Passing the quadrat: inferring biodiversity change over time and across investigators

Long-term ecological monitoring inevitably requires a passing of the quadrat from one investigator to another. Here we present the challenge and opportunity of inferring biodiversity change over time and across investigators using a rocky intertidal case study. The intertidal transect was surveyed initially by Willis Hewatt in 1931-1933, then resurveyed in detail by Rafe Sagarin, Sarah Gilman, and their mentors in 1993. In total, the transect has been resurveyed 17 times over the past three decades, with annual surveys nested within five distinct investigator eras. We addressed two goals with our dataset. First, we asked whether temporal change in several biodiversity metrics (species richness, Hill-Shannon and Hill-Simpson diversity, spatial beta diversity) was sensitive to investigator era. Second, we identified recent ecological winners and losers, relative to the historical baseline, and asked whether such designations can change over time (1993-2023). We show that investigators contributed to variable trends in species richness over time, and we hypothesize that sampling effort contributed to this effect. In contrast, we highlight declines in the biodiversity of common species (Hill-Shannon) and dominant species (Hill-Simpson) over time, in the absence of investigator effects. These declines were associated with the rise of a few numerically dominant species and a trend of increasing spatial similarity. Relative to the historical baseline, ecological winners included certain barnacles, anemones, and turban snails, while losers included mussels, urchins, limpets and a host of more cryptic taxa. However, we show that such designations can be complicated by non-linear population trajectories and discuss mechanisms related to climate change versus potential artifacts related to investigator effort. We finish with recommendations and emphasize the value of marine stations in providing a venue for sustained periodic ecological monitoring at small spatial scales, long duration, and fine taxonomic resolution.