A major challenge in biodiversity research is the incorporation of species interac-
tions into frameworks describing population and community response to global
edfnmental change (GEC). Mutualisms are a type of species interaction especially
sensitive to changing environmental conditions, and the breakdown of facilitative
species interactions could amplify the negative impacts of novel climate regimes
on focal species.
2. Here, we investigate how reciprocal interactions between two coastal founda-
tion species, the eastern oyster (Crassostrea virginica) and eelgrass (Zostera marina)
shift in sign and magnitude in response to ocean warming (+1.5°C) and acidifica-
tion (−0.4 pH) via a manipulative co-culture experiment in mesocosms.
3. Under ambient environmental conditions, oysters facilitated eelgrass leaf growth
and clonal reproduction by 35% and 38%, respectively. Simultaneously, eelgrass
decreased the oyster condition index (the ratio of tissue to shell biomass) by 35%,
indicating greater allocation of energy to shell growth instead of soft tissues at
ambient conditions. Varying sensitivities of each species to ocean warming and/
or acidification treatments led to complex shifts in species interactions that were
trait dependent. As such, community outcomes under future conditions were in-
fluenced by species interactions that amplified and mitigated species response to
environmental change.
4. Synthesis: Given that species interaction effect sizes were similar in magnitude
to effect sizes of warming or pH treatments, our results underscore the need to
identify key species and interaction types that strongly influence community re-
sponse to GEC. Specifically, for macrophyte-bivalve interactions, understanding
how physiological limitations on growth are impacted by environmental hetero-
geneity and co-culture will support the successful restoration of natural popula-
tions and the rapid expansion of aquaculture.