New research from the Integral Ecology Research Center (IERC) offers
\nsome of the clearest evidence yet that California\u2019s efforts to restrict dangerous rodenticides are paying off in the
\nwild, while also flagging a new contaminant that deserves close attention going forward.<\/p>\n
\n
<\/p>\n
The study, published in the journal Environmental Pollution, analyzed liver tissue from 1,003 Barred Owls collected
\nacross northwestern California between 2019 and 2025, the largest rodenticide biomonitoring dataset ever
\nassembled for a single wildlife species in North America. Because the owls were removed as part of an ongoing
\nregional conservation program rather than recovered as sick animals or dead carcasses, the sample is a far more
\nrepresentative picture of the population as a whole.<\/p>\n
\n
<\/p>\n
The findings are a mix of a genuine policy win and a new warning.<\/p>\n
\n
<\/p>\n
The policy win: Among young owls sampled in the study area, exposure to the most lethal class of rodenticides,
\nknown as second-generation anticoagulants, fell from about 50% of birds in 2019 to about 20% in 2024. That
\ndecline lines up closely with California\u2019s phased statewide restrictions, most notably the California Ecosystems
\nProtection Act (AB 1788), passed in 2020, along with a 2024 law extending restrictions to older-generation
\ncompounds (AB 2552). It also reflects a feedback loop central to IERC\u2019s work: supplying the science that informs
\npolicy decisions, and then documenting the real-world benefit that policy delivers within just a few years.<\/p>\n
\n
<\/p>\n
The new concern: At the same time, the study screened for cholecalciferol (vitamin D3), a different type of
\nrodenticide, for the first time in this species. It turned up in nearly one in five owls (18.5%), and detections rose over
\nthe same period that anticoagulant exposure was falling, a pattern suggesting product substitution as older
\ncompounds become harder to access. Overall, when all rodenticides were counted, as much as 46% of the owls
\nsampled carried at least one rodenticide compound in their system.<\/p>\n
\n
<\/p>\n
The Spotted Owl connection: Female Barred Owls and older adults carried the highest exposure and the widest
\nmix of compounds, consistent with pesticides accumulating over a lifetime. These are exactly the individuals,
\nbreeding-age females and established territory holders, that a population depends on most to sustain itself. Because
\nNorthern Spotted Owls are a closely related species with a higher proportion of rodents in their diets, this pattern
\nmay indicate added risk for Spotted Owl recovery.<\/p>\n
\n
<\/p>\n
\u201cNearly all rodenticide surveys rely on animals that turned up sick or dead, so they tend to capture the worst cases,\u201d
\nsaid Dr. Vitek Jirinec, lead author of the study and research scientist at IERC. \u201cOur owls were apparently healthy
\nbirds going about their lives. That gives us a much truer picture of what\u2019s circulating in the population, and more
\nconfidence that the trends we see are real.\u201d<\/p>\n
\n
<\/p>\n
Dr. Greta M. Wengert, co-founder and co-director of IERC and principal investigator on the study, has tracked
\nrodenticide contamination across the region for more than a decade. She said the results demonstrate something rare
\nin conservation science, a policy translating directly into a measurable outcome for wildlife:<\/p>\n
\n
<\/p>\n
\u201cWe\u2019ve watched illegal use and unregulated products drive rodenticide exposure in these forests for more than a
\ndecade,\u201d said Wengert. \u201cSeeing anticoagulant exposure actually decline within a few years of California\u2019s regulations taking effect is proof that policy, when it\u2019s grounded in good data from effective biomonitoring efforts,
\ncan improve outcomes for wildlife.\u201d<\/p>\n
\n
<\/p>\n
The study also reinforces and extends findings from earlier IERC research. A 2012 IERC study first documented
\nwidespread anticoagulant exposure in Pacific fishers linked to trespass cannabis cultivation on public lands, and a
\n2018 IERC study found rodenticides in 70% of sampled Northern Spotted Owls in the same region. Barred Owls,
\nSpotted Owls, fishers, and other sensitive wildlife all feed from the same contaminated food web, so this much
\nlarger dataset adds substantial weight to that earlier evidence. Across more than a decade of IERC research, the
\nsame underlying threat has persisted in multiple old-growth-dependent species, and it is now, for the first time,
\nresponding to policy.<\/p>\n
\n
<\/p>\n
The authors caution that the exact source of ongoing exposure is not fully resolved. State pesticide-use records only
\ncapture licensed commercial applicators, not household purchases or illegal use. Trespass cannabis cultivation, long
\ndocumented by IERC as a major source of rodenticide contamination on public and tribal lands, remains untracked
\nby any regulatory database, and legacy contamination from past illicit grow sites may also continue cycling through
\nthese forests well after active cultivation has stopped. Researchers say these hidden and historical sources likely
\nexplain at least part of the exposure that remains even as legal, regulated use declines.<\/p>\n
\n
<\/p>\n
About the Integral Ecology Research Center<\/p>\n
\n
<\/p>\n
Founded in Blue Lake, California, in 2004, IERC is a woman- and minority-founded nonprofit conservation science
Publication: Jirinec, V., Gabriel, M.W., Peery, M.Z., Higley, J.M., Hofstadter, D.F., Varian, C.P., Rex, A.,
\norganization dedicated to conserving wildlife and the ecosystems they rely on across northwestern California and
\nbeyond. IERC partners with tribal, state, and federal agencies to conduct field research, from GPS telemetry and
\nforensic wildlife science to toxicology and habitat monitoring, that translates directly into on-the-ground
\nconservation action for species. Learn more at iercecology.org<\/a>.<\/p>\n
\n
<\/p>\n
\nFranklin, A.B., Poppenga, R.H., Woods, L.W., Volker, S.F., Wengert, G.M. 2026. Forest food web contamination
\nby rodenticides is pervasive but responsive to regulation: Insights from Barred Owl biomonitoring.
\nEnvironmental Pollution, https:\/\/doi.org\/10.1016\/j.envpol.2026.129205<\/a><\/p>\n","protected":false},"excerpt":{"rendered":"New research from the Integral Ecology Research Center (IERC) offers
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