Mechanisms maintaining species differentiation: predator-mediated selection in aBombinahybrid zone

Abstract

Mechanisms which prevent gene flow will maintain differentiation between species, and therefore contribute to biological diversity. We describe an experimental study of such mechanisms in a hybrid zone between the fire-bellied toad Bombina bombina and the yellow-bellied toad B. variegata . In this system, preference for different breeding habitats reduces the frequency of hybridization. A comparison of habitat ecology shows that the semi-permanent ponds in which B. bombina usually breeds have higher densities of aquatic predators than the temporary puddles typically used by B. variegata . We test for behavioural adaptations in tadpoles to these different levels of predation. B. bombina tadpoles are significantly less active than B. variegata , both before and after the introduction of a predator to an experimental arena; this reduces their vulnerability as many predators detect prey through movement. Behavioural differences translate into differential survival: B. variegata suffer higher predation rates in laboratory experiments with three main predator types ( Triturus sp., Dytiscus larvae, Aeshna nymphs). This differential adaptation to predation will help maintain preference for alternative breeding habitats, and thus serve as a mechanism maintaining the distinctions between the two species.