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1.
A three-state, discrete-time Markov chain is used to model the dynamics of energy flow in a tri-trophic food web. The distribution of energy in the three trophic levels is related to the rates of flow between the trophic levels and calculated for the entire range of possible flow values. These distributions are then analysed for stability and used to test the idea that plants are resource-limited and herbivores are predation-limited. Low rates of death and decomposition, when coupled with low rates of herbivory and carnivory, tend to destabilize this food web. Food webs with higher rates of death and decomposition are relatively more stable regardless of rates of herbivory and carnivory. Plants are more prone to resource-limitation and herbivores are, in general, limited by their predators, which supports Hairston et al. (Am. Nat. 94 (1960) 421). The rate of decomposition often mediates the roles of top-down and bottom-up control of energy flow in the food web.  相似文献   

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Foregut emptying in Blennius pholis at 16–18°C is exponential ( St = So e-Rt; R=0.006 when t is in min and s in mg dry weight). In consequence, the time for the foregut to empty completely varies in proportion to ln (meal size) in a chosen size of fish or to ln (body weight) in different sized fish fed to the same relative amount (% body weight). Appetite returns rapidly in comparison with other species (4–15 h for fish of 0.5−30 g) and, because of the tidal constraints imposed on this littoral fish, voluntarily ingested meals are large (14–9% body weight depending on fish size). Studies on digestion in small, stomachless fish are not adequately made using X-radiography and must be supported by sequential gut samples after a meal.  相似文献   

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Summary The peach palm, Bactris gasipaes H.B.K., in Costa Rica, possesses unusual trichomes on the inflorescence epidermal surface. Certain cells of the trichome possess a thick, highly lignified cell wall and are consumed by the beetle Cyclocephala amazona L. before it ingests pollen from the same inflorescence. Chemical analyses show the trichome to possess no nutritive value. The thick-walled trichome cells pass intact through the beetle's digestive system, while ingested pollen is crushed. We suggest that the specialized plant cells function as gastroliths in the beetle's digestive tract.  相似文献   

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The adaptive significance of the giant form of Oxytricha bifaria has been studied. It proved to be a carnivorous phase of a bacterivorous ciliate, rather than a cannibalistic one. By such a dramatic reversal in the feeding behavior O. bifaria, a primary consumer, can reversibly become a secondary one.  相似文献   

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The digestion of various carbohydrates and synthetic substrates by the gut of Locusta migratoria was analysed quantitatively. Maltose, starch, and sucrose were found to be hydrolysed most rapidly, whereas the splitting of cellobiose, trehalose, lactose, and melecitose took place at much slower rates.The absolute carbohydrase activities in foregut and midgut are nearly equal. However, specific activities are much higher in the foregut. Only low activities were found in extracts from the hindgut and salivary glands. The latter show a pattern of sugar splitting which is different from that found in gut preparations.The distribution of carbohydrase activities between the epithelia and lumina of the foregut, midgut, and hindgut and between soluble and particulate fractions were studied. The midgut epithelium is shown to have a particularly high content of enzymes, although some carbohydrases are rather active also in the epithelium of the hindgut. During hunger periods the relative enzymatic activities of the epithelium are distinctly increased.The isolation and purification of the carbohydrases were attempted and a partial separation of individual enzymes was obtained by gel-filtration. These results indicate the presence of at least seven distinct carbohydrases in the locust gut. The molecular weights of the enzymes were estimated by gel-filtration, and KM values and pH-optima are reported.  相似文献   

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In a capture-mark-recapture fish density estimate carried out in a small freshwater pond in southern Ontario, Canada, a lift net and minnow traps were used as catching gear. Both worked well for initial captures, but most recaptures were in the traps. Density estimates within reasonable confidence limits were possible for only two species, creek chub (Semotilus atromaculatus) and common shiner (Notropis cornutus). The other 13 species present were either missed entirely or not captured or recaptured sufficiently so estimates for them were inadequate. Subsequent collection of fishes after rotenone poisoning verified the estimates for the two species, if the size selectivity of the gear and the possible effects of marking and limitations of collecting after poisoning were considered.  相似文献   

