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1.
Species loss leads to community closure 总被引:1,自引:0,他引:1
Global extinction of a species is sadly irreversible. At a local scale, however, extinctions may be followed by re-invasion. We here show that this is not necessarily the case and that an ecological community may close its doors for re-invasion of species lost from it. Previous studies of how communities are assembled have shown that there may be rules for that process and that limitations are set to the order by which species are introduced and put together. Instead of focusing on the assembly process we randomly generated simple competitive model communities that were stable and allowed for two to 10 coexisting species. When a randomly selected single species was removed from the community, the cascading species loss was recorded and frequently the resulting community was more than halved. Cascading extinctions have previously been recorded, but we here show that the relative magnitude of the cascade is dependent on community size (and not only trophic structure) and that the reintroduction of the original species lost often is impossible. Hence, species loss does not simply leave a void potentially refilled, but permanently alters the entire community structure and consequently the adaptive landscape for potential re-invaders. 相似文献
2.
In Bacteroides buccae, a hexagonally arranged periodic structure was found in the outer membrane (OM), in addition to hexagonal lattices present in its external surface layer (S-layer). This crystalline OM protein (COMP) was present as patches on the concave fracture face (the outer leaflet) of the OM in freeze-fractured cells. Occasionally, hexagonally arranged structures could also be seen on the convex fracture face of the OM as 'fingerprints' of the COMP. The OM proteins were isolated and analysed by gel electrophoresis. The major band protein had an apparent molecular mass of 17 kDa. Whether the minor band proteins are also components in the structure of the COMP remains to be elucidated. Other oral Gram-negative anaerobic rods studied did not show any periodicity in their OM. 相似文献
3.
The matrix method for calculating the overall sensitivities (including control coefficients) of a metabolic system, described by Crabtree & Newsholme [Biochem. J. 247, 113-129 (1987)], is simplified by a preliminary partitioning of the initial matrix equation. This reduces the size of the matrix to be inverted and thereby removes a major drawback with the original method. The resulting procedure is simpler and more systematic than the alternative methods currently available, especially when the system is extensively branched. 相似文献
4.
Henrik Skovgård Jonna Tomkiewicz Gösta Nachman Mikael Münster-Swendsen 《Experimental & applied acarology》1993,17(1-2):59-76
The population dynamics of the cassava green mite Mononychellus tanajoa was studied on cassava during 35 weeks (early March to first of November 1989) in an experimental field near Lake Victoria in Western Kenya. The mite population peaked at the onset of the long dry season with 1,100 mites/leaf, declined sharply to a level of about 300 individuals/leaf, not to increase again until the next rainy season commenced. An indigenous phytoseiid predator Iphiseius degenerans was abundant during the dry spell with a maximum about 9 predators/leaf.A nonlinear regression analysis revealed that food depletion in combination with I. degenerans predation limited the population growth of the mites, whereas rain intensity had no effect. The predator exhibited no aggregative response to high densities of M. tanajoa and stayed mainly in the lower part of the canopy while the spider mites preferred the top, indicating that I. degenerans is a generalist predator without capacity to control M. tanajoa alone. However, in combination with another density dependent factor, such as food depletion, the predator may have prevented the spider mites from causing complete defoliation during the dry season. 相似文献
5.
Henrik Skovgård Jonna Tomkiewicz Gösta Nachman Mikael Münster-Swendsen 《Experimental & applied acarology》1993,17(1-2):41-58
The effect of the cassava green mite Mononychellus tanajoa on the growth and yield of cassava Manihot esculenta was studied over a 10-month period in two field trials near Lake Victoria in Kenya. One plot was maintained free of mites by means of acaricide, while the other was artificially infested.The highest population density of M. tanajoa occurred during the dry season. A maximum leaf area index (LAI) of about 2 was reached at the onset of the dry season. The total leaf area of mite infested plants was reduced compared with uninfested plants during the dry spell. During the following rainy season infested plants recovered and attained the same leaf area as uninfested plants. A multiple regression model predicting the leaf area showed that 58% of the seasonal variation could be explained by plant age, soil water, and leaf injury.The net growth rate of infested plants was lower than that of uninfested plants. Maximum values of 21 (infested plants) and 49 (uninfested plants) g m-2 week-1 were attained at the onset of the second rainy season. No difference was found between uninfested and infested plants with respect to net assimilation rates per unit leaf area during the dry season. The net assimilation rates reached a maximum almost at the same time as the growth rates, but the infested plants peaked slightly earlier and at a lower level than the uninfested plants.
M. tanajoa did not affect the relative allocation of dry matter into stems and storage roots, but the absolute allocation of dry matter declined with increasing mite injury. Thus, after 10 months the dry matter of infested plants was reduced by 29% and 21% for storage roots and stems, respectively, compared with the uninfested plants. 相似文献
6.
