Main Text: Ecologists have traditionally focused on networks of
feeding relationships among species (direct trophic interactions) to
understand community structure and dynamics . Despite an increasing
awareness of the importance of non-trophic and indirect interactions
(mediated by a third species or via the environment) , integrating the
different types of interactions into a single network has proven
extremely difficult and has not yet been done empirically, despite
relatively recent efforts and developments . This is because networks
quickly become very complex, as the number of indirect interactions
increases exponentially with the number of species involved .
Amongst the non-trophic interactions, higher-order interactions (HOIs)
or interaction modifications (the modulation of a pairwise interaction
by a third species) , play a particularly important role in stabilising
species coexistence . For instance, they can attenuate negative
interactions, such as when non-prey species increase the persistence of
predation-susceptible species by modifying the ability of predators to
detect prey . This way, HOIs can promote diversity and the long-term
persistence of ecological communities . HOIs are also regulated by
feedbacks, meaning that their influence on interactions changes over
time with population densities . This context-dependency of interactions
makes it difficult to predict the effects of perturbations on ecosystems
(e.g., species loss) , with consequences for conservation and management
strategies. Thus, investigations of interaction modulation by HOIs are
unveiling a new, and previously unaccounted for, higher level of
community structuring processes. However, HOIs cannot be observed from
studying traditional pairwise networks, unlike other indirect effects
(e.g., apparent competition) , because they often involve several other
species or environmental factors beyond the interacting pair. The role
of HOIs in shaping ecological communities has mostly been explored
theoretically or by experiments with particular subsets of communities
and tractable laboratory model systems testing specific effects . How
pervasive and influential these effects are at a community level,
remains largely unknown.
Here we hypothesised that a complex network of HOIs could be constantly
modifying pairwise interactions and shaping ecological communities, and
that consequently the outcome of pairwise interactions would be a
product of many influences from distinct sources. We predicted that the
manipulation of any species (or group of species) within the community
– or even of one of their by-products (e.g. deadwood, leaf-litter,
O2, faeces) – would reverberate throughout the entire
community and thus modify apparently unrelated interactions. We
performed experimental manipulations of species living on the tropical
shrub Baccharis dracunculifolia D.C. (Asteraceae), a highly
self-contained system with a diverse arthropod fauna (for details about
the study system see the Supplementary Material). In different
treatments, we excluded all ant species or live or hatched insect galls
of the dominant galler species. Each treatment, including a control with
no species exclusion, consisted of 16 replicated plants of B.
dracunculifolia . Over two months, every week we quantified the changes
in densities of several other species or guilds (hereafter groups; ants,
herbivores, predators, and aphids) as well as changes in direct
interactions involving two gallers, such as gall induction (herbivory),
parasitism by wasps, and inquilinism (sharing occupation of the gall for
shelter and feeding) by aphids. Specifically, we combined direct
observation and gall dissection data to quantify the effects of groups
on each other’s population densities or traits, including direct trophic
and non-trophic interactions and the effect of the exclusion of a group
on another group (hereafter, density effects; Fig. 1). We also
investigated how the direct interactions changed in different contexts
defined by the density or exclusion of a third group (for detailed
methods see the Supplementary Material). This allowed the construction
of a unique “effect network” based on multiple manipulations performed
simultaneously on the same system, investigating the same direct
interactions under different contexts. Links were categorised into two
types: node modulation (node-to-node effects), which are pairwise
trophic and non-trophic interactions or density effects; and link
modulation (HOIs; node-to-link effects), which are three-way
interactions (interaction modification), or four-way interactions
(modification of an interaction modification).