We investigated the effects of the fungicide captan (cis-N-trichloromethylthio-4-cyclohexene-1,2-dicarboximide) and long-term conventional (CT) and no-tillage (NT) management on the size and composition of fungal communities from crop residues and mineral soil. Captan and reference treatments were established in CT and NT plots on a Hiwassee sandy clay loam soil (clayey, kaolinitic, thermic Rhodic Kanhapludult). At intervals, grain sorghum [Sorghum Bicolor (L.) Moench] litterbags and soils were collected for enumeration and identification of fungal hyphae and fungal colony-forming units (CFU) and to determine residue decomposition rates. Buried residues (CT) had higher densities of fungal hyphae but fewer fungal CFU than surface residues (NT). Buried residue decay rates (k = 0.0136 d-1) were 3.4 times faster than surface residues (k = 0.0040 d-1). Fungal hyphae were more abundant in NT than CT mineral soils but there were no differences in fungal CFU. Sorghum residues had a common succession of fungi including phyllosphere colonizers (e.g., Alternaria alternata, Epicoccum nigrum) and other common decomposer fungi (Cylindrocarpon, Fusarium, Mucor, and Phoma). Fungal communities were differentiated into surface residue (e.g., Alternaria, Epicoccum) and soil (e.g., Aspergillus, Trichoderma) specialists, while buried residues lacked a specialized community. Rhizopus arrhizus and Aspergillus niger showed a statistical preference for CT soils, while Penicillium verruculosum was most common in NT. Captan reduced fungal CFU and hyphal densities in residues of both NT and CT, resulting in 23 and 7% reductions in their decay rates, respectively. We found no evidence for differential effects of captan on the composition of the fungal communities in residues or soils.

The effects of local extinction on the structure of a nematode community were examined by monitoring soil nematode populations in cores that were isolated from surrounding soil. The cores were intact blocks of deciduous forest soil and litter of three different sizes (75, 250, and 920 cm3), maintained in a controlled-environment room, and sampled at 2-month intervals for 16 months.The number of nematode types per sample declined over time, from an initial average of 16 to less than 5 per 100 individuals. Types that were morphologically specialized for feeding on resources that were abundant in the cores persisted. Extinctions were associated with deficiencies of resources and with low initial population size. Nematode types that survived were generally abundant, of low trophic position and small body size. Abundance distributions at the end of the experiment were more strongly dominated by one or a few types and had higher logarithmic variances despite lower richness. Similar findings have been reported from studies of much larger, above-ground animals. Local or regional extinction appears to be a common phenomenon in ecological systems, and may affect the richness and population structure of many natural communities.

Photosynthetically fixed energy ultimately supports the great diversity of species that inhabit the world's ecosystems. Globally, natural terrestrial ecosystems fixed about 2800 * 1018 joules of energy per year (net) prior to significant human impacts. In recent years, humans have diverted or prevented about 20% to 30% of this energy from flowing through natural ecosystems by maintaining croplands (15%) and urban areas (1.8%) and by grazing livestock (2.3%). Habitat degradation, mainly in the form of desertification, has also caused a reduction (4%). Energy flow can be related to numbers of species with species-energy curves, which are relationships between species richness and the total energy flow in different regions, similar to species-area curves. By back-extrapolating along a species-energy curve, the observed reduction in natural energy flow due to human activity can be used to make a quantitative prediction of species endangerment. Given 20% to 30% reduction in energy and a speciesenergy curve exponent (z) between 0.10 and 0.20, we expect the endangerment of 2% to 7% of the world's terrestrial species. Projections to the year 2000 indicate that 3% to 9% of the world's species may be extinct or endangered by that time. These estimates are probably conservative.

(...) The addition of microbes or microbes and fauna to microcosms defaunated by microwaving caused an increase in respiration 2 d after reinoculation. The increase due to fauna plus microbes was greater than when microbes alone were added, suggesting that the fauna enhanced microbial inoculation. Fauna were also implicated as mobilizers of nutrients: amounts of ammonium and phosphate were greater in leachate from refaunated microcosms, and leaching of carbon showed a similar pattern. The mineral component of the soil had a strong influence on leachate chemistry. Mineral particles reduced phosphate leaching drastically, and retention of ammonium, carbon, and H + was also increased. (...)

