Microhabitat partitioning is driven by preferences, not competition, in two Costa Rican millipede species

The co-occurrence of similar species in a particular environment may be facilitated if they specialise on different microhabitats, reducing competition between them. In some cases, two species prefer the same microhabitat, but one is competitively excluded to its harsh margins. In this study, we assessed microhabitat preferences and competition between two species of millipedes in Costa Rica. (1) We observed them in the wild and found Nyssodesmus python most often on wood, less often on leaves, and rarely on rocks. Spirobolida was found most often on leaves, less often on wood, and never on rocks. (2) We tested their preferences in the lab and found that N. python preferred wood to rocks, wood to leaves, and rocks to leaves. Spirobolida preferred leaves to rocks, leaves to wood, and wood to rocks. (3) We tested interference competition by placing both species together in an arena in which they both had the same preference (wood vs. rocks). Both species chose to cohabitate in the same wood, indicating that one species did not directly exclude the other. In N. python and Spirobolida, co-occurrence is facilitated by differences in microhabitat preferences and not because competition forces one species out of its preferred microhabitat.