By Kathleen M. Wong, UC Natural Reserve System
Streams hide an underwater world of tiny insects in a fantastically varied web of life. Though largely hidden from human view by rushing waters, these aquatic communities are every bit as complex as anything on dry land.
Why species are found where within streams, however, remains rather mysterious. Many theories have sought to explain biodiversity patterns within ecosystems in general, and river networks in particular. But findings have been limited to specific types of rivers, such as those flowing through forests. The overarching processes driving stream ecosystem organization—knowledge critical to guide biodiversity conservation efforts—haven’t been fully understood.
To uncover organizational principles that structure stream life, UC Riverside graduate student Matthew Green and colleagues examined the aquatic invertebrates living along networks of streams and lakes in the high alpine zone of the Sierra Nevada. Many of these organisms are larval forms of insects such as stoneflies and caddisflies.
“We wanted to understand how much the environment is helping to sort organisms within a particular network,” Green says.
In the journal Ecological Monographs, Green and colleagues report finding several distinct patterns of diversity common to their stream and lake networks. Together, these patterns point toward several mechanisms that both foster diversity and sort species across aquatic environments.
The study’s roots date back about fifteen years, to a survey of aquatic invertebrates in Sequoia-Kings Canyon National Park. Research biologist David Herbst of UC Santa Barbara surveyed two stream-lake networks there for the National Park Service. Years later, Green realized that dataset would be an ideal foundation upon which to build a larger study. Green then expanded the dataset with his own stream diversity surveys, bringing the total to five networks and a whopping 118 sites sampled. Green also collected information about stream characteristics at each of his sampling sites, including data on water temperature, dissolved oxygen levels, flow rate, and type of bottom substrate.
One of the team’s major findings was that headwaters differed the most from each other in terms of species types. An existing theory called the “small but mighty headwaters hypothesis” suggested this result.
The old real estate adage of location, location, location is largely responsible for the finding. At high elevations, headwaters tend to arise in basins separated from other waterways by steep, rocky ridges. These ridges form barriers to adult insects seeking to colonize new streams to find and lay their eggs.
“Because those small streams are fairly isolated from one another, the expectation is that would lead to localized differences. Animals are not able to migrate between them and homogenize the community,” Herbst says.
Furthermore, unlike the middle reaches of streams, headwaters are unable to be colonized by species drifting downstream with the current.
The scientists also found that local diversity increased downstream from a waterway’s headwaters. In other words, they found a greater variety of animals farther away from the stream’s origins.
This result supports the “river continuum concept,” which theorizes that physical changes in a stream’s characteristics as it shifts from a headwaters tributary to a mainstem river enable it to support the most diversity in its middle reaches.
The Sierra stream sites with warmer waters and more productivity also supported the most species. The alpine location of the study, where algae growing on stream rocks is what fuels the invertebrate community, surely enhanced this result. In forest streams, where nutrient inputs were more abundant from tree leaves falling into streams, the gradient might not be so obvious.
Chutes and ladders
The exception to the “farther from the headwaters, the more diversity” finding was at the outlets of lakes. These sites, where broad lake banks constrict back into streams, harbored the fewest species of all.
That’s likely because moving from a placid lake to a flowing stream constitutes a dramatic shift in environmental conditions. The plankton drifting in lake waters gets swept up in the stream current. Insects like blackflies and net-spinning caddisflies can take advantage of the fire hose of food at lake outlets, enabling these filter feeders to form dense populations and muscle out most other types of species.
Herbst likens this exception to the river continuum concept to the board game chutes and ladders. Just as players in the game climb ladders to get to the finish line, streams build diversity as they go. But “as you hit these lake systems, the diversity basically has to restart further downstream,” he says—akin to the chutes that send game players back toward the start. It takes both distance and time for conditions to favor a wider range of species again.
A base in eastern California
Green based his field operations out of the NRS’s Sierra Nevada Aquatic Research Laboratory (SNARL). Located in Mammoth Lakes, the reserve is within easy driving distance of many of the trails Green used to hike to streams. “Having access to the reserves is really huge for research like this, for being in close proximity to field sites and having a scientific community to bounce ideas off of,” Green says.
That scientific community helped Green select a workable slate of sampling sites. “They have so much knowledge of the region. They were really helpful in saying maybe don’t do a 15 mile hike each day to go collect bugs, and maybe have a few sites that are more manageable,” Green says.
Both Green and Herbst also used the reserve’s laboratory facilities to analyze their specimens. Green consulted with Herbst to get help identifying many specimens. But even Herbst, who has decades of Sierra stream research behind him, hasn’t seen all the stream insects out there. “Other than microbes, aquatic invertebrates are far and away the most diverse life we find in the streams. They represent thousands of species that, by and large, are uncatalogued,” he says.
Beyond providing research facilities to the group, the NRS provided a fair amount of support for Green’s project over several years. A Mildred E. Mathias Graduate Research Grant and Valentine Eastern Sierra Reserve Graduate Research Grant helped with transportation and general supplies, plus reserve accommodations and lab use. A fellowship with the Institute for the Study of the Ecological Effects of Climate Impacts (ISEECI), which encouraged graduate students to document and analyze climate shifts at NRS reserves, enabled Green to purchase much of his field equipment.
Field research can be physically taxing, and this stream study was no exception. Just traveling to several of the streams required several days of backpacking at elevations from 9,000 to 12,000 feet. The scientists had to schlep not just their tents and food, but also stream sampling equipment such as nets and instruments. Green even packed in a portable boat to examine lake conditions.
To get to the most remote headwaters involves “a lot of rock hopping and trying to navigate scree fields. That can be a little challenging, but definitely fun,” Green says.
The field data the scientists amassed is immensely valuable even beyond the study’s contributions to ecological theory. For example, the specimens they collected will further scientific understanding of what species rear in alpine streams. At present, reliable keys do not exist for the immature stages of many species. Even experienced Sierra stream scientists like Herbst can identify their specimens to species only about 30 percent of the time.
“We’re finding out what lives in areas under the water’s surface where most people don’t look, and filling in some data gaps for basically unknown organisms as well,” says David Herbst, a research biologist with UC Santa Barbara.
Perhaps even more important is the snapshot of stream conditions the samples provide, especially in a period of rapid climate change. “They allow people to go back to them and ask, have we lost species or have their distributions changed?” Herbst says. Having such references can help conservationists keep tabs on mountain stream health, and ensure the biological diversity of California—including the littlest animals in the smallest streams—endures into the future.