A sucker for ecosystem engineers

Sonoran suckers, like other ecosystem engineers like beavers or salmon, substantially influence the spatial and temporal distribution of their environment’s resources. These fish create divots in soft sediments during their nightly feeding rounds in shallow waters far from where they seek refuge during the day. This feeding and these divots contribute heavily to shaping patterns of aquatic insects and suspended sediment in the Gila River, establishing the suckers as important ecosystem engineers in the river.

Source article: Booth, M.T., Hairston, N.G., & Flecker, A.S. (2020). Consumer movement dynamics as hidden drivers of stream habitat structure: suckers as ecosystem engineers on the night shift. Oikos, 129(2), 194-208. https://doi.org/10.1111/oik.06396

Ecosystem engineering

You’ve likely heard or even seen the effects of beavers on their environment: their dams block streams, creating ponds, and the gnawed-down trees and scraped mud points to the original gathering of construction materials. Beavers are among the best-known ecosystem engineers, organisms who create, maintain, significantly modify, or destroy habitats more than the average organism, but they’re far from the only ones. Others include prairie dogs, trees whose roots significantly alter the structure of streambanks and nearby stream features, salmon, and corals whose reefs provide structural support for large communities to grow among them.

Ecosystem engineers are importantly and particularly responsible for environmental heterogeneity, or the differences within a larger environment. Different organisms are specialized for different environments, so ecosystem engineers are important drivers of species richness, itself a major contributor to diversity.

A Sonoran sucker showing off the modified lips that help it dig into the stream bed.

Beavers aren’t the only ecosystem engineers that can be found in streams, however! Drs. Michael Booth, Nelson Hairston Jr., and Alexander Flecker discovered that the Sonoran sucker, Catostomus insignis, a common and dominant fish in the southwestern United States, might itself be an important engineer in its ecosystem. These relatives of carp can be greatly responsible for creating and maintaining habitat heterogeneity and increasing the amount of sediment suspended in streams through nighttime feeding routines.

Nighttime fish monitoring

The study was conducted at the Heartbar Wildlife area near the confluence of two major forks of the Gila River in western New Mexico. By capturing suckers, surgically inserting small location trackers into their bodies, and releasing the fish back to the water where they’d heal and resume normal activity, the scientists could follow the movements of the fish relate their positions to changes in the environment.

These changes were expected from the suckers’ feeding behavior: at night, they would leave their safe pools for shallow water. There, the suckers plow through soft sediments with their heads, creating “feeding divots,” and eat the invertebrates they find. This turning over of sediment leads to a change in bed composition (what used to be smoother now has divots scattered across it) and an increase in turbidity, a measure of how cloudy the water is.

The Gila River, home to many including the Sonoran sucker.

Through a set of experiments and measurements, the scientists were able to gather information on several environmental qualities:

  • How densely scattered the feeding divots were across the stream bed
  • What volume of sediment was displaced in the formation of each divot
  • How far the nearest major refuge site was from feeding grounds
  • The daily rate of divot formation
  • How divots change organic matter retention and invertebrate abundance
  • How variation in how frequently fish visited sites affected how different the disturbances were each time
  • How related sediment and carbon transport were to fish stirring up the stream bed
  • How much sediment was flowing through the river each day

The scientists were able to relate all of these different qualities and others together to assemble an idea of the effects suckers were having on their environment.

A sucker’s river

The data Drs. Booth, Hairston Jr., and Flecker collected provided great insight toward the interactions the suckers and their environment.  Almost all the fishes’ movements were during the night, during which they would leave their deep pools for shallow water. Not too picky about where they tried feeding, the suckers dispersed and dug their divots on fresh sediment and old divots alike.

In their digging, they could uncover organic material like leaves and twigs, allowing those resources to be utilized again. The sediment they stirred up would sometimes end up burying nearby organic material, and much would travel further downstream before settling onto the stream bed again. Sometimes sections of stream bed would go not even a day before more suckers would disturb it; sometimes they lasted over a week before fish revisited.

An extreme example of different turbidities: the Rio Negro, blue, meets the sediment-rich Amazon River. Suckers increase turbidity by stirring up extra sediment.

Suckers typically traveled to their feeding grounds, and the farthest sites were often the ones least frequently disturbed, but the real interruption to when and how far away the fish would venture out came in the form of low flows. Not uncommon in the arid Southwest, many organisms in those streams, like suckers, are adapted to more dry conditions, but the fish can’t swim through too-shallow waters in order to get to their regular feeding grounds. In the right alignment of conditions where flow was low but just deep enough for suckers, the fish’s feeding behavior was the most impactful on how much sediment got moved downstream: over 20% of all that material could be attributed to just these fish!

As ecosystem engineers, the effects suckers had on their environment were extended to other organisms, particularly invertebrates like those they fed on. The scientists noted that there were more bugs in the devoted areas than those undisturbed, leaving them with questions about the possibility the fish were “farming” their food by creating bug-friendly habitats. Certainly the suckers’ divots were influencing where and in what numbers bugs were to be found.

Outside of prey items, the invertebrates are also affected by the feeding through the downstream settling of sediment: these fine particles can be detrimental to certain taxa, leading to a pressure on the bugs to be resistant to them.

Thank your local engineers

Though less obvious than beavers or less magnificent than a massive salmon run, the nightly stirring up of sediment and bugs by Sonoran suckers is another case of ecosystem engineering: they make substantial changes to their environment which impact other organisms. Knowing this behavior is largely shut down by low flows, there is an extra pressure on us to utilize water resources in a sustainable way that will not hinder the behavior of other organisms, ecosystem engineers in particular due to their wide influence. The more time we spend unraveling the tightly interconnected components of ecosystems, the more we will realize how important each organism within them is—even if they suck.

Reviewed by Niranjana Krishnan (https://envirobites.org/author/nkrishnan/)

and

Carrie McDonough (https://envirobites.org/author/carrie/)

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Garrett Frandson

Garrett Frandson

I'm a master's student at the University of Missouri studying stream macroinvertebrates and the substrate they inhabit. I'm broadly interested in bugs, streams, drivers of change including climate change and other anthropogenic disturbances, and communicating the value and beauty of natural systems and the need to protect them to the general public and those who can enact policy changes. When not working, I'm probably poking around and photographing these systems and their inhabitants! IG: @frandsong

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