What is usually a lush and dense wooded area overlooking La Crosse, is now a baren leafless landscape devoid of color. With Wisconsin’s lovely seasonality to thank, a 50 degree, cloudless day in early December, looks drastically different on the Bluffs, compared to just a few weeks earlier. Along with the changing weather, the ecosystem shifts toward inactivity. Loss of leaves as photosynthesis is not efficient enough to merit their existence, sparse invertebrate activity, and even less frequent vertebrate movements. At first thought, maybe these organisms are like me and just hate the cold, but what is really at play is the larger ecological concept of energy flow. Organisms optimize for efficient use of energy and that is what we will be looking at today. A case study of ecological differences at Grandad’s bluff as the fall turns to winter.
Energy Dynamics
Ecosystems are largely shaped around primary production. The entire food web depends the amount of energy that is either a result of photosynthesis or chemical oxidation. With lower amounts of primary production, we see decreases in the amount of individuals and species that an ecosystem can maintain. This is due to largely inefficient transfer of energy, where only about 10% of what you consume is transferred to the next trophic level.
The Fall Fauna
Fall is full of the oranges and yellows that signify the decrease in primary production and the purging of the photosynthetic body of deciduous trees. The ecosystem is not yet empty of this baseline production and is still able to maintain its trophic levels. During a visit to the Grandad’s Bluff in early October I was greeted by a handful of whitetail deer (Odocoileus virginianus), as we maneuvered up in elevation, and frequent sightings of small rodents and birds. Aside from the animal life, the forest floor still has signs of green and fledging saplings fighting with the large silver maples (Acer saccharinum) for the remining sunlight. It is these small green plants that are able to nourish the aforementioned, largely herbivore species which I observed. Likewise, the presence of the second trophic level gives rise a third, carnivorous level, and even a fourth, tertiary consumer.
Early fall still sees the predator prey interactions of a secondary consumer, snake, consuming a primary consumer, chipmunk. The arms race in height between trees competing for sunlight and the presence of enough greenery to sustain a deer population.
A Reduction in Robustness
A second trip to Grandad’s bluff in early December with the goal of observing the same species that I observed only a few weeks prior, proved very insightful. No longer were there deer roaming, chipmunks scampering in the foliage, or even leaves left on the trees. The silver maple whose leaves covered the ground and blocked the sunlight from the forest floor, was now nothing more than a trunk with naked branches. The weather was changing, and the ecosystem changed along with it. Decreases in temperature and the amount of UV radiation reaching the leaves of trees and other plants derails photosynthesis. Without this primary production the beautiful, robust, ecosystem comes to a halt. The chipmunks no longer have vegetation to feed on. The snake can no longer find a chipmunk to eat and the owl can no longer find a snake as frequently.
The decrease in primary production leads the other organisms in the ecosystem into a state of inactivity and maintenance, which is hibernation in some species and just decreased metabolism in others. Fungi and detritivores now dominated the forest floor decomposing the biomass left behind from the falling leaves and trees.
Trichaptum abietinum growing on a fallen tree.
The Grand(dad’s) Scheme of Things
The ecosystem changes that I observed on Grandad’s Bluff over the course of two months are by no means limited to this ecosystem. Seasonality is a huge part of the temperate forest biome we inhabit and the ecosystems response to changes in weather are vital to survival. High activity when there is available energy and inactivity when energy is sparse. This ability to change and adapt is what makes life so vibrant and ecosystems so dynamic. Simple changes in energy distribution or presence of a top predator can cause a cascade of changes throughout the biotic components of an ecosystem.
Ultimately, the amount of primary production is a driving force in the breadth and diversity of the food web. Climates and habitats that favor higher amounts of primary production are able to support a larger number of individuals and species. At a very small scale this is what I saw at the Bluffs as biotic processes decreased with the changing weather and season.
A naked silver maple (Acer saccharinum) tree surrounded by dead and decaying organic matter.