Drosophila Courtship Evolution and Population Divergence
Investigates how cold adaptation, geographic isolation, and sexual selection drive genomic and behavioral divergence across Drosophila populations, using courtship song recording and isofemale line methods.
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Research Primer
Background
Mountain streams are among the most physically dynamic and biologically structured ecosystems on Earth. In the headwaters of the Gunnison Basin, cold, clear water tumbling out of snowmelt-fed drainages supports a tightly knit community of algae, aquatic insects, fish, amphibians, and birds. Stream ecology at the Rocky Mountain Biological Laboratory (RMBL) has long focused on how these communities are shaped by two intertwined forces: the supply of nutrients from the surrounding landscape, and the predators that move energy through the food web. Understanding these forces matters for water managers, anglers, and downstream users because the same streams that drain alpine meadows above Gothic feed the agricultural and municipal water supplies of the broader Colorado River system.
A few key concepts unlock most of the findings that follow. Stream food webs in the East River drainage basin tend to be oligotrophic, meaning they grow under chronically low nutrient supply, especially of phosphorus. Because nutrients are scarce, primary productivity by algae attached to rocks (periphytic algae) is often nutrient-limited, and small changes in nitrogen or phosphorus can have outsized effects. The diatom Didymosphenia geminata ("didymo") is a striking case: under low-phosphorus, low-flow conditions it produces extracellular stalks that coalesce into thick mats, an algae bloom that physically restructures the stream bottom. Animals also participate in nutrient cycling. Through animal-driven nutrient cycling, insect larvae excrete nitrogen and phosphorus at rates that can rival physical fluxes, and downstream transport plus repeated uptake and release is described by nutrient spiraling.
Predator effects in streams are equally subtle. Fish and predatory stoneflies kill prey directly, but they also leak chemical cues called kairomones into the water. Mayflies and other prey detect these cues and change their behavior, growth, and timing of emergence (the size and date at which larvae become winged adults). These nonconsumptive effects can ripple through to algae, biofilm succession on rocks, and ultimately to the size at emergence of insects feeding birds, bats, and trout. Stream flow itself, the hydrological regime, sets the stage by determining when habitat is available, when high water scours the bottom, and when low flows concentrate organisms and nutrients.
Foundational work
Much of the modern understanding of mountain stream food webs grew out of long-term work in Cement Creek and the East River near Gothic. Allan's surveys of benthic insects established that physical gradients and substrate complexity, more than competition among species, drive the distribution and diversity of stream insects in alpine drainages (Allan, 1975), and that brook trout diets in these streams reflect what is available in the drift, with large prey consistently overrepresented (Allan, 1981). Strikingly, a four-year experimental reduction of trout density produced no detectable change in the invertebrate community, hinting that prey exchange and behavior, not just direct mortality, govern predator impact (Allan, 1982). Allan also showed that nocturnal drift in the mayfly Baetis bicaudatus is a size-dependent predator-avoidance behavior (Allan, 1978).
The behavioral side of predator-prey interactions was opened up by Peckarsky's direct observations of stoneflies and mayflies in the East River, which demonstrated that prey use chemical and tactile cues, not vision, to detect predators (Peckarsky, 1980). Cooper, Walde, and Peckarsky then showed that the apparent strength of predation in streams depends overwhelmingly on how quickly prey are exchanged among patches (Cooper et al., 1990). Subsequent experiments revealed that even nonlethal stoneflies reduce mayfly feeding, growth, and fecundity (Peckarsky et al., 1993), and a broad synthesis tied these chemical and mechanical signals together as the dominant currency of communication in stream benthos (Dodson et al., 1994). McPeek and Peckarsky later formalized these effects into demographic models, showing that for mayflies, growth effects of predators matter more than mortality because adult size translates directly into egg number (McPeek & Peckarsky, 1998).
Key findings
A central result from RMBL streams is that fish and stonefly chemical cues reshape mayfly life histories without ever touching prey. Whole-stream additions of brook trout odor to a naturally fishless stream caused Baetis to emerge about 20% smaller (Peckarsky et al., 2002) and reduced secondary production of mayflies by 17% (Koch et al., 2020). Multiple-predator experiments demonstrated that trout odor can actually dampen the behavioral effect of stoneflies on mayflies, an interaction modification rather than simple addition of risks (Peckarsky & McIntosh, 1998). These nonconsumptive effects extend beyond prey themselves: by altering where and when grazers feed, predators indirectly change algal distributions and ecosystem properties such as trophic transfer efficiency (Schmitz et al., 2008), creating predator-induced patchiness in stream algae (McIntosh et al., 2004).
