The Importance of Prey and Habitat Structure
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Martens in a novel habitat – The importance of prey and habitat structure by Charlotte Eriksson Master degree thesis in Conservation Biology, 60 credits Department of Biology Lund University Supervisors Dr. Taal Levi Department of Fisheries and Wildlife Oregon State University Dr. Katie Moriarty Pacific Northwest Research Station U.S Forest Service June 2016 1 Abstract North American martens (Martes americana and Martes caurina) are thought to be forest specialists closely linked to late-successional forests with high canopy cover and a complex understory. The Pacific marten (Martes caurina) has only been found in two isolated populations in Oregon (one on the central coast and one on the south coast). Interestingly, despite extensive survey effort, nearly all marten observations in the Central Coastal Population of Oregon have been found in a narrow strip of partially forested coastal sand dune habitat to the west of the Pacific Coast Highway despite the presence of large areas of older forest adjacent to this coastal dune habitat. The surprising degree to which martens are isolated to this novel habitat type motivated a research effort to explain this highly restricted distribution of Pacific martens. We predicted that the range of martens might be restricted by improved foraging opportunities in the coastal dunes and an inadequate prey base in the forest, or relaxed predation and competition in the coastal dunes relative to the forest. To address these hypotheses, we characterized differences in the relative abundance and diversity of potential marten prey, and presence of other omnivorous medium-sized mammals that might compete with martens, and likely mammalian predators. We deployed 674 camera stations across four different vegetation types (beach grass, deflation plain, ericaceous forest and interior forest) within the Central Coast Ranger District area of Siuslaw National Forest (NF), Oregon, United States between October and December 2015. We also undertook vegetation surveys at each camera location to determine the difference in plant composition and structure among the vegetation types. The ericaceous forest had the highest amount of understory structure with total shrub cover and fruit-producing shrub cover being significantly higher compared to the other vegetation types. As predicted, the relative abundance and diversity of potential prey species was higher in the ericaceous forest compared to the interior forest. The presence of potential competitors and predators were often equal or higher within 2 the ericaceous forest and deflation plain compared to the interior forest. Our study suggests that martens are more flexible in their habitat use than previously believed, and that relative abundance of prey and understory habitat structure may be more important than forest age. This study provides important indications that may partly explain why martens are inhabiting the dune forest which is important for management of conservation efforts. 3 Introduction ................................................................................................................................ 5 Materials and methods ............................................................................................................... 7 Small mammal surveys ......................................................................................................... 10 Carnivore surveys ................................................................................................................. 12 Data analysis ......................................................................................................................... 12 Results ...................................................................................................................................... 14 Vegetation structure .............................................................................................................. 14 Diversity and relative abundance ......................................................................................... 15 Discussion ................................................................................................................................ 19 References ................................................................................................................................ 24 Appendix 1 ............................................................................................................................... 30 4 Introduction Carnivores are declining worldwide due to anthropogenic stressors including direct persecution, habitat loss and fragmentation (Di Minin et al. 2016). The loss of carnivores has resulted in changes in prey abundances and behavior (Ripple et al. 2001), mesopredator release (Prugh et al. 2009; Ripple et al. 2013) and facilitation of zoonotic diseases (Levi et al. 2012). However, not all species are at equal risk facing these threats. Ecological and life history traits such as energetic requirements, body size, vulnerability to predators and interspecific competition often determines how sensitive a species is to human-induced eco- system changes (Cardillo et al. 2004; Pardo Vargas et al. 2016; Linnell & Strand 2010). These factors also influence the extent to which species use different types of habitats (Pandit et al. 2015). Habitat specialists are typically more vulnerable to habitat loss or fragmentation compared to generalist species that often benefit from human disturbance (Crooks & Crooks 2002; Devictor et al. 2016; Vergara et al. 2013). North American martens (Martes americana and Martes caurina) are thought to be forest specialists closely linked to late-successional forests with high canopy cover and a complex understory (Buskirk & Powell 1994; Moriarty et al. 2015; Payer & Harrison 2003). The American marten (Martes americana) has a widespread distribution in North America from the east coast, west to Montana and north to interior Alaska while the Pacific marten (Martes caurina) occupies the west coast from southeastern Alaska south to northern California (Stone et al. 2002). In the Pacific Northwest, martens occupy coniferous forests dominated by Tsuga heterophylla, Pseudotsuga menziesii, Thuja plicata, Abies grandis and Pinus contorta (Shirk et al. 2014; Munzing & Gaines 2008; Bull & Heater 2001). Like other medium-sized carnivores, martens live in a landscape of fear where they have to balance foraging with avoiding terrestrial and avian predators (Hairston et al. 1960; Ritchie & Johnson 2009; Drew 5 1995). Complex habitat structures are therefore important as they provide protection against predators, habitat for prey species (Andruskiw et al. 2008) as well as resting and denning sites (Slauson & Zielinski 2009). As a result of their habitat requirements, martens are highly sensitive to timber harvesting (Thompson 1991). The relative abundance of prey may be particularly important to mustelids like martens. Their elongated and thin body shape make them efficient hunters in habitats under-utilized by other carnivores such as underground and arboreal refuges of prey species (Brown & Lasiewski 1972). However, a slender body with limited fat reserves and little insulation from fur, means martens have increased thermoregulatory costs and consistently high metabolic demands in comparison to other mammals (Buskirk & Harlow 1989; Scholander et al. 1950). To balance these demands martens must eat approximately 25% of their body weight daily, which, for instance, could include 7 red-backed voles (Clethrionomys gapperi) and require significant foraging effort (Gilbert et al. 2009). Not surprisingly, marten density have been strongly correlated with the abundance of prey species such as voles (Flynn & Schumacher 2009), mice and squirrels (Fryxell et al. 1999; Kleef & Wijsman 2015). Furthermore, relative to other similar sized mammals, martens have a slow life history with late sexual maturity, long interbirth intervals, and high longevity, further increasing their sensitivity to additional sources of mortality (Buskirk & Harlow 1989). The Pacific marten has only been found in two isolated populations in Oregon (one on the central coast and one on the south coast which may be connected to a population in northern California) (Zielinski et al. 2001). The populations appear to be isolated and small both in geographic extent and abundance and are therefore threatened by not only the aforementioned threats of habitat loss and fragmentation, but also high-intensity fires and disease outbreaks (Moriarty et al. in press). In addition, the Central Coastal Population may be of even further risk of local extirpation due to its immediate proximity to the ocean (Fig. 1). 6 The long-overdue predicted Cascadia subduction zone earthquake and following tsunami (Long & Shennan 1998) could potentially wipe out the whole population. Interestingly, despite extensive survey effort, nearly all marten observations in the Central Coastal Population of Oregon have been found in a narrow strip of partially forested coastal sand dune habitat to the West of the Pacific Coast Highway despite the presence of large areas of older forest adjacent to this coastal dune habitat (Fig. 1; Moriarty et al. in press). The surprising degree to which martens are isolated to this novel habitat type motivated a research effort to explain this highly restricted distribution of Pacific martens. We predicted that the range of martens might be restricted by improved foraging opportunities in the coastal dunes