Shifting Peaks: Climate change and its influence on top predators

A guest blog by Ben Higgins

A common sight our SCUBA divers encounter while surveying moray habitat!  

Lions, and tigers, and bears! Oh my! These large animals have long fascinated, if not scared, humans.  Of course, the allure of such large predators doesn’t stop at the water’s edge. Stories about enormous sharks, massive squid, and other apex predators that roam the oceans are deeply rooted in human history and vary depending on culture. Personally, I thought I was receiving sound advice during my childhood when I watched the film, “The Princess Bride,” and Vizzini warned the princess, “Do you know what that sound is, highness? Those are the shrieking eels!...They always grow louder when they’re about to feed on human flesh!” As long as these stories have been told and these films have been made, the populations of these large animals, often considered apex consumers (because they eat a lot and not a lot eats them) have been collectively declining across different ecosystems, both above and below the water’s surface. In fact, we are starting to see a pattern across a range of landscapes; populations of these large consumers are extremely sensitive to climatic and anthropogenic changes.

Over the past few decades, ecologists have become increasingly good at connecting the dots between large consumers, their prey, and their ecosystems. We have repeatedly come to a the same conclusions – the presence or absence of apex consumers have the ability to drastically shape the environment around them through food-web interactions.  Occasionally, the connection between apex consumers and their associated ecology are so firmly entwined, that just a decline in their abundance ignites a trophic cascade – a series of changes in the strengths of species’ interactions that trickle from the “big bad wolf” at the top of the food web all the way down to the very grass in which it takes its nap.  It is therefore critical to understand these multidimensional connections between apex consumers and their environments. The problem, though, is that before we can begin to study these relationships, we must understand the biology and ecology of these consumers by first asking logical life history questions about them, as the answers can greatly influence community structure and therefore, entire ecosystems.

Surprisingly, such logical questions (e.g., how many there are, where they live, how much they move, what they eat, how old they can be) were left unasked about a likely apex consumer that lives in kelp forest communities, one of the most studied ecosystems in the ocean. But because of their cryptic lifestyle they have managed to remain virtually undetected in standard species surveys, and thus have been left unstudied. Being able to study the life history of a predatory fish and begin to answer all these questions was great news and very exciting as an incoming doctoral student.  The bad news, however, was that I would have to stop listening to Vizzini’s advice. 

My research has been focused on studying the functional ecology of the California moray eel.  I can tell you straight away that luckily, they don’t shriek.  The rest of my results have come from a wide range of field and laboratory methods that span from Santa Cruz, to Catalina Island, California.  Each summer a pack of us descend upon Two Harbors, Catalina Island to trap, tag, and measure of the California moray populations.  Over the course of the past 5 summers, we’ve tagged over 1000 individuals, and each time we go back to those same coves, we continue to find new individuals.  On top of these incredible numbers, we’ve learned that the California moray moves very little, grows slowly, lives a long time (at least 22 years) and consumes many different types of prey.  All of this combined suggests that the California moray eel is an apex consumer that likely plays an important role in its ecosystem.  Much like the abundance of wolves affects that of the grass, the abundance of California morays likely influences a multitude of traits that so many of us find pleasing about kelp forests, such as the many different types of fishes observed on a weekend snorkel. Another result that has come from my research, is that even the mere presence of California morays on Catalina Island appears to be tied the occurrences of El Niño events, a significant force that has impacts all over the planet.

Using the otoliths (inner ear bones of fishes) of 33 morays, we were able to count how many years they spent in the coves of Two Harbors.  To figure this out, we looked at growth deposits on these tiny bones. The analysis is very similar to that of aging a tree – each year can be visible if you look close enough.  Then, we simply back-calculated the age of the moray to the year it settled to the rocky reefs of Two Harbors.  What we found, was that 30 of the 33 (91%) individuals we looked at, settled in Two Harbors during years when Southern California was experiencing an El Niño.  This suggests, that morays use the changes that El Niños inflict on the water currents to populate Catalina Island.  On top of this, more research has shown that the vast majority of climate models indicate that El Niño events will increase in both frequency and severity alongside global climate change.  This means that in the future, the California moray could begin to move north along the coast, outside of their historical range, and start to have impacts on populations that would have never had to worry about such an abundant but hidden predator.  These changes could influence fish populations that recreational divers and spear fishers also target, again setting the stage for some challenging coastal sustainability issues that span cultural, economic, and recreational values. 

My work shows that similar to how the lions, tigers and bears influence the environment around them, the California moray has a functional role that also helps shape what we see when we visit the kelp forest rocky reefs of Two Harbors, California – a pastime shared by millions of people each year.  It has been 78 years since Dorothy sang that song about her fear of large predators, and although we have made significant progress in understanding the ecology of apex consumers across a range of environments, we are only beginning to grasp the vast ecological challenges and associated consequences that come along with their absence or redistribution as a result of global climate change. 

Ben Higgins is a PhD candidate in Ecology and Evolutionary Biology at UCSC.