摘要:

SUMMARY1. Fungistasis in soil is a widespread phenomenon affecting most fungal propagules, though some are insensitive. In most instances, it is coexistent with the presence of living microorganisms, and is annulled by energy‐yielding nutrients. Fungistasis with characteristics similar to that in soil may also occur on leaves of plants.2. Germination and growth of bacteria and actinomycetes is also restricted in soils. The characteristics of their inhibition appear to be the same as those for fungi. Therefore, the concept of a widespread microbial inhibition in soil can be applied to all three groups of microorganisms.3. Fungistasis can be detected by various direct methods, or indirectly by methods involving the use of porous or permeable carriers. It may be expressed as a restriction on the final amount of germination (the usual parameter), germination rate (with time), and rate of germ‐tube or hyphal growth. Since the expression of fungistasis is often complete in soil, titration with nutrients may be required to distinguish between the sensitivities of different fungi.4. Fungistasis generally is expressed most strongly at soil moisture contents somewhat less than saturation. Its expression usually is maximal in neutral or slightly alkaline soils. In acidic conditions fungistasis may be lessened because of suppression of bacterial and actinomycete activity. Increased sensitivity of some fungi in soils of pH>7.0 may be caused by a directly unfavourable effect of pH on the fungus.5. Fungal species with small spores tend to be highly sensitive to fungistasis. These spores tend to germinate slowly and to require exogenous nutrients for germination. By contrast, species with larger spores and sclerotia often do not require exogenous nutrients for germination. The larger spores tend to germinate rapidly and to exhibit low sensitivity, as compared with small spores. A few nutrient‐independent spores are insensitive to fungistasis. At least a part of the difference in sensitivity is related to germination time; spores which germinate slowly compete poorly with the soil micro‐flora for their nutrients.6. Fungistasis is often temporarily annulled by enriching the soil with energy‐yielding nutrients. Usually, complex materials such as plant residues are most effective. A few weeks after such treatment, the level of fungistasis may, however, be increased. Annulment of fungistasis by compounds not utilized as energy sources has not yet been demonstrated.7. Several soils naturally suppressive toFusariumwilt diseases were more fungistatic toFusariumthan soils conducive to wilt. Potential means by which fungistasis may be manipulated to control root‐infecting fungi are (a) through stimulation of germination with nutrients, thus exposing the germ tube to lysis, and (b) by increasing the fungistatic level of soil through appropriate amendments.8. Volatile substances identified in soils, some of which are potentially inhibitory to fungi include (a) ammonia, which apparently is evolved from ammonium salts in some arid soils of high pH, (b) ethylene, which has been identified in some soils of pH<7.0 (though high levels of this gas seem to be tolerated by most fungi), (c) allyl alcohol, and (d) other unidentified substances. Non‐volatile inhibitors include high molecular weight substances revealed by molecular sieve chromatography of soil extracts. Microbial metabolites such as those present in staled fungal cultures also have been proposed to account for fungistasis. In a few soils fungistasis persists after sterilization because of the presence of inhibitory concentrations of calcium carbonate, iron or aluminium. Inherent in the proposition that inhibitory substances provide the primary mechanism of fungistasis is the concept of a highly complex phenomenon, involving various highly specific inhibitory and counteracting stimulatory substances, with the outcome for the fungus depending on the kinds and relative amounts of each present.9. By the nutrient‐deficiency hypothesis, the level of available nutrients in soil is insufficient to support germination of nutrient‐dependent propagules, except in nutrient‐rich microsites. Inhibition of nutrient‐independent propagules is explained by loss of endogenous nutrients required for germination, through microbial nutrient competition. Evidence for this hypothesis is (a) the imposition of fungistasis on numerous nutrient‐independent propagules during incubation on leaching model systems designed to simulate microbial nutrient competition in soil, (b) similar losses of endogenous nutrients occurring on soil and the leaching system, and (c) the fact that soils are chronically deficient in energy in relation to the microbial populations present, with the consequence that enforced inactivity is imposed upon most of the population at any given time for this reason alone, regardless of the presence or absence of fungistatic substances.Journal series article no. 7747 from the Michigan Agricult

ISSN:1464-7931    

DOI:10.1111/j.1469-185X.1977.tb01344.x

出版商:Blackwell Publishing Ltd    

年代:1977

数据来源: WILEY

摘要:

