Predicted distribution of a rare and understudied forest carnivore: Humboldt marten (Martes caurina humboldtensis)

Abstract

Background Many mammalian species have experienced range contractions. Following a reduction in distribution that has resulted in apparently small and disjunct populations, the Humboldt marten (Martes caurina humboldtensis) was recently designated as federally Threatened and state Endangered. This subspecies of Pacific marten occurring in coastal Oregon and northern California, also known as coastal martens, appear unlike martens that occur in snow-associated regions in that vegetation associations appear to differ widely between Humboldt marten populations. We expected current distributions represent realized niches, but estimating factors associated with long-term occurrence was challenging for this rare and little-known species. Here, we assessed the predicted contemporary distribution of Humboldt martens and interpret our findings as hypotheses correlated with the subspecies’ niche to inform strategic conservation actions. Methods We modeled Humboldt marten distribution using a maximum entropy (Maxent) approach. We spatially-thinned 10,229 marten locations collected from 1996–2020 by applying a minimum distance of 500-m between locations, resulting in 384 locations used to assess correlations of marten occurrence with biotic and abiotic variables. We independently optimized the spatial scale of each variable and focused development of model variables on biotic associations (e.g., hypothesized relationships with forest conditions), given that abiotic factors such as precipitation are largely static and not alterable within a management context. Results Humboldt marten locations were positively associated with increased shrub cover (salal (Gautheria shallon)), mast producing trees (e.g., tanoak, Notholithocarpus densiflorus), increased pine (Pinus sp.) proportion of total basal area, annual precipitation at home-range spatial scales, low and high amounts of canopy cover and slope, and cooler August temperatures. Unlike other recent literature, we found little evidence that Humboldt martens were associated with old-growth structural indices. This case study provides an example of how limited information on rare or lesser-known species can lead to differing interpretations, emphasizing the need for study-level replication in ecology. Humboldt marten conservation would benefit from continued survey effort to clarify range extent, population sizes, and fine-scale habitat use.

Structural complexity characterizes fine‐scale forest conditions used by Pacific martens

“…A high amount of shrub cover was common at the Humboldt study site only (Figure 7), consistent with the premise that shrubs are a primary feature of forests inhabited by Humboldt martens (Moriarty et al 2021).…”

“…A notable exception is the distribution of the Humboldt marten ( M. c. humboldtensis ), a Pacific marten subspecies (Schwartz et al 2020), which is in low elevation (0–1,200 m) forests of coastal California and Oregon that receive little to no snowfall (Zielinski et al 2001). Atypical of conifer forests inhabited by Pacific martens elsewhere, coastal forests inhabited by Humboldt martens can have a substantial hardwood component, and a dense, spatially extensive shrub understory (Slauson et al 2007; Moriarty et al 2019, 2021).…”

Seasonality drives the survival landscape of a recovering forest carnivore in a changing world

“…(e.g. [53]). Moreover, other putative specialists of old growth—northern flying squirrels ( Glaucomys sabrinus [54]), spotted owls ( Strix occidentalis [55]) and fishers [56,57]—have also been associated with a variety of serial stages as well as heterogeneity.…”

Conservation of rare and cryptic species: Challenges of uncertainty and opportunities for progress

“…(a) Humboldt marten ( Martes caurina humboldentensis ); (b) historical range of Humboldt marten in coastal and near‐coastal forests of Oregon and northern California, USA (USFWS, 2018), indicating the location of contemporary research efforts in California (black polygon); (c) Humboldt marten detections from the early‐ to mid‐20th century were sparse throughout the historical range and Humboldt martens were thought to be extirpated from California from 1942 to 1996; (d) Humboldt marten detections during surveys from 1989 to 1998 indicated a substantial range contraction, but also provided the first confirmed detection in California since 1942; (e) Humboldt marten detections from 1998 to 2018 suggested a broader distribution than surveys from 1989 to 1998, but still indicated a range contraction relative to historical conditions; (f) even with improved, contemporary data, disparities exist between where Humboldt marten have been documented to occur and where Humboldt martens could potentially occur based on predictive modeling (Moriarty et al, 2021). Points in panels c and d were digitized from Zielinski et al (2001) and represent approximate (within 5 km) detection locations…”

“…comm.). While recent species distribution modeling efforts indicate predicted habitat exists outside of currently designated extant population area boundaries, it is unclear whether Humboldt martens occupy this habitat, as much of the historical range remains unsurveyed (Moriarty et al, 2021; Figure 1).…”