Bottom-up and top-down controls on red-backed salamander (Plethodon cinereus) distribution : a possible landscape of fear on the forest floor

The red-backed salamander (Plethodon cinereus) is considered a forest floor keystone species and an important indicator of ecosystem health in the Northeastern United States due to its wide distribution, high-density occurrence, and established presence in the literature. P. cinereus has demonstrated a preference for eastern hemlock (Tsuga canadensis) conditions over those of mixed deciduous stands. The recent infestation of populations of T. canadensis by the invasive hemlock woolly adelgid (Adelgus tsugae) is predicted to severely impact the structure of New England Hemlock stands, and could dramatically shift the dynamics of forest floor animal communities that depend on Hemlock understory, leaf litter, and soil conditions. This study aimed to investigate the ecological niche of P. cinereus, relative to bottom-up (e.g. T. canadensis associated) and top-down (e.g. predation pressure) forces using two study sites: the Smith College Ada and Archibald MacLeish Field Station in West Whately, MA, and a private residence in Chesterfield, MA. At each location, artificial cover objects (ACOs) were placed in adjacent plots of T. canadensis and Black Birch (Betula lenta) and monitored from June to early August of 2014 for P. cinereus. In addition to assessing P. cinereus substrate preference, we also explored influence of predation pressure by the Northern Short-Tailed Shrew (Blarina brevicauda) on P. cinereus distribution, as a means of determining the relative importance of bottom-up and top-down drivers on distribution and density of P. cinereus. On average, P. cinereus was 2.2x more frequent under ACOs in T. canadensis plots than B. lenta plots, and were 2.4x and 1.9x more likely to be found under ACOs without small mammal tunnels in 5 Chesterfield and at the MacLeish Field Station, respectively. As such, the results highlight similarly strong effects of bottom-up versus top-down forces in determining the ecological niche of P. cinereus, and suggest a strong role for predation risk and enemy free space in affecting the distribution of P. cinereus. This study characterizes the interactions between P. cinereus and the most immediate members of its food chain, and puts this system into the broader context of T. canadensis decline and B. lenta successio