Purple Sea Stars, Part II

Last week I did a bit of digging into the carnivorous, brainless, neuro-sensory-cell-powered sea star of the tidepools of the Pacific Northwest.

These odd creatures are also responsible for the discovery of keystone species. Okay, the credit really goes to a scientist named Robert Paine, but the tidepools and sea stars provided him with the perfect natural laboratory. If you can believe it, prior to the 1960s, scientists though that food chains were regulated by the number of producers in an ecosystem. Further, predators were not thought to have any role in regulating species populations. It’s counterintuitive to us now, but we’re a long way from the 1960s.

Photo Credit – John Lloyd

The reason that Paine wanted a natural laboratory is because fellow scientists had questioned why, if above were true, why didn’t consumers simply devour all the plant material in a habitat? Why was there any green left? The challenge was finding an ecosystem they could control and manipulate. So when Paine discovered the tidepools, he knew he’d found the perfect place to run an experiment. He started by observing all the species in the tidepools and creating a food chain. That’s when he learned that the sea stars were the top predators in those systems. Then, to understand the role of the top predator, he removed them all from one tidepool and left them in others.

Within 18 months, the tidepools without the sea stars had changed. Within 8 years, the tidepools had but a single species left – mussels. Without the starfish to prey on the mussels, the mussels had taken over the tidepool and pushed all other species out. The mini ecosystem had collapsed without the sea stars. This was a groundbreaking discovery.

Borrowing from architecture, Paine coined the term “keystone species” that we still use today. Just as in a stone or brick arch, if the keystone is removed, everything collapses. Paine and colleagues soon also discovered that sea otters are a keystone species. In the following decades other keystone species were identified, including ones that were not top predators. These include ecosystem engineers like beavers, herbivores like bison, mutualists such as bees, and coral – a foundational species.

Purple Sea Stars, Part I

Tidepools are their own ecosystem, complete with producers, consumers, and predators. This may leave one wondering, who’s the predator? Clearly there are no fangs or talons among the tidepools’ inhabitants. In tidepools of the Pacific Northwest, it is the purple sea star. Don’t be fooled by their appearance – they are ruthless, carnivorous predators.

Photo Credit – Andrew Reding

Purple sea stars (which actually range in color from brown to orange to purple), are 5-armed invertebrates that can grow to 9 or more inches across. They live on rocky coastlines from California to Alaska. And their favorite prey includes snails, barnacles, mussels, limpets, and even crabs!

For me, this raised the question, how do they do this? Starfish do not have teeth or jaws. So to eat, they pry open shells with their arms. This is where it gets super-weird. Once the prey’s shell is pried open, sea stars stick their stomach out of their mouth. Let me repeat that: starfish push their stomach out of their mouth. Not only that, once they push their stomach out of their own body, they push it into the prey’s shell to digest it before swallowing.

So if the whole eating-stomach thing isn’t weird enough, starfish can regenerate an arm if lost, which is important because they use those arms for eating and for locomotion. Starfish also do not have a brain. Instead they have a simple nervous system that consists of a nerve ring around their mouth. It connects with nerves that extend down each arm and signals them to move. These odd creatures also have neuro-sensory cells over their whole body that relay messages. Odd indeed.

Robins Revisited

American robins one of the most common in the United States. We often associate the word “common” with ordinary, yet these birds are anything but. Which is why I’m writing about robins again.

Photo credit – Martin

For starters, American robins are often considered a sign of spring and at this time of year a welcome one. But in the lower 48 states, robins are actually year-round residents. The migration patterns of robins are complex and vary depending on the bird. Some have been known to migrate thousands of miles, while others stay within a 60 mile range year round (even among individuals living as far north as Ontario). Some even move north in the winter. Not only that, but banding studies have shown that some birds may migrate one winter but not the next. One reason we do not see robins as often in the winter (and thus associate them with spring) is that they tend to move out of open yards or other areas, and into more forested areas for shelter. This, for the record, is a simplified version of what we know about robins’ migration habits and scientists are still unraveling the mystery.

Robins are also part engineer, part architect, part artist. Of course, it’s the female who builds the nests. The male do take part, though, helping her gather materials. To build the cup-shaped nest, robins use grasses, twigs, and other plant material. They’ve also been known to use feathers, roots, flower petals, moss, and even paper for their nests. The nest is reinforced with mud which works like cement. Construction often takes place after it rains because soft mud is readily available. Yet studies have shown that these birds are resourceful in dry years. The birds have been observed taking nesting material or dry dirt to water sources, including birdbaths, to soak before returning to the building site. Once complete (after 2-6 days of construction), these sturdy nests are 6-8 inches wide, and 3-6 inches deep and the female lines the nest with soft, fine materials.

These extraordinary birds can raise 2-3 broods each season. And for every new brood, they usually build a new nest. Perhaps, if you’re lucky, you’ll have robins nesting near you soon!