4 Discussion
Our results declared that both N addition and drought reduced the
complexity of interaction network via reducing
CFG, ID and IS (Figure 2). The decreasing
CFG and ID of arthropod networks suggested that some
species have been lost (Eklof & Ebenman, 2006; Tylianakis et al., 2010)
and the decreasing IS suggested that the interdependence among
functional groups was weakened (Rzanny & Voigt, 2012). Notably, N
addition reinforced partial relationships between some herbivores and
carnivores, which broke the balance of the network and reduced its
complexity, while drought eliminated some relationships among functional
groups by filtering out some plant species that were related to
arthropods. Compared with the ever-lasting effect of N addition and
drought on arthropod communities in the peak of growing season, delayed
snowmelt had a direct effect on arthropods only in the early growing
season (Figure.S1); although delayed snowmelt had no direct effect on
arthropod community in the peak growing season, it had significant
interactive effects with N addition and/or drought on partial arthropod
functional groups (Table.1&2).
Specifically, the effect of plant N could be cascaded up to higher
trophic levels in N addition plots (Fig.1c&2), and foliar N was widely
believed to have profound influence on arthropod’s interaction both
within and among trophic levels (Ilić et al., 2021). Changes in foliar N
affect network complexity by interacting with foraging behavior of
herbivores, and such effect could also be cascaded up (Bukovinszky et
al., 2008). In our experiment, the relative abundance of plant suckers,
detritivores and vegetation hunters was increased by N addition
(Fig.1c). Generally, abundance of nitrophilous consumers increased due
to elevated N content of primary producers when they were relieved from
N restriction (Haddad et al., 2000; Elser et al., 2007; Wimp et al.,
2010). For instance, aphids or leafhoppers (plant suckers) fed by
piercing the phloem of their food plant and thus they are sensitive to
changes in plant quality (Muller et al., 1999); In particular, a
significant increase in relative abundance of plant suckers and
detritivores resulted in reinforcement of partial relationships (e.g.,
plant suckers vs. web spiders, detritivores vs. parasitoids) under N
addition treatment; however, plant chewers (e.g. grasshopper) were
insensitive to N addition (Ritchie, 2000) (Table.3). The changes of
different functional groups could be attributed to the specific
responses of their N-related traits to N addition. Thus, N addition
could weaken the net effect of other interactions thereby reducing
ecosystem network complexity by strengthening trophic chains in single
pathway (Fig.1&2).
Drought and its interaction with N addition both decreased the network
complexity as indicated by declined connectance, ID and IS (Fig.2). Our
results showed such decrease was realized not only
through
increasing foliar N, but also via reducing plant species richness
(Fig.1b&2).
Generally,
more diverse producer communities could support more diverse primary
consumers, which could be cascaded up to secondary consumers (Scherber
et al., 2010; Rzanny & Voigt, 2012). Indeed, our results highlighted
the negative effect of reduced plant richness on arthropod network
features under drought conditions (Fig.1&2). Many network properties
had been changed due to species loss and especially its resulted loss of
relationship among arthropods within the networks under drought
conditions. For example, omnivores (e.g. ants and beetles) had been
transformed into the core of networks under drought conditions because
they are better adapted to dry environments with low plant diversity.
Such dry habitats are less attractive to most other arthropods, and are
thus difficult to maintain stable interactions among arthropods and
between arthropods and plants. More importantly, omnivores were found to
be linked with many more other functional groups under experimental
drought (Fig.1b&1d), suggesting that omnivores may play a critical role
in maintaining the arthropod networks (Fig.1b&1d). Although some
species could still adapt to the dry habitats, the change of
microhabitat structure significantly impact arthropod functional groups
and could not offer a suitable environment for predation (Fig.3). For
instance, experimental drought decreased the relative abundance of web
spiders and their links with other functional groups due to decreased
vegetation height and uniformity (Figure.1). Weaving webs requires
appropriate two-dimensional or complex three-dimensional environment
(Gibson et al., 1992), but drought-induced high structural heterogeneity
and low vegetation height are disadvantageous for predation of web
spiders, leading to the disappearance of links of web spiders with plant
suckers and parasitoids. Similar results had been obtained by different
ecosystem studies measuring structure of food webs both qualitatively
and quantitatively (Ledger et al., 2012; Woodward et al., 2012;
Rosenblatt & Smith-Ramesh, 2017; Amundrud & Srivastava, 2019).
However, several researches demonstrated that plant biomass is the key
driver of arthropod community under multiple global changes in
grassland, but the plant species richness of their study sites was much
lower than ours (Wimp et al., 2010; Prather et al., 2020). Although
drought strongly impacted plant biomass in the present study (Table.1),
the spatial structure (βbal and βbal) of
plant communities did not change significantly (Fig.4). Therefore,
compared to the variation of plant biomass in grassland, the existence
of plant species under climate changes is more important and thus
appears the key factor for the arthropod ecological network stability
and complexity (Brodersen et al., 2018).
Delayed snowmelt enhanced abundance of arthropod community in June, but
showed little effect during the rest of the growing season (Fig.S1),
despite that, delayed snowmelt showed significant interactive effects
with drought or N addition on part of functional groups (Table.2&3;
Fig.S1). For example, the interaction of delayed snowmelt with drought
or N addition significantly influenced parts of carnivorous functional
groups, including web spiders and
parasitoids,
which suggests that the global change events occurring in different
seasons may interact (Table.2&3). It is conceivable that the effects of
multiple global change factors on plants may be amplified and cascaded
up to partial functional groups at higher trophic levels, ultimately
leading to the mismatch between trophic levels (Post & Forchhammer,
2008; Rosenblatt & Smith-Ramesh, 2017), which will undoubtedly affect
ecosystem stability (Baert et al., 2018; De Laendera et al., 2016; De
Laender, 2018). The responses of carnivores as a whole to the
interaction of snowmelt delay with drought or with N addition were
modest, but significant changes did occur to its finer functional groups
(including decreasing ground hunters and parasitoids). Especially, the
diversity of parasitoids and web spiders differed in the responses to
the interaction of snowmelt delay with drought (Table.2&3). It means
that the responses of arthropods to multiple global change factors might
be dependent on their specific traits (Voigt et al., 2003). Even now the
reason for the responses of these functional groups remains unclear, but
our study suggests that it is necessary to examine more complete taxa or
functional groups in future researches, with the purpose of capturing
the panorama of the responses of entire ecosystems. Meanwhile, long-term
and slow global changes such as snowmelt can cause subtle biases in
ecosystems that need to be taken into account in global change studies
(Kroel-Dulay et al., 2015).