Vielseitige Esser: Eisbären brauchen keinen Robbenhappen

Der Eisbär musste lange als Prügelknabe in der Klimadiskussion herhalten. Immer wieder wurde sein Ende vorhergesagt. Offenbar etwas vorschnell. Die Welt berichtete am 20. Juli 2016:

Klimawandel: Die Rückkehr der Eisbären auf Spitzbergen
Der Klimawandel bedroht den Lebensraum zahlreicher Tiere in der Arktis. Doch nun haben Forscher auf Spitzbergen und in Gebieten der Barentssee mehr Eisbären entdeckt als bei der Zählung von 2004. […] Einer Studie des Norwegischen Polarinstituts zufolge sind die Eisbären auf Spitzbergen in einem guten körperlichen Zustand. Und es seien trotz des massiven Eisverlustes sogar mehr Eisbären als noch im Jahr 2004. Die Norweger um den Wissenschaftler Jon Aars haben im Jahr 2015 die Eisbären auf Spitzbergen und im norwegischen Gebiet der Barentssee gezählt. Nach ihren Berechnungen besteht die Population in dieser Gegend aus insgesamt 975 Eisbären. Elf Jahre zuvor waren es nur 685.

Ganzen Artikel in der Welt lesen.

Knapp ein Jahr zuvor schwamm Die Welt noch auf der Alarmwelle und betrauerte das traurige Schicksal der Eisbären:

Die großen Verlierer des Klimawandels
Sie sind zur Ikone des Kampfs gegen den Klimawandel geworden: hungernde Eisbären auf dem schwindenden ewigen Eis. Die Weltnaturschutzorganisation wollte wissen, wie bedroht sie tatsächlich sind. […] „Das Ergebnis zeigt eine hohe Wahrscheinlichkeit, dass die globalen Eisbärenbestände in den kommenden 35 bis 40 Jahren um mehr als 30 Prozent schrumpfen werden“, erklärte die IUCN.

Nun wurde Die Welt offenbar eiskalt von rechts von den realen Zahlen überholt. Immerhin hatte die Zeitung den Mut, die unerwartet positive Entwicklung nicht zu verschweigen. Bereits damals, im November 2015, musste die IUCN-Prognose heftige Kritik einstecken, da sie auf fragwürdigen Zahlen des USGS aufbaute.

Weitere interessante Ergebnisse aus der Eisbärenforschung:

Im Dezember 2015 hatten Cronin & Cronin zeigen können, dass die Eisbären selbst die wärmsten Episoden der letzten anderthalb Millionen von Jahren überlebt haben, als das arktische Meereis vollkommen abgeschmolzen war.

Im September 2015 wurde ein Mythos entzaubert, nämlich dass Eisbären nur Robben essen könnten. Alles Quatsch. Das American Museum of Natural History gab per Pressemitteilung bekannt:

Polar bears may survive ice melt, with or without seals
New calculations indicate that land-based food sources like caribou, snow geese, and eggs might provide enough calories for bears to avoid starvation

As climate change accelerates ice melt in the Arctic, polar bears may find caribou and snow geese replacing seals as an important food source, shows a recent study published in the journal PLOS ONE. The research, by Linda Gormezano and Robert Rockwell at the American Museum of Natural History, is based on new computations incorporating caloric energy from terrestrial food sources and indicates that the bears‘ extended stays on land may not be as grim as previously suggested.

Polar bears are opportunists and have been documented consuming various types and combinations of land-based food since the earliest natural history records,“ said Rockwell, a research associate in the Museum’s Department of Ornithology who has been studying the Arctic ecology of the Western Hudson Bay for nearly 50 years. „Analysis of polar bear scats and first-hand observations have shown us that subadult polar bears, family groups, and even some adult males are already eating plants and animals during the ice-free period.“

Previous studies have predicted mass polar bear starvation by 2068, when annual ice breakup is expected to separate the bears from their sea-ice hunting grounds for a consecutive 180 days each year–creating ice-free seasons that will last two months longer than those in the 1980s. But those estimates assumed no energetic input from land food sources.

