Sea star die-off leads to kelp 'clearcut' in Howe Sound, scientists find
When a voracious predator gets wiped out, the urchin populations explode and kelp forests suffer
The massive die-off of sea stars in B.C.'s Howe Sound has had a domino effect on other creatures, resulting in the virtual clearcut of kelp forests in the area, scientists have found.
The mysterious wasting disease hit in 2013, killing sea stars from Mexico to Alaska in what has been described as one of the largest wildlife die-offs ever recorded.
In Howe Sound, 90 per cent of the sunflower star disappeared in a matter of weeks, said Jessia Schultz, lead author on a study by SFU and Vancouver Aquarium researchers published in PeerJ.
"Divers were just reporting these sea stars basically falling apart before their eyes," said Schultz.
That alarm was raised three years ago, but now the researchers have documented the knock-on effects for the ecosystem.
With their sea star predators nearly wiped out, green sea urchins quadrupled at the 20 study sites measured.
The urchins, in turn, have reduced kelp in the area by 80 per cent — essentially "clearcutting" the underwater ecosystem, said Schultz.
'Heartbreaking' change
As ecologists watched sea stars dissolve from the wasting disease, they knew there would be impacts on other creatures, because of the key role the animals play.
Make no mistake: the pretty star shape belies their true nature as voracious predators.
Sunflower stars in particular are so feared by their urchin prey that past studies have showed urchins scuttling away at the mere whiff of the sea star's arm.
"They will eat pretty much anything they can get ahold of," said Schultz, who is a master's student at SFU and Vancouver Aquarium's Howe Sound research program manager.
Without that deterrent, the urchins were free to graze the kelp forests, creating what ecologists call 'urchin barrens' at some sites.
"When you're used to seeing these luscious kelp beds that are now bare rock with urchins it's a little bit heartbreaking," said Schultz.
Kelp forests grazed
These domino effects are known as a 'trophic cascade' — a phenomenon taught in ecology textbooks, but also debated by scientists and managers, said Schultz.
What's not debated is the importance of kelp to the underwater communities, including as habitat for spot prawns and other creatures.
"Anything we can do to help understand how the system works, even as it's changing, it's going to allow us to make better management decisions," said Schultz.
Whereas she used to see an ocean floor "carpeted" in sunflower stars, now she only sees them every six or seven dives and hasn't seen a fully-grown adult in years.
How quickly they will bounce back and bring the kelp with them, remains to be seen.
