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Daniel Simberloff 《Biological invasions》2009,11(7):1735-1742
In addition to rats, nutria (Myocastor coypus) and the North American beaver (Castor canadensis) have certainly caused damage at an ecosystem level when introduced to islands, in both cases primarily by ecosystem engineering.
Of other introduced rodents successfully established on islands, the gray squirrel (Sciurus carolinensis) may be in the process of damaging entire forest ecosystems, particularly by bark-stripping. Though introduced muskrats (Ondatra zibethicus) have had ecosystem-level impacts in continental Europe, their impact on islands worldwide to which they have been introduced
has been very limited. The North American red squirrel (Tamiasciurus hudsonicus) and Barbary ground squirrel (Atlantoxerus getulus) have each had substantial impacts when introduced to particular islands, but for neither species have these impacts yet
been demonstrated to spread through an entire ecosystem. Introduced house mice (Mus musculus) may well generate ecosystem impacts on remote islands lacking rats, and it is possible that explosions of house mice on
islands after rat eradication, a common occurrence, will lead in some instances to ecosystem impacts. 相似文献
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Summary Morphologically the large intestine of the nutria resembles that of other caviomorphs, notably the guinea pig. The cecum is voluminous: it contributes 8.6% of the total intestinal length and 12.7% of the total intestinal surface area (considering the surface enlargement factor). It contains 27–32% of the wet ingesta and 20–23% of the dry matter in the gastrointestinal tract. In the colon the corresponding figures are: 21.8% of length, 12.6% of surface area, 16–21% of wet ingesta, and 16–40% of dry matter. The colon can be subdivided both structurally and functionally into two sections, the proximal and the distal colon, the border between the two being the apical flexure of a long parallel loop. The proximal colon (42% of colonic length) displays on the mucosal surface of its mesenterial side a narrow furrow bordered by ridges, which is absent in the distal colon. The ridges contain subepithelial accumulations of coiled tubuloalveolar mucoid glands, entwined by bundles of muscle fibers. Determinations of nitrogen in the contents near the furrow suggest a concentration of bacteria in this part of the lumen. It is hypothesized that the structural differentiations of the proximal colon provide mechanisms for the transport of bacteria from the proximal colon back into the cecum to maintain the fermentation function. The slopes of the longitudinal profiles for dry matter and for concentrations of sodium, potassium and calcium in the luminal contents change at the tip of the parallel loop. The electrical potential difference intestinal lumen — blood is particularly large in the proximal colon, indicating active electrogenic ion transport in this region.
SEF
surface enlargement factor
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CSM
colonic separation mechanism
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PEG
polyethylene glycol
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DM
dry matter
Dedicated to Prof. K.-E. Wohlfarth-Bottermann, Institut für Cytologie und Mikromorphologie der Universität Bonn, Bonn, Federal Republic of Germany, on the occasion of his 65th birthday 相似文献
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Effects of flooding, salinity and herbivory on coastal plant communities, Louisiana, United States 总被引:4,自引:0,他引:4
Flooding and salinity stress are predicted to increase in coastal Louisiana as relative sea level rise (RSLR) continues in
the Gulf of Mexico region. Although wetland plant species are adapted to these stressors, questions persist as to how marshes
may respond to changed abiotic variables caused by RSLR, and how herbivory by native and non-native mammals may affect this
response. The effects of altered flooding and salinity on coastal marsh communities were examined in two field experiments
that simultaneously manipulated herbivore pressure. Marsh sods subjected to increased or decreased flooding (by lowering or
raising sods, respectively), and increased or decreased salinity (by reciprocally transplanting sods between a brackish and
fresh marsh), were monitored inside and outside mammalian herbivore exclosures for three growing seasons. Increased flooding
stress reduced species numbers and biomass; alleviating flooding stress did not significantly alter species numbers while
community biomass increased. Increased salinity reduced species numbers and biomass, more so if herbivores were present. Decreasing
salinity had an unexpected effect: herbivores selectively consumed plants transplanted from the higher-salinity site. In plots
protected from herbivory, decreased salinity had little effect on species numbers or biomass, but community composition changed.
Overall, herbivore pressure further reduced species richness and biomass under conditions of increased flooding and increased
salinity, supporting other findings that coastal marsh species can tolerate increasingly stressful conditions unless another
factor, e.g., herbivory, is also present. Also, species dropped out of more stressful treatments much faster than they were
added when stresses were alleviated, likely due to restrictions on dispersal. The rate at which plant communities will shift
as a result of changed abiotic variables will determine if marshes remain viable when subjected to RSLR.
Received: 8 April 1998 / Accepted: 15 June 1998 相似文献
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Foraging behaviour of wild coypu was studied to examine two hypotheses that had been previously proposed to explain the species’ preference for aquatic plants. First, the nutritional benefit hypothesis which states that aquatic plants are more nutritional than terrestrial plants. Second, the behavioural trade-off hypothesis which states that coypus avoid foraging far from the water because of the costs associated with other types of behaviour. In order to test the nutritional benefit hypothesis, we studied the diet composition of coypus in relation to the protein content of the diet and of the plants available in the environment. Fieldwork was conducted seasonally from November 1999 to August 2000 at one study site located in the Province of Buenos Aires, east central Argentina. Behavioural observations showed that coypus remained foraging in the water and microhistological analysis of faeces indicated that their diet was principally composed of hygrophilic monocotyledons (Lemna spp. and Eleocharis spp.) throughout the year. We did not find support for the nutritional benefit hypothesis: nutritional quality (based on nitrogen content) of hygrophilic plants was not higher than that of terrestrial plants, and seasonal changes in diet quality did not match either fluctuations in vegetation quality or proportion of hygrophilic plants in the diet. Although not directly tested, the behavioural trade-off hypothesis may explain why coypus prefer to forage in or near the water as a mechanism for reducing predation risk. 相似文献
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