SUMMARY1. The introduction ofSalmo truttainto an artificial pond was followed by great reduction in the numbers of tadpoles, certain beetles andNotonecta, all species to be seen in the open water. Of the species that sheltered more securely in the plant cover, the effect on some was a curtailment of their range; on others there was little reduction of range or numbers, particularly the numbers at the end of a generation. This was attributed to self‐regulation of numbers and the creation of a reserve from which losses due to predation by fish on larger specimens could be replenished.2. Changes in the number of fish in stretches of a stony stream exerted little effect on the Ephemeroptera, but records indicate thatGammarusand fish are rarely numerous in the same stretch.3. The most abundant invertebrate carnivores in the fishpond wait for prey to come to them; of two others, a leech swims well but has poor seizing organs, a caddis‐larva the reverse. The amount consumed by such predators falls rapidly as the prey becomes scarce. Moreover the main source of prey for the common predators is from the small Crustacea which are abundant only in summer and reproduce quickly. This prey thus has properties that prevent much reduction of numbers by predation.4. On the stony substrata of Lake District lakes,Asellusand Planaria are numerous where conditions are productive, Ephemeroptera and Plecoptera elsewhere. The absence of these insects from productive places is attributed to predation.5. Planaria cannot move fast and have no efficient seizing organs, but compensate to some extent by laying trails of slime in which prey becomes entangled. AsAsellusgrows, its chances of being overpowered by Planaria decrease. Planaria, therefore, feed regularly only when prey is abundant. When it is scarce they rest, and they are able to withstand starvation for a long period.6. Planaria are preyed upon extensively by Odonata, newts and Plecoptera, and the first two keep them out of weedy ponds. The last may keep their numbers low in streams and perhaps also on stony shores of unproductive lakes, though here scarcity of food is important too. In productive lakes predation on flatworms is slight.7. Protozoa exhibit three relationships between predator and prey, two of which have been seen in larger organisms. Prey avoid predation in cover. Predators cease activity when prey becomes scarce. Prey occurs in isolated colonies which when found are destroyed by the predator, but generally not before some individuals have dispersed and founded new colonies.8. Only small invertebrates can survive predation by fish in the open water. Many also reproduce rapidly as long as conditions are favourable and enter a resting stage when they are not. Intense predation may eliminate large species. In some ponds in the Colorado mountains salamanders eliminate a large carnivorous copepod, which enables small Cladocera to survive. Absence of the copepod and presence of Cladocera of suitable size for it to feed on enable aChaoboruslarva to co‐exist with the salamander.9. Small planktivorous fish occur in the open water of some African lakes. The great size of these lakes probably makes possible the co‐existence of small fish and their predators, but also makes investigation of the relations between the two difficult.10. When species not previously present have gained access to lakes, the numbers of the native species of fish have often been greatly reduced. The exact nature of the relationship between newcomer and native has, however, not been established because other factors have been varying, observations have been scanty, or records have not been made for long enough.11. In temporary and very small bodies of water predation is mainly by invading individuals that were reared somewhere else. Characteristic organisms are phyllopods in impermanent pools and mosquito larvae in both types of water, two groups that feed in the open and away from cover, an activity possible only where predation is

ISSN:1464-7931    

DOI:10.1111/j.1469-185X.1977.tb01345.x

出版商:Blackwell Publishing Ltd    

年代:1977

数据来源: WILEY

摘要:

