Effects of an Ecosystem Engineer on Belowground Movement of Microarthropods

<div><p></p><p>Ecosystem engineers affect other species by changing physical environments. Such changes may influence movement of organisms, particularly belowground where soil permeability can restrict dispersal. We investigated whether earthworms, iconic ecosystem engineers, influence microarthropod movement. Our experiment tested whether movement is affected by tunnels (i.e., burrows), earthworm excreta (mucus, castings), or earthworms themselves. Earthworm burrows form tunnel networks that may facilitate movement. This effect may be enhanced by excreta, which could provide resources for microarthropods moving along the network. Earthworms may also promote movement via phoresy. Conversely, negative effects could occur if earthworms alter predator-prey relationships or change competitive interactions between microarthropods. We used microcosms consisting of a box connecting a “source” container in which microarthropods were present and a “destination” container filled with autoclaved soil. Treatments were set up within the boxes, which also contained autoclaved soil, as follows: 1) control with no burrows; 2) artificial burrows with no excreta; 3) abandoned burrows with excreta but no earthworms; and 4) earthworms (<i>Lumbricus rubellus</i>) present in burrows. Half of the replicates were sampled once after eight days, while the other half were sampled repeatedly to examine movement over time. Rather than performing classical pairwise comparisons to test our hypotheses, we used AIC_c to assess support for three competing models (presence of tunnels, excreta, and earthworms). More individuals of Collembola, Mesostigmata, and all microarthropods together dispersed when tunnels were present. Models that included excreta and earthworms were less well supported. Total numbers of dispersing Oribatida and Prostigmata+Astigmata were not well explained by any models tested. Further research is needed to examine the impact of soil structure and ecosystem engineering on movement belowground, as the substantial increase in movement of some microarthropods when corridors were present suggests these factors can strongly affect colonization and community assembly.</p></div

Cumulative number of microarthropods dispersing over time.

<p>(a) All microarthopods together (±SE); (b) Collembola; (c) Mesostigmata; (d) Oribatida; and (e) Prostigmata+Astigmata. N = 12 replicates per treatment.</p

Regression fit statistics for models of microarthropod abundance over time.

<p>Predictors included presence of openings (<i>Tunnels</i>), excreta (<i>Castings</i>), and earthworms (<i>Worms</i>). Time was included in all models, either on its own or in interaction with the other predictor variables. The best model has a ΔAIC_c of zero and the highest <i>w</i>AIC_c value. Models with ΔAIC_c<2 are also considered to be plausible and are shown in bold. With <i>k</i>, number of parameters; LL, log likelihood; ΔAIC_c, difference in the Akaike’s information criterion (corrected for small sample size) value between model and the most strongly supported model; <i>w</i>AIC_c, weight given by the AIC (i.e., relative strength of support for model).</p

Experimental set-up.

<p>(a) A microcosm consisting of a 750 mL “source” container, a 10 cm long “treatment” box in which the treatments were implemented, and a 120 mL “destination” container; (b) The four treatments within the boxes, including the control treatment with no earthworms, the artificial burrows treatment with two tunnels made by a dowel, the abandoned burrows treatment in which earthworms were removed before the experiment, and the earthworms present treatment in which earthworms were present throughout the experiment.</p

Total number of microarthropods in destination containers at the end of the experiment.

<p>(a) All microarthropods together (±SE) in the destination containers; (b) Collembola in the destination containers; (c) Mesostigmata in the destination containers; (d) Oribatida in the destination containers; (e) Prostigmata+Astigmata in the destination containers; and (f) Collembola, Mesostigmata, Oribatida, and Prostigmata+Astigmata in a 120 mL sample from the source containers. N = 24 replicates per treatment.</p

Shoot and root biomass in grams (± SE) for <i>Achillea millefolium</i> and <i>Campanula rotundifolia</i> with and without earthworms.

<p>Shoot and root biomass in grams (± SE) for <i>Achillea millefolium</i> and <i>Campanula rotundifolia</i> with and without earthworms.</p

Proportion of 6 mm ×6 mm grid cells with roots dying (± SE), out of all cells occupied by roots during the experiment, in soil, cracks, and burrows for <i>Achillea millefolium</i> and <i>Campanula rotundifolia</i>.

<p>Proportion of 6 mm ×6 mm grid cells with roots dying (± SE), out of all cells occupied by roots during the experiment, in soil, cracks, and burrows for <i>Achillea millefolium</i> and <i>Campanula rotundifolia</i>.</p

Schematic of experimental treatments.

<p><i>Achillea millefolium</i> and <i>Campanula rotundifolia</i> were grown individually with and without earthworms in 15 replicate pots (27 cm × 11 cm × 26 cm). Three <i>Lumbricus terrestris</i> earthworms were added to each of the earthworm treatment pots. A transparent mini-rhizotron tube (5.7 cm in diameter) ran lengthwise approximately 5 cm below the soil surface of each pot to allow mini-rhizotron images to be obtained. Each mini-rhizotron tube ran through five adjacent pots.</p

Occurrence of roots (±1 SE) in 6 mm × 6 mm grid cells containing burrows (filled circle), cracks (open circle), and soil (filled inverted triangle) for (A) <i>Achillea millefolium</i> and (B) <i>Campanula rotundifolia</i> over time.

<p>Occurrence of roots (±1 SE) in 6 mm × 6 mm grid cells containing burrows (filled circle), cracks (open circle), and soil (filled inverted triangle) for (A) <i>Achillea millefolium</i> and (B) <i>Campanula rotundifolia</i> over time.</p

Data on Dummy Caterpillars

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