Abstract
Oribatid mites represent a diverse group of soil micro-arthropods. They have evolved a broad range of defensive chemical and morphological traits (e.g. sclerotization, ptychoidy, biomineralization). Chemical defense, rather than sclerotization, can provide protection against large predators (staphylinid beetles) and many oribatid mite species are also well protected against gamasid soil mites using morphological traits (enemy-free-space hypothesis). However, since predatory mites and staphylinid beetles have different types of attacking and feeding, the adaptive values of chemical and morphological traits might differ accordingly. We used the oribatid model species Archegozetes longisetosus Aoki and the common gamasid mite Stratiolaelaps miles Berlese in a predator-prey experiment. We tested for effects of chemical defense (treatments with and without oil gland secretions) and sclerotization (treatments with unsclerotized tritonymphs and sclerotized adults) in an orthogonal design. In contrast to attacks by large predators, chemical defense was mostly ineffective against gamasid mites. Sclerotization, however, had a positive effect. Hence, in a natural environment with diverse types of predators, the enemy-free space seems only realizable by combinations of chemical and morphological protective traits.
Highlights
Oribatid mites are among the most abundant and speciose arthropods in forest soil ecosystems all over the world (Schatz, 2004; Maraun et al, 2007; Schatz et al, 2011)
We show that chemical defense was mostly ineffective against predatory mites while sclerotization had a positive effect in gamasid-oribatid mite feeding interactions
Considering all treatments, 23 of the 80 S. miles ignored the prey, while the remaining 57 predators regularly fed on A. longesitosus regardless of the treatment
Summary
Oribatid mites are among the most abundant and speciose arthropods in forest soil ecosystems all over the world (Schatz, 2004; Maraun et al, 2007; Schatz et al, 2011). A potential top-down control of soil micro-arthropods by gamasid predatory mites was suggested (Schneider and Maraun, 2009) These findings are in contrast to Peschel et al (2006) who hypothesized that adult ortibatid mites live in an "enemy-free space" [= conceptual ways of living that reduce or eliminate a species’ vulnerability agains their predators, (sensu Jeffries and Lawton, 1984)] while juveniles may not. Studies have confirmed the opisthonotal glands (= oil glands) to be defensive glands in adults and juveniles (Heethoff et al, 2011; Heethoff and Raspotnig, 2012a) against larger rove beetle (Staphylinidae) predators of the genus Stenus Latreille
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