With the last bout of sea star wasting syndrome eastern Pacific starfish took a big hit. After three years, numbers of big keystone predators like Pisaster ochraceus are still down. Removal experiments have shown we can expect changes in rocky intertidal communities when Pisaster disappears. The current starfish smackdown is a natural do over of the experimental work – on a continental scale. Wasting has been with us since 2013 so the early results are already in. If we see changes can we attribute them to loss of starfish?
The causality question is worth reflecting on because it takes more than Pisaster’s trophic influences to order the intertidal. Familiar patterns of distribution and abundance result from the interplay of upwelling, physical disturbance, settlement dynamics, temperature, acidification, and keystone trophic effects. So if you think we have a clean natural experiment going on here, think again. The thing about natural experiments is, they have a lot of uncontrolled variables.
Any conclusions I might draw about effects of starfish loss would be open to interpretation. So instead of stumbling around on the slippery Urospora-covered rocks of causality, I’ll settle for showing a few comparative photos from one of my home beaches. If we see any changes, we’ll have to be content not knowing the underlying causes.
In the pair of photos above the view is to the south. Morning sun from the top left adds shadow and glare. The blackish salt and pepper-colored layer in the upper third of both images is a Mytilus bed. Lower you see a phenomenal turn of color as anemones (2016) replace red turf (2013). The photo below is a more intimate view of the tall rectangular rock just right of center in the lower half of the view.
Barnacles, Lepas anatifera, tip this landmark. Lower is a patch of Anthopleura elegantissima, gray and packed. Sharing the middle ground, Anthopleura xanthogrammica (green) and an algae (brown) – I don’t know that one. Finally, Mazzaella splendens blankets the lowest exposed rock.
Here the view is out to sea. The morning sun is behind and over my left shoulder making for glare if it’s cloudy (2013) and shadows if it’s sunny (2016). The kelp you see in the foreground in 2013 is Laminaria sinclairii.
The view in this pair of photos is to the north. The morning sun is over my right shoulder creating annoying glare and shadows. A 2013 trench around the rock has filled in. The rich growth of Phyllospadix (green) in 2013 is reduced and partially bleached in 2016. Since the shadows make interpretation tough in the lower reaches of the 2016 shot, I’ll mention that in 2013 Phyllospadix gave way to red turf; in 2016 there is an intervening layer of Mazzella. Below is a more intimate view of the sunny southwest facing slope on the right side of the image above.
For me, the whole idea of looking for changes after the loss of a major predator like Pisaster comes from Robert Paine’s work. His foundational Pisaster removal experiments in the 1960s helped him pioneer the keystone species concept. The best celebration of Paine and the keystone species concept I know of is this short animation. I first saw it at the 2015 Ecological Society of America meeting in Baltimore. It was also the only time I met Paine.
Paine died on June 13, while I was putting together the words for this post. You can read more about him here.