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Soil organisms, including earthworms, are a key component of terrestrial ecosystems. However, little is known about their diversity, their distribution, and the threats affecting them. We compiled a global dataset of sampled earthworm communities from 6928 sites in 57 countries as a basis for predicting patterns in earthworm diversity, abundance, and biomass. We found that local species richness and abundance typically peaked at higher latitudes, displaying patterns opposite to those observed in aboveground organisms. However, high species dissimilarity across tropical locations may cause diversity across the entirety of the tropics to be higher than elsewhere. Climate variables were found to be more important in shaping earthworm communities than soil properties or habitat cover. These findings suggest that climate change may have serious implications for earthworm communities and for the functions they provide. A correction has been made for this article, which is freely available at https://science.sciencemag.org/content/369/6503/eabd9834
Background/Question/Methods Identifying the spatial distribution, abiotic and biotic associations, and ecological effects of newly introduced species is an important first step to developing management and control measures. Non-native earthworms, including species from Europe and Asia, have invaded much of the northeastern and Great Lakes deciduous forests of the USA, and the adverse ecosystem-level effects of these invasions are well-documented. However, earthworm invasions in other regions of the USA are poorly studied. We conducted a survey on the historically earthworm-free San Clemente Island (SCI), California, USA, to measure the extent, habitat affinities, and ecological effects of a newly discovered earthworm invasion. Using a stratified random sampling approach, we sampled earthworms, vegetation, and soils in 230 plots across the island. We examined the relationship between the presence of invasive earthworms and several soil, vegetation, and landscape variables. We also evaluated the effects of invasive earthworms on vegetation cover and microbial biomass, the latter of which is thought to mediate invasive earthworm impacts on plant growth. Results/Conclusions Occurrence of invasive earthworms was relatively low (5.25% of plots) and was strongly associated with distance to streams, topographic wetness index (TWI), and “northeastness” (i.e., deviation from a northeasterly direction, a variable that is a proxy for climatic conditions). We detected a significant positive association between invasive earthworm presence and percent ground vegetation cover and between invasive earthworm presence and percent grass cover. No association between invasive earthworms and microbial biomass was observed. These results and the finding of earthworms in a relatively small number of wet areas indicate that the earthworm invasion on the island is closely tied to moisture conditions. The association between invasive earthworms and total cover of ground vegetation is likely driven by the relationship between earthworms and grass cover, and in particular, by cover of a few invasive grass species (primarily Bromus spp.). Climatic variables (e.g., TWI and northeastness) may be useful for providing insight into current and future sites of earthworm invasion. It should be noted, however, that exceptionally dry conditions preceding and during the survey period likely limited our ability to detect earthworms. Therefore, the invasion may be more widespread than we estimate, and strong moisture limitation during the study period could be the driving factor for the relationships we documented. Nonetheless, follow-up studies may be required to monitor further spread and potential adverse consequences of earthworm invasion on the native flora and fauna of SCI.
A mounting body of research suggests that invasive non-native earthworms substantially alter microbial communities, including arbuscular mycorrhizal fungi (AMF). These changes to AMF can cascade to affect plant communities and vertebrate populations. Despite these research advances, relatively little is known about: (1) the mechanisms behind earthworms’ effects on AMF, and (2) the factors that determine the outcomes of earthworm-AMF interactions (i.e., whether AMF abundance is increased or decreased, and subsequent effects on plants). We predict that AMF-mediated effects of non-native earthworms on ecosystems are nearly universal because: (1) AMF are important components of most terrestrial ecosystems, (2) non-native earthworms have become established in nearly every type of terrestrial ecosystem, and (3) non-native earthworms, due to their burrowing and feeding behavior, greatly affect AMF with potentially profound concomitant effects on plant communities. We highlight the multiple direct and indirect effects of non-native earthworms on plants and review what is currently known about the interaction between earthworms and AMF. We also illustrate how the effects of non-native earthworms on plant-AMF mutualisms can alter the structure and stability of aboveground plant communities, as well as the vertebrate communities relying on these habitats. Integrative studies that assess the interactive effects of earthworms and AMF can provide new insights into the role that belowground ecosystem engineers play in altering aboveground ecological processes. Understanding these processes may improve our ability to predict the structure of plant and animal communities in earthworm-invaded regions and to develop management strategies that limit the numerous undesired impacts of earthworms.
Understanding the relative importance of environmental and anthropogenic factors in driving plant community structure, including relative dominance of native and non-native species, helps predict community responses to biological invasions. To assess factors influencing plant communities on San Clemente Island, USA, we conducted an islandwide vegetation survey in which we measured plant species richness and percent cover of native and non-native plants, as well as physical environmental variables, soil chemical properties, abundance of soil microbial functional groups (e.g., arbuscular mycorrhizal fungi [AMF]), and a human disturbance variable (distance to road). We found that total plant species richness decreased with increasing non-native plant cover, soil pH, and AMF abundance. Native plant cover increased with increasing distance to a major paved road and decreased with increasing soil moisture and pH. Non-native plant cover decreased with increasing distance to a major paved road and increased with increasing soil moisture, AMF abundance, and from southwest to northeast, a geographic/climatic gradient that represents increasing moisture. Nonmetric multidimensional scaling ordination further illustrated that trends in plant community composition were correlated with elevation, distance to a major paved road, and soil moisture, organic matter, and ammonium. These results suggest complex effects of physical environmental , soil chemical, and human-related factors on plant community structure on an oceanic island, and moreover , that different factors affect cover of native and non-native plants. Notably, our observation of apparent moisture limitation of non-native plants suggests that, in some contexts, drought conditions can limit plant invasions and may even represent an opportunity for efficient control or eradication of invasive plants. The apparent negative effect of non-native plants on native plant cover and overall plant species richness represents a conservation concern for native biodiversity on oceanic islands and suggests the potential for community reassembly as invasive species increasingly dominate due to anthropogenic disturbances.
Invasions of non-native earthworms into previously earthworm-free regions are a major conservation concern because they alter ecosystems and threaten biological diversity. Little information is available, however, about effects of earthworm invasions outside of temperate and boreal forests, particularly about invasions of islands. For San Clemente Island (SCI), California (USA) – an oceanic island with numerous endemic and endangered plant and vertebrate species – we assessed the spatial extent and drivers of earthworm invasion and examined relationships between earthworms and plant and soil microbial communities.