Introduction
Increases in species’ abundances and diversity towards the equator are
well documented (Dobzhansky, 1950; Willig et al. , 2003), and may
be associated with a heightened intensity of biotic interactions at
lower latitudes (i.e. , the “biotic interactions hypothesis”;
Mittelbach et al. , 2007; Schemske et al. , 2009). Evidence
for stronger biotic interactions in the tropics has been found for many
mutualisms and antagonisms (Schemske et al. , 2009; Zvereva &
Kozlov, 2021); however, evidence for latitudinal gradients in herbivory,
one of the most ecologically significant biotic interactions, remains
mixed (Anstett et al. , 2016; Zhang et al. , 2016).
Herbivory is particularly important because it involves a large
proportion of the biodiversity and energy flows in terrestrial
ecosystems (Becerra, 2007; Agrawal et al. , 2012). The classical
latitudinal herbivory hypothesis (LHH) suggests that plant losses to
herbivory decrease with latitude (Coley & Aide, 1991; Johnson &
Rasmann, 2011). Although this hypothesis has been popular in the
literature, its generality has been called into question in the last ten
years because latitudinal trends in herbivory tend to be highly variable
in both strength and direction (Moles et al. , 2011; Moles &
Ollerton, 2016).
Some biases among LHH studies may partly explain the inconsistency in
patterns reported thus far. The LHH was initially formulated as a
community-level hypothesis, in which herbivory was expected to be
greater in tropical versus temperate communities (Coley & Aide, 1991;
Coley & Barone, 1996). However, many recent studies have focused on
herbivory gradients within specific clades or even single species
(i.e. , species found along a latitudinal gradient), rather than
community-wide herbivory (see review in Anstett et al. , 2016).
Such non-random, species-level
measurements of herbivory are likely to bias conclusions regarding both
absolute levels of herbivory and spatial patterns in community-wide
herbivory (Zvereva & Kozlov, 2019). Furthermore, plant species may be
unevenly distributed among communities (varying in abundance), and thus
the results of LHH studies may be affected by species-specific factors,
as well as limited by species’ ranges (Anstett et al. , 2016).
Therefore, direct measurements of community-wide herbivory are better
suited to studies of the LHH, as originally proposed, and essential for
understanding the consequences of herbivory for biodiversity and
ecosystem functioning (e.g. , nutrient cycling, productivity and
trophic transfer rates) (Anstett et al. , 2016; Zvereva et
al. , 2020).
Abiotic and biotic correlates of latitude can potentially affect
community-wide herbivory through both intraspecific and interspecific
pathways. At the intraspecific level, abiotic and biotic factors can
drive variability in herbivory by directly influencing the abundance of
insect herbivores (Maron et al. , 2014) and/or by indirectly
altering plant susceptibility to herbivore damage (Moreira et
al. , 2018; Loughnan & Williams, 2019). For example, plant populations
growing under more benign climates may suffer greater herbivore damage,
as longer growing seasons increase the number of insect generations and
warmer winters result in lower insect mortality (Sakata et al. ,
2017). In addition, plants growing in high resource environments may
also invest less in defense traits (Hahn & Maron, 2016) and are
therefore more susceptible to herbivore damage.
Apart from causing intraspecific variability in herbivory, latitudinal
abiotic and biotic correlates are also predicted to alter the
composition and diversity of natural plant assemblages and to modify the
abundance of individual plant species (i.e. , community evenness)
(Yang et al. , 2011; Ma et al. , 2017), ultimately affecting
community-wide herbivory. These “species turnover effects” result from
species’ differences in susceptibility to herbivory and also levels of
adaptation to abiotic environmental conditions (Agrawal, 2007). For
example, the resource availability hypothesis posits that lower-resource
environments can select plant species with slow growth rates, but
enhanced herbivore resistance (Coley et al. , 1985; Hahn & Maron,
2016). Individual species differences lead to habitat-driven turnover in
plant community composition, potentially resulting in divergent patterns
of community-wide herbivory over space. Overall, abiotic and biotic
latitudinal correlates can affect community-wide herbivory through both
intraspecific and/or interspecific pathways. However, the relative
importance of these two pathways remains largely uninvestigated.
To test for latitudinal patterns in community-wide herbivory and to
evaluate the relative roles of intraspecific herbivory variability
versus species’ turnover effects, we conducted a survey of both
population- and community-wide herbivory along a 1,500-km latitudinal
gradient in the grasslands of the Eastern Qinghai-Tibetan Plateau.
Grasslands are the dominant ecosystem on the Qinghai-Tibetan Plateau, a
diverse region with highly variable abiotic conditions (Yao, 2019);
grassland communities in this region vary dramatically in composition,
productivity and species richness, and plant defense traits have been
shown to vary along environmental gradients within the region (Xiaoet al. , 2021). In this study, we asked: 1) Does community-wide
herbivory decrease with latitude and, if so, what is the relative
importance of intraspecific herbivory variability versus species’
turnover in driving this latitudinal pattern? 2) What abiotic
(i.e. , climatic and edaphic factors) and biotic (i.e. ,
plant community factors) factors drive latitudinal variation in
population- and community-wide herbivory, and how are these drivers
related to intraspecific herbivory variability and/or species’ turnover
effects?