Populations Can’t Grow without Homes

Sea otters are one of many charismatic species found along the California coast, yet recovery doesn’t seem to be helping them. Is it something about their habitat that is preventing population growth? (Image Credit: Mike Baird, CC BY 2.0, Image Cropped)

Gaps in kelp cover may threaten the recovery of California sea otters  (2018) Nicholson et al., Ecography, DOI:10.1111/ecog.03561

The Crux

In the 18th and 19th centuries, the fur trade was a massive industry in North America. As a result, many species were hunted and trapped to near extinction. The California sea otter (Enhydra lutris) was reduced in population to less than 50 total individuals. The enactment of the Internation Fur Treaty allowed the species (and others) to come back from the brink of extinction, and they now number over 3200 individuals and are spread across 525km of the California coast. Interestingly, although the population is recovering, it has not bounced back as quickly as other protected mammals living in the same habitat. The California sea lion, for example, has a maximum population growth rate more than twice that of the sea otter (11.7% compared to 5%).

Despite the remarkable recovery of the species, the sea otters occupy less than a quarter of their historic range and have not expanded along the coast in 20 years. The authors of this paper wanted to investigate what it is about the sea otters and their habitat that is slowing this population’s growth rate and spread along the coast.

What They Did

The authors used a dataset on sea otter strandings from 1984-2015 compiled by the Monterey Bay Aquarium, the California Department of Fish and Wildlife, The Marine Mammal Center, and many volunteers. A stranding is when a sea otter is found washed up on the beach, either alive and injured/sick or dead. When the otter is collected, data on the age, sex, date, location, body length, condition of the body, and apparent cause of stranding is compiled. The authors also collected data on the sea otter population numbers and kelp distribution over the years, as kelp is the primary shelter used by the sea otters. The researchers then analyzed the stranding events to determine what factors could be the cause.

Did You Know: Community Science

Not all of the data used in scientific studies comes from the researchers that write the papers. There are many databases compiled by amateur naturalists, enthusiasts, and other members of the non-scientific public.

A great example of this is eBird, “the world’s largest biodiversity-related citizen science project”,  an online database compiled by birders (people that like to find and look at birds) from around the world. Every year over 100 million bird sightings are entered into the database, along with data like the location and time of year. Large databases like these allow scientists access to enormous amounts of information to answer questions in their own research.

What They Found

Increases in kelp cover strongly reduced the likelihood of strandings, but the causes of stranding varied through space and time. Emaciation of females after feeding pups was the primary cause in the center of the range (due to the poorer body condition of the otters there, which is a result of greater competition in these more densely populated areas), but everywhere else the main cause of stranding was neurological disease and shark bites. Stranding rates in the last decade increased along the periphery of the otter’s range, with increasing threefold along the southern edge. Over the course of the data collection, strandings due to shark bites quadrupled. Not surprisingly, shark bite-related stranding was negatively associated with kelp cover, as the kelp provides protection from the searching sharks.

P8112237

Great White Sharks like this one are the main killer of the sea otters, and a lack of kelp cover means that they are exposed and vulnerable to these massive predators. (Image credit: Elias Levy, CC BY 2.0)

Problems

The authors point out that detection biases for these stranding events are unknown, which means that a specific cause of stranding may be more likely than the others to result in a stranded otter. Because of this, the authors have to assume that all of the causes of stranding have equal chances of resulting in an otter washing up on the shore. While this may not be the case in nature, the authors took steps to correct for these potential biases.

So What?

Although human-induced mortality has decreased since the otters were given protected status, this study has shown that it takes more than protection from humans for a population to grow and spread. The different habitats that these otters live in seem to determine the demographic rates, with more prey and sheltering habitat in the center allowing for higher population numbers, countered by the sparse kelp cover and increased predation limiting population growth at the range limits.

This study provides a great example of what scientists can do with long-term datasets that are (at least in part) maintained by the non-scientific public. Like we talked about recently with shifting baselines, we need to keep the bigger picture in mind and not just make comparisons between recent events.

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