Persistent loss of animal diversity on a rocky shore over nine decades and across multiple investigators (data and code)

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 first surveyed in 1931-1933, then resurveyed in detail in 1993. After 1993, the transect was surveyed 16 more times, with surveys nested within four distinct investigator eras. We addressed two goals with our dataset. First, we used a causal framework to detect temporal change in biodiversity (species richness, Hill-Shannon and Hill-Simpson diversity, and species evenness) in the context of investigator era and then attribute biodiversity change to seawater temperature. Second, we tested the hypothesis that population change in abundance was associated with the geographic range of species. Over thirty years (1993-2023), species richness showed no linear trend but was associated with investigator era; we hypothesize that sampling effort contributed to this effect. In contrast, we observed weak linear declines in the diversity of common taxa (Hill-Shannon), dominant taxa (Hill-Simpson), and evenness over the recent thirty years, but no evidence of investigator effects. We estimated a moderate negative effect of maximum sea surface temperature on Hill-Shannon diversity (-0.4 common taxa C-1). Relative to the historical baseline in the 1930s, there was no evidence for change in richness but strong evidence for change in diversity metrics. On average, Hill-Shannon diversity declined by 41-61%, Hill-Simpson diversity declined by 32-56%, and species evenness declined by 41-68% since the 1930s. Declines in diversity were associated with the rise of a few numerically dominant species and declines of many other species. On average, species with a southern geographic range increased in abundance more than coastwide species, reenforcing previous work on the historical transect that demonstrated a fingerprint of seawater warming. Taken together, this case study emphasizes the value of marine stations in providing a venue for sustained periodic ecological monitoring at small spatial scales, long duration, and fine taxonomic resolution.