Can sea otters help kelp under a changing climate?

By Dominique Kone1 and Sara Hamilton2

1Masters Student in Marine Resource Management, 2Doctoral Student in Integrative Biology

Five years ago, the North Pacific Ocean experienced a sudden increase in sea surface temperature (SST), known as the warm blob, which altered marine ecosystem function and structure (Leising et al. 2015). Much research illustrated how the warm blob impacted pelagic ecosystems, with relatively less focused on the nearshore environment. Yet, a new study demonstrated how rising ocean temperatures have partially led to bull kelp loss in northern California. Unfortunately, we are once again observing similar warming trends, representing the second largest marine heatwave over recent decades, and signaling the potential rise of a second warm blob. Taken together, all these findings could forecast future warming-related ecosystem shifts in Oregon, highlighting the need for scientists and managers to consider strategies to prevent future kelp loss, such as reintroducing sea otters.

In northern California, researchers observed a dramatic ecosystem shift from productive bull kelp forests to purple sea urchin barrens. The study, led by Dr. Laura Rogers-Bennett from the University of California, Davis and California Department of Fish and Wildlife, determined that this shift was caused by multiple climatic and biological stressors. Beginning in 2013, sea star populations were decimated by sea star wasting disease (SSWD). Sea stars are a main predator of urchins, causing their absence to release purple urchins from predation pressure. Then, starting in 2014, ocean temperatures spiked with the warm blob. These two events created nutrient-poor conditions, which limited kelp growth and productivity, and allowed purple urchin populations to grow unchecked by predators and increase grazing on bull kelp. The combined effect led to approximately 90% reductions in bull kelp, with a reciprocal 60-fold increase in purple urchins (Figure 1).

Figure 1. Kelp loss and ecosystem shifts in northern California (Rogers-Bennett & Catton 2019).

These changes have wrought economic challenges as well as ecological collapse in Northern California. Bull kelp is important habitat and food source for several species of economic importance including red abalone and red sea urchins (Tegner & Levin 1982). Without bull kelp, red abalone and red sea urchin populations have starved, resulting in the subsequent loss of the recreational red abalone ($44 million) and commercial red sea urchin fisheries in Northern California. With such large kelp reductions, purple urchins are also now in a starved state, evidenced by noticeably smaller gonads (Rogers-Bennett & Catton 2019).

Biogeographically, southern Oregon is very similar to northern California, as both are composed of complex rocky substrates and shorelines, bull kelp canopies, and benthic macroinvertebrates (i.e. sea urchins, abalone, etc.). Because Oregon was also impacted by the 2014-2015 warm blob and SSWD, we might expect to see a similar coastwide kelp forest loss along our southern coastline. The story is more complicated than that, however. For instance, ODFW has found purple urchin barrens where almost no kelp remains in some localized places. The GEMM Lab has video footage of purple urchins climbing up kelp stalks to graze within one of these barrens near Port Orford, OR (Figure 2, left). In her study, Dr. Rogers-Bennett explains that this aggressive sea urchin feeding strategy is potentially a sign of food limitation, where high-density urchin populations create intense resource competition. Conversely, at sites like Lighthouse Reef (~45 km from Port Orford) outside Charleston, OR, OSU and University of Oregon divers are currently seeing flourishing bull kelp forests. Urchins at this reef have fat, rich gonads, which is an indicator of high-quality nutrition (Figure 2, right).

Satellites can detect kelp on the surface of the water, giving scientists a way to track kelp extent over time. Preliminary results from Sara Hamilton’s Ph.D. thesis research finds that while some kelp forests have shrunk in past years, others are currently bigger than ever in the last 35 years. It is not clear what is driving this spatial variability in urchin and kelp populations, nor why southern Oregon has not yet faced the same kind of coastwide kelp forest collapse as northern California. Regardless, it is likely that kelp loss in both northern California and southern Oregon may be triggered and/or exacerbated by rising temperatures.

Figure 2. Left: Purple urchin aggressive grazing near Port Orford, OR (GEMM Lab 2019). Right: Flourishing bull kelp near Charleston, OR (Sara Hamilton 2019).

