Guo et al. (2019) found, from county-level pest occurrence data for 66 nonnative US forest insects and pathogens combined with 130,210 forest inventory plots, that nonnative pest richness is a hump-shaped function of native tree richness, rising with host-tree richness (facilitation) and falling with nonhost richness and biomass (dilution), with the dilution phase emerging only above roughly 15 nonhost species for generalist pests and 25 for specialists. The authors themselves identified what remains unresolved: the pattern is observational, its causal status unconfirmed, its mediating mechanisms (propagule pressure, host condition, natural enemies, pest-pest interactions) unquantified, and the shape and thresholds untested across regions, spatial scales, and time. The open problem is to establish the causal status, the mediating mechanisms, and the domain of validity of the tree-diversity-pest-invasion relationship.

Background

Nonnative forest insects and pathogens (collectively 'pests'), such as the emerald ash borer and chestnut blight, cause cascading ecological and economic damage in North American and European forests, and how the composition of native tree communities influences their establishment is a central question of invasion ecology. Two opposing effects have been proposed. Facilitation (the 'host diversity begets parasite diversity' hypothesis) holds that a community with more host species offers more niches, so more diverse tree communities accumulate more pest species. Dilution holds that in diverse communities each host species has lower average abundance and host individuals are spatially isolated by nonhost neighbors, so pests struggle to find hosts and establish; the related 'associational resistance' is the protection a host plant gains by growing among nonhost species. A pest's host status is defined relative to each pest species: trees it can feed on or infect are hosts, all others nonhosts. Specialists exploit few host species, generalists many; the distinction matters because facilitation is expected to act more strongly on specialists. Propagule pressure is the rate at which a pest is introduced to a location (in large-scale studies usually proxied by human population density or spatial contagion from already-invaded counties), and it confounds any diversity-invasion relationship because diverse regions may simply receive more introductions. Plant-community studies have long known the related 'invasion paradox' - diversity and invasibility correlate negatively at small scales but positively at large scales (Fridley et al. 2007) - and at the global scale, Liebhold et al. (2018) showed that regional plant diversity positively predicts nonnative insect invasions, i.e. net facilitation. Guo et al. (2019) addressed the intermediate, subcontinental scale for conterminous United States forests. Using United States Department of Agriculture Forest Inventory and Analysis (FIA) plot data (a quasi-systematic national grid of permanent forest plots, about one plot per 2,428 ha, species composition measured on roughly 0.067 ha plots) aggregated to 2,098 counties, and county-level occurrence records of 66 damaging nonnative forest pests (51 insects, 15 pathogens) from the Alien Forest Pest Explorer database, they regressed county pest richness on county native tree richness. The relationship was hump-shaped (unimodal), peaking near 30-35 tree species per county: pest richness rose with tree richness at low diversity (facilitation) and fell at high diversity (dilution). Pest richness increased with host-tree richness and biomass and decreased with nonhost richness and biomass; both specialist and generalist pests increased with host richness (steeper for specialists), and both showed hump-shaped relationships with nonhost richness, with the declining (dilution) phase appearing only above about 15 nonhost species for generalists and about 25 for specialists. Structural equation modeling (a multivariate path framework used to separate direct and indirect effects among correlated predictors) showed that human population density (a propagule-pressure proxy), temperature, and precipitation also influenced pest richness, though tree diversity retained significant effects on pest invasion when these factors were considered jointly. The authors drew their conclusions from observational field patterns, and stated that ultimate confirmation would require experimental manipulation, whose design and implementation across large spatial scales would be challenging. They further noted that establishment is influenced by unmeasured factors (propagule pressure beyond the population-density proxy, host age and condition, natural-enemy communities, interactions among co-occurring pests, stochastic events), that invasion time lags (between arrival, establishment, impact, and recognition) mean observed patterns can shift over time especially under ongoing introductions and climate- and land-use-driven shifts in tree ranges, and that the peak location and shape of the hump could change with climate, resource availability, spatial scale, and habitat fragmentation - so the transferability of the detected thresholds across regions and time was undetermined.

