Disentangling trophic relationships in a High Arctic tundra ecosystem through food web modeling.

Determining the manner in which food webs will respond to environmental changes is difficult because the relative importance of top-down vs. bottom-up forces in controlling ecosystems is still debated. This is especially true in the Arctic tundra where, despite relatively simple food webs, it is still unclear which forces dominate in this ecosystem. Our primary goal was to assess the extent to which a tundra food web was dominated by plant-herbivore or predator-prey interactions. Based on a 17-year (1993-2009) study of terrestrial wildlife on Bylot Island, Nunavut, Canada, we developed trophic mass balance models to address this question. Snow Geese were the dominant herbivores in this ecosystem, followed by two sympatric lemming species (brown and collared lemmings). Arctic foxes, weasels, and several species of birds of prey were the dominant predators. Results of our trophic models encompassing 19 functional groups showed that <10% of the annual primary production was consumed by herbivores in most years despite the presence of a large Snow Goose colony, but that 20-100% of the annual herbivore production was consumed by predators. The impact of herbivores on vegetation has also weakened over time, probably due to an increase in primary production. The impact of predators was highest on lemmings, intermediate on passerines, and lowest on geese and shorebirds, but it varied with lemming abundance. Predation of collared lemmings exceeded production in most years and may explain why this species remained at low density. In contrast, the predation rate on brown lemmings varied with prey density and may have contributed to the high-amplitude, periodic fluctuations in the abundance of this species. Our analysis provided little evidence that herbivores are limited by primary production on Bylot Island. In contrast, we measured strong predator-prey interactions, which supports the hypothesis that this food web is primarily controlled by top-down forces. The presence of allochthonous resources subsidizing top predators and the absence of large herbivores may partly explain the predominant role of predation in this low-productivity ecosystem.

Transvaginal reduction of an interstitial heterotopic pregnancy with preservation of the intrauterine gestation.

With use of transvaginal ultrasonographic guidance, cardiac activity in an interstitial heterotopic pregnancy at 7 weeks' gestation was terminated. The interstitial pregnancy resolved, and a healthy term infant was delivered. If an early diagnosis of an interstitial heterotopic pregnancy is made, selective reduction may allow preservation of the intrauterine gestation without surgery.

Statistical tables to detect significance between frequencies in two small samples, with particular reference to biological assays.

By simply referring to one of three tables, derived from Fisher's exact factorial method and obtained with the aid of a computer, it is possible to decide readily if differences observed between two small and equal or unequal size series are statistically significant at probability levels of 0.10, 0.05 and 0.01. The same can be very helpful for planning experiments. Examples are given in various fields of biology.