The scale of the problem becomes even more striking when compared with the size of the accessioned collections themselves. The 65 herbaria in the survey reported a combined 46,572,505 accessioned specimens alongside 1,974,206 unaccessioned ones, meaning the backlog represents about 4.1 percent of the total material held. If the estimated 104 million backlogged specimens were fully processed and catalogued, the total number of herbarium specimens available worldwide would increase by roughly 27 percent. Backlogged specimens can be found stacked on top of cabinets, in corners, warehouses and boxes, and the plants themselves may sit in field packets, newspaper wrappings, archival packets and even random household containers such as cheese cans. Because these specimens are not yet catalogued, scientists know little about what they contain, even though they may hold valuable scientific information, potentially including now extinct plant species from global biodiversity hotspots.

Not all taxonomic groups are equally trapped in the bottleneck. Bryophytes, the mosses, liverworts and hornworts, represent only 9.0 percent of the accessioned specimens reported in the survey, yet they make up 16.0 percent of the backlog. Fungi and lichens show a similar pattern, comprising 10 percent of accessioned material but 13 percent of unaccessioned specimens. This over-representation has technical roots. Unlike vascular plants, bryophytes and lichens often require microscopic examination and dissection for accurate identification, which extends processing time per specimen even when a specialist is working efficiently. Multiplying that longer identification time across large numbers of specimens creates a compounding challenge that vascular plant collections do not face to the same degree.

The state of the bryophyte backlog itself reveals how far these specimens sit from scientific usability. According to estimates from 43 respondents, 69.0 percent of backlogged bryophyte specimens are stored in field packets without labels, despite the folded archival packet with a complete label being the accepted gold standard for bryophyte curation. Field packets, paper bags, boxes and recycled envelopes are intended as temporary storage, but they frequently become long-term storage when specimens are never moved into the accessioned collection. The identification picture is equally sobering: among 35 respondents who estimated taxonomic levels, roughly three-quarters of unaccessioned bryophytes lack species-level identifications, with about a quarter identified only to division, meaning they are known simply as a moss, a liverwort or a hornwort. Such specimens cannot be filed with the accessioned collections of the same taxon, and finding an expert willing to work on them becomes harder the coarser the identification.

Why do these backlogs persist? The survey team asked 63 staff members to explain why their institutions have a bryophyte backlog and used thematic analysis to distil 22 recurring themes, organised across four scales: institutional, backlog, specimen and individual. At the institutional level, labour constraints dominated, cited by 52.4 percent of respondents, who described deficits of people, staff, curators, faculty, collections managers, volunteers and students. Lack of expertise followed at 31.7 percent, with several respondents noting that their last bryophyte or cryptogamic curator retired and was never replaced. One respondent reported that their institution’s last bryology expert retired in the 1990s and had not been replaced since. Resource constraints, mentioned by 30.2 percent, and competing institutional priorities, cited by 22.2 percent, rounded out the institutional picture, with several staff describing how digitisation of already-accessioned specimens takes precedence over integrating new material.

At the finer scales, the reasons become more granular. Backlog-level factors included specimen source, with 22.2 percent of respondents attributing their backlogs to research projects, surveys, expeditions, gifts and bequests, and backlog volume, mentioned by 19.0 percent, who described constantly playing catch-up when there are simply many more new specimens than processing capacity. Specimen-level factors included incomplete identification, cited by 22.2 percent, and missing labels, at 9.5 percent. Individual-level factors revealed a human dimension: 14.3 percent of respondents described collectors who liked to collect but did not like to curate, staff with teaching duties, and people lacking particular interest in bryophytes, making new accessions a low priority. Individual time constraints appeared at 11.1 percent, with day-to-day responsibilities requiring a huge amount of time and backlog work relegated to increasingly scarce free time.

