A guest post by Bronwen Stanford
In the bottom of every healthy stream are millions of insects. For the most part, these insects are tiny, with bodies could fit on a thumbtack. This may not be exactly what everyone likes to picture when they think of a peaceful, meandering stream, but this life is what keeps our streams clean. These tiny aquatic insects digest dead leaves, graze on algae, filter fine particles out of the water, and provide food for birds, bats, amphibians, and the many species of fish that capture people’s imagination. In coastal California streams, I have sometimes counted densities of up to fifteen thousand insects per square meter of stream bottom, from over 50 distinct insect families.
Scientists use these insects to learn about stream health. Aquatic insects are so numerous and diverse that by simply observing the numbers and kinds of insects in a stream, we can infer what types of stress a stream may be experiencing. For example, some insect species tend to increase under human-induced stresses, such as nutrients from fertilizer runoff, while other species become rare or disappear entirely. In California, stressed streams may have high numbers of blackflies, or few insects of any kind, while healthy streams have many different types of insects, including stoneflies, mayflies, and caddisflies that serve as the food source for trout and salmon.
I study aquatic insect life in the beautiful green valleys of Marin County, where cattle ranching has been the dominant land use for over 150 years. Many ranchers there are descendants of families that began raising cattle in these same locations just after the Gold Rush, and many describe land stewardship as a key motivating force in their lives. However, grazing can allow fine sediments to leach into the stream as cattle disturb and expose the soil; the increase of sediment in streams can then lead to loss of habitat as settling particles cement over the tiny nooks and crannies that fish and insects use. In response, ranchers often plant strips of trees along their stream banks to improve water quality, a technique that can also slow erosion and flooding, further benefiting aquatic life.
While there is good evidence that wider tree plantings and strategic tree placement can lead to improved stream health, we don’t know whether planting trees along longer sections of streams can have the same benefit. Benefits might include shading and cooler water, more tree roots and fallen branches to hide under and feed on, and bigger connected patches of good stream habitat. With the help of many interns, I collected and identified thousands of insects across Marin County to see how insects responded to tree plantings that extended different lengths along the stream banks. While many features of the stream, including the texture of the stream bottom, were the same from top to bottom of a strip of trees, I found that the insect communities improved with longer strips of trees. More sensitive insect species were able to live at the downstream end of tree plantings, most likely in response to subtle changes in water temperature, fine sediment, and flow associated with the trees lining the banks. This information is important, because it shows that ranchers can improve the effectiveness of their restoration efforts by planting longer runs of trees, even in cases where they aren’t able to create wider swaths of trees.
Insects are important both because they show us how stream condition is changing, and because they directly support the entire aquatic food web: they are fed on by birds, bats, and baby fish, and these in turn feed bigger fish and (in the case of Marin streams) river otters. This ecosystem snapshot can help managers decide which efforts actually succeed in improving stream health. And if people get excited about the secret world of aquatic insects in the meantime, that’s fine with me.
Bronwen Stanford is a PhD candidate in Environmental Studies at UCSC.