By Melissa Keesee

While diving with high school students in San Clemente, California that were researching an invading species, I was struck with the realization that the Sargassum weed they were researching can’t be the only invasive species spreading due to the warming temperatures in the area. As I dove deeper,  both literally and in my research, I found that there are many examples of invaders taking over the Pacific, that have warming ocean temperatures on their side.

Ocean Warming and Invasive Species, What’s the Connection?

Global climate change is not just an atmospheric issue. Surface and deep water temperatures have been reported to see changes between 0.14 and 1.0° C over the last 60 years [1]. There are a few theories on how these changing ocean temperatures can affect the movement of marine organisms. Since changes in temperatures cause stress on organisms that live in an area, they may move out of the area or die off. This creates space for new organisms to move in or take over with less competition [1]. There are also seasonal movements to consider. Some organisms move locations seasonally, and while absent, new invaders move into those areas causing increased competition when the native species return [1]. Since marine organisms in the Northern Pacific Ocean tend to move poleward with warming ocean temperatures[2], this left me many prime examples to further research. 

Diving with Sargassum Weed 

I began my journey researching invasive Pacific species by taking high school students on research dives off the San Clemente Islands in California. They conducted various research projects on the amount of Sargassum weed (Sargassum filicinum) that seemed to be replacing the kelp forests in those areas. It was disheartening to see with each dive in a new location, we rarely found lush kelp forests. I wanted to know why this happened and if it really was something to be concerned about. I learned that this weed is native to the coasts of Japan and is invading California coasts because it was likely transported via ships and currents[3]. There are, however, tropical species of Sargassum weed (Sargassum carpophyllum) that are moving north along the coasts of Asia due to warmer ocean temperatures in those areas [2].  If this weed is going from tropical locations northward, and being transported across the Pacific, this could mean more changes to kelp forests might be seen in the future. What about the organisms that rely on these kelp? Since Sargassum weeds reproduce quickly and in large quantities they could easily out-compete native kelp forests for space and nutrients [3]. Kelp provides habitat for many organisms and acts as the base of many food chains which could mean the end of many of the organisms that rely on them like urchins and sea lions. My students already felt like they were seeing fewer urchins in these areas and I was saddened with each dive that seemed to bring less and less kelp.

 Coasts that are Crawling with Green Crabs

Unlike the Sargassum weeds coming from Japan, the Green crab (Carcinus maenus) is a crustacean that is native to Europe. Just like Sargassum weed, it has been transported around the globe to many new locations by way of cargo ships [4]. The Green crab species that has invaded the Western U.S. coastlines is special though. You see, invasive species have been found to be more tolerant of temperature changes [5]. The invasive green crab seems to be more tolerant than similar crustaceans like lobster and other crab species in its newly invaded areas [5]. If ocean temperature trends continue to increase as they are, this species could continue spreading to new locations and out-competing similar, native species like its current competition with the American Lobster on the East coast of North America and the Dungeness Crab in Alaskan waters. The green crab is also a very successful predator of mussels, clams, and young crabs which could cause a crumbling of the food webs it invades and has been found to reduce the diversity in ecosystems due to its predatory style. Many states and countries are working to reduce the green crab along their coasts. The green crabs have been seen having economic effects on shellfisheries and commercial fisheries [8]. The green crab can be used as a model for what we may see in the future for many other organisms that are not yet as widespread as the green crab. If we base the movement of other invasives off of the successes we have seen for the green crab, the future for invasives is abundant. 

