Thomas Oudman

Catastrophic collapse can occur without early warning: examples of silent catastrophes in structured ecological models.

Catastrophic and sudden collapses of ecosystems are sometimes preceded by early warning signals that potentially could be used to predict and prevent a forthcoming catastrophe. Universality of these early warning signals has been proposed, but no formal proof has been provided. Here, we show that in relatively simple ecological models the most commonly used early warning signals for a catastrophic collapse can be silent. We underpin the mathematical reason for this phenomenon, which involves the direction of the eigenvectors of the system. Our results demonstrate that claims on the universality of early warning signals are not correct, and that catastrophic collapses can occur without prior warning. In order to correctly predict a collapse and determine whether early warning signals precede the collapse, detailed knowledge of the mathematical structure of the approaching bifurcation is necessary. Unfortunately, such knowledge is often only obtained after the collapse has already occurred.

Climate change and the increasing impact of polar bears on bird populations

The Arctic is becoming warmer at a high rate, and contractions in the extent of sea ice are currently changing the habitats of marine top-predators dependent on ice. Polar bears (Ursus maritimus) depend on sea ice for hunting seals. For these top-predators, longer ice-free seasons are hypothesized to force the bears to hunt for alternative terrestrial food, such as eggs from colonial breeding birds. We analyzed time-series of polar bear observations at four locations on Spitsbergen (Svalbard) and one in east Greenland. Summer occurrence of polar bears, measured as the probability of encountering bears and the number of days with bear presence, has increased significantly from the 1970/80s to the present. The shifts in polar bear occurrence coincided with trends for shorter sea ice seasons and less sea ice during the spring in the study area. This resulted in a strong inverse relationship between the probability of bear encounters on land and the length of the sea ice season. Within, 10 years after their first appearance on land, polar bears had advanced their arrival dates by almost 30 days. Direct observations of nest predation showed that polar bears may severely affect reproductive success of the barnacle goose (Branta leucopsis), common eider (Somateria mollissima) and glaucous gull (Larus hyperboreus). Nest predation was strongest in years when the polar bears arrived well before hatch, with more than 90% of all nests being predated. The results are similar to findings from Canada, and large-scale processes, such as climate and subsequent habitat changes, are pinpointed as the most likely drivers in various parts of the Arctic. We suggest that the increasing, earlier appearance of bears on land in summer reflects behavioral adaptations by a small segment of the population to cope with a reduced hunting range on sea ice. This exemplifies how behavioral adaptations may contribute to the cascading effects of climate change.

Trans-Equatorial Migration Routes, Staging Sites and Wintering Areas of a High-Arctic Avian Predator: The Long-tailed Skua ( Stercorarius longicaudus )

The Long-tailed Skua, a small (<300 g) Arctic-breeding predator and seabird, is a functionally very important component of the Arctic vertebrate communities in summer, but little is known about its migration and winter distribution. We used light-level geolocators to track the annual movements of eight adult birds breeding in north-east Greenland (n = 3) and Svalbard (n = 5). All birds wintered in the Southern Hemisphere (mean arrival-departure dates on wintering grounds: 24 October-21 March): five along the south-west coast of Africa (0–40°S, 0–15°E), in the productive Benguela upwelling, and three further south (30–40°S, 0–50°E), in an area extending into the south-west Indian Ocean. Different migratory routes and rates of travel were documented during post-breeding (345 km d−1 in late August-early September) and spring migrations (235 km d−1 in late April) when most birds used a more westerly flyway. Among the different staging areas, a large region off the Grand Banks of Newfoundland appears to be the most important. It was used in autumn by all but one of the tracked birds (from a few days to three weeks) and in spring by five out of eight birds (from one to more than six weeks). Two other staging sites, off the Iberian coast and near the Azores, were used by two birds in spring for five to six weeks. Over one year, individuals travelled between 43,900 and 54,200 km (36,600–45,700 when excluding staging periods) and went as far as 10,500–13,700 km (mean 12,800 km) from their breeding sites. This study has revealed important marine areas in both the south and north Atlantic Ocean. Sustainable management of these ocean basins will benefit Long-tailed Skuas as well as other trans-equatorial migrants from the Arctic.

Digestive capacity and toxicity cause mixed diets in red knots that maximize energy intake rate.

Among energy-maximizing animals, preferences for different prey can be explained by ranking the prey according to their energetic content. However, diet choice also depends on characteristics of the predator, such as the need to ingest necessary nutrients and the constraints imposed by digestion and toxins in food. In combination, these factors can lead to mixed diets in which the energetically most profitable food is not eaten exclusively even when it is abundant. We studied diet choice in red knots (Calidris canutus canutus) feeding on mollusks at a West African wintering site. At this site, the birds fed primarily on two species of bivalves, a thick-shelled one (Dosinia isocardia) that imposed a digestive constraint and a thin-shelled one (Loripes lucinalis) that imposed a toxin constraint. The latter species is toxic due to its symbiotic association with sulfide-oxidizing bacteria. We estimated experimentally the parameters of a linear programming model that includes both digestive and toxin constraints, leading to the prediction that red knots should eat a mixture of both mollusk species to maximize energy intake. The model correctly predicted the preferences of the captive birds, which depended on the digestive quality and toxicity of their previous diet. At our study site, energy-maximizing red knots appear to select a mixed diet as a result of the simultaneous effects of digestive and toxin constraints.

