Structure and robustness of kelp-forest food webs along a temperature gradient

Background/Question/Methods

The boom-and-bust dynamics of kelp forests have long-captured the attention of ecologists. Giant kelp (Macrocystis pyrifera) itself is sensitive to temperature, but the organisms within kelp forests vary in sensitivity, making the response of food webs difficult to predict. The Northeast Pacific is increasingly subject to warming events, such as the 2014-2015 “Blob” and the 2021 Western North America heat wave, with evidence that kelp forests in many areas are declining. The Santa Barbara Channel (SBC), California, USA is an overlap zone between the San Diegan (warm) and Oregonian (cold) floristic provinces and is characterized by variation in thermal conditions spatially and temporally. Given this variability and the diverse assemblage of species in the kelp forest food web, we asked: How will kelp forest food webs respond to changing temperatures? Specifically – how will food-web structure and robustness change? Using a recently published food web for SBC kelp forests (a metaweb for the region), we constructed food webs representing different thermal environments based on species distribution records. We calculated structural metrics and analyzed robustness for each food web under different extinction sequences. Because networks varied in size, results were compared to null models for each network.

Results/Conclusions

The metaweb for the region contained many species with a broad thermal range, with 69.3% of species present in both cold and warm provinces. Few species (3.3%) were associated only with the cooler Oregonian province, and 27% of species were associated with the warmer San Diegan province. Because a large proportion of species was present in both provinces, structural metrics only differed slightly between the warm and full food webs. However, the cold food web had lower link density, connectance, and shorter longest chain length than the other two webs, at least in part due to the smaller network size. The cold food web was slightly less robust than the warm or full food web, to loss of both the most connected species as well as random extinctions. Secondary extinctions accumulated more quickly in the cold web, but the overall shape of the extinction curves was similar for both extinction sequences. Warmer kelp forests are likely to be more complex and species rich, and this complexity adds redundancy that makes them more robust to species declines.