2023 Author: Bryan Walter | [email protected]. Last modified: 2023-05-21 22:24
California sea otter (Enhydra lutris nereis) with purple strongylocentrotus sea urchin (Strongylocentrotus purpuratus).
California sea otters are helping surviving algal forests in Monterey Bay weather an outbreak in sea urchins. After a period of record hot weather in 2014, the ecosystem underwent a shift: the herbivorous sea urchin population increased dramatically, as a result of which the dense thickets of algae turned into a mosaic interspersed with wastelands. However, sea otters also increased in number - and now they more often hunt hedgehogs. Basically, they feed on the surviving areas of forests, protecting them from excessive grazing. As noted in an article for the journal Proceedings of the National Academy of Sciences, in the future, these fragments may be a source of spores for colonizing the deserted areas of the seabed where algal forests once grew.
Kelp forests are among the richest and most productive ecosystems in the ocean. Unfortunately, human activities, including over-harvesting of marine organisms and the extermination of large marine predators, have caused them serious harm. In recent decades, one more problem has been added to these problems - heat waves that have become more frequent due to anthropogenic climate changes, which slow down the growth of algae and make them more vulnerable to grazing.
The problems of algal forests are especially noticeable in the cold waters off the coast of Central California. For millennia, there have been extensive thickets of macrocystis pyrifera algae (Macrocystis pyrifera). They fed the purple strongylocentrotus sea urchins (Strongylocentrotus purpuratus), whose numbers were controlled by the Californian sea otters (Enhydra lutris nereis) and predatory sunflower sea stars (Pycnopodia helianthoides). At the same time, sea otters in many parts of their former range were exterminated for the sake of fur, so that sea stars became the main consumers of sea urchins.
This stable system began to collapse in 2014, when the water in the Pacific Northwest warmed to anomalous values, as a result of which the growth of cold-loving macrocystis slowed sharply. At the same time, the sunflower starfish almost died out due to a mysterious disease, the spread of which was probably also triggered by unusually high temperatures. Having lost their enemies and faced with a shortage of food, sea urchins increased their numbers and switched to active feeding on live algae (before they picked up fragments of algae that fell to the bottom and tried not to show themselves to predators, hiding in crevices). As a result, continuous algal thickets turned into a mosaic landscape of remnants of forests and wastelands formed by voracious hedgehogs.
A team of researchers led by Joshua G. Smith from the University of California at Santa Cruz decided to find out how sea otters, the main enemies of sea urchins, reacted to changes in the ecosystem. To do this, they went to Monterey Bay, where the number of sea otters, thanks to protection, has been slowly increasing since the 1960s. As in other regions of Central California, here the algal forests have been replaced by a mosaic of fragments of thickets and wastelands of sea urchins. In 2017-2019, scientists conducted a series of dives to assess conditions at the bottom of the bay and calculate the population density of sea urchins. In addition, the behavior of these echinoderms was taken into account. The analysis also included data on the number of sea otters in Monterey in 2000-2013 and 2014-2018 and on their diet.
The analysis showed that the decline in the number of sunflower sea stars and the decrease in the density of brown algae coincided with a rapid increase in the number of sea urchins. This was followed by more sea otters in Monterey Bay. If in 2000-2013 a relatively stable population of about 269 individuals lived here, then after 2014 it grew to 432 individuals. According to the authors, this is due to the fact that, due to the abundance of food, more calves and young individuals began to survive. Since in one of the two neighboring populations the number of sea otters also increased, although not so sharply, and in the second it remained approximately at the same level, the migration of some of them cannot explain the increase in the number of the species in Monterey Bay.
Since 2014, sea otters of the bay began to eat more sea urchins: their share in the diet of the species increased both at the level of the entire population and among individuals that previously specialized in other types of prey. In addition, the authors began to note more individuals preferring sea urchins to other foods (p <0.01).
As observations of the behavior of hedgehogs have shown, in the wastelands they mostly feed openly, however, in the preserved areas of thickets, where the density of macrocystis exceeds one trunk per square meter, they mostly hide in cracks among stones and rocks and eat not live algae, but them fragments fallen to the bottom. At the same time, in individuals that keep in open areas, the volume of gonads is less than in their relatives from thickets (p <0, 0001). This indicates that in the wastelands the physical condition of hedgehogs is worse, although here they are more active and more numerous.
Sea otters discovered earlier than scientists that among thickets sea urchins are more nutritious. This is evidenced by the fact that in Monterey Bay these predators prefer to hunt echinoderms with large gonads that live in the remaining areas of the forest. At the same time, sea otters avoid less nutritious hedgehogs and therefore try not to hunt in the wastelands. In addition, they are reluctant to dive to great depths.
While hunting in preserved areas of algal forests, sea otters protect them from being devoured by sea urchins, Smith and his co-authors say. In the presence of predators, these echinoderms behave more cautiously and instead of openly feeding on algae, they eat their dead fragments sitting in shelters. Thus, the changed behavior of sea otters increased the stability of the surviving fragments of the ecosystem. The authors acknowledge that sea otters are unlikely to directly help algal forests reoccupy areas that have turned into wastelands, because they are extremely reluctant to feed here. However, the remaining thickets protected by these predators will become a source of spores that will one day populate areas of the seabed ravaged by hedgehogs.
Earlier, we talked about how experts evaluated the role of sea otters for the Canadian economy. It turned out that the benefits of restoring their populations outweigh the damage to trappers of crabs, sea urchins and bivalve molluscs. With the return of sea otters, the productivity of marine ecosystems will increase by 37 percent, and the profit from eco-tourism, which will be provided by those who wish to see them, will be six times higher than the monetary losses from reduced seafood production.
