The influence of habitat provisioning: use of earthworm burrows by the terrestrial salamander, Plethodon cinereus

Non-native earthworms increase the abundance and diet quality of a common woodland salamander in its northern range

Non-native earthworms found in Eastern Canada substantially affect soil properties and plant diversity, but less is known about their impacts on higher faunal species. We investigated the effects of non-native earthworms on populations of Plethodon cinereus, a common woodland salamander. We hypothesized that earthworms could adversely affect P. cinereus by consuming the forest floor, thereby decreasing soil moisture and the abundance of native preys. Conversely, earthworms could positively affect P. cinereus by providing refuge in their abandoned burrows and by being a novel prey. We installed 25 coverboards in 38 mature sugar maple (Acer saccharum) forests, 24 of which were earthworm-free. Over the next two years, we monitored earthworm and salamander populations using hot mustard extractions and visible implant elastomers, respectively. At a subset of four sites, two with and two without earthworms, we determined salamander diets in the spring (May-June), summer (July-August) and fall (September-October) seasons, using gastric lavage techniques. Forest floor depth decreased, whereas population density, body size and total prey volume of P. cinereus increased, with earthworm abundance. Earthworms, which are soft-bodied and nutritious prey, composed most of the salamander diet at sites with earthworms, volumetrically accounting for > 50% of total prey volume. Despite this, we found fewer prey items in the stomach of salamanders at earthworm-invaded sites, indicating that salamanders are getting a higher caloric intake per feeding while expending less energy. We conclude that non-native earthworms have a net beneficial effect on P. cinereus populations in Eastern Canada, mainly by improving diet quality.

Supplementary Information

The online version contains supplementary material available at 10.1007/s10530-023-03168-3.

Side-swiped: Ecological cascades emanating from earthworm invasion

Non-native, invasive earthworms are altering soils throughout the world. Ecological cascades emanating from these changes stem from earthworm-caused changes in detritus processing occurring at a mid-point in the trophic pyramid, rather than the more familiar bottom-up or top-down cascades. They include fundamental changes (microcascades) in soil morphology, bulk density, nutrient leaching, and a shift to warmer, drier soil surfaces with loss of organic horizons. In North American temperate and boreal forests, microcascades cause effects of concern to society (macrocascades), including changes in CO₂ sequestration, disturbance regimes, soil quality, water quality, forest productivity, plant communities, and wildlife habitat, and facilitation of other invasive species. Interactions among these changes create cascade complexes that interact with climate change and other environmental changes. The diversity of cascade effects, combined with the vast area invaded by earthworms, lead to regionally important changes in ecological functioning.

A global database and "state of the field" review of research into ecosystem engineering by land animals

Ecosystem engineers have been widely studied for terrestrial systems, but global trends in research encompassing the range of taxa and functions have not previously been synthesised. We reviewed contemporary understanding of engineer fauna in terrestrial habitats and assessed the methods used to document patterns and processes, asking: (a) which species act as ecosystem engineers and with whom do they interact? (b) What are the impacts of ecosystem engineers in terrestrial habitats and how are they distributed? (c) What are the primary methods used to examine engineer effects and how have these developed over time? We considered the strengths, weaknesses and gaps in knowledge related to each of these questions and suggested a conceptual framework to delineate "significant impacts" of engineering interactions for all terrestrial animals. We collected peer-reviewed publications examining ecosystem engineer impacts and created a database of engineer species to assess experimental approaches and any additional covariates that influenced the magnitude of engineer impacts. One hundred and twenty-two species from 28 orders were identified as ecosystem engineers, performing five ecological functions. Burrowing mammals were the most researched group (27%). Half of all studies occurred in dry/arid habitats. Mensurative studies comparing sites with and without engineers (80%) were more common than manipulative studies (20%). These provided a broad framework for predicting engineer impacts upon abundance and species diversity. However, the roles of confounding factors, processes driving these patterns and the consequences of experimentally adjusting variables, such as engineer density, have been neglected. True spatial and temporal replication has also been limited, particularly for emerging studies of engineer reintroductions. Climate change and habitat modification will challenge the roles that engineers play in regulating ecosystems, and these will become important avenues for future research. We recommend future studies include simulation of engineer effects and experimental manipulation of engineer densities to determine the potential for ecological cascades through trophic and engineering pathways due to functional decline. We also recommend improving knowledge of long-term engineering effects and replication of engineer reintroductions across landscapes to better understand how large-scale ecological gradients alter the magnitude of engineering impacts.

Invasive Asian earthworms of the genus Amynthas alter microhabitat use by terrestrial salamanders

Invasive earthworms are rapidly transforming detrital communities in North America. Recent studies have investigated the effects of European earthworms, whereas Asian earthworms, such as species of the genus Amynthas Kinberg, 1867, remain understudied. Amynthas is a surface-dwelling earthworm that voraciously consumes the litter layer of temperate forest floor habitats. The accumulation of detritus is important for the terrestrial Eastern Red-backed Salamanders (Plethodon cinereus (Green, 1818)) because this microhabitat provides the matrix through which salamanders travel when foraging, searching for mates, and dispersing. We examined the effect of Amynthas on the activity of “naïve” (no history of co-occurrence with Amynthas) and “experienced” (recent co-occurrence with Amynthas) salamanders in laboratory microcosms. We hypothesized that earthworms would disturb normal salamander activity through the reduction of leaf litter and physical impediment, with greater negative effects on naïve salamanders encountering this “novel” invasive species. Consistent with published studies, earthworm presence significantly decreased leaf-litter mass over time. Prior experience with earthworms did not appear to influence salamander response. However, earthworm presence had a significant effect on salamander activity, with salamanders exhibiting increased movement, cover-object use, and co-occurrence under cover objects with earthworms as the study progressed. Unnecessary movement has the potential to incur fitness costs to salamanders in the form of energetic expenditure and increased exposure to predators.

Earthworms, as ecosystem engineers, influence multiple aspects of a salamander's ecology

Ecosystem engineers create habitat that can be used by other species in multiple ways, such as refugees from predators, places to breed, or areas with increased prey resources. I conducted a series of enclosure experiments to: (1) determine if salamanders use earthworm burrows, and (2) examine the potential influence of earthworm burrow use and indirect effects on salamander intra- and interspecific competition, predator avoidance, and seasonal performance. I found that one species of woodland salamander, Plethodon cinereus, used earthworm burrows 50% of the time when burrows were present. Neither adults nor juveniles of the congeneric P. glutinosus used earthworm burrows. Intraspecific, but not interspecific, competition by P. cinereus affected salamander behavior when earthworms were absent, with P. cinereus found under cover objects >70% of the time when alone or with a P. glutinosus, but only 40% of the time when with another P. cinereus. When earthworms were present, the behavior of P. cinereus was similar across salamander treatments. Earthworms decreased the amount of leaf litter and microinvertebrates, although this did not affect salamander mass. In subsequent experiments using only P. cinereus, the refuge provided by earthworm burrows increased the survival of P. cinereus over the winter and allowed P. cinereus to avoid being consumed by the common garter snake (Thamnophis sirtalis). Because earthworm burrows provide a refuge for P. cinereus during intraspecific encounters, in the presence of a predator and over the winter, they may serve as an important belowground-aboveground linkage in eastern forests where salamanders are common.