Balancing restoration and conservation: Source material selection for restoration of an endangered Australian seagrass, Posidonia australis

Restoration of endangered species is an essential but challenging feature of the global response to biodiversity loss. Sourcing material from endangered species for restoration must not compromise conservation of existing populations, which can limit large-scale restoration opportunities. On Australia's east coast, Posidonia australis is a slow-growing endangered seagrass with limited seed production. Effective restoration relies on adult fragments sourced through direct transplanting from existing meadows, or from naturally detached “beach-cast” fragments. We assessed the relative success of these two donor sources and the effect of fragment size by monitoring shoot density, necrosis cover, and epiphyte cover over 18 months post-planting. Plots containing directly transplanted fragments maintained a shoot density of 97 %, while plots containing beach-cast fragments initially declined before stabilising at 60 % of the original density. Notably, mixed-source plots (50 % from each donor source) retained 91 % density. Fragment size (1–2 vs. >2 shoots) did not significantly impact seagrass density, indicating that smaller fragments—common in beach-cast material—remain viable for restoration. Beach-cast seagrass exhibited high necrosis in the first three months, likely due to physiological stress from beaching, but their long-term performance was comparable to directly transplanted fragments. These findings highlight a key trade-off in P. australis restoration: while direct transplantation yields higher density retention, it is constrained by legal protection and ecological concerns surrounding donor meadow harvesting. Beach-cast fragments, though more readily available, exhibit higher initial losses after planting. A mixed-source approach may provide an optimal balance, leveraging the resilience of directly transplanted fragments while minimising the ecological impact of donor harvesting. • Beach-cast fragments are viable for Posidonia australis restoration, but outcomes improve with the addition of directly transplanted shoots. • Beach-cast fragments are initially vulnerable to loss, but their longer-term performance converges towards directly transplanted fragments. • Density retention of beach-cast seagrass fragments with few shoots (1-2) is similar to larger fragments (>2 shoots).