Article: Baggini, Cecilia, et al. “Herbivore diversity improves benthic community resilience to ocean acidification.” Journal of Experimental Marine Biology and Ecology 469 (2015): 98-104.
We’ve talked a lot on this blog about the negative impacts of climate change. But as climate change science progresses, the stories are shifting more and more towards the positive, pointing out that things may not be as bad as they seem.
Many scientific studies shed light onto just how damaging warming waters, pollution, and ocean acidification can be to individual organisms (Fig. 1). Whole ecosystems are even projected to be covered by seaweed, losing critical functions and losing species diversity. But when investigating the effects of climate change, it is important to consider all the species within a community and how they are directly and indirectly connected to one another (Fig. 2). This may indicate whether or not an ecosystem has the built-in ability to mitigate change.
I wrote a piece a few months ago about a study that showed small herbivores were able to help control algal growth and takeover of seagrass habitats in areas of nutrient pollution. By having herbivores around, the ecosystem was able to maintain balance. A recent study coming out of the Mediterranean has found something similar: in ecosystems threatened by ocean acidification, the best way to prevent a seaweed takeover is to have a diverse group of organisms feed on the algae (Fig. 3). How did they find this out?
There are natural volcanic seeps in the Mediterranean that provide a unique opportunity to study the effects of ocean acidification in the field. Sites near the vent are naturally lower in pH. Researchers wanted to see what kind of role herbivores play in normal coastal systems and ones that are more acidic—sites that represent projected norms for future oceans. To investigate this, the researchers picked two sites: one reference site and one low pH site (Fig. 4). They performed surveys of the sea urchin and fish herbivores present, and their abundance at each site. It was found that two species of sea urchins were the dominant herbivores on the reference site and fish were the dominant herbivores in the low pH site (Fig. 5). This fits with what is known about urchins and acidification. Urchins struggle to calcify and maintain health in acidic waters, whereas fish don’t have it as bad. Fish likely dominate the low pH site because they don’t have to compete with the urchins for food. To summarize: there are two sites, one low pH and one normal pH, with both sites having different herbivores.
Researchers set out small tiles in the water at both sites to allow colonization by seaweed. There were three types of tiles: open ones (allowing herbivores to eat), caged ones (preventing herbivores from eating), and cage controls (these are open to eating but test whether having the cage makes a difference). They let these tiles sit for almost a year before pulling them up and checking for algal species present and biomass of the algae.
They found that by excluding herbivores, both sites saw significantly higher biomass of seaweeds. If they aren’t eaten, they will grow like crazy. But where herbivores were allowed to feed, both sites saw a reduction in algal biomass with almost identical results. No matter what type of herbivore, the seaweed was consumed at the same rate (Fig. 6).
This study helps show that ecosystems have built-in ways of handling climate change. In this case, in a natural setting, sea urchins thrive and help control the growth and spread of algae. But in low pH environments that stress urchins out, they have been “functionally” replaced by herbivorous fish which have the same ability to control the growth and spread of algae. This shows the importance of having a diverse ecosystem. If an ecosystem has diversity, there are other species that can step into a functional role. This helps to keep an ecosystem balanced. Without this replacement, algae would be able to thrive, ultimately altering the system.
It’s nice to see that climate related studies are giving us a reason for hope. More studies need to take a community approach to their studies; because we may find that they are not as fragile as we think.