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Sea Squirts, Not So Small in Numbers

Sea squirts, or ascidians, are small filter feeders that seem unassuming enough. These organisms, however, are a force to be reckoned with. Multiple species of sea squirts have been transported around the world for some time by cargo and travel. They can be found primarily in harbors and marinas where they have latched onto boats and harbor materials. Although it has been found that many species of squirts on the US Western coasts seem to be able to coexist peacefully together [6], there is something troubling researchers have noticed. The abundance of these little squirts seems to increase during warmer temperatures [6]. They are also managing to survive winter die off in some locations by staying at deeper depths [6]. These findings could mean this organism may continue to advance their range in many places where they neighbor or haven’t been found in yet [7]. In many human-dominated places, they have become a pest to recreational boaters and businesses [6]. If ocean climate continues to trend upward as they are we may begin to see the potential implications they could have on ecosystems in these areas. When coral reefs and kelp ecosystems become degraded by ocean warming and pollution, some species of ascidians tend to thrive and outcompete other species [7]. Based on their current spread, these sea squirts may have an abundant future. It seems they will experience further spread into new locations should their successes continue, and their future impacts at this time are largely unknown.  

Why Does it Matter?

So why are invading species such a big deal? Biodiversity is essential to our planet, and one of the biggest threats to biodiversity is invasive species [1]. In our efforts to protect our oceans and their biodiversity, the increasing invasion rates needs to be slowed. Although research is just getting going on the true implications in the connection between ocean warming causing invasive species, the connection between the two is undeniable. These are just a few examples of invasive species that are being aided or could be spread by warming temperatures. Doing what we can to reduce the transport and the continued spread of invasives should be a primary goal when protecting our oceans. There are many volunteer-based projects in every community that offer local residents the chance to do their part. You can participate in getting rid of invasive pests in ecosystems as well as spread education and awareness about local issues with programs like the California Department of Fish and Wildlife Services Invasive Species Program. Just check with your local government wildlife services. My students are currently working with some of the local programs in Arizona that work in citizen science to reduce invasives like the Apple Snail.  Maybe one day, my students will be diving in abundant, giant kelp forests again, and we can turn our studies to something new in the Pacific.

References

1. Occhipinti-Ambrogi, A. (2007). Global change and marine communities: alien species and climate change. Marine pollution bulletin, 55(7-9), 342-352.
2. Terazono, Y., Nakamura, Y., Imoto, Z., & Hiraoka, M. (2012). Fish response to expanding tropical Sargassum beds on the temperate coasts of Japan. Marine Ecology Progress Series, 464, 209-220.
3. Aguilar-Rosas, L. E., Aguilar-Rosas, R., Kawai, H., Uwai, S., & Valenzuela-Espinoza, E. (2007). New record of Sargassum filicinum Harvey (Fucales, Phaeophyceae) in the Pacific coast of Mexico. Algae, 22(1), 17-21.
4. Cohen, A. N., Carlton, J. T., & Fountain, M. C. (1995). Introduction, dispersal and potential impacts of the green crab Carcinus maenas in San Francisco Bay, California. Marine Biology, 122(2), 225-237.
5. Tepolt, C. K., & Somero, G. N. (2014). Master of all trades: thermal acclimation and adaptation of cardiac function in a broadly distributed marine invasive species, the European green crab, Carcinus maenas. Journal of Experimental Biology, 217(7), 1129-1138.
6. Lambert, C. C., & Lambert, G. (2003). Persistence and differential distribution of nonindigenous ascidians in harbors of the Southern California Bight. Marine Ecology Progress Series, 259, 145-161.
7. Lambert, G. (2002). Nonindigenous ascidians in tropical waters. Pacific Science, 56(3), 291-298.
8. Grosholz, E., Lovell, S., Besedin, E., & Katz, M. (2011). Modeling the impacts of the European green crab on commercial shellfisheries. Ecological Applications, 21(3), 915-924.

About the Author: Melissa Keese

I am a high school Biology and Chemistry teacher in Phoenix, Arizona. This piece was adapted from my original review paper of examples of invasive species due to warming ocean temperatures in partnership with Miami University and San Diego Zoo Global. I am a scuba diving enthusiast and take students on a research trip to San Diego each year where we dive and perform many research projects on the amount of sargassum weed in the areas. Although this project is a topic intended for students to gain experience with research methods, I couldn’t get the changes we were seeing out of my head and had to know if there were more. I have two children and a husband who loves to dive as well. We have a passion for ocean life and conservation and are trying hard to connect our desert to it any way we can.

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