The reintroduction of sea otters has been proposed as a solution to combat rising urchin populations and bull kelp loss in Oregon. From an ecological perspective, there is some validity to this idea. Sea otters are a voracious urchin predator that routinely reduce urchin populations and alleviate herbivory on kelp (Estes & Palmisano 1974). Such restoration and protection of bull kelp could help prevent red abalone and red sea urchin starvation. Additionally, restoring apex predators and increasing species richness is often linked to increased ecosystem resilience, which is particularly important in the face of global anthropogenic change (Estes et al. 2011)

While sea otters could alleviate grazing pressure on Oregon’s bull kelp, this idea only looks at the issue from a top-down, not bottom-up, perspective. Sea otters require a lot of food (Costa 1978, Reidman & Estes 1990), and what they eat will always be a function of prey availability and quality (Ostfeld 1982). Just because urchins are available, doesn’t mean otters will eat them. In fact, sea otters prefer large and heavy (i.e. high gonad content) urchins (Ostfeld 1982). In the field, researchers have observed sea otters avoiding urchins at the center of urchin barrens (personal communication), presumably because those urchins have less access to kelp beds than on the barren periphery, and therefore, are constantly in a starved state (Konar & Estes 2003) (Figure 3). These findings suggest prey quality is more important to sea otter survival than just prey abundance.

Figure 3. Left: Sea urchin barren (Annie Crawley). Right: Urchin gonads (Sea to Table).

Purple urchin quality has not been widely assessed in Oregon, but early results show that gonad size varies widely depending on urchin density and habitat type. In places where urchin barrens have formed, like Port Orford, purple urchins are likely starving and thus may be a poor source of nutrition for sea otters. Before we decide whether sea otters are a viable tool to combat kelp loss, prey surveys may need to be conducted to assess if a sea otter population could be sustained based on their caloric requirements. Furthermore, predictions of how these prey populations may change due to rising temperatures could help determine the potential for sea otters to become reestablished in Oregon under rapid environmental change.

Recent events in California could signal climate-driven processes that are already impacting some parts of Oregon and could become more widespread. Dr. Rogers-Bennett’s study is valuable as she has quantified and described ecosystem changes that might occur along Oregon’s southern coastline. The resurgence of a potential second warm blob and the frequency between these warming events begs the question if such temperature spikes are still anomalous or becoming the norm. If the latter, we could see more pronounced kelp loss and major shifts in nearshore ecosystem baselines, where function and structure is permanently altered. Whether reintroducing sea otters can prevent these changes will ultimately depend on prey and habitat availability and quality, and should be carefully considered.


Costa, D. P. 1978. The ecological energetics, water, and electrolyte balance of the California sea otter (Enhydra lutris). Ph.D. dissertation, University of California, Santa Cruz.

Estes, J. A. and J.F. Palmisano. 1974. Sea otters: their role in structuring nearshore communities. Science. 185(4156): 1058-1060.

Estes et al. 2011. Trophic downgrading of planet Earth. Science. 333(6040): 301-306.

Harvell et al. 2019. Disease epidemic and a marine heat wave are associated with the continental-scale collapse of a pivotal predator (Pycnopodia helianthoides). Science Advances. 5(1).

Konar, B., and J. A. Estes. 2003. The stability of boundary regions between kelp beds and deforested areas. Ecology. 84(1): 174-185.

Leising et al. 2015. State of California Current 2014-2015: impacts of the warm-water “blob”. CalCOFI Reports. (56): 31-68.

Ostfeld, R. S. 1982. Foraging strategies and prey switching in the California sea otter. Oecologia. 53(2): 170-178.

Reidman, M. L. and J. A. Estes. 1990. The sea otter (Enhydra lutris): behavior, ecology, and natural history. United States Department of the Interior, Fish and Wildlife Service, Biological Report. 90: 1-126.

Rogers-Bennett, L., and C. A. Catton. 2019. Marine heat wave and multiple stressors tip bull kelp forest to sea urchin barrens. Scientific Reports. 9:15050.