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Summary Four immunoreactive endocrine cell types can be distinguished in the pancreatic islets of B. conchonius: insulin-producing B cells, somatostatin-producing A1 (= D) cells, glucagon-producing A2 cells and pancreatic poly-peptide-producing PP cells. The principal islet of this species contains only a few PP cells, while many PP cells are present in the smaller islets. Except for the B cell all pancreatic endocrine cell types are also present in the pancreatic duct.At least six enteroendocrine cell types are present in the gut of B. conchonius: 1. a cell type (I) with small secretory granules, present throughout the intestine, and possibly involved in the regulation of gut motility; 2. a C-terminal gastrin immunoreactive cell, probably producing a caerulein-like peptide; these cells are located at the upper parts of the folds, especially in the proximal part of the intestinal bulb; 3. a met-enkephalin-immunoreactive cell, present throughout the first segment; 4. a glucagon-immunoreactive cell, which is rare in the first segment; 5. a PP-immunoreactive cell, mainly present in the first half of the first segment; 6. an immunoreactive cell, which cannot at present be specified, located in the intestinal bulb. The latter four cell types are mostly located in the basal parts of the folds, although some PP-immunoreactive cells can also be found in the upper parts.Most if not all enteroendocrine cells are of the open type. The possible functions of all enteroendocrine cell types are discussed.Abbreviations BPP bovine pancreatic polypeptide - CCK cholecystokinin - GEP gastro-entero-pancreatic - GIP gastric inhibitory peptide or glucose-dependent insulin releasing peptide - PPP pig pancreatic polypeptide - VIP vasoactive intestinal polypeptide  相似文献   

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SUMMARY Evolution proceeds by the selection of characters that enhance survival rates so that the long-term outcome for a species is better adaptation to its environment. These new characters are "accidentally" acquired, mostly through mutations leading to modifications of developmental events. However, changes that lead to the ectopic expression of an organ are rare and, whereas their subsequent selection for a new role is even more rare, such a scenario has apparently occurred for denticles in some teleost fish. Small, conical denticles are present, mainly on the dermal bones of the head, in a few, unrelated lineages of living teleosts. Here, I show that the morphology and structure of the denticles in Atherion elymus , an atheriniform, is similar to those of teeth inside the oral cavity. These denticles are not derived evolutionarily from odontodes of early vertebrates, nor do they represent a re-expression as such (i.e., a long-lasting ability to make odontodes outside the oral cavity). Teeth and odontodes are homologous organs but the origin of the denticles is to be found in teeth, not in odontodes. The denticles are simply teeth that form outside the mouth, probably derived from a sub-population of odontogenically pre-specified neural crest cells. These "accidental" extra-oral teeth have arisen independently in these lineages and were selectively advantageous in a hydrodynamic context.  相似文献   

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Termites consume an estimated 3–7 billion tonnes of lignocellulose annually, a role in nature which is unique for a single order of invertebrates. Their food is digested with the help of microbial symbionts, a relationship that has been recognized for 200 years and actively researched for at least a century. Although DNA- and RNA-based approaches have greatly refined the details of the process and the identities of the participants, the allocation of roles in space and time remains unclear. To resolve this issue, a pioneer study is reported using metabolomics to chart the in situ catabolism of 13C-cellulose fed to the dampwood species Hodotermopsis sjostedti. The results confirm that the secretion of endogenous cellulases by the host may be significant to the digestive process and indicate that a major contribution by hindgut bacteria is phosphorolysis of cellodextrins or cellobiose. This study provides evidence that essential amino acid acquisition by termites occurs following the lysis of microbial tissue obtained via proctodaeal trophallaxis.  相似文献   

