There is accumulating evidence that sexual interactions among species (reproductive interference) could have dramatic effects for species' coexistence.It has been shown that the fitness of individuals can be substantially reduced as a consequence of reproductive interference.This might subsequently lead to displacement of a species (sexual exclusion).On the other hand,some evolutionary and ecological mechanisms might enable species to coexist,such as the divergence of mate recognition systems (reproductive character displacement),habitat partitioning,clumped dispersion patterns or different colonization capabilities.We have previously shown that the two ground-hopper species Tetrix subulata and Tetrix ceperoi interact sexually in the laboratory as well as in the field.At sites where both species co-occur niche overlap was high,suggesting that coexistence is maintained by different niche breadths rather than by habitat partitioning.To test the hypothesis that habitat partitioning does not contribute to species' coexistence,we examined whether allotopic and syntopic populations of these two species differ in niche overlap (competitive release).Our results show that niche overlap is higher in syntopic than in allotopic populations,suggesting that the site-specific habitat structure (heterogeneity) has a stronger influence on microhabitat utilization than the presence of heterospecifics.Hence,our data do not support the hypothesis that habitat partitioning plays a substantial role for the coexistence of these sexually interacting species.
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