Invasive species are a major threat for native ecosystems and organisms living within. They are reducing the biodiversity in invaded ecosystems, by outcompeting native species with e. g. novel substances. Invasive terrestrial plants can release allelochemicals, thereby reducing biodiversity due to the suppression of growth of native plants in invaded habitats. Aside from negative effects on plants, allelochemicals can affect other organisms such as mycorrhiza fungi and invertebrates in terrestrial ecosystems. When invasive plants grow in riparian zones, it is very likely that terrestrial borne allelochemicals can leach into the aquatic ecosystem. There, the often highly reactive compounds may not only elicit toxic effects to aquatic organisms, but they may also interfere with biotic interactions. Here we show that the allelochemical 2-methoxy-1,4-naphthoquinone (2-MNQ), produced by the ubiquitously occurring invasive terrestrial plant Impatiens glandulifera, interferes with the ability of Daphnia to defend itself against predators with morphological defences. Daphnia magna and Daphnia longicephala responded with morphological defences induced by chemical cues released by their corresponding predators, Triops cancriformis or Notonecta sp. However, predator cues in combination with 2-MNQ led to a reduction in the morphological defensive traits, body- and tail-spine length, in D. magna. In D. longicephala all tested inducible defensive traits were not significantly affected by 2-MNQ but indicate similar patterns, highlighting the importance to study different species to assess the risks for aquatic ecosystems. Since it is essential for Daphnia to adapt defences to the current predation risk, a maladaptation in defensive traits when simultaneously exposed to allelochemicals released by I. glandulifera, may therefore have knock-on effects on population dynamics across multiple trophic levels, as Daphnia is a key species in lentic ecosystems.
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