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BACKGROUND AND AIMS: Roridula plants capture insects but have no digestive enzymes. It has been hypothesized that Roridula leaves absorb nitrogen from the faeces of obligately associated, carnivorous hemipterans. But rapid movement across the leaf surfaces of most plant leaves is prevented by the presence of an impermeable cuticle. However, in carnivorous plants, cuticular gaps or pores in digestive/absorptive cells allow rapid movement across the leaf surface. Recently, it was suggested that the hemipteran-plant interaction constituted a new pathway for plant carnivory. Here, a further adaptation to this pathway is described by demonstrating how Roridula plants probably absorb hemipteran faeces rapidly through their leaf cuticles. METHODS: The dye neutral red was used to document the rapidity of foliar absorption and TEM to determine the nature of cuticular discontinuities in the leaf of Roridula. KEY RESULTS: Aqueous compounds diffuse rapidly across the cuticle of Roridula's leaves but not across the cuticles of co-occurring, non-carnivorous plant leaves. Furthermore, immature Roridula leaves were unable to absorb neutral red whereas mature leaves could. Using TEM, cuticular gaps and pores similar to those in other carnivorous plants were found in the epidermal cells of mature Roridula leaves. CONCLUSIONS: The leaf cuticle of Roridula is very thin (0-120 nm) and cell wall elements project close to the leaf surface, possibly enhancing foliar absorption. In addition to these, cuticular gaps were frequently seen and probably perform a function similar to those found in other carnivorous plants: namely the absorption of aqueous compounds. The cuticular gaps of Roridula are probably an adaptation to plant carnivory, supporting the newly described pathway.  相似文献   

12.
In bacterial communities, "tight economic times" are the norm. Of the many challenges bacteria face in making a living, perhaps none are more important than generating energy, maintaining redox balance, and acquiring carbon and nitrogen to synthesize primary metabolites. The ability of bacteria to meet these challenges depends heavily on the rest of their community. Indeed, the most fundamental way in which bacteria communicate is by importing the substrates for metabolism and exporting metabolic end products. As an illustration of this principle, we will travel down a carbohydrate catabolic pathway common to many species of Bacteroides, highlighting the interspecies interactions established (often inevitably) at its key steps. We also discuss the metabolic considerations in maintaining the stability of host-associated microbial communities.  相似文献   

13.
Models are examined in which two prey species compete for two nutrient resources, and are preyed upon by a predator that recycles both nutrients. Two factors determine the effective relative supply of the nutrients, hence competitive outcomes: the external nutrient supply ratio, and the relative recycling of the two nutrients within the system. This second factor is governed by predator stoichiometry--its relative requirements for nutrients in its own biomass. A model with nutrient resources that are essential for the competing prey is detailed. Criteria are given to identify the limiting nutrient for a food chain of one competitor with the predator. Increased supply of this limiting nutrient increases predator density and concentration of this nutrient at equilibrium, while decreasing the concentration of a non-limiting nutrient. Changes in supply or recycling of a non-limiting nutrient affect only the concentration of that nutrient. Criteria for the invasion of a second prey competitor are presented. When different nutrients limit growth of the resident prey and the invader, increased supply or recycling of the invader's limiting nutrient assists invasion, while increased supply or recycling of the resident's limiting nutrient hinders invasion. If the same nutrient limits both resident and invader, then changes in supply and recycling have complex effects on invasion, depending on species properties. In a parameterized model of a planktonic ecosystem, green algae and cyanobacteria coexist over a wide range of nitrogen:phosphorus supply ratios, without predators. When the herbivore Daphnia is added, coexistence is eliminated or greatly restricted, and green algae dominate over a wide range of supply conditions, because the effective supply of P is greatly reduced as Daphnia rapidly recycles N.  相似文献   

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We model a metapopulation of predator-prey patches using both spatially implicit or mean-field (MF) and spatially explicit (SE) approaches. We show that in the MF model there are parameter regimes for which prey cannot persist in the absence of predators, but can in their presence. In addition, there are parameter regimes for which prey may persist in isolation, but the presence of predators will increase prey patch density. Predators may thus enhance prey persistence and overall abundance. The key mechanism responsible for this effect is the occurrence of prey dispersal from patches that are occupied by both prey and predators. In addition, these patches should be either long-lived, such as that occurs when predators keep prey from overexploiting its local resource, or the presence of a predator on a patch should significantly enhance the prey dispersal out of that patch. In the SE approach these positive effects of predators on prey persistence and abundance occur for even larger parameter ranges than in the MF model. Prey dispersal from predator-prey patches may thus be important for persistence of both species as a community, independent of the modeling framework studied. Comparison of the MF and SE approaches shows that local dispersal constraints can have the edge over global dispersal for the persistence of the metapopulation in regimes where the two species have a beneficial effect on each other. In general, our model provides an example of feedback in multiple-species metapopulations that can make the implementation of conservation schemes based on single-species arguments very risky.  相似文献   

