Impacts of litter type, soil source, and experience on the growth of earthworms with different foraging strategies

Background/Question/Methods

Plants can affect earthworms and other detritivores directly via the toughness and nutritional quality of the litter they deposit, and indirectly by the effects of this litter on soil chemistry and microbial communities.  Thus, invasion by an exotic plant species might affect earthworms that primarily consume relatively intact surficial litter (e.g., Lumbricus spp.) differently from those that primarily consume older organic material incorporated into the soil (e.g., Aporrectodea spp.).  We tested this hypothesis with field and greenhouse experiments.  For field experiments we collected Lumbricus and Aporrectodea juveniles in a Central Minnesota deciduous forest from a) shrub-free sites or under b) buckthorn (Rhamnus cathartica), c) Honeysuckle (Lonicera tatarica), or d) leatherwood (Dirca palustris) thickets.  We reared individual earthworms in mesh enclosures containing soils and litter derived from the same sites that we then placed back into the forest in all possible combinations of earthworm species, earthworm source, soil/litter source, and placement location.  After three weeks, we gauged earthworm performance using the relative growth rate of ash-free dry mass.  We conducted greenhouse experiments in a similar manner, using tree tubes as enclosures.  These we filled with various combinations of worm species, worm source, soil source, and litter type.

Results/Conclusions

Growth rates of juvenile earthworms in our field experiment were highly variable, but there was a significant difference in the response of Aporrectodea and Lumbricus to our treatments.  Aporrectodea grew consistently across treatments (1% – 3%/d) with few instances of mass loss, and also tended to grow more rapidly when transplanted back to their “home” soil and location.  In contrast, Lumbricus grew less consistently (–2% –7%/d), often lost mass, and tended to grow best in soils/litter derived from open sites or leatherwood thickets.  Laboratory experiments revealed that Aporrectodea juveniles grew at similar rates regardless of the type of litter provided, while Lumbricus juveniles gained mass when fed honeysuckle and basswood litter and lost mass when fed litter from red maple, sugar maple, red oak, and white oak.  Neither soil type nor litter type significantly affected the growth of Lumbricus terrestris adults.  In contrast, Aporrectodea adults collected from honeysuckle thickets grew significantly faster than did those collected from buckthorn thickets, even when reared in the same soil and fed the same litter, evincing a “soil source legacy”.  Our results suggest that different earthworm guilds may respond in diverse and history-dependent ways to vegetation change.