Flipping through field notebooks…
Explores how snowmelt timing, plant phenology, and species interactions — including ant-aphid mutualisms, predator manipulation, and nitrogen cycling — shape alpine plant and invertebrate communities in high-elevation ecosystems.
In the meadows surrounding the Rocky Mountain Biological Laboratory, some of the most consequential ecological dramas unfold at the scale of millimeters: aphids and treehoppers pierce plant stems to drink sap, ants patrol those same stems collecting sugar-rich honeydew the sap-feeders excrete, and predators like ladybeetles, lygus bugs, and parasitoid wasps try to slip past the ants to attack the herbivores. These ant-hemipteran mutualistic relationships, where insects exchange food for protection, are textbook examples of how cooperation evolves and persists, and they ripple outward through subalpine food webs to shape plant reproduction, pollinator behavior, and even what black bears eat.
Understanding these systems requires a few key ideas. Ant tending is the behavior of ants stroking aphids or treehoppers to harvest honeydew, often defending them in the process. Whether tending is truly beneficial depends on the cost-benefit ratio: ants can protect partners from predators, but they can also eat them, so the same interaction can be mutualistic in one context and antagonistic in another. Top-down control refers to the effects of predators and ants on herbivore populations from above, while bottom-up forcing refers to how plant quality, water status, and nutrients influence herbivores from below. Many of these effects are mediated by host plant phenology (the seasonal timing of flowering and fruiting) and by chemical cues that ants learn to associate with food rewards through associative learning.
For the Gunnison Basin, these interactions matter because they sit at the intersection of climate change and community ecology. Snowmelt timing, summer temperature, and drought all alter when plants flower, how stressed they are, and which insects are active when. Because higher trophic levels (predators, ants) often show greater trophic-level sensitivity to environmental change than the herbivores or plants below them, small shifts in climate can reorganize whole food webs. Long-term research at RMBL on fireweed, osha (Ligusticum porteri), aspen sunflower (Helianthella quinquenervis), and Valeriana edulis has made this region one of the world's best places to watch those reorganizations happen.
The area was opened up by a series of studies on fireweed aphids and their ant tenders in the central Colorado Rockies. Addicott showed that multiple aphid species coexist on a single host plant by differing in colonization timing, feeding position, and ant relationships (Addicott, 1978), and that these aphids compete for the limited services of ant mutualists . A landmark multispecies survey demonstrated that ant effects on aphids are density dependent and species specific: some ant species rescue small aphid colonies from decline, while others have neutral or even negative effects . Cushman and Addicott extended this by showing that ants themselves are a limiting resource for which aphids compete both within and between species . Earlier vegetation mapping in the Crested Butte area provided the environmental template against which these biotic interactions could be interpreted .
The study of how animals search for and utilize food resources in their environment
The species assemblage and relative abundances of aquatic invertebrates in a habitat, used as indicators of ecosystem condition and habitat quality
Positive species interactions such as nurse plant effects by cushion-forming species that increase reproductive success for neighboring plants
Mutually beneficial herbivore-predator associations where sap-feeding insects provide honeydew food for ants in exchange for protection against predat...
The network of feeding relationships among organisms in an ecosystem
Temporal alignment between consumer energy requirements and food source availability across trophic levels
Initial colonization phase when winged aphids establish new colonies on host plants
Controlled experiment manipulating predator presence by establishing artificial aphid colonies, protecting them from natural predators, then adding ly...
Standardized 2-hour baiting experiments using 5 different resource types arranged in pentagonal plots to sample ant communities and assess dominance p...
A controlled field experiment testing the effects of nitrogen addition and reduction on plant community composition over 8 years. Annual percent cover...
Experimental manipulation using physical barriers to exclude ants from aphid colonies while maintaining control colonies with ant access. Uses paired ...
A two-phase field protocol using training baits scented with plant chemicals followed by two-choice tests to assess whether ants form associative memo...
Plant cover measured using 1 m² quadrats divided into 1 dm² cells, counting filled cells to nearest 0.25 dm². Height measured to maximum plant extensi...
1. The abiotic environment drives species abundances and distributions both directly and indirectly through effects on multi-trophic species interacti...
