More Than Just Seagulls Munch On The Seashore

journal.pone.0068221.g005Walk along pretty much any beach and at the high tide mark will be a line of debris. There may be seaweed or shells, bits of driftwood or plastic debris. You probably won’t see any fish, though. And that’s a little odd, because fish do die, and their bodies have to go somewhere. Surely some would wash ashore.

A group of researchers in Australia think that they’ve figured out where the fish go – the fish quickly get scavenged by the critters that live along the shore. But there’s more than just seagulls finding their meals here, the team reports in PLOS One.

The scientists set up a series of experimental plots along a sandy beach on North Stradbroke Island on the east coast of Australia. They picked a spot far from humans, where dogs and beachgoers would be scarce. Then they set out 20 plots, three meters by 10 meters each, and for eight days added about five kilograms of flathead mullet fish to half the plots about two hours before sunset.

The beach was nearly picked clean. Over the eight days, 720 fish were set out and 97 percent were completely eaten. Gulls (silver gulls in this case) ate some of the mullet, but there were several other birds species as well: Torresian crows, whistling kites (b in image above), brahminy kites (a), and white-bellied sea eagles (c and d)

These avian scavengers scoured the beach most often at sunrise and in the first few hours of morning. On three occasions at night, however, red foxes (f) visited the plots, snapping up the easy meal. And on one day, a lace monitor (e) – a large, carnivorous lizard – was spotted on the beach snacking away. That was a surprise because there have been few reports of terrestrial reptiles scavenging on a beach.

One invertebrate also got in on the action. The researchers measured the diameter of ghost crab burrows and, using burrow size as a proxy for crab size, discovered that when they added fish to the experimental plots, bigger ghost crabs moved in to take advantage of the free food.

But birds were the dominant consumers of the carrion fish. And they probably play an important role in this seashore ecosystem, the researchers say, helping to transfer nutrients from the sea onto land and providing a vital link between water and soil.

Image used under Creative Commons license, Schlacher TA, Strydom S, Connolly RM, Schoeman D (2013) Donor-Control of Scavenging Food Webs at the Land-Ocean Interface. PLoS ONE 8(6): e68221. doi:10.1371/journal.pone.0068221

Petrel Bones Show How Humans Changed The Open Ocean Food Web

Pterodroma sanchwichensisIn one marine biology class I took long ago, the way we study the open ocean was compared to trying to study a forest by throwing a bucket out of a helicopter as you fly over and making conclusions based on whatever you happen to drag back up. Ocean exploration these days is a bit more sophisticated, but it’s still not easy, especially if someone wants to see how things have changed over the last several thousand years or so. But scientists figure out elegant solutions to these problems.

In a study published this week in PNAS, a research team led by the Smithsonian’s National Museum of Natural History investigated how the food web of the North Pacific Ocean has changed since humans began fishing there on a large scale by looking at the chemistry of petrel bones. Hawaiian petrels turn out to be a good proxy for what’s going on in the oceanic food web for several reasons: The birds breed only on the Hawaiian Islands, and their remains, dating back thousands of years, can be found in many archaeological and paleontological sites there. That makes the petrels special because most creatures that play a role in the oceanic food web die at sea. Even better, these bones contain a record, in the form of carbon and nitrogen isotopes, of where they foraged and what they ate when they were alive.

“Hawaiian petrels spend the majority of their lives foraging over vast expanses of open ocean,” lead author Anne Wiley of the Smithsonian Institution said in a statement. “In their search for food, they’ve done what scientists can only dream of. For thousands of years, they’ve captured a variety of fish, squid and crustaceans from a large portion of the North Pacific Ocean, and a record of their diet is preserved in their bones.”

Three decades of collecting 17,000 petrel bones have given these researchers a pretty good dataset to work with. And when they looked at the isotope information from the bones, the scientists found that from about 4,000 to 100 years ago, petrels were munching on bigger prey. But when humans intruded on the food web, snapping up the bigger fish, petrels changed their diet and gobbled up smaller creatures.

“Our bone record is alarming because it suggests that open-ocean food webs are changing on a large scale due to human influence,” coauthor Peggy Ostrom, a zoologist at Michigan State University, said in a statement.

It’s yet another case showing how humans are influencing the planet, and not necessarily for the better.

Image courtesy of Brittany Hance, Imaging Lab, Smithsonian Institution