A second major thread concerns Didymosphenia geminata blooms. Long thought to be an invasive species, fossil records show didymo has been present across most continents for centuries (Bothwell et al., 2014), and its nuisance mats are triggered by very low dissolved phosphorus (below roughly 2 ppb), not by introductions (Bothwell & Kilroy, 2014). Phosphorus enrichment increases cell division but suppresses stalk formation (Bothwell et al., 2017), and experimental work confirms that low phosphorus alone does not always produce blooms (West et al., 2020). Where mats form, they restructure invertebrate communities: the mayfly family Heptageniidae declines while chironomid midges proliferate (Brogan et al., 2020), total invertebrate density rises but biomass does not (Larson & Carreiro, 2007), and these shifts cascade to predators such as Rhyacophila caddisflies that switch toward chironomid prey (Ruiz, 2020).
A third theme links nutrients, animals, and disturbance. Insect grazers strongly depress algal biomass in these streams (Taylor et al., 2002), and macroinvertebrates contribute meaningfully to nutrient cycling, with excretion rates scaling with body mass (Cross et al., 2009). Even with a major midsummer flood, biofilm chlorophyll-to-biomass ratios remained the most consistent predictor of nitrogen and phosphorus uptake, and the seasonal trajectory of nutrient uptake was preserved despite an 180% pulse in biofilm biomass (Balik et al., 2021). Climate change is already leaving a fingerprint: long-term records show that Baetis emergence is timed by both peak stream flow and water temperature, with warming water causing earlier emergence in drought years (Harper & Peckarsky, 2006).
Current frontier
Research since 2020 has shifted from documenting predator and nutrient effects toward asking how those effects scale across river networks and respond to climate change. Recent work places small mountain streams within a broader theory of how ecosystem size constrains predator body sizes, food-web structure, and population persistence in dendritic networks (McIntosh et al., 2024). A second emerging direction is the egg-to-adult bottleneck: a synthesis across insects, fish, and amphibians argues that recruitment limitation at the egg stage can persistently shape adult populations, especially when egg-laying habitat is scarce (Downes et al., 2021). New empirical work at RMBL is testing this directly by examining which emergent rocks female insects choose for oviposition (Baker, 2025).
Didymo remains an active frontier. Nine years of survey data show consistent community shifts as didymo cover increases (Brogan et al., 2024), while side-channel experiments suggest temperature, not just nutrient ratios, may set the upper limit on bloom intensity (MacDougall, 2024). Climate-driven changes in flow and temperature are also being linked to energy subsidies between land and water, with terrestrial insect inputs becoming an increasingly important food source for trout as aquatic prey phenology shifts (Cleveland, 2021).
Open questions
Several questions stand out for the next decade. How will the combined effects of warmer water, earlier snowmelt, and lower late-summer flows interact with didymo blooms and the invertebrate communities that depend on them? Will earlier mayfly emergence cause a mismatch between insect availability and the energy demands of trout, birds, and bats? How do nonconsumptive predator effects, oviposition site limitation, and recruitment bottlenecks combine across river networks of different size to determine population persistence? And as native predators such as tiger salamanders increasingly share habitat with introduced trout in beaver-modified landscapes, can long-term monitoring at RMBL distinguish the community signatures of native versus non-native predation? Answering these questions will require continued integration of long-term datasets, whole-stream experiments, and network-scale modeling.
