Physical defence of the wild cucumber Echinocystis lobata in an invasive range changing seed removal by rodents

Invasive alien plant species in a new location usually lose their native enemies, but new interactions with local herbivores can also significantly influence their population dynamics. Seed predators have a strong effect on the seed banks, seedling recruitment, and establishment of a plant population. A given plant’s effective defence of its seeds from seed predators ensures its survival. Here, we describe a new kind of physical protection in the wild cucumber (Echinocystis lobata) in its invasive range: the production, in the central part of the fruit’s fibre frame, of 1 or 2 additional seeds which cannot be released. Research was conducted in riparian habitats in the central part of Poland. After performing three different studies to examine this phenomenon, we found that, on average, 34% of individual wild cucumber fruits contained additional trapped seeds which could not be released. Our results showed that trapped seeds have the same ability to germinate as normally released seeds, but that they differ from normal seeds in weight and shape. The frequency of removal of trapped seeds by rodents was significantly lower than that of normally released seeds. This mechanism is likely to change seed distribution in the wild cucumber’s environment.