M. Heino V. Kaitala E. Ranta J. Lindstr?m 《Proceedings. Biological sciences / The Royal Society》1997,264(1381):481-486
We explore extinction rates using a spatially arranged set of subpopulations obeying Ricker dynamics. The population system is subjected to dispersal of individuals among the subpopulations as well as to local and global disturbances. We observe a tight positive correlation between global extinction rate and the level of synchrony in dynamics among thesubpopulations. Global disturbances and to a lesser extent, migration, are capable of synchronizing the temporal dynamics of the subpopulations over a rather wide span of the population growth rate r. Local noise decreases synchrony, as does increasing distance among the subpopulations. Synchrony also levels off with increasing r: in the chaotic region, subpopulations almost invariably behave asynchronously. We conclude that it is asynchrony that reduces the probability of global extinctions, not chaos as such: chaos is a special case only. The relationship between global extinction rate, synchronous dynamics and population growth rate is robust to changes in dispersal rates and ranges. 相似文献
7.
The diurnal variation of TRH concentrations in different parts of hypothalamus was studied in 80 male rats, which were killed in groups of 5 at 3 h intervals. The hypothalamus was dissected into three parts: I) medial basal hypothalamus (MBH), II) anterior hypothalamus, and III) dorsal hypothalamus. Anterior pituitary and serum TSH concentrations were also measured. TRH concentrations were higher in MBH than in the other parts of the hypothalamus: at night 300–450 pg/mg of wet weight of tissue. When the lights were turned on, MBH-TRH levels began to decrease, reaching a nadir of 210 pg/mg at 12 noon. After 15 h, MBH-TRH levels began to increase again. The changes in TRH levels in anterior hypothalamus were usually opposite to those in MBH (r = ?0.6185). Serum TSH levels were about 800 ng/ml during the day and were decreased to about one half of these levels when the lights were turned off. Serum TSH levels were positively correlated with anterior hypothalamic TRH levels (r = 0.6457) and inversely correlated with MBH-TRH levels (r = ?0.7747). Anterior pituitary TSH levels showed small but statistically insignificant variations. In conclusion, there were statistically interrelated diurnal rhythms in anterior hypothalamic and MBH-TRH levels and serum TSH concentrations. 相似文献
8.
The study area is an urban forest remnant in the city of Tampere (Finland). The presence of vascular plant species were surveyed in 1980 and again in 2000 within the 5 × 5 m grid cells covering the whole forest. The forest fragment has been influenced by three types of urban pressure: general ecological conditions (such as eutrophication due to increase of nitrogen), edge effect and local human disturbance. A general and spatially even eutrophication was reflected in the frequencies of C- and S-strategists as a shift towards to the C-point in the CSR triangle. The total number of native species (83 in 1980, 89 in 2000), typical Vaccinium type (VT)-forest species (32 in 1980, 31 in 2000) and aliens (57 in 1980, 68 in 2000) did change, but only to a moderate extent. The number of typical VT-forest species and native species were more common in central parts of the fragment. In terms of the number of species, the forest has been relatively resistant to date to major changes. However, the average number of native plant species per grid cell increased from 13.7 to 14.4, the average number of aliens increased from 2.0 to 4.2 and the average number of typical VT-forest species decreased from 5.5 to 5.0. Natives were spatially concentrated in the centre in 1980, but in 2000 they were more evenly distributed. The edge has offered habitats for new species, but the typical VT-forest species of the original sub-xeric forest are slowly diminishing. 相似文献
9.
Karen Shapiro Woutrina A. Miller Mary W. Silver Mitsunori Odagiri John L. Largier Patricia A. Conrad Jonna A. K. Mazet 《Microbial ecology》2013,65(4):928-933
Aquatic macroaggregates (flocs ≥0.5 mm) provide an important mechanism for vertical flux of nutrients and organic matter in aquatic ecosystems, yet their role in the transport and fate of zoonotic pathogens is largely unknown. Terrestrial pathogens that enter coastal waters through contaminated freshwater runoff may be especially prone to flocculation due to fluid dynamics and electrochemical changes that occur where fresh and marine waters mix. In this study, laboratory experiments were conducted to evaluate whether zoonotic pathogens (Cryptosporidium, Giardia, Salmonella) and a virus surrogate (PP7) are associated with aquatic macroaggregates and whether pathogen aggregation is enhanced in saline waters. Targeted microorganisms showed increased association with macroaggregates in estuarine and marine waters, as compared with an ultrapure water control and natural freshwater. Enrichment factor estimations demonstrated that pathogens are 2–4 orders of magnitude more concentrated in aggregates than in the estuarine and marine water surrounding the aggregates. Pathogen incorporation into aquatic macroaggregates may influence their transmission to susceptible hosts through settling and subsequent accumulation in zones where aggregation is greatest, as well as via enhanced uptake by invertebrates that serve as prey for marine animals or as seafood for humans. 相似文献
10.
Mervi Honkatukia Maria Tuiskula-Haavisto Jesus Arango Jonna Tabell Matthias Schmutz Rudolf Preisinger Johanna Vilkki 《遗传、选种与进化》2013,45(1):31