The regression analyses added quantitative weight to these qualitative accounts. For the overall backlog, the total number of accessioned specimens and the number of full-time staff were significant predictors, with herbaria holding larger accessioned collections and employing more full-time staff having larger backlogs. For the bryophyte backlog specifically, two predictors explained backlog size: the number of accessioned bryophyte specimens and the number of staff hours per month devoted to working with bryophytes. The authors interpret these findings as supporting anecdotal reports that collectors send their specimens to larger institutions known to have full-time staff and therefore more apparent capacity to handle incoming material. The survey also found that only 29.1 percent of respondents reported having a staff member with bryological expertise, and the 72 herbaria reporting bryophyte work hours averaged just 16.6 staff hours per month, with 23 institutions reporting zero hours.

When asked how they would tackle their backlogs, the 58 staff who responded were remarkably consistent. The most frequent solution, mentioned by 67.2 percent, was simply more people, whether staff in broad terms, students, volunteers, bryologists or curators. Research suggests that repetitive tasks such as re-packaging and barcoding can be carried out by students and volunteers after initial training, and institutions such as the Chicago Academy of Sciences’ Herbarium have had success with temporary staff and dedicated volunteer forces, though volunteers require both training and supervision by existing staff. More curation and more funding each appeared in 20.7 percent of responses, while more expertise, more time, fewer competing priorities and improved workflow each appeared at around 12 to 14 percent. Respondents also suggested policy changes going forward, such as requiring full species identifications with donations and encouraging financial donations to accompany gift specimens, mirroring practices in art museums.

If resources were unlimited, how would curators prioritise? Among 52 respondents, geographic significance led at 36.5 percent, with staff wanting to accession first specimens from other countries, locally collected material, biodiversity hotspots, areas now significantly altered and areas in need of protection and conservation. Specimen significance followed at 25.0 percent, prioritising unique and understudied specimens such as types and unicates, and specimens that may represent less common, possibly rare species. Project significance, convenience, collector significance, taxonomic division and historical significance each appeared at 11 to 17 percent. The authors recommend a practical accessioning plan: inventory the backlog first, identifying prominent collectors and significant collection periods; sort specimens by identification level so that fully and partially identified material can be filed where taxonomic experts will encounter it; then curate, re-package and generate labels. Prioritising specimens with some identification, even only to family or genus, increases the likelihood that researchers examining those groups will come across them and complete the identifications in the future.

The study has limitations the authors acknowledge candidly. Backlogs remain a poorly understood part of herbaria, difficult to study when people do not acknowledge or feel prepared to discuss them. The survey was administered during the height of the Covid-19 pandemic, yielding a final sample representing roughly 2 percent of the herbaria listed by Index Herbariorum in 2020, and item non-response bias may be evident because respondents could skip questions. Still, the dataset captures voices from a geographically diverse set of institutions ranging from small herbaria with 1,000 accessioned specimens to large ones with 6,000,000, and it represents the first systematic attempt to quantify herbarium backlogs worldwide. The stakes extend far beyond housekeeping: accessioned, digitised specimens underpin biodiversity tracking, reveal where plant species were historically located and whether their distributions have changed, and help scientists identify which species need conservation. Every unaccessioned specimen is knowledge that is not yet represented in the scientific record, and the authors hope their overview will inspire institutions to approach their backlogged specimens with more confidence and to collaborate with peer institutions on moving these scientific treasures into the light.

Subject of Research: Biodiversity and Collections

Article Title: The state of herbarium specimen backlogs: Perspectives from bryophyte collections

Article References: Bendull, H., Budke, J. M., & Lewis, R. A. (2026). The state of herbarium specimen backlogs: Perspectives from bryophyte collections. Natural History Collections and Museomics, 3, 1-20. https://doi.org/10.3897/nhcm.3.182936

Image Credits: AI Generated

DOI: 10.3897/nhcm.3.182936

Keywords: herbaria, bryophytes, mosses, natural history collections, specimen backlog, biodiversity, museomics, botany, state, herbarium, specimen, backlogs