The Invisible Forest
I count 37, 38, 39 . . . I look up and blink, then focus on the beetle poster across the room. Once my eyes have adjusted, I turn to the paper next to my microscope and write "39 @ 2 mm" on the line for species number 362.
I am halfway through the leaf litter sample I collected last summer from a shady slope deep in the Ozark forest. When finished, I will have found 1,233 individuals and 68 species, all in a handful of leaf litter.
My partner, Sarah Heyman, and I study the insects, spiders, mites and other arthropods (animals with jointed legs and an exoskeleton) living in Ozark forest leaf litter. Our study is part of a multi-year project to examine the effects of different logging practices on the plants, animals and nutrient cycles of the forest.
We included leaf litter arthropods because they play a major role in transforming fallen leaves, branches and trunks into nutrients for plant growth. Although fungi and bacteria are responsible for most of the decomposition of plant materials, arthropods can double or triple the decomposition rate through their feeding activity. They also are an important part of the forest food chain, because they can concentrate proteins and minerals needed by vertebrate predators.
We have 36 5- x 5-meter permanent plots scattered randomly throughout the Conservation Department's Current River Conservation Area in Shannon County. Half are on northeast facing slopes and half on southwest facing slopes. Using random plot locations allows us to generalize from individual plots to the whole forest, since every slope has an equal chance of being picked for a plot.
Random locations are good for science, but they can be rather trying for scientists, since no thought is given to how easy it would be to find a plot the size of a couple of parking spaces in hundreds of acres of forest.
Fortunately, with over a dozen other studies taking place in the same forest we can hardly go 100 meters without bumping into some other project. Over the years, we have found our plots using pink and orange grid lines (for mapping bird locations), aluminum fences (for trapping reptiles and amphibians), white paint-marked vegetation plots and giant plastic cones (for measuring acorn production) as landmarks.
Our plots are marked with yellow flagging and paint and with plastic pipes driven into two of the corners. Using the pipes, we lay out tape measures to define the edges of the plot and to locate our sample points. Stepping softly so we don't scare any spiders and beetles away, one of us edges up to a sampling point and plunges a 3-pound coffee can over a patch of leaf litter.
Serendipitously, a 3-pound coffee can is the right size to collect a sample that is one-fiftieth of a square meter. After cutting leaves and twigs away from the can, it is tipped out of the way and the litter underneath is scooped into a labeled plastic bag until we can extract the arthropods. We take three more samples from a plot before heading to the next one.
By mid-afternoon, we will have finished nine plots and returned to our field lab to extract that day's samples. The field lab is a barn wired with over 70 electrical outlets. The outlets are for light bulbs used to dry and heat each litter sample. To avoid high temperatures and drying, most litter animals move into deeper layers or into the soil. Our litter samples are laid over screens, and as the animals move down, they fall into containers of alcohol. Once we extract the arthropods from all the samples, we go back to our lab at the University of Missouri-Columbia to count, sort and measure them.
While the actual number of arthropods will vary, we usually find around 16,000 individuals per square meter in leaf litter collected in June. The most abundant species is Onychiurus ramosus, which averages over 2,000 individuals per square meter.
This tiny arthropod is a springtail, a group of insects so primitive that they have no wings--springtails evolved before insects developed them. Instead of wings, many springtails use a forked tail to flip themselves out of a predator's grasp, hence the name "springtail." Springtails eat decaying vegetation, bacteria and fungi.
Onychiurus is small, with stubby legs and antenna, and it lacks the forked tail characteristic of other springtails. This streamlining allows it to move easily through the tiny air spaces in deep litter and soil. It is also eyeless, since it doesn't need to see in the dark world it inhabits.
In contrast, Tomocerus lamelliferous, a springtail that averages 1,000 individuals per square meter, is clearly adapted to life on top of the leaf litter. It has long antennae and functioning eyes that sense the open environment of the surface. It also has long legs and a well developed tail, so it can easily escape predators and move from litter patch to litter patch in search of food.
