Ecotone effect on assemblage-level tip-based metrics
While we expected higher diet transition rates, lower stasis time, and
shorter transition times in ecotone than core assemblages, we found
that, relative to core species, ecotone species presented 1) fewer
transitions in diet over their evolutionary history, 2) quicker
transition periods, and 3) slightly longer retention of the current
diet.
The existence of patches of favorable habitat can prevent evolution at
ecotone zones. Patches of favorable, high-quality habitat can be
ephemeral and sparsely distributed along ecotones, but can sustain large
population sizes with individuals presenting little or no shifts in
ecological, morphological and behavioral characters over time (Eckert et
al. 2008; Sexton et al. 2009). These processes result in few transitions
from ancestral diet because the retention of an optimal feeding strategy
enables species persistence in ecotone zones. This strategy could be a
generalist diet that generally evolves as an option to explore resources
from different habitats (Price et al. 2012). Also, patches of favorable
habitat along ecotones can provide the stability needed to maintain the
current diet since long time ago, perhaps since late Miocene or early
Pliocene when major sigmodontine tribes diversified (Leite et al. 2014;
Steppan and Schenk 2017) and within-clade morphological disparity
increased (Maestri et al. 2017).
Shorter stasis time at ecotone assemblages indicates that trait
evolution occurred at more regular periods of time along the
evolutionary history of ecotone species. Speciation along regular
periods prevent the accumulation of time between transition events, and
can be produced by regular cycles of environment change which are first
noticed by ecotone species (de Vivo and Carmignotto 2004; Karanth et al.
2006; Donoghue and Edwards 2014). As ecotones buffer environmental
changes, there may be thousands of years of lag between the beginning of
environmental changes and modifications of species traits. Although the
difference we found here seems to be subtle (Figs. 2-4), it represents
thousands of years of lag that may have profound influence on species
persistence and trait evolution.
We found a stasis time of around 2.5 ma for both core and ecotone
species. It is a long time period under little to no trait evolution
relative to the ~10 ma of sigmodontine presence in the
Neotropics. Although we do not know the exact geological period in which
diet stasis occurred, cooling periods such as the one embracing late
Miocene and early Pliocene (Amidon et al. 2017) may well have
facilitated diet retention over large time periods. Cooling periods
through the Cenozoic are related to speciation slowdowns across major
tetrapod clades, likely due to the influence of temperature on the
environment’s carrying capacity (Condamine et al. 2019).