Gormezano and Rockwell computed the energy required to offset any increased starvation and then determined the caloric value of snow geese, their eggs, and caribou that live near the coast of the Western Hudson Bay. They found that there likely are more than enough calories available on land to feed hungry polar bears during the lengthening ice-free seasons.

Although the exact energetic cost for a bear to hunt geese and caribou is uncertain, polar bears in Manitoba have been reported ambushing caribou with the same energetically low-cost techniques they typically use to hunt seals. The similar size of these two prey species means that bears would need to hunt for caribou only as often as they would usually hunt for seals, the researchers say.

„If caribou herds continue to forage near the coast of Western Hudson Bay when bears come to shore earlier each year, they are likely to become a crucial component of the bears‘ summertime diet,“ Rockwell said.

The eggs of snow geese are another food source for bears, and the energetic cost of obtaining eggs in ground nests is exceedingly low, the researchers say. With adequate food sources available, snow geese are known to endure polar bear egg predation without detrimental effects to the population.

Scientific consensus holds that the rapidly melting circumpolar ice reserves will increasingly prevent polar bears from hunting the seals on which they currently depend. Nevertheless, these observations of one population along the Western Hudson Bay show that bears marooned on land might, where the conditions are right, stave off starvation by turning to alternate food sources.

Wenn die Robben ausbleiben, schmecken auch Delfine ganz lecker, wie 2015 ein Fotograf dokumentierte.

Im Juni 2015 freute sich die Presse über Capuccino-Bären, eine Mischung zwischen Eis- und Braunbären. Auf Druck des Klimawandels oder ganz einfach grenzüberschreitende Tierliebe? Die University of Washington enttäuschte die Anhänger der Klimakatastrophe mit der folgenden Pressemitteilung bitterlich:

Risk of interbreeding due to climate change lower than expected

One of the questions raised by climate change has been whether it could cause more species of animals to interbreed. Two species of flying squirrel have already produced mixed offspring because of climate change, and there have been reports of a hybrid polar bear and grizzly bear cub (known as a grolar bear, or a pizzly).

“Climate change is causing species’ ranges to shift, and that could bring a lot of closely related species into contact,” said Meade Krosby, a research scientist in the University of Washington’s Climate Impacts Group. She is the lead author of a study published July 6 in Nature Climate Change that tallies the potential number of such pairings. Looking across North and South America, it finds that only about 6 percent of closely related species whose ranges do not currently overlap are likely to come into contact by the end of this century. “People have been concerned that climate change would be bringing all these species into contact, and that this could unleash a wave of interbreeding,” Krosby said. “What we found is, not so much.”

A 2010 editorial in the journal Nature suggested that northern species may begin to interbreed and create a so-called “Arctic melting pot,” and even prompted one artist’s rendition of what those new offspring would look like. The idea also worried land managers looking at how to prepare for climate change. At a workshop, land managers told Krosby they worked with very closely related species separated by small distances. What if managers linked the two areas with a wildlife corridor, and as the climate changed the species started to mix?

This study is an attempt to see how much that should be a concern. It looked at 9,577 pairs of closely related species of birds, mammals and amphibians in North and South America. For the 4,796 pairs whose ranges currently do not overlap, computer models show that only 6.4 percent of them will come into contact due to climate change by the year 2100. The most overlap among species occurred in the tropics, and among birds, likely because more species live in the tropics and birds cover wider ranges, Krosby said.

While the study suggests that climate change is unlikely to result in widespread interbreeding, wildlife biologists still need to consider their particular region and animals of interest to best protect specific populations. “Managers still need to look case-by-case at species at a local scale, but at a global scale, the big picture is that it’s probably not going to be a huge problem,” Krosby said.

