This experimental study showed that waterbirds can play an important role in dispersal of both native and alien plant species. Seeds of all native and alien plant taxa were retained for up to 24 hours in the digestive system of birds, and germinated after gut passage. We also found strong effects of seed traits on all aspects of the endozoochory process. This suggests that invasive species may have particular traits that correlate with a higher endozoochory potential, and our experimental approach can be useful to identify traits that facilitate long-distance dispersal of alien species.
Effect of passage rate and retention time
Supporting our first hypothesis, we found that alien species have a higher overall passage rate than native species. This greater seed survival of alien species might be explained by the shorter retention time of alien plant seeds in the digestive tract of waterfowl. This result also indicated that, alien wetland plant species can have higher propagule pressure, which is a significant predictor of invasiveness (Colautti et al., 2006). A high propagule pressure increases the chances of establishing self-sustaining populations in new environments (Lockwood et al. 2009). Previous works have shown that plants with smaller and rounder seeds tend to have a higher survival rate in the digestive tracts of waterfowl than large or elongated seeds (Soons et al. 2008; Lovas-Kiss et al. 2020). This might partly explain the higher passage rate and lower retention time of the alien species, although even when these seed traits were included in our models, the influence of indigenous status (i.e. native or alien) remained significant. This suggests that there might be additional seed traits of key importance which contribute to the differences between alien and native species. Such a trait could be for instance seed hardness.
Effect of gut passage in germinability
Germinability reflects seed vigour and viability (Jiménez-Alfaro et al. 2016), that has a positive effect on species establishment, spread and impact (Colautti et al., 2006). Alien species commonly have a higher germinability than native species, and can germinate under broader environmental conditions (Gioria and Pyšek 2017). These patterns were observed for the control seeds in our experiment, for which germinability of alien species was higher (Table 2). However, effect disappeared when we controlled for differences in seed traits: smaller, more elongated control seeds had a higher germinability than larger, more round seeds, as recorded in previous studies (Vera 1997; Bu et al. 2016). These effects of indigenous status and seed shape were reversed for seeds that survived the various stressors during digestion, after which native species had a higher germinability. Perhaps the native seeds were better able to resist the mechanical scarification in the gizzard (Kleyheeg et al. 2018), leading to greater breaking of physiological seed dormancy (Lovas-Kiss et al. 2015).
Interestingly, previous studies found similar contradictory effects on germinability. Soons et al. (2008) found that smaller seeds had higher germinability, while Kleyheeg et al. (2018) found the opposite, although they did not control for seed shape. Gioria and Pyšek (2017) argued that germinability under one set of controlled conditions is not a reliable predictor of invasiveness, because germination patterns are strongly dependent on environmental conditions, so that alien species can only be expected to have a competitive advantage from high germination under certain conditions. For example, changing the salinity can radically change the germination response of wetland plants, and interact with the effects of gut passage by waterfowl (Espinar et al. 2004). In addition, the fact that the inclusion of seed traits had major effects in our models of germinability underlines the crucial role of seed traits, and suggests that the remaining partial effects of indigenous status may ultimately be explicable based on additional traits (e.g. thickness and water permeability of seed coat. Therefore, further studies with a larger set of species with a more diverse set of seed traits are needed to clarify these issues.
Seed dormancy plays a key role in the germination of the seeds, and can occur in several forms. Physiological and physical dormancy are the most common forms, of which physiological dormancy is the most common in aquatic non-salt tolerant (non-halophyte) plant species such as those that we used in our experiments. For example, Bidens frondosa needs approximately 270 days to break its dormancy after a cold stratification, so its low germinability (see Table 1) may be explained by this reason (Baskin & Baskin, 2014). Only physical dormancy is considered to require gut passage by animals to be broken, leading to accelerated germination (Penfield 2017). However, our results bring the paradigm of dormancy strategies into question, since gut passage increased germinability in five of our twelve study species (Table 1). Theoretically, all the tested seeds had physiological dormancy, but they still seemed to be affected by the physical and chemical effects during their gut passage, similar to previous findings by (García-Álvarez et al., 2015). It is also important to note that despite detailed research into a range of factors (e.g. temperature, light, water, chemical concentrations) that stimulate entry into and exit from dormancy in such plants (Baskin & Baskin, 2014), the role of gut passage has so far been ignored. Further research is needed to study the interactions between gut passage and other variables driving dormancy and germination in alien plants with physiological dormancy.
Relation time, time to germination and passage rate
The time a seed spends until germination has a crucial effect on its colonization potential and success. Species with fast germination can gain competitive advantages and can speed up the time needed for recruitment of the next generation (Jiménez-Alfaro et al. 2016). We found the control seeds of alien species showed significantly faster germination compared to native species, which is in line with previous studies (Pyšek & Richardson, 2007; Gioria & Pyšek, 2017). Germination time is often decreased by gut passage, but often also varies according to retention time (Brochet et al., 2010; Garcia-Alvarez et al. 2015), which in turn is related to seed traits that can differ between alien and native species. The seeds of native species stayed longer in the digestive tract, partly due to their bigger size, and this is likely to explain why they germinated faster after gut passage than alien seeds (Brochet et al. 2010). Overall, seed traits did not influence the effect of passage on germination time. We found that elongated seeds, both for controls and gut passage, persistently needed less time for germination. This result is consistent with previous studies showing that elongated seeds tend to germinate faster, compensating for other ecological effects in which seeds of such a shape are less likely to reach the seed bank and are exposed to higher predation pressure (Grime et al. 1981; Thompson 1987; Thompson et al. 1993; Bu et al. 2016). The effect of earlier germination on the fitness of wetland plants still needs more investigation, because it can have costs from increased herbivory, as well as benefits from more rapid growth (Figuerola et al., 2005; Figuerola & Green, 2004).
Consequences for plant invasions
Dietary studies from Europe showed that ducks can disperse hundreds of plant species, including many aliens (Soons et al., 2016), and field data shows that this extends to many other waterbirds (see Introduction). Mallards (Kleyheeg et al. 2019) and other migratory waterbirds are very suitable vectors for long-distance dispersal (LDD). Although LDD events are rare, they are vital in facilitating population spread and determining local community structure (Nathan et al., 2008). We showed that waterfowl endozoochory can give alien plants an advantage for LDD by increasing their propagule pressure compared to native species, even though it can also delay germination. We also showed the important role of species traits in determining potential for endozoochory, notably through their influence on retention time and germination. Faster germination (together with a fast growth rate) can provide a competitive advantage, however, dispersal of a large number of small seeds with slower germination might also help species to avoid inappropriate environmental conditions, e.g. if seeds arrived in an early season. This combined with the faster growth rate and higher trait plasticity of alien species can ensure their overall competitive superiority (Ruprecht et al. 2014; Szabó et al. 2019). According to our results, gut passage did not affect the plasticity of germination time, however we need more information on its dependence on environmental variables. Such information supplemented with the knowledge on additional traits such as growth rate would allow us to predict the ability of alien species to expand their range.