A cascade of otters

4 04 2022

Carnivores are essential components of trophic webs, and ecosystem functions crumble with their loss. Novel data show the connection between calcareous reefs and sea otters under climate change.

Trophic cascade on the Aleutian Islands (Alaska, USA) linking sea otters (Enhydra lutris) with sea urchins (Strongylocentrotus polyacanthus) and calcareous reefs (Clathromorphum nereostratum). With males weighting up to 50 kg, sea otters have been IUCN-catalogued as Endangered since 2000. The top photo shows a male in a typical, belly-up floating position. The bottom photo shows live (pinkish) and dead (whitish) tissue on the reef surface as a result of grazing of sea urchins at a depth of 10 m. Sea otters are mesopredators, typically foraging on small prey like sea urchins, but their historical decline due to overhunting unleashed the proliferation of the echinoderms. At the same time, acidification and sea-water warming have softened the skeleton of the reefs, allowing for deeper grazing by sea urchins that eliminate the growth layer of living tissue that give the reefs their pinkish hue. Large extents of dead reefs stop fixing the excess in carbonic acid, whose carbon atoms sea water sequesters from the atmosphere enriched in carbon by our burning of fossil fuels. Photos courtesy of Joe Tomoleoni taken in Moss Landing – California, USA (otter), and on the Near Islands – Aleutian Archipelago, Alaska (reef).

For most, the decisions made by people we have never met affect our daily lives. Other species experience the same phenomenon because they are linked to one another through a trophic cascade.

A trophic cascade occurs when a predator limits the abundance or behaviour of its prey, in turn affecting the survival of a third species in lower trophic levels that have nothing directly to do with the predator in question (1).

Sea otters (Enhydra lutris) represent a text-book example of a trophic cascade. These mustelids (see video footage here and here) hunt and control the populations of sea urchins (Strongylocentrotus polyacanthus), hence favouring kelp forests  — the fronds of which are eaten by the sea urchins.

Removing the predator from the equation should lead to more sea urchins and less kelp, and this chain of events is exactly what happened along the coasts of the North Pacific (2, 3). The historical distribution of sea otters once ranged from Japan to Baja California through the Aleutian Islands (see NASA’s photo from space, and documentary on the island of Unimak), a sub-Arctic, arc-shaped archipelago including > 300 islands between Alaska (USA) and the Kamchatka Peninsula (Russia), extending ~ 2000 kilometres, and having a land area of ~ 18,000 km2.

But the fur trade during the 18th and 19th centuries brought the species to the brink of extinction, down to < 2000 surviving individuals (4). Without otters, sea urchins boomed and deforested kelp ecosystems during the 20th Century (5). Now we also know that this trophic cascade has climate-related implications in other parts of the marine ecosystem.

Underwater bites

Doug Rasher and collaborators have studied the phenomenon on the Aleutian Islands (6). The seabed of this archipelago is a mix of sandy beds, kelp forests, and calcareous reefs made up of calcium and magnesium carbonates fixed by the red algae Clathromorphum nereostratum. These reefs have grown at a rate of 3 cm annually for centuries as the fine film of living tissue covering the reef takes the carbonates from the seawater (7).

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DNA barcoding plants with citizen science

28 08 2013

hikingI was contacted recently by Oscar Jaslowski of Microryza (a web platform that allows scientists to post research  ideas and collect contributions from web visitors) about a project getting underway in Alaska by Ellen Jorgensen of Genspace. He suggested it might make a good post for ConservationBytes.com, and I agreed. Thanks for the contribution, Ellen & Oscar.

There’s nothing so final as watching the bush pilot take off in his tiny plane, leaving you stranded in the Alaskan backcountry. We had plenty of food for a three-day expedition, but no satellite phone or any other way to contact anyone. In Alaska, the phrase ‘primordial indifference’ pretty much sums up your relationship with the vast, glacier-carved landscape. Mother Nature does not care if an ant like you lives or dies.

Our destination, the Skolai Valley, is located about 480 km (300 miles) east of Anchorage, in the heart of Wrangell-St. Elias National Park. At a whopping 5.3 million hectares (13 million acres), it is the largest national park in the United States, and probably one of the least-visited. Much of its forbidding territory is snow-covered and similar to the Himalayas. In fact, the size of the massive ice fall that towers over the town of McCarthy, the origin of our flight, is exceeded only by one near Mt. Everest. But winding through the glaciers and snowfields are alpine valleys that are a backpacker’s dream. And Genspace, the nonprofit science-based organisation that I direct, was lucky enough to have received funding in 2012 to launch this expedition to Skolai.

Our  mission: to barcode wild Alaskan plant life. Two of us headed down into the river valley and the other two climbed up to the level of the mountain pass to survey more alpine vegetation. We were carrying portable plant presses – normally something too bulky for backpacking, but necessary for this trip. Read the rest of this entry »

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