Pathogens, Parasites, and Food Webs in Rocky Mountain Streams

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Research Primer

Background

Rocky Mountain streams and the meadows that surround them are alive with feeding relationships we rarely see. Mayflies graze algae on cobbles, stoneflies hunt smaller insects, trout pick prey from the current, and birds pluck emerging insects from the air. Woven invisibly through this food web is another layer of biology: the parasites, pathogens, and microbial symbionts that infect nearly every species. In the Gunnison Basin, where short summers concentrate reproduction and transmission into a few intense weeks, these hidden interactions can shape which insects emerge, which birds reproduce successfully, and which livestock herds stay healthy.

To follow the findings in this primer, a few ideas are worth introducing up front. Food web structure refers to the network of who eats whom in an ecosystem; parasites sit inside this network, sometimes altering prey selection by making hosts easier to catch. Infection prevalence is simply the proportion of individuals in a population carrying a pathogen, while transmission dynamics describe how that pathogen moves among hosts — sometimes directly, sometimes through a biting insect vector, and sometimes through vertical transmission from a mother to her offspring. Avian malaria, for example, is a blood parasite of birds transmitted by biting flies, while vesicular stomatitis is a livestock virus spread by biting midges and black flies.

Other concepts matter for specific host groups. In aphids and other insects, bacterial endosymbionts can defend their hosts against parasitoid wasps but may impose fitness costs on growth rate and demography. In bumble bees, pathogen load can vary with flower visitation patterns and the phenology of the flight season. And in stream insects, parasitic nematodes can alter emergence weight and size at maturity, with consequences that ripple outward to the predators that rely on those insects. Because climate change is lengthening vector seasons and shifting host ranges, understanding these baseline host–parasite relationships in a relatively intact mountain system like RMBL has become increasingly valuable.

Foundational work

Early dissertation and survey work at RMBL established that mountain host–parasite systems are both diverse and tractable. Murdock's doctoral studies on avian malaria in Mountain White-crowned Sparrows (Murdock, 2009) mapped the host, biting-fly vector, and blood parasite communities at RMBL field sites, and used molecular tools to link specific Leucocytozoon lineages to the black fly species that carry them. That work provided one of the first detailed pictures of hemosporidian transmission in a temperate alpine bird community and set a baseline against which future climate-driven changes could be measured.

In parallel, a pathogen survey of Rocky Mountain bumble bees (Diaz, 2008) screened Bombus workers along an elevational gradient for a suite of parasites — Crithidia bombi, Nosema bombi, Apicystis bombi, conopid flies, phorids, and tracheal mites. That survey reported that roughly 9.5% of bees carried significant infections, with N. bombi peaking late in the season and A. bombi peaking early, and found no association between the invasive flower Linaria vulgaris and pathogen intensity. Together, these early studies framed RMBL as a site where host, vector, and pathogen could be studied simultaneously across insects, birds, and the plants that connect them.

Key findings

Across hosts, parasites in this system impose measurable but context-dependent costs. In black sage aphids, the endosymbiotic bacterium Hamiltonella defensa defends its host against parasitoid wasps but carries a real demographic price: infected aphids had significantly higher mortality by the second week of the experiment, and infected populations were significantly more likely to go extinct than uninfected ones (Booster, 2010). This helps explain why the bacterium persists at only intermediate frequencies in wild aphid populations — the protection it offers is balanced by a fitness cost, a tension that shapes the evolution of insect–microbe partnerships throughout the basin.

In birds, the picture is subtler. Incubating female Mountain White-crowned Sparrows kept their nests warmer than ambient temperatures regardless of whether they carried blood parasites, and although parasitized females showed a trend toward more variable nest temperatures, the effect was not statistically significant (McCuen, 2019). Combined with the earlier community-level work on vectors and Leucocytozoon transmission (Murdock, 2009), this suggests that hemosporidian infections in alpine sparrows are common but that their fitness consequences may be modest under current conditions — a baseline that could shift as warming extends vector activity.

At the landscape scale, pathogen dynamics in Rocky Mountain livestock underscore how mountain vectors connect wild and managed systems. The 2019 vesicular stomatitis outbreak was the largest in 40 years, affecting 1,144 premises across 111 counties in eight states, and the 2020 outbreak arose from overwintering of that same virus rather than a fresh incursion (Pelzel-McCluskey et al., 2021). Strikingly, Rocky Mountain states that had extensive cases in 2019 recorded zero confirmed cases in 2020 despite strong surveillance, suggesting that local transmission conditions — vector populations, host immunity, weather — vary sharply from year to year even when the virus persists regionally.

Current frontier

The temporal trajectory of this research is revealing. Early work in the 2000s and early 2010s (Diaz, 2008; Murdock, 2009; Booster, 2010) (Murdock, 2009) (Booster, 2010) established what parasites are present and what they do to individual hosts. Work in the late 2010s (McCuen, 2019) began probing sublethal effects on reproductive behavior. The most recent landmark (Pelzel-McCluskey et al., 2021) pushes the field outward in both scale and method, using full-genome maximum-likelihood phylogenetic analysis and model selection for transmission pathways to trace a single virus across multiple states and multiple years. This integration of molecular tools, vector sampling, and spatial epidemiology marks a clear shift from site-based natural history toward landscape-scale pathogen ecology.

Emerging questions center on connecting stream, meadow, and livestock systems. Nematode infections of mayflies and stoneflies, endosymbionts in aphids, blood parasites in birds, and biting-fly-borne viruses in livestock all depend on insect vectors and hosts whose phenology is shifting. Methods such as metabarcoding of environmental DNA and integration of natural-history and community-science records through platforms like Global Biotic Interactions are beginning to let researchers reconstruct these interactions at far larger scales than single-site surveys allowed.

Open questions

Several important questions remain. How will warmer, longer summers change the transmission dynamics of avian malaria and livestock viruses in the Gunnison Basin, and will parasites that are currently mild become more costly as vector seasons lengthen? What explains the dramatic year-to-year swings in vesicular stomatitis activity in Rocky Mountain states, and can local vector or weather data predict outbreak years? How do parasitic infections of stream insects propagate upward through food webs to fish and birds, and do endosymbiont-driven shifts in aphid demography scale up to affect the plants and pollinators that share those meadows? Answering these will likely require coupling long-term RMBL monitoring with molecular and landscape-scale methods over the coming decade.

References

Booster, N. (2010). Fitness costs of the aphid endosymbiont, Hamiltonella defensa. RMBL Independent Research. →

Diaz, R. (2008). Rocky Mountain Bombus pathogen survey: Are invasive plants affecting pathogen prevalence and intensity? RMBL REU Program. →

McCuen, C. (2019). Effect of blood parasites on incubation efficiency of Z. l. oriantha in the Colorado Rocky Mountains. RMBL. →

Murdock, C. C. (2009). Studies on the ecology of avian malaria in an alpine ecosystem. Doctoral dissertation, University of Michigan. →

Pelzel-McCluskey, A., Christensen, B., Humphreys, J., Bertram, M., Keener, R., Ewing, R., Cohnstaedt, L. W., Tell, R., Peters, D. P. C., & Rodriguez, L. (2021). Review of Vesicular Stomatitis in the United States with Focus on 2019 and 2020 Outbreaks. Pathogens. →

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