Plot studies in northeastern deciduous forest have repeatedly found extremely high densities of Plethodon cinereus, sometimes on the order of one salamander or more per square metre. Because individuals are small but numerous, the aggregate biomass in classic estimates from New Hampshire and elsewhere matched or exceeded that of the breeding bird community and approached that of small mammals.
That abundance matters ecologically. Red-backed salamanders eat invertebrates in the litter layer, including detritivores such as mites and springtails, and by doing so they influence how quickly leaf litter is broken down. They are also efficient converters of food to tissue, in part because being lungless and ectothermic keeps maintenance costs low.
The biomass figures come from specific well-studied plots and should not be read as a global average; densities vary sharply with soil moisture, pH and cover objects. The species is also not equally distributed between its two colour forms, and the striped and unstriped morphs can differ in microhabitat use.