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