Each fall, trees provide a veritable Thanksgiving Feast for aquatic macroinvertebrates. They spend the whole spring and summer preparing this feast, growing it, maintaining it, making sure everything is just right, and, come fall, dropping it all in the stream. What is this feast? Leaves!
Leaves from trees and shrubs are a form of allochthonous input, or something that enters an ecosystem from outside of the system. In small headwater streams, such as those found in our area, allochthonous input is a dominant source of energy, essentially forming the base of the food chain. Generally, these small headwater streams are forested, preventing light from reaching streams, inhibiting photosynthesis, and thus limiting the growth of aquatic plants. Consequently, the main source of energy in these streams comes from trees in the form of fallen leaves.
Once leaves fall into the stream, microbes, such as bacteria and fungi, colonize leaves, starting the process of decomposition. Some macroinvertebrates, called shredders, eat the leaves themselves, feasting on the plant material. Other macroinvertebrates, called scrapers, eat the layer of algae, bacteria, and fungi that develops on fallen leaves. Both shredders and scrapers are eaten by predatory macroinvertebrates, which are in turn eaten by frogs, salamanders, fish, and other aquatic animals, moving energy up the food chain.
Leaf litter in streams also provides important habitat for macroinvertebrates. Smaller macroinvertebrates can hide from predators in the leaf litter, while predatory macroinvertebrates can hide from unsuspecting prey in the leaf litter as well.
Since leaf litter is so important for macroinvertebrate communities, any changes to the amount and type of leaf litter that reaches the stream can impact the macroinvertebrate community. In areas where trees have been cleared along stream banks, the primary energy input is no longer leaf litter. Rather, since sunlight can reach the stream, aquatic algae and vegetation will grow more readily and become the dominant source of energy. As a result, the macroinvertebrate community will shift to a community that primarily eats aquatic vegetation and algae, with consequences for the entire food chain. Similarly, if the composition of riparian vegetation changes and non-native plants become more common than native plants, the macroinvertebrate community may shift as well.
The best way to maintain and improve this Thanksgiving Feast for our aquatic life is to protect forests along streams, remove invasive species, and plant native trees along streams. Willistown Conservation Trust is working to enhance this feast at Ashbridge Preserve, where we have planted over 1000 trees along Ridley Creek. We hope that these trees will improve water quality and contribute much needed allochthonous input for the inhabitants of Ridley Creek.
By Anna Anna Willig | she/her | Watershed Conservation Associate | As part of our Watershed Protection Program, Anna assists with monthly water chemistry sampling, maintains the tree planting at Ashbridge Preserve, and analyzes water quality data from sampling and from our EnviroDIY sensors.