Although species interactions are often proposed to be stronger at lower latitudes and elevations, few studies have evaluated the mechanisms driving s...
The purpose of this study was to track year-to-year variation in aphid abundance on the host plant Ligusticum porteri (Apiaceae). We censused arthropo...
Changing phenological cues can lead to trophic mismatch for plants and herbivores, and this often shifts herbivore feeding to plant stages of lower ...
Human activities have substantially increased atmospheric nitrogen (N) deposition in ecosystems worldwide, often leading to higher plant quality for h...
The purpose of this study was to track year-to-year variation in aphid abundance on the host plant Ligusticum porteri (Apiaceae). We censused arthropo...
These early studies established three durable themes: ant-hemipteran outcomes are conditional, ant identity matters, and competition among partners is a real force in mutualisms. Work on extrafloral nectaries of aspen sunflower added a parallel system, showing that ants foraging on plant-provided nectar reduce seed damage (Inouye & Taylor, 1989), and that lycaenid butterfly caterpillars likewise depend on partner identity, with only Formica podzolica clearly reducing parasitism (Fraser et al., 2001).
A central message from the RMBL community is that ants are simultaneously friends and enemies. Experiments crossing ant access with other predators showed that ants can sharply reduce aphid numbers themselves while also displacing other predators, so the net effect is rarely additive (Billick et al., 2007). Ant nutritional state tips this balance: colonies fed protein tend aphids and visit flowers, while carbohydrate-satiated colonies become inactive and stop collecting honeydew (Petry et al., 2012). Whether a partnership functions as mutualism or predation also depends on temporal variability across years more than spatial variability across sites (Billick & Tonkel, 2003), and on aphid colony density, with intermediate-sized colonies gaining the most from ant attendance.
Plant traits structure these interactions from below. On dioecious Valeriana edulis, female plants support four times more aphids and ants than males, driven both by greater floral nectar attractiveness and by indirect effects through aphid abundance (Petry et al., 2013). Mound-building ants create "islands of fertility" with 600-800% higher soil nutrient availability around active colonies, linking insect mutualisms to ecosystem processes (Grinath et al., 2011). Ants also learn: in field assays, Formica podzolica and Tapinoma sessile both formed associative links between plant odors and sugar rewards, raising bait occupancy from 42% to 66% (Nelson et al., 2020), and a broader review documents how plant chemicals steer ant behavior across pollination, seed dispersal, and herbivore protection (Nelson et al., 2019).
Climate context is woven through all of this. Snowmelt date is the single best predictor of year-to-year aphid abundance, with earlier melt producing water-stressed host plants and smaller aphid colonies (Robinson et al., 2017) (Mooney et al., 2020). Along elevation gradients, ant-aphid mutualisms strengthen at lower, warmer, drier sites where natural enemies are more abundant and ants forage more intensely, increasing aphid survival by 66% at low elevations but having no detectable benefit at high elevations (Nelson et al., 2019). A broader review places these findings in evolutionary context, noting that ant-hemipteran mutualisms evolved independently in multiple lineages and now structure communities worldwide (Nelson & Mooney, 2022).
Early work in the late 1970s and 1980s focused on documenting conditionality and competition in ant-aphid systems. Studies in the 2000s and 2010s expanded to plant sex, nutrition, chemical communication, and elevation gradients. Since 2020, research has shifted decisively toward climate and global change drivers and toward extending the food web upward and outward. Recent experiments show that host plant phenology shapes both aphid establishment and ant recruitment, with colonies twice as likely to establish on flowering versus post-flowering plants and ant recruitment 116% higher on flowering stalks (Mooney et al., 2023). Advanced phenology of intraguild predators like lygus bugs can suppress aphid colonization before mutualisms even form (Mullins et al., 2020). Long-term nitrogen addition experiments reveal that even chronic low-level nitrogen deposition boosts ant-tended herbivores on sagebrush (Grinath, 2021) and can dampen trophic cascades between black bears and plants.
