Density‐driven bottlenecks constrain restoration of macroalgal forests

Large‐scale restoration of Mediterranean macroalgal forests is set to accelerate under new policy commitments, yet many initiatives rely on high initial settlement densities despite limited understanding of how density regulates early recovery. If early demographic bottlenecks constrain survival, growth or reproduction, restoration strategies based on maximising numbers may undermine success.

We experimentally manipulated initial settlement density of the canopy‐forming brown alga Cystoseira compressa across four densities (10, 100, 250 and 500 individuals) spanning natural to highly elevated levels. Recruits were monitored every 3 months for 1 year in the field to quantify survival, growth, reproductive development and the timing and intensity of density‐dependent bottlenecks.

Survival declined across all densities but showed strong density dependence. The lowest density maintained high survival throughout the experiment, whereas intermediate and high densities experienced rapid early mortality. At the highest densities, survival collapsed within the first 2–3 months, indicating acute early bottlenecks rather than gradual mortality. Mean final densities at the end of the experiment ranged between 0.19 and 0.44 ind. cm ⁻² .

Density strongly constrained individual growth. The lowest density rapidly progressed into larger size classes and was the only density where individuals reached reproductive maturity. Higher densities suppressed growth of surviving individuals, resulting in persistent confinement to small size classes and preventing reproductive development despite continued survival.

Quantification of survival derivatives revealed that both the timing and intensity of bottlenecks scaled predictably with initial density. Bottlenecks were absent at lower densities, prolonged at intermediate densities and short but severe at the highest densities, demonstrating that early recovery trajectories were density‐driven rather than time‐dependent.

Synthesis and applications . This single‐species and single‐site experiment provides evidence that high‐density restoration strategies accelerate mortality, suppress growth and delay reproduction, whereas low‐density approaches promote faster demographic progression and resilience. Under the conditions tested here, focusing on growth and reproductive output, rather than maximising initial numbers, reduces the need for large propagule supply and extensive receptacle production, directly supporting the conservation of donor populations while maintaining restoration effectiveness.