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Mechanisms and consequences of intra- and interspecific interference competition in Idotea baltica (Pallas) and Idotea emarginata (Fabricius) (Crustacea: Isopoda): A laboratory study of possible proximate causes of habitat segregation

1998; Elsevier BV; Volume: 227; Issue: 1 Linguagem: Inglês

10.1016/s0022-0981(97)00253-0

1879-1697

Heinz-Dieter Franke, Michael Janke,

Environmental Toxicology and Ecotoxicology

The ecologically very similar sympatric congeners Idotea baltica and I. emarginata are associated with macroalgal debris which provides the animals with both food and shelter. Habitat segregation seems to be the major factor contributing to the coexistence of these two species: I. baltica is the dominant species among surface drift weed, whereas I. emarginata occurs mainly on the sea bed among accumulations of broken decaying algae. In laboratory cultures with food in excess, there is intense intra- and interspecific competition through direct interference. Single-species cultures of both species thrive and rapidly attain an equilibrium population size which is about four times greater in I. emarginata than in I. baltica. In mixed-species cultures, I. baltica lacks a `realized niche'; independent of starting densities, I. baltica is competitively excluded by I. emarginata within a short period of time. The crucial behavioural mechanism of interference is cannibalism (intraspecific competition) and mutual predation (interspecific competition). Both intra- and interspecific interference occur mainly through density-dependent control of juvenile mortality by cannibalistic/predatory adults. Experiments on the relative strengths of intra- and interspecific interference revealed strong asymmetries in competitive relationships: Intraspecific inhibitory effects in I. baltica are much greater than in I. emarginata; I. emarginata exerts much more inhibition on I. baltica than on itself, whereas I. baltica exerts much more inhibition on itself than on its congener I. emarginata. These asymmetries account for a complete competitive dominance of I. emarginata over I. baltica, and (according to the Lotka–Volterra model of interspecific competition) inevitably result in competitive exclusion of I. baltica by I. emarginata. All types of asymmetry are due to the fact that juveniles of I. emarginata are much better protected from being cannibalized and preyed upon by conspecifics and heterospecifics, respectively, than juveniles of I. baltica. This protection is probably based on an active chemical defense mechanism that becomes ineffective immediately after death. The possibility is discussed that current interference competition is an important factor in maintaining habitat segregation in I. baltica and I. emarginata in the field. Extrapolation of the laboratory results to the field suggests an essentially unidirectional effect, with I. emarginata itself being largely unaffected by I. baltica but competitively excluding the latter from zones of potential overlap.