Introduction

To quantify the impact of biodiversity loss on human well-being, ecological research has measured biodiversity-ecosystem functioning (BEF) relationships in experiments and in the field (Tilman et al. 2014; Duffy et al. 2017). Even though the importance of biodiversity for providing ecosystem functions is supported by increasing empirical evidence, the quantitative relationships vary remarkably across communities and sites (Cardinale et al. 2007; Duffy et al. 2017; van der Plas 2019), calling for a systematic understanding of the underlying mechanisms.
Many studies argue that complementarity in how plants use abiotic resources is the main driving force behind positive plant diversity-productivity relationships (Barry et al. 2019). However, the productivity of plants not only depends on how they access and compete for resources, but is also strongly influenced by interactions with herbivores and animals of higher trophic levels (Schneider et al. 2016; Barnes et al. 2020; Albertet al. 2022). In addition, research on BEF relationships did not systematically address the consequences of spatial structures such as spatial heterogeneity in plant distribution and resource availability as well as spatial integration by local and large-scale movement of animals. While resource-based interactions between plants are spatial processes constrained to a plant’s immediate neighbourhood (Chesson 2000a), recent evidence draws attention to community assembly processes that affect biodiversity maintenance in BEF experiments based on the meta-community (Bannar‐Martin et al. 2018; Furey et al.2022), highlighting the importance of also considering processes at larger spatial scales. This includes interactions of plants with animals at higher trophic levels that integrate local effects over larger spatial distances (McCann et al. 2005; Ryser et al. 2021). Thus, this raises the question of how the interactions between animal- and resource-based mechanisms and the different scales they are associated with explain BEF patterns, such as the plant diversity-productivity relationship, and their variance at the community scale?
Traditionally, BEF research focuses on the relationship between plant diversity and productivity emerging at the community level (Cardinaleet al. 2007). Only recently, investigating the implications of local interactions between plant individuals and their immediate neighbours (hereafter: neighbourhood scale; Fig.1; Sapijanskas et al. 2013; Fichtner et al. 2018) has started. At this scale, individual plants access different parts of the total available resources (e.g., the resource pools in the soil) depending on their resource acquisition strategies (e.g., functional traits) and the proportion of space they can access (e.g., spatial spread of their roots). The latter adds a spatial component to plants’ resource-use. Reducing the spatial resource overlap between neighbouring plant individuals (Fig. 1A) makes them complementary in their access to resources as it reduces the strength of their competitive interactions and thereby renders competitive exclusion less likely (Chesson 2000b). While this spatial segregation of plants’ resource-use facilitates coexistence, it potentially imposes constraints on resource acquisition and productivity. For example, if two plants have mostly complementary resource requirements, they may benefit from having a spatial resource overlap. These arguments suggest that an increased spatial resource overlap could increase productivity at the community scale at the cost of a higher likelihood of local competitive exclusion. As competitive exclusion results in lower plant diversity, this can have negative feedback on plant community productivity, calling for a more systematic understanding of resource-mediated interactions between plants at the neighbourhood scale and their importance for plant diversity- productivity relationship.
While plants can interact through a local spatial resource overlap, animal movement couples even distant plants, for instance when herbivores move to switch resources. This movement of herbivores yields apparent competition between plants (Fig. 1C, spatially-non nested), which can impose strong negative effects on the productivity and survival of the two resource plants (Holt 1977). At higher trophic levels, populations of larger species such as top predators with large home ranges (Tucker et al. 2014; Hirt et al. 2021) will integrate energy fluxes across sub-food webs assembled from populations of plants, herbivores and smaller consumers. This creates a spatially nested food web structure with local food webs nested in the home range of top predators (Fig. 1C). As a result, apparent competition emerges among less mobile herbivores due to a shared, more mobile predator. This spatial structure of natural food webs opposes the widespread classic concepts that assume well-mixed and therefore spatially non-nested food webs. Instead, the spatially nested food webs will display much higher levels of complexity. Additionally, a spatial coupling of energy fluxes from sub-food webs by top predators can have stabilizing effects (McCannet al. 2005). As food web stability also increases the realized diversity of plants and eventually the productivity of plant communities (Schneider et al. 2016; Albert et al. 2022), spatially nested food web structures should also increase the productivity of the plant community. Considering the strong impact animals can have on plant community composition and functioning, the consequences of representing food webs either as spatially nested or non-nested could be substantial as they significantly differ in how they couple individuals and populations.
Processes at different spatial scales, ranging from competition for abiotic resources between neighbouring plants to apparent competition and large-scale integration of food webs by top predators, simultaneously affect functions within an ecosystem. Recent studies emphasized the importance of integrating such processes that act at different spatial scales in meta-communities (Furey et al. 2022) and meta-ecosystems (Gounand et al. 2018), especially when considering their implications for BEF relationships (Gonzalez et al. 2020; Furey et al. 2022). Despite their importance for community dynamics and functioning, the interactions among these processes have yet to be explored. As a result, our mechanistic understanding of how spatial interactions between plants via their resources or through higher trophic levels affect community-level functions is severely limited.
To address this issue, we introduce a spatially-explicit model of plant individuals that can access local resource pools of their direct neighbours. By integrating this plant-resource model with a spatially-explicit food web model, we investigate how resource competition and multi-trophic interactions interact across spatial scales to shape diversity-productivity relationships in plant communities. We hypothesize that, (1) positive diversity-productivity relationships can only emerge when plants are able to interact through a spatial resource overlap. Further, a spatially nested food web structure will introduce processes at different spatial scales. We therefore expect that (2) herbivore-induced apparent competition will have negative effects on plant productivity, whereas (3) spatial integration of sub-food webs by top predators should balance local dynamics and increase apparent competition between herbivore populations, minimizing competitive exclusion of plants and leading to an increase in their diversity and productivity.