How complete are insect inventories? An assessment of the British butterfly database highlighting the influence of dynamic distribution shifts on sampling completeness

Much recent scientific, media and public attention has focussed on the evidence for and consequences of declines in insect biodiversity. Reliable, complete inventories can be used to estimate insect trends accurately, but incomplete data may distort assessments of biodiversity change. Thus, it is essential to understand the completeness of insect inventories. Assessing the database of Great Britain butterfly occurrences, likely the most complete database for any group of insects in the world (with 10,046,366 records for 58 butterfly species), we found that only 62% of the cells have complete inventories at the finest scale evaluated. The dynamic nature of butterfly distributions in response to climate change could explain this result, as the distribution of completeness values is related to the increasing occurrence of some species at higher latitudes as a consequence of recent range expansions. The exceptional quantity of information collected in Great Britain about this appealing group of insects is insufficient to provide a complete picture. Consequently, we cannot expect to build complete inventories for less popular taxa, especially in less comprehensively sampled countries, and will require other techniques to understand the full extent of global biodiversity loss.