The trophic role of a forest salamander: Impacts on invertebrates, leaf litter retention, and the humification process

Abstract

Woodland (Plethodontid) salamanders are the most abundant vertebrates in North American forests, functioning as predators on invertebrates and prey for higher trophic levels. We investigated the role of Ensatina (Ensatina eschscholtzii ) in regulating invertebrate numbers and leaf litter retention in a northern California forest. Our objective was to examine how salamander predation on invertebrates affects leaf litter retention and the amount available for soil-building and carbon capture at the litter-soil interface. We used field enclosures to quantify the effects of Ensatina on invertebrates and litter retention over two wet seasons, using moisture as a covariate. In the first year Ensatina reduced Coleoptera (beetles) and Diptera (flies) larvae <2 mm, adult Coleoptera, Collembola (springtails), and Formicidae (ants), and increased Oribatid mites, larvae <2 mm (Diptera and Coleoptera), Diplopoda (millipedes), and Aranaea (spiders) <2 mm by reducing their competitors and predators. A single Ensatina in a 1.5m2 enclosure increased litter retention by 13.3%±3.6% (mean 6 SE) compared to controls, facilitating the capture of 200 kg/ha of carbon. At a similar density range-wide this would equate to 72.3 metric tons/yr of carbon in one season potentially sequestered in forest soil rather than entering the atmosphere. In the second year invertebrate densities doubled in response to early rains such that while salamanders reduced the numbers of the same taxa, effect sizes were lower compared to year one, producing biological effects that failed to achieve statistical significance. However, three taxa did significantly increase in year two (Annelida [worms], Psocoptera [barklice], and Chelonethida [pseudoscorpions]). Litter retention in year two was greater on treatment plots by 5.6%±4.6%; however, high variability across plots precluded statistical significance. Ensatina suppressed some invertebrate taxa, released others, increased leaf litter retention, and facilitated greater carbon capture in both years; however, the strength of the effects were modulated by the bottom-up effects of the timing and amount of precipitation in year two. © 2014 Best and Welsh.