Decomposers Are Natures Garbage Men

By Scott Shalaway
HAVE YOU ever noticed how quickly road kills disappear? Sometimes vultures or crows are responsible, but often it just seems to vanish. It’s the work of decomposers–bacteria, fungi and myriad invertebrates whose ecological niche is to recycle organic matter. Without these organisms, the planet would quickly become a giant compost pile.

Though decomposers perform an invaluable ecological service, their role in nature is often minimized or even dismissed. Biologists prefer to concentrate their attention on higher organisms, especially those that don’t stink.

But it may be that the odors associated with decomposers may be more than by-products of their actions. It may be that decomposers compete with larger scavengers by producing repugnant chemicals that actually discourage larger consumers.

This notion was first suggested by ecologist Dan Janzen in a paper published in 1977 entitled, “Why Fruits Rot, Seeds Mold, and Meat Spoils.” He suggested that microbes make dead stuff distasteful or even harmful to other animals, thus minimizing competition with higher organisms.

This is a fascinating twist on our typical attitude toward decomposers. It vaults them from simple, easy-to-ignore nutrient recyclers to sophisticated ecological competitors.

A paper in the current issue of Ecology reports on a series of field and lab studies that test this notion in a marine ecosystem. Biologists from Georgia Institute of Technology (Georgia Tech) led by biology professor Mark Hay demonstrate that microbes do indeed affect the appeal of dead flesh to scavengers.

To test whether aged meat attracts fewer consumers than fresher meat, researchers baited crab traps with Menhaden (a bait fish) that had been rotting in a pool of warm water. They also baited other traps with freshly thawed Menhaden, which contained relatively few microbes. Then they set the traps in salt marshes and caught hundreds of Stone Crabs, as well as other crab species, fishes and snails.

Many more animals were attracted to the freshly thawed bait than the rotten fish.

Counting the scavengers found in the traps confirmed the attractiveness of the various baits, but it didn’t necessarily test feeding, Hay noted. “It could be that the rotten food is just as good, but a lot of the good smells have leached out in the water, so maybe it’s just food that’s harder for predators to find.”

Researchers addressed their questions about feeding by conducting laboratory experiments. To eliminate food avoidance because of texture, they fed Stone Crabs, Lesser Blue Crabs and Striped Hermit Crabs noodle-like test foods made from pureed forms of either the freshly thawed Menhaden or the rotten bait. They found that, no matter the rotten bait’s texture, Stone Crabs avoided eating the rotted, microbe-laden food, but readily consumed the freshly thawed Menhaden containing few microbes.

“Even when the stone crabs were handed the rotten fish, they didn’t want to eat it,” Hay said.

Next, researchers tested whether microbes directly affected the palatability of microbe-laden, rotting food. They placed Menhaden in two pools for two days–one group in seawater where microbes were allowed to grow naturally and the other in seawater with an antibiotic added to suppress microbe growth. In the lab, Stone Crabs readily ate both freshly thawed Menhaden and fish that had soaked in water with antibiotics, but refused to eat the rotten fish not protected from microbial attack.

To determine if reducing bacterial growth affected an animal’s ability to find the bait, researchers also repeated the trapping experiment in the marsh, but used newly thawed fish, fish soaked in antibiotic treated water and fish aged without antibiotics. They found that both freshly thawed bait and aged, antibiotic-treated bait attracted animals more frequently than traps containing aged, microbe-laden Menhaden.

Microbes are everywhere, and Hay argues that they are not passive scavengers. He hopes this research will make ecologists think more critically about the broad role of microbes in ecosystems. Microbes are often omitted or relegated to a minor role in food web diagrams, but they should be depicted as direct competitors with larger animals.

The next step is for someone to do experiments with road kills to determine if terrestrial decomposers operate similarly. My hunch is that they do. –Pittsburgh Post Gazette