Tegner, M. J., and L. A. Levin. 1982. Do sea urchins and abalones compete in California? International Echinoderms Conference, Tampa Bay. J. M Lawrence, ed.

Burning it down

By Leila S. Lemos, PhD Candidate in Wildlife Sciences, Fisheries and Wildlife Department, OSU

As you might know, the GEMM Lab (Geospatial Ecology of MARINE Megafauna Laboratory) researches the marine environment, but today I am going to leave the marine ecosystem aside and I will discuss the Amazon biome. As a Brazilian, I cannot think of anything else to talk about this week than the terrifying fire that is burning down the Amazon forest in this exact minute.

For some context, the Amazon biome is known as the biome with the highest biodiversity in the world (ICMBio, 2019). It is the largest biome in Brazil, accounting for ~49% of the Brazilian territory. This biome houses the biggest tropical forest and hydrographic basin in the world. The Amazon forest also extends through eight other countries: Bolivia, Colombia, Ecuador, Guiana, French Guiana, Peru, Suriname and Venezuela. To date, at least 40,000 plant species, 427 mammals, 1,300 birds, 378 reptiles, more than 400 amphibians, around 3,000 freshwater fishes, and around 100,000 invertebrate species have been described by scientists in the Amazon, comprising more than 1/3 of all fauna species on the planet (Da Silva et al. 2005, Lewinsohn and Prado 2005). And, these numbers are likely to increase; According to Patterson (2000), one new genus and eight new species of Neotropical mammals are discovered each year in the region.

I feel very connected to the Amazon as I worked as an environmental consultant and field coordinator in 2014 and 2015 (Figs. 1 and 2) along the Madeira river (or “Wood” river) in Rondonia, Brazil (Fig. 3). I monitored Amazon river dolphins (Inia geoffrensis; Fig. 4), a species considered endangered by the IUCN Red List in 2018 (Da Silva et al. 2018). The Madeira river originates in Bolivia and flows into the great Amazon river, comprising one of its main tributaries (Fig. 3).

Figure 1: Me, working along the Madeira river, Rondonia, Brazil, in 2015.
Source: Laura K. Honda, 2015.

Figure 2: Me, helping to rescue a sloth from the Madeira river, Rondonia, Brazil, in 2014.
Source: Roberta Lanziani, 2014.

Figure 3: The Amazon hydrographic basin, with the Madeira river highlighted.
Source: Wikipedia, 2019.

Figure 4: Amazon river dolphins (I. geoffrensis) along the Madeira river, Rondonia, Brazil.
Source: Leila S. Lemos, 2014; 2015.

Here is also a video where you can see some Amazon river dolphins along the Madeira river:

Source: Leila S. Lemos, 2014; 2015.

In addition to the dolphins, I witnessed the presence of many other fauna specimens like birds (including macaws and parrots), monkeys, alligators and sloths (Fig. 5). The biodiversity of the Amazon is unquestionable.

Figure 5: Macaws (Ara chloropterus), parrots (Amazona sp.) and the Guariba monkey or brown howler (Allouatta guariba) along the Madeira river, Rondonia, Brazil.
Source: Leila S. Lemos

Other than its great biodiversity, the Amazon is known as the “lungs of the Earth”, which is an erroneous statement since plants consume as much oxygen as they produce (Malhi et al. 2008, Malhi 2019). But still, the Amazon forest is responsible for 16% of the oxygen produced by photosynthesis on land and 9% of the oxygen on the global scale (Fig. 6). This seems a small percentage, but it is still substantial, especially because the plants use carbon dioxide during photosynthesis, which accounts for a 10% reduction of atmospheric carbon dioxide. Thus, imagine if there was no Amazon rainforest. The rise in carbon dioxide would be enormous and have serious implications on the global climate, surpassing safe temperature boundaries for many regions.

Figure 6: Total photosynthesis of each major land biome. This value is multiplied by 2.67 to convert to total oxygen production. Hence total oxygen production by photosynthesis on land is around 330 Pg of oxygen per year. The Amazon (just under half of the tropical forests) is around 16% of this, around 54 Pg of oxygen per year.
Source: Malhi 2019.

