Hurry Up and Wait: A Strategy for Survival in Intermittent Streams

By Paul G. Johnson, Pinnacles National Park Wildlife Biologist

February 2018 - It’s fairly easy to imagine the basic life cycle of an aquatic insect living in a stream that flows year round: the adult insect lays eggs in the stream, the eggs hatch, the immature stages eat and grow, and then the adults emerge and continue the cycle. But how do insects carry out their life cycle in a stream that dries up for most of the year? What if the stream experiences summer flash floods, with the streambed being picked up and jumbled by the water?

Michael Bogan has been studying intermittent streams since 2003, and since 2014 at Pinnacles National Park. In order to better understand how aquatic insects survive periods of stream drying, he focused on a tiny winter-flying stonefly, Mesocapnia arizonensis. This species was known from only 22 locations in Arizona and New Mexico, but he found them at nearly 100 intermittent stream sites including Pinnacles National Park, a range extension of 500 miles.

Adult male Mesocapnia arizonensis on a human hand for scale.

© Michael Bogan

Bogan hypothesized that these stoneflies survive the dry season by having a dormant stage that can persist in dry stream bed sediments. To test this, he dug up sediment to a depth of one foot during the dry season at one Arizona stream, and wetted the sediment in a lab to see if stoneflies would emerge. None did.

However, Bogan found abundant stoneflies in the same stream within days of winter flow resuming. This suggests that soon after hatching from eggs, the stoneflies crawl deep into the stream bed where they pass the dry season and avoid flash floods. Because intermittent streams can have ‘false starts’ and quickly dry again during winters with little rain, some portion of each stonefly generation likely remains dormant to ‘wake up’ and crawl back to the surface in subsequent years when winter rains might be better.

One prediction of climate change models for our area is that droughts will be more frequent and prolonged, while flooding may become more severe. While these changes may lead to the extirpation of many aquatic invertebrate species at Pinnacles, Mesocapnia arizonensis and other species with similar adaptations to flashy intermittent streams could benefit.

See the full article about this project for additional details.