Primary producers provide energy and habitat that is vital for maintaining biodiversity. Understanding linkages between different types of primary producers and the species that depend on them would allow us to predict how productivity losses due to disturbance or climate change affect biodiversity and ecosystem function. To understand the relationship between primary production and biodiversity, we used a high-resolution food web to gain a mechanistic understanding of energy flow via trophic interactions along with a remotely-sensed kelp biomass and phytoplankton production time series. We first asked, what food-web structures should be supported by kelps vs phytoplankton? Second, which food-web state is more vulnerable to extinctions? Finally, how often are these conditions occurring in space and time? Using a published meta-web for kelp forests of the Santa Barbara Channel, CA, we constructed food webs representing different sources of primary production sources. We determined the number of consumers feeding directly on each production source and assessed the vulnerability of consumers to extinction based on the number of resources available after a given production source declined. We calculated tracked co-extinctions following loss of consumers relying on focal production sources, as well as random extinctions. We then identified annual averages of kelp-canopy biomass and Net Primary Productivity (NPP) for each site (annual sites) that had a threshold abundance of kelp-canopy area using remotely-sensed data.
Food webs without kelps or diatoms were not substantially different in structure. More consumers relied directly on diatoms than kelps (135 vs. 39). However, there were more immediate co-extinctions when kelps and kelp detritus were removed from the web (7 species) than when diatoms were removed (2 species). When extinction sequences were based on usage of these resources, the extinction of kelp-consumers led to more co-extinctions than loss of species feeding on diatoms, even though more species consumed diatoms. One kelp-consumer in particular, a small snail species, triggered many co-extinctions as it serves as an essential host for many parasite species. Throughout Southern California, the majority of annual sites had lower-than-average kelp production and NPP (8176 annual sites), while the second most common state was low kelp production and higher-than-average NPP (5451 annual sites). Only 3441 annual sites had both higher-than-average NPP and kelp production, while 3708 annual sites had high kelp production and lower-than-average NPP. This suggests that the majority of annual sites are vulnerable to local extinction of kelp consumers and subsequent co-extinction of associated species.