Problem Statement

The problem is to establish the causal status, the mediating mechanisms, and the domain of validity of the relationship between native tree diversity and invasion by nonnative forest insects and pathogens - the concurrent facilitation and dilution reported by Guo et al. (2019) for the conterminous United States, where county pest richness is a hump-shaped function of native tree richness (peak near 30-35 tree species per county), increasing with host-tree richness and biomass (facilitation, steeper for specialist than generalist pests) and decreasing with nonhost richness and biomass (dilution, dominating only above about 15 nonhost species for generalist and about 25 for specialist pests). Three linked aspects frame the open question, because all three concern the same observed pattern and its interpretation. These are independently addressable sub-questions: progress on any one of them counts as partial progress, and all three are required for a full resolution, while the mediators, scales, regions, and time periods listed within each aspect describe the scope to be examined rather than separate mandatory studies. (1) Causal status: determine whether the facilitation and dilution effects are causal rather than artifacts of observational confounding (propagule pressure, climate, spatial contagion, and land-use history). (2) Mechanisms: quantify the degree to which the candidate mediators - propagule pressure independent of human-population-density proxies, host condition (e.g., age and individual host susceptibility), the abiotic environment, natural-enemy communities, the presence of and interactions among co-occurring pests, and stochastic events - mediate or modify the facilitation and dilution effects. (3) Transferability (spatial and temporal): test whether the hump-shaped richness-invasion relationship and its dilution thresholds hold across other regions, spatial scales (plot to subcontinental), and time periods, under ongoing introductions, arrival-establishment-impact time lags, and climate- and land-use-driven shifts in tree species ranges. Together, the three aspects ask when, where, and through which mechanisms native tree diversity regulates - or fails to regulate - nonnative forest pest invasion.

References

  1. Guo Q, Fei S, Potter KM, Liebhold AM, Wen J (2019) Tree diversity regulates forest pest invasion. Proceedings of the National Academy of Sciences 116(15):7382-7386. doi:10.1073/pnas.1821039116, https://doi.org/10.1073/pnas.1821039116
  2. Liebhold AM, Yamanaka T, Roques A, Augustin S, Chown SL, Brockerhoff EG, Pyšek P (2018) Plant diversity drives global patterns of insect invasions. Scientific Reports 8:12095. doi:10.1038/s41598-018-30605-4, https://doi.org/10.1038/s41598-018-30605-4
  3. Fridley JD, Stachowicz JJ, Naeem S, Sax DF, Seabloom EW, Smith MD, Stohlgren TJ, Tilman D, Von Holle B (2007) The invasion paradox: reconciling pattern and process in species invasions. Ecology 88(1):3-17. doi:10.1890/0012-9658(2007)88[3:TIPRPA]2.0.CO;2, https://doi.org/10.1890/0012-9658(2007)88[3:TIPRPA]2.0.CO;2
  4. Staab M, Schuldt A (2020) The influence of tree diversity on natural enemies - a review of the 'enemies' hypothesis in forests. Current Forestry Reports 6(4):243-259. doi:10.1007/s40725-020-00123-6, https://doi.org/10.1007/s40725-020-00123-6
  5. Ward SF, Liebhold AM, Fei S (2022) Variable effects of forest diversity on invasions by non-native insects and pathogens. Biodiversity and Conservation 31(11):2575-2586. doi:10.1007/s10531-022-02443-4, https://doi.org/10.1007/s10531-022-02443-4
  6. Gougherty AV, Davies TJ (2024) Evolutionary history of host trees amplifies the dilution effect of biodiversity on forest pests. PLOS Biology 22(2):e3002473. doi:10.1371/journal.pbio.3002473, https://doi.org/10.1371/journal.pbio.3002473
  7. Wang P, Wu D, Wang Y, Shen Z, Li B, Shu Z, Chu C (2024) Tree species richness suppresses red imported fire ant invasion in a subtropical plantation forest. Journal of Applied Ecology 61(11):2751-2761. doi:10.1111/1365-2664.14786, https://doi.org/10.1111/1365-2664.14786
  8. Guo Q, Qian H, Zhang J (2023) Does regional species diversity resist biotic invasions? Plant Diversity 45(3):353-357. doi:10.1016/j.pld.2022.09.004, https://doi.org/10.1016/j.pld.2022.09.004
  9. Sánchez-Dávila J, Galiana N (2025) From local to regional scales: ecological drivers of forest insect pest diversity across Europe. Journal of Environmental Management 396:128164. doi:10.1016/j.jenvman.2025.128164, https://doi.org/10.1016/j.jenvman.2025.128164
  10. Trombik J, Šenfeldová S, Ward SF, Atkinson TH, Liebhold AM (2026) Does biotic resistance govern forest invasions by bark and ambrosia beetles? Ecography 2026(3):e08164. doi:10.1002/ecog.08164, https://doi.org/10.1002/ecog.08164
  11. Clipp HL, Potter KM, Peters MP, Prasad AM, Matthews SN, Gougherty AV (2026) Warming temperatures and shifting precipitation patterns may exacerbate pest damage in North American forests. Nature Ecology & Evolution 10(5):893-906. doi:10.1038/s41559-026-03039-9, https://doi.org/10.1038/s41559-026-03039-9