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Summary Enteroendocrine cells containing glucagon-, substance P-, neurotensin- and VIP-like substances have been demonstrated immunocytochemically in the gut of Barbus conchonius. Mainly based on the distribution of the immunoreactive endocrine cells in this and a previous* study, at least eight different enteroendocrine cell types appear to be present in this stomachless fish: 1. C-terminal-gastrinimmunoreactive cells*, predominantly present in the upper parts of the folds of the proximal part of the intestinal bulb. 2. Metenkephalin-immunoreactive cells*, basally located in the folds of the first segment. 3. Pancreatic polypeptide (PP)-immunoreactive cells*, mainly present in the first half of the first segment. 4. Glucagon-like-immunoreactive (GLI) cells that are basally located in the folds of the first segment and that contain a different polypeptide (possibly glicentin) than pancreatic glucagon cells. 5. Substance P-immunoreactive cells, present in the upper parts of the folds throughout the gut. 6. C-terminal-neurotensin-immunoreactive cells, basally located in the folds throughout the first segment. 7. Vasoactive intestinal polypeptide (VIP)-immunoreactive cells, present in small numbers in the proximal part of the intestinal bulb. 8. Nonspecifically-immunoreactive cells*, found throughout the intestinal bulb. Many VIP-immunoreactive nerves have been demonstrated in the smooth muscle layer and myenteric plexus of the gut; furthermore some of them are peptide histidineisoleucine (PHI)-immunoreactive. Substance P-, somatostatin-, neurotensin- and met-enkephalin-immunoreactive nerves are also found. Thus, at least partial sequences of four different mammalian neuropeptide hormones (VIP, substance P, neurotensin, met-enkephalin) occur both in endocrine cells and enteric nerves of the gut of B. conchonius.  相似文献   

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Background and Aims

Ibicella lutea and Proboscidea parviflora are two American semi-desert species of glandular sticky plants that are suspected of carnivory as they can catch small insects. The same characteristics might also hold for two semi-desert plants with glandular sticky leaves from Israel, namely Cleome droserifolia and Hyoscyamus desertorum. The presence of proteases on foliar hairs, either secreted by the plant or commensals, detected using a simple test, has long been considered proof of carnivory. However, this test does not prove whether nutrients are really absorbed from insects by the plant. To determine the extent to which these four species are potentially carnivorous, hair secretion of phosphatases and uptake of N, P, K and Mg from fruit flies as model prey were studied in these species and in Roridula gorgonias and Drosophyllum lusitanicum for comparison. All species examined possess morphological and anatomical adaptations (hairs or emergences secreting sticky substances) to catch and kill small insects.

Methods

The presence of phosphatases on foliar hairs was tested using the enzyme-labelled fluorescence method. Dead fruit flies were applied to glandular sticky leaves of experimental plants and, after 10–15 d, mineral nutrient content in their spent carcasses was compared with initial values in intact flies after mineralization.

Key Results

Phosphatase activity was totally absent on Hyoscyamus foliar hairs, a certain level of activity was usually found in Ibicella, Proboscidea and Cleome, and a strong response was found in Drosophyllum. Roridula exhibited only epidermal activity. However, only Roridula and Drosophyllum took up nutrients (N, P, K and Mg) from applied fruit flies.

Conclusions

Digestion of prey and absorption of their nutrients are the major features of carnivory in plants. Accordingly, Roridula and Drosophyllum appeared to be fully carnivorous; by contrast, all other species examined are non-carnivorous as they did not meet the above criteria.Key words: Roridula gorgonias, Drosophyllum lusitanicum, Proboscidea parviflora, Ibicella lutea, Cleome droserifolia, Hyoscyamus desertorum, phosphatase, phosphomonoesters, fruit flies, N, P, K, Mg uptake from prey  相似文献   

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