The study likely overestimates how many species could be at risk of interbreeding because it assumes that all species will be able to access new habitats that become available due to climate change. In fact, natural barriers prevent animals from reaching all potential new habitats, and humans have created new barriers such as highways, farms, and cities that can block migrations to more hospitable places. “The number one strategy for helping biodiversity respond to climate change is to increase connectivity, to link up habitats that have been fragmented by human activity, so species can move, and track climate as it shifts to stay comfortable,” Krosby said. “If people are worried that wildlife corridors and other ways to increase connectivity could bring these species into contact, we’re saying: That’s probably not going to happen, and allowing species to move is far more important.”

Krosby did her doctoral work looking at how historic climate changes affected species in the past, including how the end of the last ice age led to interbreeding among West Coast songbirds. Now she focuses on contemporary climate change, to see how species are responding and how land managers can best protect biodiversity under faster, human-driven changes to Earth’s climate. Co-authors include Joshua Lawler, an associate professor in the UW’s School of Environmental and Forest Sciences; Joshua Tewksbury, a UW professor of biology; postdoctoral researchers Theresa Nogeire and Julie Heinrichs in the School of Environmental and Forest Sciences; and former UW researchers Chad Wilsey, now at the National Audubon Society; Jennifer Duggan, now at California State University Monterey Bay; and Jenny McGuire, now at the Georgia Institute of Technology. The research was funded by the Wilburforce Foundation, the Doris Duke Foundation and the David and Lucile Packard Foundation.

Interessant auch die Übersicht von Susan Crockford die in einem Übersichtsbeitrag auf WUWT 10 fehlgegangene Eisbärhorrorszenarien entlarvt.

Im Mai 2016 gaben auch York et al. Entwarnung: Den kanadischen Eisbären geht es gut, dokumentierten sie in einem Paper im Fachblatt Ecology and Evolution.

Zuguter letzte: Eisbären sind viel bessere Schwimmer, als man lange dachte. Wenn ihnen das Eis zum Wandern fehlt, kraulen sie einfach durchs Wasser. In einem Fall dokumentierten Forscher einen Eisbären, der innerhalb von neun Tagen vierhundert Kilometer schwimmend zurücklegte. Die Zoological Society of San Diego gab am 19. April 2016 per Pressemitteilung bekannt:

Study indicates polar bears are swimming more as sea ice retreats

A study undertaken by scientists from the University of Alberta and Environment and Climate Change Canada to understand swimming behavior in polar bears is showing an increase in this behavior related to changes in the amount and location of summer sea ice. Lead author Nicholas Pilfold, now a postdoctoral fellow at San Diego Zoo Global, said „the pattern of long-distance swimming by polar bears in the Beaufort Sea shows the fingerprint of climate change. Swims are occurring more often, in association with sea ice melting faster and moving farther from shore in the summer.“

The study, published in a recent issue of the journal Ecography, was accomplished using satellite-linked telemetry-tracked populations of polar bears in the Beaufort Sea and Hudson Bay. Results of the study show an increase in swimming associated with reduced ice, due to climate change. In 2012, the year in which Arctic sea ice hit a record low, 69 percent of the tracked adult females in the Beaufort Sea swam more than 31 miles (50 kilometers) at least once.

„Recent studies indicate that swimming may be energetically costly to polar bears,“ said Nicholas Pilfold. „Given the continued trend of sea ice loss, we recognize that an increased frequency in the need to engage in this behavior may have serious implications for populations of polar bears living around the Arctic Basin.“

Swimming frequency and other movement factors varied between individual bears and showed differences dependent on age, sex, body size and geographic features of the region. Swims occurred more frequently in the Beaufort Sea than in Hudson Bay. Researchers noted that females with young cubs tended to swim less to avoid submersion of youngsters in cold waters, while lone subadults swam as frequently as lone adults. The longest recorded swim in the study was by a subadult female that traveled over 249 miles (400 kilometers) in nine days.