along one trail, where foraging was primarily nocturnal, the match between ant mass and load mass was compared for laden ants at night and during the day. Laden ants at night were larger, carried relatively heavier loads, and showed a higher degree of matching between their mass and load mass than those foraging during the day.
5. A comparison of load masses of ants coming down a local tree and of ants picking up marked fragments from along their trail suggested that the diel difference in load mass and in the match between ant mass and load mass were related to the greater proportion of ants carrying freshly cut leaf fragments at night. Fresh fragments weighed more due to higher water content, and the match between ant mass and load mass was greater for ants cutting fresh fragments than for ants picking up abandoned fragments from along their trail.
6. Possible explanations for the diel changes in forager size and activity are discussed.     

Relationships between the effects of insect herbivory and sheep grazing on seasonal changes in an early successional plant community

Summary The effects of spring grazing by sheep and of natural levels of insect herbivory were studied in 1985 on a limestone field abandoned from arable land for four years. A split-plot design was adopted in which paddocks, arranged in Latin squares, were either left ungrazed or heavily grazed by sheep for ten days in April. Within each paddock plots were either sprayed regularly with Malathion-60 or untreated.Natural levels of insect herbivory, compared to the reduced levels in insecticide-treated plots, had effects of similar magnitude to those from the short burst of spring grazing. Many attributes of the grazed/insecticide-treated sward were either increased or decreased by a factor of two within a season. Both types of herbivore caused changes in the direction of plant succession as well as in its rate. Effects on early successional species were large and similar when caused by either type of herbivore. Effects on later successional species were often smaller, but also showed differences in the action of the two herbivore types, as did effects on sward height, species richness and total cover. The effects of sheep and insect herbivory were not always additive or in the same direction.The results suggest that manipulations of both mammal and insect herbivores may be powerful tools for directing changes in plant community composition.     

Contributions of C3 and C4 plants to higher trophic levels in an Amazonian savanna

We studied the energy flow from C₃ and C₄ plants to higher trophic levels in a central Amazonian savanna by comparing the carbon stable-isotope ratios of potential food plants to the isotope ratios of species of different consumer groups. All C₄ plants encountered in our study area were grasses and all C₃ plants were bushes, shrubs or vines. Differences in δ¹³C ratios among bushes (xˉ = −30.8, SD = 1.2), vines (xˉ = −30.7, SD = 0.46) and trees (xˉ = −29.7, SD = 1.5) were small. However the mean δ¹³C ratio of dicotyledonous plants (xˉ = −30.4, SD = 1.3) was much more negative than that of the most common grasses (xˉ = −13.4, SD = 0.27). The insect primary consumers had δ¹³C ratios which ranged from a mean of −29.5 (SD = 0.47) for the grasshopper Tropidacris collaris to a mean of −14.7 (SD = 0.56) for a termite (Nasutitermes sp.), a range similar to that of the vegetation. However, the common insectivorous and omnivorous vertebrates had intermediate values for δ¹³C, indicating that carbon from different autotrophic sources mixes rapidly as it moves up the food chain. Despite this mixing, the frogs and lizards generally had higher values of δ¹³C (xˉ = −21.7, SD = 1.6; xˉ = −21.9, SD = 1.8, respectively) than the birds (xˉ = −24.8, SD = 1.8) and the only species of mammal resident in the savanna (xˉ = −25.4), indicating that they are generally more dependent on, or more able to utilise, food chains based on C₄ grasses. Received: 7 May 1998 / Accepted: 30 November 1998     

黑蚁对雪松长足大蚜集群时空变化的取食反应

我们研究了黑蚁 (Lasiusniger)如何开发距离其巢不同距离的多种食物资源 ,以及黑蚁的照看活动是否与雪松长足大蚜 (Cinaracedri)种群的增长有关 ,以及 /或者受空间分布的影响。笔者定期调查了Cinaracedri集群的空间分布 ,以及在雪松树枝上黑蚁取食的时间变化 ,发现只有 2 5 %的蚜虫集群在研究后数周内保持在同样的位置 ,这表明蚜虫集群的高度运动性迫使黑蚁改变活动路线以获取事物资源。实际上 ,黑蚁集群能选择最近的食物资源以及蚜虫数量最多的地点     

