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.