I spent last week at the annual Ecological Society of America conference in Sacramento, California. It’s a huge gathering of ecologists, and it’s amazingly diverse. Ecology is as big as the planet, and the range of presentations and conversations at the meeting are as big as ecology. I like going. It gets me excited about science, it challenges me and pushes me to understand research that’s unfamiliar to me, and it gives me the chance to interact with other people who work on very different organisms and research questions. But it’s a lot of time to spend inside, especially in the summer, so I always try to take a day or two at the end of the conference each year and go someplace quiet and wild. This year I drove east from Sacramento, out of the Central Valley and up through the hills, watching the landscape change, until I was up in the conifer trees and granite rock on the crest of the Sierra Nevada at Donner Pass. I parked the car and went for a walk. I saw lots of great things up there, from tiny plants growing in crevices, to craggy mountains I’ve hiked on past trips through the area.
I also saw this lying by the trail:
And that’s what this post is about.
The squiggles on this log were made by beetle larvae, eating their way through the outer layers of the wood, hidden under the bark, while this tree was still standing, or just after it fell. That’s the easy story, but it only tells part of what went on here, only a small fraction, really. These squiggles are footprints of a huge set of interactions.
The adult female beetles that laid their eggs here probably didn’t choose this tree randomly. Plants get stressed by a huge variety of factors: changes in water or temperature; a hard winter or summer; diseases or pests; pollution; damage from fires or storms; or just old age. And the plants put out chemical signals of stress that many insects are adapted to detect. These beetles came here because they picked up that signal on the breeze.
There’s a huge variety of beetles that live in trees. Some of them eat wood; some eat the inner layers of bark, others excavate galleries under the bark to rear fungi that the female brought in with her and left behind along with her eggs. The tree becomes a habitat for growing a fungal garden. Other fungi, drifting on the wind, will colonize the tree too, finding a way through the protective bark in the holes made by the beetles. Bacteria show up as well, and add to the community living in the beetle galleries.
The beetles themselves give off chemical signals that other organisms pick up. Parasitoid insects use a range of cues and clues to locate host insects inside the tree and lay eggs in, on or near the hosts, turning the beetle larva into a food source for their offspring. Predatory insects colonize the galleries and hunt down the beetles, while fungus feeding insects or bacterial grazers move in and feed on the fungi and microbes. Some insects specialize in feeding on frass — the nutrient-rich droppings and debris created by the wood-feeding beetles moving through their galleries. Bigger players move in as well – woodpeckers and other birds pick insects from the bark and wood, and small mammals dig into fallen logs. It’s a diverse little food web there under the bark, and many of the players will, in their own way, contribute to breaking down the bark, and the underlying wood, recycling the tree and speeding up its decay.
You might look at this log and see only beetle galleries. And if that’s your main research focus then that would be a perfectly reasonable perspective. Somebody else might look at the log and see the fungi, or the woodpeckers, or the parasitoids, or the little frass-eating flies, or the cycling of nutrients that goes on in and under the decaying wood. Or, you could step back and see the big picture, the whole complicated, messy food web.
Ecology, and our ecology conferences, are kind of like that. We’re interested in different questions, and we like different organisms. Some of us do most of our research in the field, some of us do our work at the lab bench, and some of us at the computer. We wear different clothes to work, and we use different tools. But, in a sense, we’re all looking at the same log.