Response of Salvinia molesta to insect damage: changes in nitrogen,phosphorus and potassium content

Summary When adults and larvae of the weevil Cyrtobagous salviniae destroyed buds and tunnelled through rhizomes of Salvinia molesta, the plant responded by producting new growth which contained higher concentrations of nitrogen than in undamaged plants or in the older parent tissue of the same plant. Damage to leaves by the moth, Samea multiplicalis did not induce the same response. C. salviniae fed on the new growth and the higher nitrogen intake would have increased its reproductive capacity and enhanced its action as a biological control agent.Damage by both insects resulted in potassium leaching from the plant but no change in concentrations of phosphorus. The results support earlier suggestions that damage by C. salviniae might improve the qualtity of the host plant for this herbivore.     

In praise of the everyday: Trust and the art of social living in an Amazonian community

The article states the high evaluation that an Amazonian people, the Piaroa, place upon the artful skills of everyday existence. It is argued that their emphasis upon the creativity of daily practice is forthcoming from a powerful and egalitarian social philosophy. The difficulties of translating such a philosophy, where the human self is contextualised within a wider cosmic setting, are raised. The aim of translation would be to enable us to engage in dialogue with the Piaroa about common concerns (upon the relation of the individual to the collectivity, for instance, or upon the idea of freedom, or the question of the relation of customs to rational decision making). These are a people who overtly shun the idea of a social rule, yet strongly value sociality, their own customs, and the mutuality of the ties of community. At the same time they demonstrate even more forcefully an ‘obstinate individualism’. A major puzzle to be discussed is the notion that personal autonomy is understood as a social capacity, and a cultural one as well: the volitional I, the social relation, and the cultural artifice are an associated set of values. The centrality of the notions of reflective reason and personal trust to this particular egalitarian ethics will be discussed.     

Seed bank and vegetation composition of forest stands of varying age in central Belgium: consequences for regeneration of ancient forest vegetation

Vegetation composition differs significantly between ancientand recent forest, due to slow colonization capacity of typical forest speciesand the higher abundance of early successional species in recent forest.However, little is known about differences in persistent seed bank compositionbetween ancient and recent forest and about the interaction between seed bankand vegetation in relation with forest age. We surveyed the seed bank and theunderstorey vegetationcomposition in transects from ancient to recent forest. Seed bank and fieldlayer vegetation characteristics and similarity between seed bank andvegetationwere analysed in relation to recent forest age and distance to the ancientforest. A total of 39 species and 14,911 seedlings germinated, whichcorresponds with a seed density of 12,426 seeds/m².Total seed density is significantly higher in the youngest recent forest parcel(55 years). Also the seed bank composition in the youngest forest parceldifferssignificantly from the other parcels. After a longer period of reforestation,the seed bank approaches that of the ancient forest, suggesting seed bankdepletion, although the seed bank is permanently replenished to some extent byseed bank forming species from local disturbances. Seed bank composition doesnot change significantly with distance to the ancient forest. Similaritybetween seed bank and vegetation composition, nomatter the forest age, is very low, but decreases with increasing forest age.The most frequent species in the vegetation are absent in the seed bank andvice versa. The contribution of forest species is highin the vegetation and they almost not occur in the seed bank, while species offorest edges and clearings, and species of disturbed environments are morefrequent in the seed bank. The seed bank is mainly composed of earlysuccessional species of former forest stages or species which temporary occurinsmall-scale disturbances. The seed bank may enhance the negative effects ofearlysuccessional, mainly competitive species to the forest species richness in therecent forest. In this respect, forest management should minimise forestdisturbances, to prevent germination of competitive species form buriedseeds.     