Box mites, which average 400 individuals per square meter, belong to a group of arachnids (eight-legged arthropods--mites, ticks, spiders and harvestmen) called oribatid mites. Unlike other arachnids, oribatids are herbivorous. Together with the springtails, the oribatids help speed up the rate of decomposition by feeding on dead plant material and fungi.
Oribatids also are interesting because their exoskeletons are hardened with calcium carbonate. The shell-like exoskeletons help protect the slow-moving oribatids from predators. The box mite has taken this innovation one step further by closing the shells covering its body the same way a box turtle does, so that its soft parts are completely protected.
One of the predators the oribatids must protect themselves from are mesostigmatid mites, which average over 900 individuals per square meter. Like most litter species, this one still needs to be identified by a taxonomist. Until then, it is known to us as species #154. Its mouth parts are needlelike, adapted for exploiting any cracks in oribatid exoskeletons.
Pseudoscorpions are another important predator. These animals also are arachnids. Their name comes from the two large claws at the front of the body that give them the appearance of scorpions. However, there is no stinging tail, so pseudoscorpions rely on the dexterity of their claws to capture prey. They average 300 individuals per square meter.
Both mesostigmatid mites and pseudoscorpions feed on springtails, insect larvae and other arachnids--anything they are able to catch and hold onto.
Together these five species make up 30 percent of the total number of individuals in the leaf litter. This may suggest that leaf litter is a pretty simple community composed of only a few species, but we typically find over 50 species in each sample, and we have distinguished over 800 species altogether.
Sometimes, as I am looking at a new sample, I can't help thinking of an African landscape. Swift and active surface-dwelling springtails remind me of antelope, the abundant and deep-dwelling springtails and fly larvae burrow into the litter like rodents into soil, the stolid and slow moving oribatids graze through the litter like hippos through a marsh, and the predatory mites, pseudoscorpions, spiders, beetle larvae and ants, stalk their prey like lions, jackals and hyenas. The same dramas that unfold in the African savanna are played out a hundred times a day in each of our samples.
While our long-term goal is to find out how logging affects leaf litter arthropods, we already know some interesting things. We know that mites and springtails prefer cool, moist northeast slopes, that predators are more abundant on southwest slopes and that flies appear to favor more acidic soils. We also know that the group with the most species depends on whether you are talking about samples (it's mites) or about the whole forest (it's beetles).
Figuring out what causes the differences in the distribution patterns of these groups will help us understand how ecological communities work, an understanding that will help us keep ecological communities functioning in the face of ever increasing environmental stress.
Toward the end of A Sand County Almanac, Aldo Leopold writes: "Daniel Boone's reaction depended not only on the quality of what he saw, but on the quality of the mental eye with which he saw it. Ecological science has wrought a change in the mental eye. It has disclosed origins and functions for what to Boone were only facts. It has disclosed mechanisms for what to Boone were only attributes."
I would have liked to see the forests that Boone saw, the huge trees and the diversity of plant and animal life that existed before European settlement. But I have a clear and beautiful vision of a forest that Boone never saw, and it would be hard to decide which of us is more satisfied.
Visit Our Web Site
Keeping track of hundreds of different species is no easy task. To make identification of species faster and simpler, we decided to take photographs of each species and store the photos on a computer for easy access. Sarah Heyman photographs our arthropods through the microscope using a digital camera, edits the images and passes them on to me so I can put them into a computer database.
The Bay Foundation and the University of Missouri gave us funds to buy a camera, computer and software for our project. While we were still figuring out the best way to handle the pictures, the world wide web took off, and we decided a web page would be a great way to display the pictures, allowing us to search for species with a click of a mouse. The other advantage was that we could make this key to our arthropods available to anyone interested in leaf litter arthropods.
Our web page has a brief introduction to arthropods, a guide to identifying them for novices and an introductory key to major arthropod groups. We also have finished our springtail pages and are getting ready to start on fly larvae. Visit our web site.