Historical marine ecology using non-traditional data sources reveals the impact of local and global processes over half a century

Human impacts on earth span centuries, yet scientific studies cover a fraction of this time. Historical records and citizen scientist data are a useful resource for the long-term studies needed to understand and respond to pressures on nature, yet their quality and validity have been challenged. To explore how such non-traditional sources can be used to understand historical ecological change, we studied a site (Shiprock) with long-term citizen science activity in the Sydney, Australia region. We analysed approximately 6000 taxonomic records and reports revealing substantial ecological changes between 1965 and 2020, including the local disappearance of some fish and invertebrate species and declines in the abundance of many taxa including kelp. We found indications of potential range extensions more frequently from the north than the south, consistent with patterns expected from the global processes of climate change. We compared the relative advantages and limitations of the two main citizen science data collection modes: structured surveys and opportunistic presence records. Structured surveys provided broadly scientifically useful ecological information including species richness, populations, community structure and temporal change. Opportunistic data had the potential to provide long-term retrospective community information and species presence, but were limited in the ability to provide species absence, biomass, populations, community structure and coverage of remote areas. Our study allowed the development of scientifically- and managerially-relevant insights encompassing foundation, threatened, protected and invasive species, community shifts and the impacts of local and global processes over historical timescales.