Kelp forests along the Central and Southern coast are being destroyed by hungry sea urchins, leading UC Santa Barbara doctoral students to investigate what this could mean for the marine environment.
Purple sea urchins are devouring the kelp forest in what’s typically a balanced relationship between the two. As a result, kelp forests along the state's Central and South Coasts are depleting rapidly.
UC Santa Barbara doctoral student Mae Rennick co-wrote a study asking, why are urchins chomping through the forests?
“There has historically been co-existence between urchins and kelp for a long time," she said. "One of the reasons we were so interested in this topic is because of a more recent trend of kelp being degraded along our coast because of this urchin herbivory — and really quickly.”
Typically, urchins eat detritus, which are scraps of dead kelp that break off over time. Bart DiFiore, the study’s other co-author, compares the shedding of kelp to leaves falling off of a tree.
“Think of this as a tree that's sort of constantly losing some of its leaves. And so these leaves, in the case of a tree, in the case of a kelp, the blades of the kelp are kind of raining down off of the kelp toward the seafloor," he said.
"In a healthy system, the idea is that the urchins are basically stuck in the cracks and crevices, and they're taking this detrital kelp as it drifts by, and then they're eating that detrital kelp," DiFiore said.
It may seem simple enough — the kelp breaks off into pieces, then urchins eat these broken-off segments. However, urchins are now cutting straight to the source, eating the live kelp.
Difiore said this dynamic has shifted in many ecosystems along the coast, leading to kelp forest barrens.
“When you add up all how much each individual urchin can eat, and that rate is greater than the rate that the kelp plants are shedding blades and basically providing detritus to those urchins, that's when we see these big flips," he said.
According to DiFiore, this problem extends beyond there simply being too many urchins. In these barrens, the team found that each individual urchin’s consumption exceeds the amount of detritus available.
In their experiment, DiFiore and Rennick found a strong relationship between the amount of detritus available versus how much living kelp there was.
“When the consumptive capacity of the urchin populations was greater than the amount of detritus the kelp forests was producing, there was a up to 50-fold decline in the amount of living kelp biomass," DiFiore said.
Rennick shares some of the consequences if there's no intervention in these unhealthy ecosystems.
“We're trying to figure out how to solidify these management efforts and really target certain areas that need it, in order to maximize kelp biomass," Rennick said. "Because it's really necessary for productivity, and carbon sequestration, and tourism and fishing. And it has so many implications for our coast.”
Rennick said the first step to reviving kelp forests is educating people on how to address their decline.
“I am a big proponent of education. Awareness of the fact of what's going on in our backyard is a huge first stepping point.”
