Significance

In a world of rapid environmental change, effective biodiversity conservation and management relies on our ability to detect changes in species occurrence. While long-term, standardized monitoring is ideal for detecting change, such monitoring is costly and rare. An alternative approach is to use historical records from natural history collections as a baseline to compare with recent observations. Using a database of opportunistic museum and citizen science records, we found significant declines in occupancy in eight and significant increases in six species in the period 1900-2013, consistent with estimates of change obtained using more standardized resurvey data. Our findings indicate that our approach enables robust estimates of temporal trends from opportunistic specimen and observation data, thus facilitating the use of these data in biodiversity conservation and management.