Unfortunately, this scenario is not really far from us. Even though deforestation indices have fallen in the last 15 years, fire incidence associated with droughts and carbon emissions have increased (Aragão et al. 2018; Fig. 7).

Figure 7: Linear trends (2003–2015) of annual (a) deforestation rates, and (b) active fires counts in the Brazilian Amazon. Red circles indicate the analyzed drought years by Aragão et al. (2018).
Source: Aragão et al. 2018.

Since August 2019, the Amazon forest has experienced extreme fire outbreaks (Figs. 8 and 9). Around 80,000 fires occurred only in 2019. Despite 2019 not being an extreme drought year, the period of January-August 2019 is characterized by an ~80% increase in fires compared to the previous year (Wagner and Hayes 2019). The intensification of the fires has been linked to the Brazilian President’s incentive to “open the rainforest to development”. Leaving politics aside, the truth is that the majority of these fires have been set by loggers and ranchers seeking to clear land to expand the agro-cattle area (Yeung 2019).

Figure 8: The Amazon in July 28: just clouds; and in August 22: choked with smoke.
Source: NOAA, in: Wagner and Hayes, 2019.

Figure 9: Images showing some of the destruction caused by the fires in the Amazon region in 2019.
Source: Buzz Feed News 2019, Sea Mashable 2019.

Here you can see some videos showing the extension of the problem:

Video 1 – by NBC News:

Video 2 – a drone footage by The Guardian:

I consider myself lucky for the opportunity to have worked in the Amazon rainforest before these chaotic fires have destroyed so much biodiversity. The Amazon is a crucial home for countless animal and plant species, and to ~900,000 indigenous individuals that live in the region. They are all at risk of losing their homes and lives. We are all at risk of global warming.


Aragão LEOC, Anderson LO, Fonseca MG, Rosan TM, Vedovato LB, Wagner FH, Silva CVJ, Silva Junior CHL, Arai E, Aguiar AP, Barlow J, Berenguer E, Deeter MN, Domingues LG, Gatti L, Gloor M, Malhi Y, Marengo JA, Miller JB, Phillips OL, and Saatchi S. 2018. 21stCentury drought-related fires counteract the decline of Amazon deforestation carbon emissions. Nature Communications 9(536):1-12.

Buzz Feed News. 2019. These Heartbreaking Photos Show The Devastation Of The Amazon Fires. Retrieved 1 September 2019 from

Da Silva JMC, Rylands AB, and Da Fonseca GAB. 2005. The Fate of the Amazonian Areas of Endemism. Conservation Biology 19(3):689-694.

Da Silva V, Trujillo F, Martin A, Zerbini AN, Crespo E, Aliaga-Rossel E, and Reeves R. 2018. Inia geoffrensis. The IUCN Red List of Threatened Species 2018: e.T10831A50358152. Downloaded on 27 August 2019.

ICMBio. 2019. Amazônia. Retrieved 26 August 2019 from deconservacao/biomas-brasileiros/amazonia

Lewinsohn TM, and Prado PI. 2005. How Many Species Are There in Brazil? Conservation Biology 19(3):619.

Malhi Y. 2019. does the amazon provide 20% of our oxygen? Travels in ecosystem science. Retrieved 29 August 2019 from

Malhi Y., Roberts JT, Betts RA, Killeen TJ, Li W, Nobre CA. 2008. Climate Change, Deforestation, and the Fate of the Amazon. Science 319:169-172.

Patterson BD. 2000. Patterns and trends in the discovery of new Neotropical mammals. Diversity and Distributions, 6, 145-151.

Sea Mashable. 2019. The Amazon forest is burning to the ground. Here’s how it happened and what you can do to help. Retrieved 1 September 2019 from

Wagner M, and Hayes M. 2019. Wildfires rage in the Amazon. CNN. Retrieved 26 August 2019 from

Wikipedia. 2019. Madeira river. Retrieved 29 August 2019 from

Yeung J. 2019. Blame humans for starting the Amazon fires, environmentalists say. CNN. Retrieved 26 August 2019 from