SUMMARY1. Eggs of Crocodilia and Chelonia, like those of birds, have a pair of egg membranes separating a thick layer of albumen from the calcareous shell. In contrast, eggs of oviparous Lepidosauria have only a single shell membrane, upon which relatively small amounts of calcium carbonate are deposited; and the volume of albumen in eggs is extraordinarily small at the time of oviposition.2. With the possible exception of certain geckos and some chelonians, eggs of oviparous reptiles seem always to absorb water from the substrate during the course of normal incubation. In so far as the rate of water absorption exceeds the rate of water loss by transpiration from exposed surfaces, the eggs swell during incubation. The term ‘cleidoic’ cannot be used to describe eggs of this type.3. Embryos of lizards and snakes influence the water potential of extra‐embryonic fluids contained within their eggs, thereby maintaining or increasing the gradient in water potential that drives water absorption.4. Embryos of Crocodilia and Chelonia obtain a substantial portion of the calcium used in ossification of skeletal elements from the inner surfaces of the eggshell. In contrast, embryonic lizards and snakes draw upon extensive reserves of calcium present in the yolk, and obtain little (if any) calcium from the eggshell.5. All reptilian embryos seem to produce substantial quantities of urea as a detoxification product of protein catabolism. Contrary to expectation, uricotelism may not be common among reptilian embryos, even in those few instances where development takes place within a hard, calcareous egg.6. In eggs of Crocodilia and Chelonia, respiratory gases seem to pass by diffusion through pores in the calcareous eggshell and through spaces between the fibres of the pair of egg membranes. No pores have been observed in the shell of lepidosaurian eggs, and so gases presumably diffuse between the fibres of the single (multilayered) shell membrane.7. Metabolism of reptilian embryos is temperature‐dependent, as is true for most ectothermic organisms. For each species, there appears to be a particular temperature at which embryonic development proceeds optimally, and departures from this optimum elicit increases in developmental anomalies and/or embryonic mortality.8. Viviparity has evolved on numerous occasions among species of the Squamata, but seemingly never among Crocodilia or Chelonia. Since the evolution of viviparity entails a progressive reduction in the eggshell, only those organisms whose embryos do not depend upon the eggshell as a source of calcium may have the evolutionary potential to become viviparous.9. Evolutionary transitions from oviparity to viviparity could have been driven by selection related to (i) thermal benefits to embryos consequent upon retention of eggs within the body of a parent capable of behavioural thermoregulation; (ii) protection of the eggs from nest predators and/or soil microbes; and (iii) more effective exploitation of a seasonal food resource by early emergin

ISSN:1464-7931    

DOI:10.1111/j.1469-185X.1977.tb01346.x

出版商:Blackwell Publishing Ltd    

年代:1977

数据来源: WILEY

摘要:

SUMMARY1According to ‘Gause's hypothesis’ a corollary of the process of evolution by natural selection is that in a community at equilibrium every species must occupy a different niche. Many botanists have found this idea improbable because they have ignored the processes of regeneration in plant communities.2Most plant communities are longer‐lived than their constituent individual plants. When an individual dies, it may or may not be replaced by an individual of the same species. It is this replacement stage which is all‐important to the argument presented.3Several mechanisms not involving regeneration also contribute to the maintenance of species‐richness:(a). differences in life‐form coupled with the inability of larger plants to exhaust or cut off all resources, also the development of dependence‐relationships,(b) differences in phenology coupled with tolerance of suppression,(c) fluctuations in the environment coupled with relatively small differences in competitive ability between many species,(d) the ability of certain species‐pairs to form stable mixtures because of a balance of intraspecific competition against interspecific competition,(e) the production of substances more toxic to the producer‐species than to the other species,(f) differences in the primary limiting mineral nutrients or pore‐sizes in the soil for neighbouring plants of different soecies, and(g) differences in the competitive abilities of species dependent on their physiological age coupled with the uneven‐age structure of many populations.4The mechanisms listed above do not go far to explain the indefinite persistence in mixture of the many species in the most species‐rich communities known.5In contrast there seem to be almost limitless possibilities for differences between species in their requirements for regeneration, i.e. the replacement of the individual plants of one generation by those of the next. This idea is illustrated for tree species and it is emphasized that foresters were the first by a wide margin to appreciate its importance.6The processes involved in the successful invasion of a gap by a given plant species and some characters of the gap that may be important are summarized in Table 2.7The definition of a plant's niche requires recognition of four components:(a) the habitat niche,(b) the life‐form niche,(c) the phenological niche, and(d) the regeneration niche.8A brief account is given of the patterns of regeneration in different kinds of plant community to provide a background for studies of differentiation in the regeneration niche.9All stages in the regeneration‐cycle are potentially important and examples of differentiation between species are given for each of the following stages:(a) Production of viable seed (including the sub‐stages of flowering, pollination and seed‐set),(b) dispersal, in space and time,(c) germination,(d) establishment, and(e) further development of the immature plant.10In the concluding discussion emphasis is placed on the following themes:(a) the kinds of work needed in future to prove or disprove that differentiation in the regeneration niche is the major explanation of the maintenance of species‐richness in plant communities,(b) the relation of the present thesis to published ideas on the origin of phenological spread,(c) the relevance of the present thesis to the discussion on the presence of continua in vegetation,(d) the co‐incidence of the present thesis and the emerging ideas of evolutionists about differentiation of angi

ISSN:1464-7931    

DOI:10.1111/j.1469-185X.1977.tb01347.x

出版商:Blackwell Publishing Ltd    

年代:1977

数据来源: WILEY