The Effect of Digestive Capacity on the Intake Rate of Toxic and Non-Toxic Prey in an Ecological Context

Digestive capacity often limits food intake rate in animals. Many species can flexibly adjust digestive organ mass, enabling them to increase intake rate in times of increased energy requirement and/or scarcity of high-quality prey. However, some prey species are defended by secondary compounds, thereby forcing a toxin limitation on the forager’s intake rate, a constraint that potentially cannot be alleviated by enlarging digestive capacity. Hence, physiological flexibility may have a differential effect on intake of different prey types, and consequently on dietary preferences. We tested this effect in red knots (Calidris canutus canutus), medium-sized migratory shorebirds that feed on hard-shelled, usually mollusc, prey. Because they ingest their prey whole and crush the shell in their gizzard, the intake rate of red knots is generally constrained by digestive capacity. However, one of their main prey, the bivalve Loripes lucinalis, imposes a toxin constraint due to its symbiosis with sulphide-oxidizing bacteria. We manipulated gizzard sizes of red knots through prolonged exposure to hard-shelled or soft foods. We then measured maximum intake rates of toxic Loripes versus a non-toxic bivalve, Dosinia isocardia. We found that intake of Dosinia exponentially increased with gizzard mass, confirming earlier results with non-toxic prey, whereas intake of Loripes was independent of gizzard mass. Using linear programming, we show that this leads to markedly different expected diet preferences in red knots that try to maximize energy intake rate with a small versus a large gizzard. Intra- and inter-individual variation in digestive capacity is found in many animal species. Hence, the here proposed functional link with individual differences in foraging decisions may be general. We emphasize the potential relevance of individual variation in physiology when studying trophic interactions.

Marine biorhythms: bridging chronobiology and ecology

Marine organisms adapt to complex temporal environments that include daily, tidal, semi-lunar, lunar and seasonal cycles. However, our understanding of marine biological rhythms and their underlying molecular basis is mainly confined to a few model organisms in rather simplistic laboratory settings. Here, we use new empirical data and recent examples of marine biorhythms to highlight how field ecologists and laboratory chronobiologists can complement each others efforts. First, with continuous tracking of intertidal shorebirds in the field, we reveal individual differences in tidal and circadian foraging rhythms. Second, we demonstrate that shorebird species that spend 8–10 months in tidal environments rarely maintain such tidal or circadian rhythms during breeding, likely because of other, more pertinent, temporally structured, local ecological pressures such as predation or social environment. Finally, we use examples of initial findings from invertebrates (arthropods and polychaete worms) that are being developed as model species to study the molecular bases of lunar-related rhythms. These examples indicate that canonical circadian clock genes (i.e. the homologous clock genes identified in many higher organisms) may not be involved in lunar/tidal phenotypes. Together, our results and the examples we describe emphasize that linking field and laboratory studies is likely to generate a better ecological appreciation of lunar-related rhythms in the wild. This article is part of the themed issue ‘Wild clocks: integrating chronobiology and ecology to understand timekeeping in free-living animals’.

Colony-Breeding Eurasian Spoonbills in the Netherlands: Local Limits to Population Growth with Expansion into New Areas

It has been suggested that in most colony-breeding birds, food availability in the feeding areas surrounding the colonies limits, and thereby regulates, population size. However, population size is also determined by adult survival, which will additionally be influenced by circumstances outside the breeding season. Most Eurasian Spoonbills Platalea leucorodia leucorodia in The Netherlands breed on the Wadden Sea barrier islands. After 30 years of exponential growth, the breeding population in the Dutch Wadden Sea area is now levelling off towards a maximum of nearly 2000 nests. For these Spoonbills, density-dependent effects on survival by the different age-classes and in the different seasons have already been demonstrated. However, the mechanisms underlying the densitydependent survival of juveniles before and after fledging remain unclear. To examine whether these density-dependent effects reflect limitations at the colony level, we compared colony growth, chick condition and reproductive success among the Wadden Sea colonies. Population growth rates from 1988 to 2015 varied widely between the 10 existing colonies, and so did the statistically predicted maximum colony sizes. Chick condition, measured for 781 chicks in six different colonies between 2011 and 2015, was lower in stable colonies than in growing colonies, although not for the very late chicks, and reproductive success tended to be lower as well. Over the longer period of 1991 to 2011, reproductive success showed a strong negative relationship with colony size. We propose that the levelling off of colony sizes in the Wadden Sea is caused by local food limitations, and suggest further research in this direction. The continuing growth of the Dutch population is now being fuelled by exponentially increasing numbers of Spoonbills breeding in the Delta area.