Introduction
A large body of research in the last several decades has investigated
potential factors that can promote the structural and dynamical
stability of complex food webs and their constituent populations. These
factors include hierarchically ordered feeding ), characteristic
predator-prey body mass ratios (Brose et al. 2006), allometric
degree distributions of feeding links (Otto et al. 2007),
compartmentalization (Stouffer & Bascompte 2011), weak interaction
links including weak omnivory and reduced predation pressure at low
densities
(, pairwise negative correlation between interaction strengths , and
self-regulation (e.g., cannibalism, intraspecific interference, ), among
others (reviewed by ). More recently, studies have started incorporating
different types of interactions in complex food webs (multiplex or
multi-layer networks; ) to account for multiple types of ecological
interactions, such as mutualisms and parasitism, in which organisms
simultaneously engage in natural communities.
A ubiquitous feature of natural systems is that almost all organisms
grow in size during their lifetime and switch diets, trophic positions,
species interacting with, and habitats as they grow . Such ontogenetic
development introduces life-history stages and flows of biomass between
the stages through growth and reproduction to food webs, collectively
forming complex multi-layer ecological networks. Studies have shown that
ontogenetic diet shifts have far-reaching effects on competitive and
predator interactions, population dynamics, and community structure in
small food web modules . The persistence of consumers can be enhanced in
life-history structured communities through biomass overcompensation in
consequence of ecological asymmetry between different stages (e.g.,
juveniles are better competitors than adults; ). Such asymmetry,
however, can also be expected to destabilize populations by inducing
cohort cycles or alternative stable states without a predator . Research
on how these effects in small food web modules may scale up to an entire
complex food web is still in its infancy, and so are the tools to
generate life-history structured complex food webs in a biologically
justifiable manner.
Studies have reported the mixed effects of including a stage structure
on the stability of complex food webs . Rudolf & Lafferty (2010) found
that, using static topological models of food webs, structural
robustness to species removal was lower with a stage structure than
without. They pointed out that species might be more sensitive to
resource loss when ontogenetic stages were sequential resource
specialists. Bland et al. (2019) used population dynamical models of
complex food webs and showed that non-stage-structured food webs lost
twice as many consumer taxa as stage-structured webs, while the
variability of biomass dynamics did not differ. Mougi (2017) also used
similar population dynamical models and concluded that species
persistence (the fraction of species persisting in a food web) increased
as the proportion of stage-structured species increased in the food webs
and that the effect was more pronounced in food webs with a greater
number of species and interactions. More studies are needed to elucidate
the role of a stage structure on persistence and stability and how it
may come about in complex food webs.
Rudolf & Lafferty (2010) and Bland et al. (2019) used the niche model
(Williams & Martinez 2000) to generate network topologies and split a
node into stages to create a stage-structured taxon (nodes represent
taxa, and interacting taxa are connected by links in ecological
networks). The niche model has a demonstrated ability to produce many
observed structural properties of empirical food webs despite its
simplicity and has been the most widely used food web structural model.
Splitting a node, as in Rudolf & Lafferty (2010) and Bland et al.
(2019), can nontrivially modify the food web topology generated by the
niche model, likely compromising the desirable properties of the food
webs. Therefore, it is unclear how realistic the modified food webs in
these studies would still have remained after new nodes and links were
added to incorporate a life-history structure. Firstly, minimizing the
alteration of the network topology generated by the niche model is
desirable because it is capable of producing realistic food web topology
and because food web data resolved to life-history stages to verify the
topology of food webs with stage-structured taxa are currently very
scarce. Secondly, the niche model generates a “trophic species,” which
is a functional group defined to consist of one or more taxa (e.g.,
species, genus, ontogenetic stages) that share the same sets of
predators and prey . Life-history stages of a species are distinguished
for their distinct ecological roles, at least partly by their
characteristics related to feeding, so that a life-history stage can be
considered as a whole trophic species (not a fraction of it). Based on
these interpretations and the observation that ontogenetic diet shifts
are widespread in nature (Werner & Gilliam 1984), a plausible
alternative approach is to instead group nodes generated by the niche
model to assemble a stage-structured taxon. This approach would allow
preserving mostly the original topologies of the food webs from the
niche model. No study has investigated this approach before.
We took the grouping approach to introduce a stage structure into
complex food webs. Following Bland et al. (2019), we applied the
allometric trophic network (ATN) model of biomass dynamics to the
stage-structured food webs on which we linked stages by biomass flow via
growth and reproduction. We motivated the food webs studied here from
aquatic communities at temperate and northern latitudes. It is well
established that consumer-resource interactions are hierarchically
structured largely by body size in aquatic communities because of the
indeterminate growth of fishes and gape-limited predation . We found
that food webs with stage-structured consumers tended to be less
dynamically variable and supported a greater number of species than food
webs with non-stage-structured consumers.