Impact of the 2011 Tohoku earthquake on the species diversity of rocky intertidal sessile assemblages

The impacts of large-scale disturbance events on the species diversity of rocky intertidal sessile assemblages across multiple spatial scales are not well understood. To evaluate the influence of the 2011 Tohoku Earthquake on alpha and beta diversities of rocky intertidal sessile assemblages, we censused sessile assemblages in the mid-shore zone from 2011 to 2019. The census was conducted across 22 study sites on five rocky shores along 30 km of the Sanriku Coast of Japan, which is located 150–160 km north–northwest of the earthquake epicenter. Alpha diversity was measured with three Hill numbers (H₀, H₁, and H₂), which represent the number of equally common species that would exist in a community with the same diversity as the sampled community, with higher values of the subscript indicating more weight placed on abundant species. Beta diversity was measured with two metrics (BD_total at two spatial scales). Values were compared between the years 2011–2019 and the pre-earthquake period (2003–2010). The results show that the Tohoku Earthquake significantly altered the species diversity of intertidal sessile assemblages across multiple spatial scales. All diversity metrics obtained at multiple spatial scales (i.e., alpha diversities: H₀, H_1, and H₂; beta diversities: BD_total at the shore and regional scales) decreased immediately after the earthquake and then increased in subsequent years. Two years after the earthquake, H₀ recovered to within the range of pre-earthquake values and H₁ and H₂ became significantly higher than pre-earthquake values. Most metrics of alpha and beta diversities recovered to pre-earthquake levels after several years, but regional BD_total remained low for a longer period.