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.