Weeds are selected to produce overwhelming propagule pressure, and while
vertebrate and invertebrate seed predators destroy a large percentage of
seeds, their ecosystem services may not be sufficient to overcome
germination site limitations. Cover crops are suggested to facilitate seed
predation, but it is difficult to disentangle reductions in weed recruitment
attributable to granivores from those due to plant competition. Using common
lambsquarters as a focal weed species, we used experimental seed subsidies
and differential seed predator exclusion to evaluate the utility of
vertebrate and invertebrate seed predators in fallow, killed cover crop, and
living mulch systems. Over two growing seasons, we found that seed predators
were responsible for a 38% reduction in seedling emergence and 81% reduction
in weed biomass in fallow plots following simulated seed rain, suggesting
that granivory indeed overcomes safe-site limitation and suppresses weeds.
However, the common lambsquarters densities in ambient seedbanks across
fallow and cover crop treatments were high, and seed predators did not
impact their abundance. Across the study, we found either neutral or
negative effects of vertebrate seed predators on seed predation, suggesting
that invertebrate seed predators contribute most to common lamnsquarters
regulation in our system. These results imply that weed seed biocontrol by
invertebrates can reduce propagule pressure initially following senescence,
but other tools must be leveraged for long-term seedbank management.