The unseen invaders : introduced earthworms as drivers of change in plant communities in North American forests (a meta-analysis)

Globally, biological invasions can have strong impacts on biodiversity as well as ecosystem functioning. While less conspicuous than introduced aboveground organisms, introduced belowground organisms may have similarly strong effects. Here, we synthesize for the first time the impacts of introduced earthworms on plant diversity and community composition in North American forests. We conducted a meta-analysis using a total of 645 observations to quantify mean effect sizes of associations between introduced earthworm communities and plant diversity, cover of plant functional groups, and cover of native and non-native plants. We found that plant diversity significantly declined with increasing richness of introduced earthworm ecological groups. While plant species richness or evenness did not change with earthworm invasion, our results indicate clear changes in plant community composition: cover of graminoids and non-native plant species significantly increased, and cover of native plant species (of all functional groups) tended to decrease, with increasing earthworm biomass. Overall, these findings support the hypothesis that introduced earthworms facilitate particular plant species adapted to the abiotic conditions of earthworm-invaded forests. Further, our study provides evidence that introduced earthworms are associated with declines in plant diversity in North American forests. Changing plant functional composition in these forests may have long-lasting effects on ecosystem functioning.Peer reviewe

Soil faunal and structural responses to the settlement of a semi- sedentary earthworm Lumbricus terrestris in an arable clay field

We studied the soil community and habitat consequences of introducing a deep-burrowing, sedentary life-style earthworm, Lumbricus terrestris (dew-worm) into arable zero-till clay. Seventeen years after introduction, which was originally aimed at improving the macroporosity and permeability of the heavy clay soil, we returned to the gradient of L. terrestris dispersal (well established, more recent and no settlement) and investigated the temporal development of the ecosystem engineering impacts on different faunal groups (earthworms, enchytraeids and nematodes) and soil porosity. The faunal re- sponses were examined both at L. terrestris midden scale (individual living site) and field scale. We found that L. terrestris middens sustained elevated densities of all three faunal groups. In the case of earth- worms, there was evidence for temporal development of the engineering impact as the difference be- tween the midden and non-midden areas was more pronounced in the well-established areas than close to the leading edge of dispersal. The earthworm community composition was not altered at L. terrestris midden sites. The settlement of L. terrestris had no discernible effects on field-scale earthworm and nematode abundances, but enchytraeids were practically absent beyond the leading edge of the dispersal. This effect might, however, be partly explained by a gradient of increasing clay content. Soil macroporosity at L. terrestris midden sites did not increase with the age of L. terrestris settlement. Our results suggest that L. terrestris settlement in a clay soil can significantly increase the spatial patchiness of soil fauna, but may not, except in the case of enchytraeids, affect their field-scale abundances or the macroporosity of the soil in the vicinity of L. terrestris living sites

Earthworm communities in relation to tree diversity in a deciduous forest

Migge-Kleian S, Cesarz S, Fahrenholz N, Platner C, Schaefer M. Earthworm communities in relation to tree diversity in a deciduous forest. EUROPEAN JOURNAL OF SOIL BIOLOGY. 2007;43:S61-S67.The understanding of belowground biodiversity is still rather incomplete and interactions between above- and belowground systems have rarely been looked at when explaining belowground biodiversity patterns. The present study presents results of the influence of tree species diversity on the earthworm community in a central European deciduous mixed forest. Within the Hainich National Park three replicate sites were chosen within three levels of tree diversity ranging from one species (beech, DL1) to three species (beech, lime, ash, DL2) to five species stands (beech, lime, ash, hornbeam, maple, DL3). Earthworms were extracted by heat from soil and litter samples at four sampling dates at each site. In addition, in spring four samples per site were sorted by hand and identified to species level. Generally, earthworm densities were positively correlated with tree diversity in May and November but negatively in February. Sites with low tree species diversity had earthworm populations which were more stable due to a more permanent and deeper litter layer. Increasing proportions of high quality litter in DL2 und DL3 were correlated with higher densities of both epigeic and endogeic earthworm species, while litter in DL1 sites was mainly inhabited by epigeic species. Generally, there was a positive correlation between earthworm and tree species diversity indicating the importance of diverse food qualities for the decomposer fauna at the local scale especially in springtime. (C) 2007 Elsevier Masson SAS. All rights reserved

Earthworm invasion in North America: Food resource competition affects native milliped survival and invasive earthworm reproduction

The invasive non-nativeearthwormAmynthas agrestis (Goto and Hatai, 1899) has recently been documented invading forests of the Appalachian Mountains in the southeastern United States. This epigeic earthworm decreases the depth of organic soil horizons, and this may play a role in the decrease of millipede richness and abundance associated with A. agrestisinvasion. To investigate the mechanisms behind these effects, A. agrestis and the millipede Sigmoria ainsliei (Xystodesmidae) were placed into microcosms with soil and either L horizon, F and H horizon, or a combination L/FH treatment. Microcosms were destructively sampled and reconstructed with the same treatments every four weeks to assess faunal fresh weight change and survival. Soils from earthworm treatments were wet-sieved for cocoons to assess treatment effects on reproduction. On average, millipede mortality occurred 88 days sooner in treatments that did not have FH horizon material, and within all litter treatments millipedes tended to survive longer when A. agrestis was absent. Earthworms maintained higher fresh weight in L/FH than FH or L treatments. With a single exception, no A. agrestis cocoons were recovered from microcosms that also contained S. ainsliei. The results suggest that A. agrestis and S. ainsliei may compete for foodresources, particularly the smaller particle material in the FH horizons of the forest floor. Millipedes may exert some biotic resistance to A. agrestisinvasion, as diminished earthworm fecundity was observed in experimental units containing both species

Habitat association and coexistence of endemic and introduced ant species in Galápagos Islands

1. We investigated ant communities in all main vegetation zones of the model island of Santa Cruz in the Galápagos archipelago (155 collection points, spread over 21 sites; 28 ant species collected), and evaluated the distribution, coexistence, and effect of environmental factors in a community composed of endemic, probably endemic, and introduced ants of the New World and exotic origin. 2. Introduced species were the most frequent, occurring in 98% of the samples, yet endemic and probably endemic species still occurred in 54% of the samples, and constituted one of three most common species. The present study revealed that the habitat type along with altitude and the tree cover are the primary factors shaping ant community composition. Little evidence was found for a competitively structured assemblage of ant species. 3. The present study confirmed the predominance of two dominant invasive species, Solenopsis geminata Fabricius and Wasmannia auropunctata Roger, whose abundances are negatively correlated. The abundance of S. geminata is positively correlated with the overall species richness, and with the proportion of other introduced species. The presence of both invasive ants is associated with a low evenness of ant communities. 4. The present study (i) stresses the dominance of introduced species and the relative resistance of endemic species, (ii) highlights the on-going processes of species introductions and (iii) points out the need for adequate monitoring and conservation of the pristine and threatened environments that constitute the Galápagos Islands.FLWINSCOPUS: ar.jinfo:eu-repo/semantics/publishe