Sagebrush Ecosystems: Structure, Arthropods, and Spatial Ecology

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Research Primer

Background

Sagebrush ecosystems blanket vast stretches of western North America, including the steppe landscapes that border the East River valley around Gothic, Colorado. At the heart of these systems is big sagebrush (Artemisia tridentata), a foundation species — meaning a plant whose physical presence and abundance literally builds the habitat that countless other organisms depend on. When sagebrush thrives, it provides shelter, food, and microclimate refuge for an entire community of arthropods, birds, and mammals. When sagebrush is stressed, the consequences ripple outward through the food web. Understanding how sagebrush systems are structured, who lives on them, and how those residents interact with one another is therefore central to managing the Gunnison Basin's open lands.

Much of the research in this area zooms in on the small-bodied residents of sagebrush and rabbitbrush — aphids, treehoppers (membracids), and the thatching ants that tend them. These insects participate in mutualisms, partnerships in which both species benefit: sap-feeding insects produce sugary honeydew that ants drink, while ants in turn protect the sap-feeders from predators and parasitoid wasps (specialist wasps that lay eggs inside other insects, eventually killing the host). These small interactions matter because they connect upward to larger animals, including black bears, which dig up ant nests as a seasonal food source, and downward to plants, whose seed production can rise or fall depending on which insects are feeding on them.

To measure these systems, researchers combine careful field sampling — visual counts of arthropods on individual shrubs, 1-square-meter quadrats to estimate plant cover — with newer remote-sensing tools such as LiDAR, a laser-based technology that produces detailed three-dimensional maps of vegetation structure. Together, these approaches allow ecologists to ask how nitrogen pollution, drought, grazing, and predator activity reshape the sagebrush community across both fine and broad spatial scales.

Foundational work

Early RMBL research established that ant-tended insects are central players in sagebrush food webs and that their interactions with ants are surprisingly nuanced. Reithel and Billick (Reithel & Billick, 2006) showed that thatching ants increased treehopper nymph abundance on rabbitbrush but not on Wyethia, because the latter plant senesced too early in the summer for the mutualism to pay off. Follow-up work demonstrated that ant exclusion before the fourth treehopper instar dramatically reduced nymph size and survivorship, pinpointing when in development the ant partnership matters most (Erickson, 2006), and that aggregation size and host plant identity jointly shaped survival in ways that helped explain why generalist host use evolves (Reithel & Campbell, 2008).

A parallel line of work revealed that ants are not simply protectors. Billick et al. (Billick et al., 2007) experimentally showed that thatching ants reduced aphid numbers by roughly 42 percent — acting simultaneously as predators and as protectors against other enemies. Abbot et al. (Abbot et al., 2008) added a nutritional dimension, finding that host plant identity, not ant tending, drove herbivore nitrogen content. Together, these landmark studies framed sagebrush arthropod communities as bottom-up systems in which plant quality sets the stage, while ants modulate the cast of characters.

Key findings

A recurring theme is that ants behave as friends, enemies, or both, depending on context. Ant colonies fed protein-rich diets tended aphids far more often than colonies fed sugar, suggesting ants switch between mutualism and predation based on their own nutritional state (Petry & Mooney, 2009). Aphid colonies grew fastest at intermediate densities when ants were present (Roos, 2013), and treehoppers that switched host plants suffered higher mortality, consistent with the idea that ants recognize their partners through chemical cues that match the host plant (Sanchez, 2013). Close proximity to thatching ant nests was associated with greater aphid and herbivore abundance and, through complex indirect pathways, increased seed production in lupine (Bayer, 2010).

These small-scale interactions scale up into trophic cascades that reach black bears and the plants themselves. Grinath (Grinath, 2018) showed that in ambient conditions, bears digging up thatching ant nests indirectly increased plant reproduction by removing the ants that protected harmful herbivores. Crucially, low-level nitrogen deposition — the chronic, diffuse nitrogen pollution drifting in from distant cities and agriculture — disrupted this cascade: under nitrogen enrichment, bear-induced ant nest inactivity no longer benefited plants. A longer-term follow-up confirmed that nine years of low-level nitrogen addition increased total arthropod abundance on sagebrush, driven specifically by ant-tended sap-feeders, even though sagebrush cover itself did not change (Grinath, 2021).

Methodological work has supported these ecological findings. Grinath (Grinath, 2019) demonstrated that simple linear regression using plant cover or volume — not height — produced the most accurate biomass estimates in sagebrush–rabbitbrush communities, giving field crews a practical tool. Wilson et al. (Wilson et al., 2018) also showed that structured undergraduate research programs at sites like RMBL substantially increase students' likelihood of pursuing PhDs and producing scientific output, anchoring the human infrastructure that sustains long-term sagebrush research.

