Date Published: January 31, 2019
Publisher: Public Library of Science
Author(s): Katie M. Moriarty, Jake Verschuyl, Andrew J. Kroll, Raymond Davis, Joshua Chapman, Bruce Hollen, Alessio Mortelliti.
Forest management guidelines for rare or declining species in the Pacific Northwest, USA, include both late successional reserves and specific vegetation management criteria. However, whether current management practices for well-studied species such as northern spotted owls (Strix occidentallis caurina) can aid in conserving a lesser known subspecies—Humboldt martens (Martes caurina humboldtensis)–is unclear. To address the lack of information for martens in coastal Oregon, USA, we quantified vegetation characteristics at locations used by Humboldt martens and spotted owls in two regions (central and southern coast) and at two spatial scales (the site level summarizing extensive vegetation surveys and regionally using remotely sensed vegetation and estimated habitat models). We estimated amount of predicted habitat for both species in established reserves. If predicted overlap in established reserves was low, then we reported vegetation characteristics to inform potential locations for reserves or management opportunities. In the Central Coast, very little overlap existed in vegetation characteristics between Humboldt martens and spotted owls at either the site or regional level. Humboldt martens occurred in young forests composed of small diameter trees with few snags or downed logs. Humboldt martens were also found in areas with very dense vegetation when overstory canopy and shrub cover percentages were combined. In the South Coast, Humboldt martens occurred in forests with smaller diameter trees than spotted owl sites on average. Coastal Humboldt martens may use stands of predicted high quality spotted owl habitat in the Pacific Northwest. Nonetheless, our observations suggest that coastal Humboldt martens exist in areas that include a much higher diversity of conifer size classes as long as extensive dense shrub cover, predominantly in the form of high salal and evergreen huckleberry, are available. We suggest that managers consider how structural characteristics (e.g., downed logs, shrub cover, patch size), are associated with long-term species persistence rather than relying on reserves based on broad cover types. Describing vegetation may partially describe suitability, but available prey or predation risk ultimately influence likelihood of individual Humboldt marten use. Guidelines for diversifying vegetation management, and retaining or restoring appropriate habitat conditions at both the stand level and regionally, may increase management flexibility and identify forest conditions that support both spotted owls and Humboldt martens.
The “umbrella species” concept posits that habitat conservation for one species with certain overarching characteristics of behavior and habitat use with larger home ranges will provide habitat for species with similar, or more narrow, niches [1, 2]. Accurate descriptions of habitat conditions for a target (or well-studied) species can guide management activity but often information is lacking for rare or little-known species . Assumptions underlying the umbrella species concept must be validated, and over-simplistic or qualitative descriptions of habitat conditions (e.g., old growth forest) for both the umbrella and other species can result in management that does not provide attributes necessary for persistence or recovery of lesser known species (e.g., rare species recovery, ).
We surveyed for Humboldt martens throughout coastal Oregon with the highest sampling densities in areas where Humboldt martens had been previously detected [10, 26]. Our contemporary surveys suggest two populations of Humboldt martens exist in the South and Central Coast . An ongoing northern spotted owl telemetry study occurred adjacent to and between these populations. Thus, for this analysis, we had two Humboldt marten study areas: South (42.5°N, -124.2°W) and Central Coast (43.9°N, -124.1°W) and one spotted owl study area near Coos Bay (43.3°N, -123.9°W) (Fig 1). The maritime climate in all study areas was characterized by cool dry summers (average low and high July temperature = 12, 18°C, 6–9% total precipitation) and mild, wet, winters (average low and high January temperature = 4, 12°C, averages of 100–300 cm of annual precipitation) interspersed with fog and cloud cover year round .
We collected vegetation data within 537 vegetation plots within the 50% utilization distribution of 13 northern spotted owls home ranges (x = 41.3 plots/individual, range 11–79).
Conservation efforts for single species largely influence forest management decisions on public lands in the Pacific Northwest, USA (e.g., [25, 68]). Our analyses indicated that two late successional associated species were not predicted to use the same areas. Sites used by both northern spotted owl and Humboldt marten had overlapping vegetation characteristics, such as overstory cover, but the species were associated with differing sizes and amounts of large trees, snags, and logs. Thus, areas used by spotted owls for nesting and roosting represented only a portion of the broader vegetation conditions used by Humboldt martens. In contrast to areas used by owls, Humboldt martens in both the South Coast and Central Coast regions used areas with dense and diverse shrub communities. Shrub cover may be a surrogate for the structural complexity typically provided by downed logs, both of which provide protective cover and foraging opportunities for the small-bodied terrestrial carnivore . Humboldt martens may also use areas predicted for spotted owls and older forests, but we did not observe such overlap in coastal Oregon. Our study locations may be more diverse and have a different management history compared to locations where Humboldt martens were found in northern California [56, 70]. If critical habitat elements were not strongly correlated with vegetation cover, but instead associated with more ephemeral resources such as prey  and predation risk , then we would predict Humboldt martens to be associated in areas with such biotic elements as much or more than areas with vegetation cover . Understanding the proportional importance of individual habitat elements was beyond the scope of this study, but experimental studies elucidating such information would benefit future management and conservation (e.g., ).
We can use our descriptions of vegetation structure to identify forest types and ownerships where heterogeneity can be achieved; broadly classified in three areas for these species: areas with Humboldt martens but not spotted owls (e.g., the Central Coast Humboldt marten study area), areas with Humboldt martens and spotted owls (e.g., the South Coast Humboldt marten study area), and areas with spotted owls but not Humboldt martens (e.g., the Coos Bay spotted owl study area).