The existence and strength of higher order interactions is sensitive to environmental context

One strategy for understanding the dynamics of any complex system, such as
a community of competing species, is to study the dynamics of parts of the
system in isolation. Ecological communities can be decomposed into single
species, and pairs of interacting species. This reductionist strategy
assumes that whole-community dynamics are predictable and explainable from
knowledge of the dynamics of single species and pairs of species. This
assumption will be violated if higher order interactions (HOIs) are
strong. Theory predicts that HOIs should be common. But it is difficult to
detect HOIs, and to infer their long-term consequences for species
coexistence, solely from short-term data. I conducted a protist microcosm
experiment to test for HOIs among competing bacterivorous ciliates and
test the sensitivity of HOIs to environmental context. I grew three
competing ciliate species in all possible combinations at each of two
resource enrichment levels, and use the population dynamic data from the
one- and two-species treatments to parameterize a competition model at
each enrichment level. I then compared the predictions of the
parameterized model to the dynamics of the whole community (three-species
treatment). I found that the existence, and thus strength, of HOIs was
environment-dependent. I found a strong HOI at low enrichment, which
enabled the persistence of a species that would otherwise have been
competitively excluded. At high enrichment, three-species dynamics could
be predicted from a parameterized model of one- and two-species dynamics,
provided that the model accounted for nonlinear intraspecific density
dependence. The results provide one of the first rigorous demonstrations
of the long-term consequences of HOIs for species coexistence and
demonstrate the context-dependence of HOIs. HOIs create difficult
challenges for predicting and explaining species coexistence in nature.