AUSABLE WATER WISE: Ecosystem engineers of the river channel

Silk nets created by caddisflies trap food particles and hold the streambed in place. (Photo provided)

If you’ve ever seen beaver take up residence in a stream, you’ll know the incredible power they have to alter their environment. While it can be an inconvenience for landowners, the transformation from bubbling stream to mountain pond, to open meadow, and back into a stream long after the beaver has moved on is an incredible thing to witness.

Many a cartoon strip has depicted beavers as architects and engineers. In the science world, we refer to beaver as ecosystem engineers. The term ecosystem engineer can be applied to any animal or plant that creates or modifies the environment around them to better suit their needs. Under the water, crayfish use their claws to move gravel around to find food and create burrows for shelter. A stream full of crayfish can alter an entire stream environment. But who else is responsible for changes under the water’s surface?

Flyfishing anglers spend much of their time creating and using lures that mimic three major groups of aquatic insects: mayflies, stoneflies and caddisflies. They tie various flies out of feathers, fur, shiny threads and beads to imitate these flies’ underwater larval stages and the transitional form they take as they move from water to air as adult flying insects. These insects, because of their abundance, are prime food sources for freshwater fish. One particular family of caddisflies, however, have gained a reputation as ecosystem engineers.

This locally abundant family of caddisflies, called the Hydrophsychids, spend most of their water bound time spinning nets of silk that they put out into the river current to catch drifting food particles. The silk is not as strong as spider silk but is strong enough to withstand high water velocities. It is a very efficient way of collecting food, so much so that this family of insects thrives in both wild and heavily human altered rivers. But the nets do more than just providing food for the caddisflies.

Recent research suggests that the Hydropsychid caddisfly family may actually be engineering stream systems. We already knew that their nets slow the water velocity just above the streambed. This creates suitable, lower velocity habitat for many other species of aquatic insects. Just like a beaver dam creates extra habitat for fish, amphibians, dragonflies, and birds, the Hydropsychid caddisflies open up more habitat for more diverse species to move in and shelter or feed from the faster currents. But wait, there’s more.

When these aquatic insects put out their silky nets into the current, the ends adhere to pieces of gravel on the streambed. Looking more closely, river ecologists realized the silk can actually hold gravel and cobble substrate together, preventing movement and erosion during moderate to high flow events.

These silky webs actually stabilize the streambed. Ongoing studies in Pennsylvania and Montana hope to understand just how much of a stabilizing effect these caddisflies can have on the stream environment. It’s possible that by looking at their presence and abundance in a stream, we could better estimate how much gravel and fine sediment would move in a flood or build up over time. Building habitat and stream resilience in floods — it’s a significant achievement for the smallest of engineers.