Amensalism via webs causes unidirectional shifts of dominance in spider mite communities

Abstract

Competitive displacement is considered the most severe consequence of interspecific competition; if a superior competitor invades the habitat of an inferior species, the inferior species will be displaced. Most displacements previously reported among arthropods were caused by exotic species. The lack of investigation of displacement among native species may be due to their apparently harmonious coexistence, even if it is equivalent to an outcome of interspecific association. A seasonal change in the species composition of spider mites, from Panonychus ulmi to Tetranychus urticae, is observed in apple trees worldwide. Previous laboratory experiments have revealed amensal effects of T. urticae on P. ulmi via their webs. Using manipulation experiments in an orchard, we tested whether this seasonal change in species composition occurred as the result of interspecific competition between these spider mites. Invasion by T. urticae prevented an increase in P. ulmi densities throughout the experimental periods. Degree of overlap relative to the independent distribution on a leaf-surface basis (omega (S)) changed from positive to negative with increasing density of T. urticae. T. urticae invasion drove P. ulmi toward upper leaf surfaces (competitor-free space). The niche adjustment by P. ulmi occurred between leaf surfaces but not among leaves. Our findings show that asymmetrical competition between T. urticae and P. ulmi plays an important role in this unidirectional displacement and that the existence of refuges within a leaf produces the apparently harmonious coexistence of the mites and obscures their negative association.

Tolerance to Solar Ultraviolet‐B Radiation in the Citrus Red Mite, An Upper Surface User of Host Plant Leaves

“…For instance, during a 2‐year field study in northern Japan, Osakabe et al . found that most T. urticae settled on the lower leaf surfaces of apple trees throughout the experimental period, while less than 1.4% of adult females occurred on the upper leaf surfaces. Such biased distribution may be a consequence of the adaptation of T. urticae adult females to avoid solar ultraviolet radiation by sheltering on the lower leaf surfaces .…”

“…Nevertheless, larvae of the European red mite, Panonychus ulmi (Koch), that hatched on upper leaf surfaces immediately move to lower leaf surfaces . This may be a major mechanism of coexistence by which T. urticae constrains the population development of P. ulmi in the field ; the solar UVB radiation could enhance this effect.…”

Benthic community-mediated sediment dynamics

“…However, Snelgrove & Butman (1994) falsified most claims of the theory of trophic amensalism, mainly on the ground that, through modifications applied in several examples, it lacks generality. In terrestrial ecology, the term 'competitive displacement' is used to describe this type of organism-environment -organism interaction (Osakabe et al 2006). Although not addressed in this study, the exceptionally high abundances of P. elegans also might have had an adverse effect on the larval settlement of other species, either directly through predation or indirectly (Cummings et al 1996).…”

The succession of phytophagous and phytoseiid species in a newly planted apple orchard without insecticide applications

“…Since insecticides were not applied to the orchard, the populations of T. urticae in the apple orchard are likely to have disappeared due to the occurrence of P. ulmi. The interaction between P. ulmi and T. urticae in the field should be investigated because previous experiments have shown the opposite situation, T. urticae had been superior species to P. ulmi (Foott, 1963;Osakabe et al, 2006). In this study, although the P. ulmi population was influenced by the occurrence of T. vulgaris, the predation of P. ulmi by T. vulgaris was insuffi cient to prevent P. ulmi from becoming abundant (Toyoshima, 2003).…”

“…The effect of ground cover on the T. urticae population was considered to be negligible because seasonal fluctuations in the ground cover of unsprayed orchards were similar to changes in the ground cover observed in sprayed orchards where T. urticae occurred. Rather, the decrease in T. urticae abundance may have arisen through interactions with P. ulmi, even though P. ulmi has been demonstrated to be a competitively inferior species to T. urticae in manipulation experiments conducted in an apple orchard (Foott, 1963;Osakabe et al, 2006). Factors and mechanisms responsible for the exchange from T. urticae to P. ulmi were not clarifi ed in this study.…”