Morphological and reproductive responses of dominant plant species to local conditions in herbaceous successional stages of a calcareous hillside

Secondary succession on calcareous hillsides, initiated following the abandonment of agro-pastoral practices, is characterized by the transformation of initial short sward into tall herbaceous vegetation and finally woody formations. These structural changes are accompanied by modifications in ecological conditions but some species are able to persist and flower along the successional gradient. In this study, we compare reproductive and morphological traits of seven perennial species present in three successional stages [short grassland (SG), tall grassland (TG), and encroached grassland (EG)] to test if plant species present modifications along the successional gradient. The results show that morphological traits as height, leaf area and leaf dry mass increase for all studied species. Along the successional gradient, Teucrium chamaedrys increases ramet biomass more than twice (+145%) while Brachypodium pinnatum increases it even more (+340%). Maximum specific leaf area was observed in Brachypodium pinnatum (SLA = 20.4 mm² mg⁻¹) in SG, whereas Helianthemum nummularium and Teucrium chamaedrys have both rising SLA and falling leaf dry matter content in TG and EG. Concurrently, Helianthemum nummularium, Hippocrepis comosa and Teucrium chamaedrys show a clear decrease in sexual reproduction, with between 18 and 40% fewer flowers during the progress of succession. By contrast, Festuca lemanii, Sesleria caerulea and Brachypodium pinnatum increase their flower numbers per inflorescence (×3.6, ×3.3, and ×3.5, respectively). Festuca lemanii and Sesleria caerulea increase seed production to a maximum in TG, and Brachypodium pinnatum and Carex flacca increase seed output in TG and EG (×4.5, ×5.5, respectively). Changes in plants traits underscore the idea that species allocate resources differently probably because of increasing competition and decreasing edaphic constraints. This study also implies the potential existence of specific responses allowing plants to maintain existence within a plant community and explaining their contrasting performance during the succession.     

Plant-mediated interactions between insects and a fungal plant pathogen and the role of plant chemical responses to infection

Diverse organisms simultaneously exploit plants in nature, but most studies do not examine multiple types of exploiters like phytophagous insects and fungal, bacterial, and viral plant pathogens. This study examined patterns of induction of antipathogenic peroxidase enzymes and phenolics after infection by the cucurbit scab fungus, Cladosporium cucumerinum, and then determined if induction mediated ecological effects on Colletotrichum orbiculare, another fungal pathogen, and two insect herbivores, spotted cucumber beetles, and melon aphids. Peroxidase induction occurred in inoculated, `local,' symptom-bearing leaves 3 days after inoculation, and in `systemic,' symptom-free leaves on the same plants 1 day later. Phenolics were elevated in systemic but not in local leaves 3 days after inoculation. Detached systemic leaves from plants inoculated with C. cucumerinum developed significantly fewer and smaller lesions after challenge with C. orbiculare. Spotted cucumber beetles did not show consistently significant preferences for infected versus control leaf disks in comparisons using local or systemic leaves, but trends differed significantly between leaf positions. In no-choice tests, beetles removed more leaf area from local but not from systemic infected leaves compared to control leaves, and melon aphid reproduction was enhanced on local infected leaves. In the field, cucumber beetle and melon aphid densities did not differ between infected and control plants. Antipathogenic plant chemical responses did not predict reduced herbivory by insects. Other changes in metabolism may explain the positive direction and spatially dependent nature of plant-mediated interactions between pathogens and insects in this system. Received: 28 September 1997 / Accepted: 9 February 1998     

Relative availability of nitrogen in host plants of invertebrate herbivores: three possible nutritional and physiological definitions

Summary It has been postulated that free amino acids in the diet of herbivores are a more readily available source of nitrogen than are amino acids in intact proteins. When plants are subjected to any of a number of stress factors the concentration of free amino acids in their tissues may increase relative to the protein content. It has been further postulated that such changes in available nitrogen can increase the survivorship of very young herbivores feeding on the tissues of stressed plants. In this paper, the concept of readily available nitrogen is given more specific definitions in the effort to uncover an underlying nutritional or physiological mechanism responsible for such changes in larval survivorship. The first hypothesis considered is that amino acids are more readily available than proteins in the sense that, as they occur in plant tissue, they are a more nutritious source of needed nitrogenous compounds. The second hypothesis considered is that amino acids are absorbed into the insect gut free from interference by proteinase inhibitors, and thus are more chemically available than are proteins. Finally, free amino acids, because of their solubility and greater mobility, may be more available to herbivores because of physical factors restricting the movement and digestion of proteins from plant tissue into the insect gut. The applicability of each hypothesis to certain types of insects feeding on particular plant tissues is discussed.The investigation reported in this paper (No. 86-7-224) is in connection with a project of the Kentucky Agricultural Experiment Station and is published with the approval of the Director     

