Hidden Stars of the Sea: How the Sunflower Sea Star Protects Rocky Reefs
By Stephanie Tsui
Sunflower sea stars may not look like the cuddly sea otters we love to marvel at in aquariums, but both species play similar roles in their natural habitats. They are found in kelp forests, which are one of the most productive ecosystems on Earth. Kelp forests are vital ecosystems that can provide habitat for over 200 species (Graham 2004). Scientists have already studied the effects of otters on urchins, but little is known about the importance of sea stars. Sea otters are known as keystone predators, which are species that have a disproportionately large effect on the balance of an ecosystem by feeding. Without the keystone predator, the system can fall out of equilibrium. In terms of sea otters, these animals protect kelp forests by eating and controlling urchin numbers.
A little bit of history
In the past, sea otters were hunted extensively for their fur. As a result, their populations dwindled, and scientists observed something bizarre — kelp forests started to decline. When urchin populations grow out of control, they feed voraciously on kelp, creating seemingly empty landscapes known as urchin barrens. Once these habitats have been altered, it is difficult for them to bounce back to their original state.
Since a lot of attention has been put on the power of sea otters, Dr. Jenn M. Burt and her colleagues were curious to see if other species have a similar impact. They investigated the effect of sunflower sea stars on urchins and kelp. The story behind the sunflower sea star is interesting. In 2013, the northeast Pacific was hit with an epidemic of Sea Star Wasting Disease (SSWD). This disease decimated sea star populations of over 20 species, including the sunflower sea star. It causes sea stars to develop lesions, deflate, lose arms, and dissolve to death (Kohl et al. 2016; Menge et al. 2016). Because of this massive die-off, scientists were able to explore how urchin populations and kelp densities changed before and after the disease.
The study design
Dr. Burt and her colleagues surveyed 11 different sites along the central coast of British Columbia, Canada from 2013 to 2016. Sea otters were initially present at five of these sites and appeared during the course of the study at three other locations. At each location, scientists measured the density of sea stars, red urchins, and kelp within a given area. They chose to focus on the red urchin species because they were the most abundant urchin species in the area. While the outbreak of SSWD began in 2013, the disease only appeared in this region in January 2015. Thus, they were able to compare how urchin populations changed two years before and two years after the arrival of the disease, as well as with or without the presence of sea otters.
In this study, researchers also compared the different sizes of urchins eaten by otters and sea stars. They separated urchins into three size classes: small, medium, and large. Otters tend to eat the largest urchins while sea stars eat medium-sized urchins. Unlike large urchins, which can flee, or small urchins, which can hide under rocks or other urchins, medium-sized urchins are at a disadvantage when dealing with predators. Using the collected data, they produced models to determine if otters or sea stars had a larger effect on urchin populations of the different size classes.
How do sunflower sea stars and sea otters compare?
The results from this study showed that although sea otters have greater control of urchin populations, sunflower sea stars also help support rocky reef habitats. When the sea otters returned, sea urchin populations declined. Urchin numbers grew, however, when SSWD hit sea star populations. In terms of size, researchers found that sea otters have the greatest impact on large-sized urchin densities while sea stars influence medium- and small-sized urchins. Otters require more energy; therefore, they prefer to eat large urchins or other high-calorie prey. Sea stars are slow-moving predators that are too small to feed on large urchins.
Since urchin populations are directly affected by the presence of sea otters and sea stars, the amount of kelp consumed increased when SSWD arrived. Sunflower sea stars were not present to control the medium- and small-sized urchin populations. The presence of sea stars alone is not enough to prevent urchins from overgrazing on kelp and creating urchin barrens. Thus, sea otters are still essential for protecting kelp densities on rocky reefs.
Sea stars, sea otters, and conserving kelp forests
This study is not perfect. Even though urchins are directly consumed by sea otters and sea stars, it is possible that urchins also change their behavior in the presence or absence of predators. With fewer sea stars present, medium- and small-sized urchins may be less likely to hide and therefore consume more kelp than usual. This idea does not change the results of their study. Instead, it implies that the mechanism behind the sunflower sea star’s role in rocky reefs is unclear. Is it the mere presence of the sea star that controls urchin populations, or is it the act of predation? Regardless, sunflower sea stars play a vital role in these habitats.
If we want to protect and conserve kelp forests from overgrazing, we need to better understand who contributes to managing urchin populations. The role of sunflower sea stars has been overlooked in the past, yet future studies can further explore their importance. With this knowledge, we can build better management decisions and make stronger predictions about kelp-dominated habitats. Not only should we focus on the rehabilitation and reintroduction of sea otter populations, but we should also put our efforts into restoring sea star populations damaged by SSWD. Without sunflower sea stars, sea otters lose their trusty sidekicks.
References
Burt, J. M., Tinker, M. T., Okamoto, D. K., Demes, K. W., Holmes, K. and Salomon, A. K. 2018. Sudden collapse of a mesopredator reveals its complementary role in mediating rocky reef regime shifts. Proceedings of the Royal Society London B 285: 20180553.
Graham, M. H. 2004. Effects of local deforestation on the diversity and structure of southern California giant kelp forest food webs. Ecosystems 7: 341–357.
Kohl, W. T., McClure T. I., and B. G. Miner. 2016. Decreased temperature facilitates short-term sea star wasting disease survival in the keystone intertidal sea star Pisaster ochraceus. PLoS One 11(4): e0153670.
Menge, B. A., Cerny-Chipman, E. B., Johnson, A., Sullivan, J., Gravem, S., and F. Chan. 2016. Sea star wasting disease in the keystone predator Pisaster ochraceus in Oregon: insights into differential population impacts, recovery, predation rate, and temperature effects from long-term research. PLoS ONE 11(5): e0153994.
