1 Introduction
In natural ecosystems, the complex
interactions among arthropods play a key role in maintaining ecosystem
functions, but the current situation of arthropod diversity is worrying
(Cardoso et al., 2020; Eggleton, 2020; Wagner, 2020; Wagner et al.,
2021). The key point of maintaining biodiversity has shifted from the
protection of individual species to that of their interactions (Harvey
et al., 2017). To understand the strength and complexity of arthropod
interactions, it is necessary to expand the focus from single
interactions to the network analysis for all the interactions. More
importantly, the ecosystem complexity embodied in the interaction
networks is declining rapidly
(Pocock et al., 2012; Moreno-Mateos
et al., 2020). Considering the sensitivity of interactions among
arthropods to multiple global drivers (Tylianakis et al., 2008;
Rosenblatt & Smith-Ramesh, 2017; Hamann et al., 2020), ecological
networks may be facing more serious challenges than biodiversityper se (Valiente-Banuet et al., 2014).
Arthropods are encountering novel
suites of biotic and abiotic conditions as a result of rising N
deposition, more frequent drought events and warming-induced snowmelt
advancement (Bobbink et al., 2010; Huang et al., 2015; Zhu et al.,
2019). As a dimension of food
web-based cascading process, the bottom-up effects of plant communities
have been demonstrated to have a strong impact on most arthropods
(Abrams, 1995; Scherber et al., 2010). In fact, it is practically
impossible for us to observe all interactions of multi-trophic
communities at the same time. The interaction between consumers and
resources may be regulated by the characteristics of resources (variety,
quality and quantity), and arthropods also have to adapt to new habitats
due to global change. Thus, it is
urgent to understand the process of bottom-up effects of plant
communities that affected arthropods under the scenarios of global
change (Hortal et al., 2015; Sage, 2020).
First, changes in plant richness had
been demonstrated to have a strong positive bottom-up effect on
arthropods (Scherber et al., 2010; Castagneyrol & Jactel, 2012; Wan et
al., 2020), and thus it is crucial to maintain the complexity and
stability of multi-trophic interactions (Rzanny & Voigt, 2012). N
deposition and drought filtered out plant species in different ways and
then cascaded up to the loss of species at higher trophic levels (Dunne
& Williams, 2009; Stevens et al., 2010; Pocock, Evans & Memmott, 2012;
Craven et al., 2016). Second, plant nutrient quality is also the major
factor to influence arthropod communities. The N concentration in
host-plant tissue can consistently serve as one of the best predictors
for insect herbivores (Throop & Lerdau, 2004). The elevated plant N
concentration due to N deposition could significantly improve individual
herbivore performance, and induce changes in plant-herbivore
relationship through decreased plant defenses (Mattson, 1980; Throop &
Lerdau, 2004). Drought can also weaken plant defense and increase the
concentration of nutrients (Luo et al., 2018), and ultimately result in
the increase of chewing herbivory (e.g. grasshopper, but toxic in higher
N) (Franzke & Reinhold, 2011; Gutbrodt et al., 2011). Earlier snowmelt
can reduce herbivore abundance by shaping plants of low nutrient and
water content, even cascade up to affect the predation and mutualism of
higher trophic levels (Wipf & Rixen, 2016; Mooney et al., 2020). Third,
the enhanced primary productivity could increase consumer abundance and
diversity, especially in low plant diversity communities (Srivastava &
Lawton, 1998; McCary et al., 2021). However, such positive correlation
has been reversed under experimental drought (Prather et al., 2020) and
N addition (Haddad et al., 2000). Delayed snowmelt could decrease
primary productivity (van Wijk et al., 2003; Gamon et al., 2013; Kelsey
et al., 2021), and therefore potentially affect arthropod communities
(Wirta et al., 2015; Penczykowski et al., 2017). Finally, the changes of
microhabitat, such as the soil moisture and vegetation structure,
especially induced by N deposition, drought and advanced snowmelt, could
directly impact arthropods (Rosenblatt, 2016; Hamann et al., 2020). For
example, drought can reduce soil moisture and increase water loss from
arthropods, thus filter out drought intolerant species (Jamieson et al.,
2012; Barnett & Facey, 2016; Torode et al., 2016). N deposition is
found to have negligible direct impacts on arthropods (Throop & Lerdau,
2004; Johnson & Jones, 2017). Moreover, drought and N deposition may
alter the microenvironments for arthropods by changing vegetation
height, and ultimately affect the performance of arthropods. For
arthropods, the type of predation risk was modulated by vegetation
height, i.e., short or tall vegetation structure provide different
abiotic conditions and predation risk (Langellotto, 2004; van Klink et
al., 2015). Compared with low-vegetation environments, high-vegetation
environments can assure arthropod development and survival by
maintaining more stable soil temperature and reducing extreme climate
events (Cherrill & Brown, 1992; Bourn & Thomas, 2002; Bourn & Thomas,
2002; Roy & Thomas, 2003). On the contrary, some arthropods (e.g.,
beetles) hunt more efficiently in low-vegetation ground, while there are
more traps (e.g. Thomisidae and Araneidae) and parasitic (e.g.
Hymenoptera) risk in high-vegetation habitat (Gibson et al., 1992;
Morris, 2000; Langellotto & Denno, 2004).
In the last decade, the importance of
ecological networks was recognized again, and was proposed that the
strength and complexity of interactions were determinants of the
topological properties of the networks (Ings et al., 2009; Tylianakis et
al., 2010; Tylianakis & Morris, 2017). With the increasing awareness of
ecological networks, the analysis of species-based and functional
group-based interaction networks had greatly improved our understanding
of ecosystem processes (Ings et al., 2009; Tylianakis et al., 2010;
Rzanny & Voigt, 2012; Giling et al., 2019). Compared with species-based
method, functional group-based approach allows the comparison between
communities composed of different species and further help us better
understand the niche changes under global changes. In addition, most
studies of global changes focused on single factors, but recent
meta-analysis clearly indicated that the consequences of multiple global
change agents may be better understood if studied in concert (Hamann et
al., 2020; Wilson & Fox, 2020). Thus, we conducted a field experiment
in the natural grassland of northeast China with the intention to detect
the effects of N addition, simulated drought, and delayed snowmelt on
grassland arthropod ecological network. We modularized the whole
arthropod communities and characterized them as an interaction network.
We attempt to answer the following questions: (1) how does the network
complexity change under different global changes? (2) how does the
global change factor modify the interaction pattern of arthropod
functional groups?
2 Materials and methods