Life-history structure and food web stability
How a life-history structure may affect the stability and persistence of
complex food webs has not been much studied. It is not immediately clear
whether it is stabilizing or destabilizing based on existing theories.
We can expect multiple aspects of stage-structured populations to
contribute to instability. As discussed by Rudolf & Lafferty (2010),
when stage classes have smaller subsets of the feeding range than the
species as a whole, resources essentially become less substitutable,
especially when the overlaps between feeding ranges are small. Thus, if
resources for one stage become scarce, the persistence of the entire
species is greatly endangered unless growth and reproduction can
constantly replenish the dwindling stage. Similarly, as stages become
more specialized, consumer-resource interactions may become less diffuse
and some of the remaining interactions may strengthen. Because weak
interactions tend to stabilize trophic interactions (McCann 2012;
Gellner & McCann 2016), specialized stages likely reduce stability of
food webs. Also, a stage structure introduces delays and asymmetry
between stages into population models, both of which are known to often
cause population instability in the forms of cohort cycles and
alternative stable states . Therefore, the odds seem to be against
increased food web stability by introducing life-history stages.
Stages in structured populations can subsidize dwindling stages through
growth and reproduction, which is probably one of the main reasons why a
stage structure in food webs could enhance the persistence of
stage-structured populations and other dependent populations.
Furthermore, biomass flow via growth or reproduction between competing
stages with overlapping diet might moderate the destabilization effects
of exploitative competition. showed that the exploitative competition
module reduced food web persistence as it increased in frequency in
dynamical models of complex food webs. This effect of diet overlap may
appear contradictory to the result from Rudolf & Lafferty (2010), which
showed that, when feeding niches were overlapping by more than about
30%, the inclusion of stages increased the robustness of food webs.
Because they studied the robustness of static food webs (only topology,
no dynamics), diet overlap reduced reliance of a stage-structured
population on any particular resource, as the authors explained. In
dynamic models, exploitative competition can ensue and drive one of the
competitors and possibly other populations to extinction, but if the
competitors are ontogenetic stages of the same species, biomass flow
between the stages could alleviate competitive exclusion. In the study
by Stouffer & Bascompte (2010), the frequency of the tritrophic food
chain module had positive effects on persistence in large food webs. In
a sense, life-history structured populations contain a biomass flow
chain inside. We conjecture that this might also contribute to food web
stability. In addition to the possible adverse effects on stability we
discussed above, ontogenetic asymmetry may also help some populations
persist in food webs. showed in stage-structured food web modules that
persistence of consumers could be promoted in communities with
stage-structured prey through emergent facilitation due to biomass
compensation in the prey population. It seems reasonable to state that
the effects of life-history stages on the stability of complex food webs
are complex and contingent on the balance of the effects of different
processes.