Effects of temperature on sexual competition in three kelp species, Pterygophora californica , Nereocystis luetkeana , and Macrocystis pyrifera : Implications for range shifts in foundation species

SUMMARY Kelp forests are threatened by rising ocean temperatures. However, the effects of warming on early life stage interactions among co‐occurring kelp species, particularly competitive dynamics and reproductive timing, remain poorly understood. This study investigates the effects of temperature on embryonic sporophyte competitive interactions and reproductive timing of three co‐occurring kelp species: Macrocystis pyrifera , Pterygophora californica , and Nereocystis luetkeana through laboratory‐based sporophyte density and gametogenesis experiments. Results indicate that temperature significantly affects kelp sporophyte competitive dynamics. P. californica sporophytes outcompeted M. pyrifera sporophytes at lower temperatures (8 and 12°C), while M. pyrifera sporophytes outcompeted both P. californica and N. luetkeana sporophytes at 16°C. N. luetkeana exhibited consistently lower sporophyte density compared to the other species. Reproductive timing also varied with temperature, with earlier egg production observed at higher temperatures for all three kelp species. These findings suggest that climate‐induced warming could alter the competitive landscape of kelp forests, potentially favoring M. pyrifera over other species. Understanding these complex interactions is crucial for predicting the ecological consequences of ocean warming and developing effective conservation strategies for kelp ecosystems.