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Trait-mediated indirect effects and complex life-cycles in two European frogs

Summary: Trait-mediated indirect effects and complex
life-cycles in two European frogs
Res Altwegg*
Institute of Zoology, University of Zürich, Winterthurerstrasse 190, CH-8057 Zürich, Switzerland
Most animals actively avoid predators. If such a reaction reduces competitive ability, for
example by reducing food intake, predator presence can lead to trait-mediated indirect effects.
Because predator avoidance typically leads to reduced growth rather than reduced survival, its
effect on population processes is difficult to assess. This is especially true for organisms with
complex life-cycles, where predator avoidance during one stage is expected to lead to trait-
mediated indirect effects if it has effects reaching into the following life stages. I experimentally
investigated the effect of caged (thus non-lethal) dragonfly larvae on the competition between
tadpoles of two frog species (Rana lessonae and R. esculenta) and on juvenile frog survival
during the subsequent terrestrial stage. In response to caged predators, R. lessonae delayed
metamorphosis more than R. esculenta, but they both metamorphosed heavier. These
differences suggest the possibility of a competitive disadvantage for R. lessonae in the
presence of predators, which could lead to trait-mediated indirect effects. However, the presence
of predators did not modify competitive effects and had no measurable consequences on
terrestrial survival. Regardless of the presence of predators, competition during the larval stage
had large effects on metamorphosis and led to strongly decreased survival in the subsequent


Source: Altwegg, Res - Avian Demography Unit, Department of Statistical Sciences, University of Cape Town


Collections: Environmental Sciences and Ecology