Leaf nitrogen and phosphorus concentrations of woody plants differ in responses to climate,soil and plant growth form

Leaf chemistry is important in predicting the functioning and dynamics of ecosystems. As two key traits, leaf nitrogen (N) and phosphorus (P) concentrations set the limits for plant growth, and leaf N:P ratios indicate the shift between N‐ and P‐limitation. To understand the responses of leaf chemistry to their potential drivers, we measured leaf N and P concentrations of 386 woody species at 14 forest sites across eastern China, and explored the effects of climate, soil, and plant growth form on leaf N, P and N:P ratios. In general, leaf N and P were both negatively related to mean annual temperature and precipitation, and positively related to soil N and P concentrations. Leaf N:P ratios showed opposite trends. General linear models showed that variation in leaf N was mainly determined by a shift in plant growth form (from evergreen broadleaved to deciduous broadleaved to conifer species) along the latitudinal gradient, while variations in leaf P and N:P were driven by climate, plant growth form, and their interaction. These differences may reflect differences in nutrient cycling and physiological regulations of P and N. Our results should help understand the ecological patterns of leaf chemical traits and modeling ecosystem nutrient cycling.     

Perceptions, use and availability of woody plants among the Gourounsi in Burkina Faso

The present study was conducted in five villages around the Nazinga Game Ranch, Burkina Faso. Fifty informants of the Gourounsi ethnic group were used. Eighty-one useful woody species were identified, out of a total of 110 woody species in the area. Woody plant use was quantified as: edible fruits (28 species), vegetable sauce (22 species), firewood (33 species), construction (29 species) and medicine (64 species and 167 remedies). Further, species accumulation curves were used to estimate that more than 650 remedies for medicine are used in the area. The community's knowledge of plants was analysed in relation to age, gender, village of residence and amount of intercultural visits. The knowledge pattern was remarkably uniform, with only two variations: (1) men generally identified more edible fruit trees than women, and (2) one village reported more firewood species than the other four villages. The informants had a variety of opinions concerning the availability of useful plants, but the majority found the availability to be fine and 47% of the informants searched for useable products in the savanna on a daily basis. No correlation was found between an informant's impression of useful products availability and his/her frequency of visits into the savanna. The results show that the Gourounsi people live in intimate relation with the savanna surrounding their villages. They are aware of the environment and are willing to learn and adopt new conservation practices.     

How plants shape the ant community in the Amazonian rainforest canopy: the key role of extrafloral nectaries and homopteran honeydew