Previous articleNext article No AccessNotes and CommentsA Simple, Stable Model of Mutualism Incorporating Handling TimeDavid Hamilton WrightDavid Hamilton WrightPDFPDF PLUS Add to favoritesDownload CitationTrack CitationsPermissionsReprints Share onFacebookTwitterLinkedInRedditEmail SectionsMoreDetailsFiguresReferencesCited by The American Naturalist Volume 134, Number 4Oct., 1989 Published for The American Society of Naturalists Article DOIhttps://doi.org/10.1086/285003 Views: 27Total views on this site Citations: 64Citations are reported from Crossref Copyright 1989 The University of ChicagoPDF download Crossref reports the following articles citing this article:Shohei Kawata, Gaku Takimoto Pollinator predation stabilizes plant–pollinator mutualisms through the modification of pollinator behavior, Ecological Research 38, no.22 (Dec 2022): 360–366.https://doi.org/10.1111/1440-1703.12376Fengde Chen, Zhong Li, Lijuan Chen Dynamic Behaviors of a Stage Structure Commensalism System with Holling type II Commensalistic Benefits, WSEAS TRANSACTIONS ON MATHEMATICS 21 (Dec 2022): 810–824.https://doi.org/10.37394/23206.2022.21.93Xu Li Stationary distribution and ergodicity of stochastic Wright's mutualism model with regime switching, Stat 11, no.11 (Apr 2022).https://doi.org/10.1002/sta4.440Luciano Stucchi, Javier Galeano, Juan Manuel Pastor, Jose María Iriondo, José A. 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Intact cores of agricultural soil planted with Sorghum bicolor were treated with selective biocides or combinations of biocides to manipulate soil organisms. Half the replicates of each biocide treatment were also given N fertilizer. The plants were maintained in a greenhouse, where growth and nutrient content and soil-organism populations were monitored over 16 weeks.

A quantitative technique for estimating extinctions due to clearing of natural ecosystems is described. Applied on a global scale, the method yields preliminary figures on extinctions of flowering plants, butterflies, land birds and land mammals ranging from 5.4 to 15.3% for the period from the beginning of agriculture until the year 1980. Actual numbers of extinctions of mammals and birds to date are much lower, possibly in part due to a tendency for the technique to overestimate species loss at the global scale. However, delayed extinctions of species whose populations have been reduced but not exterminated by habitat destruction are likely, suggesting that human impacts may be more serious than they currently appear.

The composition and dynamics of foraging assemblages of bees were examined from the standpoint of species-level arrival and departure processes in patches of flowers. Experiments with bees visiting 4 different species of flowers in subalpine meadows in Colorado gave the following results: 1) In enriched patches the rates of departure of bees were reduced, resulting in increases in both the number of bees per species and the average number of species present. 2) The reduction in bee departure rates from enriched patches was due to mechanical factors-increased flower handling time, and to behavioral factors-an increase in the number of flowers visited per inflorescence and in the number of inflorescences visited per patch. Bees foraging in enriched patches could collect nectar 30-45% faster than those foraging in control patches. 3) The quantitative changes in foraging assemblages due to enrichment, in terms of means and variances of species population sizes, fraction of time a species was present in a patch, and in mean and variance of the number of species present, were in reasonable agreement with predictions drawn from queuing theory and studies in island biogeography. 4) Experiments performed with 2 species of flowers with different corolla tube lengths demonstrated that manipulation of resources of differing availability had unequal effects on particular subsets of the larger foraging community. The arrival-departure process of bees on flowers and the immigration-extinction process of species on islands are contrasted, and the value of the stochastic, species-level approach to community composition is briefly discussed.

Seven species of freshwater algae were grown in defined media in monoalgal and mixed cultures, to test whether species interactions and community efficiency were predictable on the basis of ecological theory. Results supported the prediction of competitive interactions, although no differences were detectable in the strength of competition in two and three species mixed cultures, nor was the severity of competitive effects predictable on the basis of relatedness at the level of phyla. On a community level, cultures with two and three species fixed a greater proportion of available carbon than cultures with only one species, implying a more efficient overall use of available resources. Implications for the design of artificial closed ecosystems are discussed.

La theorie selon laquelle l'apport energetique necessaire a une biocenose limite la capacite de cette biocenose a contenir. Les especes permet d'expliquer 70 a 80% de la variation du nombre d'especes dans des iles extremement diverses par leur surface et leur environnement. On examine, en particulier, certaines donnees sur le nombre d'especes d'Angiospermes et d'oiseaux terrestres ou d'eau douce sur des iles du monde entier