New directions push the system in fresh ways. Researchers are asking how black bears digging up Formica obscuripes nests for protein link insect mutualisms to vertebrate nutrition, with ant nests offering greater fat and pupal mass in June than in July (Hunter, 2023). Others are revisiting what "dominance" means in ant communities, finding that behavioral, numerical, and ecological dominance are not interchangeable and only behavioral dominance trades off with discovery ability (Nelson & Mooney, 2025). Parasitoid wasps, deer browse, and cattle grazing are being folded into multitrophic models of these meadows (Rittler, 2024) (Wright, 2024).
Major uncertainties remain. How will continued advances in snowmelt and summer warming reshape the match between aphid, predator, and ant phenologies over decades, and will mutualisms persist or break down as phenological mismatches accumulate? What roles do honeydew chemistry, aphid endosymbionts, and ant associative learning play in determining which partnerships strengthen under stress? How do nitrogen deposition, large herbivore grazing, and bear foraging interact to reroute energy through these small-bodied networks? And can the strong elevational patterns documented in the Gunnison Basin be used to forecast lowland communities' futures? Answering these questions will require continuing RMBL's hallmark combination of long-term monitoring, factorial field experiments, and chemical and isotopic tools applied across the full multitrophic community.
Addicott, J. (1978). Competition for mutualists: aphids and ants. Canadian Journal of Zoology. →
Addicott, J. (1978). Niche relationships among species of aphids feeding on fireweed. Canadian Journal of Zoology. →
Addicott, J. (1979). A multispecies aphid-ant association: density dependence and species-specific effects. Canadian Journal of Zoology. →
Billick, I., Hammer, S., Reithel, J. S., Abbot, P. (2007). Ant-aphid interactions: are ants friends, enemies, or both? Annals of the Entomological Society of America. →
Billick, I., Tonkel, K. (2003). The relative importance of spatial vs. temporal variability in generating a conditional mutualism. Ecology. →
Cushman, J. H., Addicott, J. (1989). Intra- and interspecific competition for mutualists: ants as a limited and limiting resource for aphids. Oecologia. →
Fraser, A. M., Axen, A. H., Pierce, N. E. (2001). Assessing the quality of different ant species as partners of a myrmecophilous butterfly. Oecologia. →
Grinath, J. B. (2021). Chronic, low-level nitrogen deposition enhances abundances of ant-protected herbivores inhabiting an imperiled foundation species. Acta Oecologica. →
Grinath, J. B., Inouye, B. D., Underwood, N., Billick, I. (2011). Impact of mound-building ants on ecosystem properties create islands of fertility in alpine meadows. →
Hunter, A. (2023). Breakfast of champions: Spatiotemporal variation in the quality of ant nests for bear consumers. →
Inouye, D. W., Taylor, O. R. (1989). Seed protection by ants foraging on the extrafloral nectaries of the aspen sunflower, Helianthella quinquenervis. →
Langenheim, J. H. (1962). Vegetation and environmental patterns in the Crested Butte area, Gunnison County, Colorado. Ecological Monographs. →
Mooney, K. A., Nelson, A. S., Mullins, A. (2023). Host plant phenology shapes aphid abundance and interactions with ants. Oikos. →
Mooney, K. A., Pratt, R., Mullins, A. (2020). Early snowmelt reduces aphid abundance Aphis asclepiadis by creating water stressed host plants Ligusticum porteri and altering interactions with ants. Arthropod-Plant Interactions. →
Mullins, A., Nelson, A. S., Mooney, K. A. (2020). Advanced phenology of intraguild predators shifts herbivore host plant preference and performance. Ecological Entomology. →
Nelson, A. S., Carvajal Acosta, N., Mooney, K. A. (2019). Plant chemical mediation of ant behavior. Current Opinion in Insect Science. →
Nelson, A. S., Mooney, K. A. (2022). The Evolution and Ecology of Interactions Between Ants and Honeydew-Producing Hemipteran Insects. Annual Review of Ecology, Evolution, and Systematics. →
Nelson, A. S., Mooney, K. A. (2025). Different aspects of dominance are not equivalent when testing for trade-offs in ant communities. Ecology and Evolution. →
Nelson, A. S., Pratt, R. T., Pratt, J. D., Smith, R. A., Symanski, C. T., Prenot, C., Mooney, K. A. (2019). Progressive sensitivity of trophic levels to warming underlies an elevational gradient in ant-aphid mutualism strength. Oikos. →
Nelson, A. S., Symanski, C. T., Hecking, M., Mooney, K. A. (2020). Are ants botanists? Ant associative learning of plant chemicals mediates foraging for carbohydrates. Ecological Entomology. →