Ant-plant interactions in the canopy of a lowland Amazonian rainforest of the upper Orinoco, Venezuela, were studied using a modified commercial crane on rails (Surumoni project). Our observations show a strong correlation between plant sap exudates and both abundance of ants and co-occurrence of ant species in tree canopies. Two types of plant sap sources were compared: extrafloral nectaries (EFNs) and honeydew secretions by homopterans. EFNs were a frequent food source for ants on epiphytes (Philodendron spp., Araceae) and lianas (Dioclea, Fabaceae), but rare on canopy trees in the study area, whereas the majority of trees were host to aggregations of homopterans tended by honeydew-seeking ants (on 62% of the trees examined). These aggregations rarely occurred on epiphytes. Baited ant traps were installed on plants with EFNs and in the crowns of trees from three common genera, including trees with and without ant-tended homopterans: Goupia glabra (Celastraceae), Vochysia spp. (Vochysiaceae), and Xylopia spp. (Annonaceae). The number of ant workers per trap was significantly higher on plants offering one of the two plant sap sources than on trees without such resources. Extrafloral nectaries were used by a much broader spectrum of ant species and genera than honeydew, and co-occurrence of ant species (in traps) was significantly higher on plants bearing EFNs than on trees. Homopteran honeydew (Coccidae and Membracidae), on the other hand, was mostly monopolised by a single ant colony per tree. Homopteran-tending ants were generally among the most dominant ants in the canopy. The most prominent genera were Azteca, Dolichoderus (both Dolichoderinae), Cephalotes, Pheidole, Crematogaster (all Myrmicinae), and Ectatomma (Ponerinae). Potential preferences were recorded between ant and homopteran species, and also between ant-homopteran associations and tree genera. We hypothesize that the high availability of homopteran honeydew provides a key resource for ant mosaics, where dominant ant colonies and species maintain mutually exclusive territories on trees. In turn, we propose that for nourishment of numerous ants of lower competitive capacity, Philodendron and other sources of EFNs might be particularly important.     

Nickel hyperaccumulation as an anti-herbivore trait: considering the role of tolerance to damage

Metal hyperaccumulation is a striking trait exhibited by many plant species, but the evolutionary ecology of metal hyperaccumulation is poorly understood. It has been widely hypothesized that metal hyperaccumulation evolved to protect plants from herbivory. However, there is currently little evidence that metal hyperaccumulation enhances the fitness of plants in the presence of herbivory. In this study, we conducted a multi-factor greenhouse experiment to examine the effects of two soil nickel concentrations (unamended (0 μg/g) and nickel amended (600 μg/g)), and three levels of artificial damage (0, 10 and 50%) on the growth of plants from two populations of Thlaspi montanum var. montanum. We observed a significant interaction between soil nickel and artificial damage. An a posteriori analysis of this interaction revealed that the presence of nickel significantly improved the ability of T. montanum to tolerate the negative effects of intense damage. Our results indicate that metal hyperaccumulation could benefit T. montanum by increasing its tolerance to damage. This study suggests that there is a potential for the evolution of metal hyperaccumulation in response to intense herbivory on T. montanum.     

以龟裂地植丛发生演替为例探讨植物对环境的反应与反馈效应

准葛尔盆地荒漠龟裂地植丛发生演替的研究结果表明：在荒漠龟裂地发生演替的初始阶段,环境因子对植物发生具有极大的限制作用。但是,一旦有先锋植物侵入,植物就会通过自身的生命活动不断改善其生存环境,为新植物的进入创造条件,新植物的定居又会反作用于环境,使其生存条件进一步改善,如此往复,推动整个微生态系统由简单至繁杂呈顺序性、方向性发展。生命的存在及其活动过程对环境的反应和对于环境的反馈作用,是导致生态系统顺向发展的根本动力。该研究将为如何改善生境质量提供理论基础,对植被恢复及受损生态系统的重建,特别是对加深全球变化环境下气候与植被关系的理解,提高预测陆地生态系统对全球变化反应的能力有重要的意义。     

Allometry and development in herbaceous plants: functional responses of meristem allocation to light and nutrient availability

We examined the relationship between meristem allocation and plant size for four annual plant species: Arabidopsis thaliana, Arenaria serphyllifolia, Brassica rapa, and Chaenorrhinum minus. Gradients of light and nutrient availability were used to obtain a range of plant sizes for each of these species. Relative allocation to reproductive, inactive, and growth meristems were used to measure reproductive effort, apical dominance, and branching intensity, respectively. We measured allocation to each of these three meristem fates at weekly intervals throughout development and at final developmental stage. At all developmental stages reproductive effort and branching intensity tended to increase with increasing plant size (i.e., due to increasing resource availability) and apical dominance tended to decrease with increasing plant size. We interpret these responses as a strategy for plants to maximize fitness across a range of environments. In addition, significant differences in meristem response among species may be important in defining the range of habitats in which a species can exist and may help explain patterns of species competition and coexistence in habitats with variable resource availability.