Associational defence reduces grazing and deforestation in a kelp forest ecosystem: Implications for restoration

Kelp forests are declining globally under warming seas, altered predator–prey dynamics and intensified herbivory, with sea urchins often driving shifts from canopy‐dominated forests to persistent urchin barrens. Management commonly relies on labour‐intensive urchin removals, with variable success, underscoring the need for complementary, ecosystem‐based tools that directly reduce urchin grazing pressure.

Focussing on declining far northern California kelp forests, emblematic of general trends in global kelp loss, we combined subtidal surveys, field experiments and laboratory feeding assays to assess whether the understorey seaweed Desmarestia herbacea can provide associational defence for the canopy‐forming kelp ( Nereocystis luetkeana ) by physically deterring sea urchin ( Strongylocentrotus purpuratus ) grazing and offering low nutritional pay‐off for these grazers.

Surveys in intact forests revealed a strong positive association between D. herbacea and N. luetkeana . In urchin barrens, live D. herbacea and algal‐mimic barriers reduced urchins by up to 98% inside plots and by >60% in the surrounding area extending 0.25 m from the plot boundary, and decreased kelp tissue loss at the higher‐grazing site. In a 6‐week experiment, natural D. herbacea stands sustained protection of N. luetkeana from urchin grazing relative to removals, depressing urchin densities over time and reducing encroachment from nearby barrens, yielding a net increase in N. luetkeana abundance by Week 4, and extending kelp survival by ~1.7 weeks. In 24‐h feeding assays, urchins consumed D. herbacea and another understorey alga ( Pterygophora californica ) at roughly half the rate of N. luetkeana . Over 30 days, urchins consumed similar amounts of D. herbacea and P. californica , but urchin gonadosomatic indices were lower on the D. herbacea diet, indicating reduced energetic payoff.

Synthesis and application. Together, these results show that D. herbacea can protect N. luetkeana by limiting urchin access in the field and by depressing urchin condition when consumed. Incorporating D. herbacea and functionally similar understorey seaweeds into kelp forest restoration strategies highlights the value of more holistic, nature‐based approaches, such as leveraging associational defences to maximize recovery of focal foundational species and improve restoration outcomes in northern California and elsewhere.