Intraspecific herbivory variability, rather than species’
turnover effects, determined latitudinal variation in community-wide
herbivory
The evidence found here for greater herbivory at lower latitudes is in
agreement with the LHH, and concurs with studies of other systems
including salt marsh plants (Pennings & Silliman, 2005; Penningset al. , 2009) and terrestrial plants (Anstett et al. ,
2014; Baskett & Schemske, 2018). However, to the best of our knowledge,
our study is the first to show a decrease in community-wide herbivory
with latitude. Several studies have conducted multispecies analyses by
sampling diverse sets of representative species in two or more locations
(e.g. , Pennings et al. , 2009; Zvereva et al. ,
2020); however, this is no substitute for the direct evaluation of
community-wide herbivory in most instances (Zvereva et al. ,
2020). When considering single species, a decline in herbivory at higher
latitudes might also result from a reduction in abundance; abundance is
a well-studied species-level feature positively affecting herbivory (the
so-called “resource concentration effect”; Underwood et al. ,
2014; Kim & Underwood, 2015). Therefore, earlier studies, which did not
account for the abundance of individual plant species, likely yielded
biased estimates of community-wide herbivory and, consequently, of the
overall effects of background herbivory on ecosystem-level processes
(Anstett et al. , 2016; Zvereva et al. , 2020; Moreiraet al. , 2021).
Latitudinal variation in community-wide herbivory is expected to result
from two additive components, intraspecific herbivory variability and
species’ turnover effects. In this study, we found that the decline in
community-wide herbivory with latitude mirrored similar declines in both
intraspecific herbivory variability and observed herbivory levels for
three common plant species. This indicates that latitudinal variation in
community-wide herbivory was mainly driven by decreases in herbivory on
individual plant species, rather than by shifts in community
composition. This relatively weak role for species’ turnover effects is
inconsistent with a recent study of boreal forests (taiga) (Zverevaet al. , 2020). Zvereva et al. (2020) found that
latitudinal shifts in community composition (the proportion of deciduous
plants increased with latitude) counterbalanced a simultaneous decrease
in herbivory on deciduous species; the net result was a lack of
latitudinal variation in community-wide herbivory (Zvereva et
al. , 2020).
This inconsistency among studies may be attributed to the higher plant
species richness in grasslands (c . 20–30 plant species in a 0.5
× 0.5 m2 quadrat) and correspondingly low abundance of
each plant species (e.g. , 96.3% of 54 plant species had percent
cover < 5%; Ma et al. , 2017); for species showing
significant latitudinal gradients in herbivory, there were no
corresponding latitudinal shifts in abundance. For example, the three
most broadly-distributed plant species (P . macrophyllum ,S . pulchra and T . mongolicum ) showed
latitudinal gradients in herbivory, but the biomass of each species did
not vary with latitude. The relatively high herbivory levels observed
here could affect plant fitness, and thus competitive ability (Agrawalet al. , 2012); as a result, a compensatory increase in abundance
or biomass may have been lacking. Hence, we conclude that the abundance
of each plant species in a community should be taken into account when
studying latitudinal variation in community-wide herbivory and
detangling the relative importance of intraspecific herbivory
variability versus species’ turnover.