References
Allan, J. D. (1975). The distributional ecology and diversity of benthic insects in Cement Creek, Colorado. Ecology. →
Allan, J. D. (1978). Trout predation and the size composition of stream drift. Limnology and Oceanography. →
Allan, J. D. (1981). Determinants of diet of brook trout (Salvelinus fontinalis) in a mountain stream. Canadian Journal of Fisheries and Aquatic Sciences. →
Allan, J. D. (1982). The effects of reduction in trout density on the invertebrate community of a mountain stream. Ecology. →
Baker (2025). Understanding aquatic insect oviposition to increase aquatic insect recruitment rates. →
Balik et al. (2021). High-discharge disturbance does not alter the seasonal trajectory of nutrient uptake in a montane stream. Hydrobiologia. →
Bothwell & Kilroy (2014). The Didymo story: the role of low dissolved phosphorus in the formation of Didymosphenia geminata blooms. →
Bothwell et al. (2014). The origin of invasive microorganisms matters for science, policy, and management: the case of Didymosphenia geminata. →
Bothwell et al. (2017). Blooms of benthic diatoms in phosphorus-poor streams. →
Brogan (2020). The impact of Didymosphenia geminata on the community structures of invertebrates in streams around the Rocky Mountain Biological Lab. →
Brogan et al. (2024). Consequences of nuisance algal blooms of Didymosphenia geminata on invertebrate communities in Rocky Mountain streams. Freshwater Science. →
Cleveland (2021). Energy fluxes of terrestrial and aquatic invertebrates in the East River. →
Cooper et al. (1990). Prey exchange rates and the impact of predators on prey populations in streams. Ecology. →
Cross et al. (2009). Macroinvertebrate excretion rates and their contribution to nutrient cycling in a Rocky Mountain stream. →
Dodson et al. (1994). Non-visual communication in freshwater benthos: an overview. Journal of the North American Benthological Society. →
Downes et al. (2021). From insects to frogs, egg-juvenile recruitment can have persistent effects on population sizes. Annual Review of Ecology, Evolution, and Systematics. →
Harper & Peckarsky (2006). Emergence cues of a mayfly in a high-altitude stream ecosystem: potential response to climate change. Ecological Applications. →
Koch et al. (2020). Nonconsumptive effects of Brook Trout predators reduce secondary production of mayfly prey. Freshwater Science. →
Larson & Carreiro (2007). Effects of the nuisance diatom Didymosphenia geminata on invertebrates in a Rocky Mountain stream. →
MacDougall (2024). The effects of temperature and N:P ratios on didymo algae growth. →
McIntosh et al. (2004). Predator-induced resource heterogeneity in a stream food web. →
McIntosh et al. (2024). Ecosystem-size relationships of river populations and communities. Trends in Ecology & Evolution. →
McPeek & Peckarsky (1998). Life histories and the strengths of species interactions: combining mortality, growth, and fecundity effects. Ecology. →
Peckarsky (1980). Predator-prey interactions between stoneflies and mayflies: behavioral observations. Ecology. →
Peckarsky & McIntosh (1998). Fitness and community consequences of avoiding multiple predators. Oecologia. →
Peckarsky et al. (1993). Sublethal consequences of stream-dwelling predatory stoneflies on mayfly growth and fecundity. Ecology. →
Peckarsky et al. (2002). Predator chemicals induce changes in mayfly life history traits: a whole-stream manipulation. →
Ruiz (2020). Exploring effects of proliferation of Didymosphenia geminata on abundance and coexistence of Rhyacophila species (Trichoptera). →
Schmitz et al. (2008). From individuals to ecosystem function: toward an integration of evolutionary and ecosystem ecology. Ecology. →
Taylor et al. (2002). Reach-scale manipulations show invertebrate grazers depress algal resources in streams. →
West et al. (2020). Didymosphenia geminata blooms are not exclusively driven by low phosphorus under experimental conditions. Hydrobiologia. →
Concept (4) →
courtship displays
Behavioral displays performed by animals during mating season to attract mates or compete for breeding opportunities
biogeographic variation
Morphological differences in the same species across different geographic regions
population genomic divergence
Genetic differentiation between populations across the genome, measured by allele frequency differences and population genetic statistics like FST
microsatellite DNA analysis
Molecular technique using highly polymorphic DNA markers to assess genetic relationships among individuals
Protocol (2) →
Drosophila courtship song recording via wing vibration (Drosophilidae)
Recording male Drosophila courtship songs by placing flies in Petri dish chambers and capturing wing vibration sounds with directional microphone posi...
Isofemale line method (Drosophilidae)
Wild-caught flies are used to establish laboratory breeding lines maintained under standardized conditions. Lines are kept in multiple vials with cont...