Article: Vergés, Adriana, et al. “Long-term empirical evidence of ocean warming leading to tropicalization of fish communities, increased herbivory, and loss of kelp.” Proceedings of the National Academy of Sciences (2016): 201610725.
The world’s oceans are warming and it’s no big secret. Warmer waters cause direct problems for many marine species, but one of the indirect (but significant) impacts is new interactions between species. As waters get warmer, species migrate. On land and in the ocean, researchers have found direct evidence of poleward migration, meaning that as the world warms up, species are moving towards the north or south pole (depending on what side of the equator they’re starting on) in search of more favorable conditions (Fig. 1). As species migrate and enter new habitats, they are going to start interacting with the species that are already established, and these interactions could have major implications.
Observing and modeling species migration under climate change has been relatively well-studied (Fig. 2). However, few studies have looked into what happens after the migration occurs and how species will interact with one another and how that may restructure a community. For example, imagine a pod of dolphins findings that the waters they inhabit are becoming too warm so they start to migrate to colder waters. Once they are in a new habitat they’ll have to eat. Fish populations (which haven’t had to deal with dolphins in the past) are likely going to crash, which could have a ripple effect on the rest of the community. The interactions between dolphins and fish will be important to understand if we want to know how or if the community can persist as is. So while research to this point has shown migrations to be common, fewer studies have looked at the impact that new species’ interactions will have.
Starting in 2002, a group of researchers in Australia set out to monitor fish populations in transition sites between coral reefs and temperate kelp forests, knowing that they would likely capture migrations. While monitoring their sites over a ten-year period, they realized they weren’t just documenting changes in fish communities, but they unexpectedly noticed that once thriving kelp forests had become decimated over the same period of time. Knowing the importance of kelps as habitat providers, researchers pivoted and began to explore what was happening to these communities as a result of increased migration.
Researchers working off the eastern coast of Australia set out underwater video devices at 12 sites baited with fish food to monitor changes over time in the fish community (Fig. 3). They continued to monitor these sites with video for ten years. Over this period of time they noticed that the kelp forests they were filming in had completely disappeared, so researchers went back to the video footage to track changes in kelp abundance, evidence of fish eating kelp, and changes in fish populations. In order to determine what caused the decline in kelp, researchers also looked into how factors such as temperature and nutrients were changing over the ten years. In addition, researchers compared kelp abundance between the sites of kelp loss and inshore sites where kelp still remained.
Between 2002 and 2011, all of their video sites experienced a complete loss of kelp. Each year of the study, evidence of kelp feeding increased. In 2002, herbivory was evident in <10% of the sites monitored, but this number increased to 50% in 2004 and >70% by 2008 (Fig. 4). At the same time, tropical fish found at these sites increased from <10% in 2002 to >30% in 2010 and this included a significant increase in herbivorous fishes, the ones that eat the kelp (Fig. 5)! Over this period, researchers found that temperatures at their sites had increased by 1.1 degrees F. Temperature and other non-biological factors, did not have an impact on kelps themselves, rather, temperature indirectly harmed kelp by increasing the amount of herbivores.
Researchers compared fish populations and herbivory between the sites where kelp deforestation occurred and sites close by that still had kelp present. They found that the amount of herbivorous fish was much higher in offshore sites and when they transplanted kelp to both sites, consumption only occurred at the offshore sites (Fig. 6). This likely occurred because herbivory and grazing is rare in temperate sites compared to tropical sites. Here, sites that showed increases in tropical fish were the sites where kelp was consumed at a high rate.
We know that as the world warms, species are migrating. But we don’t know a whole lot about what that means for a community. This research helps shed some light on how migrations can impact ecosystems. Grazing is an important part of tropical ecosystems, where fish can help keep excess algal growth in check, allowing corals to thrive and continue to provide for the rest of the ecosystem. But when these fish move to more temperate waters and kelp forests, their grazing is actually hurting the ecosystem. Here, researchers have found direct evidence of tropical fish migration over a period of time and have quantified how this migration will impact foundational species like kelps (Fig. 7).