Plant dispersal syndromes are unreliable as predictors of zoochory and long-distance dispersal by ungulates and waterbirds

Plant dispersal syndromes are allocated based on diaspore morphology and used to predict mechanisms of dispersal. Many authors assume that only angiosperms with endozoochory, epizoochory or anemochory syndromes have a long-distance dispersal (LDD) mechanism. Too much faith is often placed in classical syndromes to explain historical dispersal events and to predict future ones. The "endozoochory syndrome" is actually a "frugivory syndrome" and has often diverted attention from endozoochory by non-frugivores (e.g. waterbirds and large herbivores) that disperse a broad range of angiosperms, for which they likely provide the maximum dispersal distances. Neither the endozoochory nor the epizoochory syndromes provide helpful predictions of which plants non-frugivores disperse, or by which mechanism. We combined data from Albert et al. (2015a), Soons et al. (2016) and Julve (1998) to show that only 4% of European plant species dispersed by ungulate endozoochory belong to the corresponding syndrome, compared to 36% for ungulate epizoochory and 8% for endozoochory by migratory ducks. In contrast, the proportions of these species that are assigned to an "unassisted syndrome" are 37%, 31% and 28%, respectively. Since allocated syndromes do not adequately account for zoochory, empirical studies often fail to find the expected relationship between syndromes and LDD events such as those underlying the colonization of islands or latitudinal migration. We need full incorporation of existing zoochory data into dispersal databases, and more empirical research into the relationship between plant traits and the frequency and effectiveness of different dispersal mechanisms (paying attention to unexpected vectors). Acknowledging the broad role of non-frugivores in facilitating LDD is crucial to improve predictions of the consequences of global change, such as how plant distributions respond to climate change, and how alien plants spread. Networks of dispersal interactions between these vertebrates and plants are a vital but understudied part of the Web of Life. The datasets we present here illustrate these limitations of syndromes, and include data from Brochet et al. (2010) regarding the syndromes of plants dispersed by Eurasian Teal via epizoochory or endozoochory.