Tuesday, July 16, 2013

URCHIN BARRENS! Aka the Trouble with Tribbles (=sea urchins!) Post!

Purple Urchins
Image by Annie Crawley
Sea urchins are among the best known, most heavily published on, and most "important" of echinoderms. People eat them and they are studied in marine ecology pretty heavily. Most marine biologists I know think highly of sea urchins. They're pleasant animals with an unusual appearance

But the truth is, no matter how adorable or fuzzy, useful and/or cute an animal may be, TOO many of them is nothing but trouble! True for Star Trek tribbles and for sea urchins!
(disclaimer: Tribbles are science fiction, sea urchins are not)

*Tribble factoid: Someone has ACTUALLY given tribbles a scientific name: Polygeminus grex! don't believe me? go see Memory Alpha!)

Tribbles are actually a GREAT introduction for today's topic: SEA URCHIN BARRENS!

What are Sea Urchin Barrens??  These are places where a sea urchin species' abundance increases dramatically to the point where the urchin devours EVERYTHING in its path, effectively leaving all else 'barren' except for more hungry sea urchins.
Purple Urchins
Image by Annie Crawley
Images above by AndyOlsson
This is not far removed from the imagined "ecology" of Star Trek's tribbles (A good essay applying real population math about tribble populations can be found here, but this image from the famous ST:TOS episode hopefully gives you the general idea!)
Image from TrekNews.net
The gist of it is simple:  TOO MANY URCHINS and they EAT TOO MUCH. But unlike tribbles (which were eradicated by Klingons-yes I know they're not real), in the case of sea urchins, we can actively study the ecological interactions and conditions which have caused the populations to explode in number.
Image by AndyOlsson
Here is a video showing tons and tons of Red Urchins (S. franciscanus) on a barren in Southern California. Thee bottom is essentially devoid of all but more hungry urchins!

What causes urchin barrens? 
Um. Its complicated but the common thread seems to be that there is an association between barrens and the absence of sea urchin predators.

In many of the papers I've read about Northern Hemisphere species, the loss of a major sea urchin predator seems to be one of the immediate attributed causes of the runaway population growth, but as we've seen with other species such as the Crown of Thorns (Acanthaster planci) the story is often complicated....

Most of the studies involve temperate-cold water urchins in the Strongylocentrotidae, specifically Strongylocentrotus purpuratus (purple urchin), S. franciscanus (red urchin), S. droebachiensis (green urchin) and S. polyacanthus.  Literature was abundant, but this paper by Nathan Stewart & Brenda Konar provided much of (but not all) the info for this post.

In one of the most familiar studies from the Pacific Northwest coast, the main predators were sea otters (in many cases, I assume Enhydra lutris-some papers did not mention species).

The fundamental ideas outline the notion that as sea otter populations decline, predation pressure decreases and with nothing to keep the populations at a controlled level sea urchin populations dramatically increase and began to devour kelp (and really everything else!)  to the extent that they effectively clear the bottom.
Urchin Barren
Image by Santa Monica Bay Restoration Foundation
Purple Urchins
Image by Annie Crawley
In Stewart & Konar's paper, individuals from these population explosion urchins were compared against "healthy" urchins which occurred naturally in kelp forest habitats.  Some dynamics:
  • Urchin densities were SEVEN times greater than those elsewhere
  • Kelp forest (vs. 'barren') urchins were larger and more robust
  • "Barren' urchins were smaller with less tissue
  • "Barren urchins had little to no reproductive tissue compared to kelp forest urchins
Different species of Strongylocentrotus (as well as other urchin species!) live in different places and have different predators!

On the North Atlantic coast, there is a similar population explosion of the Green Urchin, Strongylocentrotus droebachiensis, which from the look of it, is pretty severe

Here's a video that shows just WOW... a lot of them..

I have briefly written about the impact of this many Green Sea Urchins. They all POOP! This actually has a pretty serious ecological impact. 

Some, such as this paper, have proposed that these population increases have been caused by the loss of lobsters (Homarus americanus) which feed on green sea urchins. But in all liklihood, as the system is better understood the more complicated the explanation.
Northern Lobster, Gulf of Maine
Image by AJmart
Other predators, such as wolf eels and starfish, also feed on green sea urchins and well.. it can get messier...

Now, in the Southern Hemisphere we have a similar, parallel situation with a completely different family and species of sea urchin: Centrostephanus rodgersii (Diadematidae).
Sea Life: Long Spined Sea Urchin
Image byEdward Vella
Climate Change Enters the Picture! 
A paper by Ling et al. 2009, in the distinguished Proceedings of the National Academy details  a scenario with some important dynamics
  1. The range of the urchin is dramatically expanded because of increasingly warm waters in/around the eastern Tasmanian region.
  2. The lobster Jasus edwardsii is one of the primary predators of Centrostephanus and has been heavily overfished. The BIG lobsters that would feed on urchins are taken for food leaving the urchins to run amok!
Its important to note how significant the human factor has played into these dynamics. Climate change and overfishing are thought to be the primary agents responsible for urchin "barrens" in these circumstances.

This issue has been conveniently summarized in this video...

The takeaway lesson: Predator loss seems pretty strongly associated with urchin "barrens" aka population explosions. But all sorts of environmental factors, including warmer waters, and multiple predator interactions can be important..

So we have a LOT of sea urchins. Couldn't we uh..just eat them? 


BUT, you can after all, only fish so much. After you've taken the lobsters, the urchins and the kelp what else have you got left? A good answer seems to lie with good sustainable fisheries management..but we shall see how this works out...


jebyrnes said...

And then there's the CRAZY behavior of white urchins on barrens as documented by Coyer et al. 1987. That always blows me away.

ChrisM said...

D'OH! I KNEW I saw that somewhere! I was looking for that other predation paper last week. Now I gots it! thank you!

AnnaD said...

Friends just mentioned that a recently placed artificial reef, the USS Mohawk was entirely covered with urchins when they dived it in May. There were very few when they dived it in November. Those critters work fast!

Chris said...

This is cool!