SAN LUIS VALLEY— Every fall, after the leaves, twigs and other natural debris fall to the ground, a rich community of animals springs to life—weird, prehistoric-looking creatures with names like water bears, sow bugs, springtails and harvestmen. It’s a mind-blowing world filled with herbivores, predators, parasites, parasites who feed on other parasites (hyperparasites) and symbiosis.
Some, like pill bugs and mites chow down on the leaf litter. Others, like spiders and pseudoscorpions (think tiny lobsters), chow down on other animals. Still others use the leaf layers for warmth during the cold winter months, emerging in springtime to begin the life cycle again.
Not only does leaf litter create habitat for wildlife, it’s vital for gardens, farms, ranches and the overall environment. “Leaf debris gives shelter to soil microbes and insects living in that sphere,” said San Luis Valley soil scientist Patrick O’Neil with Soil Health Services— and it’s that rich microbiota that plays a vital role in soil health.
“Leaf litter also protects the soil from being beaten by the sun and moisture from evaporating as quickly,” O’Neil added, “which is important in this sunbaked valley.”
The process begins as caterpillars and other insects dine on foliage in the spring and summer months. Their nutrient-rich excrement, called insect frass, drops to the ground, mixing with leaf litter and other natural debris in the fall. Moisture becomes trapped in the layers, creating the perfect habitat for worms, snails, centipedes, spiders, mites and a host of microscopic animals.
The invertebrates shred and eat the debris, making it easier for bacteria and fungi to decompose the matter into rich humus. Nutrients such as nitrogen, calcium and sulfur are recycled into the soil and eventually used as food by trees, shrubs and other plants. The cycle creates, in effect, nature’s home-made fertilizer. New plants sprout from the enriched soil, which, depending on the ecosystem, can take hundreds to thousands of years to produce.
O’Neil helps farmers and ranchers throughout the Valley with soil health. This year, he couldn’t help but notice the effects of drought, like more dead lawns than normal and bare patches in yards. “That’s because there was no cover,” he said, “no snow to insulate the soil and keep it from desiccating and warming up and waking up plants too soon.” While leaf litter doesn’t have the same insulating effect as snow against the cold, it keeps the soil from drying out.
When conditions are right in the leaf litter, animals and other life necessary for the break down process come naturally. “Reptiles, mammals, insects and birds can all be members of a litter ecosystem,” said Denver-based ecologist Susan Sherrod. “Over 100 species of beetles alone have been counted in the detritus of one Colorado ecosystem.” Bumblebee queens often hibernate under the warm, moist layers. After awakening in spring, they forage on the early blossoms of dandelions, iris and wild strawberry. Back under the leaf litter, they form balls of pollen and wax and lay eggs. Then they build waxy pots that they fill with the nectar and pollen balls, often called beebread. The stored food allows sustenance for the queen while she broods on her eggs and for her eyeless, legless larvae upon emergence.
Some butterfly and moth species benefit from fallen leaves, including Colorado natives such as the brown elfin and nais metalmark. The pretty Colorado hairstreak lays her eggs on gambel oak leaves in late summer, where the eggs remain until spring. The western banded skipper lays eggs on the ground near asters and thistles, while the Aphrodite fritillary lays them near violets. Fallen leaves and other dead vegetation provide protection for the butterfly eggs through winter and are a food source for emergent caterpillars in spring. Some insects seek out pine needles. The pine white butterfly, for example, lays her eggs on the needles of ponderosa pine, Douglas fir and other conifers. Later, the larvae eat the needles.
Then there are the water bears and daddy longlegs. Water bears are microscopic critters in the leaf litter that feed on algae, bacteria, nematodes and organic detritus. They need a film of water around their bodies or they shrivel up and remain alive but motionless until they can replenish their liquid shell. Daddy longlegs are called harvestmen because they feed on both live and dead matter. Long-legged harvestmen live in the loose outer layers, while short-legged harvestmen hide deeper in the leaves.
But the most plentiful of all are mites and springtails—minuscule critters found in leaf litter all over the world. Several hundred thousand can exist in one square meter of soil, and they’re rock stars when it comes to breaking down detritus and turning it into nutrient-rich humus. They feed mostly on bacteria and fungi growing on leaf litter, said Timothy Seasted, professor of ecology and evolutionary biology at the University of Colorado. “Think of scraping the icing off of an Oreo cookie. They like the filling.” New bacteria and fungi grow and eat away at the debris’ surface, resulting in more decay after each meal.
For folks who let fallen leaves linger, the first creatures they’ll observe hanging around leaf piles are birds in springtime. That’s because birds feed their young a hodgepodge of insects rather than berries, seeds and nuts. The leaf litter provides a virtual buffet for avian parents during a time when insect populations are low and the need is high.
Birds such as robins, woodpeckers and a variety of sparrows hunt for food in the litter. It’s fun to watch different sparrow varieties—spotted towhees, dark-eyed juncos, fox sparrows and chipping sparrows—pick through the leaves by scratching, hopping backward and repeating the process until they discover a wiggly treat. Birds and other animals, like chipmunks, will also use leaf litter for cover.
Though leaf litter may harbor garden pests, it also provides habitat for animals that keep pests under control. Damsel bugs, for example, overwinter as adults in the fallen leaves. They’re voracious predators of aphids, true bugs, spider mites and small caterpillars, Sherrod said. “The biota that process leaf litter also inhabit the underlying soil, keeping it aerated with their burrowing and tunneling activities, and mixing high-nutrient surface soils with deeper layers.”
Leaf litter also serves as mulch that helps prevent erosion and keep weeds at bay.  “Leaf litter is basically the soil’s armor,” O’Neil said. “Take that off and it exposes the soil surface to evaporation and radiation.”
Rather than raking all the leaves from the yard this fall, consider keeping some around for the critters and soil health. Also consider keeping a few pine needles, which work great for suppressing weeds and lining walking paths.
The Valley’s notorious winds can make it difficult to maintain the right conditions for breakdown and keep leaf layers in place. “You’ll need something to help those leaves begin to decay,” O’Neil said. Adding compost and moisture to the mix encourages the right conditions for the breakdown process and helps leaves and other debris anchor into the soil. Making sure decay is well on its way before winter sets in is also important in preventing pathogens from developing that are harmful to lawns. “You want the degradation process so that more pores, small particles and mulch-like materials develop instead of just a solid sheet of matted-together-leaves that become an impediment to air flow.”  
Leaf litter is temporary so try not to worry about aesthetics. “Farmers are challenged with that perception as well,” O’Neil said. “But 10 years ago, farmers who maintained more residue in their fields were called trash farmers. Today they’re lauded as the most conservation-minded farmers in the nation.”