Developing together: The elementome and biogeochemical niche of the mutualistic occupants of a fig microcosm

In brood-site pollination mutualisms, where flowers provide nutrition and
shelter to pollinator offspring in exchange for pollination, resource
allocation to inflorescences is directly related to the fitness of plants
and pollinators. We determine resource allocation to components of an
enclosed monoecious Ficus inflorescence or syconium that, besides seeds,
also houses and provides nutrition to pollinator wasp offspring, each
developing within individual uniovulate galled flowers. Along with
biomass, we determine elemental concentrations as parameters of resource
allocation. For the first time, we apply the biogeochemical niche (BN)
concept to a mutualism and construct the BN of syconial occupants using
the elementomes and stoichiometric ratios of plant, seed, and pollinator
tissue. We predicted that BNs of seeds and galls containing wasps should
differ due to differences in tissue type and to facilitate their
co-development. We also measure trophic stoichiometric ratios (TSRs) for
various elements to determine resource mismatch between consumers and
resources. We found that the syconium wall, which insulates and protects
developing seeds and wasps, constituted 58% of syconial biomass.
Individual pollinators and their galls were significantly heavier than
seeds indicating that their development is resource- intensive. As
predicted, seeds and adult female pollinators had significantly different
BNs, highlighting differences in nutritional needs of these mutualistic
occupants within a shared nutrient-providing resource. Pollinators had
significantly lower C:N and C:P ratios compared to the syconial wall
indicating limitation of N and P within host resources. The BN of
pollinator wasps was distinguished by significantly higher concentrations
of nitrogen, phosphorus, zinc and sulphur compared to the syconium wall or
seeds. TSR of >4 for nitrogen and sulphur highlight the heightened
resource mismatch that pollinators likely face for these elements during
their development. We found no overlap in the BNs of male and female
pollinator wasps, likely due to their starkly different anatomical and
functional traits. Overall, our study demonstrates how BN and TSR can
reveal trading of resources within mutualisms highlighting non-overlapping
requirements of elements and the potential limitations it can pose for
resource providers and consumers. These parameters can serve as common
currencies for comparisons across mutualistic interactions.