Petry, W. K., Perry, K. I., Fremgen, A., Rudeen, S. K., Lopez, M., Dryburgh, J., Mooney, K. A. (2013). Mechanisms underlying plant sexual dimorphism in multi-trophic arthropod communities. Ecology. →
Petry, W. K., Perry, K. I., Mooney, K. A. (2012). Influence of macronutrient imbalance on native ant foraging and interspecific interactions in the field. Ecological Entomology. →
Rittler, A. (2024). Associations Between Deer Browse and Aphid Colonization in a Long-Term Monitoring Study of Liguisticum porteri. →
Robinson, E. A., Nelson, A. S., Pratt, R., Mooney, K. A. (2017). Multitrophic interactions mediate the effects of climate change on herbivore abundance. Oecologia. →
Wright, A. (2024). Variation of wasp behavior patterns and pollination behavior on Ligusticum porteri. →
The formation of predictive relationships between contingent stimuli in the environment
Photosynthetically active radiation levels measured as percentage of full-sun radiation
Transmission of endosymbionts from mother to clonal offspring through the maternal lineage.
Nectar-secreting organs located on leaf laminae, petioles, rachis, bracts, stipules, pedicels, or fruit that attract arthropods
Introduction of natural enemies to reduce pest population density through environmentally friendly means
Distribution of plant growth and resources between aboveground (shoots) and belowground (roots) structures
Chemical signals that organisms use to obtain information about their environment
Experimental design testing multiple factors simultaneously in all combinations
Method for quantifying the proportion of overall variance explained by different factors or pathways using R² calculations
How male and female plants of dioecious species differ in their interactions with insects
The sharing of liquid food resources between nestmates through specialized anatomical and behavioral adaptations
The hypothesis that species that are competitively dominant have reduced ability to discover new resources quickly
Having the greatest foraging success relative to abundance in the environment
The excretion of sugar-rich liquid waste by herbivorous hemipterans feeding on plant sap
The balance between costs and benefits that determines whether species interactions are mutualistic or antagonistic
Aphid colony growth rate calculated as ln(n1)-ln(n0)/t where n1 and n0 are final and initial colony sizes
The abundance or availability of resources in a given area
Experimental design where subjects choose between two alternative options to assess preferences
Experimental technique to prevent predator access while maintaining other ecological interactions
Non-living environmental factors such as climate, light, and temperature that influence organism performance
Behavioral patterns of wasps that parasitize other arthropods, particularly aphids
Chemical compounds with therapeutic or biological activity
Sampling method using buried containers with killing solution to capture ground-dwelling invertebrates
The accumulation of substances like sodium in organisms through their food chain, allowing carnivores to obtain adequate salt from prey
Field-based study of ant interactions with pollinators on Helianthella quinquenervis, involving ant identification and behavioral observations. Protoc...
Exhaustive visual surveys of arthropod abundance on randomly selected male and female plants conducted across multiple years to quantify sex-biased co...
Flowers are bagged to prevent natural parasitism, then exposed to controlled introductions of parasitoid wasps or ambient parasitoid activity, followe...
Calculation and statistical analysis of plant biomass allocation between vegetative and reproductive tissues using arcsin transformations and three-wa...
Statistical analysis using subset of main data to assess repeatability of measurements.
PERMANOVA testing of volatile composition differences across species and years, with NMDS visualization of chemical dissimilarity patterns.
Three dominance scores calculated for each species at each site where it occurred to assess competitive hierarchies.
The purpose of this study was to track year-to-year variation in aphid abundance on the host plant Ligusticum porteri (Apiaceae). We censused arthropo...
The purpose of this study was to track year-to-year variation in aphid abundance on the host plant Ligusticum porteri (Apiaceae). We censused arthropo...
The objective of this study is to understand how climate cues affect the abundance and phenology of aphids and the arthropods with which they interact...