Current frontier

Research from 2020 onward has shifted toward longer time horizons and broader trophic context. Grinath's (Grinath, 2021) nine-year nitrogen experiment marked a move from short-term manipulations to chronic exposure studies, the timescale most relevant to real-world atmospheric pollution. Hunter (Hunter, 2023) extended the bear–ant link by quantifying how the nutritional value of ant nests changes seasonally, with June nests offering significantly more fat and pupal mass than July nests because of the production of large-bodied sexual ants. Most recently, Joseph (Joseph, 2024) tested whether the bear-to-plant cascade extends to mule's ears sunflowers and found no clear cascade, but did detect that nitrogen addition weakened the treehopper–ant mutualism and that cattle preferentially grazed sunflowers in nitrogen-enriched plots — an unexpected interaction between pollution and large-herbivore behavior.

The trajectory of this work is toward integrating multiple stressors — nitrogen deposition, drought, grazing, and predator loss — within spatially explicit frameworks. Billick et al. (Billick et al., 2018) articulated this "ecology of place" agenda, arguing that ecological and evolutionary processes should be mapped onto specific landscapes using tools like LiDAR-derived vegetation structure. Methodological advances in spatial abundance modeling (Conlisk et al., 2012) and reactive transport modeling (Maher & Mayer, 2019) hint at the kinds of cross-disciplinary tools likely to inform the next decade of sagebrush ecology.

Open questions

Several important questions remain. How will sagebrush arthropod communities respond when chronic nitrogen deposition intersects with intensifying drought, and at what point do these stressors push the system past a tipping point? Why do some bear-mediated cascades propagate to plants while others, like the sunflower system, do not — is it a matter of plant identity, herbivore life history, or pollution context? How do cattle grazing preferences interact with nitrogen pollution to reshape plant communities over decades? And can spatially explicit tools such as LiDAR be paired with long-term arthropod surveys to predict where in the Gunnison Basin sagebrush systems are most vulnerable to combined stressors? Answering these will require sustained, place-based research of the kind RMBL is uniquely positioned to support.

References

Petry & Mooney (2009). A balanced diet: Effects of ant nutritional state on the balance between mutualism and predation upon aphids. →

Abbot, P. et al. (2008). Insect herbivore stoichiometry: the relative importance of host plants and ant mutualists. Ecological Entomology. →

Roos (2013). Benefits of ant attendance for aphid colonies of varying density. →

Billick, I. et al. (2018). Rocky Mountain Biological Lab. Ecology of Place. Mountain Views. →

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. →

Sanchez (2013). Chemical camouflage and the consequences of changing host plants in a treehopper-ant mutualism. →

Conlisk, E. et al. (2012). The shape of a species' spatial abundance distribution. Global Ecology and Biogeography. →

Grinath, J.B. (2018). Short-term, low-level nitrogen deposition dampens a trophic cascade between bears and plants. Ecology and Evolution. →

Grinath, J.B. (2019). Comparing predictive measures and model functions for estimating plant biomass. Plant Ecology. →

Grinath, J.B. (2021). Chronic, low-level nitrogen deposition enhances abundances of ant-protected herbivores inhabiting an imperiled foundation species. Acta Oecologica. →

Hunter (2023). Breakfast of champions: Spatiotemporal variation in the quality of ant nests for bear consumers. →

Joseph (2024). Indirect effect of black bears on sunflowers in nitrogen-polluted and pristine steppe. →

Maher, K., Mayer, K.U. (2019). Tracking diverse minerals, hungry organisms, and dangerous contaminants using reactive transport models. Elements. →

Reithel, J.S., Billick, I. (2006). Bottom-up mediation of an ant-membracid mutualism: effects from different host plants. Evolutionary Ecology. →

Reithel, J.S., Campbell, D.R. (2008). Effects of aggregation size and host plant on the survival of an ant-tended Membracid. Annals of the Entomological Society of America. →

Crain (2005). The Effect of Ant Colony Proximity on Membracid Survivorship. →

Erickson (2006). The timing of the ant-effect on nymph size and survivorship in an ant-treehopper mutualism. →

Wadgymar, S.M. et al. (2017). Identifying targets and agents of selection. Methods in Ecology and Evolution. →

Bayer (2010). What's for lunch: deciphering ant omnivory on lupine. →

Wilson, A.E. et al. (2018). Assessing science training programs: Structured undergraduate research programs make a difference. BioScience. →

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