Little long-term change in regional species richness of tropical butterflies over the past 166 years masks turnover in community composition.

Most information on biodiversity changes is from the last few decades despite species responding to environmental changes for centuries. Longer-term information is needed to contextualize whether recent changes reflect longer-term trends. We focus on tropical regions, which are exceptionally biodiverse but contain many species that are currently threatened. We integrate historical and contemporary data from museum collections and online records for 45 butterfly species from Sulawesi (Indonesia) to explore species richness trends over the past 166 years (1857-2022), test whether recent trends mirror longer-term trends and examine whether species that are endemic, forest-dependent and/or host-plant specialists have declined the most. Over the 166-year time period, we found no systematic decline in overall species richness, despite shorter-term multi-decadal changes (positive, stable and negative trends). Recent trends generally did not match longer-term trends. Contrary to expectation, we found long-term increases in some species, particularly those that are non-endemic or open-habitat tolerant, whereas endemic and/or forest-dependent species showed more mixed trends, either stable or declining. We find long-term stability in regional species richness, but this masks composition changes that include more non-endemic and open-habitat species over time. Short-term fluctuations, spanning a few decades, did not reflect longer-term patterns, highlighting challenges in determining robust patterns of biodiversity change.