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101.
102.
Null model analysis of species nestedness patterns 总被引:6,自引:0,他引:6
Nestedness is a common biogeographic pattern in which small communities form proper subsets of large communities. However, the detection of nestedness in binary presence-absence matrices will be affected by both the metric used to quantify nestedness and the reference null distribution. In this study, we assessed the statistical performance of eight nestedness metrics and six null model algorithms. The metrics and algorithms were tested against a benchmark set of 200 random matrices and 200 nested matrices that were created by passive sampling. Many algorithms that have been used in nestedness studies are vulnerable to type I errors (falsely rejecting a true null hypothesis). The best-performing algorithm maintains fixed row and fixed column totals, but it is conservative and may not always detect nestedness when it is present. Among the eight indices, the popular matrix temperature metric did not have good statistical properties. Instead, the Brualdi and Sanderson discrepancy index and Cutler's index of unexpected presences performed best. When used with the fixed-fixed algorithm, these indices provide a conservative test for nestedness. Although previous studies have revealed a high frequency of nestedness, a reanalysis of 288 empirical matrices suggests that the true frequency of nested matrices is between 10% and 40%. 相似文献
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104.
Coastal eutrophication is thought to cause excessive growth of epiphytes in eelgrass beds, threatening the health and survival
of these ecologically and economically valuable ecosystems worldwide. Mesograzers, small crustacean and gastropod grazers,
have the potential to prevent seagrass loss by grazing preferentially and efficiently on epiphytes. We tested the impact of
three mesograzers on epiphyte biomass and eelgrass productivity under threefold enriched nutrient concentrations in experimental
indoor mesocosm systems under summer conditions. We compared the results with earlier identical experiments that were performed
under ambient nutrient supply. The isopod Idotea baltica, the periwinkle Littorina littorea, and the small gastropod Rissoa membranacea significantly reduced epiphyte load under high nutrient supply with Rissoa being the most efficient grazer, but only high densities of Littorina and Rissoa had a significant positive effect on eelgrass productivity. Although all mesograzers increased epiphyte ingestion with higher
nutrient load, most likely as a functional response to the quantitatively and qualitatively better food supply, the promotion
of eelgrass growth by Idotea and Rissoa was diminished compared to the study performed under ambient nutrient supply. Littorina maintained the level of its positive impact on eelgrass productivity regardless of nutrient concentrations. 相似文献
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Ants belong to the most important groups of arthropods, inhabiting and commonly dominating most terrestrial habitats, especially
tropical rainforests. Their highly collective behavior enables exploitation of various resources and is viewed as a key factor
for their evolutionary success. Accordingly, a great variety of life strategies evolved in this group of arthropods, including
seed harvesters, gardeners, and planters, fungus growers, nomadic hunters, life stock keepers, and slave makers. This study
reports the discovery of a new lifestyle in ants. In a Southeast Asian rainforest habitat, Euprenolepis procera is specialized in harvesting a broad spectrum of naturally growing mushrooms, a nutritionally challenging and spatiotemporally
unpredictable food source. While unfavorable to the vast majority of animals, E. procera has developed exceptional adaptations such as a shift to a fully nomadic lifestyle and special food processing capabilities,
which allow it to rely entirely on mushrooms. As a consequence, E. procera is the most efficient and predominant consumer of epigeic mushrooms in the studied habitat and this has broad implications
for the tropical rainforest ecosystem. 相似文献
110.
Madagascar provides some of the rare examples where two or more primate species of the same genus and with seemingly identical
niche requirements occur in sympatry. If congeneric primate species co-occur in other parts of the world, they differ in size
in a way that is consistent with Hutchinson’s rule for coexisting species, or they occupy different ecological niches. In
some areas of Madagascar, mouse lemurs do not follow these “rules” and thus seem to violate one of the principles of community
ecology. In order to understand the mechanisms that allow coexistence of sympatric congeneric species without obvious niche
differentiation, we studied food composition of two identical sized omnivorous mouse lemur species, Microcebus griseorufus and M. murinus with the help of stable isotope analyses (δ
15N and δ
13C). The two species are closely related sister species. During the rich season, when food seems abundant, the two species
do not differ in their nitrogen isotope composition, indicating that the two species occupy the same trophic level. But they
differ in their δ
13C values, indicating that M. griseorufus feeds more on C4 and CAM (Crassulacean-acid-metabolism) plants than M. murinus. During the lean season, M. murinus has lower δ
15N values, indicating that the two species feed at different trophic levels during times of food shortage. Hybrids between
the two species showed intermediate food composition. The results reflect subtle differences in foraging or metabolic adaptations
that are difficult to quantify by traditional observations but that represent possibilities to allow coexistence of species. 相似文献