Chubs are prolific ecosystem engineers. Photo by B. Peoples

Chubs (Nocomis spp.) are large minnows that are common throughout eastern North America. In spring, adult male chubs carry gravel in their mouths to construct large, mound-shaped nests for spawning. Nests can be 1.5 m in diameter and over a half-meter tall.

Chub nests are unique features on the streambed. They’re constructed of a very specific gravel size, in habitats with perfect flow velocity. Chub  nests can be the only sources of concentrated gravel in stream habitats that are starved of sediment because of anthropogenic hydrologic alteration.

Chubs can be particularly important because they facilitate the reproduction of many other minnow species—over 30 throughout their native distribution. These ‘nest associates’also lay their eggs in chub nests. A chub nest being swarmed by hundreds of bright red, yellow and orange associates is one of the most colorful sights to be seen in aquatic ecosystems—rivaling even the most flamboyant coral reef species.

Associates don’t take care of their own babies after spawning,so  they benefit from nest guarding and egg burying by male chubs. Chubs also keep nests free of silt, which smothers eggs and larvae. Nest association can be mutualistic because high proportions of associate eggs can reduce the probability of chub eggs being preyed upon. However, nest association outcomes can also range from being commensalistic or even parasitic, depending on identities and behaviors of participants.

Nest association behavior ranges from largely facultative for some associate species, to entirely obligate for others. Accordingly, nest association provides an excellent system for studying patterns of increasing interest to ecologists: context-dependency in interaction outcomes.

To understand context dependency in this system, we conducted a large-scale instream experiment to investigate the effects of predator density and spawning habitat quality (heavily silted, control) on the associative behavior and reproductive success of Bluehead chub and Mountain redbelly dace. Dace wouldn’t even try to spawn without chubs. Although chubs built nests in the absence of dace, they hatched much more larvae on nests where dace eggs were present. Moreover, the relationship switched from being commensalistic to mutualistic as habitat quality improved.

Nest association interactions are much more complex than the two-species example we used in the  experiment. Firstly, chubs aren’t the only species that build nests in these streams. Secondly, multiple associate species can be involved in any given spawning event (i.e. up to 6 species in the New river). Associates can also switch from spawning with one host to another. This level of behavioral diversity can have strong implications at the community level.

We used the chub-associate system to examine a rapidly-developing ecological model: the stress-gradient hypothesis (SGH). Briefly, the SGH predicts that the frequency of competitive interactions should be most important in relatively benign habitats, but that positive interactions (e.g. facilitation by chubs) should prevail in harsher habitats. We took the SGH a step further, and asked whether the proportional representation of beneficiary species (e.g. chubs nearly-obligate nest associates) would increase with physical stress (e.g. disturbance). As deduced by the SGH, proportions of chubs and nearly-obligate associates increased with stress, but other groups (e.g. facultative  and non-associates) did not. To date, the SGH has been used almost exclusively in plant communities. However, our work and others is beginning to show that this model may be much more widely applicable than previously understood.

Lastly, we used chubs and associates to try and untangle the relative roles of biotic interactions and abiotic features for determining species co-occurrence. We used two-species occupancy modeling, which allowed us to compare hypotheses specifying the importance of interactions, habitat, or both for co-occurrence of chubs and Mountain redbelly dace or Rosyside dace. Both nest associates exhibited positive co-occurrence with chubs, and overwhelming evidence suggested that interactions alone drive co-occurrence of chubs and rosyside dace. However, strong evidence suggested that positive co-occurrence between chubs and mountain redbelly dace is mediated by habitat factors associated with stream size. This work adds to the growing evidence that biotic interactions can be evident even at large spatial scales.

Nest association is an incredibly interesting aspect of many fishes’ natural history. It can also provide a very tractable study system for investigating some of ecology’s most interesting questions. Many questions still linger, ranging from the basic ecology of these fishes, to more theoretical issues. So far, we’ve barely seen below the surface.

by Brandon Peoples