Global warming leads to increased intensity and frequency of weather extremes. Such increased environmental variability might in turn result in increased variation in the demographic rates of interacting species with potentially important consequences for the dynamics of food webs. Using a theoretical approach, we here explore the response of food webs to a highly variable environment. We investigate how species richness and correlation in the responses of species to environmental fluctuations affect the risk of extinction cascades. We find that the risk of extinction cascades increases with increasing species richness, especially when correlation among species is low. Initial extinctions of primary producer species unleash bottom-up extinction cascades, especially in webs with specialist consumers. In this sense, species-rich ecosystems are less robust to increasing levels of environmental variability than species-poor ones. Our study thus suggests that highly species-rich ecosystems such as coral reefs and tropical rainforests might be particularly vulnerable to increased climate variability.

Many ecological responses to climate change have been documented. However, due to indirect effects, some responses can be complex and difficult to predict. For example, our understanding of effects on consumers involving responses on several trophic levels is limited. Here, combining the knowledge of trophic interactions in the EU's fourth largest lake with long-term climate and catch data, we analyse potential drivers of change in this system's apex predator. We show that warm winters correlate with later poor catches of great Arctic charr (Salvelinus umbla), and that in recent years predator-prey cycles involving this species have disappeared. The likely mechanisms are trophic mismatches directly and indirectly affecting two stages of charr, the fry and the juveniles, respectively. Our study illustrates how a long-lived consumer may be subjected to double jeopardy from the effects of warming across trophic levels, and that a food web approach can aid in disentangling the chain of mechanisms responsible.

Summary 1. Many fish stocks have declined, because of overharvesting, habitat destruction and introduced species. Despite efforts to rehabilitate some of these stocks, not all are responding or are recovering only slowly. 2. In freshwater systems, introduced crayfish are often problematic, and it has been suggested that their egg predation could reduce recruitment in depleted stocks of native fish. 3. Here, we report the results of a field experiment, using experimental cages, on the extent of predation on eggs of great Arctic charr (Salvelinus umbla) in Lake Vättern, Europe’s fifth largest lake. Here, the great Arctic charr has declined dramatically and is listed as critically endangered. 4. We were able to partition the total loss rate of eggs into background mortality, predation by introduced signal crayfish (Pacifastacus leniusculus) and predation by native fish. The mortality rate of charr eggs because of crayfish was estimated at more than five times that because of native fish. Of the total loss of eggs, 80% is believed to be caused by crayfish and 14% by fish, with 6% being natural background mortality. 5. In a worst case scenario, our data infer that only 25% of the original number of eggs would survive, compared with 75% in the absence of crayfish. This could impair recovery of the stock of the endangered great Arctic charr in Lake Vättern. 6. Contrary to earlier claims that crayfish predation on eggs of great Arctic charr is insignificant, our results indicate that crayfish predation may exceed fish predation and suggest that the abundance of signal crayfish on the spawning sites of great Arctic charr should be managed.

Estimation of fish stock size distributions from survey data requires knowledge about gear selectivity. However, selectivity models rest on assumptions that seldom are analyzed. Departures from these can lead to misinterpretations and biased management recommendations. Here, we use survey data on great Arctic char (Salvelinus umbla) to analyze how correcting for entanglement of fish and nonisometric growth might improve estimates of selectivity curves, and subsequently estimates of size distribution and age-specific mortality. Initial selectivity curves, using the entire data set, were wide and asymmetric, with poor model fits. Removing potentially nonmeshed fish had the greatest positive effect on model fit, resulting in much narrower and less asymmetric selection curves, while attempting to take nonisometric growth into account, by using girth rather than length, improved model fit but not as much. Using simulations we show that correcting for both entanglement and size selectivity produces accurate estimates of mortality rates, while correcting for size selectivity only does not. Our study demonstrates an approach that increases the accuracy of estimates of fish size distributions and mortality rates from survey data.Résumé : L'estimation de la distribution des tailles au sein de stocks de poissons à partir de données tirées d'évaluations nécessite une connaissance de la sélectivité des engins. Les modèles de sélectivité reposent toutefois sur des hypothèses qui sont rarement analysées. Des écarts par rapport à ces dernières peuvent se traduire par des interprétations erronées et des recommandations de gestion biaisées. Nous avons utilisé des données d'évaluations relatives à l'omble chevalier (Salvelinus umbla) pour analyser comment des correctifs visant à tenir compte de l'enchevêtrement de poissons et de la croissance non isométrique pourraient améliorer les estimations de courbes de sélectivité et, ainsi, les estimations de la distribution des tailles et de la mortalité selon l'âge. Les courbes de sélectivité initiales obtenues en utilisant l'intégralité des données étaient larges et asymé-triques, et l'ajustement du modèle n'était pas bon. Le retrait des poissons potentiellement non enchevêtrés a eu le plus grand effet positif sur l'ajustement du modèle aux données, produisant des courbes de sélection beaucoup moins larges et asymé-triques, alors que la prise en compte de la croissance non isométrique en utilisant le tour du corps plutôt que la longueur a amélioré l'ajustement du modèle, mais dans une moindre mesure. Nous démontrons, à l'aide de simulations, que le fait de corriger pour l'enchevêtrement et la sélectivité des tailles produit des estimations exactes du taux de mortalité, contrairement au fait de corriger seulement pour la sélectivité des tailles. L'étude démontre qu'une telle approche accroît l'exactitude des estimations de la distribution des tailles et du taux de mortalité des poissons basées sur des données d'évaluations. [Traduit par la Rédaction]