Ecological drivers influence the distributions of two cryptic lineages in an earthworm morphospecies
Introduction
Soils contain a wealth of invertebrate biodiversity recognised for their important contributions to ecological processes (Bardgett and van der Putten, 2014, Fitter et al., 2005, Giller, 1996). One key group of species are the “ecosystem engineers”: those organisms that modify the physical state of the soil and resource availability for other species. Earthworms are known as a key group of ecosystem engineers in many habitats. They perform a range of physical (aeration, bioturbation, litter fragmentation) and biological (microbial interactions, exudate production) roles in soil (Blouin et al., 2013, Lavelle et al., 1997, Sackett et al., 2013, Umarov et al., 2008). Because of their functional importance, earthworms have emerged as a major taxon for biomonitoring and biomarker assessments of human induced pressures on soil communities (Cluzeau et al., 2012, Rutgers et al., 2009).
As soil invertebrate species, including earthworms, have been shown to be sensitive to a range of land use change and pollution impacts (Bundy et al., 2007, Cluzeau et al., 2012), different soil taxa have become a natural focus for research on the relationships between environmental pressures, biodiversity and soil functioning (Bartlett et al., 2010, Leveque et al., 2015, Rutgers et al., 2016). For community studies, a major constraint relates to current uncertainties in earthworm taxonomy. Traditionally earthworm identification has relied on morphology, but the paucity of suitable local keys and problems with application to juveniles has also recently encouraged the use of molecular methods (Dominguez et al., 2015, Emerson et al., 2011, Klarica et al., 2012). These genotyping studies have begun to challenge current understanding of diversity through the identification of genetically distinct cryptic lineages within previously established morphospecies.
Earthworm species in which cryptic lineage diversity has to date been identified include Eisenia fetida/andrei (Römbke et al., 2016), Lumbricus terrestris (James et al., 2010), Aporrectodea caliginosa (PerezLosada et al., 2009), Allolobophora chlorotica (King et al., 2008), Amynthas gracilis/Amynthas cortici (Novo et al., 2015) and Lumbricus rubellus. For L. rubellus, genotyping studies based on mitochondrial cytochrome oxidase I and II markers have identified as many as 6 cryptic lineages across Europe (Giska et al., 2015), two of which are found in the UK (Andre et al., 2010, Kille et al., 2013). The two UK lineages have 10–15% divergence for the mitochondrial COI and COII sequences. While this implies they may actually be cryptic species, recent analysis of multiple nuclear markers using RADseq has not supported this interpretation, instead suggesting that different L. rubellus lineages may actually correspond to a single highly polymorphic species (Giska et al., 2015). Comparative studies of the two lineages in the UK have, nonetheless, identified physiological differences between them, including variation in pheromone production (Jones et al., 2016), maturation time (Anderson et al., 2013), metabolic profiles (Liebeke et al., 2014), mechanism of arsenic adaptation (Kille et al., 2013), trace element metabolism (Andre et al., 2010), and microbiome complement (Pass et al., 2015).
Despite known biological differences, the extent to which differences in distribution and physiology are related to different geographical, climate and soil physicochemical preferences between the two known UK lineages of L. rubellus is not established. The two lineages found co-occur at some, but not all, sites meaning that they have some likely niche divergence that facilitates coexistence (Andre et al., 2010, Giska et al., 2015, Kille et al., 2013). We aim to better understand the nature of the spatial and geochemical drivers of lineage relative abundance, and so here we test the hypothesis that the site distribution of the two cryptic L. rubellus lineages is based on one or more geographical, climatic, physiochemical or biotic drivers. We collected and genotyped morphotype L. rubellus at multiple well-characterized sites that differed in their properties to investigate the relationships that determine lineage distributions. Tissue metal concentrations were also measured to assess if trace metal levels could also influence distributions, as could be the case if the two lineages had different sensitivity to specific contaminants.
Section snippets
Site selection
Twenty six sites located across England and Wales (Fig. 1) were visited between four times (for Devon Great Consols Mine and Control, Shipham Mine and Control, Cwmystwyth Mine and Control) and a single visit (for Porton Down, Parys Mountain, Castell, Clydach, Roman Gravel, Didcot) over four separate sampling events from Spring 2011 to Spring 2014. The chosen sites were selected to capture a range of the habitats and soil conditions under which morphotype L. rubellus can be collected. Land-uses
Results
High quality L. rubellus COI sequences were obtained from DNA samples taken from 787 earthworms for assignment as either Lineage A or Lineage B individuals. The maximum number of sequences from any one site was 73, from Cwmystwyth Mine, and the minimum 3, from Avonmouth Control (Fig. 1). In total, 457 individuals were assigned as Lineage A, 58% of the number collected. The remaining 330 (42%) were assigned as Lineage B. Eight sites (Avonmouth Savalco, Avonmouth Incinerator, Clydach Smelter,
Discussion
Species distributions can be affected by a range of environmental drivers, including physiological tolerances, dispersal constraints, biotic interactions and anthropogenic influences (Dennis and Hellberg, 2010, Gaston, 2003). Among earthworms, species show preference for certain habitats, for example common compost earthworm species such as Eisenia fetida, Perionyx excavatus and Eudrilus eugeniae preferentially occupy organic matter rich habitats associated with animal manure or composting
Conclusions
Earthworms represent ‘super-sentinels’ exploited for environmental monitoring and ecotoxicology, as well as being keystone soil engineers essential for soil quality. The identification of possible drivers of species and lineage distributions has potential implications for their use in environmental assessment as well as in studies of ecosystem service delivery. For example, when assessing biodiversity effects of pollution and land-use change it may be valuable to consider the occurrence of
Acknowledgments
This study was supported by the Natural Environment Research Council (NERC), UK, under grant number NE/H00973/1. We thank Dr Rachael Madison and Dr Judith Garforth for help with earthworm and soil collection and the Forestry Commission, ADAS and Countryside Commission for Wales for allowing access to Environmental Change Network (ECN) sites at Alice Holt, Snowdon and Drayton Experimental Frame respectively.
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Current address: Department of Symbiosis, Max Planck Institute for Marine Microbiology, Bremen, Germany.